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WiFi-VoIP

Prepared By : Ala’ KhalifehUniversity of California-Irvine, 2006

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Industry Approach

Most of the mobile companies are trying to enhancing their mobile sets to make them VoIP capable.

Besides, the industry is now looking for a new architecture for integrating the Data Network with the Mobile Network thought a new architecture called “IMS”.

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Milestones

Part I : VoIP Benefits Requirements

Part 2 : WiFi Technology summary Access Problems

Part 3 Vo802.11

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Voice over IP (VoIP) benefits

VoIP Definition Using the Internet to carry phone conversations, known as

Internet telephony or voice over IP (VoIP). Portability

Telephone number is associated with an IP phone, not a location Take phone with you when move offices - simply plug into VoIP-

ready jack long distance calls

It will save you money on long distance and international phone calls

Access Numbers. Get a ‘virtual presence’ in multiple cities with phone numbers for

each city area code all routed through to your number

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Voice over IP (VoIP) benefits

Simple to set up and to use To build and administrate only one data network, which is

capable of carrying both voice and data, instead of establishing two separate infrastructures, one for voice and the other one for data.

Many new Advanced Features can improve how you use your phone.

Many Business opportunities eBay is acquiring Skype for between $2.6 - 4.1 billion.! (skype is

just a four-year old company !)

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Quality of Service Requirements

 In order to have a good VoIP service that may constitute an acceptable alternative to the PSTN network. It is extremely important that such service must meet some quality of service regulations and standards such as:

End-to-end delay, or called latency( Not to exceed 150 ms). Packet loss (Not to exceed 20%), Delay variation called jitter delay

802.11 Protocols Standards

IEEE 802 Network Technology Family Tree 802.11 is a member of the IEEE 802 family, which is a series of specifications for local area network (LAN) technologies. Figure 1 shows the relationship between the various components of the 802 family and their place in the OSI model.

Figure 1: The IEEE 802 family and its relation to the OSI model

How To Access the Network ?

Random Access Protocols When node has packet to send

transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes “collision”, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:

If channel sensed idle: transmit entire frame

If channel sensed busy, defer transmission

Human analogy: don’t interrupt others!

CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmission

collision:entire packet transmission time wasted

CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage

collision detection: easy in wired LANs: measure signal strengths, compare

transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting

human analogy: the polite conversationalist

CSMA/CD collision detection

Ethernet uses CSMA/CD

No slots adapter doesn’t transmit if it senses that some other adapter is

transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is

transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time,

that is, random access

IEEE 802.11: multiple access

Like Ethernet, uses CSMA: random access carrier sense: don’t collide with ongoing transmission

Unlike Ethernet: no collision detection – transmit all frames to completion acknowledgment – because without collision detection, you don’t

know if your transmission collided or not Why no collision detection?

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

can’t sense all collisions in any case: hidden terminal, fading Goal: avoid collisions: CSMA/C(collision)A(voidance)

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then - transmit entire frame (no CD)

2 if sense channel busy then - start random backoff time- timer counts down while channel idle- transmit when timer expires- if no ACK, increase random- backoff interval, repeat 2

802.11 receiver

if frame received OK

- return ACK after SIFS (ACK needed due

to hidden terminal problem)

Wireless network characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access):

AB

C

Hidden terminal problem• B, A hear each other• B, C hear each other• A, C can not hear each othermeans A, C unaware of their

interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal fading:• B, A hear each other• B, C hear each other• A, C can not hear each other

interferring at B

RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

sender first transmits small request-to-send (RTS) packets to AP using CSMA RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

Collision Avoidance: RTS-CTS exchange

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Voice over Wireless

VoIP over 802.11 (Vo802.11)

The emerging popularity of VoIP in the enterprise market coupled with 802.11 LANs raises the following questions:

Can voice be transported over an 802.11 network? If voice can travel over a wired IP network, why could it not

travel over a wireless network? If transporting voice over an 802.11 network has limitations,

what are they?

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Voice over Wireless

Two major technical problems that stand in the way are

1) low VoIP capacity in WLAN.

2)unacceptable VoIP performance in the presence of coexisting traffic from other applications

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Voice over Wireless

IEEE 802.11b, which can support data rates up to 11Mbps.

A VoIP stream typically requires less than 10Kbps.

Ideally, the number of simultaneous VoIP streams that can be supported by an 802.11b WLAN is around 11M/10K = 1100, which corresponds to about 550 VoIP sessions, each with two VoIP streams.

However, it turns out that the current WLAN can only support no more than a few VoIP sessions. For example, if GSM 6.10 codec is used, the maximum number of VoIP sessions that can be supported is 12.

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Voice over Wireless

This result is mainly due to the added packet-header overheads as the short VoIP packets traverse the various layers of the standard protocol stack, as well as the inefficiency inherent in the WLAN MAC protocol, as explained below.

A typical VoIP packet at the IP layer consists of 40-byte IP/UDP/RTP headers and a payload ranging from 10 to 30 bytes, depending on the codec used.

So the efficiency at the IP layer for VoIP is already less than 50%. At the 802.11 MAC/PHY layers, the drop of efficiency is much

worse. Consider a VoIP packet with 30-byte payload. The transmission

time for it at 11 Mbps is 30 * 8 / 11 = 22 μ sec

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Voice over Wireless

The transmission time for the 40-byte IP/UDP/RTP header is 40 * 8 / 11 = 29 μ sec .

