Besides the PHY and MAC overheads, there is another problem in VoWLANthat reduces the system capacity even further. An 802.11 BSS typicallyconsists of an AP and stations. ( A typical example is Internetbrowsing. )
The 802.11 standardspecifies that both the AP and the station use the same MAC.Specifically, the AP and the stations use the same mediacontention/access scheme and back-off periods (contention windowvalues).
This means that if an AP and station both compete to get access tothe channel, both are equally likely to get access to the medium.However, in a BSS, thereis one AP and multiple stations. This means that if multiple stationsand the AP compete for the media, it is more likely that one of thestations gets access to the media.
This inherent fairness among the stations and the AP works finefordata communication since most data applications are highly asymmetrical- i.e., they have high download (from the AP to the station) trafficand low uplink (from the station to the AP) traffic. (Since stations rarely compete to accessthe media in data communication, the AP can easily get access to themedia when it needs it. )
However, voice communication is bidirectional and highlysymmetrical; i.e., the downlink traffic and the uplink traffic are verysimilar in terms of bandwidth requirements. This means that stationsneed to access the media much more often than in data communication.
Given that stations and AP are competing on an equal footing andthat there are multiple stations involved in voice communication in aBSS, the AP is much less likely to get access to the channel than allother stations combined.
Put another way, since the AP is as likely to get access to thechannel as any other station, the probability of a station gettingaccess to the media is higher than the probability of the AP gettingaccess to the media if more than one station is competing for themedia.
Combine this with the fact that the AP is handling much more trafficthan any station and you have a system where the node handling the mosttraffic (AP) is not given priority over other nodes. ( Since all traffic must pass through theAP, the AP is almost handling as much load as the combined load of allstations in the BSS. )
This leads to a single point of back-up and congestion in a VoWLANBSS.
In practice, if there are N wireless IP phones in a BSS making callsto wired networks, the AP is handling N times the load as compared toany other node in the BSS. However, fairness in 802.11 would allow theAP to access the medium only as much as any other node.
The bottom line is that the AP will not be able to transmit thetraffic that it is receiving. Bad, as this may sound, things get worse.Since we are dealing with real-time traffic, a packet which getsdelayed beyond a limit waiting in the AP queue is rendered useless.
From the 802.11 MAC perspectives, this situation arises because802.11 requires that every station that finishes a transmission AND hasa packet waiting in its queue MUST perform the random back-off. In thebuild up to a congested network, the AP will almost always have morethan one packet in its queue (sinceall packets must go through it ) so it will be backing off.
For an 802.11b network, this backing off will, on average, add adelay of 320 µs6 to its effective packet-transmission time (DIFS= 20 microseconds; CWavg = 32/2 = 16; Delay = DIFS * CW). All phones,however, in most cases will rarely have more than one packet totransmit unless congestion is really heavy or there are PHY layerproblems (such as moving out of rangeof the AP ).
The previous sections have argued that the maximum number of voicecalls that can exist in a 802.11b BSS is severely limited. Analysis hasquantized this observation. as shown in Table5.1 below.
|Table5.1: VoIP Capacities for 802.11b|
This analysis has been confirmed by observation too. Put briefly,the number of VoIP connections that can exist in a BSS increases withthe use of larger packetization periods. However, higher payload sizesmean larger end-to-end delays in VoIP.
So, this increased system capacity comes at the cost of an increasedend-to-end delay in VoWLAN systems. Hence, simply using very largepacketization periods is not a viable solution for increasing systemcapacity in VoWLAN networks.
Earlier in Section 5.3.1, we explained that stations transmittingsmaller packets are at a loss with respect to stations transmittinglarger packets, since DCF (the default 802.11 MAC protocol) specifiesthe same back-off times for both regardless of the payload size theyintend to use.
Now, consider an 802.11 BSS where some stations are being used forVoWLAN communication whereas others are being used for dataapplications like Internet browsing. Since VoWLAN stations would beusing smaller payload sizes, they are inherently at a loss whencompeting with data stations for access to the media.
This situation is in fact the exact opposite of what is desired.Voice communication requires low delay and low jitter and it shouldtherefore be given a higher priority. In 802.11 we have, in effect,voice traffic being given lower priority than data traffi c. What isneeded then is a way to prioritize voice over data traffic.
The IEEE 802.11 group were aware of the problems with using DCF forreal-time traffic. They therefore defined the point coordinationfunction (PCF) to be used with real-time traffic (Figure 5.6 below ).
The PCF divides time into superframes. Each superframe is furthersubdivided into a CFP (contention free period) followed by a CP(contention period).
|Figure5.6. PCF Timing|
Therefore, in the PCF, CFP and CP alternate over time with theadditional requirement that the CP be long enough to ensure thedelivery of at least one MSDU. During the CP, channel access is stillcontrolled as in DCF. However, in the CFP, channel access is controlledexclusively by the PC (point coordinator), which is a logical entitytypically colocated at the AP.
