Nortel Networks 1 ユーザーズマニュアル
Voice over Wireless LAN Solution Guide
v1.0
December 2005
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Page 37
and SVP in combination on the WLAN 2300 series. The WLAN Handsets 2210/11/12 also
support WMM, but note that as of today, the officially supported VoWLAN solution between
handset and WLAN 2300 series requires that you not enable the WMM features. That is, you
must turn off WMM on the WLAN 2300 series when using WLAN 2210/11/12 handsets on the
network.
support WMM, but note that as of today, the officially supported VoWLAN solution between
handset and WLAN 2300 series requires that you not enable the WMM features. That is, you
must turn off WMM on the WLAN 2300 series when using WLAN 2210/11/12 handsets on the
network.
The following sections describe SVP and WMM and discuss the operational differences on the
WLAN 2300 series when WMM is enabled or disabled.
WLAN 2300 series when WMM is enabled or disabled.
2.5.1.1 SVP
description
SVP functions can be broken down into two categories: those functions requested or required by
an access point, and those that are implemented on the handsets and WTM 2245 that go beyond
the capabilities of an access point. The SVP specification requests a number of behaviors from
any access point, mostly related to priority processing of voice and SVP packets. An access point
must service SVP packets before all other packets, while taking care not to reorder high priority
packets to or from the same device. Access points should also use a zero backoff contention
window size for transmitting these high priority packets. This ensures that the AP can transmit
before any other device that is using the medium. Lastly, retransmission of a corrupted packet
should not delay other voice frames that are queued for other voice devices. Normally when an
error occurs in transmission, a longer backoff period is used, followed by the retry. Any frames in
the queue normally must wait for this transmission to be completed or fail after the maximum
number of retries. Because this creates a situation in which one problematic high priority device
can impact all other high priority devices, SVP also requests that the AP move on to service other
high priority devices before returning to retransmit the corrupted frame. The WLAN 2300 series
implements all elements required by SVP, and is certified through lab testing as an SVP
compatible product.
an access point, and those that are implemented on the handsets and WTM 2245 that go beyond
the capabilities of an access point. The SVP specification requests a number of behaviors from
any access point, mostly related to priority processing of voice and SVP packets. An access point
must service SVP packets before all other packets, while taking care not to reorder high priority
packets to or from the same device. Access points should also use a zero backoff contention
window size for transmitting these high priority packets. This ensures that the AP can transmit
before any other device that is using the medium. Lastly, retransmission of a corrupted packet
should not delay other voice frames that are queued for other voice devices. Normally when an
error occurs in transmission, a longer backoff period is used, followed by the retry. Any frames in
the queue normally must wait for this transmission to be completed or fail after the maximum
number of retries. Because this creates a situation in which one problematic high priority device
can impact all other high priority devices, SVP also requests that the AP move on to service other
high priority devices before returning to retransmit the corrupted frame. The WLAN 2300 series
implements all elements required by SVP, and is certified through lab testing as an SVP
compatible product.
The remaining SVP capabilities are implementations of the WTM 2245 and WLAN Handsets
2210/11/12. The WTM 2245 has an admission control feature that enables it to keep track of the
AP to which WLAN Handsets 2210/11/12 are associated. The WTM 2245 therefore knows how
many active calls are on each AP, and takes action. After the maximum number of per-AP calls is
determined by the engineering/design process, that number is programmed into the WTM 2245.
When an AP reaches that number of calls, the WTM 2245 prevents additional calls from using
that AP. An idle device that is associated to the “full” AP and setting up a new call will be
instructed to roam before making the call. A device with an established call that is attempting to
roam to the “full” AP will be instructed not to roam to that AP.
2210/11/12. The WTM 2245 has an admission control feature that enables it to keep track of the
AP to which WLAN Handsets 2210/11/12 are associated. The WTM 2245 therefore knows how
many active calls are on each AP, and takes action. After the maximum number of per-AP calls is
determined by the engineering/design process, that number is programmed into the WTM 2245.
When an AP reaches that number of calls, the WTM 2245 prevents additional calls from using
that AP. An idle device that is associated to the “full” AP and setting up a new call will be
instructed to roam before making the call. A device with an established call that is attempting to
roam to the “full” AP will be instructed not to roam to that AP.
Furthermore, the WLAN Handsets 2210/11/12 work together with the WTM 2245 to synchronize
communications in order to avoid collisions. The normal 802.11 contention avoidance mechanism
uses a statistical method to keep transmissions from colliding. Even the Request-to-Send/Clear-
to-Send (RTS/CTS) function can collide with another transmission because the RTS frame relies
on the same statistical collision avoidance mechanism. There will always be some percentage of
collisions when you rely on the standardized 802.11 collision avoidance techniques. The greater
the number of devices using the medium, the greater is the probability of collisions. Beyond a
certain threshold of collisions, in terms of percentage, call quality begins to degrade. Hence, SVP
provides an additional mechanism that helps to prevent collisions more effectively than the
normal 802.11 mechanisms. First, handset transmissions are synchronized with the received
voice stream, so that packets are sent at an offset from the received voice packets. This ensures
that upstream and downstream voice packets will not collide with each other within the same
voice call over the half-duplex medium. Downstream (from AP to handset) packets to multiple
devices are naturally collision free because transmission from an AP is serialized. Upstream
voice packets from multiple handsets are also synchronized by SVP in such a way that handset
transmissions are very unlikely to take place at the same time. Because of this, all voice packets
to and from handsets and to and from APs experience fewer collisions than would occur if they all
simply relied on normal 802.11 collision avoidance mechanisms.
communications in order to avoid collisions. The normal 802.11 contention avoidance mechanism
uses a statistical method to keep transmissions from colliding. Even the Request-to-Send/Clear-
to-Send (RTS/CTS) function can collide with another transmission because the RTS frame relies
on the same statistical collision avoidance mechanism. There will always be some percentage of
collisions when you rely on the standardized 802.11 collision avoidance techniques. The greater
the number of devices using the medium, the greater is the probability of collisions. Beyond a
certain threshold of collisions, in terms of percentage, call quality begins to degrade. Hence, SVP
provides an additional mechanism that helps to prevent collisions more effectively than the
normal 802.11 mechanisms. First, handset transmissions are synchronized with the received
voice stream, so that packets are sent at an offset from the received voice packets. This ensures
that upstream and downstream voice packets will not collide with each other within the same
voice call over the half-duplex medium. Downstream (from AP to handset) packets to multiple
devices are naturally collision free because transmission from an AP is serialized. Upstream
voice packets from multiple handsets are also synchronized by SVP in such a way that handset
transmissions are very unlikely to take place at the same time. Because of this, all voice packets
to and from handsets and to and from APs experience fewer collisions than would occur if they all
simply relied on normal 802.11 collision avoidance mechanisms.