Effects of Delay and Motion in Satellite Communications

Slide Number 1Rev -, July 2001

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada


Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat CanadaTechnical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

3.6.1 Echo Cancellation



Vol 3: Satellite Communication Principles

Sec 6: Dealing With the Effects of Delay and Motion

3.6.1: Echo Cancellation

Echo CancellationIn telephony applications, speech quality is the benchmark for assessing the overall quality of an end-to-end voice connection. Therefore, in order to provide a quality-of-service expected by the subscribers, the objective is to effectively remove echo primarily generated and exasperated by the following elements:

• Two-wire/four-wire hybrid devices (echo reflection)

• Acoustic feedback from speaker phone (echo reflection)

• Digital processing delays (delay element)

• Transcoding processing delays (8 kbps voice processing delay element)

• Transmission delays (propagation delay element)



How Echo is ProducedHybrid DevicesThe primary source of echo is from 2Wire/4Wire hybrid devices. These devices are used to interface a two-wire telephone line to a four-wire network.

The two-wire port carries both directions of the conversation. The hybrid splits the 2-wire loop into two separate pairs of wires, one for the send-path and one for the receive-path. The 4-wire network is where amplification and equalization is implemented.

Unfortunately, the hybrid is by nature a leaky device. Therefore, in order to reduce the amount of echo, the 2-wire and 4-wire ports must be impedance matched.

Vol 3: Satellite Communication Principles, Sec 6: Effects of Delay and Motion

Part 1: Echo Cancellation

3.6.1.1: How Echo is Produced



How Echo is ProducedNormally, hybrids are equipped with variable or switched resistance and capacitance elements in order to reduce the echo return loss (ERL) at each port. The actual amount of signal that is reflected back depends on how well the balance network of the hybrid matches the 2-wire line.

As a rule-of-thumb, if the round trip echo has a total delay of less than 32 milliseconds, no echo cancellers will be required. In fact, if echo is returned to the subscriber handset with up to a 32 millisecond delay it generates a sense that the call is live by adding sidetone, which is like hearing your own voice. This makes a positive contribution to the quality of the call, as perceived by the subscriber.

If the round trip echo delay exceeds 32 milliseconds, however, the sidetone becomes intrusive and objectionable and echo cancellers must be employed.






How Echo is ProducedAcoustical EchoAcoustical echo is not as prevalent as echo caused by hybrids. Nevertheless, it is encountered in telecommunications networks.

Acoustical echo is usually caused by poor isolation between the microphone and speaker during hands-free applications. Usually hands-free speakerphones incorporate an internal echo-control circuitry to ensure that echo is not a problem.

Video/audio conferencing studios create a multi-path acoustical echo, which occurs when the audio signal is reflected from many surfaces and directed back at the microphone. This type of echo is usually suppressed at the source by sophisticated internal echo-control circuitry.






How Echo is ProducedEcho delay

Echo delay is exacerbated by the following elements in a network:

Transmission Facility Delay (Milliseconds/Kilometer)

T1 Carrier over copper 1/160

Fiberoptic cable 1/160

Microwave radio 0.7/160

Non-Loaded copper cable 2/160

Loaded copper cable 33/160








Voice Coder Equipment One-way Delay(Milliseconds)

8k ACELP (G.729) 1/160

16k LDCELP (G.7.28) 1/160

32k ADPCM 0.7/160

4to1 VBR_ADPCM T1/E1 2/160 Transcoder








Transmission Equipment One-way Delay(Milliseconds)

ATM Switch 1 to 6

Digital Cross-Connect 0.25

PCM Channel Bank 00.125 to 0.50

Geosynchronous Satellite 140

MEO Satellite 70

LEO Satellite <10 milliseconds (includes

processing delays)Vol 3: Satellite Communication Principles, Sec 6: Effects of Delay and Motion





Satellite Delay Effect on EchoBy far, the propagation delay introduced by a satellite link contributes most to the generation of echo.

However, speech-processing delays by some bandwidth-efficient voice coders also introduce objectionable transmission delays and therefore warrant the use of echo cancellers.



3.6.1.2: Satellite Delay Effect on Echo



Echo Cancellation MethodsVia Net Loss (VNL)One of the earliest forms of echo control was achieved by a VNL design.

