Chapter 8: ISDN & Transaction Processing

X.25 & ISDN

Packet today has become the almost perfect vehicle for extending and exploiting the power of ISDN. It offers:

Dedicated user dial-out capabilities.
Because most ISDN telephones and terminal adapters contain PADs, virtually every ISDN telephone, PC or data terminal can be a dedicated host station on the NYNEX X.25 network. It can direct packet messages to - and receive messages from - virtually any location on any public packet network anywhere in the world.

Fast connections.
Because D-channel devices are linked directly to the X.25 network, there is no dialing into modem pools, no waiting for a connection. Credit authorizations that took 12-45 seconds on analog lines are now completed in 2-7 seconds.

Low access costs.
Packet network access costs from $2-4 a month for each D channel, and up to eight primary devices can be linked to an ISDN BRI . In addition, several "access controllers" exist that let multiple point-of-sale terminals send and receive through the D channel simultaneously.

Low transmission costs.
Transmission costs are based on the amount of data transmitted, not time or distance. A typical credit-card authorization, for example, involves less than 50 characters of data, and costs about a penny.

More than adequate speeds.
A typical POS transaction contains less than 100 characters. At 9.6Kbps, or about 1,200 alphanumeric characters a second, most are transmitted in less than a tenth of a second.

Almost perfect accuracy.
POS devices face hazards that range from rain at a gas pump to spilled soda on a check-out card-swipe. Thus the error-checking and correction capabilities of X.25 still represent a major advantage.

Simple host connections.
A single ISDN B channel can support hundreds of X.25 connections to a bulletin board, information service, database or transaction system.

Simplified host equipment needs.
A host's permanent B-channel packet link to the X.25 network can totally eliminate the need for modems at the host site.

All this and telephones, too.
While transactions take place on the D channel, B channels of a BRI can simultaneously carry telephone and fax calls, video meetings, and more.