Scalable voip mobility intedration and deployment- P22 pptx

210 Chapter 6 www.newnespress.com beacon. That doesn’t guarantee that the client will hear the beacon at exactly that time, however. Beacons can be delayed if the air is occupied at that time. Furthermore, because beacons are sent out as broadcasts, the client might just miss the beacon or the beacon can be collided with. If the client does hear the beacon, it can then go to sleep so long as no traffic is buffered for it. Clients may also skip beacons. They would do this to save additional battery, at the expense of increasing the amount of time the frames would be buffered. Clients usually let the access points know how many beacons they will skip by sending a listen interval in their Association Request messages. A listen interval of 1 means that the client will wake for every beacon; a listen interval of 10 means that the client will wake only for every tenth beacon. Be careful, however; some clients do not follow the listen interval they state, waiting either for more or less beacons than they advertise. The client signals that it is going to sleep by using the power management bit in any unicast frame it sends to the access point (except for non-Action management frames). The power management bit is in the Frame Control field for the frame. When the client sends a frame Access Point PSPoll Mechanism Client Device Beacon Null (PM=1) Null (PM=0) PS Poll Data 1 Beacon 0 TIM Bit for Client In Power Save Mode Time ACK Awake Awake Access Point PSNonPoll Mechanism Client Device Beacon Null (PM=1) Null (PM=1) Null (PM=0) Null (PM=0) Data 1 Beacon 0 TIM Bit for Client In Power Save Mode Time ACK Figure 6.1: Wi-Fi Legacy Power Save Voice Mobility over Wi-Fi 211 www.newnespress.com with the power management bit set and when it gets an Acknowledgement in response, it knows that the access point has heard the client’s change of state and can now go to sleep. From this moment on, the access point will buffer frames, until the client sends any frame to the access point with the power management bit not set. That signals that the client is now awake, and can be sent packets as usual. While the client is in power save mode, and it wakes to find that its TIM bit is set to signify that it has frames available for it, the client has two choices on how to gather those frames. The first choice is known as the PSPoll mechanism, and uses the Power Save Poll (PS Poll) frames. After the beacon with the client’s TIM bit set, the client would send a PS Poll frame to the access point. This frame, which is usually acknowledged right away, triggers the access point to deliver exactly one of the buffered frames for the client. That buffered frame is put into the transmit queue, using the appropriate access category for WMM. The frame that is sent also has its More Data bit in the Frame Control field set if there are subsequent frames that are buffered. Once the client has the frame, it can chose to send another PS Poll to get another frame. This one-PS-Poll/one-data-frame exchange continues until the access point’s buffer is drained or the client wishes to sleep more. The other option the client has is to use the PSNonPoll mechanism. This mechanism is quite simple: the client simply sends a data frame, usually a Null data frame, stating that it is no longer sleeping, by clearing the power management bit. The access point will proceed to queue all of the buffered frames, each using its own WMM access category. The client can then wait for a certain amount of time, hoping that it got all of the frames it was going to get, after which it can send another Null data frame, signifying it is going back to sleep. Any frames that may have still been in a transmit queue might get buffered again by the access point, for a later PSNonPoll exercise. The advantage of the PSNonPoll mechanism is that it is simple and doesn’t require a significant back-and-forth. The disadvantage is that the client has no way of knowing if there are any remaining frames for it, without going to sleep and waiting for the next beacon. The choice between PSPoll and PSNonPoll modes is often left up to the client’s software implementation, and not exposed to you. However, some clients do give a choice up front, or have specific behavior where they will use one method or the other, depending on how aggressive you set its power save settings to (using a slider, say). It should be clear that neither mode is good for quality-of-service traffic, because the client can be forced to wait as much as a beacon interval (times its listen interval) before it finds out traffic is available. If the beacon interval is set to the typical 100 milliseconds, and the listen interval is 10, then that can be up to a second of delay. Broadcast and multicast frames are also covered in the legacy scheme. However, no polling is necessary for those frames to be delivered. Instead, the access point sets aside a certain number of the beacons for multicast traffic. If any client on the access point is in legacy 212 Chapter 6 www.newnespress.com power save mode, the access point will buffer all multicast traffic. The special beacons known as Delivery Traffic Indication Messages (the poorly named DTIM) are just like regular beacons, except that they come every so many beacons—when the next one is coming is signaled as a part of the TIM in every beacon—and they signal if multicast traffic is buffered. If multicast traffic is buffered, the TIM has the zeroth bit, corresponding to AID 0, set. If clients receive a beacon with that bit set, they know that the next frames coming from