VNL works by introducing signal loss in each direction of the transmission path. Therefore, the echo signal will get attenuated twice.



3.6.1.3: Echo Cancellation Methods

Figure 3.6.1.3a Via Net Loss

This technique was not effective for circuits that traverse long distances because VNL results in an unacceptable reduction in speech levels.



Echo Cancellation MethodsEcho SuppressorThe echo suppressor achieved another form of echo control.

Echo suppressors control echo by opening, or inserting a large amount of attenuation, into the send-path. Ideally, the suppressor should open the send-path only when the caller is talking and not when the caller is listening. A speech detector is used to perform this function.

However, even the best echo suppressors often removed the caller’s echo and some of the speech of the person the caller was trying to listen to. This meant that a polite conversation protocol had to be observed by both parties in order to limit caller interjection (double-talk), essentially forcing a half-duplex scenario. VNL and echo suppressors became obsolete with the introduction of echo cancellers.






Echo Cancellation MethodsEcho SuppressorVNL and echo suppressors became obsolete with the introduction of echo cancellers.




Figure 3.6.1.3b Echo Suppressor



Echo Cancellation MethodsEcho CancellerToday, all service providers use single- and multi-channel echo cancellers.

The echo canceller basically subtracts the echo signal and does not insert any attenuation into the circuit. Therefore, both callers can speak simultaneously (double-talk) without the lockout and clipping that plagued echo suppressors.

An echo canceller monitors speech from the far end that passes through its receive-path and uses this information to compute an estimate of the echo that is then subtracted from its send-path. Therefore, echo is eliminated or cancelled, and only the near-end speech is sent to the far end.









Figure 3.6.1.3c Echo Cancellor

Echo Cancellation MethodsEcho Canceller



Echo Cancellation MethodsDrop-side Echo Return Loss (ERL)The drop-side of the circuit contains the hybrid and is called the end-path. The amount of ERL present in the end-path must never exceed the maximum end-path ERL specified by the manufacturer. If this ERL is exceeded the echo canceller will not be able to perform to the specification defined by the manufacturer.

ERL is specified as the minimum difference between the send and receive levels at the drop side of an echo canceller device at which the echo canceller device will perform satisfactorily. Minimum ERL is approximately 6 dB.






Echo Cancellation MethodsTail circuit end-path delayTail circuit end-path delay is very low if a handset is connected to the echo canceller via a 2W/4W converter. In this application the hybrid in the telephone set and the hybrid in the 2W/4W converter are co-located near the echo canceller device with the result that no detectable tail circuit propagation delays can be measured.

These types of subscriber signaling, foreign exchange subscriber (FXS) applications are implemented when a loop-start/ground start telephone set or a loop-reversal payphone set is connected directly to the echo canceller.

However, there are foreign exchange office (FXO) applications where the two-wire line extends from the echo canceller to the PSTN.






Echo Cancellation MethodsThis two-wire loop-start or ground-start line is connected to the echo canceller via a 2W/4W converter. In this application the PSTN hybrid is located many kilometers away from the echo canceller device; therefore, tail circuit propagation delays must be taken into account. The FXO line normally has a tail circuit propagation delay, which must not exceed the echo canceller requirements.

The affect of tail circuit delay is measured from R(out) to the S(in) on the drop-side. Echo cancellers that comply to ITU-T G.165 standards will support tail circuits delays up to 48 milliseconds. Some higher performance echo cancellers will support tail circuit delays up to 128 milliseconds.






Echo Cancellation MethodsLine-side Echo Return Loss Enhancement (ERLE)The line-side of the circuit contains the bulk of the delay and is called the longhaul.

Since the echo to be cancelled occurs in the hybrid, the amount of longhaul delay is irrelevant. However, keep in mind that an echo canceller will be required if the total round trip circuit delay (longhaul to end-path to longhaul) exceeds 32 milliseconds.

Echo cancellation occurs between the S(in) and S(out) ports. The total amount of echo attenuation that an echo canceller provides is measured in dB’s and is called ERLE.






Echo Cancellation MethodsEcho Cancellation MethodAn echo canceller monitors speech from the far end as it passes through its receive-path and uses this information to compute an echo estimate, which is then subtracted from its send-path.

