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Data Communication Modes Functional Description

AMD

4–4

4.2.2

Synchronous Transmission

Synchronous transmission requires that clocking information be transmitted along with

the data, either by a method of encoding data that contains clocking information, or by a

modem that encodes clock information in the modulation process. In either case, data are

sent at a defined rate which is controlled by a timing source at the transmitter.

Synchronous communication channels send data faster with less overhead than asyn-

chronous channels but are more expensive to design than asynchronous channels. In

synchronous communication, a timing reference, or “clock”, is used to control the transfer

of information. This clock specifies to the receiver when to sample the data (bit synchroni-

zation) in order to ascertain which data value (‘0’ or ‘1’) was transmitted. The optimum

sample times usually correspond to the middle of the bit cell to minimize error. This clock

signal is encoded along with the data sent so the receiver must be able to decode the

Figure 4–1. Asynchronous Format incoming clock signal. A circuit called a “phase-locked

loop” is typically used for this purpose.

In addition, since data rates are usually higher and data are typically sent with no gaps

between characters, synchronous communication requires some level of buffering at both

the transmitter and receiver.

Once bit synchronization has been established, the next phase for the receiver is to know

what group of bits constitute a character (character synchronization). This requires that

the receiver search the receive bit stream on a bit-by-bit basis for a character synchroniz-

ing pattern in order to determine which set of bits in the bit stream defines the first char-

acter transmitted.

Synchronous communication channels are found in many mainframe data networks. The

greater throughput of the synchronous channel is required in mainframe environments

where many terminals are connected to the computer and multiplexed onto one channel.

The synchronous protocols used may be either character-oriented or bit-oriented.

4.2.2.1

Synchronous Character-Oriented Protocol

In a Character-Oriented Protocol (COP) data are transmitted in message blocks and re-

quire that each block be preceded by either an 8- or 16-bit predefined “sync character”.

In addition, COPs are typically restricted to half-duplex operation and depend heavily on

special control characters or character sequences, such as SOH or DLE STX and ETX, to

determine the start and end of a particular field within a message block. IBM BISYNC is

an example of a COP. MONOSYNC, on the other hand, is a character count protocol

where both ends of the communication link keep track of the number of characters sent

and received. This solves the problem of having to use special control characters for field

delineation as used in the BISYNC protocol. The DDCMP (Digital Data Communication

Message Protocol) from DEC is another example of a character count protocol in use to-

day. MONOSYNC and BISYNC message formats are shown in Figures 4–2 and 4–3, re-

spectively.

Since sync characters are only appended to the start of a message block, additional sync

characters may be inserted within a transmission at distinct time intervals or during a

pause in order to maintain synchronization.

4.2.2.2

Synchronous Bit-Oriented

Bit-Oriented Protocols (BOP) may be used in half- or full-duplex operation and are less

dependent on special control characters. BOPs rely instead on the position of bits within

specific fields.

The most common BOPs in use today are High-Level Data Link Control (HDLC) and Syn-

chronous Data Link Control (SDLC). These two protocols are nearly identical except for

minor differences in the use of the Address and Control fields.

All SDLC information is sent in frames and follow a standard format as shown in Figure

4–4. SDLC frames begin and end with the 8-bit flag sequence, “01111110.” All stations

on the link search continuously for this flag sequence which indicates the start of a frame

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