the access point will be all of the multicast frames buffered. Each multicast frame, except for the last one, will have the More Data bit set. Thus, clients can stay awake to collect all multicast traffic, and then go back to sleep after the last multicast data frame, with the cleared More Data bit, comes through. (Of course, if that last frame is lost, being multicast, the clients will have to decide on their own when to return to sleep.) The consequence of the all-or-nothing multicast buffering is that multicast traffic on Wi-Fi when anydeviceisinpowersaveisnotgenerallysuitableforreal-timetrafc!Lookfor architectures that provide solutions for this problem if real-time multicast is a priority for your network. Finally, I haven’t gone into details on how the TIM bits are compressed. It is not easy to read the TIM bits by hand, but a good wireless protocol analyzer will be able to read them for you, and let you know which AIDs are set in any beacon. 6.0.2.2 WMM Power Save To provide power saving while the mobile device is in a call, the Wi-Fi Alliance came up with the second power saving technique, WMM Power Save. This technique, based on the quality-of-service additions in the 802.11e amendment to the standard, acts as a parallel scheme to the legacy one, using similar concepts but in a way that avoids having to wait for beacons and can apply on a per-access-category basis. If you notice, there is nothing in the standard that prevents clients that are using the legacy power save scheme from ignoring beacons, for the most part, and sending PS Polls whenever they want. If the client were sure that there is going to be a packet for it waiting every so often—say, 20 milliseconds—then it could just send PS Polls every 20 milliseconds, collect its data, and have real-time power save. Of course, this doesn’t happen for legacy power save, because the client has no guarantee that it won’t get some other frames rather than what it is looking for. However, this is the concept that WMM Power Save builds on. WMM Power Save is optional, and support for it is signaled by the WMM information elements in the Association messages and the beacons. Unlike with legacy power save, WMM Power Save (capitalized, as it is a formal name) is aware of the WMM access categories and can apply to a subset of them. The two subsets are delivery-enabled access categories and trigger-enabled access categories. Voice Mobility over Wi-Fi 213 www.newnespress.com First, let’s start with the polling protocol. The client no longer checks the beacons to see if there is traffic. Instead, it is responsible for knowing that traffic is waiting for it, and how often. For phones, this is not a problem, as voice is bidirectional and consistent. Instead of sending a PS Poll frame, or using the PSNonPoll mechanism, the phone sends data frames in access categories that it has specified to be trigger-enabled. The access point looks for those data frames, and uses that as a trigger—just as it does in legacy with Power Save Poll frames—sending packets in response from the power save buffer. Those packets, however, can only come from the delivery-enabled access categories. Which categories are delivery- and trigger-enabled are usually specified in the Association Request from the client—there, a bitmask specifies which categories are legacy and which are delivery and trigger enabled together—or in TSPEC messages, which we will come to later. Here’s a common example. The phone associates, and tells the access point that it wantsthevoicecategory(AC_VO)tobedelivery-andtrigger-enabled.Thatmeansthatthe other three categories work on the legacy scheme. If packets come in for those other categories while the client is asleep, the TIM bit on the beacon will be set and the client will use legacy power save mechanisms to get the frames. But when a voice packet is sent to the access point, the access point silently holds onto the packet. The only way the client can get the voice packet is to send a voice packet of its own. When it does, that causes the access point to respond with one or more voice packets in its buffer. Unlike with legacy power save, the client can ask for more than one packet at a time. Using the concept of a service period, which is set at Association time by the client and specifies the number of frames the client wants to get for every trigger (either two, four, six, or all), the access point will send out the correct number of frames. The last frame, whether because the buffer is empty or the service period has been exceeded, will have a special end of service period (EOSP) bit set in the QoS header. Once the client gets that frame, it can go back to sleep. As you can see, the legacy and WMM Power Save schemes operate simultaneously and independently. The only overlap is that the client goes into to power save mode for both schemes simultaneously. This means that devices that are actively using WMM Power Save should never use the PSNonPoll method during that time, because the client waking up from power save mode will cause the access point to send all frames, whether they are from the legacy or WMM Power Save access categories. The capability to support WMM Power Save should be considered nearly mandatory for most voice equipment. Some mobile devices use proprietary mechanisms that may or may not be supported by every access point, but the trend is towards using WMM Power Save. Of course, the problem with WMM Power Save is that it works well only for voice, but that is not a concern for us in this book. 214 Chapter 6 www.newnespress.com 6.1 Technologies that Address Voice Mobility with