Part 2: Packet Transmission
Packets, frames
Local area networks (LANs)
Wide area networks (LANs)
Hardware addresses
Bridges and switches
Routing and protocols
Packets, frames and error
detection
• Packets and frames
• Control data and byte stuffing
• Error detection - parity bits, checksums and
cyclic redundancy checks
Packets
• Most networks transmit data in small blocks
called packets
– helps to detect transmission errors
– gives fair access for a shared connection
between many computers
• These are packet networks or packet
switching networks
An example of fair access
A wants to send
a 5 megabyte
file to C
B wants to send
a 10 kilobyte
file to D
network speed is
56,000 bits per second
Packets and time division
multiplexing
• Packet networks use a form of TDM
Packets and hardware frames
• Each type of hardware defines its own
format/wrapping for a packet called a frame
Simple example of framing
• A frame may include “unused” control data
values to mark both its beginning and end
• Advantage is error detection:
– transmitter crashes => eot will not arrive
– receiver crashes => soh marks next valid frame
• Disadvantages:
– requires two extra characters per frame
– cannot carry arbitrary values (e.g, soh and eot)
Byte stuffing
• So control characters can be included in data
• Reserve a special character to mark the
occurrence of control characters
• Transmitter scans data block and replaces all
occurrences of control characters. Receiver
performs the reverse mapping.
Character in data Characters sent
soh
esc x
eot
esc y
esc
esc z
An example of byte stuffing
Frame before byte stuffing
Frame after byte stuffing
Transmission Errors
• Much of the complexity of networks arises
from susceptibility to interference that can
cause:
– transmitted data to be lost or changed
– random data to appear
• Single-bit errors versus burst errors
Error Detection and Correction
• First we need to detect errors
– Sender includes some extra (redundant) information
that summarises the original data
– Receiver checks this
– Various schemes, differing in complexity, data
overhead and robustness
• Then we need to decide what to do
– Error correction
– Retransmission
Parity bits and parity checking
• Count number of 1 bits in the data and add
an extra parity bit to make this odd or even
– even parity - parity bit is set so that total
number of 1s is even - 1011001 => parity bit 0
– odd parity - total 1s should be odd - 1011001
=> parity bit 1
• Transmitter calculates and adds, receiver
calculates and checks
• Introduces additional costs
• Only detects limited types of errors
Other error detection methods
• Other methods include checksums and
cyclic redundancy checks
• These can be compared according to:
– amount of extra data to be transmitted
– amount of extra computation involved
– types of errors that are detected
• Note the difference between detecting that
an error occurred and knowing how to fix it
Checksums
– Interpret the data as if it were a sequence of
integers and add them together to get an integer
result called a checksum
– Add in any carry bits too
– Append the checksum to the frame
– 16 and 32 bit checksums are common and are
usually computed for a whole packet
Example checksum
Evaluation of checksums
• Data overhead - 16 or 32 bits
• Computational overhead - simple additions
• Undetected errors - some periodic reversal
of bits (e.g., reversing one bit in each of
four data items)
Example failure of checksums
data item
(binary)
0001
0010
0011
0001
totals
checksum
value
1
2
3
1
7
data item
(binary)
0011
0000
0001
0011
totals
checksum
value
3
0
1
3
7
Cyclic redundancy checks
(CRCs)
• Detects more errors than checksums and
only requires simple hardware
• Based on binary division rather than
addition
Overview of CRC
• Uses binary division instead of addition
• Sender wants to send D, a piece of data d bits long
• Sender and receiver agree a generator, G, a bit
pattern that is r + 1 bits long
• Sender appends R (an additional sequence of r
bits) to D so that the resulting sequence is exactly
divisible by G using binary (modulo 2) arithmetic
• Receiver divides the received bit pattern by G and
checks whether the remainder is 0
d bits
r bits
D: Data bits
R: CRC bits
• Can detect burst errors of less than r +1 bits
and odd number of bit errors
• Can detect burst errors of length greater
than r + 1 with probability 1 – 0.5r
CRC - hardware components
Exclusive or (xor) unit
CRC - hardware components
Shift register
CRC - combining components
• Combine 3 shift registers and 3 xor units
• Initialise registers and then feed in the bits
of the message one at a time
• Final state gives the CRC - calculated by
both the transmitter and receiver
Evaluation of CRCs
• Data overhead - 16 or 32 bits
• Computational overhead - low - combines
simple hardware devices
• Types of errors - good for burst errors changes to several bits in one location that
may be caused by a sudden interference
(e.g. by lightening)
Why do CRCs work?
• Mathematical analysis is beyond our scope
• Intuition
– each single bit of the message dramatically
affects the whole CRC (feeds into three places)
– the effect of each bit loops through the process
several times (the registers are connected into a
cycle)
Revised frame format
• Frames now contain a data block, framing
information and error detection information
• Transmitter does: data -> byte stuffing ->
framing -> error detection and receiver
does the reverse
Summary
• Packets and frames
• Control data and byte stuffing
• Error detection - parity bits, checksums and
cyclic redundancy checks