Program Emulation Role in Satellite Delay Compensation.

The use of alternative communications techniques has significantly increased over the past two years. There are several reasons: the breakup of Ma Bell; the fear of dramatic increases in leased-line charges resulting from current investigations by the FCC and Bellcore; the fact that troubleshooting is much more difficult because of all the operating companies and vendors the user must go to; and the new regulatory and technology factors that are appearing with increasing frequency.

Today's business world relies on data communications. Businesses such as banks, insurance companies and human services--including police departments, travel agencies and commercial airlines--are especially dependent on data communications.

New operating concepts such as distributed data processing, local area networks, X.25 public data networks and satellite transmission have promoted the use of data communications and increased the demand for less-expensive and more-reliable facilities and equipment.

Satellite transmission is being used or contemplated by many companies as a method to reduce transmission costs, as a backup and as disaster recovery for prolonged terrestrial communications line outages. In some areas of the country, satellites are the only viable alternative to terrestrial communications lines.

If a corporation is going to use satellite communications it must first choose to contract with a common carrier and obtain site-to-site service, or contract with a satellite or transponder owner for a space segment and build and own the corporate ground segment. Transponders are divided into C-band and Ku-band. C-band is 4/6 Ghz and Ku-band is 12/14 GHz.

Data transmission by satellite, while fine for some companies, has been a disappointment for others--especially for those using delay-sensitive protocols such as IBM's 3270 Bisync protocol. This is due to the unavoidable effect of satellite propagation delay, the result of satellites orbiting 23,000 miles above the earth.

Some protocols can work with extensive delays. But in the case of IBM's 3270 Bisync, a positive-acknowledgement protocol, these delays can be overwhelming to the point of rendering multidrop lines useless.

Overcoming The Delays

The challenge to today's data communications managers and applications engineers is to understand the applications and effects of satellite transmission delays and the possible corrections.

To overcome inherent delay problems, some users may be able to migrate their network to a protocol that is more immune to the effects of satellite delays than 3270 Bisync.

X.25 and SNA/SDLC are well-suited for satellite transmission because they can support multiple outstanding messages.

These protocols use an acknowledgement sequence number, Nr and Ns, that starts at zero and can increment to seven before cycling to zero again. This is called "modulo-8." It can be expanded to "modulo-128," where 127 messages can be outstanding before cycling to zero. However, most of today's equipment can not handle modulo-128, and it is rarely used.

The fact is, most users already have a large installed base of equipment that cannot be converted. They may want to consider another alternative due to the expense of new equipment and the even greater expense and time delay of retraining users.

Delay-compensation devices are the logical and most cost-effective way to overcome propagation delay problems. One form of satellite delay compensation is program emulation, which provides local acknowledgement and response to the front-end processor and control units. This eliminates time-outs and efficiency loss due to satellite propagation delay.

Program emulation can be handled on a line-by-line basis using external individual "black boxes." But a much more organized and cost-effective way is through program emulation in networked statistical multiplexing, which provides for multiple lines and greater network efficiency.

Some statistical multiplexing devices also provide statistics, redundancy and other capabilities not available in the "black-box" solutions. For example, Infotron provides program emulation through what it calls "Bisync Emulation," and incorporates it into its 790 Network Concentrator and 990 Network Processor multiplexing products to compensate for satellite delays and to increase communications link efficiency. Total throughput can be increased by as much as 40 percent with the Bisync emulation feature due to bandwidth savings.

Bisync emulation removes non-productive polling traffic from the communications lines and handles all poll/select requests and acknowledgements locally. This can provide support for additional users without adding equipment, reduce the number of network facilities typically required to expand a system, compensate for satellite-caused delay time-outs and even decrease response time from the user's point of view.

to a user, response time is the amount of time the keyboard is locked. With local acknowledgement, the user's terminal is unlocked very quickly.

The Bisync emulation feature contines to provide all the error-control correction and diagnostics available on all standard IBM 3270 Bisync systems. Data compression is incorporated into the Bisync emulation feature to further reduce bandwidth usage and improve communications link efficiency.

Normal IBM 3270 Bisynchronous network operation controls the flow of data transmission by using polling schemes. Polling increases the amount of data traffic over a network because the control characters are transmitted in addition to the normal data-consuming bandwidth on the transmission facility. The Bisync emulation feature controls the flow in a Bisync environment and cuts down on the overhead.

All IBM 3270 and 790NC/990NP control and diagnostics remain intact, giving the user full data integrity.

A unique feature of the 790NC and 990NP is that they allow a user to combine async, Bisync, SDLC and other multiple-protocols in the same unit.

With the addition of the ANM800 network monitor, a user can accumulate statistics on the amount of compression in the network, peak and average characters per second, NAKs and other systemwide, technical-control network-management information.