Wi-Fi The introduction of WMM into Wi-Fi allowed voice to now have a prioritized way of being carried over the air. But other basic elements of providing a toll-quality voice system needed to be put in place. Many of these newer techniques borrow from how things are done on the cellular networks, and work is only beginning now to try to standardize certain parts of them. How vendors—access point and phone—implement these features goes a long way towards determining how well the voice mobility network will work. 6.1.1 Admission Control: The Network Busy Tone Therstconceptthatisneededisprovidinga“networkbusy”tone.Everynetworking technology has its capacity limits, and given the discussion in Section 6.0.1.1, Wi-Fi can have some fairly severe ones. As the number of voice calls exceeds the network capacity, the air becomes crowded with aggressive, high-priority voice packets. This causes increased loss and can end up hurting the quality of every active call on the air in that region. The solution is to not let in the calls that cause the capacity to be exceeded. The goal is to provide the caller with that network busy signal. (If you have never heard a network busy tone before, on standard telephones, they sound like the usual caller busy tone, but they beep at a much faster rate.) When used for voice, admission control is often called Call Admission Control (CAC), pronounced“cack.”TherearetwomethodscurrentlyinuseforWi-Fitoprovidethis. 6.1.1.1 SIP-Based Admission Control The first method is to rely on the call setup signaling. Because the most common mechanism today is SIP, we can refer to this as SIP-based admission control. The idea is fairly simple. The access point, most likely in concert with a controller if the architecture in use has one, uses a firewall-based flow-detection system to observe the SIP messages as they are sent from the phones to the SIP servers and back. Specifically, when the call is initiated, either by the phone sending a SIP Invite, or receiving one from another party, the wireless network determines whether there is available capacity to take the call. If there is available capacity, then the wireless network lets the messages flow as usual, and the call is initiated. On the other hand, if the wireless network determines that there is no room for the call, it will intercept the SIP Invite messages, preventing them from reaching the other party, and interject its own message to the caller (as if from the called party, usually), with one of a few possible SIP busy statuses. The call never completes, and the caller will get some sort of failure message, or a busy tone. Voice Mobility over Wi-Fi 215 www.newnespress.com Other, more advanced behaviors are also possible, such as performing load balancing, once the network has determined that the call is not going to complete. The advantage of using SIP flow detection to do the admission control is that it does not require any added sophistication on the mobile devices than they would already have with SIP. Furthermore, by having that awareness from tracking the SIP state, the network can provide a list of both calls in progress and registered phones not yet in a call. The disadvantage is that this system will not work for SIP calls that are encrypted end-to-end, suchasbeingcarriedoveraVPNlink. 6.1.1.2 WMM Admission Control Building on even more of the specification in the 802.11e quality-of-service amendment is WMM Admission Control. This specification and interoperability program from the Wi-Fi Alliance,whichisrequiredtoachieveVoiceEnterprisecertication(seeSection 6.3), uses an explicit layer-2 reservation scheme. This scheme, in a similar vein as the lightly used RSVP protocol (RFC 2205), requires the mobile device to reach out and request resources explicitly from the access point, using a new protocol built on top of 802.11 management frames. This protocol is heavily dependant on the concept of a traffic specification (TSPEC). The TSPEC is created by the mobile phone, and specifies how much of the air resources either or both directions of the call (or whatever resource is being requested) will be taken. The access point processes the request as an admission controller (a function often placed literally on the controller, by coincidence), which is in charge of maintaining an account of which clients have requested what resources and whether they are available. The overall protocol is rather simple. The mobile device, usually when it determines that it has a call incoming our outgoing, will send an Add Traffic Stream (ADDTS) Request message (a special type of Action management frame) to the access point, containing the TSPEC that will be able to carry the phone call. The access point will decide whether it can carry that call, based on whatever scheme it uses (see following discussion), and send an ADDTS Response message stating whether the stream was admitted. WMM Admission Control can be set to mandatory or optional for each access category. For example, WMM Admission Control can be required for voice and video, but not for best effort and background data. What this would mean is that no client is allowed to transmit voice or video packets without first requesting and being granted admission for flows in those access categories, whereas all clients would be allowed to freely transmit best effort and background data as they see fit. Which access categories require admission control is signaled as a part of the WMM information element, which goes out in beacons and some other frames. 