The Integrated Services Digital Network (ISDN) is a set to international standards that define data transmission across digital circuits. The original telephone network followed an analog system that used wires to connect users. This system is prone to breakdown and is not reliable over long distances. ISDN enables you to overcome the drawbacks of the original telephone wires by using digital circuit.
ISDN provides Wide Area Network (WAN) services in incremental values of 64 kbps. ISDN digitizes the telephone lines enabling us to transfer voice, text, graphics, music, and video data over telephone lines.
The Features of ISDN are: -
Speed: - ISDN requires only 2 second to complete a call as compared to analog signals that requires 30 seconds. You can combine channels for ensuring high data transfer rate. The data rate for each connection is minimum of 64 kbps.
Multiple Device Support: - ISDN enables multiple devices to share a single line. You need not have a separated line for data, voice and video data. For example, if you are talking to a person on the phone and need to transfer an image to that person, you can transfer it using the same line.
INexpensive: - ISDN provides an inexpensive mode of data transfer. Moderns limit the data speed and accuracy since they need to inter convert analog and digital signals.
ISDN requires certain devices that are compatible with the ISDN protocol. The devices are connected using RJ-45 connectors. The pin 3 and 6 in the connector are used for transmitting data. The pins 4 and 5 are used for receiving data.
Network Termination Device 1 (NT 1): - Provides an interface between the ISDN Basic Rate Interface :( BRI) line and other devices. The NT1 devices act as a testing point for troubleshooting. Some terminal adapters and routers have the NT1 functionality built in his reduces the installation cost, since you do not need to install the NT1 devices separately.
Network Termination Device (NT 2): - Communicates with the ISDN protocols. Every ISDN device has a built in NT 2 device.
ISDN Router: - Enables multiple computers on a LAN to share a single ISDN BRI connection. The routers support analog voice, modern, and fax applications, ISDN routers are expensive than terminal adapters.
Terminal Equipment 1 (TE 1): - Consists of ISDN devices that understand the ISDN standards.
Terminal Equipment 2 (TE 2): - Consists of non-ISDN terminals that require a Terminal Adapter (TA) to connect to an ISDN network.
Terminal Adapter (TA): - Converts some other form of signalling to ISDN to enable non-ISDN devices, such as the TE 2 to work with the 2-wire ISDN network.
ITU-T has grouped the ISDN Protocols according into the following areas:
· Protocols that being with “E” are for telephone network standards for ISDN. : For example, the E, 164 protocols describes international addressing for ISDN.
· Protocols that being with “I” deal with concepts, terminology, and general methods. The I.100 series includes general ISDN concepts and the structure of other I-series recommendations; I.200 deals with services aspects of ISDN; I 300 describe network aspects; I.400 describes how the User-Network Interface (UNI) is provided.
· Protocols beginning with “Q” cover how switching and signalling should operate. The term “ signalling” in this context means the process of call set used. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the ISO/OSI reference model. Q.931 specifies ISO/OSI reference model Layer 3 functions.
ISDN is a channelized service. A channelized service splits a wire into many logical channels or time slots. Each time slots have their own bandwidths and time on the wire. The division of the wire into logical time slots is termed as Time Division Multiplexing (TDM). Some of the common terms that are used in channelized connections are:
Digital Signal 0 (DS 0): - Represents a 64 kbps channelized connection
Digital Signal 1 (DS 1): - Consists of a group of channelized connections. There are two types of DS1s.
* T1 : Consists of a group of 24 DS0s
* E1: Consists of a group of 32 DS0s
The ISDN interfaces provide access to the uses. The interfaces differ based on the number of bearer (B) and delta (D) channels. The B channels transmit data over the network. They operate at a speed of 64 kbps. The speed of data transfer through B channels depends on the service provider. The D channel is the signalling channel. When the router needs to create a connection using B channel, the phone number to which it wants to connect is sent using the D channel. ISDN supports two types of connections, the Basic Rate Interface (BRI) and Primary Rate Interface (PRI)
Basic Rate Interface (BRI)
Consists of two 64 kbps bearer B channels and one 16 kbps delta d channel. The BRI physical layer specification is based on International Telecommunication Union Telecommunications Standards Section (ITU-T) L.1430. The BRI is also referred as a 2B+D network.
The B channels are used for digitized speech transmission of for relatively high speed data transport. The B channel is the elemental circuit switching unit.
The D channel carries signalling information (call setup) to control calls on B channels at the UNI. In addition to carrying signalling information, the D channel is used to carry subscriber low-rate packet data, such as alarm systems. Traffic over the D channel employs the LAPD data-link-level protocol. LAPD is based on High-Level Data Link Control (HDLC)
Primary Rate Interface (PRI)
Consists of multiple B and D channels, PRIs in Europe have 30 B ad 1 D channel while the PRIs in U.S have 23 B and 1 D channel. The total bit rate offered by PRI is 1.544 Mbps.