Types of Error Control

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6.2 TYPES OF ERROR CONTROL

Before we discuss the details of structured redundancy, let us describe the two

basic ways such redundancy is used for controlling errors. The first, error detection

and retransmission, utilizes parity bits (redundant bits added to the data) to detect

that an error has been made. The receiving terminal does not attempt to correct

the error; it simply requests that the transmitter retransmit the data. Notice that a

two-way link is required for such dialogue between the transmitter and receiver.

The second type of error control, forward error correction (FEC), requires a oneway

link only, since in this case the parity bits are designed for both the detection

and correction of errors. We shall see that not all error patterns can be corrected;

error-correcting codes are classified according to their error-correcting capabilities.

6.2.1 Terminal Connectivity

Communication terminals are often classified according to their connectivity with

other terminals. The possible connections, shown in Figure 6.6, are termed simplex

(not to be confused with the simplex or transorthogonal codes), half-duplex, and

full-duplex. The simplex connection in Figure 6.6a is a one-way link. Transmissions

Figure 6.6 Terminal connectivity classifications. (a) Simplex. (b) Halfduplex. (c) Full-duplex.

are made from terminal A to terminal B only, never in the reverse direction. The

half-duplex connection in Figure 6.6b is a link whereby transmissions may be made

in either direction but not simultaneously. Finally, the full-duplex connection in

Figure 6.6c is a two-way link, where transmissions may proceed in both directions

simultaneously.

6.2.2 Automatic Repeat Request

When the error control consists of error detection only, the communication system

generally needs to provide a means of alerting the transmitter that an error has

been detected and that a retransmission is necessary. Such error control procedures

are known as automatic repeat request or automatic retransmission query (ARQ)

methods. Figure 6.7 illustrates three of the most popular ARQ procedures. In each

of the diagrams, time is advancing from left to right. The first procedure, called

stop-and-wait ARQ, is shown in Figure 6.7a. It requires a half-duplex connection

only, since the transmitter waits for an acknowledgment (ACK) of each transmis-

 

[+] Enlarge Image

Figure 6.7 Automatic repeat request (ARQ). (a) Stop-and-wait ARQ (halfduplex). (b) Continuous ARQ with

pullback (full-duplex). (c) Continuous ARQ with selective repeat (full-duplex).

 

sion before it proceeds with the next transmission. In the figure, the third transmission

block is received in error; therefore, the receiver responds with a negative acknowledgment

(NAK), and the transmitter retransmits this third message block

before transmitting the next in the sequence. The second ARQ procedure, called

continuous ARQ with pullback, is shown in Figure 6.7b. Here a full-duplex connection

is necessary. Both terminals are transmitting simultaneously; the transmitter is

sending message data and the receiver is sending acknowledgment data. Notice

that a sequence number has to be assigned to each block of data. Also, the ACKs

and NAKs need to reference such numbers, or else there needs to be a priori

knowledge of the propagation delays, so that the transmitter knows which messages

are associated with which acknowledgments. In the example of Figure 6.7b,

there is a fixed separation of four blocks between the message being transmitted

and the acknowledgment being simultaneously received. For example, when message

8 is being sent, a NAK corresponding to the corrupted message 4 is being received.

In the ARQ procedure, the transmitter “pulls back” to the message in error

and retransmits all message data, starting with the corrupted message. The final

method, called continuous ARQ with selective repeat, is shown in Figure 6.7c. Here,

as with the second ARQ procedure, a full-duplex connection is needed. In this procedure,

however, only the corrupted message is repeated; then, the transmitter

continues the transmission sequence where it had left off instead of repeating any

subsequent correctly received messages.

The choice of which ARQ procedure to choose is a trade-off between the requirements

for efficient utilization of the communications resource and the need to

provide full-duplex connectivity. The half-duplex connectivity required in Figure

6.7a is less costly than ful-duplex; the associated inefficiency can be measured by

the blank time slots. The more efficient utilization illustrated in Figures 6.7b and c

requires the more costly full-duplex connectivity.

The major advantage of ARQ over forward error correction (FEC) is that

error detection requires much simpler decoding equipment and much less redundancy

than does error correction. Also, ARQ is adaptive in the sense that information

is retransmitted only when errors occur. On the other hand, FEC may be

desirable in place of, or in addition to, error detection, for any of the following

reasons:

1. A reverse channel is not available or the delay with ARQ would be excessive.

2. The retransmission strategy is not conveniently implemented.

3. The expected number of errors, without corrections, would require excessive

retransmissions.

Relevant NI products

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