PACKETS, FRAMES AND ERROR DETECTION

INTRODUCTION:
The previous chapters of data communication described how bits are transmitted
across a physical network using a transmission medium.
This chapter introduces the concept of packets of data rather than bits for
communication.

CONCEPT OF ‘PACKET’:
Network systems divide data in small blocks or junks called packets, which they
send individually. Why we need packets rather than bits? The answer to this question is
because a sender and receiver need to coordinate to detect transmission errors. Also the
individual connection between each pair of computers is not possible. That’s why to
solve these problems shard network connections are made among many workstations.

PROBLEMS WITH SHARING:
The demand of sharing is very high because many computers need to use the shared
networks. In addition to this some applications have large data transfer. In this way they
hold the network for long time. But on the other hand some applications cannot wait so
long. So we need a mechanism for fairness.

SOLUTION FOR FAIRNESS:
To the fairness, the solution is to divide the data into small block or chunks called
‘PACKETS’. Computers take turns to send one packet at a time over the shared
connection.
Because each packet is small so no computer experiences a long delay.
Example:
In the figure one reason for using the packets is illustrated. We see that in a shared
resource when one pair of computer communicates, the other must wait. To understand
the use of packet here, let’s suppose a transmission with packets in the figure.

WITHOUT PACKETS:
A 5MB file transferred across network with 56Kpbs capacity will require 12 minutes.
This means that all that computers will be forced to wait for 12 minutes before initiating

other transfers.WITH PACKETS:

Now if the file is broken into packets, other computers must only wait until packet (not
entire file) has been sent.
Suppose file is broken into 1000 byte packets.
Now each packet takes less than 0.2 seconds to transmit. Here other computers must only

wait for 0.14 sec before beginning to transmit.
Note: - if both files are 5MB long, each now takes 24 minutes to transmit. But if the
second file is 10MB long it sill be transmitted in only 2.8 seconds while 5MB file still
takes roughly 12 minutes.

PACKETS AND TDM:
Dividing data into small packets allow time division multiplexing. In TDM each packet
leaves the source and is switched on the shared communication channel through a
multiplexer. At the destination the packet is switched through a demultiplexer to the
destination.
In the figure this process is illustrated with a multiplexing circuit shown.