ITU Centres of Excellence for Europe

Mobile Broadband: LTE/LTE-Advanced,

WiMAX and WLAN

Module 3:

WLAN/WiFi broadband access networks

MODULE 3:

WLAN/WiFi broadband access networks Contents WLAN/WiFi broadband access networks..............................................................1 3 Introduction ........................................................................................................3 3.1 IEEE 802.11 standard (2012)..........................................................................3 3.2 Broadband WLAN/WiFi: the IEEE 802.11n.....................................................9 3.3 WLAN network design ..................................................................................19 3.4 Quality of Service (QoS) and Security in WLAN ...........................................34 3.5 WiFi hotspot solutions...................................................................................41 3.6 4G mobile data traffic offloading to WLAN (from LTE/LTE-Advanced, and from Mobile WiMAX) ...........................................................................................45 3.7 Cognitive resource management in future wireless networks .......................48 3.8 Business and regulatory aspects of WLAN/WiFi...........................................52 3.9 Reference list ................................................................................................54

3 Introduction Wireless Local Area Networks (WLANs) represent a broadband access technology which provides high data rates on limited coverage, with great fidelity. That is why WLAN is usually seen as a complement technology to LTE or WiMAX used to serve hot-spots or locations where the demand for network capacity is extraordinarily high. The most proliferated WLAN technology is the one based on IEEE 802.11 standard, also known as WiFi (Wireless Fidelity). Considering its successful history and proved reliability, WiFi broadband access networks are still an active and evolving technology, which deserves significant attention among the plethora of wireless broadband technologies. This third module of the course discusses the latest moments of the WiFi technology evolution. We will start with the analysis of the newest standard edition IEEE 802.11 (2012), and its latest and most famous amendment 802.11n. This module also involves discussions regarding some inevitable aspects of WLAN network design, Quality of Service (QoS) and Security in WLAN. We also analyze the modern trends in WiFi hotspot solutions and 4G mobile data traffic offloading to WLAN. Finally Module 3 provides some visions of the cutting-edge developments in WLAN technologies represented by cognitive resource management in future wireless networks and the business and regulatory aspects of WLAN/WiFi technology.

3.1 IEEE 802.11 standard (2012) The purpose of IEEE 802.11 standard is to provide wireless connectivity for fixed, portable, and moving stations within a local area. This standard also offers to regulatory bodies a means of standardizing access to one or more frequency bands for the purpose of local area communication. The scope of this standard is to define one medium access control (MAC) and several physical layer (PHY) specifications for wireless connectivity for fixed, portable, and moving stations (STAs) within a local area. The original IEEE 802.11 standard was published in 1999 and reaffirmed in 2003. The first revision was published in 2007, which incorporated into the 1999 edition the following amendments: IEEE Std 802.11a™-1999, IEEE Std 802.11b™-1999, IEEE Std 802.11b-1999/Corrigendum 1-2001, IEEE Std 802.11d™-2001, IEEE Std 802.11g™-2003, IEEE Std 802.11h™-2003, IEEE Std 802.11i™-2004, IEEE Std 802.11j™-2004 and IEEE Std 802.11e™-2005. The latest revision IEEE 802.11-2012 specifies technical corrections and clarifications to 802.11 standard for wireless local area networks (WLANs) as well as enhancements to the existing medium access control (MAC) and physical layer (PHY) functions. It also incorporates Amendments 1 to 10 published in 2008 to 2011.

In particular, the current revision, IEEE Std 802.11-2012, incorporates the following amendments into the 2007 revision:

— IEEE Std 802.11k™-2008: Radio Resource Measurement of Wireless LANs (Amendment 1): Mostly used by AP manufacturers, this amendment makes additional radio and network information available to WLAN devices. This information is used to make real-time decisions about WLAN management, typically for better load balancing.

— IEEE Std 802.11r™-2008: Fast Basic Service Set (BSS) Transition (Amendment 2): This amendment is a good example of how various amendments interact, and sometimes even conflict with each other to degrade instead of enhance 802.11 performance. A critical aspect of Wi-Fi is mobility, and this typically involves a client device moving from AP to AP to maintain adequate signal strength as the user moves around. This movement creates a “transition” or a “hand-off” and it needs to take place relatively quickly for services to continue uninterrupted, especially time-sensitive services like VoIP over wireless (VoFi). As more and more amendments were added to 802.11 this transition time degraded significantly, causing problems for services like VoFi. This amendment addresses this degradation, making transitions as fast as they can be.

— IEEE Std 802.11y™-2008: 3650–3700 MHz Operation in USA (Amendment 3): 802.11y specifies a “light-licensing” scheme for U.S. users to take advantage of spectrum in the 3650 – 3700 MHz band, at power levels that are significantly higher than those used in the 2.4 or 5GHz bands. The use case for this technology will typically be for longer distance, point-to-point, backhaul communication using 802.11.

— IEEE Std 802.11w™-2009: Protected Management Frames (Amendment 4): 802.11w specifies methods to increase the security of 802.11 management frames. Management frames are 802.11 packets that control communication on the WLAN, but do not contain data. Examples include beacons, RTS/CTS, probe responses, acknowledgements, etc. Currently management frames are sent “in the clear” making them potentially vulnerable to malicious manipulation.

— IEEE Std 802.11n™-2009: Enhancements for Higher Throughput (Amendment 5): this amendment will be discussed in Section 3.2.

— IEEE Std 802.11p™-2010: Wireless Access in Vehicular Environments (Amendment 6): 802.11p deals with data exchange between high-speed vehicles, and between vehicles and a yet-to-exist roadside WLAN infrastructure based on licensed spectrum in the 5.85 – 5.925GHz band. Though the plans for this technology seem quite grandiose, activity in this area has been quite limited to date, and obviously has no affect on enterprise WLAN users. Any WLAN analysis capabilities for 802.11p are likely to be very specific to this particular application.

— IEEE Std 802.11z™-2010: Extensions to Direct-Link Setup (DLS) (Amendment 7): Direct link setup (DLS) allows WLAN client devices to connect directly to each other, bypassing the typical link through an infrastructure AP. This has many benefits, including an increase in speed (between the clients), an increase in network throughput (for all users), and an increase in overall service delivery, especially for multimedia (like a computer to DVR connection or a laptop to projector connection).

— IEEE Std 802.11v™-2011: IEEE 802.11 Wireless Network Management (Amendment 8): 802.11v provides a mechanism for wireless clients to share information about the WLAN environment with each other and APs to improve WLAN network performance in real time. Most client devices do not yet take advantage of this new capability, but as compatible devices (both client and infrastructure) come to market the need to analyze 802.11v packets will become very important, especially in determining if the information shared is really resulting in anticipated improvements.

— IEEE Std 802.11u™-2011: Interworking with External Networks (Amendment 9): This is an extremely hot topic in mobile computing, and one that will continue to get tremendous attention. It also requires solutions to some pretty difficult problems, including discovery, authentication, authorization, and compatibility, hence progress towards implementation has been slower than anticipated.

— IEEE Std 802.11s™-2011: Mesh Networking (Amendment 10): Mesh networking specifies an architecture and protocol for WLANs that use radio-aware metrics over self-configuring multi-hop topologies. Essentially, 802.11s enables the creation of high-performing, scalable, ad-hoc networks, often with no wired network access at all. One of the most widely discussed use cases is in emergency services, like those provided by the Federal Emergency Management Agency, where a scalable network needs to be set up very quickly in a remote area for a finite period of time. It is highly unlikely that enterprise customers will take advantage of mesh technology. WLAN analysis of mesh environments is likely to be highly specialized and mostly transient, so there’s no need for the typical enterprise-focused WLAN analysis system to deal with 802.11s, at least not anytime soon.

As a result of publishing the 802.11-2012 revision, all of the previously published amendments and revisions are now retired. In IEEE Std 802.11-2012, the order of clauses and annexes has also been revised. The result of this revised order on the numbering of clauses and annexes is summarized in Figure 1.

Figure 1 - Changes in clause numbers and annex letters from 2007 revision to

2012 revision

The IEEE 802.11-2012 standard has wide range of purposes. In particular, the standard:

— Describes the functions and services required by an IEEE 802.11™-compliant device to operate within independent and infrastructure networks as well as the aspects of STA mobility (transition) within those networks.

— Describes the functions and services that allow an IEEE 802.11-compliant device to communicate directly with another such device outside of an independent or infrastructure network.

— Defines the MAC procedures to support the MAC service data unit (MSDU) delivery services.

— Defines several PHY signaling techniques and interface functions that are controlled by the IEEE 802.11 MAC.

— Permits the operation of an IEEE 802.11-conformant device within a wireless local area network (WLAN) that may coexist with multiple overlapping IEEE 802.11 WLANs.

— Describes the requirements and procedures to provide data confidentiality of user information and MAC management information being transferred over the wireless medium (WM) and authentication of IEEE 802.11-conformant devices.

— Defines mechanisms for dynamic frequency selection (DFS) and transmit power control (TPC) that may be used to satisfy regulatory requirements for operation in any band.

— Defines the MAC procedures to support local area network (LAN) applications with quality-of-service (QoS) requirements, including the transport of voice, audio, and video.