The transmission of a beacon frame marks the beginning of asuperframe. The beginning of the superframe also marks the beginning ofthe CFP. During this time, PC has complete and unilateral access to thechannel and no other station may try to access it. In the CFP, the PCpolls the stations for a chance to transmit data.
A polled station is allowed to transmit a single MSDU when it ispolled. This polling continues till the end of the CFP, which isindicated by the transmission of a CF-End packet.
Ideally, the centralized tighter control of channel access by the PCshould have allowed time-sensitive applications like voice to satisfytheir QoS (delay and jitter requirements). However, there are manyloopholes in PCF which make it unusable.
Consider what happens if the PC polls a station when the CFP is justabout to end. Since the 802.11 standard allowed stations to start datatransmission even if the MSDU delivery could not finish before theupcoming TBTT (target beacon transmission time), this basically meantthat the CFP may actually end up the transmission of the next beacon.
This unpredictability in TBTT stemming from the unknown transmissiondurations of the polled stations during a CFP is a serious limitationof PCF since it has serious repercussions not only for the QoS of otherstations but also for power management in stations.
Consider another case where the PC polls a station and the stationhas an MSDU to transmit. Since the maximum allowed size of the MSDU is2304 bytes, this station can effectively capture the channel for a longtime, which would in turn delay the transmissions of other stations inthe BSS.
Note that this loophole stems from PCF specifying the polledstation's channel access in terms of the number of MSDUs rather thanabsolute time. These problems have made PCF practically unusable and ithas seen very little (if any) deployment in the industry.
5.5 Admission Control
Most networks require that a user/endpoint request permission to usethe network before it actually starts using the network resources. Thisis known as admission control and can be broadly classified into twocategories: authentication-based and resources-based.
Authentication-based admission control refers to a procedure inwhich the network ensures the authenticity of the user/endpoint beforeallowing it to access the network. This is usually done for thepurposes of security and billing.
The 802.11 standard allows for authentication-based admissioncontrol where the network/AP may reject the admission request of astation if the station fails the authentication process. (The fact that this authentication schemeis weak and suffers from a lot of loopholes is another matter, )
Resource-based admission control has its roots in telephony networkslike PSTN and is related to the concept of QoS. The underlyingphilosophy is that, if a network admits a user to the network, it mustallocate/reserve resources for this user to ensure service to the user.
In the case of telephony, we do not want the destination phone toring if we can't ensure that resources for the call will be available(this is known as a call defect, and has a more stringent failure raterequirement in toll-quality phone networks).
Since the network has limited resources, the network should keeptrack of what resources it is currently using and allow users to usethe network only if there are enough resources available to servicethis user. Hence, admission control in this case is a matter of networkcapacity and usage.
Since the 802.11 standard has its roots in data communications,where the network does not reserve any resources for any users, it doesnot provide any resource-based admission-control procedures.
This, combined with the fact that the capacity of WLANs in terms ofVoWLAN is extremely limited, means that congestion in VoWLAN networksis not handled gracefully. In most scenarios, a congested VoWLANnetwork severely degrades performance for all nodes in the system.
Security in 802.11 networks is a complex issue and has been a focus ofa lot of attention. Our concern here of course is that while we mightnot care if someone snoops to see what web pages we are downloading, wedo care if someone can snoop on our conversations.
5.7 Power Save
As discussed previously, the 802.11 power save mechanism required thatthe AP buffer packets destined for a dozing station and inform thestation about buffered packets in the beacon. This means that packetsdestined from the AP to the station may be delayed for as long as thebeacon period.
Since beacon periods are typically configured to be of the order ofhundreds of milliseconds, using this power-save approach for real-timetraffic would lead to delays of hundreds of milliseconds in the voicepath.
Arguably, smaller beacon periods would make this approach suitableeven for low-delay applications like VoWLAN; but smaller beacon periodsalso means that when a Wi-Fi handset is not in use, it will have towake up more often, thus leading to more power consumption and lowerbattery lives. Clearly, this is unacceptable. Hence, we conclude thatthe 802.11 doze mode is not suitable for real-time applications likeVoWLAN.
5.8 Roaming/Handoffs in 802.11
The wireless medium is a harsh medium for signal propagation. Signalsundergo a variety of alterations as they traverse the wireless medium.Some of these changes are due to the distance between the transmitterand the receiver, others are due to the physical environment of thepropagation path, and yet others are due to the relative movementbetween the transmitter and the receiver.
Attenuation refers to the drop in signal strength as the signalpropagates in any medium. All electromagnetic waves suffer fromattenuation. For radio waves, if r is the distance of the receiver fromthe transmitter, the signal attenuation is typically modeled as 1/r2.It is important to emphasize that this is radio modeling we are talkingabout.
Such models are used for simulation and analysis. In real life,radio propagation is much harsher and the signal strength and qualityat any given point depend on a lot of other factors too. Attenuation ofsignal strength predicts the average signal strength at a givendistance from the transmitter::
Attenuation explains why all wireless transmissions have a limitedgeographical range. The signal strength at the receiver decays as thedistance between the transmitter and receiver increases. This decay insignal strength means receivers far away from the transmitter are moreprone to suffer from transmission errors. To operate reliably in theharsh wireless medium, the 802.11 MAC protocol requires each packet tobe individually ACKed by the receiver.
