Design and Implementation of Wi-Fi MAC Transmitter for Low Power Applications

Ms.PadmaRatna. G. K.S.Rao, Mr.P.Rajasekharbabu, Padma.neni85@gmail.com rsbabu.peketi@gmail.com

Abstract — Wireless communication is one of the fastest growing technologies. Wireless Fidelity, or Wi-Fi, is a term used to describe the technology of wireless local area networks (WLAN) based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. The main aim of WiFi is to provide communications with high bandwidth, with low cost and easy availability of the chipsets. Mobile stations in a WLAN have to compete for a shared multiple access medium, in that situation there is possibility of collisions. To avoid collisions and to increase speed of WiFi, we are designing MAC layer functions as hardware for WiFi transmitter using IEEE 802.11b. The best known and most widely used variation of the 802.11 wireless LAN standards is 802.11b. The main cores of the IEEE 802.11b standard are the CSMACA, Physical and MAC layers.

Keywords— WiFi, WLAN, MAC layer, CSMA/CA, IEEE 802.11, IEEE 802.11b.

I. INTRODUCTIONA. Overview of Wireless LANSA wireless LAN (WLAN) is a data transmission system designed to provide location-independent network access between computing devices by using radio waves rather than a cable infrastructure.WLAN utilizes spread-spectrum technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network.For the home user, wireless has become popular due to ease of installation, and location freedom with the gaining popularity of laptops. The majority of future wireless LAN growth is expected in

healthcare facilities, educational institutions, and corporate enterprise office spaces. B. Overview of Wireless Fidelity Wifi stands for Wireless Fidelity. Wi-Fi is based on the IEEE 802.11 family of standards and is primarily a local area networking (LAN) technology designed to provide in-building broadband coverage. WiFi is a universal wireless networking technology that utilizes radio frequencies to transfer data. Wi-Fi is more commonly used in point-to-multipoint (PMP) environments to allow extended network connectivity of multiple portable devices such as laptops, telephones, or PDAs. Wi-Fi also allows connectivity in point-to-point (P2P) mode, which enables devices to directly connect and communicate to each other. WiFi networks operate in the unlicensed 2.4 radio bands, with an 11 Mbps (802.11b) or 54 Mbps (802.11a) data rate, respectively.There are three most important items which makes Wi-Fi working in your laptop or desktop. These are: Radio Signals Wi-Fi Card which fits in your laptop or computer.

Hotspots which create Wi-Fi Network.Below Fig1. Illustrate the components of a common

Wi-Fi network.

Fig 1. Common Wi-Fi Network

wi-fi also can be used to create a wireless mesh network, which is a decentralized, reliable, resilient, and relatively inexpensive solution that can support areas of lacking or destroyed network infrastructure

(e.g., connectivity for a field office or emergency command center).

C. Benefits of Wireless FidelityThe popularity of wireless LANs is a testament primarily to their convenience, cost efficiency, and ease of integration with other networks and network components.The benefits of WiFi include:1) Convenience - The wireless nature of such networks allows users to access network resources from nearly any convenient location within their primary networking environment (home or office). 2) Mobility – With the emergence of public wireless networks, users can access the internet even outside their normal work environment. 3) Deployment - Initial setup of an infrastructure-based wireless network requires little more than a single access point4) Expandability - Wireless networks can serve a suddenly-increased number of clients with the existing equipment. In a wired network, additional clients would require additional wiring.5) Cost - Wi-Fi chipset pricing continues to come down, making Wi-Fi a very economical networking option and driving inclusion of Wi-Fi in an ever-widening array of devices.

D. Disadvantages of Wireless FidelityFor a given networking situation, wireless LANs have following limitations.1) Security - Security concerns have held back Wi-Fi adoption in the corporate world. It can be easy to crack the current security technology, known as wired equivalent privacy (WEP), used in most Wi-Fi connections. A hacker can break into a Wi-Fi network using readily available materials and software.2) Range - The typical range of a common 802.11b Wi-Fi network is on the order of tens of meters. It will be insufficient in a larger structure. 3) Reliability- Like any other radio frequency transmission, wireless networking signals are subject to a wide variety of interference, as well as complex propagation effects such as multipath. 4) Speed- The speed on most wireless networks (typically 1-108 Mbps) is reasonably slow compared to the slowest common wired networks.

