ISDN PRI Technologies

In this post, we will understand the nuances between the following concepts related to ISDN PRI:

  • TDM links and interfaces
  • T1, T1 PRI,
  • E1, E1 PRI,


E1 and T1 are two TDM technologies; they leverage Time Division  Multiplexing to transmit and receive information. E1 TDM link provides a total 2.048 Mbps bandwidth. T1 TDM link provides a total 1.544 Mbps bandwidth. Both the E1 TDM and the T1 TDM links are segmented into channels, or timeslots. Each timeslot allows a bandwidth of 64 kbps.

E1 TDM and T1 TDM have been used in the past to carry digitized voice. But since carrying voice and data is the same thing -from a transmission perspective- then T1 TDM and E1 TDM have been leveraged to carry data, voice and even video.

E1 TDM, T1 TDM and WAN Protocols

E1 TDM and T1 TDM can be used to carry PPP traffic.

Also, E1 TDM and T1 TDM are used as a Link Layer transport protocol for ISDN PRI technology. That’s why we find in literature E1 PRI and T1 PRI. And since ISDN technology makes ISDN calls to transmit information, then we can say that on each timeslot, an ISDN call is made, and that each ISDN call has 64 kbps of bandwidth.

So when we have ISDN PRI over T1 or E1 link, we are faced with the concept of timeslots or channels. It’s not important here to distinguish what type of information is carried at each timeslot. What we need to know is that an ISDN call is dialed at each timeslot.

How Many Channels Are In An ISDN PRI Connection?

There is a difference in terms of number of channels between running ISDN PRI over E1 TDM and running it over T1 TDM. When ISDN PRI runs over a E1 TDM link, we have 30 channels dedicated for ISDN calls and 1 channel for ISDN PRI signaling. And when it runs over a T1 TDM link, we have 23 channels for ISDN calls and the 24th channel is for ISDN signaling.


There is a third TDM link: ISDN BRI. This TDM link has two time slots for ISDN calls and one time slot for ISDN signaling. So at each time slot an ISDN call is dialed.

So next time you read about TDM links in the Voice world, you know that we are talking about E1, T1 and ISDN BRI.

Configuring ISDN PRI E1 controller

Configuring Framing Type

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Configuring Linecode

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Configuring Clock Source

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Configuring time slots

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This is a H.323 gateway. If it were a MGCP gateway, we would have added the “service mgcp”:

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ISDN E1 and T1 IOS Show Commands

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This command shows nothing because the router has not T1 cards.e1-t1-2016-05-25 17_48_50

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The E1 controller is down. Notice the “Transmitter is sending remote alarm” message:

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Troubleshooting an ISDN E1 PRI Trunk

In this scenario, the E1 PRI controller is down.

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Let’s diagnose it with “loopback diag” controller command. Make sure the terminal console logging is on:

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Let’s check the state of the E1 PRI controller:

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Let’s stop the diagnostic with “no loopback diag”

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There is no way to clear counters on the controller. Even a reset of the controller does not:

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So there is clearly an issue with the Telco provider.

Popular Cisco ISDN PRI Commands

Here are a couple of popular ISDN PRI commands you can invoke on you Cisco router connected to a PRI circuit.

Show ISDN History

Show isdn history is the third command of the show ISDN trilogy. It displays a list of past calls made on the PRI interface.

If you are familiar with the output of the show isdn active command, then let me tell you that show isdn history has a similar format, but it is longer in number of lines.

Show Call Active Voice Brief

I called into a corporate national phone number 731114124 from my mobile phone. Here is the output of the show call active voice brief:

Output of the show call active voice brief command, part 1
Output of the show call active voice brief command, part 2

Show ISDN Active

The show isdn active command allows to see the active calls on a PRI interface. In the same time, you can determine how many channels are used on the PRI, because each call occupies a channel.

You can use the show isdn active command on any gateway that is connected to a PRI trunk interface.

Note that Call Type refers to the direction of the call: inbound (In) or outbound (Out), from the perspective of the PRI interface.

Show Voice Busyout

Finding a Good ISDN PRI Simulator

For my CCNP BSCI studies, I needed to have some hands on ISDN technology in order to remember the IOS commands. All simulation software that I worked with (Boson Netsim, Packet Tracer, RouterSim Network Visualizer) were great for CCNA studies. However, they do not include any support for ISDN. It was clear that I needed to find another simulator.

While searching forums, I found about the Adtran Atlas 550. It’s a ISDN simulator that’s used among CCIE candidates and Telecom engineers. However, it costs around 640$. I did not have the means to buy one of those. Besides, there were strict import regulations that allow only business owners to receive equipment from abroad. As a regular Cisco certification student, this was not possible. I thought of borrowing some equipement from my company, but I found out that we did not have Cisco routers with BRI interfaces (although we had Nortel ARN and ASN!).

One day I sat down and opened Notepad. I wrote down all ISDN-related IOS commands. Then, I repeated the cycle until I was recalling them fast. It was a small discovery: how did I not think of this earlier? Notepad is my friend! This is so easy to do. But, this method still does not enable you to learn all the ISDN scenarii.


The ISDN PRI technology was a popular way to benefit from a high bandwidth connection for companies using voice while keeping the bill controlled. Nowadays companies are migrating more and more to fiber optic transport links and benefitting from more attractive prices.

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