Feature Descriptions; Sip Proxy Redundancy - Linksys 900 Series Administrator's Manual

Feature Descriptions

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Feature Descriptions
SPA900 Series IP phones are full featured, fully programmable IP phones that can be custom
provisioned within a wide range of configuration parameters. This chapter contains a
high-level overview of features to provide a basic understanding of the feature breadth and
capabilities of SPA900 Series IP phones.
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SIP Proxy Redundancy

In typical commercial IP Telephony deployments, all calls are established through a SIP proxy
server. An average SIP proxy server may handle tens of thousands of subscribers. It is
important that a backup server be available so that an active server can be temporarily switched
out for maintenance. SPA900 Series IP phones support the use of backup SIP proxy servers so
that service disruption should be nearly eliminated.
A simple way to support proxy redundancy is to configure a static list of SIP proxy servers in
the SPA900 Series IP phone configuration profile, where the list is arranged in order of priority.
The SPA900 Series IP phone attempts to contact the highest priority proxy server whenever
possible.
The dynamic nature of SIP message routing makes the use of a static list of proxy servers
inadequate in some scenarios. In deployments where user agents are served by different
domains, for instance, it would not be feasible to configure one static list of proxy servers per
covered domain into every SPA900 Series IP phone. One solution to this situation is through
the use of DNS SRV records. SPA900 Series IP phones can be instructed to contact a SIP proxy
server in a domain named in SIP messages. The SPA900 Series IP phone consults the DNS
server to get a list of hosts in the given domain that provides SIP services. If an entry exists,
the DNS server returns an SRV record that contains a list of SIP proxy servers for the domain,
with their host names, priority, listening ports, and so on. The SPA900 Series IP phone tries to
contact the list of hosts in the order of their stated priority.
If the SPA900 Series IP phone is currently using a lower priority proxy server, it periodically
probes the higher priority proxy to see whether it is back on line, and attempts to switch back
to the higher priority proxy whenever possible.
Linksys 900 Series IP Phone Administrator Guide
1-6
End-to-end delay—End-to-end delay does not affect voice quality directly but is an important factor
in determining whether subscribers can interact normally in a conversation taking place over an IP
network. A reasonable delay figure should be about 50–100 ms. End-to-end delay larger than
300 ms is unacceptable to most callers. SPA900 Series IP phones support end-to-end delays well
within acceptable thresholds.
SIP Proxy Redundancy, page 1-6
Supported Codecs, page 1-7
Other Features, page 1-8
Chapter 1 Introducing Linksys 900 Series IP Phones
Document Version 3.1