5) Energy - Power consumption is fairly high compared to some other standards, making battery life and heat a concern.To overcome these disadvantages of WiFi we are designing controller which controls the accessing of medium at the transmitter of WiFi using IEEE 802.11 standards.

II. BACKGROUND For computer networks, a seven layer –layer ISO (International Standards Organization) OSI (Open Systems Interconnection) reference model is widely used. The communication sub network can be described by the lower three layers (i.e., physical, data link, and network layers). Existing LAN’s, MANs (metropolitan area networks) do utilize broadcast channels rather than point-to-point channels for information transmission. Therefore, a simple modification of OSI model is done by adding the so called MAC (medium access control) sub layer in data link layer. The MAC sub layer protocols, usually known as the multiple access protocols, are primarily a set of rules that communicating terminals need to follow, and these are assumed to be agreed upon a priori.

Fig. 2 802.11 OSI model

A. 802.11 Operating Modes:The 802.11 standard defines two modes: Infrastructure mode and ad hoc mode. In infrastructure mode, the wireless network consists of at least one access point connected to the wired network infrastructure and a set of wireless end stations. This configuration is called a Basic Service Set (BSS). An Extended Service Set (ESS) is a set of two or more BSSs forming a single subnetwork.

Ad hoc mode (also called peer-to-peer) mode or an Independent Basic Service Set, or IBSS) is simply a set of 802.11 wireless stations that communicate directly with one another without using an access point or any connection to a wired network

B. Important WLAN StandardsIEEE 802.11 standard is of Wireless LAN (WLAN) standards. IEEE 802.11b defines the medium access control layer [MAC] for WiFi to avoid collisions. In general, the MAC Layer manages and maintains communications between 802.11 stations (radio network cards and access points) by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium.

III. RELATED WORK

A. Medium access basics:IEEE 802.11 wireless LANs use a media access control protocol called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Wi-Fi systems are half duplex shared media configurations where all stations transmit and receive on the same radio channel. The fundamental problem this creates in a radio system is that a station cannot hear while it is sending, and hence it impossible to detect a collision. Because of this, the developers of the 802.11 specifications came up with a collision avoidance mechanism called the Distributed Control Function (DCF).According to DCF, A Wi-Fi station will transmit only if it thinks the channel is clear. All transmissions are acknowledged, so if a station does not receive an acknowledgement, it assumes a collision occurred and retries after a random waiting interval.The incidence of collisions will increase as the traffic increases or in situations where mobile stations cannot hear each other.

B .Carrier -sensing and Network Allocation VectorCarrier sensing is used to determine if the medium is available or not. As a condition to accessing the medium, the MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the

amount of time that the previous frame needs to send its frame. The NAV must be zero before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame’s length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. This process reserves the medium for the sending station.

Fig. 3 Using NAV for Virtual Carrier

Short interframe space (SIFS): The SIFS is used for the highest-priority transmissions, such as RTS/CTS Frames and positive acknowledgements. High-priority transmissions can begin once the SIFS has elapsed .Once these high priority transmission begin, the medium becomes busy, and so frames transmitted after SIFS has elapsed have priority over frames that can be transmitted only after long intervals.DCF interframe space (DIFS): The DIFS is the minimum medium idle time for contention –based services. Stations may have immediate access to the medium if it has been for a period longer than the DIFS.

C. Exponential Backoff AlgorithmBackoff is a well known method to resolve contention between different stations willing to access the medium, the method requires each station to choose a Random Number (n) between 0 and a given number, and wait for this number of Slots

before accessing the medium, always checking whether a different station has accessed the medium before. The Slot Time is defined in such a way that a station will always be capable of determining if other station has accessed the medium at the beginning of the previous slot. This reduces the collision probability by half. Exponential Backoff means that each time the station chooses a slot and happens to collide; it will increase the maximum number for the random selection exponentially. The 802.11 standard defines an Exponential Backoff Algorithm, which must be executed in the following cases: (a)If when the station senses the medium before the first transmission of a packet and the medium is busy,(b) After each retransmission, and (c) After a successful transmission (d) The only case when this mechanism is not used is when the station decides to transmitWe are dealing with only 802.11 MAC layer for this paper using 802.11b standard due to its ease of operating standards. The following figure shows a schematic of the access mechanism:

