CME - Configuring ISDN PRI

This section will run through the process of configuring CME to use an ISDN PRI connection over a T1 line (E1 is very similar).

Typical single channel T1 module.

T1 48 channel Network Module with PVDMs installed.

Packet Voice and Data Modules come in two versions PVDM and PVDM2.

In order to connect a T1 line to a CME router, you require a T1 module. Fortunately, they are very affordable and readily available on ebay. On my routers, the first T1 port is located in chassis 0, module 2 and port 0 or 0/2/0. Your configuration may be different.

On some Cisco voice platforms, one or more Packet Voice and Data Modules (PVDMs) are needed to support the voice interface cards. If no PVDMs are present, the voice ports may be missing in the active configuration. The PVDMs hold digital signal processors (DSP farms) that make the VICs fully functional, and are installed on the motherboard of the Cisco router series or on the network modules. The PVDMs are rated by the number of DSP channels that they have: 8, 16, 32, 48 and 64.

To check if you have a PVDM module, issue the "show diag" command. On 2811 routers, there are two PVDM status LEDs on the front lower right corner that indicate the presence (lit) or absence (dim) of PVDM modules.

The number of channels on a PVDM card will determine the number of channels that you can use on your T1 card. For example if you only have a PVDM-16 card, then even though the T1 line has 24 channels, you would only be able to use 16. If you have FXO and FXS modules then each port on these cards will be using up a channel also and reducing the number of PVDM channels availabler for your T1 card.

The Physical Connection

You will be connecting to a Telco service provider who will provide the Network Termination (NT) connection. Your T1 line will act as the Customer Premise Equipment (CPE). These terms are used to indicate who provides the master clock. The NT provides the master synchronization clock for the connection and it is always the telco. The configuration shown here may not match what your Telco expects. The number one problem with T1/ISDN PRI lines is matching your configuration with what the Telco expects. Contact the Telco for their expected parameters.

There are many different connectors used for T1 lines and the latest is the 8P8C connector which is the same connector used by Ethernet. It is often called an RJ45 connector but that is actually the wiring standard for Ethernet using an 8P8C connector. For a T1 line, the wiring standard is RJ48 using an 8P8C connector.

8P8C Modular Connector.

RJ48 Wiring Standard - T1 lines uses Pins 1/2 and 4/5

There are 4 steps in configuring a T1 ISDN PRI rate interface:

  1. Configuring the Router's T1 Channel
  2. Configure the T1 for ISDN PRI
  3. Configure the digital voice module
  4. Configure a dial-plan to use the ISDN PRI line

1. Configuring the Router's T1 Channel

The first step is to clear the router's counters so that we can see any new errors later. Then connect to your Telco's provided T1 connection.

Router# clear counters

Next, configure the WIC T1 module to act as a Customer Premise Equipment (CPE) and receive clock from the Network Termination (NT)

Router(config)# network-clock-participate wic 2

Configure the ISDN PRI global parameters:

isdn switch-type primary-ni
isdn voice-call-failure 0

Configure the T1 digital line settings using North American standards:

controller T1 0/2/0
description T1 line configuration 
clock source line 		
framing esf
linecode b8zs
cablelength short 133
pri-group timeslots 1-12,24

network-clock-select 1 t1 0/2/0


Framing esf: T1 Extended SuperFrame
Linecode: b8zs - bipolar with eight-zero substitution replaces each string of 8
consecutive zeros with a special pattern to ensure synchronization
Cablelength: Sets the line level depending on the length of the cable
Pri-group: Channels 1-23 are data B channels , 24 is the control channel D.
We are only using channels 1-12 for data which is the capacity of my router's DSP/PVDM

Network-clock-select sets the T1 line as the main clock source for the router

Basic layer 1 test

You should have a green light on the T1 port. Verify the operation of the T1 digital line by using the "show controllers T1" command – there should be no errors and the T1 should be the priority clock (1).

show controllers T1
show network-clocks

2. Configure the T1 for ISDN PRI

Configure ISDN PRI to use the T1 digital line:

interface Serial0/2/0:23
description ISDN PRI to FX-T1-Server
no ip address
no ip redirects
encapsulation hdlc
isdn switch-type primary-ni
isdn incoming-voice modem
isdn supp-service name calling
no cdp enable


  • At the Serial0/2/0 interface the T1 channel numbers start at 0 and go to channel 23. Cisco has channel 23 assigned as the D channel (control). Only channel 23 needs to be configured as it controls the data channels 0-22.
  • Notice the conflict where the T1 configuration starts at channel 1 and goes to 24.>
  • This T1 digital line will be used for digitized voice so an IP address is not required.
  • The data link layer encapsulation is HDLC - N.A. standard
  • The switch-type is National - N.A. standard
  • The incoming-voice channel data is voice (speech) and not a modem or fax
  • "Name calling" sends out call display information
  • Sending Complete tells the switch that all the digits and information necessary for the call are contained in this Setup message.

