Products for ISDN technology from different vendors even with similar features and options may create some compatibility issues. CCITT after good deliberations over the years published the first significant ISDN standards in a number of red binders in 1984 and they were simply known as the Red Book standards. The group subsequently met four years later which culminated in the publication of the 1988 Blue Book standards. These international publications were the foundation for the evolving ISDN national standards. The CCITT eventually was reformed into the group, which is now called the ITU- T. The standards used to define ISDN make use of the OSI reference model with the first three layers of this OSI reference model.
There are two standard ISDN connectors:
- For accessing basic rate ISDN, a RJ-45type plug and socket (similar to a telephone plug) is used using unshielded twisted pair cable.
- Access to primary rate ISDN is through a coaxial cable.
The ISDN passive bus, which can be a maximum of I km in length, is a cable in user premises. It enables up to eight user devices to be attached to the basic rate ISDN interface. Since there are only two B-channels, only two of the eight devices can communicate at anyone time. For this reason, each device must contend for access to the passive bus.
The equipment available for ISDN includes:
- Terminal Adapters
- ISDN internal computer Terminal Adapter cards
- Video Conferencing PC cards, and
- LAN access gateways or bridges some of which are based on PC cards or stand alone boxes.
It is possible to avoid all the problems of API standards for internal computer adapters by using an external ISDN Terminal Adapter. Since the speed of most serial ports on a PC has been limited to about 19.2Kbps until recently, this approach has not been viable. However, recently, internal PC cards, which will work asynchronously up to 115Kbps, have appeared, which could have applications in multimedia work when used with an appropriate external Terminal Adapter.
ISDN is accessed through one of two services, named by the CCITT as Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides 144Kbps using the existing twisted pair copper telephone cable.
BRI includes 2B channels and I-D channel. This may be written as 2B+D. B channel (bearer) provides 64 Kbps data transmission and can carry voice or data. D channel (Delta) operates at 16 Kbps and is used for control, i.e. for signaling between user devices and the ISDN. Therefore, the total data rate of BRA is therefore 144 Kbps. Audio digitized using pulse code modulation (PCM).
PRI can carry 30 independent voice or data channels, each at 64 Kbps. The structure has a 64 Kbps D-channel for singling between devices and the network, and a 64 Kbps channel for synchronization and monitoring. The total data rate of PRI is 2.048 Mbps. PRI is popularly referred to as '30B+D' because of the number of B-channels and D-channels, or 1.421 because of the CCITT recommendation from which it is taken. This form of access is primarily intended for use in situations which require a large transmission capacity, such as when organizations make voice and data calls through an Integrated Services PBX.
ISDN Internetworking Equipment
There are different internetworking devices that can use ISDN:
- Terminal Adapters (TAs) these are external devices that connect a conventional data interface, such as X.21, to an ISDN circuit, allowing non-ISDN equipment to use the ISDN. Terminal adapters are widely used by internetworking manufacturers who do not have an approved native ISDN interface for their devices.
A disadvantage of this solution is that not all the information from the D-channel passes through the TA, so the non-ISDN equipment cannot take full advantage of ISDN facilities, such as Calling Line Identification.
- ISDN Bridges Because of its simplicity, bridging is one of the most popular ways of linking LANs. The big problem with ISDN bridging is controlling the bridge's use of the ISDN network. Bridges are simple to set up and use because they will forward data, such as broadcasts, by default. Over ISDN, this means that calls will be made to send largely unnecessary data. Over a period of time, this can prove very expensive.
To avoid this, bridges can be configured to block broadcasts from specific addresses, and to understand particular protocols. But the bridge loses its major benefit of simplicity. Bridges are ideal for ISDN backup.
- ISDN Routers Routing is a far more effective way to utilize ISDN for LAN internetworking. This is the approach being taken by the entire internetworking vendors. Data is only sent over the ISDN network when it is really needed. There are no unnecessary broadcast messages to transmit, so the bandwidth is used more efficiently than with bridges and the configuration can actually be simpler. Filters may be used to block out all unnecessary traffic.
- Quality ISDN connections are very low error rate digital pipes.
- Flexibility ISDN can be thought of as a configurable leased line. Connections can be established at any time between any two locations where ISDN is available. It offers a very fast (almost transparent) call set-up, so its dialup nature is transparent to most users.
- Economy ISDN is charged for rent like a telephone call. Usage costs are identical to the telephone service. In general, ISDN is extremely cost-effective for intermittent LAN to LAN connectivity.
- Availability ISDN is now becoming very widely available because the initiatives taken by the government of various countries.
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