— Defines mechanisms and services for wireless network management of STAs that include BSS transition management, channel usage and coexistence, collocated interference reporting, diagnostic, multicast diagnostic and event reporting, flexible multicast, efficient beacon mechanisms, proxy ARP advertisement, location, timing measurement, directed multicast, extended sleep modes, traffic filtering, and management notification.

— Defines functions and procedures aiding network discovery and selection by STAs, information transfer from external networks using QoS mapping, and a general mechanism for the provision of emergency services.

— Defines the MAC procedures that are necessary for wireless multi-hop communication to support wireless LAN mesh topologies.

The new IEEE 802.11 release is the product of an evolutionary process that has played out over five years and drawn on the expertise and efforts of hundreds of participants worldwide. More than 300 voters from a sweeping cross-section of

global industry contributed to the new standard, which has roughly doubled in size since its last published revision. In general, many telecommunication experts claim that IEEE 802.11-2012 revision has been expanded significantly the WiFi technology towards supporting communications between devices and networks that are faster and more secure, while offering improved Quality of Service and, improved cellular network hand-off. IEEE 802.11 standards already underpin wireless networking applications around the world, such as wireless access to the Internet from offices, homes, airports, hotels, restaurants, trains and aircraft around the world. The standard’s relevance continues to expand with the emergence of new applications, such as the smart grid, which augments the facility for electricity generation, distribution, delivery and consumption with a two-way, end-to-end network for communications and control, as well as applications dedicated to manufacturers, healthcare workers and retail service providers around the world. Regarding the technical novelties delivered by the new 802.11-2012 we can emphasize several of them such as, new support for 3.65 and 3.7GHz bands, to avoid clashing with 2.4GHz or 5GHz networks, as well as better support for direct linking, faster cellular hand-offs, in-car networks, roaming and mesh networking. 802.11-2012 is also expected to provide 600Mbps throughput. The PHY (physical layer) and MAC (a software layer) components of 802.11-2012 will be reworked to provide that impressive speed. Those changes will also allow for new additions like "mesh" networking, changes in security, broadcast/multicast/unicast data delivery and additional network management features.

3.2 Broadband WLAN/WiFi: the IEEE 802.11n The IEEE 802.11n standard offers several technical benefits over previous technology generations, which result in improved throughput to 802.11n-based clients, as well as greater reliability for legacy 802.11a/b/g clients. It is important to say that 802.11n is much more than just a new radio for 802.11. In addition to providing higher bit rates (as was done in 802.11a, b, and g), 802.11n makes dramatic changes to the basic frame format that is used by 802.11 devices to communicate with each other. This subsection will describe the changes incorporated in 802.11n, including MIMO, radio enhancements, and MAC enhancements. Environmental characteristics and network density plays a significant role in the ultimate performance of a network. In well-designed networks, each access point can serve well over 150 Mbps of TCP throughput to clients using 802.11n technology, and multiple radios can operate simultaneously to provide several gigabits of throughput. The following table presents an overview of the used techniques for improvement of network performance along with the corresponding effects and opportunities that are provided by each of them. Table 1 – Overview of the techniques for network performance improvement

Multiple-input multiple-output is the heart of 802.11n. The following technical discussion of MIMO provides a basis for understanding how 802.11n can reach data rates of 600 Mbps. MIMO technology takes advantage of several techniques to improve the SNR at the receiver. One technique is transmit beamforming. When there is more than one transmit antenna, it is possible to coordinate the signal sent from each antenna so that the signal at the receiver is dramatically improved. This technique is generally used when the receiver has only a single antenna and when there are few obstructions or radio-reflective surfaces, e.g., open storage yards. To understand transmit beamforming, consider a radio signal as a wave shape, with a wave length that is specific to the frequency of the signal. When two radio signals are sent from different antennae, these signals are added together at the receiver’s antenna (see Figure 2). Depending on the distance that each radio signal travels, they are very likely to arrive at the receiver out of phase with each other. This difference in phase at the receiver affects the overall signal strength of the received signal. By carefully adjusting the phase of the radio signals at the transmitter, the received signal can be maximized at the receiver, increasing SNR. This is what transmit beamforming does: It effectively focuses the transmitters on a single receiver, as shown in Figure 3.

Figure 2 – Destructive interference

Figure 3 – Constructive Interference with Transmit Beamforming

Transmit beamforming cannot easily be done at the transmitter without information from the receiver about the received signal. This feedback is available only from 802.11n devices, and not from 802.11a, b, or g devices. To maximize the signal at the receiver, feedback from the receiver must be sent to the transmitter so that the transmitter can tune each signal it sends. This feedback is not immediate and is only valid for a short time. Any physical movement by the transmitter, receiver, or elements in the environment will quickly invalidate the parameters used for beamforming. The wavelength for a 2.4 GHz radio is only 120mm, and only 55mm for a 5 GHz radio. Thus, a normal walking pace of 1 meter per second will rapidly move the receiver out of the spot where the transmitter’s beamforming efforts are most effective. Transmit beamforming is useful only when transmitting to a single receiver. It is not possible to optimize the phase of the transmitted signals when sending broadcast or multicast transmissions. For this reason, in general networking applications, the utility of transmit beamforming is somewhat limited, providing improved SNR at the receiver for only those transmissions that are sent to that receiver alone. Transmit beamforming can increase the data rate available at greater distances from the AP. But, it does not increase the coverage area of an access point, since that is determined, in large part, by the ability to receive the beacons from the access point. Beacons are a broadcast transmission that does not benefit from transmit beamforming. In typical indoor WLAN deployments, (e.g., offices, hospitals, and warehouses) the radio signal rarely takes the shortest and most direct path from the transmitter to the receiver. This is because there is rarely “line of sight” between the transmitter and the receiver. Often there is a cube wall, door, or other structure that obscures the line of sight. All of these obstructions reduce the strength of the radio signal as it passes through them. Luckily, most of these

environments are full of surfaces that reflect a radio signal as well as a mirror reflects light. Imagine that all of the metallic surfaces, large and small, that are in an environment were actually mirrors. Nails and screws, door frames, ceiling suspension grids, and structural beams are all reflectors of radio signals. It would be possible to see the same WLAN access point in many of these mirrors simultaneously. Some of the images of the access point would be a direct reflection through a single mirror. Some images would be a reflection of a reflection. Still others would involve an even greater number of reflections. This phenomenon is called multipath (see Figure 4).

Figure 4 – Multipath interference

When a signal travels over different paths to a single receiver, the time that the signal arrives at the receiver depends on the length of the path it traveled. The signal traveling the shortest path will arrive first, followed by copies or echoes of the signal slightly delayed by each of the longer paths that the copies traveled. When traveling at the speed of light, as radio signals do, the delay between the first signal to arrive and its copies is very small, only nanoseconds. (A rule of thumb for the distance covered at the speed of light is roughly one foot per one nanosecond.) This delay is enough to be able to cause significant degradation of the signal at a single antenna because all the copies interfere with the first signal to arrive. A MIMO radio sends multiple radio signals at the same time and takes advantage of multipath. Each of these signals is called a spatial stream. Each spatial stream is sent from its own antenna, using its own transmitter. Because there is some space between each of these antennae, each signal follows a slightly different path to the receiver. This is called spatial diversity. Each radio can also send a different data stream from the other radios. The receiver has multiple antennas as well, each with its own radio. Each of the receive radios independently decode the arriving signals (see Figure 5). Then, each radio’s received signal is combined with the signals from the other receive radios. With a lot of complex math, the result is a much better receive signal than can be achieved with either

a single antenna or even with transmit beamforming. One of the two significant benefits of MIMO is that it dramatically improves the SNR, providing more flexibility for the WLAN system designer.

Figure 5 – Spatial multiplexing

MIMO systems are described using the number of transmitters and receivers in the system—for example, 2×1 is “two by one,” meaning two transmitters and one receiver. 802.11n defines a number of different combinations for the number of transmitters and the number of receivers, from 2×1, equivalent to transmit beamforming, up to 4×4. Each additional transmitter or receiver in the system increases the SNR. However, the incremental gains from each additional transmitter or receiver diminish rapidly. The gain in SNR is large for each step from 1×1 to 2×1 to 2×2, the improvement with 3×3 is not quite as large, and beyond 3x3 is more moderate. The use of multiple transmitters provides the second significant benefit of MIMO: the ability to use each spatial stream to carry its own information, providing dramatically increased data rates. 802.11n Enhancements In addition to MIMO technology, 802.11n makes a number of additional changes to the radio to increase the effective throughput of the WLAN. The most important of these changes are increased channel size, higher modulation rates, and reduced overhead. In the following we will describe each of these changes and the effect they have on WLAN throughput.