216 Chapter 6 www.newnespress.com For WMM Admission Control, it is worth looking at the details of the concepts. The main concept is one of a traffic stream itself, and how it is identified and recognized. Traffic streams are represented by Traffic Identifiers (TID), a number from 0–7 (the standard allows up to 15, but WMM limits this to only 7) that represents the stream. Each client gets its own set of eight TIDs to use. Each traffic stream, represented by its TID, maps onto real traffic by naming which of the eight priority values in WMM will belong to this traffic stream (see Table 6.1). Thus, if the phone intends to send and knows it is going to receive priority 7—recall that this is the highest of the two voice AC priorities—it can establish a traffic stream that maps priority 7 traffic to it, and get both sides of the call. In order for that to work, the client can specify whether the traffic stream is upstream-only, downstream-only, or bidirectional. It is possible for the client to request both an upstream-only and downstream-only stream mapping to the same priority (different TIDs, though!), if it knows that the airtime used by the downstream side is different than the upstream side—useful for video calls—or it may request both at once in one TID, with the same airtime usage. All of this freedom leads to some complexity, but thankfully there is a rule preventing there from being more than one downstream and one upstream flow (bidirectional counts as one of each) for each access category. Thus, the AC_VOvoiceaccesscategorywillonlyhaveoneadmittedbidirectionalphonecallinitat any given time.* The client requests the traffic stream using the TSPEC. Table 6.3 shows the contents of the TSPEC that is carried in an ADDTS message. There’s quite a lot of information in a TSPEC, so let’s break it down slowly, using the example of a 20 millisecond G.711 (nearly uncompressed) one-way traffic flow: • TheTS Info field (see Table 6.4) identifies the TID for the stream, the priority of the data frames that belong to this stream, what direction the stream is going in (00 = up, 01 = down, 10 = reserved, 11 = bidirectional), and whether the AC the stream belongs to is to be WMM Power Save delivery enabled (1) or not (0). The rest of the fields are not used in WMM Admission Control, and have specific values that will never change (Access Policy = 01, the rest are 0). • TheNomimal MSDU Size field mentions the expected packet size, with the highest- order bit set to signify that the packet size never changes. G.711 20ms packets are 160 * Of course, there had to be a catch. Some devices can carry two calls simultaneously, if they renegotiate their one admitted traffic stream to take the capacity of both. Because WMM Admission Control views flows as being only between clients and access points, the ultimate other endpoint of the call does not matter. However, this is not something you would expect to see in practice. Voice Mobility over Wi-Fi 217 www.newnespress.com bytes of audio, plus 12 bytes of RTP header, 8 bytes of UDP header, 20 bytes of IP header, and 8 bytes of SNAP header, creating a data payload (excluding WPA/WPA2 overhead) of 208 = 0xD0. Because the packet size for G.711 never changes, this field would be set to 0x80D0. • TheMaximum MSDU Size field specifies what the largest a data packet in the stream can get. For G.711, that’s the same as the nominal size. There is no special bit for fixed sizes, so the value is 208 = 0x00D0. This can also be left as 0, as it is an optional field. • TheInactivity Interval specifies how long the stream can be idle—no traffic matching it—in microseconds, before the access point can go ahead and delete the flow. 0 means not to delete the flow automatically, and that’s the common value. • TheMean Data Rate specifies, in bits per second, what the expected throughput is for the stream. For G.711, 208 bytes every 20 milliseconds results in a throughput of 83200 bits per second. Table 6.3: WMM admission control TSPEC TS Info Nominal MSDU Size Maximum MSDU Size Minimum Service Interval Maximum Service Interval Inactivity Interval Suspension Interval Service Start Time … 3 bytes 2 bytes 2 bytes 4 bytes 4 bytes 4 bytes 4 bytes 4 bytes … Minimum Data Rate Mean Data Rate Peak Data Rate Maximum Burst Size Delay Bound Minimum PHY Rate Surplus Bandwidth Allowance Medium Time 4 bytes 4 bytes 4 bytes 4 bytes 4 bytes 4 bytes 2 bytes 2 bytes Table 6.4: The TS info field Traffic Type TID Direction Access Policy Aggregation WMM Power Save Priority TSInfo Ack Policy Schedule Reserved Bit: 0 1–4 5–6 7–8 9 10 11–13 14–15 16 17–23 218 Chapter 6 www.newnespress.com • TheMinimum Data Rate and Peak Data Rate specify the minimum and maximum throughput the traffic stream can expect. These are optional and can be set to 0. For G.711, these will be the same 83,200 bits per second. • TheMinimum PHY Rate field specifies what the physical layer data rate assumptions are for the stream, in bits per second. If the client is assuming that the data rate could drop as low as 6Mbps for 802.11ag, then it would encode the field at 6Mbps = 6,000,000bps = 0x005B8D80. • TheSurplus Bandwidth Allowance is a fudge factor that the phone can request, to account for that packets might be retransmitted. It’s a multiplier, in units of 1/8192nds. A value of 1.5 times as an allowance would be encoded as 0x3000 = 001.1000000000000, in binary. • Theothereldsareunusedbytheclient,andcanbesetto0. In other words, the client simply requests the direction, priority, packet size, data rate, and surplus allowance. The access point gets this information, and churns it using whatever algorithms it wants— this is not specified by the standard, but we’ll look at what