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Integrated Services Digital Network (ISDN): ISDN means Integrated Services Digital Network. ISDN was developed by ITU-T in the 1980s. These are digital telephony and data transport services offered by regional telephone operators. ISDN includes the physical, data link, and network layers of the OSI model.

The main objective of ISDN is to provide fully integrated digital services to the users. ISDN is actually a set of communication protocols that were offered by telephone corporations. These include the digital services of the telephone network. ISDN allows the data to be sent over existing telephone cables, and these data are video, audio, text, and file in the form. The main goal of the ISDN is provided to the end-to-end connectivity in the worldwide from of the WANs. Another goal of ISDN is also to provide a combination of voice and non-voice services.

Narrowband ISDN is the first generation of the ISDN. Narrowband ISDN is based on the basic unit of the switching, and this unit is 64 kbps. The frame relay is the primary device of the Narrowband ISDN. Broadband ISDN (B-ISDN) is a second generation of the ISDN. The second generation of ISDN supports very high data rates, usually hundreds of Mbps. Asynchronous Transfer Mode (ATM) is the main technical contribution of the B-ISDN, which is also known as the cell relay.

May 14, 2016 ISDN Stands for 'Integrated Services Digital Network.' ISDN is a telecommunications technology that enables the transmission of digital data over standard phone lines. It can be used for voice calls as well as data transfers. ISDN stands for INTEGRATED SERVICES DIGITAl NETWORK. ISDN networks are capable of generating higher transmission speed of 1.4Mbps. Although 128Kbps speed is more of standard in digital technology. The medium used by ISDN lines are unshielded twisted pair cables (UTP) for communication. Primary Rate Interface ISDN (PRI-ISDN) connections consist of 24 digital channels divided between 23 B channels and a single D channel. The B channels carry the voice or data between the customer premises and the telco’s central office (CO), while the D channel is used for establishing connections and signaling. Integrated Services Digital Network (ISDN) is an all-digital dial-up (on-demand) service that can simultaneously carry high-quality voice, data, and video transmissions over existing Plain Old Telephone Service (POTS) telephone wires of the subscriber’s local loop connection at speeds of 128 Kbps or higher. ISDN or Integrated Services Digital Network is a circuit-switched telephone network system that transmits both data and voice over a digital line. You can also think of it as a set of communication standards to transmit data, voice, and signaling. These digital lines could be copper lines.

The Architecture of ISDN shown in below:

Advantages of ISDN

1. ISDN can operate the video, voice, text, and data simultaneously.

2. The call setup of the ISDN is faster than the analog modem.

3. It provides higher data rates.

4. ISDN provides fully digital services, so the probability of error is very low.

ISDN provides three types of digital services:

1. Bearer Services: Bearer services offer the path for transfer data between the users without the network manipulating the content of that data. Bearer services are related to the Physical,>
2. BRI (Basic Rate Interface)
3. PRI (Primary Rate Interface)

BRI: BRI stands for Basic Rate Interface. In the ISDN, BRI provides two B-channels and one D-channel to initiate connections, i.e. (2B + D). The B-channels work at the 64 Kbps for each channel, while the D-channel works at the 16 Kbps.

PRI: PRI stands for Primary Rate Interface. In the ISDN, PRI provides 23 B-channels and 1 D-channel. The PRI offers an aggregate data rate of 1.544 Mbps. The PRI services are very expensive, and it is used to connect many computers and devices on a large scale.

The ISDN channels table shown below with each transmission data rates:

The Integrated Services Digital Network was the first telecommunications service designed specifically for digitaldata communication. ISDN was designed to run over standard voice digital telephone systems already in place. Consequently ISDN conforms to specifications found in the telecom's digital voice network. However, it took so long for ISDN to be standardized that it was never fully deployed in the telecommunications networks it was intended for.

ISDN Channel Types

ISDN uses two bearer channels and one data channel.

Bearer Channels

The first channel type is a 'bearer' channel or 'B' channel. These are the channels that carry the end-user's actual data. Bearer channels are always 64kbps in the US, and 56kbps elsewhere.

Data Channels

To confuse things, the telcos called the second channel type the 'data' channel or 'D' channel. This channel contains the data THEY consider important such as signalling errors, framing and various control and managment signals. The D channel's speed is 16 kbps for a BRI, and 64 kbps for a PRI.

ISDN Service Types

ISDN provides two types of service, Primary Rate Interface (PRI) and Basic Rate Interface (BRI). These speed and number of channels for these two types of service vary depending upon whether you are in the United States or are elsewhere.

Primary Rate Interface

In the US, a PRI consists of 23, 64kbps 'B' channels and one 64kbps 'D' channel for a total of 1.544 Mbps. Elsewhere in the world a PRI consists of 30 'B' channels running at 56kbps and one 'D' channel also running at 64kbps. The reason for this difference is that overseas systems use a 7-bit word (7 bits x 8000 sample rate = 56,000 bps) and no echo cancellation. The US uses an 8-bit word (8 bits x 8,000 sample/second rate = 64,000 bps) with echo cancellation in the voice network.

