Delay and Response Times

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Delay and Response Times

THE FACTS - In interactive satellite networks, propagation or path delay has presented unique problems for voice communications and earlier generations of data protocols. These problems have been successfully overcome by the use of equipment and protocols specifically suited to the requirements of satellite transmission.

DELAY - Satellite path delay is approximately 270 milliseconds (the time required for the signal to travel 35,800 km into space and return). If associated signal processing time through baseband equipment is included, total path delay is closer to 320 milliseconds.

In voice communications, the most noticeable effect of path delay has been echo.

ECHO - Echo results from the imbalance when two-wire twisted pair local telephone circuits connect to a four-wire long distance transmission line, causing part of the transmitted signal to return along the line from the far end of the circuit. In land-based networks, echo is less noticeable due to the shorter delay time for the returning echo signal, typically in the order of 50 milliseconds. However, in satellite networks the echo returns to the sender's handset about half a second after transmission.

Digital echo cancellers are now used to eliminate echo by storing a replica of the digitized voice and superimposing the replica in inverted form over the returning echo. The echo problem will disappear entirely when integrated digital services are adopted throughout the telephone industry - since integrated digital networks require four wire circuits throughout which eliminate unbalanced lines.

In data communications, satellite propagation delay has prevented use of older binary synchronous or block-by-block protocols. Protocols such as IBM Bisynchronous Link Control and ISO Basic Mode operate at throughput efficiencies of less than 50 per cent over satellite, since the delay time in receiving an acknowledgment is comparatively long--particularly when small block sizes are used.

THE SOLUTION - The answer to propagation delay for satellite data networks is found in the use of advanced protocols and/or delay compensators. They provide acknowledgments locally before data is transmitted over the satellite, thus eliminating the lag time for protocol handshakes. The new generation of Very Small Aperture Terminal (VSAT) earth stations and some multiplexers have built-in delay compensators and protocol convertors known as PADs (Packet Assembler-Disassembler) which perform handshakes locally and convert protocols to satellite-efficient versions.

Advanced protocols such as High Level Data Link Control (HDLC) and Synchronous Data Link Control (SDLC), use a technique known as continuous Automatic Retransmission Request (ARQ) that allows several blocks of data to be sequentially transmitted in a "window" with only one acknowledgment required. In terrestrial application of these advanced protocols, a 7-block window is most commonly used. The International Standards Organization (ISO), however, has standardized a window size of 127 block specifically for satellite networks.

Two other requirements are necessary to push satellite throughput efficiency up to the level demanded for most data communications networks. First, bit error rate (BER) must be low enough to limit retransmission of data blocks to the smallest attainable number. The satellite transmission medium itself presents a very acceptable channel error environment. Telesat's Tl transmission service. for example, has a threshold BER of 1 in 10

, with a typical BER of 1 in 10

Secondly, block sizes should be in the order of 10 kilobits for 7-block windows, but can be a full magnitude lower for 127-block windows, and still achieve a throughput efficiency comparable to landline networks. In fact, throughput efficiencies approaching 100 per cent are achievable with satellite networks depending on transmit speed, block size and BER.

Propagation delay does not pose any insurmountable problems in the design and operation of a satellite data network - except when attempts are made merely to replace one transmission medium with another.

DESIGN CONSIDERATIONS - In designing a satellite network, consideration must be given to the most appropriate protocol for the satellite environment as well as other factors (such as local polling rather than remote polling on multi-drop networks) that promote the efficiency of satellite communications. Companies with a major investment in one particular protocol can utilize the techniques of protocol spoofing available with advanced MUXES, PADS, and VSAT terminals.

When these considerations are addressed in tandem with the many strategic advantages of satellite networks (such as distance insensitivity, capacity and flexibility), the costs and benefits of satellite communications can be more clearly weighed.

Even with propagation delay, satellite communications can actually improve response times in some instances and offer far less variance in average response time. In fact satellite propagation delay is often no more pronounced than the internal network delay in terrestrial public packet switched data networks.