Integrated Services Digital Network (ISDN)


Integrated Services Digital Network, a circuit-switching network used for voice, data and video transfer over existing copper telephone lines. ISDN is a bit similar to the normal telephone system but it is faster and needs less time to setup a call. ISDN runs on the bottom three layers of the OSI reference model.

There are several types of ISDN channels, the two main being the 64 Kilobits per second B-channel for data, and the D-channel for control information. Two B-channels + one D-channel make up ISDN BRI (Basic-Rate Interface), some Remote Access servers support a feature called multilink allowing both B-channels to be combined in a single virtual link of 128 Kbps. In SOHO networks often 1 B-channel is used for data (an internet connection for example) and 1 B-channel is used for voice (connected to a digital telephone for example). The US and Japanese version of ISDN PRI (Primary-Rate Interface) is made up of 23 B-channels (total rate of 1.472 Mbps) and 1 D-channel. The European and Australian version supports 30 B-channels (total rate of 1.984 Mbps) and 1 D-channel.
A common implementation of these two types of ISDN is a remote access solution with ISDN PRI at the corporate network supporting 23 dial-in connections for employees with ISDN BRI at home. Also an ISDN BRI connection is often implemented as a backup line between routers in WANs such as in a Frame Relay network as shown in the following image:

isdn bu fr Integrated Services Digital Network (ISDN)

Besides this dial-up ISDN configuration for backup and other Dial on Demand Routing (DDR) configurations another service offered are ISDN BRI leased-line connections, the difference is they always use both data channels for the connection to the ISDN service provider and ISDN BRI leased-lines are always active.

ISDN Function groups

The ISDN function groups represent the devices in an ISDN environment such as terminals, terminal adapters, network-termination devices and line-termination equipment. The following table lists these devices:

TE1 (Terminal Equipment 1) Specialized ISDN terminals that understand the ISDN standards, for example an ISDN telephone.

TE2 (Terminal Equipment 2) Non-ISDN Terminals that need a Terminal Adapter (TA) to connect to an ISDN network, for example a regular telephone.

TA (Terminal Adapter) Converts some other form of signaling to ISDN to allow non-ISDN devices (TE2) to work the 2-wire ISDN network.

NT1 (Network Termination 1) Connects TE1 or TA devices to the ISDN network. In the US, the NT1 is located at the customer’s premises and owned by the customer. In other parts of the world the NT1 is usually provided by the carrier (typically a telephone company).

NT2 (Network Termination 2) The NT2 is a physical device that interfaces the NT1 to different types of devices (TE1 or TA). In most cases it is a PBX at the customer’s premises.
Take for example an apartment building or campus, if have a demand for ISDN lines from your renters (customers) you can order an ISDN PRI and connect it to your local PBX. You can then extend the ISDN service to any place in the building(s).

The following image shows the various function groups and reference points.

isdn ref func Integrated Services Digital Network (ISDN)

The following image illustrate some real-life situations. As you can see the NT2 is left out, most NT1 adapters today have a U interface on one side and an s/t on the other so you simply plug your TE1 or TA into the NT1 and you’re good to go.

isdn Integrated Services Digital Network (ISDN)

The following image shows two type of routers, the upper is usually used in North America where the demarcation point between the customer premises and the carrier’s network is the U reference point, this router is actually a TE1 with a built-in NT1 and is also known as a ‘U router’. The other router is used in most other parts of the world where the NT1 is provided by the telco, this router is actually a TE2 with a built-in TA and is also known as a ‘S/T router’.

isdnrouters Integrated Services Digital Network (ISDN)

ISDN Reference points

ISDN specifies four reference points that define the logical interfaces/connections between function groups (also represented in the mage below):
R defines the reference point between non-ISDN equipment (TE2) and a TA.
S defines the reference point between and an NT2.
T defines the reference point between NT1 and NT2 devices.
U defines the reference point between NT1 devices and line-termination equipment in a carrier network. Relevant in North America where the NT1 function isn’t provided by the carrier network.

ISDN protocols

ISDN protocols are defined in ITU protocols that operates on the Physical, Data Link and Network layer of the OSI model. There are several series of protocols dealing with different issues:
E series defines the use of ISDN on the existing telephone network.
I series deals with concepts, aspects, and services.
Q series covers switching and signaling. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921. LAPD is the signaling protocol used on the D-channel in ISDN BRI and PRI.

Configure ISDN BRI and Legacy DDR

Configuring ISDN may seem to be complex but is rather simple in basic situations. The diagram below shows a typical setup connecting two remote offices using an ISDN dial-up configuration.

isdn log 2routers Integrated Services Digital Network (ISDN)

First the ISDN switch type must be configured and should match the carrier’s equipment. You can use the isdn switch-type command in both global config mode (required) and interface configuration mode (optional if different per interface). For example:
Router(config)#isdn switch-type basic-dms100
The correct switch type should be supplied by the carrier. Click here for a table at listing the ISDN BRI service provider switch types. If you change the switch-type, you must reload the router for the new switch type to take effect.