Physical Layer Enhancements The original 802.11 direct sequence radio and the 802.11b extension to the base standard use a radio channel spacing that is 22 MHz wide. 802.11a and 802.11g use 20 MHz wide radio channel spacing. Because 802.11g is an extension to 802.11b, 802.11g spaces its channels just as 802.11b does, every 22 MHz. The size, or bandwidth, of the radio channel is an important measure of the efficiency of the radio. This is called the spectral efficiency and is measured in bits per Hertz. The spectral efficiency of 802.11b is one-half the bits per Hertz (for example, 11 Mbps in 22 MHz). 802.11a and 802.11g have higher spectral efficiency, as much as 2.7 bits per Hertz at 54 Mbps. Using exactly the same technology as 802.11a and 802.11g, some proprietary WLAN systems are available that provide up to 108 Mbps. These proprietary systems use a simple technique to double the data rate of 802.11a and 802.11g. They use two channels at the same time. This is called channel bonding. With channel bonding, the spectral efficiency is the same as 802.11a and 802.11g, but the channel bandwidth is twice as great. This provides a simple way of doubling the data rate.

Figure 6 - 20MHz and 40MHz channels

802.11n uses both 20 MHz and 40 MHz channels. Like the proprietary products, the 40 MHz channels in 802.11n are two adjacent 20 MHz channels, bonded together. When using the 40 MHz bonded channel, 802.11n takes advantage of the fact that each 20 MHz channel has a small amount of the channel that is reserved at the top and bottom, to reduce interference in those adjacent channels. When using 40 MHz channels, the top of the lower channel and the bottom of the upper channel do not have to be reserved to avoid interference. These small parts of the channel can now be used to carry information. By using the two 20 MHz channels more efficiently in this way, 802.11n achieves slightly more than doubling the data rate when moving from 20 MHz to 40 MHz channels (see Figure 6).

802.11n continues to use OFDM and a 4-microsecond symbol, similar to 802.11a and 802.11g. However, 802.11n increases the number of subcarriers in each 20 MHz channel from 48 to 52. This marginally increases the data rate to a maximum of 65 Mbps, for a single-transmit radio. 802.11n provides a selection of eight data rates for a transmitter to use and also increases the number of transmitters allowable to four. For two transmitters, the maximum data rate is 130 Mbps. Three transmitters provide a maximum data rate of 195 Mbps. The maximum four transmitters can deliver 260 Mbps. In total, 802.11n provides up to 32 data rates for use in a 20 MHz channel. When using 40 MHz channels, 802.11n increases the number of subcarriers available to 108. This provides a maximum data rate of 135 Mbps, 270 Mbps, 405 Mbps, and 540 Mbps for one through four transmitters, respectively. Similarly, there are eight data rates provided for each transmitter, 32 in total, for the 40 MHz channel. 802.11n can also use a short guard interval that is 400 nanoseconds long, instead of 800 nanoseconds. This slightly increases the maximum data rates, for example in 40 MHz channels, to 150 Mbps per transmitter. A four-transmitter 802.11n radio operating with 40 MHz channels and using the short guard interval can therefore deliver a maximum of 600 Mbps. The 802.11n standard includes the ability for the receiver to combine the received signals from multiple antennas to reassemble a single spatial stream. Multipath echoes in an environment can lead to frequency selective fading, in which certain subcarriers within a 20 MHz or 40 MHz signal are stronger than others. Maximal-ratio combining (MRC) enables the receiver to correlate the signal reception from multiple antennas and select the strongest of each antenna before decoding a particular subcarrier. The result is illustrated in Figure 7: the aggregate signal of several antennas offers a stronger, more consistent result than any individual antenna. This effectively increases the receive sensitivity, solely through the use of digital signal processing.

Figure 7 – Maximal Ratio Combining

MAC Layer Enhancements There is a significant amount of fixed overhead in the 802.11 MAC layer protocol, and in the interframe spaces and acknowledgements of each frame transmitted, in particular. At the highest of data rates, this overhead alone can be longer than the entire data frame. In addition, contention for the air and collisions also reduce the maximum effective throughput of 802.11. 802.11n addresses these issues by making changes in the MAC layer to improve on the inefficiencies imposed by this fixed overhead and by contention losses. Every frame transmitted by an 802.11 device has fixed overhead associated with the radio preamble and MAC frame fields that limit the effective throughput, even if the actual data rate was infinite (see Figure 8).

Figure 8 – Frame Overhead

To reduce this overhead, 802.11n introduces frame aggregation. Frame aggregation is essentially putting two or more frames together into a single transmission. 802.11n introduces two methods for frame aggregation: Mac Service Data Units (MSDU) aggregation and Message Protocol Data Unit (MPDU) aggregation. Both aggregation methods reduce the overhead to only a single radio preamble for each frame transmission (see Figure 9).

Figure 9 – Aggregated frame

Because multiple frames are now sent in a single transmission, the number of potential collisions and the time lost to backoff is significantly reduced. The maximum frame size is increased in 802.11n, as well, in order to accommodate these large, aggregated frames. The maximum frame size is increased from 4 KB to 64 KB. One limitation of frame aggregation is that all the frames that are aggregated into a transmission must be sent to the same destination; that is, all the frames in the aggregated frame must be addressed to the same mobile client or access point. Another limitation is that all the frames to be aggregated have to be ready to transmit from the client or access point at the same time, potentially delaying some frames to wait for additional frames, in order to attempt to send a single aggregate frame. A third limitation of aggregation is that the maximum frame size that can be successfully sent is affected by a factor called channel coherence time. Channel coherence time depends on how quickly the

transmitter, receiver, and other items in the environment are moving. The faster things are moving the smaller the maximum frame size can be as the data rate is reduced, i.e., the time for the transmission must be less than the channel coherence time. There are slight differences in the two aggregation methods that result in differences in the efficiency gained. These two methods are described here. MSDU (MAC Service Data Units) aggregation is the more efficient of the two aggregation methods. It relies on the fact that an access point receives frames from its Ethernet interface, to be translated to 802.11 frames and then transmitted to a mobile client. Similarly, most mobile client protocol stacks create an Ethernet frame, which the 802.11 driver must translate to an 802.11 frame before transmission. In both these cases, the “native” format of the frame is Ethernet, and it is then translated to 802.11 format for transmission. Theoretically, MSDU aggregation allows frames for many destinations to be collected into a single aggregated frame for transmission. Practically, however, MSDU aggregation collects Ethernet frames for a common destination, wraps the collection in a single 802.11 frame, and then transmits that 802.11-wrapped collection of Ethernet frames (see Figure 10). This method is more efficient than MPDU aggregation, because the Ethernet header is much shorter than the 802.11 header.

Figure 10 – MSDU Frame Aggregation

For a mobile device, the aggregated frame is sent to the access point, where the constituent Ethernet frames are forwarded to their ultimate destinations. For an access point, all of the constituent frames in the aggregated frame must be destined to a single mobile client, since there is only a single destination in each mobile client. With MSDU aggregation, the entire, aggregated frame is encrypted once using the security association of the destination of the outer 802.11 frame wrapper. A restriction of MSDU aggregation is that all of the constituent frames must be of the same quality-of-service (QoS) level. It is not permitted to mix voice frames with best-effort frames, for example. MPDU (Protocol Data Units) aggregation is slightly different from MSDU aggregation. Instead of collecting Ethernet frames, MPDU aggregation translates each Ethernet frame to 802.11 format and then collects the 802.11 frames for a common destination. The collection does not require a wrapping of another 802.11 frame, since the collected frames already begin with an 802.11 MAC header (see Figure 11).

Figure 11 – MPDU Aggregation

MPDU aggregation does require that all the 802.11 frames that constitute the aggregated frame have the same destination address. However, this results in the same behavior as MSDU aggregation, since the destination of all frames sent by a mobile client is that client’s access point, where the 802.11 frames are translated to Ethernet and forwarded to the ultimate destination. Similarly, the destination of any frame sent by the access point is a single mobile client. With MPDU aggregation, it is possible to encrypt each constituent frame independently, using the security association for each individual 802.11 destination address. This does not have any effective difference from the encryption done in MSDU aggregation, as all frames sent by a mobile client are encrypted using the security association for the access point, and all frames sent by the access point are encrypted using the security association for the single mobile client that is the intended recipient of the frame. Similar to MSDU aggregation, MPDU aggregation requires that all of the constituent frames be of the same QoS level. The efficiency of the MPDU aggregation method is lower than that of the MSDU aggregation method, because of the extra overhead of the individual 802.11 frame headers for each constituent frame. The efficiency is further reduced when the encryption is used. Encryption adds overhead to each of the constituent frame in MPDU aggregation, where MSDU aggregation incurs overhead for a single encryption of the outer 802.11 wrapper.

3.3 WLAN network design While there have been great advances made in the speed and easy of implementation of Wi-Fi networks, the basic nature of radio frequency (RF) is generally unchanged. Designing an optimal WLAN with high number of users who can access the WLAN in a small physical space (high-density WLAN) remains a challenge. High-density WLAN design refers to any environment where client devices will be positioned in densities greater than coverage expectations of a normal enterprise deployment, for example a traditional, carpeted office. For reference, a typical office environment has indoor propagation characteristics for signal attenuation. User density is the critical factor in the design. Aggregate available bandwidth is delivered per radio cell, and the number of users and their connection characteristics (such as speed, duty cycle, radio type, band, signal, and SNR) occupying that cell determines the overall bandwidth available per user. A typical office environment, Figure 12, may have APs deployed for 2500 to 5000 square feet with a signal of -67 decibels in millowatts (dBm) coverage and a maximum of 20 to 30 users per cell. That is a density of one user every 120 square foot (sq. ft.) and yields a minimum signal of -67 dBm.

