Understanding ISDN

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.

OK, so lets get a bit more acquainted with ISDN.

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.

Generally we see 2 types of ISDN, Primary Rate and Basic Rate.
Depending on who you talk to you will find a large range of terms used when speaking of ISDN Primary and Basic rates.
Basic Rate is also referred to as BRI (Basic Rate Interface), BRA (Basic Rate Access), S0 Bus, ISDN 2 and 2B + D.

Primary Rate (E1) is often refereed to as PRI (Primary Rate Interface), PRA (Primary Rate Access), S2 Bus, ISDN 30 and 30 B + D. 

Primary Rate comes in two sizes, E1 and T1. E1 is a 30 channel circuit where as T1 is a 23 channel circuit. 

ISDN speeds are set at transmission speeds up to 64 Kbps per channel. Primary Rate E1 is often referred to a a 2 Mb circuit. This is because 32 Channels at 64 Kbps is 2,048 Kbps (or 2 Mbps). Hang-on, didn’t we just say 30 channels for E1? Correct, however there is also the signalling channel (usually channel no. 16) as well as a “channel” that control the synchronization. Thus 30 B (bearer) channels represent the capacity and the D (Data) represents the signalling and sync info for the circuit.

Basic rate on the other hand has just 2 B channels at 64 Kbps and a single D channel for signalling and synchronization at 16 Kbps.

In an enterprise voice environment each B channel can carry one voice conversation.

ISDN runs at 8000 Hz, this means that every second the channels make 8000 round trips between Telco and ISDN terminating equipment. Because of this it is very important that a clocking system is in place to provide a reference point to make sure that the channels remain in phase.

Telcos are generally the best sources of accurate clocks and will be much more accurate than any hardware that generates its own clock speed on site. 


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

Basic Rate Pin outs

Primary rate use Pins 1 & 2 for RX and Pin 4 &5 for TX

Primary Rate Pin outs

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


About Paul B

My name is Paul Bloem and I am employed at Lexel Systems in New Zealand as a Principal Consultant for Unified Communications. I have been working on enterprise voice solutions for over 20 years. My first 10 years were spent working for a Telco in South Africa (Telcom SA). This is where all the groundwork happened as I was exposed to just about every aspect of telecommunication you could imagine. I develop an interest in PBX technologies and eventually became the go-to guy. Next, I had a 10 year run at Siemens South Africa, most of my time there was as a Technical Trainer. During this time VoIP hit the world stage, I had the privilege of introducing VoIP both as H.323 and later SIP across the Siemens HiPath 4000 solution stack. In 2008 I immigrated to New Zealand with my newly attained MCSE, I was ready to go where no PBX Techie had gone before. I was employed to explore OCS 2007 and that was pretty much the beginning of the end for me. I have been working on OCS and Lync ever since. My current role focuses exclusively on Lync and associated technologies.. That includes pre-sales, consulting, architecture and design, training and support. I even get to play in the development space from time to time - focus on play ;-) I was nominated as a Microsoft VTSP for Lync early in 2013 and also awarded Microsoft's MVP award for Lync in 2014.
This entry was posted in ISDN Cable, ISDN PIN outs, Understanding ISDN. Bookmark the permalink.

2 Responses to Understanding ISDN

  1. Suresh R says:

    Very useful article. Thank you for posting this.


  2. Paul Bloem says:

    Thank you Suresh, you are welcome.


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