An echo canceller consists of four major building blocks:• Convolution processor • Double-talk detector

• Subtractor • Nonlinear processor

Echo cancellers have four connections, or ports, two on the line-side and two on the drop-side. The four ports are:

• Receive-in (Rin) • Send-in (Sin)

• Receive-out (Rout) • Send-out (Sout)






Echo Cancellation MethodsConvolution processorThe convolution processor is the principle component of the echo canceller. It normally resides within a very powerful digital signal processor (DSP). The processor has two registers: the X-register and the H-register.




Figure 3.6.1.3d Echo Cancellation Method

The DSP performs a mathematical process called convolution, which is the multiplication of the contents of the X-register with the contents of the H-register.The resulting product, the echo estimate, is fed to the subtractor.



Echo Cancellation MethodsThe first step in echo cancellation occurs when the caller’s voice enters the Rin port. The convolution processor receives samples of the voice signal every 125 microseconds (PCM sample rate of 8000/sec) and stores the samples in the X-register.

The caller’s voice then travels from the Rout port to the hybrid, where most of the signal gets transferred to the 2wire loop connected to either an FXS or FXO interface. Some of the far-end caller’s voice is also transferred across the hybrid to the Sin port of the echo canceller. This is the echo to be cancelled.

The objective of the cancellation process is for the contents of the H-register to become a mathematical representation of the hybrid’s impulse response. The output of any device can be accurately predicted once the device’s impulse response and input are known.






Echo Cancellation MethodsIf the Rout were a single short burst of energy, the impulse response of the hybrid would be immediately known. However, Rout speech signal normally consists of many different frequency components spread over varied periods of time. Therefore, the impulse response must first be learned and then accurately stored in the H-register.

Once the impulse response is accurately represented in the H-register, the convolution of its contents will produce an accurate estimate of the hybrid’s echo. With an accurate estimate of the echo, the end-path (tail circuit) delay and the ERL of the hybrid can be derived.

There are two types of convolution processors:• Full-tapped• Windowed






Echo Cancellation MethodsThe full-tapped convolution processor uses an H-register large enough to model the impulse response of the entire end-path, and can simultaneously model an unlimited number of echoes. An E1 echo canceller supporting 64-millisecond end-path (tail circuit) delay operates at approximately 270 MIPS. A T1 echo canceller operates at approximately 210 MIPS.

The windowed convolution processor uses an H-register much smaller than the actual end-path and can only cancel those echoes that fit within the H-register’s window.






Echo Cancellation MethodsSubtractorThe subtractor subtracts the echo estimate developed by the convolution processor. The resulting output is residual echo that is passed on to the nonlinear processor and is also fed back to the convolution processor as the error signal.

The echo canceller converges, or minimizes, the error signal as the H-register develops a model of the end-path’s impulse response. The process of modeling the impulse response is called “adaptation”. The initial echo estimate will not resemble the actual echo; however, the objective is to use the error signal to adjust the H-register’s contents to minimize the error signal.






Echo Cancellation MethodsConvergenceThe process of minimizing the error signal is called “convergence”.

Convergence occurs within a few milliseconds of the beginning of speech. How quickly the H-register adapts or learns the hybrid’s impulse response determines the convergence speed of the echo canceller. This is in turn dependant upon the speed of the processor and the efficiency of the code.

The objective is for the echo canceller to converge as quickly as possible. High performance echo cancellers converge within 50 milliseconds or better. The CCITT specifies a convergence time of less than 500 milliseconds.






Echo Cancellation MethodsDouble-talk DetectorThe double-talk detector senses simultaneous near-end and far-end speech.

When this occurs, the detector sends a signal to the convolution processor to ignore the error signal from the subtractor, therefore causing the contents of the H-register to freeze.

Consequently, adaptation is halted when double-talk is detected. The echo canceller still continues to cancel echo during double-talk. Once double-talk ends the detector instructs the convolution processor to once again use the error signal to adapt the H-register to the impulse response of the hybrid.






Echo Cancellation MethodsDivergenceIf the H-register were allowed to adapt during double-talk, the error signal would get very large and the impulse response model would be erroneously adjusted.

Misadjustment of the impulse response model can result in clear or distorted echo.

This phenomenon is also known as “divergence”.