Figure 12 – Typical office WLAN

In planning and deploying such a WLAN, an AP is typically placed in an area expected to have a higher user density, such as in a conference room, while common areas are left with less coverage. In this way, pre-planning for high density areas is anticipated. Conference rooms are often placed in clusters, so it is best to design for the maximum capacity of the area. For example, maximum occupancy for the three rooms is 32, so user density would be one user per 28 square feet, Figure 13.

Figure 13 – Calculating user density

Planning The WLAN design process can begin in many ways but generally it begins with an expressed desire to provide connections to a specific area where a number of users will participate in a focused activity. To evaluate what is possible, it is first necessary to understand what is required as well as what is possible. There is generally a primary application that is driving the need for connectivity. Understanding the throughput requirements for this application and for other activities that will take place on the network will provide the designer with a per-user bandwidth goal. Multiplying this number by the number of expected connections yields the aggregate bandwidth that will be required. The required per connection bandwidth will be used to drive subsequent design decisions. Design Point #1: Establish and Validate a Per-Connection Bandwidth Requirement How much bandwidth does each user require on average? In Table 2, the nominal throughput requirements for several popular applications and use cases in a higher education setting are shown. Table 2 – Bandwidth requirements per application

In all cases, it is highly advisable to test the target application and validate its actual bandwidth requirements. Software designers are often required to pick just one average number to represent the application’s requirements when there are actually many modes and deployment decisions that can make up a more accurate number. It is also important to validate applications on a representative sample of the devices that are to be supported in the WLAN. Additionally, not all browsers and operating systems enjoy the same efficiencies, and an application that runs fine in 100 kilobits per second (Kbps) on a Windows laptop with Microsoft Internet Explorer or Firefox, may require more bandwidth when being viewed on a smart phone or tablet with an embedded browser and operating system. Once the required bandwidth throughput per connection and application is known, this number can be used to determine the aggregate bandwidth required in the WLAN coverage area. To arrive at this number, multiply the minimum acceptable bandwidth by the number of connections expected in the WLAN coverage area. This yields the target bandwidth needed for the need series of steps. Design Point #2: Calculate the Aggregate Throughput Required for the Coverage Area If this course was for a wired rather than wireless network, calculating aggregate throughput requirements would entail dividing the aggregate capacity by the channel bandwidth available. Then, the number of channels would be established and these would be plugged into a switch. But in a WLAN, a channel’s speed is effected by multiple factors including protocols, environmental conditions, and operating band of the adapter. Before calculating aggregate throughput, several things must be considered. In the aggregate throughput calculation, the connections instead of the seats were used as the basis for calculation. The number of connections in a cell is what determines the total throughput that will be realized per connection instead

of the number of seats. Most users today carry both a primary computing device (such as a tablet computer, or laptop) as well as a second device (such as a smartphone). Each connection operating in the high-density WLAN consumes air time and network resources and will therefore be part of the aggregate bandwidth calculation. An increase in numbers of device connections is one of the primary reasons older WLAN designs are reaching oversubscription today. Wi-Fi is a shared medium. Much like an un-switched Ethernet segment, it operates as a half duplex connection. Only one station can use the channel at a time and both the uplink and downlink operate on the same channel. Each channel or cell used in a Wi-Fi deployment represents a potential unit of bandwidth much like an Ethernet connection to a hub. In Ethernet, switching technology was developed to increase the efficiency of the medium by limiting the broadcast and collision domains of a user to a physical port and creating point-to-point connections between ports on an as-needed basis, dramatically increasing the overall capacity. Users and applications also tend to be bursty (a measure of the unevenness or variations in the traffic flow) in nature and often access layer networks are designed with a 20:1 oversubscription to account for these variances. Application and end user anticipated usage patterns must be determined and also accounted for. Some applications, such as streaming multicast video, will drive this oversubscription ratio down while others may drive this factor even higher to determine an acceptable SLA for each cell’s designed capacity. For 802.11 wireless networks or any radio network in general, air is the medium of propagation. While there have been many advances in efficiency, it is not possible to logically limit the physical broadcast and collision domain of an RF signal or separate it’s spectrum footprint from other radios operating in the same spectrum. For that reason, Wi-Fi uses a band plan that breaks up the available spectrums into a group of non-overlapping channels. A channel represents a cell. Using the analogy of Ethernet, a cell represents a single contiguous collision domain. How many users can access an AP comfortably? Hundreds. But the question should not be how many users can successfully associate to an AP but how many users can be packed into a room and still obtain per-user bandwidth throughput that is acceptable. 802.11 Scalability: How much bandwidth will a cell provide? To scale 802.11 networks to reliably deliver consistent bandwidth to a large number of users in close proximity, it is important to examine certain WLAN fundamentals under reasonably ideal conditions. Once the rules are understood, the ways to manipulate them to maximum advantage will be presented. In real WLANs, the actual application throughput is what matters to the end user, and this differs from the signaling speed. Data rates represent the rate at which data packets will be carried over the medium. Packets contain a certain amount

of overhead that is required to address and control the packets. The application throughput is carried as payload data within that overhead. Table 3 shows average application throughput by protocol under good RF conditions. Table 3 – Average application throughput by protocol

Are 802.11n Data Rates Dependable? Today, many clients are 802.11n ready and this can provide throughput and efficiency increases in a high-density deployment. Most WLANs, however, will support a mix of client protocols. Evaluating the historic average client mix in a WLAN is possible by either looking at the WLAN controller graphical user interface (GUI) and using this historic mix of information for planning purposes. Unless the WLAN is very unique, most environments will likely be dealing with a diverse mix of clients and protocols for the near future. Consider that other factors, such as the number of connections, can also be expected to vary over time and for these reasons it is often a best practice to build in some buffer to smooth the long term results. The raw speed advantage of 802.11n high throughput (HT) rates is impressive and boosts the overall efficiency and capacity of the design by permitting more users or higher speeds to be realized on the same channel. Figure 14 shows mixed client protocol capacities for a given cell.

Figure 14 – Mixed wireless client protocol performance in a cell (802.11a/g/n)

The graph above shows throughput rates under varying mixes of HT20 modulation coding scheme-15 (MCS15) 2SS data rates and legacy 802.11a/g (for the purpose of this discussion 802.11a and 802.11g are the same protocol – different bands and are considered equal) data rates within a single isolated cell. • With either all MCS15 or all 802.11a/g clients, the difference in throughput is 480 percent • With a 50/50 mix, there is a 400 percent increase over legacy throughput • With a drop to just 25 percent of MCS15 clients, the increase is 300 percent In this example using 30 connections, the application throughput to the end user would be 833 Kbps with all legacy connections or 3.9 Mbps with all 802.11n connections. A mix drives throughput down. Other variables, such as user density or environmental noise, can and likely will change over time and will effect the throughput as well. Using legacy data rates as a nominal value, Table 4 shows the relationship between cell bandwidth and per connection bandwidth. Table 4 – data throughput and user connections per wireless protocol

A mixed cell containing both 802.11b and 802.11g traffic results in a throughput rate that is less than double that of 802.11b alone and roughly half of 802.11g alone. A similar effect was seen when 802.11n and legacy 802.11a/g rates were compared. Until the inclusion of 802.11n, all advances in Wi-Fi technology have come through incremental increases in encoding technology. 802.11n changed the encoding and streamlined the logistics of bonding 20 MHz channels and increasing the available channel bandwidth. In implementing new technology, it is also necessary to provide a mechanism that allows the old and the new protocols to coexist. It is this mechanism that reduces the overall efficiency of the channel due to additional overhead. An 802.11b modem was not designed to speak 802.11g. In order to avoid collisions, the 802.11b radios must be informed that the channel is needed by 802.11g for a period of time. What is Co-Channel Interference and Why is it Important in High-density WLANs? CCI is a critical concept to understand when it comes to understanding the behavior and performance of 802.11 WLANs. It is a phenomenon where transmissions from one 802.11 device bleed into the receive range of other 802.11 devices on the same channel, causing interference and reducing the available spectrum and resulting performance. CCI can cause channel access delays as well as collisions in transmissions that corrupt frames in transit. Figure 15 illustrates how APs on the same channel interfere with each other.

Figure 15 – Co-channel interference

802,11 networks are contention based and rely on Clear Channel Assessment (CCA) mechanisms to judge the medium state (if busy we wait, when free we transmit). In the example above, this client’s performance is being impacted because it can hear both APs. To this client, the two AP cells are coupled or acting as one super cell. For the uplink, both APs’ transmissions will be seen as a busy channel by the client and the client will simply wait for an opportunity to transmit. Worse yet, on the downlink, transmissions from either AP will potentially collide and retries will increase the contention for the medium and continue to drive the data rates down overall. The effects of CCI are not limited to just the AP cell. In a high-density environment, the clients themselves will have the effect of increasing the overall cell size. CCA is based on a receive threshold that evaluates the carrier for activity. It is generally a good practice to consider -85 decibels per milliwatt (dBm) as that threshold. Figure 16 shows a coverage model based on data rates. Higher data rates do not propagate as far. If the distances look long in this model, it is because it was calculated using an outdoor open space model rather than an indoor model which assumes attenuating factors in the environment. There are not many walls between the APs and clients in most high-density deployments.

