Understanding Packets, Frames, and Error Detection in Computer Networks, Slides of Computer Networks

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COMPUTER

NETWORKS

Lecture-

CHAPTER No. 7

PACKETS, FRAMES

and

ERROR DETECTION

The Concept Of Packets

 The problems:

• A sender and a receiver need to

coordinate to detect transmission

errors

• Individual connections between

each pair of computers is not

possible

The Concept Of Packets

 The solution:

• Shared network connections among

many workstations.

Use of Packets

Problems with the Sharing

 Demand high – Many computers need

to use the shared networks

 Some applications have large data

transfers – Holds the network for a

long time

 Some applications cannot wait

 Need mechanism for fairness

Example

Example

 WITHOUT PACKETS

• 5MB file transferred across network with 56Kbps capacity will require 12 minutes:
• All other computers will be forced to wait 12 minutes before initiating other transfers

5x10 bytes * 8 bits / byte 60 secs / minute * 56x10 bits / second

11.9 minutes

6 3 =

Advantage

 Other computer must only wait .143 seconds before beginning to transmit

 Note:

• If both files are both 5MB long, each now takes 24 minutes to transmit
• BUT if second file is only 10KB long, it will be transmitted in only 2.8 seconds, while 5MB file still takes roughly 12 minutes

Packets and TDM

 Dividing data into small packets allows Time-

Division Multiplexing

 Each packet leaves the source and is

switched onto the shared communication channel through a Multiplexer

 At the destination, the packet is switched

through a Demultiplexer to the destination

Packets and Frames

 Packet is ``generic'' term that refers

to a small block of data

 Each hardware technology uses

different packet format

 Frame or hardware frame denotes a

packet of a specific format on a

specific hardware technology

Frame Format

 Need to define a standard format for data

to indicate the beginning and end of the frame

 Header and trailer used to “frame” the data

Framing in Practice

 Incurs extra overhead - soh and eot take time to transmit, but carry no data

 Accommodates transmission problems:

• Missing eot indicates sending computer crashed
• Missing soh indicates receiving computer missed beginning of message
• Bad frame is discarded

Data Stuffing

 Sometimes, the special characters (i.e.

soh or eot ) may appear in the data

 Problem: soh and eot as part of data will

be misinterpreted as framing data

 Solution: Data stuffing