Nortel Networks Recording Equipment 1 User Manual
Voice over Wireless LAN Solution Guide
v1.0
December 2005
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Page 18
MLT to the L2 switch. In this configuration, all APs in the branch must be DAPs powered by the
L2 switch or a separate PoE injector.
L2 switch or a separate PoE injector.
In most cases, you should disable spanning tree on the switch port to which a DAP is connected.
If spanning tree is not disabled, there is a possibility that the AP will never connect to a WSS
2300 because of timing differences between spanning tree and the AP switch detection timers. In
the case of the ERS 8300, spanning tree does not put the port in forwarding mode before the AP
reboots to reinitiate a new connection attempt. The failed connection-reboot cycle repeats
indefinitely until spanning tree is turned off.
If spanning tree is not disabled, there is a possibility that the AP will never connect to a WSS
2300 because of timing differences between spanning tree and the AP switch detection timers. In
the case of the ERS 8300, spanning tree does not put the port in forwarding mode before the AP
reboots to reinitiate a new connection attempt. The failed connection-reboot cycle repeats
indefinitely until spanning tree is turned off.
2.2.2.3
RF design constraints
The WLAN Handset 2210/ 11/12 and most PDAs that support the MVC 2050 are currently
802.11b-only devices. Most APs shipping today are 802.11g capable, and the IP Softphone 2050
and MCS Client run on PCs that have 802.11b, 802.11g, and 802.11a interfaces readily available.
This creates some interesting dynamics and interesting choices for network deployments. The
following points lay the groundwork for a discussion of these choices.
802.11b-only devices. Most APs shipping today are 802.11g capable, and the IP Softphone 2050
and MCS Client run on PCs that have 802.11b, 802.11g, and 802.11a interfaces readily available.
This creates some interesting dynamics and interesting choices for network deployments. The
following points lay the groundwork for a discussion of these choices.
1. Separation of devices by multiple Service Set Identifiers (SSID) on the same radio does
not create multiple shared mediums—the devices still transmit and receive using
common radio resources on a common channel.
common radio resources on a common channel.
2. Current QoS mechanisms in the industry are most effective at protecting and
prioritizing traffic on the downstream, that is, from AP to Mobile Unit (MU). Wi-Fi
Multimedia (WMM) improves upstream prioritization by giving a statistical edge to
different classes of devices so they are more likely to transmit ahead of lower class
devices. Still other devices may cheat on the contention window to gain a statistical
advantage, though there are drawbacks to this method. The bottom line is that there is
no real arbitration or coordination between multiple devices that need to transmit
packets upstream.
Multimedia (WMM) improves upstream prioritization by giving a statistical edge to
different classes of devices so they are more likely to transmit ahead of lower class
devices. Still other devices may cheat on the contention window to gain a statistical
advantage, though there are drawbacks to this method. The bottom line is that there is
no real arbitration or coordination between multiple devices that need to transmit
packets upstream.
3. The 802.11g devices in a mixed 802.11b/g network are statistically favored by a 2:1
ratio over 802.11b devices. This means that if there is one 802.11g device and one
802.11b device and both are trying to saturate the medium with a data transfer, for
example, the 802.11g device will transmit, on average, two frames for every one frame
from the 802.11b device. If there are two 802.11g devices for every one 802.11b
device, then on average four 802.11g transmissions will occur before one 802.11b
transmission.
802.11b device and both are trying to saturate the medium with a data transfer, for
example, the 802.11g device will transmit, on average, two frames for every one frame
from the 802.11b device. If there are two 802.11g devices for every one 802.11b
device, then on average four 802.11g transmissions will occur before one 802.11b
transmission.
4. The 802.11g transmissions take less time than 802.11b transmissions due to the higher
data rates. So even though 802.11g devices transmit more often, they spend less time
transmitting packets, which mitigates the effect of 802.11g devices being favored
statistically from the perspective of the 802.11b clients. Having too many 802.11g
devices relative to 802.11b devices upsets this balance though, so beware.
transmitting packets, which mitigates the effect of 802.11g devices being favored
statistically from the perspective of the 802.11b clients. Having too many 802.11g
devices relative to 802.11b devices upsets this balance though, so beware.
Should you enable 802.11g or maintain an 802.11b-only network? There isn’t an easy answer. If
there is a significant amount of upstream traffic from data devices, then the question becomes
unimportant. The best course of action in that case is to keep data devices off the 802.11b/g
network entirely. Large numbers of 802.11g devices can also cause problems with 802.11b
handsets on the medium. However, if instead you make those 802.11g devices actually use
802.11b for communication, the situation will likely become worse. Disabling 802.11g support and
maintaining a dual mode 802.11a/b network can make 802.11a more attractive for dual mode
data clients and reduce the amount of data devices using the 2.4 GHz spectrum. Enabling
802.11g support may increase the number of data devices sharing the 2.4 GHz channels, which
is detrimental to voice devices. As a general policy, when you have large amounts of data, use
802.11a for data and 802.11b for voice, but leave 802.11g disabled.
there is a significant amount of upstream traffic from data devices, then the question becomes
unimportant. The best course of action in that case is to keep data devices off the 802.11b/g
network entirely. Large numbers of 802.11g devices can also cause problems with 802.11b
handsets on the medium. However, if instead you make those 802.11g devices actually use
802.11b for communication, the situation will likely become worse. Disabling 802.11g support and
maintaining a dual mode 802.11a/b network can make 802.11a more attractive for dual mode
data clients and reduce the amount of data devices using the 2.4 GHz spectrum. Enabling
802.11g support may increase the number of data devices sharing the 2.4 GHz channels, which
is detrimental to voice devices. As a general policy, when you have large amounts of data, use
802.11a for data and 802.11b for voice, but leave 802.11g disabled.
On the other hand, if you only have a handful of 802.11b/g capable (non-802.11a capable) data
devices and the WLAN is to be used primarily for voice, then enabling 802.11g support is
devices and the WLAN is to be used primarily for voice, then enabling 802.11g support is