Integrity in Data
Communications
Packets, Frames and Error
Detection
More About Packets

networks
are called amounts of
Networks do not These
transfer
arbitrary
‘packet networks’ or packet
data for 2 reasons:switching networks’
1. Errors in large blocks cause large delays.
-Senders and receivers have to coordinate
transmission. Errors often occur.
-Dividing data into small blocks allows the
sending and receiving computers to make fast retransmission.
More About Packets
2.

Computers have to share underlying connections
in hardware.
-Communications channels are expensive so
sharing allows all to be treated equally.
Early networks allowed an application to hold a
resource until finished.
Packets and Time-Division
Multiplexing

Computers take turns sending and receiving
small packets of data.
1
Computer 1 using channel to send packet.
2
Multiplexing occurs here.
3
Packets and Time-Division
Multiplexing

Computers take turns sending and receiving
small packets of data.
1
Computer 2 using channel to send packet.
2
Multiplexing occurs here.
3
Packets and Time-Division
Multiplexing

Computers take turns sending and receiving
small packets of data.
1
Computer 3 using channel to send packet.
2
Multiplexing occurs here.
3
Packets and Time-Division
Multiplexing

Computers take turns sending and receiving
small packets of data.
A source with a small total amount will finish
promptly.
Larger amounts will take longer.

Data are arriving at two or more computers
simultaneously.
Packets and Hardware Frames



Packet = small block of data (General)
Hardware technology defines size.
Frame = term used for specific hardware
``packet``.
Packets and Hardware Frames



Packet = small block of data (General)
Hardware technology defines size.
Frame = term used for specific hardware
``packet``.
soh
block of data in frame
eot
Packets and Hardware Frames



Packet = small block of data (General)
Hardware technology defines size.
Frame =Unprintable
term used for specific hardware
ASCII
``packet``.
Characters
soh
Hex 01
block of data in frame
eot
Hex 04
Packets and Hardware Frames





Packet = small block of data (General)
Hardware technology defines size.
Frame = term used for specific hardware
``packet``.
Disadvantage is overhead.
Advantage is reliability.
Byte Stuffing



Most networks cannot afford to reserve
characters.
Systems never confuse data with control
information.
So extra bits or bytes are inserted to change
data for transmission.
i.e. bit or byte stuffing

The esc character is Hex 1B
Byte Stuffing
Character
Characters
in Data
Sent
___________________________________
soh
eot
esc
esc x
esc y
esc z
Transmission Errors


Parity Checking (RS 232)
Checksums
 16

bit checksums
Break data into 16 bit (2byte) segments
 Sum
the values
 Send the sum in with the transmission
 Receiver compares answers after transmission
Transmission Errors

Checksums
H
e
l
l
o
w
o r
l
d
.
Transmission Errors

Checksums
H
48
e
65
l
l
o
6C
6C
6F
20
w
o r
77
6F
72
l
d
.
6C
64
2E
Transmission Errors

Checksums
H
48
e
65
l
l
o
6C
6C
6F
20
w
o r
77
6F
72
l
d
.
6C
64
2E
4865 + 6C6C + 6F20 + 776F + 726C + 642E + carry = 71FC
Ethernet

Bus topology (10/100 Megabits)





Gigabit Ethernet (Uses fibre as well)
Hardware monitors bus for carrier
No carrier – Sender transmits
Carrier – sender waits
Collisions
 Senders use a random number generator to
calculate delay time
 If collision occurs again the range of the
random number generator is increased.
ATM Technology

Designed for voice, video & data
Voice and video require low delay and jitter
Video also requires much higher data rate
ATM Technology

Packets should maximize payload
8
Kbytes is common in some networks
 Phone systems use an 8 bit audio sample
every 125 microsecond (millionth of second)
 Sender must delay more than a second to
accumulate enough samples to fill a packet
 Telephone systems employ echo cancellation
techniques
 Large packets also create an echo problem
ATM divides all data into fixed ``cells``
 48 octets for data
 5 octets of header information
ATM Technology




Nortel has developed 6.4 Tbps
Commercial platform of 6.4 Tbps was available
in 2001.
Uses Dense-wavelength division multiplexing.
Designed to deliver 99.9999% reliability
The End.