Published by Keyboard Banger on

In this post, we will understand the nuances between the following concepts:

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

E1 and T1

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 litterature 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.

There is a difference in terms of number of channels between running ISDN PRI over E1 TDM and running ISDN PRI 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 ISDN PRI 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 timeslots for ISDN calls and one timeslot for ISDN signaling. So at each timeslot 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 E1 controller

Configuring framing type

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Configuring linecode

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Configuring clock source

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Configuring timeslots

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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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Sample E1 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:

e1-t1-2016-05-25 17_51_00

Troubleshooting a E1 PRI trunk

In this scenario, the E1 PRI controller is down.

e1-pri-t1-2016-05-27 20_51_09

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


e1-pri-t1-2016-05-27 20_55_23

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

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

e1-pri-t1-2016-05-27 20_56_38

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

e1-pri-t1-2016-05-27 21_00_14

e1-pri-t1-2016-05-27 21_01_09

So there is clearly an issue with the Telco provider.

Categories: Networking Basics

Keyboard Banger

Keyboard Banger is a network engineer from Africa. He has been working in network support and administration since 2008. He started writing study notes about certification exams and technology topics a couple of years ago. When he's not writing articles, he can be found wandering on technical forums.


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