The Data Link Layer

1
The Data Link Layer

• Chapter 3

2
Data Link Layer Design Issues

• Main topics the algorithms for achieving
  reliable, efficient communication between two
  adjacent machines at the data link layer.
• By adjacent, we mean the two machines are
  physically connected by a communication channel
  (e.g., a coaxial cable or a phone line) that acts
  conceptually like a wire.
• The essential property of a channel that makes it
  wire-like'' is that the bits are delivered in
  exactly the same order in which they are sent.
• Why need data link layer software ?
• Communication circuits make errors occasionally,
• they have a finite data rate,
• there is a nonzero propagation delay, and
• the adjacent machines have finite and maybe
  different processing speeds.

3
Functions of the Data Link Layer

• Provide service interface to the network layer
• Dealing with transmission errors
• Regulating data flow slow receivers not swamped
  by fast senders

4
Relationship between packets and frames
5
Services Provided to Network Layer
The principal service transferring data from the
network layer on the source machine to the
network layer on the destination machine.

• Virtual communication. Actual
  communication.

• Three possible services
• Unacknowledged connectionless service.
• Acknowledged connectionless service.
• Connection-oriented service.

6
Placement of the data link protocol

• An example a subnet consisting of routers
  connected by point-to-point lines.
• When a frame arrives at a router, the hardware
  verifies the checksum and passes the frame to the
  data link layer software.
• The data link layer software, which may be
  embedded in a chip on the network adaptor board,
  checks to see if this is the frame expected. and
  if so, gives the packet in the payload field to
  the (network layer) routing software.
• The routing software chooses the appropriate
  outgoing line and passes the packet back down to
  the data link layer software.
• The data link layer software transmits it over
  the selected outgoing line.
• Many of the principles at the data link layer,
  such as error control and flow control, are also
  found in transport and other layers and protocols
  as well.

7
Framing

• What is framing ?
• The process of breaking the bit stream offered by
  the physical layer into discrete segments
  (frames).
• Why framing ?
• For computing the checksum for each frame.
• For acknowledging the lost frames.
• How to do framing ?

8
A character stream

• Without errors. (b) With one error.

What happens if the count is garbled by an error
?
9
Flag bytes

• A frame delimited by flag bytes. (b) Four
  examples of byte sequences before and after
  stuffing.

Disadvantages closely tied to 8-bit
characters and the ASCII character code.
10
Bit stuffing

• (a) The original data.
• (b) The data as they appear on the line.
• (c) The data as they are stored in receivers
  memory after destuffing.

11
Physical layer coding violations

• The IEEE 802 standard LAN uses the following
  (Differential Manchester) encoding scheme a 1
  bit is encoded as HL, and a 0 bit as LH.
• The invalid encodings - HH and LL - can be used
  for framing.
• Character count can be combined with others for
  extra safety.

12
Error control and flow control

• Error control the goal (of a reliable
  connection-oriented service) to ensure each
  frame is ultimately passed to the network layer
  at the destination exactly once, no more and no
  less.
• The measures
• Acknowledgements from the receiver to the sender.
  
• Timers for preventing senders from hanging
  forever.
• Sequence numbers for dealing with duplicated
  frames.
• Flow control What to do with a sender that
  systematically wants to transmit frames faster
  than the receiver can accept them ?
• Some kind of feedback mechanism is needed so that
  the sender can be made aware of whether or not
  the receiver is able to keep up.
• This is frequently integrated with error handling
  for convenience.

13
Error Detection and Correction

• Transmission errors are a fact of life.
• Sources of errors (local loop and wireless
  links)
• Thermal noise and electromagnetic interference
  broad-spectrum background noise level.
• The frequency dependency of the amplitude,
  propagation speed, and phase of signals
  distortion.
• Echoes, off-course birds, etc.
• Errors tend to come in burst rather than singly.

14
Error-Correcting Codes
Basic idea Including redundant information along
with each block of data sent to enable the
receiver to deduce what the transmitted character
must have been. An unit containing m bits of
data and r bits of redundancy, or check bits, is
referred to as an m r bits codeword. The
number of bit positions in which two codewords
differ is called the Hamming distance. Two d
apart codewords require d single-bit errors to
convert one into the other. The minimum
Hamming distance between any two codewords from a
list of codewords is the distance of the complete
list. To detect d errors, a distance of d
1 is needed because with such a code there is no
way that d single-bit errors can change a valid
codeword into another valid codeword. To
correct d errors, a distance of 2d 1 is needed
because that way the legal codewords are so far
apart that even with d changes, the original
codeword is still closer than any other codeword,
so it can be uniquely determined.
15
Hamming code

• At the sending side
• The bits of the codeword are numbered
  consecutively, starting with bit 1 at the left
  end.
• The bits that are powers of 2 (1, 2, 4, 8, 16,
  etc.) are check bits. The rest (3, 5, 6, 7, etc.)
  are filled up with the data bits.
• Check bit forces the parity of the following bits
  to be even (or odd)
• the check bit itself, and
• all data bits whose position number has an
  element of 2i when it is expended into a sum of
  powers of 2. E.g., for check bit 1, the following
  data bits are checked 3 1 2, 5 1 4, 7
  1 2 4, ...
• One data bit is checked by all and just those
  check bits occurring in its expansion.

