[OpenSIPS-Users] OpenSip SIP, SIP-I e SIP-T

[Alex Balashov]

Adrian Georgescu wrote:

> Why should SIP-T still exist? Is it cheaper than having a gateway? What 
> is the practical use case for investing in such technology?
> 
> I am eager to learn

We've used it extensively in work with CLECs that operate TDM switches 
such as the Metaswitch, Lucent LCS/Telica, etc.

When a carrier operates more than one switch, SS7 interconnection 
between them is generally required so, for the same basic reasons an 
internal iBGP mesh or partial mesh (confederation) between two border 
routers is required for IP.   One switch must be aware of numbers routed 
or ported into the other switch, and so on.

The reason for its existence is that if both network elements support 
SIP-T, it allows you to replace an SS7 IMT (inter-machine trunk) with an 
IP-based mechanism for this interconnection.  This allows you to move 
the traffic over a data network and get all the benefits that this 
brings;  economies of scale through decreased facilities, 
oversubscription, etc.  The main benefit is the elimination of TDM trunk 
exhaust;  SS7 IMTs are physically bundles (trunk groups/TCICs) of DS0s, 
usually consisting of one or more T1s, and sometimes DS3s or more.  That 
means that when a large volume of calls is running between the two 
switches, you could burn up all your SS7 trunks.  Running the calls as 
SIP-T allows you to use something like a gigabit network core to make 
that problem go away somewhat -- a key benefit of VoIP in most other 
scenarios with which you are familiar with.

At the same time, the switches still need ISUP attributes carried in SS7 
IAMs and ACMs for billing, because that's just the information they 
operate on internally.  SIP-T provides an IP-based way to encapsulate 
that information.

SIGTRAN (essentially, SS7-over-IP) is another way to do this.  However, 
SIP-T is lightweight and easier to deploy.  It also allows you to use 
existing SIP network elements (proxies, session border controllers, 
etc.) to route and manage the traffic.   For example, if you were using 
OpenSIPS + ACC + FreeRADIUS as a CDR catcher, you could run the "SS7" 
calls between two switches and log the appropriate information as custom 
attributes.  There are no good open-source implementations for SIGTRAN - 
nothing as turn-key as Kamailio or OpenSIPS.  SIP is high-level and much 
easier to deal with and manipulate using a far wider range of tools.

SIP-T is also becoming an attractive external interconnect option.

-- 
Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
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