Integrated Services Digital Network (ISDN) is a technology that allows digital communication over a traditional analog phone line, so both voice and data can be digitally transmitted over the PSTN. ISDN never had the popularity that it was expected to have because it came along at a time when other alternative technologies were developed.
There are two flavours of ISDN:
- ISDN BRI (Basic Rate Interface)
- ISDN PRI (Primary Rate Interface)
The ISDN-speaking devices are called terminal emulation equipment and the devices can be categorised into native ISDN and non-native ISDN equipment. The native ISDN equipment is made up of devices that were built to be ISDN-ready and are called TE1 devices (Terminal Equipment 1). Non-native ISDN equipment is made up of TE2 devices. Non-native ISDN equipment can be integrated with native ISDN equipment by using special Terminal Adapters (TAs), meaning only TE2 devices require TA modules.
Moving towards the ISDN provider, you will find Network Termination 2 (NT2) devices and Network Termination 1 (NT1) devices. These are translation devices for media, transforming five-wire connections into two-wire connections (the local loop). The local loop is the user connection line and it is a two-wire link.
An interesting thing about the network termination devices is that in North America the customer is responsible for NT1 devices, while in other parts of the world this is the service provider’s responsibility. Because of this issue, some Cisco routers provide built-in NT1 functionality and they will feature a visible “U” under the port number so that the user can quickly see this capability. The “U” notation comes from the ISDN reference points terminology that describes where you might have a problem in the ISDN infrastructure, as shown in Figure 41.5 below:
Figure 41.5 – ISDN Reference Points
These reference points are important during the troubleshooting or maintaining process of an ISDN network. The ISDN switch is usually located at the service provider location. The different ISDN reference points are as follows:
- U reference point – between the ISDN switch and the NT1 device
- T reference point – between the NT2 and the NT1 devices
- S reference point – between terminals (TE1 or TA) and NT2 devices
- R reference point – between non-ISDN native devices and TAs
The ISDN Basic Rate Interface (BRI) connectivity contains two B (bearer) channels for carrying data and one D (delta) channel for signaling. The BRI connection is abbreviated as 2B+D to remind you about the number of channels each provides. Each of the bearer channels in ISDN will operate at a speed of 64Kbps. Multilink PPP can be configured on top of these interfaces to allow the user to reach a bandwidth of 128Kbps. This bandwidth is considered to be very low according to modern networks requirements.
The delta (D) channel in BRI ISDN is dedicated to 16Kbps for control traffic. There are also 48Kbps overall for framing control and other overhead in the ISDN environment, meaning the total ISDN bandwidth for BRI is 192Kbps (128Kbps from the B channels + 16Kbps from the D channel + 48Kbps overhead).
ISDN Primary Rate Interface (PRI) has 23 B channels and one D channel in the US and Japan. The bearer channels and the delta channels all support 64Kbps. If you include the overhead, the total PRI bandwidth is 1.544Mbps. In other parts of the world (i.e., Europe and Australia) the PRI connection contains 30 B channels and one D channel.
ISDN PRI connections are commonly used as connectivity from the PSTN to large phone systems (PBX). Each of the 23 or 30 B channels can be used as a single phone line, so the entire PRI connection can be considered a trunk that carries multiple lines. The main advantage of using a PRI connection instead of multiple individual lines is that it is easier to manage and it offers scalability.
The technologies described above are called Time Division Multiplexing (TDM) technologies. TDM refers to being able to combine multiple channels over a single overall transmission medium and using these different channels for voice, video, and data. Time division refers to splitting the connection into small windows of time for the various communication channels.
In a PSTN, you need to be able to transmit multiple calls along the same transmission medium, so TDM is used to achieve this goal. TDM actually started in the days of the telegraph and later on gained popularity with fax machines and other devices that use TDM technologies.
When you have leased lines (buying dedicated bandwidth), the circuits that are sold are measured in terms of bandwidth. A DS1 or T1 circuit in North America provides 24 time slots of 64Kbps each and a 9Kbps control time slot (for a total of 1.544Mbps, as mentioned earlier). TDM terminology is tightly connected with the leased-line purchasing process.