Fig. 4 Backoff Access Mechanism

D. 802.11 MAC Layer FunctionsThe following summarizes primary 802.11 MAC functions, especially as they relate to infrastructure wireless LANs:1) Authentication: Authentication is the process of proving identity, and the 802.11 standard specifies two forms: Open system authentication and shared key authentication. In this paper we use shared key authentication only. Shared key authentication is a

process that bases authentication on whether the authenticating device has the correct WEP (wired equivalent privacy) key.2) Association: Once authenticated, the radio NIC must associate with the access point before sending data frames. Association is necessary to synchronize the radio NIC and access point with important information, such as supported data rates. 3) RTS/CTS: The optional request-to send and clear-to-send (RTS/CTS) function allows the access point to control use of the medium for stations activating RTS/CTS. 4) WEP: With the optional WEP enabled, the wireless NIC will encrypt the body (not header) of each frame before transmission using a common key, and the receiving station will decrypt the frame upon receipt using the common key. The 802.11 standard specifies a 40-bit key and no key distribution method, which makes 802.11 wireless LANs vulnerable to eavesdroppers.5) Fragmentation: The optional fragmentation function enables an 802.11 station to divide data packets into smaller frames. This is done to avoid needing to retransmit large frames in the presence of RF interference. The bits errors resulting from RF interference are likely to affect a single frame, and it requires less overhead to retransmit a smaller frame rather than a larger one. As with RTS/CTS, users can generally set a maximum frame length threshold whereby the radio NIC will activate fragmentation. If the frame size is larger than the threshold, the radio NIC will break the packet into multiple frames, with each other.

IV. DESIGN ASPECTSA. Block Diagram Explanation1) CLK Generation Block: The clock generation block is used to generate 1MHz clock frequency from 50MHz that is available from the board. This generated clock is easy to use to the backoff block and difs counter blocks which are sub multiples of 1MHz clock frequency.2) DIFS Block: The DIFS counter block is operated by the main control machine which initiates the counter, the estimated period for the DIFS with respect to PHY layer considering 802.11b is 50 micro seconds, so the counter is made work with 50 as counter under clk_1MHz clock frequency to

obtain a delay of 50 micro seconds. As soon as the block gets completed it intimates the control machine for further back off operation.

Fig .5 Block Diagram of WiFi MAC Transmitter

3) Backoff Block: This block is operated only when the phy_idle signal that is provided by the PHY layer becomes logic 0 before the DIFS gets completed. The start of backoff is initiated by control machine, and initially the random value is taken as 30, that is defined by number of time slots.

In this case the slot time is taken as 20 micro seconds.

4) Control Block:   Back off state is an important operation in wireless LAN MAC that decides which Node to access the channel, this is executed if the phy_ilde becomes low within DIFS counter period

else not, in our case the state transition has moved DIFS to backoff state to test the states transition operation. The further states are for the transfer of the header frames and probe frames , auth frames association frames and rts and data frames ,that are processed in sequence, by enabling the header block with header_En so that the data_in byte of input that get transfers framing it as the header frame through the MPDU multiplexer block to the physical layer, very similar to the header block is the probe block is enabled in the probe state that activates the probe block only making all the other blocks inactive, this makes only probe block to get the data_in data in bytes to pass the data to the multiplexer block, so with the remaining frames get transmitted, with authentication, association and RTS, data frames ,but each frames are the request frames and so as soon the request frame is transmitted the response frame is received that is processed with the receiver block of WLAN MAC and give the signal of no error for the transmitted frames as prob_reply, asso_reply, auth_reply, rts_reply, and data_reply, for their respective request frame transmission in the MAC transmitter block. So each and every frame transmission in transmitter of WLAN MAC is transferred only if the previous frame’s response is received correctly.5) Header Block: The header block defines the type of frame that is transmitted, the contents of the frame has type, subtype values that define which frame body to transmit, and also the add1 recipient address of 6 bytes and the transmitter address which is MAC address of transmitting node, duration of 2 bytes.6) Probe Block: The probe block has the contents as SSID 32 bytes and supported data rate and extended data rates of 8 bytes that are need to transmit in a sequence. 7) Authentication Frame Block: The authentication block transmits the frame body of the auth frame byte by byte. The content algorithm no, authentication transaction number status code and challenge text, that are send in sequence of bytes through the block to MPDU block to PHY layer.8) Association Frame Block: The association frame block capability information, listen interval, SSID of 32 bytes, supported rates are transmitted in a sequence of byte by byte to MPDU block.

