Introduction.
 
    In earlier days, telephone switching and trasmission was analog in nature. With the evolution of digital technology, computers changed this and it led to the digital networks. Today multiple digital networks are deployed, providing specific services. The digital technology not only limited to telephone networks, but has found its way in business and residential environments as well.

Service Requirements.
The minimum  abilites that are expected from today's ISDN network are :
 * Handle voice, video, audio, interactive or bulk data and full-motion video
 * Allocation of bandwidth on demand
 * Provide low bit error rates, low delay, low nondelivery rates
 * Provide various levels of security
 * Handle various range of transmissoin speeds and call durations

ISDN Services.

An ISDN user device obtains a network connection by requesting service over the D-channel. The requested message over the D-channel contains a set of parameters identifying the desired service. ISDN services are categorized based upon their scope and the source of the service.
 

  The Services are
 1.  Bearer services.
 2.  Teleservices.
 3.  Supplementary services.

Bearer services : are those which allow user to send information from one device on the network to another. They allow information transfer and involve the lower 3 layers of the OSI model, depending on the service. Users may agree to use any of the higher layer protocols and is transparent to ISDN.

Teleservices : are value added services provided by network, can provide end-to-end services and are charecterised by their lower layer attributes and higher layer attributes. Examples are teletex, videotex etc.

Supplementary services : are enhancements to bearer services or offer facilites before or after bearer services; they are considered call feautres. They include capabilites such as call forwarding, call transefer, 3-way conferencing etc.
 

1.1  BEARER SERVICES AND ATTRIBUTES

Bearer services supported by ISDN are described in ITU-T recomendations I.210 and I.230. They are defined in terms of access, information and general attributes. Access attributes are those charecteristics describing how a user accesses network functions. Information transfer attributes are those which tells about the charecteristics of transfer of information across networks. General attributes describe other charecteristics of the service, such as supplementary services, quality-of-service parameters and internetworking.
 
 

1.1.1  Information Transfer attributes

There are roughly six information transfer attributes. They describe the charecteristics of the connection between users across ISDN.

a) Information transer mode:- There can be three modes circuit, packet and the frame mode. Circuit mode is generally used for delay-sensitive applications.
b) Information transfer rate:- It is the throughput required for the requested connection across the network. Information rates of 64,384,1536 Kbps correspond to B, H0, and H11 channels respectively.
c) Information transfer capability:- refers to type of information that is being transferred across the network. This attribute provides information about the requirements that users are requesting from network regarding information transfer. Example audio,video, UDI, etc.
d) Establishment of communication:- This attribute describes when the requested service should be granted. Possible values are "Demand", "Permanent" and "Reserved".
e) Symmetry:- This tells whether the flow of information is one way or two way. It also describes about whether the speed in both directions is the same or not.
f) Communication Configuration:- attribute describes the configuration of the connection between the end users of this service. Point-point service involves two users, such as a simple telephone call. Multipoint describes services initiated by one user to several other users, such as a conference call.
 
 

 
1.1.2  Access Attributes

They describe characteristics of connection between the user and a network. These attributes provide information about the type of channel and protocols that a user will employ to access the desired network service. The first attribute is access channel and rate over which user access to network should be granted. User may request service on either a D, H or a B channel. The access protocols describe which particular protocol may be used for the requested service. They may be either signalling access protocols or information access protocols.

1.1.3  General attributes and Supplementary Services :

General attributes further specify characteristics of the individual bearer services, such as quality of service, internetworking possiblities etc. Supplementary Sevices allow the network to provide the user with more dynamic and flexible control of how they use the network, above and beyond the mere transportation of bits. They are described in ITU-T Recommendations I.250. Few of them are briefly described below :-

* Calling Line Identification: Delivers the calling party's ISDN number to the called party.
 
* Malicious Call Identification (MCI): Allows the called party to identify the calling party in those cases of crank or other malicious calls.
 
* Call Transfer(CT): Allows user to transfer an established call to third party

* Call Hold (CH): Allows a user to interrupt communications on an existing call and then subsequently reestablish the connection.

* Three-Party Service (3PTY): Allows a user to place an active call on hold and  place a new call to a third party. The user can then switchback and forth  between two calls, join the calls together to form 3-way conversation.
 
* Reverse Charging (RC): Provides a mechanism so that the calling party may request that the charges be made to the called party.
 
 
 

1.2   TELESERVICES

Teleservices are Value-Added-Services(VAS) that may be provided on the top of the bearer services and are described in the I.240-series recommendations. These services are end-to-end higher layer services (Layers 4 to 7 of OSI). The services may be offered by either the user or by the network.In the former case the network merely transports the bits; in the later case the network provides VAS directly. Examples of teleservices are teletex, videotex etc.

1.3  BROADBAND SERVICES

Broadband ISDN services are those that require speed greater than what can be delivered from a single PRI. These services include all of telephone and conversational messaging and retrieval services. The conversational services provide two-way, real-time, end-to-end transfer of information between two users. Possible applications are videotelephony, high-speed LAN, broadband video conferencing. The other type of B-ISDN service is called distribution service and examples of this type are High-Definition TV, broadcast videography, broadcast video etc.