Home > Lync EV, Uncategorized > To E1 or to T1? – that might be the question

To E1 or to T1? – that might be the question

I was recently working on a Lync engagement that involved deploying Lync Enterprise Voice to connect to a legacy PBX. In order to route calls to and from the legacy PBX, we needed to configured a media gateway and a Q.SIG trunk. Q.SIG is a protocol that runs on a T1 PRI. It was originally invented as an industry standard to allow PBX’s from different vendors to route calls to each other. Prior to Q.SIG, if you have a Nortel PBX, (“switch”), and an Avaya PBX, and you wanted to route calls, there was no way to do this because each legacy PBX vendor used their own proprietary protocols to route and communicate with their own PBXs.

Q.SIG was developed to allow different vendor PBXs to route and communicate calls. Q.SIG runs on a standard T1 PRI. The Q.SIG protocol contains various pieces of information such as the Called Number, (who you are trying to dial), and the Caller ID, (who is calling). In order to connect two PBXs with a Q.SIG trunk, each PBX is installed and configured with a Q.SIG “card” so that the T1 PRI cable, (i.e. what we call a standard RJ45 Ethernet cable), can connect the two. The connection can also be a leased line T1 but for simplicity I am using the cabling example.

It turns out that media gateways from the major vendors also support Q.SIG, so that if you want to integrate a legacy PBX supporting Q.SIG with Lync, then you can use a media gateway to do the protocol conversion by connecting the Q.SIG port on a media gateway to the legacy PBX, and then configure the Ethernet “side” of the media gateway to communicate with Lync.

Q.SIG running over a T1 PRI only supports 23 concurrent calls occurring between the media gateway and the legacy PBX. The reason that it is only 23 calls, is that a T1 is a 1.5 MB “pipe”. Since each call is dedicated 64 kb/s of bandwidth you then divide 1.5 mb/s by 64 kb/s and you get the number 23. You might notice that the “maths” does not quite come out as “23” but there is another channel called the “D” channel that provides the signaling, (i.e. start the call, stop the call, etc.). The “D” channel is the telephony version of SIP, whereas the voice channels are the media streams like RTP in the Lync “world”.

But what if you wanted to get a few more calls from your Q.SIG connection?

Turns out that the T1 standard is used in the US, and that the E1 standard is used outside the US. The E1 standard has 30 channels instead of 23, (still 64 kb/s) and total bandwidth of 1.5 MB/s. When Q.SIG was developed, the designers accounted for both the T1 and E1 standards. This means that when you set up the Q.SIG card on the PBX, you can have the PBX administrator set it to “E1” instead of “T1”, and on the media gateway set the gateway to use Q.SIG with “E1” instead of “T1”, and you will then get up to 30 concurrent calls between the PBX and the gateway & Lync.

Categories: Lync EV, Uncategorized
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