The Link Layer

1
The Link Layer

• Has 2 main Components
• The Data Link Layer
• Medium Access Control (MAC)
• ExamplesSLIP, PPP, Ethernet.

2
Logical Link Layer Function

• Provides service to the network layer
• Has two Components
• Data link layer
• Framing
• Error Handling
• Flow control
• Medium access control
• In the case of a shared medium, this function
  provides means for sharing optimally the medium

3
Framing

• What is it? breaking up the bits into discrete
  frames and checking the total number of received
  bits (the checksum)
• Why? to ensure that the number and the value of
  the sent and the received bits are exact
• How? character count, Start/Stop characters,
  Start/stop flags

4
Character Count
Character count
Character count
Character count
5 1 2 3 4 5 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3
Frame 1 with 5 char
Frame 4 with 8 char
The problem with this method is
New Character counts
Transmission Error
5 1 2 3 4 7 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3
Frame 1 with 5 char
5
Start/Stop characters
Stuffed DLE
A
DLE
B
A
DLE
DLE
B
STX
DLE
ETX
DLE
A
DLE
B

• STX is Start of TeXt
• DLE is Data Link Escape
• ETX End of TeXt

6
Start/stop flags
7
Error Handling

• What is it? A mechanism to make sure all frames
  are delivered to the right destination and in the
  right order and only once
• Why? The sent data may be corrupted or vanish in
  the network
• How? Give feedback to the source machine by
  acknowledging the reception and using temporal
  flags (i.e. a timer) Then after correcting the
  errors

8
Error Handling

• Error detection
• Parity bit
• 2D/Crossed parity
• Hamming code
• Error correction
• Hamming Code

9
Parity bit

• The sender adds a parity bit to the end of the
  word such that the total number of bits
  (including the parity bit) is even (Even parity)
• 01110110 becomes 011101101
• Detects only odd number of corrupted bits Why?

10
2D/Crossed Parity
11
Hamming code (Detecting Correcting)

• Hamming distance of two WORDS is the number of
  bits that differ (exclusive OR between the 2
  words)
• Example 10001001 and 10110001, the Hamming
  distance is 3
• The Hamming distance of a list of codewords is
  the smallest distance between any pair of
  codewords on this list

12
Hamming code (Detecting Correcting)

• Assume the following list of valid codewords
• 0 0 0 0 0 0 0 0 0 0
• 0 0 0 0 0 1 1 1 1 1
• 1 1 1 1 1 0 0 0 0 0
• 1 1 1 1 1 1 1 1 1 1
• What is the hamming distance of this list?
• It is 5 since the smallest distance between any
  two codewords of this list is 5

13
How can we use hamming codes in errors
detection/correction

• Codeword Data Checkbits
• N bits M bits R bits
• Given the structure codewords, we can build a
  complete list of valid codewords
• A Hamming distance of d will allow to detect up
  to (n-1) corrupted bits Why?
• A Hamming distance of d will allow to correct up
  to (n/2) or ((n-1)/2) corrupted bits Why?

14
Hamming code (Detecting Correcting)

• Hence
• we need a distance of d1 to detect d errors
• we need a distance of 2d1 to correct d errors
• A hamming distance of 5bits can
• detect errors of 4bits
• correct errors of 2bits
• See the example in page 185
• Generally, Checkbits bits are put at power of 2
  positions (1, 2, 4, 8, etc.) and Data elsewhere...

15
Building a Hamming Code of Distance 3
16
Family of Hamming Codes
17
Flow Control (FC)

• What is it? Synchronization mechanism to allow
  faster machines and slow machines to coexist on
  the net
• Why? The flow of faster machines is higher than
  the one of slow machines
• How? Throttling the sender into sending no more
  than the receivers capacity
• The basic principle Send n frames now but,
  after they have been sent, dont send any more
  until getting an OK to continue The protocol
  contains well defined rules about when a sender
  may transmit the next frame

18
Elementary FC protocols fundamentals

• Read carefully pages 191..193 for the basic
  definitions of the standard variables and
  functions that are used

19
Elementary FC protocols example 1

• The unrestricted Simplex Protocol The Utopia
  Protocol (fig. 3-9, page 196)

20
Elementary FC protocols Example 2

• A simplex protocol for noisy channels The stop
  and wait Protocol (fig. 3-10, page 198)

21
Elementary FC protocols Example 3
A positive acknowledgement with retransmission
protocol
22
Elementary FC protocols Example 4
See fig. 3-13 page 205