whai is ISDN ?? Part 2

This architecture provides a

 number of integrated services currently provided by separate networks.

ISDN adds capabilities not found in standard phone service. The main feature is that instead

of the phone company sending a ring voltage signal to ring the bell in your phone, it sends

a digital package that tells who is calling (if available), what type of call it is

(data/voice), and what number was dialed (if multiple numbers are used for a single line).

ISDN phone equipment is then capable of making intelligent decisions on how to answer the

call. In the case of a data call, baud rate and protocol information is also sent, making

the connection instantaneous. [Griffiths] ISDN Concepts:

 With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth

 of 64 kbit/s each. A delta channel (D channel) handles signalling at 16 kbit/s or 64

 kbit/s. H channels are provided for user information at higher bit rates. [Stallings] There

 are two types of ISDN service: Basic Rate ISDN (BRI) and  Primary Rate ISDN (PRI).

BRI: consists of two 64 kbit/s B channels and one 16 kbit/s D channel for a total of 144

kbit/s. The basic service is intended to meet the needs of most individual users. PRI:

intended for users with greater capacity requirements. Typically the channel structure is 23

B channels plus one 64 kbit/s D channel for a total of 1.544 Mbit/s. H channels can also be

implemented: H0=384 kbit/s, H11=1536 kbit/s, H12=1920 kbit/s. [Stallings]

 In this paper, I will concentrate on defining the specifics of Basic Rate ISDN for local

 loop transmission.  I will provide an in depth view of ISDN as it relates to layer 1 to 3

 of the seven layer OSI model.  I will also provide the specification for communication at

 the S/T customer interface.

Basic Rate ISDN:

Basic Rate Interface (BRI) - The BRI is the fundamental building block of an ISDN network.

It is composed of a single 16 kbit/s "D-channel" which is used for call setup and control

and two 64 kbit/s "B-channels". The B-channels can be used to carry voice and both circuit

mode and packet mode data traffic. The D-channel may also be used to carry X.25 packet

traffic if the network supports that option. [Griffiths]

Basic Rate Interface D Channel - In the analog world, a telephone call is controlled

in-band.  Tones and voltages are sent across lines for signalling conditions.  ISDN does

away with this.  The D channel becomes the vehicle for signalling.  This signalling is

called common channel since a separate channel for signalling is used by two or more bearer

channels. [Hopkins]

 User - Network protocols define how users interact with ISDN networks.  Between the user

 equipment and network equipment is a set of defined interfaces. The U interface is between

 the central office and the customer premise.  This interface carries information on the

 twisted pair of wires between the customer and the central office.  At the S/T interface

 located at the customer location, two pairs of wires (one for transmitting, one for

 receiving) are used.  The intermediate device between the U and the S/T interface is known

 as an NT1.  The NT1 is a hybrid that converts from four wire to two wire and also

 transforms the 2B+D signal into a different bit stream format. [Griffiths]

ISDN and the OSI Model - The OSI (Open Systems Interconnect) seven layer protocol was

developed to promote interoperability in the data world.  ISDN, which followed OSI, was

designed to be a network technology inhabiting the lower three layers of the OSI model.

Consequently, an OSI end system that implements an OSI seven layer stack can contain ISDN at

the lower layers.  Also, services such as TCP/IP (Internet Transmission Control Protocol)

can use the ISDN network.