ISDN network equipment can generally be broken down into two groups – devices that understand and are capable of communication via ISDN, and those that do not natively support it. For example, a regular analog phone would not be capable of communicating over the ISDN network without some type of intermediate device converting its signals to the digital standard used by ISDN. However, some devices are already ISDN-capable, such as routers that ship with an ISDN interface. In the word of ISDN, devices are assigned a code that describes their capabilities.
The first type of device that you should look at is known as Network Termination 1 (NT1). In North America, an NT1 device sits at the customer premises, and connects to the service provider’s network, as shown in the figure below. The local loop wiring between the service provider and a customer is based on 2 wires. The purpose of an NT1 is to convert this 2-wire interface into the 4-wire interface used by ISDN. Once installed, the NT1 can be used to connect up to 8 additional ISDN-aware devices on the customer network. As such, any customer (in North America) that wants ISDN connectivity must have a local NT1 device. In Europe, NT1 equipment is located at the service provider’s facilities. Many Cisco router models provide an ISDN interface with a built-in NT1, allowing them to connect directly to a service provider’s ISDN network.
The next devices that you need to be aware of are known as Terminal Equipment (TE). Two types of Terminal Equipment exist, known as TE1, and TE2. TE1 devices are those designed to work with ISDN, using the 4-wire interface. An example might be an ISDN telephone, or a router with a built-in ISDN interface.
TE2 devices are those that pre-date ISDN standards, or are simply not ISDN capable. Examples would include a traditional analog phone, fax, or even a computer. In order for TE2 equipment to connect to the ISDN network, an additional piece of equipment is required. This device is referred to as a Terminal Adapter (TA). A Terminal Adapter is often an external device that connects to TE2 equipment over a serial port. It can also take the form of an expansion card, to install within a PC, for example. In either case, TE2 equipment always needs a TA to connect to an ISDN network. The figure below illustrates a small network with both TE1 and TE2 equipment connecting to an NT1 device.
Another ISDN device that you should be familiar with is known as Network Termination 2 (NT2). An NT2 device is much less common, but is used to provide switching, concentrating, and multiplexing services for ISDN lines. NT2 devices typically take the form of Private Branch Exchange (PBX) equipment. In reality, most NT1 devices also include NT2 capabilities in their design. The placement of an NT2 device is illustrated in the figure below.