Polycom H340 - Manual (Page 26)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
6 Subnets, Network Performance and DHCP
Subnets are used to create a boundary between network segments. Although these boundaries are logical, they
become like a physical boundary for mobile network devices moving throughout the enterprise. When a device with
an established IP data stream (such as with an active phone call) attempts to roam across a subnet boundary, it
must obtain a valid IP address within the new subnet. During this process, the data stream cannot be re-established
automatically and the connection (voice call) is dropped. In the case of SpectraLink Wireless Telephones, the
handsets should be power cycled to obtain a new DHCP IP address. The handsets can automatically recover in the
new subnet from a lost network connection with the original subnet, but the 40-second failure and recovery time
generally warrants cycling the power. Please note that in order for the phone to continue functioning in the new
subnet the DHCP scope must contain the appropriate DHCP options to allow the phone to regain connectivity with
the voice infrastructure.
Some APs, Ethernet switches and third-party devices have implemented methods to facilitate subnet roaming. While
these methods are transparent to the client device and are fundamentally a good approach to accommodating
multiple subnets, they often cause enough delay and jitter to manifest poor voice quality and the tradeoffs might
make such solutions unattractive for voice applications.
Since the push-to-talk feature of the SpectraLink i640 Wireless Telephones use multicast IP packets, a PTT call will
generally be isolated to a single subnet. With the deployment of IP multicast routing it is possible for the multicast
traffic that is normally pruned at the network boundary to be passed into one or more other subnets. Please review
your network manufacturer’s documentation for information on how to properly configure multicast routing.
There are additional subnet requirements for Wireless Telephones based on the infrastructure components that are
used, as described in the following sections.
6.1 Subnets and Telephony Gateway Interfaces
SpectraLink Wireless Telephones, Telephony Gateways, SVP Server(s) and the APs generally must reside on the
same subnet. This is required because SpectraLink handsets use IP multicast messages to initialize the handset
registration on the Telephony Gateways. In addition, The Telephony Gateways and SVP Server(s) use multicast to
discover each other and stay synchronized. Most routers deployed in multi-subnet Ethernet environments are
configured to filter out multicast and broadcast messages. Unless a router is configured for multicast routing, if a
handset is powered up on a different subnet than the Telephony Gateway to which it is registered, the multicast
message will not reach the Telephony Gateway.
6.2 Subnets and IP Telephony Server Interfaces
With an IP telephony interface, the SVP Server can be placed on a separate subnet from either the APs or call
server. The handsets will find the SVP Server and call server on another subnet through the default gateway option
statically configured in the handset or via DHCP option 3 when using a DHCP server for IP addressing.
SpectraLink Wireless Telephones can be deployed across multiple subnets when used with an IP telephony server if
the performance requirements outlined below are met. One of two deployment scenarios described in this section
can be used, depending on needs and infrastructure capabilities. Keep in mind that the handsets will never actively
roam across a subnet boundary without power-cycling the handsets unless a VIEW Certified layer-3 roaming
infrastructure is used in accordance with the VIEW deployment guidelines.
In one deployment scenario for accommodating multiple subnets, each subnet is treated independently with respect
to the SVP Servers and wireless network, but each subnet can still provide service to a single IP telephony server.
One or more SVP Server(s) can be deployed on each subnet just as with a single subnet system, including
identifying the registration SVP via DHCP option 151 or static configuration. In the second scenario, a single SVP
Server (or set of SVP Servers with one registration SVP) is deployed, generally on the same subnet as the IP
telephony server. The single (Registration) SVP Server is identified to all phones via DHCP option 151 or static