ISDN was designed and implemented to be an upgrade from slower analog modems. The first difference between ISDN and analog modems is that an ISDN connection is pure digital from end to end. The main advantage of ISDN over analog modems is faster speed and simultaneous services, like voice and data. Another advantage is out-of-band signaling. Unlike analog modems, the clocking for an ISDN connection is performed on a separate connection, called a D channel. This means the actual connection that transmits and receives data does not have to waste bandwidth synchronizing the digital signal.

This Daily Feature will focus on the basics of ISDN and the different components involved in setting up and configuring ISDN connections. Future articles will describe ISDN services, as well as configuration using different ISDN components.

The ISDN components
The most confusing parts of ISDN are the many components that are part of the basic configuration. However, since more open ISDN standards have become prevalent, it is not as confusing or as hard to configure as it used to be. The hardest part in the configuration these days is just getting the correct information from the provider.

Found in ISDN networks are Terminal Equipment types, Network Termination types, and Terminal Adapters. You need to understand each type before learning how to configure ISDN.

  • Terminal Equipment Type 1 (TE1): This is a device that understands the SS7 signaling used by the provider’s ISDN switch. This could be a router interface, host, or telephone.
  • Terminal Equipment Type 2 (TE2): This is a device that does not understand the SS7 signaling used by the provider’s ISDN switch. This could be a router serial interface or other type of WAN equipment used for serial connections.
  • Network Termination Point 1 (NT1): This is a device that converts a four-wire network to the ISDN two-wire network. ISDN only uses two wires and a TE1 sends an ISDN signal out on four wires. The NT1 is needed to convert the four-wire network to the ISDN two-wire network. The NT1 is used in North America and is part of the Customer Premise(s) Equipment (CPE), which basically means the customer buys and installs the hardware. In other parts of the world, the NT1 is supplied by the provider.
  • Network Termination Point 2 (NT2): Used for devices that do not understand ISDN standards.
  • Terminal Adapter (TA): For a TE2 to communicate on the ISDN network, you must use a terminal adapter to convert the non-ISDN signal to an SS7 ISDN signal. However, it must still be converted from a four-wire network to a two-wire network. The TA must connect into an NT1 for this conversion before connecting to an ISDN network.

The ISDN reference points
Reference points are used in an ISDN network to locate troubleshooting locations throughout the network. The following reference points are used:

  • R: This reference point is used to specify the location between an NT2 and a terminal adapter.
  • S/T: This reference point is the network point that understands ISDN signaling, but is still a four-wire network. This will be found between a TE1 and NE1. Typically, these points are referenced together, but really the S reference point is the point between the user terminals and NT2, while the T reference point is found between the NT1 and NT2 devices. However, since the services of an NT1 and NT2 are usually found in one box, you can refer to this reference point as S/T.
  • U: This reference point is used to define a network point that understands ISDN signaling techniques and is also a two-wire network. This reference point plugs directly into the ISDN switch. Most router interfaces will now ship as a U reference point, which illuminates the need for a separate hardware device before plugging into the ISDN network.

ISDN switch types
The ISDN switch type is used to configure the interface of the router to the provider’s ISDN switch. Since the ISDN standards were not accepted until recently, there were many different types of ISDN switches manufactured. When the basic-ni1 ISDN switch was introduced a few years ago, it became the de facto standard and most North American providers now use this switch type. You need to verify this configuration with your provider. Some routers auto-sense the ISDN switch type, but you should verify with your provider anyway.

Basic Rate Interface (BRI)
A Basic Rate Interface (BRI) consists of two DS0s (Digital Signal 0) of 64 K each; they are called Bearer channels, or B channels. This provides a customer with 128 Kbps of data bandwidth. ISDN uses what is called out-of-band signaling as a separate channel for clocking. This channel is called a Data channel, or D channel, and is 14 Kbps. This means a BRI is 128 Kbps+14 Kbps or 142 Kbps total bandwidth. The advantage of the D channel is that actual user bandwidth is not affected by nondata traffic.

Primary Rate Interface (PRI)
A PRI is 24 DS0s, with 23 B channels and one D channel of 64 Kbps. This is a total of 1.544 Mbps. Sound familiar? We call this a T1 in North America.

ISDN Service Provider IDs (SPIDs)
This is what identifies your DS0s to the provider. It’s an important configuration parameter on the router, although some routers do auto-sense this configuration, as well. A SPID is usually the phone number of the B channel, with a 0101 or 1111 following the number. Here is an example of a BRI interface configuration on a Cisco router:
interface BRI0
 ip address negotiated
 isdn switch-type basic-ni
 isdn spid1 72098130121111
 isdn spid2 72098130131111

This configuration has the IP address of the interface provided by DHCP and each SPID is followed by a 1111. The switch type is basic-ni, which is configured as basic-ni1 or just basic-ni. Cisco routers can take either command.

ISDN Layer Two services
ISDN uses a protocol called Link Access Procedure, D channel (LAPD), which is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB); these are both Data Link layer connection-oriented point-to-point serial encapsulation methods. LAPD runs only on the D channel and is used for signaling and flow control services. LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.

ISDN Layer Three services
ISDN uses two ITU-T specifications at the Network layer of the OSI model for signaling on the D channel: ITU-T Q.930 and ITU-T Q.931. These two specifications support point-to-point, circuit-switched (dial-up), and packet switched technologies. Both specifications provide setup, connection, releaser, user information, cancel, status, and disconnect messages to the network.

ISDN is a great WAN technology, if you can get it. However, if you have the option of DSL or cable modem, those are faster, cheaper technologies. You would still use ISDN if you needed voice, video, data, and special application services, which are not provided by DSL and cable modems.