ISDN Protocol Components

This topic is an overview of the ISDN technology components, which are relevant to the Cisco network engineer and network administrators.

ISDN is a layer 3 technology that is based on the following protocols:

  • Q.931
  • LAPD

So ISDN is a collection of protocols.

ISDN Q.931 Message Format

The format of the Q.931 message is as follows:

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  • The Protocol Discriminator value is 8, for an ISDN Q.931.
  • Call Reference identifies a particular communication on a B channel. Remember that the D channel sends and receives signaling information for all bearer channels of the ISDN link. So Call Reference is unique for each communication.
  • Message Type can either be on one byte or two bytes.
  • Information Elements, aka IEs, are chunks of useful information, such as Caller Number, Called Number, Bearer Capability,…
    • There are mandatory Information Elements and optional Information Elements
    • they can be either one-byte long, or multi-byte long.

ISDN Implementations

ISDN protocols are implemented for BRI and PRI technologies. And for each one of these, there is the North American way, the European way, the Japanese way,… depending on which part of the world you are in.

For example, if you are in Germany, the configuration you need to implement on your ISDN gateway router follows the ETSI specifications, or the European Telecommunications Standards Institute. However, if you are in Canada, you will have to configure your ISDN router according to the North American standards.

The symmetrical implementation of ISDN is called QSIG.

ISDN Supplementary Services

ISDN provides basics services such as type of data, calling number, called number, data rate, transfer mode, call establishment, call teardown…

Another type of services called Supplementary Services were developed such as abbreviated dialing and call waiting.

Like we saw in the previous paragraph, the Supplementary Services are developed for BRI and PRI, and there are region-specific implementations.

For example, here is the list of North American NI-1 supplementary services for PRI:

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and here is the equivalent for European countries:

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To benefit from a supplementary service, an originating gateway requests it by sending a Facility Q.931 message. Facility is a message type. The terminating gateway replies with either Facility Acknowledge or Facility Reject.

Debug ISDN Q931 On Cisco Routers

We are going to explore the output of debug isdn q931 command. We make an outbound ISDN call and observe the behaviour of the ISDN protocols on the gateway.

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The gateway sends an ISDN Q.931 message out of the serial 0/2/0:23 interface, which is the D channel.

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This is an outbound packet, which is demonstrated by the TX -> sign

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The outbound packet is a SETUP message. This is a known message type in the Q.931 message format.

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The protocol discriminator has a value of 8, which confirms that this is a Q.931 message.

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The call reference value comes next.

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Now come the Information Elements: Bearer Capability, Channel ID, Progress Indicator, Display, Calling Party Number and Called Party Number.

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Here is a trick to distinguish Information Elements: notice the “i” letter after Bearer Capability, Channel ID,…? that means it’s an IE.

A Call Proceeding packet is received. This is depicted by the “RX<-” symbol. This packet is part of the same ISDN call, which is depicted by the same value of the Call Reference:

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Another packet is received. It is a Call Alerting packet. And it belongs to the same communication.

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ISDN Q.931 Protocol Overview

ISDN Protocol

ISDN: The Integrated Services Digital Network

ISDN Processing Overview

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