16
Hamming code

• At the receiver side
• A counter is initialized into zero.
• Each check bit k ( 1, 2, 4, ...) and its checked
  data bits are examined to see if the parity is
  correct.
• If not, k is added into the counter.
• If the counter is zero at the end, the codeword
  is accepted as valid. Otherwise, it contains the
  number of the incorrect bit.
• E.g., if the parities were wrong when examining
  check bits 1, 2, 4, then the counter-value 7
  1 2 4 , we can conclude that data bit 7 was
  inverted since bit 7 is the only one checked by
  bits 1, 2, and 4.

17
Use of a Hamming code to correct burst errors
The following figure shows some 7-bit ASCII
characters encoded as 11-bit codewords using a
Hamming code. The data bits are found in bit
positions 3, 5, 6, 7, 9, 10, and 11.

• To correct burst errors by means of Hamming codes
  
• A sequence of codewords are arranged as a matrix,
  one codeword per row.
• The data is transmitted one column ( bits) at a
  time, starting with the leftmost column.
• The matrix is reconstructed at the receiving
  side.
• If a burst error of length occurs, at most 1 bit
  in each codeword will be effected, which can be
  corrected by the Hamming code.

18
Error-detecting codes

• Error-correcting codes are suitable for simplex
  channels where no retransmission can be
  requested.
• Error-detection plus retransmission is often
  preferred because it is more efficient.
• For comparison, consider a channel with error
  rate of 10(-6) per bit. Let block size be 1000
  bits.
• To correct a single error (by Hamming code), 10
  check bits per block are needed. To transmit 1000
  blocks, 10,000 check bits (overhead) are
  required.
• To detect a single error, a single parity bit per
  block will suffice. To transmit 1000 blocks, only
  one extra block (due to the error rate of
  10(-6) per bit) will have to be retransmitted,
  giving the overhead of only 2001 ( 1000 1001)
  bits.
• Under what conditions is error-correction more
  efficient ?

19
Cyclic Redundancy Code (CRC) method

• A k-bit frame is regarded as the coefficient list
  for a polynomial with k terms, ranging from
  x(k-1) to x0.
• Polynomial arithmetic is done modulo 2. Both
  addition and subtraction are identical to
  EXCLUSIVE OR
• 10011011 01010101
• 11001010 - 10101111
• ---------------
  ---------------
• 01010001 11111010
• The basic idea of the CRC method
• The sender appends a checksum to the end of the
  frame in such a way that the polynomial
  represented by the checksummed frame is divisible
  by a generator polynomial G(x).
• When the receiver gets the frame, it tries
  dividing it by the same G(x). If there is a
  remainder, there must have been an error and a
  retransmission will be requested.
• How to compute the chechsum for a given frame ?

20
Calculation of the CRC checksum

• In any division problem, if you diminish the
  dividend by the remainder, what is left over is
  divisible by the divisor.
• Detectable errors by CRC
• All single errors are detectable if G(x)
  contains two or more terms.
• All double errors are detectable if G(x) does not
  divide xk 1 for any k up to the maximum
  frame length.
• All odd number of errors if G(x) contains a
  factor of x 1 .
• All burst errors of length ? r (the degree of
  G(x) ) or the number of check bits.
• The probability of a burst error of length r 1
  being accepted as valid is (0.5) (r 1).
• The probability of a bad frame, garbled by a long
  burst or multiple shorter bursts, getting through
  unnoticed is (0.5)r.
• Simple hardware has been built to compute and
  verify the CRC checksum.

21
Elementary Data Link Protocols

• Assumptions
• The physical layer, data link layer, and network
  layer are independent processes.
• The network layer contacts the data link layer by
  generating proper events. There exist suitable
  library procedures for the data link layer to
  delivery a packet (To_network_layer), and to
  fetch a packet (from_network_layer).
• The data link layer provides a reliable,
  connection-oriented service to the network layer.
  
• The physical layer computes/checks checksums in
  outgoing/incoming frames, and contacts the data
  link layer by generating proper events. There
  exist suitable library procedures for the data
  link layer to send a frame (to_physical_layer),
  and to fetch a frame (from_physical_layer).