Although ISDN supports several upper-layer protocols such as IP, IPX and Appletalk, typically IP is used and this is also the one relevant to the CCNA exam. Configuring an IP address on an ISDN BRI interface is done in the same way as configuring an IP address for any other interface such as Ethernet or Serial:
Router(config)#interface bri 0 (to enter interface config mode)
Router(config-if)#ip address

Some service providers require the use of SPIDs for your ISDN device to be able to place or receive calls. A SPID is usually the telephone number of the channel with some optional numbers which can be used to identity the service(s) the customer is subscribed to. The SPID numbering scheme depends on the service provider and the switch-type. For example, the DMS-100 switch type requires a SPID for each B channel.
Router(config-if)#isdn spid1 5055551234 0111 (B1 channel)
Router(config-if)#isdn spid2 5055551235 0111 (B2 channel)

The default encapsulation type for each B-channel is HDLC, however PPP encapsulation is recommended over HDLC in order to allow the use of CHAP authentication. The encapsulation type can be configured using the following command in interface configuration mode:
Router(config-if)#encapsulation ppp

Now to configure the actual part that maps the link to the network layer using the dialer map command, it defines the remote host where the calls are going, specifies whether broadcast messages will be sent and the dialing string to use to set up the call. Here’s the syntax of the command:
Router(config-if)#dialer map protocol next-hop-address name remote-name speed 56|64 dial-string
We’ll break down the command using example options:
Router(config-if)#dialer map ip name RouterB speed 64 broadcast 55588613213

- The IP address of the remote router’s BRI interface used in this command is the next hop. In the global configuration you will have to define a static route to the remote network pointing to the next hop address used in the dialer map command. The use of static routes is very important, since you don’t want to use dynamic routing protocols for this type of connection because the routing updates will keep the link up.
- The remote name in name remote-name is the hostname of the other router.
- speed defaults to 64 (in kilobits) but you may need to set it to 56 in some situations.
- The broadcast option specifies whether broadcast packets such as routing updates are sent.
- The dial-string is the telephone number that should be dialed when making an outgoing connection. You can leave out this number to configure the interface to only accept incoming connections.

The following commands will define “interesting” traffic that will cause the router to place a call make the connection. For example if you want the router to dial-in for all IP traffic you need to configure a dialer-list and bind it to the BRI interface:
Router(config)#dialer-list 1 protocol ip permit
Router(config)#int bri0
Router(config-if)#dialer-group 1

You can also use regular or extended access lists to permit all traffic except HTTP/HTTPs for example. Instead of using the options in the dialer-list command above you would specify the access list:
Router(config)#dialer-list 1 protocol ip list 101

The following command makes the router disconnect calls that haven’t had any interesting traffic for the configured time:
Router(config-if)#dialer idle-timeout seconds

To add some level of security and to identify the router when it dials out, you should use the Challenge Handshake Authentication Protocol (CHAP). The hostname of the router is used to identify the router to another router when sending messages.
Router(config-if)#ppp authentication chap

The global configuration username command is required when CHAP is used to specify the CHAP secret message to use when challenged by another router. Important to know is that the two routers that need to talk must share the same password.
Router(config)#username routerB password password

PPP Multilink

Multilink is a feature that enables the use of both B-channels combined for one call. To turn on multilink use the following command:
Router(config-if)#ppp multilink

Use the following command to specify when the second B-channel should kick-in (bandwidth on demand). When the total load for this connection reaches this threshold, it brings up the other B channel. This value represents a utilization percentage; it is a number between 1 and 255, where 255 is 100 percent.
Router(config-if)#dialer load-threshold 60


Here are some commonly used show commands used to monitor and troubleshoot ISDN:

Router(config)#show interfaces bri number
Displays information about the physical attributes of the ISDN BRI B and D channels.

Router(config)#show controllers bri number
Displays protocol information about the ISDN B and D channels. Checks Layer 1 (physical layer) of the BRI.

Router(config)#show isdn {active | history | memory | status | timers}
Displays information about calls, history, memory, status, and Layer 2 and Layer 3 timers.

Router(config)#show dialer interface bri number
Obtains general diagnostic information about the specified interface. Checks Layer 3 (network layer).

Router(config)#show isdn status
Use to verify that ISDN BRI Layer 1 is ACTIVE, LAYER 2 State is MULTIPLE_FRAME_ESTABLISHED, and the service profile identifiers (SPIDs) are valid.

Router(config)#debug q921
Checks Layer 2 (data link layer).

The following three commands offer more advanced methods to check Layer 3 (network layer) operation:

Router(config)#debug isdn events
Router(config)#debug q931
Router(config)#debug dialer

Author: Johan Hiemstra