Echo Cancellation MethodsNon-linear Processor (NLP)The inaccurate representation of speech samples by PCM modulators (quantization error) makes it difficult to develop a perfect echo estimate. Non-linear echoes can be caused by:

• Clipped speech signals• Speech compression• Poor quality speakerphones

Consequently, it is extremely difficult to develop an accurate echo estimate of these nonlinear echoes because the echo canceller’s linear impulse response model cannot be correlated with these nonlinear echoes. The practical amount of echo cancellation achievable is about 35 dB. The residual echo from the subtractor is reduced to an inaudible level by the NLP.






Echo Cancellation MethodsThe NLP has a suppression threshold that is typically adaptive, based on Rin and Sin levels. The NLP is also known as the “center clipper” because it removes the middle or center of any signal that exceeds the NSP’s threshold: signals above the threshold are allowed to pass, while signals below the threshold are removed. The threshold is made adaptive because, if the NLP simply blocked all signals in the send-path, there would be noticeable clipping of speech, especially the first syllable of the near-end speech.

Total Echo CancellationThe total echo cancellation is provided by the following elements:

• Hybrid (ERL)• Convolution processor (ERLE)• Non-linear processor (Center Clipper)






Echo Cancellation MethodsThe diagram below highlights the amount of echo level attenuation associated with each of the three elements.

The echo canceller also performs per-call control by monitoring the presence of MODEM or FAX tones and automatically disabling the echo cancellation process when this type of traffic is detected.




Figure 3.6.1.3e



3.6.2 Doppler effect



Description and Cause of the EffectAustrian physicist and mathematician Christian Johann Doppler explained the Doppler effect principle in 1842.

“It is the change in the frequency of a wave observed at a receiver whenever the source or the receiver is moving relative to each other or to the carrier of the wave”.

In LEO satellite mobile communications, the motion of the satellite as it rises over the horizon will cause a progressive upwards shift of the radio “center frequencies”, perhaps by as much as 40 kHz. There will be a corresponding drop in the frequencies when it passes overhead and is setting. The Doppler shift is more apparent when LEO satellites pass over at high versus low elevations.


Part 2: Doppler Effect

3.6.2.1: Description and Cause of the Effect



Description and Cause of the EffectThe apparent frequency relative to the actual frequency can be calculated by the following formula:

Fapp = Fact(1 + V/C)

C = 300,000,000 m/s

V = radial speed component of the observer relative to the source (LEO motion)

Fact = actual frequency generated at the source

Fapp = Doppler shifted frequency






Description and Cause of the EffectIn GEO satellite systems the satellite operators that maintain the station-keeping parameters will define the residual movement of the satellite. The typical dimensions are 0.1o in longitude and latitude.

Therefore, the satellite will move within a box of the order of 75 km X 75 km X 35 km. This introduces an altitude variation of approximately 35-km with a periodicity of 24 hours.

The nominal altitude of geo-synchronized satellite is 35,786 km. Therefore there will be a variation in single-hop propagation time of approximately 234 microseconds, which simply amounts to the propagation delay difference between minimum and maximum satellite altitude.






Mitigation by BufferingA geostationary satellite should be positioned directly over the equator and orbit with a duration of 24 hours. In practice, the earth, moon, and sun’s gravity influence the exact inclination of the satellite (relative to the equator). Station keeping is therefore required to maintain the orbital position.

When viewed from the Earth, the satellite appears to prescribe and ellipse in space, degrading to a "figure 8" as the angle of inclination increases.

The orbit of the satellite can result in a peak-to-peak altitude variation and a resultant variation in propagation delay. Some data elasticity must be built into the Earth Station receive system so that the slow drift in data arrival times will not cause a loss of synchronization.

This elasticity is provided by a device called a “buffer”. Vol 3: Satellite Communication Principles, Sec 6: Effects of Delay and Motion


3.6.2.2: Mitigation by Buffering



Mitigation by BufferingA buffer is simply a block of RAM that is used in a First In, First Out (FIFO) manner. It is usually positioned at the front end of the baseband portion of the receive system. It is filled by data incoming from the satellite link and is emptied by the remainder of the receive system.To operate, the buffer is allowed to fill half way before any data is clocked out. If there were no doppler shift, the buffer would remain half full, data being clocked in and out at the same rate.In the presence of doppler shift, however, the data arrives slightly faster or slower than the clock-out rate. The buffer will clock this data in as it arrives and will alternately fill past the 50% full mark, or empty below the 50% mark while maintaining the constant clock-out rate required by the receive system. Buffers must be correctly sized so that they can accept one full cycle of satellite motion without exceeding RAM capacity.






