ISDN Services

ISDN user interfaces

International ISDN support

ISDN Services

Integrated Services Digital Network or ISDN is the ITU-T (formerly CCITT) term for the digital public telecommunications network.  The ISDN user interface is known as the Basic Rate Interface (BRI). 

ISDN Basic Rate Interface (BRI) 

The Basic Rate Interface provides two 64 Kbps B-channels and one 16Kbps D-channel and is often referred to as 2B+D.  It is designed to provide service to individual users or small businesses and could, for example, provide transmission of a telephone conversation on one B-channel and a 64Kbps data session on the second B-channel with the call control and signalling for both channels carried out on the 16Kbps D-channel. 

This is the type of interface most likely to be encountered in Inmarsat-B HSD applications. 

B-Channels

A B-channel is a single, duplex 64Kbps channel.  A B-channel can carry any type of digital information (voice, data, or video) with no restrictions on format or protocol imposed by the ISDN carrier.  An Inmarsat-B HSD channel provides a single, duplex B-channel. 

D-Channels 

A D-channel is a signalling channel.  It carries the information needed to connect or disconnect calls and to negotiate special calling parameters (e.g. Calling Line ID, call waiting, data protocol).  The data rate of a D-channel of a Basic Rate Interface (BRI) operates at 16Kbps. 

D Channel Protocols

The ITU-T D-Channel protocols are the basis for all ISDN D-Channel protocols. Real world ISDN network implementations are derived from the ITU-T, but differ in varying degrees. The relevant ITU-T standards are: Q.931 for Layer 3; Q.921 for Layer 2, and I.430 for layer 1. 

ISDN D-Channel Protocols include: DSS1(ETSI and ITU-T), 1TR6, DMS-100, AT&T Custom and NI‑1

ISDN User Interfaces

ISDN is normally implemented using existing two-wire, twisted-pair conductors from the local telephone exchange to the customer premises up to a maximum distance of 5.5km from the exchange.  At the customer premises the point of termination of the incoming ISDN telephone line is known as the U-interface.  The incoming ISDN line is terminated at the U-interface by a Network Termination device known as an NT1.  

In North America it is the responsibility of the customer to provide the NT1 but in most of the rest of the world the NT1 is provided and installed by the telephone company.  Physically the NT1 is like an oversized telephone line-box and is normally wall-mounted.  

The customer interface of the NT1 is known as the S/T-bus or S0 interface and utilises an eight-wire RJ-45 interface.  In point-to-point applications, the S/T bus can connect equipment up to 1000 metres apart.  When used in a passive bus configuration (i.e. connecting up to eight terminals) it can span a distance of up to 500 metres.  The name S/T-bus comes from the letters used in the ISDN specifications used to refer to two reference points, S and T.  Point T refers to the connection between the NT1 device and the customer supplied equipment (e.g. data terminal, telephone, fax machine).  Customer equipment can connect directly to the NT1 at point T, or there may be a PBX (Private Branch Exchange, i.e. customer owned telephone exchange).  When there is a PBX present, point S refers to the connection between the PBX and customer terminal equipment.  The specification for the T interface is identical to the specification for the S interface.  In ISDN terminology “terminal” can mean any sort of end-user ISDN device, such as data terminal, telephone or fax machine. 

Interface conversion between the terminal equipment and S/T-bus is carried out using a Terminal Adapter (TA).  The Terminal Adapter is typically packaged in a similar fashion to modems i.e. either as a stand-alone unit or as a built-in PC-card or module for various types of communications equipment (such as bridge/routers or PBX’s).  Stand-alone TA’s are supplied configured with a standard serial communications interface such as RS.232, V.35 or X.21.  The interface is normally specified by the customer according to the application.  Nowadays most equipment capable of being used with ISDN is supplied with a built-in ISDN Basic Rate Interface (BRI) as standard or may be ordered with a BRI as an option. 

ISDN Connectors

ISDN networks present an NT1 connection to the network subscriber. North America is the exception, where the U interface is presented to the network subscriber. 

Both the NT1 and U interface are RJ45 connectors, so care must be taken when connecting ISDN equipment (which may have an interface connection for the NT1 or U interface) and the network (which may be an NT1 or U interface).

International ISDN support

The links below indicate which countries and telephone companies support ISDN. Telephone companies supporting ISDN are listed below each country.