[asterisk-users] Question about SS7

[Alex Balashov]

mark morreny wrote:
> Hi,
>  
> I have read about SS7 recently and learnt that it is a signalling 
> protocol used in PSTN for call management, setup, etc.  The thing that I 
> don't understand is how SS7 plays a role in VOIP.  When I make calls 
> between landline and Asterisk via PSTN, I don't need to do anything with 
> SS7.  Is it because the SS7 signalling is already done by Asterisk 
> already?  From the prespective of implementing Asterisk, what kind of 
> SS7 support is needed?  Is SS7 something needs to be concerned about 
> when using Asterisk with T1/E1?

That depends entirely on the application of Asterisk.

In general, as others have commented, SS7 does not play a role in VoIP 
and does not concern you as an Asterisk user.

SS7 is an out-of-band signaling system is used inside telcos for 
trunking between network elements -- mainly softswitches or traditional 
big-iron TDM switches (such as the Lucent 5E, Nortel DMS, etc.) -- and 
to provide certain database access features used by that type of 
equipment and service provider, such as LNP (Local Number Portability 
information), CNAM (directory listings), certain billing processes, etc.
SS7 operates on a carrier level and at global dimensions.

In other words, the phone company's switches talk to each other -- and 
to other phone companies' switches -- using SS7.  That is why it is 
irrelevant to your PSTN experience as an end-user.  You never see any SS7.

SS7 is _not_ used at the network edge, and is _not_ commonly used in 
handoff to users, even ones who order very large amounts of access 
circuits from a telco.  These are typically done as PRI and ISDN PRI 
signaling is used to control them;  that is a completely different 
animal than SS7 signaling.  You can still aggregate PRIs into trunk 
groups, couple T1s full of B channels into one logical signaling group 
controlled by a single D channel using NFAS, etc.  There is no need to 
do SS7 outside of an inter-carrier environment.  So, while I don't know 
exactly what sort of service you are envisioning when you say "with 
T1/E1," if it's available to you, it probably does not use SS7.

Where SS7 can become somewhat relevant is if you are a CLEC trying to 
get away with using Asterisk in some switching capacity, and are wanting 
to interconnect with a carrier in the full sense of the term (as in, 
actual interconnection, not access circuits).  That's typically done 
over SS7.  But Asterisk is completely the wrong tool for that job anyway.

In other words, for all practical intents and purposes, you don't need 
SS7 and do not need to know about it.

-- 
Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
Tel    : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
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