Figure 16 – WLAN Coverage model based on data rates

In any Wi-Fi design, the effects of CCI can be limited by isolating the individual cells from one another through the use of non-overlapping channels and natural environment attenuation (walls, ceilings, file cabinets and cubes). We would not place two APs on the same channel directly next to one another intentionally. In a normal design, the environment and distances we are covering generally permit adequate coverage without a lot of CCI. But in a high-density network design, the distances are going to be constrained and propagation will be good, as such cell coupling and resulting CCI will become much more likely. Design Point #3: Choose a High Minimum Data Rate to Support Increased Efficiency, Lower Duty Cycle, and Reduce the Effective Size of the Resulting Cell CCI is not only an issue that will be faced in aggregating channels within the high-density deployment but something that must be kept in mind regarding existing deployments of surrounding areas. Lecture halls and classrooms tend to be co-located in the same facility, so overall design must be considered. In 2.4 GHz there are three non-overlapping channels with which to work in achieving isolation. The RF properties of 2.4 GHz signals give it better range and less attenuation than signals in 5 GHz. In a high-density environment, there is often only one clean channel reuse within a 10,000 square foot area. Channel reuse in such an area is opportunistic at best and it is not possible to estimate without careful advanced survey techniques. Results will vary from no increase in bandwidth to modest gains and will differ from site to site. If faced with such a challenge, consult with a professional with experience in advanced engineering techniques specific to a high density RF deployment. Adding more APs can reduce the number of users per cell and may appear to give more coverage when the space is empty. But once it fills up, the effect will be that of one large

super cell covering the room with limited bandwidth and sporadic connections for all. In contrast to 2.4 GHz, 5 GHz has many more channels with which to work. As many as 20 channels can be received in the United States and between five and 21 in the rest of the world. Most regions have between 19 and 21 channels. But all 5 GHz channels are not created equally. Limitations on maximum power for parts of the band are not of concern, but Dynamic Frequency Selection (DFS) channels represent some challenges that must be addressed. DFS was implemented so that APs and clients can share the band with radar devices. DFS details how radar is detected and what should be done in the event of detection. APs operating on DFS channels must first listen to a channel for 60 seconds to determine if there is a radar present before transmitting any energy. If an AP is operating on a DFS channel and detects a radar (real or false) it must shut down operations on that channel and abandon it for 30 minutes before that channel can be evaluated again for use. 802.11n can operate in a 40 MHz channel by bonding two 20 MHz channels together and this significantly increases throughput. However, this is reserved for burst mode transfers only. It is only practical to do this in 5 GHz because 2.4 GHz is already limited by the number of channels available. If there are enough 5 GHz channels to achieve the WLAN goals using a bonded channel plan (9 in the U.S. if using available DFS channels) to meet throughput goals, consider it. If forced to reuse 5 GHz channels, more consistent results will be delivered using strictly 20 MHz channels and avoiding loss of efficiency due to CCI. The essential question for a high-density design is how many channels for each band will be needed to match the client base? This can be a tricky question since even dual band capable clients do not always select the faster 5 GHz band. Since bandwidth in 2.4 GHz is going to be limited, 5 GHz must be relied on to reach the goal. Dual band adapters have been shipping with most laptops for some time. This does not mean that every laptop is a dual band client, but many are. Simply having a dual band client does not guarantee that it will choose 5 GHz over 2.4 GHz. The Microsoft Windows operating system defaults to a Wi-Fi channel search that starts with the 5 GHz channel 36 and continues searching through all of the 5 GHz channels that the client is capable of. If no 5 GHz AP is found then it will continue the search in 2.4 GHz starting at channel 1. Unless the Windows default is changed or the user has chosen a third party Wi-Fi utility to set spectrum preference to 2.4 GHz, the client radio will first try to associate to a 5 GHz AP. Apple Computer’s latest release for Atheros and Broadcom chipsets also searches 5 GHz first. Design point #4: Determine the Number of Channels and Cells Needed A sample high-density WLAN project may include a design that yields 300 Mbps consistently to support 300 concurrent users. Under optimal conditions, 802.11g and 802.11a data rates yield 25 Mbps throughput. However, a high-density environment will be less than optimal from a SNR standpoint. A better number to use is 20 Mbps throughput. Table 5 provides a quick reference using 20 Mbps

per cell and per channel as the throughput value. Looking strictly at 5 GHz and assuming no channel reuse at this point, it is clear that 1 Mbps per user with 15 channels and 15 cells can be easily supported. Table 5 – Reference guide for channels

Design Point #5: Account for and Manage all Energy within the Operating Spectrum to Ensure all of it is Available for Use The discussion until now has centered on a use case where every client in the room will be competing for bandwidth simultaneously. This is the case when the users in the room simultaneously access a resource on queue. However there are many instances where the design requirement is to offer access to resources or the Internet for casual use at an event or within a venue such as a sports arena. Planning and sizing for these types of events can be quite different and will be based on expected Client Duty Cycle. At a sporting event, for example, there are certain areas that will require ubiquitous and instant access during the entire event. Ticketing, vendor sales, staff, and press areas will generally require the highest amount of access. Of these, the press area is the only one that requires a high level of capacity in the arena itself. For the fans attending the event, only a percentage will be active on the WLAN at any one time. We see a 20 to 30 percent take rate with some well defined peaks occurring during period breaks. During play, very few fans are accessing the WLAN. However, this is changing as applications such as video replay, instant stats, and concession orders from the seat become more commonplace. Observation and understanding of the requirements of WLAN users and situational requirements will guide the development of reasonable design goals. 500 users in a room who require simultaneous access to a single resource is a different design challenge than 1000 or 1500 users who only occasionally use the wireless network. Also, be aware that user patterns can and do change with time. This has been seen with the increase in the number of network clients per user. Monitoring network access and keeping good statistics will allow wireless engineers to stay on top of user trends on the university campus. Often one of the biggest challenges in a high-density environment is access and aesthetics. A large meeting hall is impressive because of its size and a great deal goes into the aesthetics of the environment. The best approach to engineering a specific space is to do a qualified sight survey. Once the APs are mounted, physical adjustments become a lot more complex, so it is best to test while installing and make certain that the coverage that has been defined in the design is what is installed. APs have evolved rapidly in short period of time. If an AP with external antenna capabilities is to be used, it is essential that an antenna that was designed for that AP also be used. MIMO or 802.11n APs need MIMO antennas to perform properly. Even if HT rates are not being counted on, the antenna and the radios are a system and the system is designed to perform with all of these elements. Mounting APs or directional antennas directly overhead in an environment may not be acceptable to the building owners. There are several ways to solve this problem and, depending on the environment and restrictions imposed, several methods may be used together.

The best approach to engineering a specific space for a high-density WLAN is to first do a thorough, active site survey to determine how and where the APs should be optimally installed. This will clarify what is possible in the space and provide a design to work from. Any changes to the optimal placement imposed by restrictions will require another survey because the final throughput for the space will likely change. If the environment and requirements neccesitate the use of directional antennas, remember that once the APs are mounted, physical adjustments can become a lot more complex, depending on the mounting location. So it is best to test them while they are being installing to ensure that the anticipated coverage results from what is installed. Use of directional antennas and downtilt One challenge often faced in a lecture hall or auditorium is the need to provide more bandwidth than a single use of the channels available in 2.4 GHz will allow. Using a directional antenna can provide cell-to-cell isolation if placed, mounted, and adjusted properly. One aspect of using directional antennas is the concept of mechanical downtilt. Downtilt involves adjusting the antenna down to change the coverage pattern that is created. The coverage pattern can be adjusted by changing the mounting height or the mechanical downtilt angle, Figure 17.

Figure 17 – Adjusting directional antenna using downtilt

• H = height of the antenna • A = downtilt angle • BW = the 3 dB horizontal beamwidth of the antenna

By adjusting the downtilt of the antenna, it is possible to “dial in” " or add WLAN coverage" to specific areas within the coverage zone. APs and RF energy operate much like light cast by lighting fixtures. It is possible to light an entire warehouse with a bare bulb on the ceiling, but the result is low levels of light in some areas. But if there are multiple fixtures, including some with higher patterns of luminosity to illuminate larger, the result is comprehensive overall lighting. RF is invisible, so measuring the coverage and adjusting it appropriately requires tools to measure the coverage. For each antenna placement, simply walking the area below it and adjusting the antenna to change the pattern based on Received Strength Signal Indication (RSSI) levels to match coverage requirements is generally all that is required at the initial installation. Antennas hear the same as they transmit. If measuring and adjusting are done carefully, using consistent measurements and tools, good results can be achieved. Any additional tuning can be managed with power threshold adjustments through RRM. In all cases, a full site survey to compare the results to the plan is required once all assets are installed. AP Placement Options The most common method of achieving even coverage is to evenly space the APs directly over the clients they will serve. There are multiple options to accommodate overhead mounting of the APs in an unobtrusive manner. Although many people do not consider any AP to be a welcome stylistic addition to a room, APs with internal antennas can be flush mounted to a variety of surfaces and offer an option with less impact on a room’s aesthetics. In these cases, a flush mount antenna can be much less obtrusive. External antennas increase the cost and complexity of the installation slightly, but can be justified if the end result is the ability to cover the room at a sufficient density and meet aesthetic requirements. Once the decision is made to incorporate external antennas, numerous options are opened for shaping the RF cell through the use of directional antennas. Channel reuse in 2.4 GHz can be achieved in smaller spaces by using directional antennas overhead. Ceiling height and antenna choice will determine cell boundaries and taking measurements is required. In Figure 18, assuming the room is 9000 square feet, using the internal antenna enables AP nine channels of 5 GHz, and three channels of 2.4 GHz to be provided comfortably. Using an external omnidirectional antenna, the results would be much the same. Using omnidirectional antennas on 5 GHz and directional antennas on 2.4 GHz, one, two, or three additional 2.4 GHz channels could be added within this space. Throughput improvements would largely be gained by more even client distribution and less resulting CCI at the client. Some additional capacity will be gained, but only to the extent that CCI can be eliminated between the cells and this will depend on ceiling height, antenna pattern, and power levels in 2.4 GHz.