22
Protocol Definitions
Continued ?
Some definitions needed in the protocols to
follow. These are located in the file protocol.h.
23
Protocol Definitions(ctd.)
Some definitions needed in the protocols to
follow. These are located in the file
protocol.h.
24
Unrestricted Simplex Protocol
25
Simplex Stop-and-Wait Protocol
26
A Simplex Protocol for a Noisy Channel

• A scheme using timer and acknowledgement
• The sender starts a timer after sending a frame.
• The receiver sends back an acknowledgement only
  when the incoming frame were correctly received.
• The sender will retransmit the frame if the timer
  expired before receiving an acknowledgement.
• What is the fatal flaw in the above scheme ?
• Loss of an acknowledgement duplicated frame.
• How to distinguish a frame being seen for the
  first time from a retransmission ?
• Use a sequence number in the header of each
  frame.

Continued ?
27
A Simplex Protocol for a Noisy Channel (ctd.)
A positive acknowledgement with retransmission
protocol.
28
Sliding Window Protocols

• How to achieve full-duplex data transmission ?
• Two separate simplex data channels -- waste of
  bandwidth.
• One circuit for data in both directions and using
  the field in the frame header to distinguish a
  data from an acknowledgement.
• Piggybacking temporarily delay outgoing
  acknow-ledgements so that they can get a free
  ride on the next outgoing data frame. Save
  bandwidth, number of interrupts and buffers.
• How long should the receiver wait for an outgoing
  frame onto which to piggyback the
  acknowledgement ?
• An ad hoc scheme waiting a fixed number of msec
  (timer).

29
Basics of sliding window protocols

• A sequence number is assciated with each
  transmitted frame. Sequence numbers range from 0
  up to some maximum (2n - 1) circularly.
• A window is a list of consecutive sequence
  numbers. The size of a window is determined by
  two pointers the lower edge and the higher edge.
  The size can be fixed or dynamically extended and
  shrunk.
• The sender maintains a sending window with
  consecutive sequence numbers corresponding to
  frames sent but has yet not acknowledged.
• Whenever a new packet arrives from the network
  layer, it is given the next highest sequence
  number, and the upper edge of the window is
  advanced by one.
• When an acknowledgement comes in, the lower edge
  is advanced by one.
• The size of the sender's window is a dynamical
  one.
• The receiver maintains a receiving window with
  consecutive sequence numbers corresponding to
  frames it is permitted to accept.
• When a frame whose sequence number is equal to
  the lower edge of the window is received, it is
  passed to the network layer, an acknowledgement
  is generated, and the window is rotated by one.
• The receiver's window always remains at its
  initial size.

30
A sliding window of size 1, with a 3-bit sequence
number

• (a) Initially.
• (b) After the first frame has been sent.
• (c) After the first frame has been received.
• (d) After the first acknowledgement has been
  received.

31
A One-Bit Sliding Window Protocol

• The maximum window size is 1.
• Stop-and-wait.

Continued ?
32
A One-Bit Sliding Window Protocol (ctd.)
33
A One-Bit Sliding Window Protocol (2)

• Two scenarios for protocol 4. (a) Normal
  case. (b) Abnormal case. The notation is (seq,
  ack, packet number). An asterisk indicates where
  a network layer accepts a packet.

34
A Protocol Using Go Back N

• A tacit assumption in previous protocols the
  time in the following period is negligible
• The transmission time for a frame to arrive at
  the receiver.
• The processing time for the receiver to handle
  the incoming frame.
• The transmission time for an acknowledgement to
  come back at the sender.
• Consequently, the sender is blocked during the
  above period, which can be very large in channels
  with long round-trip propagation delay.
• How to solve this problem ?
• The pipelining solution Set the maximum
  sender's window size (the number of outstanding
  frames) so large that the sender is allowed to
  transmit multiple frames until the
  acknowledgement for the first frame comes back.
  Thereafter, acknowledgements will arrive one
  after another, so the sender always gets
  permission to transmit.
• What to do if a frame in the middle of a long
  stream is damaged or lost ?

35
Pipelining and error recovery

• (a) The go back n strategy
• The receiver discards all subsequent frames,
  sending no acknowledgements. The receiver's
  window size is 1.
• After time out, the sender retransmits all
  unacknowledged frames in order, starting with the
  damaged or lost one.

• (b) The selective repeat strategy
• The receiver buffers correct subsequent frames,
  but sending no acknowledgements (or Nak). The
  receiver's window size is larger than 1.
• After time out (or receiving Nak), the sender
  retransmits the damaged or lost frame.

