INTEGRATED SERVICES DIGITAL NETWORK (ISDN)

Most digital data processing devices have limited data transmission capability. They generate a simple digital signal and mostly for a shorter distance and limited data transmission.

In most cases, the communication devices such terminal or computers are not directly attached to the transmission or networking devices.

In communication, the equipment consisting of digital end instruments that convert the user information into data signals for transmission, or reconvert the received data signals into user information such computers and terminals are generally referred to as Data Terminal Equipment (DTE).

The equipment that provides the signal conversion and coding between the DTE and the transmission medium is known as Data Circuit-terminating Equipment (DCE). The DCE may be separate equipment or an integral part of the DTE or of intermediate equipment. The DCE may perform other functions that are normally performed at the network end of the line. The DTE makes use of the transmission system through the mediation of this DCE. Example of the DCE is a modem.

Figure 1: Data communication interfacing

The DCE is responsible for transmitting and receiving bits one at a time over a transmission medium or network. On the other side the DCE must interact with the DTE. In general, this requires both data and control information to be exchanged. This is done over the set of wires known as interchange circuits. To achieve this, the high degree of cooperation is required.

To facilitate the task of data processing equipment for manufacturers and users, standards have been developed that specifies the exact nature of interface between the DCE and the DTE. Thus, the ISDN is one of the existing physical interfaces.

ISDN was developed by ITU-T in 1976. It is a set of protocols that combines digital telephony and data transport services. The whole idea is to digitize the telephone network to permit the transmission of audio, video and text over existing telephones lines.

ISDN is an effort to standardize subscriber services, provide user/network interface and facilitates the internetworking capabilities of existing voice and data networks.

The goal of ISDN is to form a wide area network that provides universal end-to-end connectivity over digital media. Integrating all the separate transmission services into one without adding new links or subscriber lines can do this.

What is ISDN?
ISDN is an acronym for Integrated Services Digital Network. A high-speed digital communication network evolving from existing telephone services. The goal in developing the ISDN was to replace the current telephone network, which requires digital-to-analog conversions, with facilities totally devoted to digital switching and transmission, yet advanced enough to replace traditionally analog forms of data, ranging from voice to computer transmission, music and video.

ISDN is available in two forms known as: -

1. BRI (Basic Rate Interface)
2. PRI (Primary Rate Interface)

The BRI consists of two: -

• B (bearer Channel) that carries data at 64 Kbps.
• D (Data Channel) that carries control and signal information at 16 Kbps.

SERVICES:
The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into three categories:

1. Bearer Services
2. Teleservices and
3. Supplementary services

Figure 2. ISDN services

Bearer services
Bearer services provide the means to transfer information (voice, data and video) between users without the network manipulating the content of the information. The network does not need to process the information and therefore does not change the content. In the OSI model, Bearer services belongs to the first three layers and are well defined in the ISDN standard.

Teleservices
In teleservicing, the network may change or process the contents of the data. These services correspond to layers 4 to 7 of the OSI model. Teleservices rely on the facilities of bearer services and are designed to accommodate complex users needs without the user having to be aware of the details of the process. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing. Although the ISDN defines these services by names, they have not yet become standards.

Supplementary services
Supplementary services are those services that provide additional functionality to the bearer services and Teleservices. Examples of these services are reverse changing, call waiting and message handling all familiar from today's telephone company services.

SUBSCRIBER ACCESS TO THE ISDN
To allow flexibility, digital pipes between customers and the ISDN office (the subscriber loops) are organized into multiple channels of different sizes. The ISDN standard defines three channel types each with a different transmission rate: -

1. Bearer Channel
2. Data Channel
3. Hybrid Channel

B Channel
It is defined at a rate of 64 Kpbs. It is a basic user channel and can carry any type of digital information in full-duplex mode as long as the required transmission rate does not exceed 64 Kbps.

D Channel
It can be either 16 or 64 Kbps depending on the needs of the user. Although the name says data, the primary function of a D Channel is to carry control signaling fro B Channels.

H Channels
The Hybrid Channels are available with data rates of 384 Kbps (H0), 1536 H11), or 1920 (H12). These rates suit H channels for high data rate applications such as video, teleconferencing and so on.

PRI - Primary Rate Interface
The usual PRI specifies a digital pipe with 23 B channels and one 64 Kbps D Channel. See figure below: - Figure 3. PRI

Functional grouping
In the ISDN standard, the devices that enable users to access the services of the BRI or PRI are described by their functional duties and collected in functional groupings. Subscribers choose the specific devices best suited to their needs from these groupings. The ISDN defines only the functional behavior of each group. The standard does not say anything about implementation. Each functional grouping is a model that can be implemented using devices or equipment chosen by the subscriber. Functional groupings used at the subscriber's premises include network terminations (type 1 and 2), terminal equipment (type 1 and 2) and terminal adapters.

Network Terminal 1 (NT1)
This device controls the physical and electrical termination of the ISDN at the user's premises and connects the user's internal system to the digital subscriber loop. NT1 organizes the data streams from a connected subscriber into frames that can be sent over the digital pipe, and translates the frames received from the network into a format usable by the subscriber's devices. To this end it performs the basic multiplexing functions of byte interleaving, but it is not a multiplexer. An NT1 synchronizes the data stream with the frame-building process in such a way that multiplexing occurs automatically.

Network Terminal 2 (NT2)
This device performs functions at the physical, data link and network layers of the OSI model (layers 1 2 and 3). They provide multiplexing (layer 1), flow control (layer 2) and packetizing (layer 3). NT2 provides intermediate signal processing between the data-generating devices and NT 1. The NT 1 is still required to provide a physical interface to the network. There must a point-to-point connection between an NT2 and an NT1. The NT2 are used primarily to interface between a multi-user system and NT1 in PRI.