9) MPDU Block: The MPDU block is a multiplexer block that passes the bytes of information from each frame block to the physical layer only one at a time, depending on the selection signals that are generated from the control state machine.

V. SIMULATION RESULTS

Fig. 6 DIFS Block outputFig.6 shows the output of difs block of transmitter.

Fig .7 Backoff Block outputFig.7 shows the output of backoff block of transmitter.

Fig .8 Authentication Block outputFig.6 shows the output of Authentication block of transmitter

Fig .9 Association Block outputFig.9 shows the output of Association block of transmitter

Fig .10 Data Block outputFig.10 shows the output of data block of transmitter

Fig .11 Mpdu Block outputFig.11 shows the output of Mpdu block of transmitter

VI. CONCLUSIONWe have designed and verified functionally individual blocks for WiFi MAC Transmitter which

implements medium access control (MAC) layer as hardware using VHDL. They are 1) To design the transmitter we have utilized CSMA/CA algorithm to overcome the disadvantages of Wireless Local Area Networks which supports WiFi standards.2) The design has been synthesized using FPGA technology from Xilinx. This design is targeted to the device family→spartan3, device→xc3s4000, and package→cs144 and speed grade→ 6. The device belongs to the Vertex-E group of FPGAs from Xilinx.

(a)By defining MAC layer functions as hardware in this project the advantages are:1. The speed of data transmission is increased by avoiding the collisions of data at transmitter by implementing backoff algorithm of MAC layer.2. Reliability of transmitter is increased by implementing Association function of MAC layer.3. More Security of data is provided by implementing Authentication function of MAC layer.4. Range of the network can be increased by using repeaters at transmit section.5. By avoiding collisions, retransmissions of data are reduced which results in less power consumption.

(b) Disadvantages of Wi-FI can be described as below: they are 1. The range is very limited. The repeaters or access points which can be used to increase the range are very cost effective.2. Even by implementing MAC layer functions as hardware the speed and security of data transmissions are very less when compared to wired networks.

VII. FUTURE SCOPE1. We can apply the MAC layer functions for WiFiRe protocols to extend the range of WLANs and to improve efficiency.2. We can extend this technology for WiMax WMANs to improve efficiency. 3. We can define new protocols for improving efficiency and for reducing the cost of wireless devices.

VIII. APPLICATIONSWi-Fi technology has been highly utilized for many applications such as voice communications, video communications, and various data services without any loss in quality. Voice applications over WLANs allow users to always stay connected, thus reducing response times in emergency situations. As the Wi-Fi standard continues to improve, voice and video services can be applied differently to better serve the community.Another utilization of Wi-Fi technology growing in popularity is in regard to video surveillance. Many cities are considering employing video surveillance cameras on streetlamps to decrease criminal activities. Previously, data gathered on surveillance cameras was easily lost with damage to the tape. Wi-Fi cameras will now be able to transmit video in real-time to a central database, guaranteeing better reliability in saving information.

IX. REFERENCES1. T. S. Rappaport, Wireless Communications,

Principles and Practice, Prentice Hall, 1996.2. ANSI/IEEE Std 802.11, 1999 Edition, Wireless LAN

Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Adopted by the ISO/IEC and redesigned as ISO/IEC 8802-11:1999(E).

3. LAN MAN Standards Committee of the IEEE Computer Society, Wireless LAN medium access control (MAC) and physical layer (PHY) specification, IEEE, New York, NY, USA, IEEE Std 802.11-1999 edition, 1999.

4. Wei Ye and John Heidemann, Medium Access Control in Wireless Sensor Networks, USC/ISI TECHNICAL REPORT ISI-TR-580, OCTOBER 2003.

5. IEEE P802.11, the Working Group for Wireless LANs.http://grouper.ieee.org/groups/802/11.

6. Wi-Fi Planet. (2002) the Big Question: 802.11a or 802.11b. Retrieved Feb 6, 2007, from http://www.wifiplanet.com/columns/article.php/96118

7. IEEE 802.11 Working Group. (2007). IEEE 802.11 Wireless Local Area Networks - The Working Group for WLAN Standards. Retrieved February 6, 2007, from http://www. ieee802. org/ 11/.

8. Reynolds, Janice. (2003). Going Wi-Fi: A Practical Guide to Planning and Building an 802.11 Network. Kansas: CMP Books.