If a packet is not ACKed, the transmitter must retransmit thepacket. However, this protocol will work only up to a certain thresholddistance, say R (range). If the distance between the transmitter andthe receiver is increased beyond R, the received signal strength willbe too low to achieve any communication. The value of R, therefore,depends upon three things:
1) TransmittedPower Level: Increasing transmission power will increase therange. However, 802.11 operates in the unlicensed frequency spectrumand most governments restrict the maximum transmission power level inthis spectrum to less than 1 watt.
2)Wavelength/Frequency: Higher frequencies are more prone toattenuation than lower frequencies. Therefore 802.11b/g networks, whichoperate at 2.4 GHz, have a greater range than 802.11a networks, whichoperate at 5 GHz.
3) AntennaGains: Since transmitted power level and operating frequencyare restricted by laws and standards, respectively, many 802.11equipment manufacturers concentrate on antenna gains to increase theirrange.
Most 802.11 equipment uses omnidirectional antennas since they aredesigned for general-purpose networking. However, it is possible tocustom-design WLAN products per specifi c requirements to increase therange of 802.11 networks. Use of intelligent antennae and MIMO(multiple in multiple output)is also getting considerable attention in the industry for thispurpose.
The bottom line, however, is that an 802.11 BSA has limitedgeographical range, typically a few hundred feet. Given that mobilityis an inherent expectation in wireless networking, the question is howto provide seamless connectivity to a mobile user in 802.11 networks.
This is where roaming comes in. When a station moves out of therange of its AP and enters into the range of another AP, a handoff issaid to have occurred. (The termshandoff and roaming are used interchangeably in 802.11 networks. )
It is important to realize that an 802.11 handoff is a Layer-2 process.An analogy might help make things clearer. Consider a laptop which isplugged into a corporate LAN socket using an Ethernet cable. Thislaptop has a valid IP address, which applications use to access theintranet and the Internet.
Now, if the Ethernet cable is plugged out from its current LANsocket and plugged into another socket belonging to the same corporateLAN, the applications on the laptop can continue to use its IP address.
In fact, the applications are not even aware of what has happened.The 802.11 handoff is a similar process where the plugging in and outof the Ethernet cable from sockets is analogous to disconnecting froman AP and connecting to another AP.
Therefore, if a higher-layer application uses a reliabletransport-layer protocol like TCP, it does not even need to be aware ofthe 802.11 handoff—the transport layer will take care of retransmittingpackets that were lost during the handoff.
A web browser running HTTP (HyperText Transfer Protocol)is such an example. Consider a mobile user running a web browser on hislaptop which connects to the Internet via an 802.11 network. When theuser clicks on an HTTP link while a handoff is in progress, theunderlying transport protocol, TCP, conceals the delay/packet loss dueto the handoff by using retransmissions. In other words, higher layers (HTTP, the web browser and the user )are unaware of the handoff and can continue without disruption.
On the other hand, if a higher-layer application uses an unreliabletransport layer protocol like UDP , it would “see” the handoffas temporarily increased delay or packet loss. As long as theapplication can tolerate and recover from this delay or loss, an 802.11handoff would be “transparent” to the user too.
The challenge for VoWLAN is that voice is extremely sensitive todelay. The end-to-end delay budget for voice is 250 ms; this means thatthe accumulative delay between the two endpoints involved in a voicecall must not exceed 250 ms. This 250 ms must include the totaltransmission delay, propagation delay, processing delays in the networkand and codec delays at both endpoints.
In WLANs, the 802.11 MAC introduces an extra transmission delay inthe WLAN due to the contentious nature of the MAC protocol.( The situation is worse if both endpointsin the voice call use VoWLAN .)
This wireless transmission delay increases as networks become morecongested or suffer from interference. The bottom line is that thebudget for each component of the accumulative delay will typically bespecifi ed by the service provider, but we can't assume that we wouldhave all 250 ms available for 802.11 delays. A good VoWLANimplementation would aim to keep the 802.11 delays limited to 40″50 ms.
Given that VoWLAN operates on a very restricted delay budget, the802.11 handoff times on the order of a few hundred milliseconds areunacceptable for voice. (Typicallybetween 200 and 500 ms. )Thus, handoff times are anotherimportant area of product differentiation and vendors are competing tominimize handoff times in their products.
To read Part 1, go to The many faces of VoWLAN
Used with the permission of the publisher,Newnes/Elsevier, this two part series is based on material from “Wi-FiTelephony: Challenges and Solutions for Voice over WLANs,” byPraphul Chandra and David Lide.
PraphulChandra is currenty senior research scientist at HP Labs, India, which focuseson “technological innovation for emergining countries.” David Lide iscurrently a senior member of the Technical Staff at Texas Instruments, Inc., and has workedon various aspects of Voice Over IP for the past eight years.