36
Sliding Window Protocol Using Go Back N
Continued ?
37
Sliding Window Protocol Using Go Back N
Continued ?
38
Sliding Window Protocol Using Go Back N
Continued ?
39
Sliding Window Protocol Using Go Back N
40
Sliding Window Protocol Using Go Back N (2)

• Simulation of multiple timers in software.

41
A Sliding Window Protocol Using Selective Repeat
Continued ?
42
A Sliding Window Protocol Using Selective Repeat
(2)
Continued ?
43
A Sliding Window Protocol Using Selective Repeat
(3)
Continued ?
44
A Sliding Window Protocol Using Selective Repeat
(4)
45
A Sliding Window Protocol Using Selective Repeat
(5)

• (a) Initial situation with a window size seven.
• (b) After seven frames sent and received, but not
  acknowledged.
• (c) Initial situation with a window size of four.
• (d) After four frames sent and received, but not
  acknowledged.

46
Protocol Verification

• Finite State Machined Models
• Petri Net Models

47
Finite State Machined Models

• (a) State diagram for protocol 3. (b)
  Transmissions.

48
Petri Net Models

• A Petri net with two places and two transitions.

49
Petri Net Models (2)

• A Petri net model for protocol 3.

50
Example Data Link Protocols

• HDLC High-Level Data Link Control
• The Data Link Layer in the Internet

51
High-Level Data Link Control
The nice thing about standards is that you have
so many to choose from. At the data link layer,
we have IBM SDLC (Synchronous Data Link Control),
ANSI ADCCP (Advanced Data Communication Control
Procedure), ISO HDLC (High-level Data Link
Control), CCITT LAP (Link Access Procedure), etc.
If you do not like any of them, you can just
wait for next year's model. All are bit-oriented
and using bit stuffing. All use the same frame
structure

• The frame is delimited with flag sequence
  01111110.
• Address can be used to identify a terminal on
  multidrop lines or to distinguish commands from
  responses for point-to-point lines.
• Control is used for sequence numbers,
  acknowledgements, and others.
• Data can be arbitrarily long.
• Checksum uses CRC-CCITT.

52
High-Level Data Link Control (2)

• Three kinds of frames distinguished by the
  control field
• (a) An information frame.
• (b) A supervisory frame.
• (c) An unnumbered frame.

53
The Data Link Layer in the Internet

• There are two situations in which point-to-point
  communication occurs in Internet
• Router-router leased line connection between
  Internet routers.
• Host-router dial-up line connection between home
  computers and the Internet service providers'
  routers, as shown in the following Figure.

• A home personal computer acting as an internet
  host.

54
PPP Point to Point Protocol

• PPP provides three features
• A framing method that unambiguously delineates
  the end of one frame and the start of the next
  one. The frame format also handles error
  detection.
• A link control protocol for bringing lines up,
  testing them, negotiating options, and bringing
  them down again gracefully when they are no
  longer needed. This protocol is called LCP (Link
  Control Protocol).
• A way to negotiate network-layer options in a way
  that is independent of the network layer protocol
  to be used. The method chosen is to have a
  different NCP (Network Control Protocol) for each
  network layer supported.

55
A typical scenario of PPP

• A typical scenario of a home user calling up an
  Internet service provider to make a home PC a
  temporary Internet host
• Physical connection setup phase
• The PC calls the provider's router via a modem.
• The router's modem answers the phone and
  establishes a physical connection.
• Data link layer options negotiation phase
• The PC sends the router a series of LCP packets
  in the payload field of one or more PPP frames.
  These packets and their responses select the PPP
  parameters to be used.
• Network layer options negotiation phase
• A series of NCP packets are sent to configure the
  network layer and to assign an IP address for the
  PC (if the PC wants to run a TCP/IP protocol
  stack).
• Data communication phase
• The PC sends and receives IP packets over the
  established connection.
• Connection release phase
• When the PC is finished, NCP is used to tear down
  the network layer connection and free up the IP
  address.
• The LCP is used to shut down the data line layer
  connection.
• The computer tells the modem to hang up the
  phone, releasing the physical connection.

56
The PPP full frame format for unnumbered mode
operation
The PPP frame was chosen to closely resemble the
HDLC frame, as shown below
In summary, PPP is a multi-protocol framing
mechanism suitable for use over modems, HDLC
bit-serial lines, SONET, and other physical
layers. It supports error detection, option
negotiation, header compression, and optionally,
reliable transmission using HDLC framing.
57
PPP Point to Point Protocol (2)

• A simplified phase diagram for bring a line up
  and down.

58
PPP Point to Point Protocol (3)

• The LCP frame types.