Mitigation by BufferingBuffer sizingThe example below highlights the buffer size required for a 64 kbps link, assuming geo-synchronous station-keeping parameter of 0.05o. Therefore a buffer depth of 1 milliseconds is sufficient to cope with Doppler shift effect.

31.4 µs Path variation from station A to satellite62.8 µs Path variation from station A to B.125.6 µs Path variation from station A to B back to A.251.2 µs Buffer size exercised.502.4 µs Minimum buffer size selected.






Mitigation by BufferingBuffer Sizing

The buffer may re-center once after carrier acquisition (ie. after rain fade).

Example:64 kbps link double hopped.

125.6 µs * 64000 b/s = 8.04 bits.

Buffer required to hold bits = 9 (8.04 rounded up) x 2 = 18 bits

Smallest buffer to accommodate a reset = 36 bits

Therefore, the smallest buffer will be 64 bits (±32) since a 32 bit buffer will overflow.






Mitigation by BufferingIntroduction of latencySynchronization between Earth Stations in the network is of paramount importance. In TDMA networks the objective is to avoid bust overlaps from others in the frame. Burst overlaps occur for the following reasons:

• Slant range of each remote earth station is different (distance from earth station to satellite)

• Doppler shift effect

• Earth Station clock frequency offset

Earth Station clock frequency offset is corrected by employing an automatic frequency control mechanism, which is usually managed and controlled by the reference Earth Station equipped with a network control facility.






Mitigation by BufferingTo overcome the slant range and Doppler effect, TDMA networks continuously perform frame synchronization tests. Employing one of the following techniques can perform synchronization:

• Closed loop synchronization

• Open loop synchronization

See Subject 3.3.3.3 for information concerning these two synchronization methods.






3.6.3 Clocking Schemes



Clocking SchemesDigital communications requires timing control in order to identify the rate at which bits are transmitted, and to identify the start and end of each bit. In satellite communications the timing signal is embedded with the transmitting data and can therefore be extracted by the receiver, if desired, in order to synchronize its internal clock.

Generally, synchronous digital satellite networks can employ the following four clocking options in order to transfer data from the source MODEM to the destination MODEM.

Use an external clock (Cesium standard terrestrial clock) Use an internal clock (MODEM generated clock) Use a single clock source (Master/Slave operation) Use independent clock source (Plesiochronous)

Vol 3: Satellite Communication Principles

Sec 6: Dealing With the Effects of Delay and Motion

3.6.3: Clocking Schemes



Loop Timing SchemesThe advantage of a loop-timing scheme is that the destination receiver can synchronize its internal clock from the clock generated by the source transmitter.

The destination MODEM will use the recovered clock to time the reception of data from the satellite and time the transmission of data to the data terminal equipment.

This type of clocking scheme will ensure synchronous transmission of data from source to destination.


Part 3: Clocking Schemes

3.6.3.1: Loop Timing Schemes



Loop Timing SchemesThe clock source can either be generated by the source MODEM or can be generated by a very stable external clock, such as a cesium standard noted to have a stability rating of 1 X 10-12. Usually MODEM clocks have a stability rating ranging from 1 X 10-7 to 1 X 10-9.

In a typical point-to-point digital satellite network the source MODEM us usually connected to the terrestrial backbone that employs very accurate and stable cesium standard clock. In this topology the backbone clock is nearly always used to synchronize the source MODEMs own internal clock.



3.6.3.1: Loop Timing Schemes



Independent Timing SchemesIf the source and destination MODEM in a synchronous point-to-point digital satellite network each use their own internal clocks, or each use their own external clock from their respective timing island, a clock slip at a constant rate will occur.

A clock slip will occur even if the source and destination clocks are very close in frequency to each other.

This type of synchronous transmission is referred to be Plesiochronous. In order to prevent any loss of data, plesiochronous networks are normally equipped with data buffers, similar to Doppler shift buffers.

The buffers are designed with sufficient depth to ensure data overflow and data underflow does not occur.