Figure 18 – Nine AP WLAN deployment

3.4 Quality of Service (QoS) and Security in WLAN WLAN QoS QoS refers to the capability of a network to provide differentiated service to selected network traffic over various network technologies. QoS technologies provide the following benefits:

o Provide building blocks for business multimedia and voice applications used in campus, WAN, and service provider networks

o Allow network managers to establish service-level agreements (SLAs) with network users

o Enable network resources to be shared more efficiently and expedite the handling of mission-critical applications

o Manage time-sensitive multimedia and voice application traffic to ensure that this traffic receives higher priority, greater bandwidth, and less delay than best-effort data traffic.

With QoS, bandwidth can be managed more efficiently across LANs, including WLANs and WANs. QoS provides enhanced and reliable network service by doing the following:

o Supporting dedicated bandwidth for critical users and applications o Controlling jitter and latency (required by real-time traffic) o Managing and minimizing network congestion o Shaping network traffic to smooth the traffic flow o Setting network traffic priorities

In the past, WLANs were mainly used to transport low-bandwidth, data-application traffic. Currently, with the expansion of WLANs into vertical (such as retail, finance, and education) and enterprise environments, WLANs are used to transport high-bandwidth data applications, in conjunction with time-sensitive multimedia applications. This requirement led to the necessity for wireless QoS. Several vendors have supported proprietary wireless QoS schemes for voice applications. To speed up the rate of QoS adoption and to support multi-vendor time-sensitive applications, a unified approach to wireless QoS is necessary. The IEEE 802.11e working group within the IEEE 802.11 standards committee has completed the standard definition, but adoption of the 802.11e standard is in its early stages, and as with many standards there are many optional components. Just as occurred with 802.11 security in 802.11i, industry groups such as the Wi-Fi Alliance are defining the key requirements in WLAN QoS through their Wi-Fi MultiMedia (WMM) ensuring the delivery of key features and interoperation through their certification programs. An example deployment of wireless QoS is shown in Figure 19.

Figure 19 – WLAN QoS Deployment solution

QoS is defined as the measure of performance for a transmission system that reflects its transmission quality and service availability. Service availability is a crucial element of QoS. Before QoS can be successfully implemented, the network infrastructure must be highly available. The network transmission quality is determined by latency, jitter, and loss, as shown in Table 6. Table 6 – QoS parameters

WLAN Security Security was the IT manager’s main concern in the past and the reason why WLANs were not implemented. However, as the ubiquity of wireless devices

drove demand from end users, evolving wireless standards have solved these security issues to the point where a properly implemented wireless network is more secure than most wired networks. The following is a brief summary of the previous issues and resolutions. Wired Equivalent Privacy (WEP) The original standard for wireless security was proven to be crackable. Now standards are more secure. With the introduction of 802.11i in 2004, encryption became effectively uncrackable in its current form; however, this can come with some complexities. 802.1X is the strongest form of authentication, but it is more expensive and difficult to set up and maintain. Using a pre-shared key (PSK) can be strong, but using a single passphrase limits security to its weakest link — the “human factor,” and requires IT managers to replace passphrases manually on a regular basis for security purposes. For best user experience and in the interest of saving IT manager time with troubleshooting and maintenance, it is important to consider a balance between strong 802.1X and PSK. Dynamic PSK is the best balance for usability and security in that it does not involve IT time and removes the user from handing out their unique key. Figure 20 shows the WLAN security options.

Figure 20 – WLAN security options

Rogue access points (APs) Rogue APs have long been a bane to IT. Network administrators wasted countless hours tracking down unauthorized devices. Two fundamental issues drove rogue APs: lack of corporate Wi-Fi and cheap home APs that required little network knowledge to install. The later made it simple for employees to bring rogue APs to the workplace. Rogue APs are possibly the first case of consumer products critically impacting tightly controlled corporate networks. Rogue AP detection and location services in modern enterprise-class WiFi systems have given IT powerful tools to combat these devices. WLAN Security is part and parcel of Network Security Wireless security is network security. The WLAN exists to provide mobile end users with access to the wired network, both for internal resources and Internet access. Whether the organization has a simple flat single subnet network connected only to an Internet gateway, or is segmented by multiple routers and firewalls, the WLAN has to integrate with the network as a whole and all WLAN traffic can be expected to wind up on the wired network. Virtually all wired networks have a firewall implemented between the corporate network and the Internet. Some larger or more security conscious users may also have a firewall segmenting the corporate network from the rest of the user groups within the enterprise. In either case, the existing wired network security implementation can be leveraged to ensure that both the wired and wireless networks are secure. For example, a security conscious business or larger enterprise with the resources to manage a robust security network would route all of the network traffic from the Internet into the corporate network through a firewall at the DMZ. Further, they would have another firewall segmenting the corporate network from differing end user groups on the wired network via different corporate VLANs with 802.1x authentication. Another corporate VLAN set-up would be created for the wireless network with a corporate SSID, and another SSID could be set-up for guest users wishing to only access the internet which would circumvent the internal firewall and corporate network. See the diagram below.

Figure 21 – WLAN implementation

In this example, the WLAN implementation layered on the existing wired network and existing security implementation negates the need for another firewall on the WLAN controller, which would simply be more software to learn and maintain without any added benefit. WLAN Authentication and Encryption Because radio waves cannot be isolated to a narrow location, the central concerns of WLAN security from the beginning have been access control (who can use the WLAN) and encryption (how to keep WLAN data private). These are essentially the same concerns that came up with VPN access to secure networks over the insecure Internet. In fact, because of flaws in the original WEP security standard, many early WLAN deployments were completely separated from the primary network and used VPN connections for authentication and encryption to corporate resources. This architecture treated the WLAN as if it were an incoming Internet connection. It was and is a very secure architecture, but it required additional equipment, such as additional VPN concentrators beyond the scale required by the organizations remote access needs. The earlier flawed WEP security standard has been replaced by 802.11i/WPA2, which is much more robust and allows for full integration of the WLAN into the

LAN. Although a detailed description of encryption options follows, it boils down to, if at all possible, use WPA2 security with AES encryption. WPA2 options – PSK The 802.11i standard defines two authentication methods, commonly called by their Wi-Fi alliance certifications: WPA2-Enterprise and WPA2-Personal (or WPA2-PSK for ‘pre-shared key’). WPA2-Enterprise is extremely secure and is built around the 802.1X port authentication standard. Note that even though 802.1X frequently comes up in discussion of Wi-Fi networks, it is a port based authentication protocol and was originally designed for wired networks. WPA2-Enterprise requires a RADIUS server (or RADIUS proxy) and a database of users with their corresponding credentials. However, 802.1X can be challenging to deploy. For less complex WLAN deployments, WPA2-PSK uses a single passphrase, the Pre Shared Key, or PSK, to access the WLAN. From an end user point of view, and even a deployment point of view, it is virtually the same as WEP — the user chooses the SSID of the WLAN and is asked for the passphrase, which will be cached by the OS for future automatic connections. However, unlike WEP, WPA2-PSK corrects implementation weaknesses, can use AES encryption and cannot be broken unless a weak passphrase was chosen. The security concerns with WPA2-PSK do not stem from the quality of the encryption implementation, but from issues of managing the passphrase. Because the passphrase is shared, all users of the WLAN must know it. The more people that know a secret, the less secret it is. Well meaning folks tend to give the PSK to visitors who wish to check their email. Key rotation (periodically changing the passphrase) almost never happens, even when someone leaves the organization, because it is so cumbersome. When it does happen, informing users of the new PSK may wind up being by email or posted signs – not great ways to preserve a secret. Although the encryption is strong, a poorly chosen or weak passphrase is subject to brute-force dictionary attacks that attempt to guess the passphrase. Figure 22 presents the topology and communication when dynamic PSK is implemented.

