ISDN Stands for Integrated Service Digital Network, and As the Name Suggests It Allows

ISDN

Introduction

ISDN stands for Integrated Service Digital Network, and as the name suggests it allows digital communication. This is favourable as digital technology is a lot faster, and more accurate than the old analogue lines as they no longer require the process of modulation and demodulation. ISDN relies on already existing copper cable systems, causing it’s integration into our existing communications system to be smoother and less disruptive.

According to ITU-T(formerly CCITT) "an ISDN is a network, in general evolving from a telephony IDN, that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have addressed by a limited set of standard multi-purpose user-network interfaces." The key point of this definition is the ability to support voice services adequately, this has not been achieved using any other concept. ISDN has four major aspects; telephone network; integrated services; digital; network.

The concept of the Integrated Services Digital Network (ISDN) originated from the desire to provide integrated services through better use of the bandwidth potential in existing telecommunications subscriber loops (loop plant). Advances in VLSI technology and digital signal processing have made high rate digital transmission to the subscriber's premises not only possible, but also economical. ISDN is the rationalization of the architecture of a telecommunications network to support and utilize this improved capability. One of the cornerstones of the ISDN concept is international compatibility. The CCITT has addressed this requirement by defining a rigid structure for subscriber access to an ISDN, at several reference points. A corresponding set of specifications has been generated facilitating the development of generic subscriber interfaces to ISDN's for all applications. The acceptance and timely implementation of ISDN’s will be enhanced by the ubiquitous connectivity provided by such standardized interfaces. Further, the economics of scale and competition will reinforce this trend through reduced costs of implementation. Figure 1 shows the ISDN User Network Access Model.

ISDN Protocol Architecture

ISDN components include terminals, terminal adapters (TA), network-termination devices, line-termination equipment and exchange-termination equipment.

·  ISDN terminals come in two types:

·  Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1).

·  Non - ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2).

TE1 are connected to the ISDN network through a four-wired twisted-pair digital link.

TE2 are connected to the ISDN network through a terminal adapter.

The ISDN TA can either be a stand-alone device or a board inside TE2. If implemented as a stand-alone device, the TE2 is connected to the TA via a standard physical layer interface
(for example, EIA232, V.24 or V.35).

Beyond the TE1 and TE2 devices, the next connection point in the ISDN network, is the NT1 or NT2.
These are network-termination devices that connect the four-wired subscriber wiring to the conventional two-wired local-loop.

In North America, the NT1 is a customer premises equipment(CPE) device.

The NT2 is a more complicated device, typically found in digital private branch exchange (PBXs), that performs layers 2 and 3 protocol functions and concentration services.

An NT1/2 device also exists .
It is a single device that combines the functions of an NT1 and an NT2.

A number of reference points are specified in ISDN.
These reference points define logical interfaces between functional groups such as TAS and NT1s.
ISDN reference points include R (the reference point between non-ISDN equipment and a TA), S (the reference point between users' terminals and the NT2), T (the reference point between NT1 and NT2 devices) and U (the reference point between NT1 devices and line-termination equipment in the carrier network).

The U reference point is relevant only in North America, where the NT1 function is not provided by the carrier network.

A sample of ISDN configuration is shown in the next figure. This figure shows three devices attached to an ISDN switch at the central office.
Two of these devices are ISDN - compatible, so they can be attached through an S reference point to NT2 devices.
The third device (a standard non - ISDN telephone) is attached through the R reference point to a TA.

Any one of those devices could also be attached to an NT1/2 device, which would replace both the NT1 and the NT2. Similar users' stations are attached to the right ISDN switch (although not shown).

LAYERS

LAYER 1

ISDN physical-layer (Layer 1) frame formats that differ depending on whether the frame is outbound (from a terminal to a network) or inbound (from a network to a terminal).
Both physical-layer interfaces are shown in the next Figure.

The frames are 48 bits long, of which 36 bits represent data.
The F bits provide synchronization.
The L bits adjust the average bit value.
The E bits are used for contention resolution when several terminals on a passive bus contend for a channel.
The A bit activates devices.
The S bits have not yet been assigned.
The B1, B2 and D bits are for users' data.

