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Run VoIP through your wireless LAN (cont)

Posted: 08 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:wi-fi? design basics? phone system? 802.11b? phy?


The HCCA approach can be characterized as an N-body synchronization scheme, whereby the AP sets up a polling schedule for N stations that attempt to stay synchronized with the AP in spite of schedule perturbations. It's fair to characterize this as an N-body problem because timing anomalies with any station on the polling for either uplink or downlink traffic effect the timing of the other N-1 participants. The timing interdependence of polling must be compared to the timing independence power-save methods applied to EDCA, which is the other QoS extension to Wi-Fi defined within WMM.

EDCA scenario

The EDCA access method provides for prioritized channel access. Each station will select from four sets of priority-controlling parameters for best-effort packets (normal), background packets (very low priority), video traffic, and voice traffic (highest priority). The AP has the same set of priority-controlling parameters, but may also transmit one time period earlier than any station. This is the secret behind HCCA polling (the AP can always transmit before any station) as well as one of the important principles in EDCA (the AP always wins contention for the channel).

Unlike HCCA, where the station must slave itself to the AP's polling schedule, the EDCA station may operate in a specialized power-save mode called unscheduled APSD, or UPSD. In this mode, the station sleeps until it has a voip frame ready to transmit (Fig. 2). The AP is expecting this behavior because of a prior signaling handshake conducted between the station and AP.

2. Pictured is the EDCA station's specialized power-save mode, called Unscheduled APSD, or UPSD.

The power-up procedure at the station can occur with perfect timing, i.e., there's no schedule right-shift, or waiting for the poll, or timing effects caused by other stations or conflicting schedules. The station comes to full power and transmits the VoIP frame using the highest priority parameters available to it. It's reasonable to expect the uplink frame to launch with less than 2ms of power-consuming delay. It's assumed that the AP is configured to avoid long bursts or other behavior that would increase this delay. This assumption must be taken for both HCCA and EDCA. Without it, either scheme will experience a schedule right-shift or a delay in the UPSD frame exchange.

The uplink frame is ACKed by the AP. The station can retransmit if necessary, and will stay awake until the AP sends down a VoIP frame or a null indication (meaning that there are no VoIP packets to send). Implementations on conventional AP hardware show that the turnaround time at the AP can be bounded to values less than 100ms, and improvements to this value should be expected. The lesson is that the effort to add this functionality to an off-the-shelf AP is minimal, especially when compared to the many complexities of maintaining a CBR polling schedule.

As a practical matter, the UPSD scheme will have about the same 75 percent power-save efficiency for a 20ms CBR scenario as the HCCA scheme. With a 30ms CBR interval, the efficiency should improve to 83 percent. The principal difference is that the N-body synchronization problem is traded for a one-body exercise.

Complexity arguments favor the EDCA scheme, especially because the power-save efficiency of the two approaches are similar. Two other points of comparison between HCCA and EDCA should be mentioned although they're only indirectly related to power-save procedures. These are the hidden node problem and AP interference.

Hidden nodes are stations that can receive frames from an AP, but may not be able to receive frames from all other stations associated with the AP. Without any mitigating procedures, hidden nodes violate the basic premise of CSMA. That is, the stations should sense the medium before transmitting. If sensing is imperfect because they're hidden, they'll create interference when transmitting.

Fortunately, EDCA is robust in the presence of interference and collisions from all sources including hidden nodes. Also, hidden nodes are rarely a factor in small or enterprise environments where there's good AP coverage. In outdoor environments, directional antennas can eliminate most of the hidden nodes.

It's often claimed that HCCA is immune to hidden node effects because stations transmit only when polled, thus avoiding collisions. This is true. But the polled stations must also be EDCA stations to communicate with the AP for non-CBR traffic. Thus, a hidden node remains hidden to some extent with HCCA. It's also true that HCCA APs and a CBR polling discipline are particularly vulnerable to interference from nearby APs and other stations. Any appreciable interference rate will right-shift the schedule whereas EDCA will adapt without effort. The bottom line is that the one-body solution with EDCA/UPSD is preferred to an N-Body problem with HCCA.

About the author

Dr. Greg Chesson is the director of protocol engineering at Atheros Communications. Chesson earned degrees in computer science at the University of Illinois.

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