Test your layer 2 configuration by issuing the:

show isdn status 

Look for Layer 1 as ACTIVE and Layer 2 as MULTIPLE_FRAME_ESTABLISHED to indicate that the signaling channel functions correctly and exchanges Layer 2 frames with the Telco switch

OSI Layers

ISDN PRI resides in both the Network (Q.931) and Data Link layer (Q.921) of the OSI model. T1 resides in the Physical layer.

Test Q.921

This step is not necessary but is provided to aid in troubleshooting. Here's how to debug ISDN with good communications and a test with bad communications:

debug isdn q921

*Apr 12 04:14:43.967: ISDN Se0/2/0:23: RX <- SABMEp c/r=1 sapi=0 tei=0

*Apr 12 04:14:43.971: ISDN Se0/2/0:23: TX -> UAf c/r=1 sapi=0 tei=0

Unplug the T1 line then connect it. If Layer 2 is stable, the router and switch must begin to synchronize with each other. The Set Asynchronous Balanced Mode Extended (SABME) message appears on the display. This message indicates that Layer 2 tries to initialize with the other side. Either side can send the message and try to initialize with the other side. If the router receives the SABME message, it must send back an Unnumbered Acknowledge frame (UAf). The router then changes the Layer 2 status to MULTIPLE_FRAME_ESTABLISHED.

If the switch receives and recognizes the UAf, both devices synchronize, and periodic keepalives are exchanged between the router and the ISDN switch. These messages are in the form of Receiver Ready (RRf and RRp). These keepalives are seen ten seconds apart, and ensure that both sides are able to communicate with each other. For example:

*Apr 12 05:19:56.183: ISDN Se0/2/0:23: RX <- RRp sapi=0 tei=0 nr=18

*Apr 12 05:19:56.183: ISDN Se0/2/0:23: TX -> RRf sapi=0 tei=0 nr=18

Disconnect the T1 cable at the network termination. Wait for the T1 port to show the alarm LED.

T1 Loopback Test

Use a T1 loopback plug (Tx connected Rx) and connect it to the loose end of the T1 cable. If it is a cable with an RJ48 wired end, then make a T1 loopback using a female connector. If it is wired into a patch panel, then make a male loopback. And if it is wired into a punch block, cross pair 1 (pin 4/5) with pair 2 (1/2).

T1 loopback crosses Pins 1/2 and 4/5

The debug isdn q921 output must indicate that the SABME was sent, and this message was received:

	RX <- BAD FRAME(0x00017F)Line may be looped!

When you are finished turn off debugging:

no debug isdn q921 

3. Configure the digital voice module

Enable the Digital Signaling Processing services for the Wic T1 card:

voice-card 0
dsp services dspfarm

Enable the voice-port on the Wic T1 card (only have to configure the D channel):

voice-port 0/2/0:23

Examine the voice-port status:

Show voice port summary		
Show voice port 0/2/0:23

4. Configure a dial-plan to use the ISDN PRI line

Create an outbound dial-plan to use the ISDN PRI interface:

dial-peer voice 6 pots
description T1 PSTN
numbering-type unknown
destination-pattern 780.......
port 0/2/0:23
prefix 780

Note: In this example, the destination-pattern "780......." matches any 10 digit number beginning with 780, but it strips off the 780 and then sends out the remaining 7 digits (represented by the 7 periods). The "prefix 780" command adds the 780 back onto the beginning of the 7 digits.

From one of your phones, dial the an outside phone number using the T-1 line. It should connect.

Direct Inward Dial Configuration

You have the option of having incoming calls go directly to specific phones using the Direct Inward Dial (DIDs) numbers assigned by your Telco (780-410-22xx to 2299) or you can have the incoming calls go to an Auto Attendant. If you have DIDs assigned by the Telco and they start at 2200, you can add the following into the telephony-service section:

dialplan-pattern 1 78041022.. extension-length 4 extension-pattern 22..

From an external phone, dial one of your DID numbers (ex. 780-410-2201) and verify that you can connect.

5. Connect to the Service Provider

The biggest problem when connecting to a service provider's T1 line is "how do you test that you are actually connected?". It is simple, use a T1 loopback connector and test your end as discussed earlier at the demarcation point. The demarcation point is the point in the network where your responsibility ends and the service provider begins. This tests your end.

Demarcation Point

Connect the service provider's T1 by attaching the T1 loopback to their end. Tell the service provider that you have a T1 loopback connector and ask them to test their T1 line. When both test works, you know that both ends have T1 connectivity.

If you connect your T1 to the service provider and it doesn't work then it must be a either a wiring error: commonly Rx and Tx wired on reversed pins - the connection to the service provider is straight thru not a cross-over! Check the color of the alarm, if RED that means a wiring error.

OR a mismatched configuration issue: you must confirm that the T1 and ISDN settings that you have configured agree with what the service provider expects.

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Copyright July 2013 Eugene Blanchard