3.6.3.2: Independent Timing Schemes



3.6.4 Effect of Delay on Data Systems



When Using ARQ SystemsAutomatic repeat request or automatic retransmission query (ARQ) uses an error detecting code together with a feedback channel to initiate a retransmission of any blocks of bits received in error. It is obviously not suited for voice communications, but can be used for data transmission.

With ARQ, data messages are built up at the originating end on a packet or block basis. Each block or packet has appended to it a “block check count” or parity tail.

At the receiving end, a similar processing technique is used and the locally derived parity symbols are compared to the received parity symbols. If they are the same the message is said to be error free. If they are not the same the block or packet is in error.


Part 4: Effect of Delay on Data Systems

3.6.4.1: When Using ARQ Systems



When Using ARQ SystemsThere are two principal performance criteria to evaluate ARQ systems:

The probability of delivering erroneous data to the user The throughput, defined as the ratio of the information bits

delivered to the user to the total number of symbols transmitted in the channel

Powerful error-detecting codes exist, which can detect nearly all types of error patterns, regardless of how these errors occur on the channel. Consequently, the key figure of merit in the evaluation of ARQ systems is the throughput.

There are three principal ARQ schemes: Stop and Wait ARQ Continuous ARQ Selective Repeat ARQ






When Using ARQ SystemsStop-and-Wait ARQStop and Wait ARQ is the simplest and the most widely used.

After sending a block, the transmitting terminal waits for a positive or negative acknowledgement from the received terminal.

If it is positive acknowledgement (ACK), it sends the next block of data.

If it is a negative acknowledgement (NACK), it resends the same block of data.






When Using ARQ SystemsContinuous ARQWith continuous ARQ, the transmitting terminal does not wait for an ACK after sending a block of data. It immediately sends the next block of data.

While the blocks are being transmitted, the transmitting terminal examines the stream of ACKs. If the transmit terminal receives a NACK, or fails to receive an expected ACK, it must associate the NACK to a specific block of data.

Each ACK or NACK will contain the sequence number of the transmitted block so that the transmitter can cross-reference the sequence number to a specific block of data.

Upon receiving an NACK, the transmitter will re-transmit the corrupt block of data plus, unfortunately, all the blocks of data that followed.






When Using ARQ SystemsSelective Repeat ARQA more efficient scheme is to re-transmit only the block of data that was corrupted, and not those blocks of data that followed it.

This single block re-transmission requires that the block be identified and therefore needs more logic circuitry and buffers in the transmitting and receiving terminals.

The larger buffers are required because the blocks of data may not be received in serial order and must therefore be re-formatted.








3.6.4.2: Spoofing

SpoofingIn interactive satellite networks, propagation delay has presented unique problems for legacy data protocols. These problems have been successfully overcome by the use of equipment and protocols specifically suited to the requirements of satellite transmission.

Satellite propagation delay and signal processing delay can introduce a total point-to-point transmission delay of approximately 320 milliseconds. ACK or NACK responses will therefore contribute a 640 milliseconds round trip delay. This will result in very inefficient throughput.

Satellite propagation delay has prevented the use of older binary synchronous or block-by-block protocols.



SpoofingProtocols 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 answer to propagation delay problems for satellite data networks is found in the use of advanced protocols and/or delay compensators. These provide acknowledgments locally before data is transmitted over the satellite, thus eliminating the lag time for protocol handshakes.

The new generation of VSAT Earth Stations, and some multiplexers, have built-in delay compensators and protocol converters known as PADs (Packet Assembler-Disassembler) which perform handshakes locally and convert protocols to satellite-efficient versions.



3.6.4.2: Spoofing



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

Two requirements are necessary to push satellite throughput efficiency up to the level demanded for most data communications networks:

The bit error rate (BER) must be low enough to limit retransmission of data blocks to the smallest attainable number

127-block window must be implemented, and each block size can be 1 kilobit



3.6.4.2: Spoofing



SpoofingThroughput efficiencies approaching 100 per cent are achievable with satellite networks when transmit speed, block size and BER are considered in the link design.

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.

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.



3.6.4.2: Spoofing



SpoofingCompanies 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.



3.6.4.2: Spoofing

Engineering

Effects of Delay and Motion in Satellite Communications