·  F = Framing bit

·  L = Load balancing

·  E = Echo of previous D bits

·  D = D channel (4 bit x 4000 frames/sec = 16 Kbps)

·  A = Activation bit

·  S = Spare bits

·  B1= B1 channel bits

·  B2= B2 channel bits

Multiple ISDN user devices can be physically attached to one circuit.
In this configuration, collisions can occur if two terminals transmit simultaneously.

ISDN therefore provides features that determine link contention.
When an NT receives a D bit from the TE, it echoes it back in the next E bit position. The TE expects the next E bit to be the same as its last transmitted D bit.

Terminals can not transmit in the D channel unless they first detect a specific number of "1" bits (indicating "no signal") corresponding to a pre-established priority.
If the TE detects a bit in the echo (E) channel, that is different from its D bit, it should stop transmission immediately.

This simple technique ensures that only one terminal will transmit its D bit at a given time.
After a successful D message transmission, the terminal's priority is reduced by requiring it to detect more "1" bits in a row before transmitting.

Terminals may not raise their priority until all the other devices on the line have had an opportunity to send a D message.
Telephone connections have higher priority than all other services, and signaling information has a higher priority than non-signaling information.

LAYER 2

Layer 2 of the ISDN signaling protocol is Link Access Procedure, D channel, also known as LAPD.

LAPD is similar to High-level Data Link Control (HDLC) and Link Access Procedure Balanced (LAPB) .

As LAPD's acronym indicates, it is used across the D channel to ensure that control and signaling information flows and has been received properly.

LAPD's frame format is very similar to that of HDLC, as shown in the next figure, and like HDLC it uses supervisory, information and unnumbered frames.

The LAPD protocol is formally specified in CCITT Q.920 and CCITT Q.921.

·  SAPI = Service Access Point Identifier (6 bits)

·  C/R = Command / Response bit

·  EA = Extended Addressing bits

·  TEI = Terminal End-point Identifier.

The LAPD flag and control fields are identical to those of HDLC.
The LAPD address field can be either one or two bytes long.
If the extended address bit of the first byte is set, the address is one byte long, otherwise, the address field's length is two bytes.

The first address field byte contains the Service access point identifier (SAPI) , which identifies the port at which LAPD services are provided to Layer 3. The C/R indicates whether the frame contains a command or a response.
The Terminal End-point Identifier (TEI), identifies either a single terminal or multiple terminals.
A TEI of all "1" indicates a broadcast.

LAYER 3

Two Layer 3 specifications are used for ISDN signaling: CCITT I.450 (also known as CCITT Q.930) and CCITT I.451 (also known as CCITT Q.931).
Together, these protocols support user to user, circuit switched and packet switched connections. A variety of call establishment, call termination, information and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS and DISCONNECT .

These messages are functionally similar to those provided by the X.25 protocol.

The next figure from CCITT I.451, shows the typical stages of an ISDN circuit switched call.

ISDN System Architecture

Digital Switching

Architecture

As can be seen from Figure 1, there are points intend-to-end transmission of information through an ISDN, where information must change from one type of transmission scheme to another. These points are at the Terminal Equipment

(TE),the Network Terminations (NT), the Loop Terminations(LT), the Exchange Termination (ET) etc., shown in Figure 1.Rather than create a different interface component for each unique type of carrier-to-carrier transition, it is more practical to design an interface between each type of transmission method and some intermediate format. T h e intermediate format called for is a common mechanism to transfer information between the interface devices. The ST-BUS, developed by Zar link Semiconductor, is a component interface designed

specifically for this purpose. This application note describes the ST-BUS structure as it applies to both the ISDN model and the ISDN

components developed by Zar link Semiconductor.