Basic Rate Interface

BRI's consist of two bearer channels and one data channel. The Bearer channels are either (US) 64 or 56 kbps (elsewhere) and the data channel is 16 kbps, providing up to 144 kbps of data transmitted and 128 kbps of usable user data.

Digital Calling Services

Calling Line ID

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Called Number

Automatic Number Identification

Wiring & Connectors

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Primary and basic rate ISDN both use .25 guage copper twisted-pair telephone wire; however, cable TV companies and other organizations have used this protocol as well over coaxial cable. The only difference between PRI and BRI is that time-division multiplexing is used to combine several BRI's into a PRI making the PRI a trunk.

Several Terms used by telecommunications companies and the ITU are used to describe he device, interface and connection points.

ISDN Equipment - Grouped by Function

Terminal Equipment 1 (TE1)

This is any device that understands ISDN signalling standards.

Terminal Equipment 2 (TE2)

This is any device that does NOT understand the ISDN signalling standard.

Network Terminator Type 1 (NT1)

When you connect a TE1 device to the ISDN network, you use an NT1. An NT1 will connect any 2-wire TE1 device to the ISDN network.

Network Terminator Type 2 (NT2)

Devices using 4 wire cables must use an NT2 to convert the 4 wire connector to a 2 wire connector that can be physically connected to the ISDN network through an NT1.

Terminal Adaptor (TA)

A terminal adaptor allows a non ISDN device (a device that is a TE2) to communicate with the ISDN network via an NT1. This is typically needed where the device uses a non-polar electrical signalling system. ISDN's electrical signalling is bipolar, thus a converter is needed.

Local Termination (LT)

This is an NT1 at the ISP's side of the connection.

Exchange Termination (ET)

This is the connection between the customer's last mile (local loop) connection and the service provider's ISDN network. Usually, this is the line card in the ISDN switch at the provider's local exchange office.

ISDN Reference Points

R

This is the connection reference point designating the connection interface between an ISDN Terminal Adaptor and a non-ISDN device. There really aren't any standards for this reference point as this reference point was designated for devices that allow non-ISDN devices to communicate with the ISDN devices. Clearly there are many proprietary ways to do this, none of which are part of any standard.

S/T

This is the connection reference point designating the connection interface between an ISDN capable device and a Network Terminator 1. Reference point 'S' is for user terminals that connect to the ISDN network. Reference point 'T' defines connections between NT1 and NT2 devices. An S/T reference point combines the functions of the S and the T reference points. S/T is governed by the ITU I.430 specification.

U

This is the connection reference point designating the connection interface between an ISDN NT1 and the ISDN services presented by the ISDN switch.

V

This is the connection reference point between the line termination equipment and the exchange termination equipment.

SIGNALLING

The signalling scheme used on ISDN is called '2 Binary, 1 Quaternary' or 2B1Q. This signalling scheme allows twice as many bits to be transmitted in the same time-slice as a normal digital voice line. This is achived by allowing a wider range of voltage states than is used on an ordinary voice line. Link Access Protocol D-channel or LAPD protocol (aka CCITT Q.920/921) runs on top of 2B1Q and provides much of ISDN's 'intelligence' for the reliable transfer of data. LAPD runs in Asynchronous Balanced Mode (ABM) so that both ends of the connection can perform call setup and teardown, bearer channel supervision, error recovery and data transmission. LAPD is also necessary to run ISDN over Frame Relay.

LAPD

T1 based PRI uses B8ZS for encoding, ESF for bit framing with digital signalling 2B1Q as the initial input sample data. This differs from voice lines, which use PCM encoded data.

E1 based PRI uses HDB3 (high density bipolar order 3); CRC 4 for framing

Link Access Procedure, Data (LAPD)

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Link Access Procedure, Data or LAPD is the protocol used to transfer data on an ISDN circuit. LAPD is used on the D channel of ISDN PRI or BRI interfaces.

flag, address, control, information, CRC, flag

Flag

Binary value 01111110 - signals the start of the frame.

Address

service access point identifier (SAPI); command-response;Address extension0; Terminal Endpoint Identifer

Control

Signals call control values (11 of them)

Information

This is the payload of the LAPD frame, but contains Q.931 data; information field, Protocol discriminator, Length of call reference value, Call reference value; message type, mandatory and optional information elements

CRC check

This is a check of the data using CRC checking to allow for error correction and recovery of data upon receipt.

Flag

011111110 - signals the end of the frame.

ISDN Related Standards

ITU Standards

ITU Q.921

ITU Q.931

SPID - Service Profile Identifer - composed of: Prefix, directory number, suffix. A SPID is sometimes required in order to use ISDN with certain service providers. This is based on the type of ISDN switch used.

Call Setup

1. SETUP
2. CALL PROCEEDING
3. ALERTING
4. CONNECT
5. CONNECT ACK

Call Teardown

1. DISCONNECT
2. RELEASE
3. RELEASE COMPLETE

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Cisco ISDN configuration (IOS)

Isdn Network Topology