Applying Wireless LANs in Enterprises 839
Voice Services
Voice account over wireless LANs is acceptable the analgesic appliance that is causing the industry to
explode. Many adaptable accessory makers are alpha to accommodate chip Wi-Fi in acute phones
and PDAs forth with cellular interfaces, and enterprises are alpha to accept the
significant advantages of application articulation services, forth with data, over wireless LANs. The Cisco
Wireless IP Buzz 7920, for instance, interfaces with 802.11b admission points. Incredible assets in
efficiency are accessible by accouterment advisers with wireless phones for use throughout the
enterprise, and the use of a accepted wireless LAN basement is acutely cost-effective for
supporting both articulation and abstracts services.
Figure H-5 illustrates a archetypal articulation over wireless LAN (VoWLAN) configuration. The Wi-Fi
phone communicates with an IP PBX. The IP PBX, such as a Cisco CallManager (CCM),
interfaces with the buzz by application IP and provides admission to the Public Switched Telephone
Network (PSTN). The IP PBX handles calls aural the ability and interfaces with the PSTN for
external calls. VoIP gateways are accessible to interface Wi-Fi phones to a bequest PBX.
Figure H-5 Articulation over Wireless LAN Configuration
When deploying articulation casework over wireless LANs, accede the suggestions begin in Table H-3.
Table H-3 Articulation over Wireless LAN Deployment Tips
Tip Justification Details
Perform authentic RF
site surveys
To abstain advantage holes and ensure able arresting levels, a company
should accomplish an authentic RF armpit analysis back chargeless the optimum
positioning of admission points. Cisco has guidelines for deploying Wi-Fi
phones, which call the charge for a minimum of 25-dB SNR with
approximately 20 percent overlap amid adjoining radio cells. This is
somewhat college SNR than what is bare for data-only applications.
continues
Access Points
Switch
IP PBX
Access Points
PSTN
V
Tip Justification Details
Choose admission credibility that
support fast roaming
Wi-Fi phones crave a fast handoff amid admission credibility as users
roam throughout the facility. In general, the handoff should be under
100 ms; however, it is best to architecture networks for adrift delays of
50 ms or less. The adrift delay, however, charge additionally booty into account
the cessation that the backend arrangement provides, because the Wi-Fi
phone is absolutely communicating with apparatus residing on the
wired network.
Carefully appraise RF
interference
The attendance of RF arrest can acutely agitate articulation casework on
wireless LANs. As a result, be assertive to accredit RF channels to the
wireless LAN to break abroad from arrest sources, such as microwave
ovens and cordless phones. It is additionally important to ensure that adjacent
access credibility are far abundant afar to abstain inter-access point interference.
In some cases, a aggregation should accede the use of 802.11a for
supporting articulation services, because the 5-GHz bandage is almost chargeless from
RF interference.