Having just written a long (ish) post on a forum, I thought I would post it here for future reference.
These are high level methods of connecting OCS 2007 R2 or Lync 2010 to the PSTN.
There are three options;
In terms of capability of connection to the PSTN (Telephone network) functionally nothing much has changed in Lync 2010 compared to OCS 2007 R2 other than the need for SRTP support in gateways.
The lack of change is not a bad thing, as you really already have plenty of options!!
Traditionally people have ISDN connections to the telephone network which in the USA are called T1 lines which give you a potential 24 channels (calls). Lync 2010 Server has no way of connecting to these ISDN lines without some form of interface. This could be putting an ISDN interface card into your server, but in general is by using a gateway device from the likes of Audiocodes or NET Quintum.
The gateway terminates the ISDN lines and then translates the audio into RTP streams (SRTP for Lync) and SIP for signalling (i.e. setting up who is calling who).
Generally this is the most used option as by terminating the ISDN lines on the gateway it is then possible to route calls either to Lync or to an existing PBX system
The next most common option is to use an existing PBX to terminate the ISDN lines and have it talk to Lync, either through a gateway (kind of a reverse of the order above) or, the PBX might be able to talk SIP, and use TCP to talk over Ethernet to Lync.
Instead of dealing with traditional ISDN lines the Lync server will connect over IP (TCP or UDP) Port 5060, to an ITSP (Internet Telephony Service Provider). For example people like Verizon, BT, Global Crossing etc.
This allows OCS to route and receive calls directly to the PSTN without the need for any legacy telephony equipment.
Sometimes, even with the above solution, a gateway can be useful, which can be used more like a session border controller, giving options to manipulate the traffic as it passes from Lync to the ITSP.
Hope that clears things up.
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