Avaya Communication Server 1000 Installation And Commissioning Manual page 64

Planning
network deployments. The following list describes some of the points to consider when
determining network deployment:
• Separation of devices by multiple SSIDs on the same radio does not create multiple
shared mediums—the devices still transmit and receive using common radio resources
on a common channel.
• 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
sometimes cheat on the contention window to gain a statistical advantage, though there
are drawbacks to this method. There is no real arbitration or coordination between multiple
devices that need to transmit packets upstream.
• The 802.11g devices in a mixed 802.11b/g network are statistically favored by a 2:1 ratio
over 802.11b devices. For example, 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, the
802.11g device transmits, 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, on average, four
802.11g transmissions occur before one 802.11b transmission occurs.
• Although 802.11g devices transmit more often, because of higher data rates, they spend
less time transmitting packets. This means that 802.11g devices are not necessarily
favored in the network. Having too many 802.11g devices relative to 802.11b devices
upsets this balance.
There is no easy way to determine whether to maintain an 802.11g-only network or an 802.11b-
only network. If there is a significant amount of upstream traffic from data devices, the best
course of action 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
you force the 802.11g devices to use 802.11b for communication, the situation can 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 can increase the number of data devices sharing
the 2.4 GHz channels, which is detrimental to voice devices. As a general policy, for large
amounts of data, use 802.11a for data and 802.11b for voice, but leave 802.11g disabled.
Alternately, if there are few 802.11b/g data devices and the WLAN is to be used primarily for
voice, consider enabling 802.11g support. The goal is to carefully control the number of data
devices that share radio resources with voice devices.
For example, if a large number of laptops exist in a campus and if 802.11g mode is enabled,
it is probable that a large proportion of those laptops use 802.11g (2.4 GHz) for connectivity,
which makes it much more difficult to provide good quality voice for handsets. If 802.11g is
disabled, it is probable that a large proportion of those laptops use 802.11a (5 GHz) because
it offers much higher throughput compared with 802.11b, and voice quality benefits.
64 Avaya WLAN IP Telephony Installation and Commissioning November 2010