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Tue Sep 5 14:32:44 MST 2006


First we had analog lines.  Then ATT started grouping 24 analog lines to
form a T1.  Inband signalling was used in each channel.  Time studies
indicate that these channels can be more effectively used if the signalling
(ringing, busy, etc) is removed out of each channel and put into a common
data channel, or D Channel as they call it in ISDN parlance.  So for a North
American PRI line, a T1 with 24 channels is sectioned off into a 1 D
channel, which is used for signalling, and 23 B channels (bearer channels)
which are used strictly for voice traffic.  So ISDN/PRI lines use meta
signalling to control the voice channels.

SS7 is strictly a signalling and control protocol.  It carries no voice
traffic, but controls how voice is routed between locations.  For instance,
some one picks up the telephone and dials a number across the country.  The
local telco switch signals the telco switch at the other end to ring the
destination phone.  This signalling is handled by SS7.  If the destination
party picks up, it is at that moment when the the source telco and the
destination telco open up a voice circuit and connect the two parties.  If
the destination party does not pick up, and the source party hangs up, no
voice channels have been opened up, and the telco enjoys a cost saving by
not having to dedicate a resource to the conversation, such as like back in
the good old days of RBS (Robbed Bit Signalling) T1's.

SS7 also handles the CLASS series of value-add signalling services.  SS7 is
therefore useful for handling the signalling on large channel volumes
(loosing a B channel to a D channel in a PRI for every 24 channels is
expensive overhead), and is good for geographically distributed dialling

So, to wrap up, SS7 is a meta meta signalling protocol.  It controls the
signals going down a PRI which signals the PBX on what it needs to do with
the call.

If any others on this list can contribute their thoughts and experiences, it
would be greatly appreciated.

Ray Burkholder

> -----Original Message-----
> From: asterisk-users-admin at
> [mailto:asterisk-users-admin at] On Behalf Of Brad Waite
> Sent: October 27, 2003 15:22
> To: asterisk-users at
> Subject: Re: [Asterisk-Users] SS7 signaling/Softswitch
> CW_ASN - Gus wrote:
> > Anyway, in certanly implemetations you don't need CCS7 to=20
> connect to CO. You
> > always can connect with PRI... same speed and same=20
> functionalities to user
> > side. In fact, CCS7 is the support for ISDN-PRI avanced=20
> features. If you
> > could connect with Lucent 5ESS you can have a PRI treated=20
> as route...
> Gus,
> I'm not following you here when you say, " can have a=20
> PRI treated as=20
> route..."  Can you clarify?
> I'm trying to determine what AIN features may be available on=20
> a PRI D channel.=20
> I know the D channel is a near extension to SS7, but I don't=20
> know what subset of=20
>   queries/commands are available between the two.
> Brad Waite
> W Cubed
> _______________________________________________
> Asterisk-Users mailing list
> Asterisk-Users at
> --=20
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