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Voice and Signaling Features
SIPv2 –
Session Initiation Protocol Version 2 (RFC 3261-3265)
SIP Proxy Redundancy
– Static or Dynamic via DNS SRV
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 subscribers. It is important that a backup server
is available so that an active server can be temporarily switched out for maintenance. The SPA supports
the use of backup SIP proxy servers so that service disruption should be next to non-existent.
SIP Proxy Dynamic Redundancy:
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 an SPA. One
solution to this situation is through the use DNS SRV records. The SPA can be instructed to contact a SIP
proxy server in a domain named in SIP messages. The SPA shall consult the DNS server to get a list of
hosts in the given domain that provides SIP services. If an entry exists, the DNS server will return a SRV
record which contains a list of SIP proxy servers for the domain, with their host names, priority, listening
ports, etc. The SPA shall try to contact the list of hosts in the order of their stated priority.
Re-registration with Primary SIP Proxy Server
If the SPA is currently using a lower priority proxy server, it should periodically probe the higher priority
proxy to see if it is back on line and attempt to switch back to the higher priority proxy whenever possible.
It is very important that switching proxy server should not affect calls that are already in progress.
Codec Name Assignment
Negotiation of the optimal voice codec is sometimes dependent on the SPA device’s ability to “match” a
codec name with the far-end device/gateway codec name. The SPA allows the network administrator to
individually name the various codecs that are supported such that the correct codec successfully
negotiates with the far end the equipment.
Voice Algorithms:
The SPA-841 supports the following voice algorithms.
G.711 (A-law and mµ-law)
This very low complexity codec supports uncompressed 64 kbps digitized voice transmission at one
through ten 5 ms voice frames per packet. This codec provides the highest voice quality and uses the
most bandwidth of any of the available codecs.
G.726
This low complexity codec supports compressed 16, 24, 32 and 40 kbps digitized voice transmission at
one through ten 10 ms voice frames per packet. This codec provides the high voice quality.
