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Will 802.11e provide good QoS for voice?

Oct 06, 20033 mins
Network Security

Q: Will 802.11e provide adequate quality of service (QoS) for voice in a dense enterprise environment? How many quality voice calls can an access point handle? – James, Chicago.

A: 802.11e provides two different means of supporting QoS. First there is the extension of the widely deployed distributed coordination function (DCF) that makes use of carrier sense multiple access with collision avoidance (CSMA/CA) as the access mechanism. The enhanced DCF (EDCF) mechanism will add four levels of statistical access priority, enabling the separation of frames into different priority levels. EDCF provides statistical priority only. It does not guarantee that low priority frames will always wait until all higher priority frames are transmitted.

The second 802.11e QoS mechanism is an extension of the point coordination function (PCF) option of the original 802.11 standard. This mechanism uses negotiated connections between an access point and mobile device, along with specifically assigned transmit times for every frame under the control of the access point. This mechanism is the hybrid coordination function (HCF). Queue priority is also specified for a minimum of four priorities. HCF will provide priority guarantees, always transmitting higher priority frames before lower priority frames.

Through these QoS methods, 802.11e will definitely provide sufficient support for Voice over wireless LAN (VoWLAN) in dense user environments.

However, you may not need to wait for the 802.11e standard to become official if you are looking for QoS today. Several WLAN systems exist that can handle priority queuing and connection negotiation, as well as offer a migration path to 802.11e.

How many quality voice calls an access point can handle depends on a few factors. First, what physical layer is being used? The 54M bit/sec. of 802.11a or 802.11g will support several more simultaneous calls as 802.11b (which only has up to 11M bit/sec). While most WLAN handsets are built with 802.11b today, there are handsets that will soon arrive that incorporate 802.11g and 802.11a. Second, consider the actual data rate used by each individual handset when it communicates with the access point. Even though 802.11a and 802.11g may support data rates up to 54M bit/sec, lower data rates are typically achieved as the handset moves farther away from the access point. Even 802.11b has this characteristic.

The more densely you deploy your access points, the higher the data rate your handsets will achieve. For 802.11b, you can expect to support up to 15 simultaneous voice calls if all the handsets are sending at 11M bit/sec, However, if your handsets are sending at only 1M bit/sec, you can expect to support only four or five simultaneous calls. If you notice that this doesn’t result in a constant ratio, you’d be right. The reason for this is that there is a lot of constant overhead involved with every 802.11 frame, regardless of the data rate.