Satellite Data Modems Offer Greater Networking FLexibility.
Growing customer demand for videoconferencing and high-speed data, the move toward integrated services digital networks (ISDN) and the search for increased flexibility is causing many large users to implement networking via satellite communication. For this reason the network-management challenge will grow exponentially as the quantity of terminals increases. It is therefore important to consider the features necessary to accomplish a degree of future network control.
Single-channel-per-carrier (SCPC) data modems use a frequency-division multiplex approach of allocating transponder bandwidth. At any one time, each data modem is assigned a different set of transmit/receive carrier frequencies. Spacing of the carriers depends on the modulation technique, the data rate and the amount of redundant bits allocated to forward error correction. The vast majority of systems use quaderature phase-shift keying (QPSK), 12-percent error redundancy and carriers spaced at 0.8 times the data rate (45 kHz for 56 kb/s).
Remote control of satellite modems can be accomplished by using the terrestrial telephone switched network or a supplementary low-speed channel to gather information from the modem or to issue networking commands.
To imagine a single control center managing a nationwide data network brings forth visions of a mountain of software. Indeed this can be true, however, the context here is merely a central location where at first the network controller will be a simple CRT terminal. The network elements would have the software humanized, with the use of menu screens, English commands and so forth. As the need increases, a personal computer can be substituted to automate the process.
Following is a list of network control and monitor functions intended to serve as an example of networking through satellites and how to relate these network requirements to product features.
* Carrier frequency--Allows a centrally stored pool of frequencies to be assigned in the network on a need basis, provided the modems are frequency-agile. Frequencies will be assigned and transmitted from the network control center (NCC) to a modulator (uplink) and a demodulator (downlink) to step up a half-duplex link. A second set of frequencies would be necessary to set up a full-duplex link.
* Transmit power level--Centrally controlling the transmit power allows maximum use of transponder capacity by setting power levels based on actual receive levels. Transponders are power limited rather than bandwidth limited. Different carriers tend to generate intermodulator products due to non-linearities in the satellite's traveling-wave-tube amplifiers. To reduce the interfering effects of intermodulation products, the aggregate transmit power is set below full power. Thus, it is important to transmit with the lowest transmit power and still meet the signal-to-noise objectives of the network. Thus, less uplink power results in lower recurring satellite costs.
* Eb/No Measurement--Measurement of signal-to-noise (Eb/No) ratio at the demodulator is the controlling element in determining the transmit power (see above) to achieve the signal-to-noise objective of the network. Eb/No measurements are conducted in real time at each demodulator and will be transmitted to the NCC. If Eb/No is below the network objective, commands to increase the uplink power will restore the proper link performance.
* Code Rate--Modern satellite modems use forward error-correcting codes to compensate for the very high path attenuation that gives rise to low signal-to-noise ratios and high bit-error rates. The amount of error correction relates to the amount of redundant information transmitted within the data. Hence, under severe signal-to-noise conditions, increasing transmit power will sometimes be insufficient and it will become desirable to increase the power of the forward error correction.
* Modulator 2047 encoder--This commands any modulator in the network to transmit the industry-standard 2047 test pattern. A test demodulator tuned to that frequency would be somewhere in the network or at the NCC. This allows routine testing of all modulators in the network.
* Demodulator 2047 decoder--In a similar manner, the NCC can instruct any demodulator to tune to the frequency of the modulator that is transmitting the 2047 pattern and to count errors in the pattern. This error-count register can then report to the NCC the quantity of errors, thus the bit-error rate of any link can be measured.
* Demodulator analog parameters--Remote sensing of the following parameters can be very useful when attempting to isolate faults. These parameters should be in digital form and in the proper engineering units: carrier frequency, input level, Eb/No, carrier frequency offset
* Alarm status--Any alarm conditions should be reported to the NCC and indicate at least the following: output-level failure, failure of modem-control processor, loss of input level, power-supply voltage variations and unlocked synthesizer. Additional Control Commands:
The following miscellaneous commands can assist in controlling the network: front-panel lockout, enable/disable RTS control of carrier, enable/disable scrambler, transmit one second of pure carrier and force one error in the 2047 test.
Although the procedures to control the behavior of a communications network are difficult to detail in advance, and should be developed in stages, preparation can be made. Even if network monitor and control has been deferred, most of the software and hardware hooks can be provided with virtually no increase in initial equipment cost. You can have a simple centralized network control center terminal consisting of a CRT and a modem. You will have the mechanism in place to manage the expansion you know is coming.
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|Date:||Mar 1, 1985|
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