Even though we would like to believe that SIP Trunking is becoming more prevalent, the current status (at least in New Zealand) is that many are still using ISDN to deliver PSTN connectivity. This means that when deploying a Lync Enterprise Voice Solution we will need to intercept the ISDN with a gateway\SBC (Session Border Controller). The SBC will then bridge the ISDN to a SIP Trunk configured to connect to Lync Mediation Server.
What is ISDN?
ISDN (Integrated Services Digital Network) is a system of digital phone connections that has been designed for sending voice, video, and data simultaneously over digital or ordinary phone lines.
ISDN PIN outs
Both ISDN Primary Rate and Basic rate are 2 copper pair connections (4 physical wires).
One pair is for Transmit (TX) and the other for receive (RX).
Its BEST to ensure that the actual wires used for transmit form a pair, don’t split them as this can cause hard to diagnose issues. For Example Blue & Blue\White would make a suitable TX pair, while Orange and Orange\White makes a good RX pair.
Also, each pair has a + (called “tip”) and a – (called “ring”). So the Rx pair has a RX+ and a RX- wire as does the TX pair. The pairs are not polarity sensitive.
Looking at a RJ45 socket you will notice that Basic Rate and Primary Rate use different pin outs.
Basic Rate use Pins 3 & 6 for TX and Pins 4 & 5 for RX
Primary rate use Pins 1 & 2 for RX and Pin 4 &5 for TX
Straight or Cross over?
The endpoint side of the ISDN circuit (Usually a PABX) is also referred to as the Terminal Endpoint (TE). The network side of the circuit (Usually the Telco) is also called Network Terminator (NT).
When connecting a NT to a TE the cable needs to be cross over. This has nothing to do with ethernet cross over..
Cross over simply means that the TX pair on the NT side is wired to the RX pair on the TE side and visa versa.
Intercepting ISDN PRI to a PABX with a SBC will need a cross over cable from the SBC to the PABX.
The image depicts cross over, a standard ether net cable may be used as a straight cable.
Basic Rate is always wired as cross over regardless of the terminating equipment
Although the images above indicate the wiring of all the pins, only the TX and RX pins are used.
For a deeper dive see this tutorial