Describe the various modes of data transmission. Or Explain the following with the help of examples – (i) Simplex (ii) Half duplex ( iii) Full duplex.

Describe the various modes of data transmission. Or Explain the following with the help of examples – (i) Simplex (ii) Half duplex ( iii) Full duplex.

Ans. Data communications links are configured to fulfill the particular

needs for a given system. In the simplest link, it contains a single primary and

a single secondary station connected to node end points of the link. The primary

station originates the communication link and maintains control over that link

until all data transfers are completed. The answering responsibility is of the

secondary station. Both stations must use the same protocol, data rates, and

data codes for data to be correctly transmitted or received. The actual technique

of sending and receiving data is further classified into three types- simplex,

half duplex, and full duplex.

(i) Simplex A system or a particular data transmission can be

configured to transmit and receive data in one direction only from primary to

secondary [as shown in fig. 2.7 (a)]. This transmission is known as simplex

transmission. This type of data transfer link can be performed by the system

of fig 2. 7, if, for example, the primary would always send data to the secondary

and the secondary would not be needed to respond or transmit anything in return.

simplex and half duplex

communication directions

Simplex transmission is used in environments, in which large quantities

of data are transmitted without acknowledgement of the reception. The data

can be transmitted rapidly and continuously.

An example, application of a simplex system would be an inventory dump

from a warehouse to a company’s central offices. Once the inventory was

taken and entered into a computer, the files containing the inventory data can

be transmitted to the accounting department. This data can be transferred or

dumped late at night when the system would be less occupied. There would

be no immediate acknowledgement of receipt of the inventory data because

no one is working in the accounting department at the time. This is the ideal

use for a simplex transmission. If an error should take place, there would be

no way for the secondary to inform the primary that it occurred.

(ii) Half Duplex In the half duplex mode, bidirectional data transfer

is possible. Half duplex permits transmission of data in both directions between

primary and secondary stations, but restricts these transfers to one direction

it a time as shown in fig. 2.7 (b). The primary might start by transmitting a

message from its transmitting circuits to the secondary’s receiving units, which

receives and stores the data. When the message is completely transmitted, the

secondary can answer with a message to its own from its transmitter to the

primary’s receiver.

(iii) Full Duplex in the full duplex mode, both stations can transmit

to each other and receive each other’s messages simultaneously as shown in

fig. 2.7 (c). One way to obtain this is for the primary and secondary to send

binary data using two different sets of audio tones for logic I and 0. As an

c:-.ample, the primary might transmit data using 1250 Hz and 1750 Hz signals

tor mark and space while the secondary would use 2050 Hz and 2750 Hz. The

receiver portions of both station’s modems would be tuned to the suitable set

of tones in order to differentiate between those it transmitted and those it


A full-full duplex mode is also sometimes used for communication. Similar to fill duplex, data can be transmitted and received simultaneously. The difference is that in a full-full duplex system, data is transmitted to one location and received  from a different location. This mode of transmission is shown in

Fig. 2.7 (d).

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