This article is part four in a five-part series from contributor Michael Finneran. Read the first three:
Step 1: Planning for capacity, not just coverage
Step 2: Moving to 802.11a
Step 3: Assessing security enhancements.
Critical Step 4: Incorporating quality of service
Most organizations are looking toward carrying voice on their WLAN at some point, so one of the critical elements to include in the planning is quality of service (QoS) support to insure that voice packets are given higher priority access to the channel. The important development in this area is the emerging 802.11e MAC protocol. The 802.11e standard will include two operating modes, either of which can be used to improve service for voice:
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- Wi-Fi Multimedia Extensions (WME)/Enhanced Digital Control Access (EDCA) (mandatory)
- Wi-Fi Scheduled Multimedia/Polled Access (optional)
The WME/EDCA option is an enhanced version of the Distributed Control Function (DCF) defined in the original 802.11 MAC. The "enhanced" part is that EDCA will define eight levels of access priority to the shared wireless channel. Like the original DCF, the EDCA access is a contention-based protocol that employs a set of waiting intervals and back-off timers designed to avoid collisions. However, with DCF, all stations use the same values and hence have the same priority for transmitting on the channel. With EDCA, each of the different access priorities is assigned a different range of waiting intervals and back-off counters. Transmissions with higher access priority are assigned shorter intervals. The standard also includes a packet-bursting mode that allows an access point or a mobile station to reserve the channel and send three to five packets in sequence.
While EDCA does not include a mechanism to deliver true consistent delay, it can insure that voice transmissions wait less than data transmissions. True consistent delay services can be provided with the optional Polled Access. Polled Access operates like the little used Point Control Function (PCF) defined with the original 802.11 MAC. In Polled Access, the access point periodically broadcasts a control message that forces all stations to treat the channel as busy and not attempt to transmit. During that period, the access point polls each station that is defined for time sensitive service.
To use the Polled Access function devices must first send a traffic profile describing bandwidth, latency, and jitter requirements. If the access point does not have sufficient resources to meet the traffic profile, it will return a "busy signal." The reason Polled Access is being included as an optional feature is that all access points must be able to return a "service not available" response to stations' profile requests. The 802.11e specification is going through its final review cycles and should be ratified by mid-2004.
If voice is in your WLAN planning horizon, it is absolutely essential that you confirm the vendor's plans regarding 802.11e support. There are pre-standard protocol enhancements that have been developed by VoWLAN vendors, however you would be better served with a standards-based solution.
To read part 5, click here.
About the author:
Michael Finneran is an independent telecommunications consultant specializing in wireless networks and technologies. Besides his research and consulting activities, he writes a regular column called "Network Intelligence" for Business Communications Review and teaches their seminars on wireless technologies and wireless LANs. He can be reached at email@example.com.
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