White Paper
Page 18
6.1 Subnets
and
NetLink
Telephony
Gateway
Interfaces
NetLink Wireless Telephones, NetLink Telephony Gateways, NetLink
SVP Server(s), and the wireless APs must reside on the same subnet.
One reason for this requirement is that the NetLink Wireless Telephones
use IP multicast messages to initialize the Wireless Telephone
registration on the NetLink Telephony Gateways. Most routers deployed
in multi-subnet Ethernet environments are configured to filter out
multicast and broadcast messages. If a NetLink Wireless Telephone is
powered up on a different subnet than the NetLink Telephony Gateway
to which it is registered, the multicast message will never reach the
NetLink Telephony Gateway.
6.2
Subnets and IP
Telephony
Server
Interfaces
Although not recommended, NetLink Wireless Telephones can be
deployed across multiple subnets when used with an IP telephony
server interface. This can help facilitate subnet roaming when the
subnets are geographically separated by defined boundaries.
Each subnet must have its own NetLink SVP Server. This is necessary
because Ethernet packets containing voice as their payload have short
interesting lifetimes, making the timely delivery of voice packets
essential. Routers can introduce latency and delay between the NetLink
SVP Server and the APs, which manifests as poor voice quality.
Ethernet connectivity between the NetLink SVP Server and the IP
telephony server should never exceed 100 ms of network delay and 10
ms of network jitter regardless of the physical properties of the link. The
ability to cross a subnet boundary exists in this scenario, but the NetLink
handsets will need to be power-cycled to obtain new IP address within
the new subnet. In addition, other configuration considerations need to
be addressed. Because users will not want to re-administer the
Wireless Telephones to get them to work on another subnet, the
ESSIDs should be broadcast using the “Learn Always” mode, the WEP
key should be the same or turned off, and DHCP should be used.
7.0 Conclusion
Voice telephony over a wireless LAN represents the convergence of
voice and data technology in the wireless environment. There are some
specific network design criteria that must be applied to effectively
implement a wireless telephony solution that is suitable for the
demanding requirements of both voice users and data network
administrators. With a little background study, both network and
telephony professionals will be able to easily and confidently design and
deploy a SpectraLink Wi-Fi telephony solution.