ISDN transmission Channels

Information travels on ISDN transmission media in the form of serial digital data

streams .Each information stream can support multiple communication channel s through Time Division Multiplexing (TDM). The CCITT has defined several types of information channels, each with a defined bit rate. Signalling information, telemetry information and low speed packet communications are carried on D

channels, which have a bit rate of 16 kilobit / s(D(16))and a bit rate of 64 kilobit/s (D(64)). Information that will be termed “user information” in this note , is carried on B- channel s , H0- channel s , H11-channelsand H12-channels. This information can be circuit switched data, packet switched data ,audio ,facsimile, telex or video information. The bi t rates for user information channels are as follows:- The interface between ISDN components must support al l the different

types of information channels. The component interface, therefore, must use TDM techniques, based on the 64kbi t /s channel rate common to al l channel types.

Multiple 64 kbit/s channels can be used to construct the higher bandwidth H0, H11 and H12-channels.At the 16kbit/s rate, the data fits into two bits of a 64

kbit/s channel. The maximum throughput supported by the current recommendations is 2048 kbit/s (32X64kbit/s channels), specified at the 2048 kbit/s S1 reference point (I series recommendation I.431). The capacity of the component interface should encompass the bandwidths required by all current reference point specifications, implying that the minimum number of 64 kbit/s channels supported by the component interface should be 32. This philosophy allows for maximum connectivity between all types of ISDN interface components.

ISDN Interfaces

The ISDN recommended standard provides a small set of compactable interface

which are intended to economically support a wide range of user applications.

The standard recognizes that different interfaces are required for applications

that take different information rates and requirements. Functional grouping are a

set of capabilities needed in an ISDN user access interfaces specific functions

Within a functional grouping may be performed by multiple pieces of equipment

and software .

Reference points are the points dividing the functional groups. Usually a

reference point corresponds to a physical interface between pieces of

equipment.

Functional groups

Access Points

The ISDN also provides access points. The definitions of the access points are as follows:-Access point1 and Access point3 are the access points for bearer services supported by the ISDN .Functional grouping NT1 include functions equivalent to physical layer of OSI reference model.

The NT1 can define the boundary of the carrier ISDN; It may be controlled by the network carrier. It provides the user a fixed standard interface into the ISDN. The NT2 functions are equivalent to the physical layer and higher layers of the OSI model. Example for an NT2 functions are telephone branch exchange, local area networks and terminal or cluster controllers. In other words NT2 functions as an end user equipment terminates into the NT2 through an S connection reference point.

The TE functions represent the end user equipment (DTES). They include not only this, but other devices such as end user digital telephones and integrated workstations found in offices. ISDN defines two types of TES (Terminal Equipment Type) that works with the ISND network using an ISDN interface.

The TE2 function requires the more conventional interface such as E1A-232-D , one of the V or X series standards.

The TA (Terminal Adapter) is actually a protocol converter which changes existing interface such as E1A-223-D into the standard ISDN interface. The ISDN standard allows the 1A function to be combined with an end user DTE device as well. Its principle function is supporting an ISDN connection for a TE2 device.

BISDN

Narrowband ISDN has been designed to operate over the current communications infrastructure, which is heavily dependent on the copper cable. B-ISDN however, relies mainly on the evolution of fiber optics. According to CCITT B-ISDN is best described as ‘a service requiring transmission channels capable of supporting rates greater than the primary rate.’ Behind this statement lies the plan for a network and services that will have far more impact on the world we know today, than ISDN ever would.

ISDN is Telephone Network

When ISDN is referred to as a network it is to be considered a telephone network, not a computer network. Broadband ISDN allows its users to communicate over high speed, high quality digital channels. The media is supports include Telex, fax, voice telephone, video telephone, audio, high definition TV and computer networking.

ISDN is Integrated Services

In the past video, audio, voice and data services needed different types of communication channels. One of the main advantages of ISDN is the ability to integrate these features over the same network and cable plant. Not only is this possible using ISDN technology but the quality of the transmission is better also. In the past four networks were needed and video was distributed on coaxial lines, audio over balanced lines, voice used copper cable pairs and data services required coaxial or twisted pair cables. Using one network allows reductions in installation costs, as well as easier installation. Other features available include demand networking, automatic bandwidth and on the fly connectivity. Advances in the services available are due to ISDN being digital.