sorts of considerations tend to be used in Section 6.1.1.3. Normally, we’ll assume that the access point knows what percentage of airtime is available. The access point will then decide how much airtime the requested flow will take, as a percentage, and see whether it exceeds its maximum allowance (say, 100% of airtime used). If so, the flow is denied, and a failing ADDTS Response is sent. If not, the access point updates its measure of how much airtime is being used, and then allows the flow. The succeeding ADDTS Response has a TSPEC in it that is a mirror of the one the client requested, except that now the Medium Time field is filled in. This field specifies exactly how much airtime, in 32-microsecond units per second, the client can take for the flow. The definition of how much airtime a client uses is based on what packets are sent to it or that it sends as a part of a flow. Both traffic sent by the client to the access point and sent by the access point to the client are counted, as well as the times for any RTSs, CTSs, ACKs,andinterframespacingsthatarebetweenthoseframes.Anotherwayofthinking about it is that the time from the first bit of the first preamble to the last bit of the last frame of the TXOP counts, including gaps in between. In general, you will never need to try to count this. Just know that WMM Admission Control requires that the clients count their usage. If they exceed their usage in the access category they are using, they have to send all subsequent frames with a lower access category—and one that is not admission control enabled—or drop them. One advantage of WMM Admission Control is that it works for all traffic types, without requiring the network to have any smarts. Rather, the client is required to know everything Voice Mobility over Wi-Fi 219 www.newnespress.com about the flows it will both send and receive, and how much airtime those flows will take. The network just plays the role of arbiter, allowing some flows in and rejecting others. Thus, if the client is sufficiently smart, WMM Admission Control will work whether the protocol is SIP, H.323, some proprietary protocol, or even video or streaming data. The disadvantage of that, however, is that the client is required to be smart, and all of its pieces—from wireless to phone software—have to be well integrated. That pretty much eliminates most softphones, and brings the focus squarely on purpose-built phones. Furthermore, the client needs to know what type of traffic the party on the other side of the call will send to it. Some higher-level signaling protocols can convey this, such as with SDP within SIP, but doing so may be optional and may not always be followed. For a phone talking to a media gateway, for example, the phone needs to know exactly how the media gateway will send its traffic, including knowing the codec and packet rate and sizing, before it can request airtime. That can lead to situations in which the call needs to be initiated and agreed to by both parties before the network can be asked for permission to admit the flow, meaning that the call might have to be terminated by the network midway through ringing, if airtime is not available. Because WMM Admission Control is so new—by the time of publication, WMM Admission Control should be launching shortly and large amounts of devices may not yet be available—it remains to be seen how well all of the pieces will fit together. It is notoriously difficult for general-purpose devices to be built that run the gamut of technologies correctly, and so these new programs might be more useful for highly specific purpose-built phones. 6.1.1.3 How the Capacity Is Determined Through either admission control scheme, the network needs to keep track of how much capacity is available. From the previous discussions on the effects of RF variability and cellular overlap, you can appreciate that this is a difficult problem to completely solve. As devices get further away from the access points, data rates drop. Changing levels of interference, from within the network or without, can cause increasing retransmissions and easily overrun surplus bandwidth allowances. In the end, networks today adopt one of two stands, and may even show both to the user. The more complicated stand for the network—but simpler for the user—is for the network to automatically take the variability of RF into account, and to determine its own capacities. In systems that do this, there is no notion of a static maximum number of calls. Instead, the system accepts however many calls as it can handle. If conditions change, and fewer calls can be handled in the system, the network reserves the right to proactively end a client’s reservation, often in concert with load balancing. The other stand, simpler for the network but far more complicated for the user, is for the administrator to be required to enter the maximum number of calls per access point (or some other static metric). The idea here is that the administrator or installer is assumed to . retransmissions and easily overrun surplus bandwidth allowances. In the end, networks today adopt one of two stands, and may even show both to the user. The more complicated stand for the network—but. the WMM access categories and can apply to a subset of them. The two subsets are delivery-enabled access categories and trigger-enabled access categories. Voice Mobility over Wi-Fi 213 www.newnespress.com First,. are delivery- and trigger-enabled are usually specified in the Association Request from the client—there, a bitmask specifies which categories are legacy and which are delivery and trigger enabled