NT2 can be implemented by a variety of equipment types i.e., PBX can be NT2; it coordinates transmission from a number of incoming links (user phone lines) and multiplexes them to make them transmittable by an NT1. LAN can also function as an NT2.

Terminal Equipment 1 (TE1)
The terminal equipment is used by the ISDN standard to mean the same things as DTE in other protocols. It refers to digital subscriber equipment. TE1 is any device that supports the ISDN standards like digital telephones, integrated voice/data terminals, and digital facsimiles

Figure 4: TEs and NTs

Terminal Equipment 2 (TE2)
The ISDN standard defines a second level of terminal equipment called terminal equipment 2 to provide backward compatibility with a customer's equipment. TE2 is any non-ISDN device such as a terminal, workstation, host computer or regular telephone.

Terminal adaptor (TA)
TA converts information received in non-ISDN format from a TE2 into a format capable of being carried by the ISDN.

THE ISDN LAYERS
It is difficult to apply the simple seven-layers architecture specified by the OSI to the ISDN. One reason is that the ISDN specifies two different channels (B and D) with different functionalities. B Channels are for user-to user communication (information exchange). D Channels are predominantly for user-to- network signaling. The subscriber uses the D channels to connect to the network, then the B channels to send information to another user. These two functions require different protocols from each other at many of the OSI layers.

The ISDN also differs from the OSI standard in its management needs. A primary consideration of the ISDN is global integration. Maintaining the flexibility required to keep the network truly integrated using public services requires a great deal of management.

For these reasons, the ITU-T has devised an expanded model for the ISDN layers. Instead of a single seven-layer architecture like OSI, the ISDN is defined in three separate planes:

1. The User plane
2. The Control plane
3. The management plane

All three planes are divided into seven layers that correspond to the OSI model.

At the physical layer, the B and D channels are alike. They use either the BRI or PRI interfaces devices.

At the data link layer, the B channel uses LAPB or some version of it.

At the network layer, the B channel has many options. B channels (and D channels acting like B channels) can connect to circuit-switched networks, packet-switched networks (X.25), Frame Relay networks and ATM networks, among others.

The user-plane options for layers 4 through 7 are left to user and are not defined specifically in the ISDN.

Simplified layers of ISDN

	B Channel		D Channel
Layers 4, 5, 6 and 7	User's choice		End-to-end user signaling
Network	X.25 and other		Call control Q.931
Data link	LABP and others		LAPD
Physical	BRI (I.430) and PRI (I.431)

Physical Layer
The ISDN physical layer specifications are defined by two ITU-T standards: I.430 for BRI access and I.431 for PRI access. These standards define all aspects of the BRI and PRI. Of these aspects, four are of primary importance:

• The mechanical and electrical specifications of the interfaces R, S, T and U.
• Encoding
• Multiplexing channels to make them carriable by the BRI and PRI digital pipes
• Power supply.

Physical layer specifications for BRI
R Interface
The R interface is not defined by ISDN. A subscriber can use any of the EIA standards such as EIA-232 or V such V.24 or X series such X.21.

S Interface
The ITU-T specifies the ISO standard, ISO 8887. This standard calls for four-, six-, or eight-wire connections. At least four wires are necessary to support full-duplex communication over every B and D channel. The plugs for these connections, along with the electrical specifications for each wire are shown below: -

Figure 5. S Interface

The power can come from a battery or power outlet, or it can come from the ISDN center to the NT. In this case, only four connections are needed to connect the TE and NT (wires c, d, e, and f)

S Interface pins

Name	TE	NT
a	Power source 3	Power sink 3
b	Power source 3	Power sink 3
c	Transmit	Receive
d	Receive	Transmit
e	Receive	Transmit
f	Transmit	Receive
g	Power sink 2	Power source 2
h	Power sink 2	Power source 2

U Interface
For the U interface (digital subscriber or local loop), the ITU-T specifies a single-pair twisted-pair cable in each direction.

Physical connection:
The physical connection is made between terminal equipment (TE) and network-terminating equipment (NT). The physical connection defined in ISO 8877, specifies that the NT and TE cables shall terminate in matching connectors that provide for eight contacts.

In the ISDN diagram, the two pins are used to provide data transmission in each direction. These contact pins are used to connect twisted pairs leads coming from the NT and TE devices. Since there are no specific functional circuits, the transmit/receive circuits are used to carry both data and control signals. The control information is transmitted in the form of messages.

The specification provides for the capability to transfer power across the interface. The direction of power depends on the application. I a typical application, it may be desirable to provide for power transfer from the network side toward the terminal to, that, maintain a basic telephony service in the event of failure of the locally provided power. This power transfer can be accomplished using the same leads used for digital signal transmission (c, d, e, f) or on additional wire, using access leads g – h. the remaining two leads are not used in the ISDN configuration but may be useful in other configurations.

ELECTRICAL SPECIFICATION
The ISDN electrical specification dictates the use of balanced transmission. With balanced transmission, signals are carried on a line, such as twisted pair, consisting of two conductors. Signals transmitted as a current that travels down one conductors and returns on the other, the two conductors forming a complete circuit.

For digital signals, this technique is known as differential signaling, as binary value depends on the direction of the voltage difference between the two conductors.

Unbalanced transmission, which is used on older interfaces such as EIA-232, uses a single conductor to carry the signal, with ground providing the return path.

The balanced mode tolerates more, and produces less noise than unbalanced mode. Ideally, interference on a balanced line will act equally on both conductors and not affect the voltage difference. Because unbalanced transmission does not possess these advantages, it is generally used only on coaxial cable. When it is used on interchange circuits, such as EIA-232, it is limited to very short distance.

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