Figure 22 – Dynamic PSK

3.5 WiFi hotspot solutions Wireless hotspots are springing up all over the world. They are typically located in areas where travelers need to wait or want to take a break. These include but are not limited to: motorway service areas, railway stations, airport lounges, hotel lounges, coffee shops and internet cafes. Anyone with a broadband connection and a wireless device can set up an open wireless area. The key to a successful wireless hotspot is to make sure only paying guests can gain access. The larger hotspot operators are using billing mechanisms similar to those used for mobile phones: either pre-paid or billed monthly. But unlike mobile phones, few roaming agreements have been put in place which means these accounts are only valid at hotspots supported by individual operators. A more flexible billing mechanism is to sell access by the hour or minute. At the point of sale, a ticket is issued with the appropriate login details. The user enters these details when they connect to the wireless gateway and can use the facilities for the amount of time purchased. This simple method only requires a few moments of a staff member’s time to issue a ticket from the point-of sale ticket printer. It means any user can access the system and they only pay for what they need. Many coffee shops, bars and restaurants are offering wireless internet access as a way of not just attracting but also keeping visitors for longer. Visitors that stay longer will also spend more on drinks and food. Add the extra sales to the money made from selling the wireless internet access and it becomes a win-win business proposition. The wireless hotspot can also be combined with an existing Internet café installation using fixed PCs and ordinary wiring as shown in Figure 23.

Figure 23 – Coffee shop/Internet café architecture

Business travelers on the road can catch up with their email or download important documents for their next sales meeting while having a break at a motorway service area equipped with a wireless hotspot. Similarly, air travelers can respond to that important email or work on their sales plans while sitting in the airport lounge waiting for their flight. Depending on the size and layout of the terminal, the wireless area can be extended using wireless bridges so that users can roam from café to restaurant to lounge while still logged in to the single wireless gateway account. In this way, longer sessions can be sold to users and more clients can share the same services as shown in Figure 24.

Figure 24 – Road, Rail or transportation Hub

Hotel lobbies and lounges are one of the favorite watering holes for road warriors. They like to use them for impromptu meetings or as refueling stops both for the body and email. A wireless hotspot can increase the appeal for hotel facilities and encourage guests to stay longer and spend more on food and beverages. In addition, the wireless hotspot can be combined with a business centre facility to offer both wired and wireless access for travelling professionals. It can also be combined with an in-building xDSL solution which provides high speed internet access to guest rooms using the hotels internal telephone wiring. A combined wireless hotspot, business centre and xDSL solution is shown in Figure 25. The same accounting and authorization system can be used for all three types of users. This helps spread the investment costs and increases the revenue potentials of each solution.

Figure 25 – Hotel lounge or lobby

A business case for a wireless hotspot A wireless hotspot solution can be a surprisingly lucrative investment. It can make good economic sense either as a new venture or as an additional service added on to an existing Internet café or hotel internet service. The competitive rates for high speed broadband links and wireless broadband equipment mean that anyone with a suitable location and a steady stream of visitors can benefit from a wireless hotspot solution. In the following simple example for a coffee shop, we have assumed that on average 4, 12 or 24 users a day make use of the service and spend on average half an hour connected. The cost of an hour’s access is €5 and it takes a staff member less than a minute to serve a wireless client. The cost to set-up and run the hotspot and the potential revenue stream are shown in Table 7. Table 7 - Wireless Hotspot Business Case

As can be seen, a small investment can generate a reasonable revenue stream. Staffing and running costs are kept low as staff is already in place to provide the food and drinks. Advertising can be kept minimal e.g. simply a sign or leaflets and flyers. And the higher the advertising budget, the more customers you will

get per day. So even with a modest number of customers using the facilities, the investment soon pays for itself and leads to profit within a year. Wireless networks have been available for many years. It is simple to set up an open wireless network that can be used by anyone with a wireless card in the vicinity. In fact many companies have set up open wireless networks without realizing it. The key differences between a wireless hotspot and an open wireless network are the following important features: • Only registered users are allowed to use the network and are charged for what they use • The network must be secure and not allow users to see each other’s data or access each other’s machines • The wireless network must be able to cover a large area and the reach of the network should be controllable to prevent signal leakage • The network connection should not become a bottleneck as more users are added or power users require more bandwidth • Free access to some sites should be allowed to allow clients to login and to provide extra advertising opportunities When a client first tries to access the Internet in a wireless hotspot, the client’s browser is redirected to a registration screen where they enter their login details. These can be an existing pre-paid or billable account set up with the wireless ISP. Alternatively, it can be a temporary account generated on purchase of the access units required at the point of sale. Once the account details have been authorized against the authentication database, the client is granted access to the connection. The session is closed either when their pre-paid time has elapsed or when the client closes the connection. Security is essential for the successful operation of a wireless hotspot. There are various aspects of security that have to be considered: • Protection of data: Most wireless receivers and transmitters can be set-up with data encryption so that snoopers cannot eavesdrop on email or other people’s conversations. • Prevention of unauthorized access: A wireless network should be secured against unauthorized access by disabling the broadcast of the wireless station beacon and by ensuring all clients are authenticated against the authorization database. • Isolation of wireless clients: Wireless clients must be isolated so that they cannot access other clients’ computers or see their data. • Protection from hackers and snoopers: The wireless hotspot should include an industrial strength firewall between the wireless network and the internet connection to prevent malicious access from external internet users. Internet addresses should be hidden behind the firewall using the Network Address Translation (NAT) protocol.

3.6 4G mobile data traffic offloading to WLAN (from LTE/LTE-Advanced, and from Mobile WiMAX) With the advent of network technologies such as 3G and 4G (LTE/LTE advanced and mobile WiMAX) mobile data traffic has been growing at an unprecedented pace and increasingly outstripping the network capacity. The introduction of smart-phones that offer ubiquitous internet access allied to a host of other internet capable devices including tablets, consoles, laptops and navigators - all becoming internet capable has been driving the data growth. The network operators are also increasingly introducing data services as a result of data increasingly contributing to the service revenues, but data traffic consumption is outpacing the data revenue. The proliferation of these mobile broadband devices along with the unlimited data bundles from the operators has led to network congestion and deteriorating network quality. Figure 26 presents the major data growth factors.

Figure 26 – Data growth factors

In this section we have considered both the aspect of data migration from Mobile to Fixed which we term - Data Offload and also data migration from Fixed to Mobile which we term - Data Onload. We recognize that even though data offload alleviates some of the operator’s network congestion using micro cells, a significant proportion of the offload could itself be offset by fixed to mobile migration of data as demonstrated in Figure 27 below.

Figure 27 - Data Migration from Mobile to Fixed and Fixed to Mobile

The whole offloading market has been a well understood market for a long time now, and by making the cell smaller the carriers will have more capacity. The challenges have always been how to execute the deployment of microcells and to find all the places to deploy the necessary access points and other factors including mounting; power supply; and backhaul to interconnect them to mobile carrier’s core network. The following list summarizes the key inhibitors for Mobile Data Offloading.

o Seamless user experience: WiFi connections need to provide a seamless connection and need to be of higher strength for the users to offload from the carrier’s network to the WiFi network.

o This means once connected to the WiFi network users must be able to seamlessly connect to the same network when in range.

o Legal and Regulatory Issues: According to the FCC’s (Federal Communications Commission) lawful intercept requirements, any traffic originating from a licensed spectrum should be sent over the carrier’s core network and has to be managed by the carrier which would ideally include the femtocell traffic. Conversely in WiFi, data traffic does not have to advance through many core network elements.

o Branding and Marketing approach: Brand is everything for a mobile carrier along with the user experience associated with the brand. As found, mobile network operators have to become increasingly innovative in their branding for a product.

Conducted reports provide five year forecasts for mobile data offload and onload, across eight regions of the world: North America, Latin America, Western Europe, Central & Eastern Europe, Far East & China, Indian Sub-Continent, Rest of Asia Pacific and Africa & Middle East. The forecasts within the full report cover several aspects: total mobile data usage, data growth over the cellular network, data offloaded via femtocells and WiFi, data onload from fixed to mobile. As the mobile data to be offloaded from the operator’s cellular network depends on the data that is primarily intended for the cellular network, we need to consider the total data traffic generated by mobile devices whose primary usage is via the operator’s cellular network. The following chart shows the forecast for total mobile data traffic generated.

Figure 28 - Mobile Data Traffic Generated by Smart-phones, Feature Phones &

Tablets The amount of data traffic that is being offloaded from the operator networks to other complementary networks will increase from 43.1% in 2010 to 63.2% by the end of 2015. Developed markets including North America and Western Europe are forecast to witness the highest offload factor (%) throughout the forecast period, reaching 76% and 74% by the end of the forecast period respectively. Developing markets are forecast to witness higher growth over the forecast period with Indian Sub Continent at 137.8% CAGR (Compound Annual Growth Rate), Latin America and Africa & Middle East at 126% CAGR.

