What is data transmission ? Explain the parallel transmission and serial transmission. Or Write short notes on parallel and serial transmissions.

What is data transmission ? Explain the parallel transmission and serial transmission. Or Write short notes on parallel and serial transmissions.

Ans- There is always a need to exchange data, commands and other

control information between a computer and its terminals or between two

computers. This information is in the form of bits. Data transmission refers

to movement of the bits over some physical medium connecting two or more

digital devices. There are two options of transmitting the bits, namely, parallel

transmission, and serial transmission.

Parallel Transmission – Uinary data, consisting of 1s and Os, can be

organized into groups of n bits each. Computers generate and consume data in

groups of bits much as we conceive of and use spoken language in the form

of words rather than letters. By grouping, n bits of data can be transmitted at

a time instead of one. This is known as parallel transmission.

mechanism of parallel transmission

Fig. 2.1 shows the mechanism for parallel transmission which is

conceptually simple one- use n wires to transmit n bits at one time. That way

each bit has its own wire, and all n bits of one group can be transmitted with

each clock pulse from one device to another. In fig. 2.1, parallel transmission

for n = 8 is shown. Typically, the eight wires are bundled in a cable with a

connector at each end.

The main benefit of parallel transmission is speed. All else being equal,

parallel transmission can increase the transfer speed by a factor of n over

serial transmission. But, cost is a significant disadvantage of parallel

transmission. Parallel transmission needs n communication lines just to send

the data stream. The parallel transmission is usually limited to short distances

because of its expensiveness.

Serial Transmission – In serial transmission, one bit follows another.

Therefore, only one communication channel is required rather than n to send

data between two communicating devices.

The benefit of serial transmission over parallel transmission is that with

only one communication channel, serial transmission reduces the cost of

transmission over parallel by roughly a factor of n. Fig. 2.2 shows the

mechanism of serial transmission.

mechanism of serial transmission

Because the communication within devices is parallel, conversion devices

are needed at the interface between the sender and the line (parallel-to-serial)

and between the line and the receiver (serial-to-parallel). Serial transmission

takes place in one of the two ways-asynchronous or synchronous.

In serial transmission, the sending and receiving devices have an additional

complexity. The sender must determine the order in which the bits are sent.

For example, when sending eight bits from one byte the sender must determine

whether the high-order or low-order bits are sent first. Similarly the receiver

must know where to place the first received bit within the destination bytes. It

may seem like a tribal issue, but different architectures may number the bits.

in a byte differently and if the protocols do not agree on how to order the bits

the information will be transmitted incorrectly.

The receiver then assembles the incoming bit stream into characters.

Serial transmission presents two synchronization problems related to the

receiver first achieving bit synchronization and then achieving character


Serial transmission is also slower as compared to parallel transmission

because the bits are sent one at a time. However, serial transmission is cheaper

and more reliable than parallel transmission over long distances.

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