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