3.7 Cognitive resource management in future wireless networks During the last decades development of wireless communication systems has been an immensely successful endeavor. We have witnessed an explosion especially in popularity of technologies utilizing the license-exempt ISM frequency bands. Both Wireless LANs and Bluetooth are now available in practically every larger mobile terminal. The dark side of this success story is the over-crowdedness one is already witnessing in these popular ISM bands. In the technology-homogeneous case the standard medium access control procedures can relatively effectively be used to share the wireless medium amongst communicating nodes. However, this solution is not anymore sufficient when numerous different technologies compete from the same spectrum. This is already a severe problem for new technologies especially in the 2.4 GHz ISM band. It is also arguable that more intelligent (“cognitive” and self-learning) methods could lead to better MAC utilization. In addition to “traditional” interoperability solutions, such as the ones developed to mitigate WLAN-Bluetooth interference, more radical approaches have also been suggested. In particular spectrum agility techniques originally developed to reclaim momentarily unused parts of the strictly regulated radio spectrum can be exploited in the ISM bands as well. Key idea in these approaches is to reconfigure communicating radios with very rapid response times to use frequencies that are at the moment not used by their primary users. Most interesting of these proposals is perhaps the suggestion of using adaptive machine learning techniques. These “cognitive radio” solutions were originally suggested for enabling dynamic spectrum management in licensed frequency bands to regain some of the unused spectrum but are suitable for solving the co-existence problems introduced above. In a sense we can view cognitive radios as extreme spectrum agile radios that in the ISM band case “borrow” spectrum from other users sharing the band, instead of a primary user authorized by some governing body. Major concerns with respect to the methods proposed so far are twofold: 1) It is very difficult to guarantee that heterogeneous population of cognitive radios that attempt to optimize their own spectrum usage can achieve even satisfactory global use of wireless resources. Collaboration in communication systems on single layer, network or link, has been studied in detail especially in game theory formalism. However, overall system analysis considering cross-layer aspects remains to be done. 2) Straightforward optimization of link and physical layer performance can lead to vast amounts of wasted capacity if no higher layer protocol or entity is able to benefit from it. We argue that “cognitive” techniques also offer an alternative to traditional cross-layer optimization which has proven to be more problematic than the initial optimistic works indicated.

There are proposals for extending the scope of the cognitive radio research towards more holistic approach. The idea is to introduce a framework for cognitive resource manager (CRM) enabling autonomic optimization of the communication stack as a whole, instead of focusing solely on the spectrum problem and thus going well beyond simplistic RRMs (Radio Resource Managers) and medium access control techniques. Thus section further discusses the exchange of network information between the CRMs as a method to avoid harmful interactions arising from local optimization methods leading into globally unsatisfactory solutions. Communication between CRM instances could finally be used to federate individual cognitive radios to become cognitive wireless networks. This way the CRM would offer a systematic approach as a framework for distributed cross-layer optimization. In this section we discuss about the functions of the Cognitive Resource Manager (CRM) with more details. Its conceptual architecture is shown in figure 29. We see the CRM as a multi-functional software entity that will primarily carry out cross-layer optimization using a toolbox of advanced reasoning methods and a great variety of information from the application layer, the underlying networking and data link layer as well as the operating system. Based on the collected knowledge the CRM can for example, optimally manage spectrum resources, flexibly adapt MAC and link parameters and allow the best possible settings for the applications running on top. In addition the CRM could consider policies such as proposed by the DARPA XG project during all optimization processes. Later on the CRM can return its recommendations for policy updates based on experiences and observed behavior of other network nodes.

Figure 29 - Conceptual architecture of the CRM

The CRM can also function as a “connection manager” deciding upon the frequency channels as well as the type of communication technology to be used (IEEE 802.11, Bluetooth, UMTS, etc.) in case a variety of interfaces and networks are available. In this context, different services such as voice-call, audio- and video-conferencing can experience higher quality if their specific QoS requirements, e.g. in terms of delay and bit rate, are carefully taken into account. Since the CRM will perform multidimensional optimizations using substantial amount of data, the traditional numerical methods might not be fast and scalable enough if applied to the full dataset. Accordingly, alternative approaches such as genetic algorithms and simulated annealing are used inside a toolbox as depicted in figure 29. These natural optimization methods are interesting candidates since they are proven to be successful in solving problems with large number of variables, can work with numerically generated or experimental data, and so on. In order to more efficiently handle the large amount of knowledge data (including historical data), sorting and clustering of the available information used in the CRM is required. Classical techniques like k-means can be considered, however due to the high degree of data variety and dynamicity, more advanced algorithms will be needed. Promising candidates include neural networks based approaches such as self organizing maps (SOMs). SOMs have already been successfully applied e.g., to perform unsupervised traffic pattern classification and estimation without a priori information. Additionally, time series analysis can be used, for example, in finding periodicity and compensating for the missing data to be able to provide reasonable estimates. In order to achieve a reliable operation of the CRM, quality of the data used in the decision process should be ensured. Accordingly, data filtering techniques to handle the linear and non-linear noise are required. Techniques such as Bayesian reasoning and statistical learning theory can be deployed to deal with uncertainty and ensure the reliability of the data and inference. Although it is represented as a single block in the architecture, the CRM usually has a modular and extensible structure. For example, the toolbox itself is not limited to the enabling optimizing techniques mentioned above. Further methods could be added in a plug-and-play fashion. In fact, from an implementation point of view, the CRM could be seen as a micro kernel with additional software modules where the scheduling and time synchronization mechanisms of different optimization and reasoning processes are carried out. One of the key challenges for CRM is its distributed structure. Figure 30 presents a behavioral model of the cognitive radio and how the steps are realized using a CRM.

Figure 30 - Behavioral model of the cognitive radio and how the steps are

realized using CRM

3.8 Business and regulatory aspects of WLAN/WiFi Regulation can be defined as: A written rule made by a government or another authority which is intended to control the way something is made or done. Or: Regulation is the process of making rules which govern behavior. Regulation exists in many forms: economic, health, safety, technologic. In this section the focus is towards economic and business regulation of WLAN/WiFi. Regulators have always been interested in the development of the telecommunications industry and infrastructure. In the past, the main role of the telecommunications regulation was in protecting customers from the monopoly power of vertically integrated operators. Deregulation in the 90s changed the industry and the role of the regulators. The technological convergence on the other hand is bringing together the telecom, broadcasting, and information services regulation. The main rationales behind regulation are: effective use of resources, competitive markets, customer rights, preventing abuses like monopolies and cartels, redistribution of wealth. The regulators have a so called narrow window to guide business models – they can not make any drastic decisions which could affect the industry overnight. New business models may arise either accidentally or deliberately. Regulation may in the worst case seriously hinder business activities, if planned negligently. Regulators face always challenges when new technologies are introduced. The legislation is always a little bit late. The regulators need to define how they apply the existing laws to new technologies and businesses. EU does not impose specific legal provisions on public WLAN. It establishes a regulatory framework and defines the tasks of National Regulatory Agencies (NRA). The telecommunications regulatory framework recognizes that much of telecommunications regulation exists as a means of addressing potential and actual abuses of market power. With that in mind, the EU attempts a comprehensive, technology neutral approach to regulation. The European Commission defines a series of relevant telecommunications markets, and provides a set of guidelines for determining the presence or absence of market power. Within each market the NRA determines whether one or more parties possess Significant Market Power (SMP). If SMP exists, the NRA will impose appropriate obligations. Basically EU seeks to move completely away from technology-specific and service-specific legislation. Many countries have not considered the legal status of WLAN Networks because WLAN has not yet become sufficiently common. Some countries have dealt with the matter only from the viewpoint of frequencies and licenses. Most countries that have considered the matter further, share the opinion Finland has taken. For example, Spain, Italy, Switzerland, Turkey, Hungary and Estonia considered that service offered by a café or a hotel to its customers is not public telecommunications.

On the other hand, there may be obligations imposed on service providers on the basis of other laws. For example in Italy, service providers must authenticate users on the basis of the anti-terrorism law. In the US the legal and regulatory framework is very different than in EU. The latest revision of the Telecommunications Act of 1934, of 1996, separates telecommunication services from information services. The Act defines an information service as “the offering of a capability for generating, acquiring, storing, transforming, processing, retrieving, utilizing, or making available information via telecommunications, and includes electronic publishing, but does not include any use of any such capability for the management, control, or operation of a telecommunication system or the management of a telecommunication service.” It underpins the US deregulatory policy toward the Internet. The Internet should be viewed as an enhanced service, and that the Internet consequently should not itself be subject to significant regulation. In the US regulators seem to lack authority and the people tend to trust the companies more than the government – at least when compared to Europe. The American attitude to large companies has always been somewhat ambivalent – they worry about the power of large corporations wield, and yet at the same time they appreciate the potential benefits associated with the economies of scale and the scope that they command. It is not held to be a problem for a firm to possess market power; rather the abuse of the market power is problematic. FCC has a limited power to collect confidential information and it lacks the ability to protect that information. Although regulation in the U.S. is multilevel with federal, state and municipal bodies, the FCC has taken a position that the Internet is interstate. There’s a huge interest in WLAN in the US and they are cautious to introduce any laws that might jeopardize the growth of the wireless network infrastructure. On the other hand the Americans are increasingly concerned about cyber-security. They have noticed that the nature of connection is very different from traditional LAN – people can appear and disappear from sight. As they are opening up an additional 255 megahertz of spectrum in the 5-gigahertz band, the biggest concern is whether or not it could affect military radars. Table 8 – Regulation comparison (US, EU)

Neither the US nor EU has taken public WLAN networks into deeper consideration. In the Table 8 there are few major differences that affect the way regulation is applied generally.

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