However, the 802.11 MAC/PHY layers have additional overhead of more than 800 μ sec , attributed to the physical preamble, MAC header, MAC backoff time, MAC acknowledgement, and intertransmission times of packets and acknowledgements.

As a result, the overall efficiency drops to less than 3%

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Voice over Wireless

In an enterprise WLAN or public WLAN hotspot, supporting VoIP becomes even more complicated, since the WLAN needs to simultaneously support other applications besides VoIP.

Even when the number of VoIP sessions is limited to just half of the capacity in an 802.11b WLAN, interference from just one TCP connection will cause unacceptably large increases in the delay and packet loss rate of VoIP traffic.

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Voice over Wireless

Another way to increase the number of audio streams is to utilize a higher performing standard, such as 802.11a for the WLAN backbone.

802.11a has the capacity to handle approximately four times as much voice traffic as 802.11b.

The problem with 802.11a, though, is that other non-voice users of the network may only be equipped with 802.11b. As a result, consider installing access points that include both 802.11a (for voice users) and 802.11b (for data users).

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Voice over Wireless

A WLAN can support voice if you implement the system with high performance and quality of service (QoS) in mind.

The problem is that the 802.11 standard doesn't support QoS yet.

The 802.11e group is currently working on a QoS upgrade, but the ratification of the standard is still a ways off.

802.11e will prioritize traffic on the network, making data give way to voice packets.

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Voice over Wireless

Until 802.11e is available, WLANs need to deploy proprietary QoS mechanisms to enable effective blending of voice and data. That's a problem when trying to support voice traffic over a public WLAN

Research Ideas

1) Study the effect of TCP/UDP traffic on the Voice trsffic over the Wireless channels.

2) Improving the network access mechanisms.

Literarture survey and Future Readings

1)Solutions to Performance Problems in VoIP over 802.11 Wireless LAN 1 ,Wei Wang, Soung C. Liew Department of Information Engineering ,The Chinese University of Hong Kong

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 1, JANUARY 2005

2)TCP Fairness in 802.11e WLANs.pdf,IEEE COMMUNICATIONS LETTERS, VOL. 9, NO. 11, NOVEMBER 2005

3)Experimental Evaluation of TCP Performance and Fairness in an 802.11e Test-bed.pdf, SIGCOMM’05 Workshops, August 22–26, 2005, Philadelphia, PA, USA.

4)Supporting VoIP Traffic in IEEE 802.11 WLAN with Enhanced Medium Access Control (MAC) for Quality of Service, Avaya Labs Research

5)Scalability and Performance Analysis of IEEE 802.11a,CCECE / CCGEI, Saskatoon IEEE, May 2005

Literarture survey “Related Papers”

D. Chen, S. Garg, M. Kappes, and K. Trivedi, “Supporting VBR VoIP

traffic in IEEE 802.11 WLAN in PCF mode,” Avaya Laboratories,

Basking Ridge, NJ, Tech. Rep. ALR-2002-026, 2002.

M. Veeraraghavan, N. Cocker, and T. Moors, “Support of voice services

in IEEE 802.11 wireless LANs,” Proc. INFOCOM’01, vol. 1, pp.

488–497, Apr. 2001.

R. O. Bladwin, N. J. Davis IV, S. F. Midkiff, and R. A. Raines, “Packetized

voice transmission using RT-MAC, a wireless real-time medium

access control protocol,” Mobile Comput. Commun. Rev., vol. 5, no. 3,

pp. 11–25, 2001.

S. Garg and M. Kappes, “On the throughput of 802.11b Networks

for VoIP,” Avaya Laboratoriess, Basking Ridge, NJ, http://www.research.

avayalabs.com/techreportY.html, Tech. Rep. ALR-2002-012.

Literarture survey “Related Papers”

“An experimental study of throughput for UDP and VoIP traffic in

IEEE 802.11b networks,” Proc. IEEE WCNC’03, vol. 3, pp. 1748–1753,

Mar. 2003.

T. Hiraguri, T. Ichikawa, M. Iizuka, and M. Morikura, “Novel multiple

access protocol for voice over IP in wireless LAN,” presented at the

IEEE Int. Symp. Computers and Communications, Taormina, Italy, Jul.

2002.

A. Banchs, X. Perez, M. Radimirsch, and H. Stuttgen, “Service differentiation

extensions for elastic and real-time traffic in 802.11 wireless

LAN,” in Proc. IEEE Workshop High Performance Switching and

Routing, May 2001, pp. 245–249.

Literature survey “Related Papers”

H.-H. Liu and J.-L. C. Wu, “Packet telephony support for the IEEE

802.11 wireless LAN,” IEEE Commun. Lett., vol. 4, no. 9, pp. 286–288,

Sep. 2000.

N. Prasad and A. Prasad, WLAN Systems and Wireless IP for Next Generation

Communications. Norwood, MA: Artech House, 2002.

A. Prasad, “Performance comparison of voice over IEEE 802.11

scheme,” Proc. IEEE VTC’99, vol. 5, pp. 2636–2640, Sep. 1999.

D. Chen, S. Garg, M. Kappes, and K. Trivedi, “Supporting VoIP traffic

in IEEE 802.11 WLAN with enhanced medium access control (MAC)

for quality of service,” Avaya Laboratories, Basking Ridge, NJ, Tech.

Rep. ALR-2002-025, 2002.