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Checklist for Planning a Voice Over Wi-Fi Network: Quality of Service (Part 3 of 4)

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Back to the future with this Airheads Online article from 2007.


Quality of Service

QoS is an end-to-end requirement, and each segment in the voice call must be properly analysed.  The LAN requirement was identified above:  this section refers to the Wi-Fi link.

      • Identify the path to infrastructure and clients supporting the WMM standard.  The new QoS certification from the Wi-Fi Alliance is known as WMM (Wireless MultiMedia);  it is based on 802.11e from the IEEE.  Few handsets support WMM today, but the next generation certainly will and any new WLAN infrastructure should be WMM-capable.  WMM allows clients in the Wi-Fi cell to gain preferential access for high-priority traffic (e.g. voice) while lower-priority traffic may be delayed.  It is particularly useful in the uplink direction:  for downlink traffic, Aruba supports multiple queues in the AP to ensure priority.
      • Notwithstanding the above, many successful voice over Wi-Fi networks do not use WMM.  They either rely on proprietary QoS mechanisms, or police the downlink in the Wi-Fi cell (see above).  This has proven adequate in Enterprise applications, as control of downlink traffic regulates overall bandwidth usage. However, WMM is the correct, standard approach to over-the-air QoS, and WMM support should be the goal.
      • It is  helpful to have a mechanism to monitor the usage of QoS priorities.  This is because WMM allows badly-behaved clients to claim improperly high priority at-will.  The best WLANs can monitor and police the use of high-priority in WMM.
      • QoS mapping for voice and data streams should be automatic.  Many WLAN implementations use a special voice SSID which maps to a voice WLAN.  This was an adequate solution when handsets generated only data or only voice traffic, but as converged voice-and-data clients proliferate, it is not possible to separate traffic types by SSID and VLAN.  The WLAN must deal with voice AND data streams from the same device, which requires a more sophisticated architecture.
      • As noted above, the LAN and all other segments in the voice call should be QoS-compliant, with 802.1p or DSCP.• RF scanning inhibit is a useful feature for improved QoS. 
      • Most WLAN APs provide service on one RF channel while time-slicing on other channels to perform RF monitoring, intrusion detection and location functions.  This scanning should be inhibited when there is an active call on the AP, or dropped frames and voice quality problems will result.

Call Admissions Control
Since a Wi-Fi cell is usually the bandwidth bottleneck for voice calls, it is important to limit the number of active calls on any single AP.  This can be achieved by a CAC algorithm.

    • Lab tests show limits in the order of 12-15 calls for 802.11b (some WLANs can go as high as 22 calls before voice traffic exceeds available bandwidth and quality suffers).  However, lab tests exist in a parallel, ideal universe, and in practical networks a figure of 10 calls per AP is a reasonable maximum for 802.11b.  This leaves some bandwidth ‘reserved’ for data traffic and allows for handsets connecting at lower data rates than 11 Mbps.
    • If 802.11g or 802.11a handsets are used, the situation is much easier:  more than 30 simultaneous calls on an AP are possible with these handsets.  Lab tests show up to 76 calls when all clients connect at 54 Mbps.
    • A quick capacity check will show whether these figures are reasonable for the expected client density.  Take a Wi-Fi cell on the floorplan, estimate the peak number of voice clients (e.g. for a cell covering 12 offices/cubicles, perhaps 24 voice clients peak), and then take an estimate of the peak active calls (e.g. if 33% of 24 voice clients are on-call = 8 calls peak).  Be sure this does not greatly exceed the capacity of the cell (e.g. 10 calls for 802.11b).  Remember that the CAC will limit calls and maintain quality if thresholds are exceeded, and that load-balancing to adjacent cells will usually accommodate excess offered traffic.
    • TSpec-ready infrastructure will allow accurate, standards-compliant CAC in the future.  Another part of the 802.11e standard, TSpec signalling provides RSVP-like bandwidth reservation, and will be the new standard for CAC.  It will soon be adopted by the Wi-Fi Alliance as the WMM-AC certification, and any Wi-Fi infrastructure installed today must be TSpec-capable, or have an upgrade path to WMM-AC.
    • Graceful call rejection at the CAC limit is an important feature.  When the CAC limit is reached, there are different ways to deny excess calls.  Good WLAN implementations either load-balance idle voice clients, or send signalling messages (using SIP) to indicate to the client that the call cannot be completed.
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