1. Chapter 3
The Data Link Layer

2. Data Link Layer Design Issues
Services Provided to the Network Layer
Framing
Error Control
Flow Control

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. Functions of the Data Link Layer (2)
Relationship between packets and frames.

5. Services Provided to Network Layer
(a) Virtual communication.
(b) Actual communication.

6. Services Provided to Network Layer (2)
The data link layer can be designed to offer various services:
Unacknowledged connectionless service
Acknowledged connectionless service
Acknowledged connection-oriented service

7. Services Provided to Network Layer (3)
Placement of the data link protocol.

8. Framing data link layer must use physical layer.
physical layer accept a raw bit stream and attempt to deliver it to the destination.
In destination, some bits may have different values and the number of bits received may be less than, equal to, or more than the number of bits transmitted.
Data link layer must detect and, if necessary, correct errors.
Framing:
break up the bit stream into discrete frames and add checksum

9. Framing (2) Framing methods: Byte count Flag bytes with byte stuffing
Flag bits with bit stuffing
Physical layer coding violations

10. A character stream. (a) Without errors. (b) With one error.
Framing (3)
Byte count
A character stream. (a) Without errors. (b) With one error.

11. PPP (Point-to-Point Protocol)
Framing (4)
Flag bytes with byte stuffing
(a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing.
PPP (Point-to-Point Protocol)

12. Framing (5) Flag bits with bit stuffing Bit stuffing
Each frame begins and ends with a special bit pattern,
HDLC (High level Data Link Control) protocol
Bit stuffing
(a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in receiver’s memory after destuffing.

13. Error Control reliable delivery Using feedback Ack (Acknowledgements)
NAck (Negative Acknowledgements)
Timer
sequence numbers

14. Flow Control feedback-based flow control rate-based flow control
‘‘You may send me n frames now, but after they have been sent, do not send any more until I have told you to continue.’’

15. Error Detection and Correction
Type of errors
Single bit
Burst
Error-Correcting Codes
FEC (Forward Error Correction)
Error-Detecting Codes
For highly reliable links, such as fiber

16. Error-Correcting Codes
n-bit Codeword:
n = m + r
Code rate:
m/n
Hamming Distance:
To reliably detect d errors, you need a distance d + 1 code
to correct d errors, you need a distance 2d + 1 code

17. Error-Correcting Codes
Example:
, , , and
Hamming distance ???
Correct and detect ???
is Received. What was original frame?
we want to design a code with m message bits and r check bits that will allow all single errors to be corrected.

18. Error-Correcting Codes
Hamming code

19. Error-Correcting Codes
Use of a Hamming code to correct burst errors.

20. Error-Correcting Codes
Convolutional code
are specified in terms of their rate and constraint length
Are used as part of the GSM mobile phone system, in satellite communications, and in
the NASA convolutional code of r = 1/2 and k = 7

21. Error-Detecting Codes
Parity
Checksums
Cyclic Redundancy Checks (CRCs)

22. Error-Detecting Codes
Parity
Problem with burst error
Using rectangular matrix n bits wide and k bits high
Interleaving
A burst of length n + 1 will pass undetected

23. Error-Detecting Codes
Checksums
groups of parity bits

24. Error-Detecting Codes
CRC
Calculation of the polynomial code checksum.

25. Error-Detecting Codes
CRC
All single-bit errors will be detected.
By making x + 1 a factor of G(x), we can catch all errors with an odd number of inverted bits.
a polynomial code with r check bits will detect all burst errors of length ≤ r
Use in Ethernet
detects all bursts of length 32 or less
detects all bursts affecting an odd number of bits

26. Elementary Data Link Protocols
An Unrestricted Simplex Protocol
Error-free
Unrestricted buffers
Without delay
A Simplex Stop-and-Wait Protocol
Flow control protocol
Acknowledgement
Data traffic
Simplex
Half duplex

27. Elementary Data Link Protocols
A Simplex Protocol for a Noisy Channel
Frames may be either damaged or lost completely
Using Ack
Using timer
Using Sequence number
How many bits are required ?????
PAR (Positive Acknowledgment with Retransmission)
ARQ (Automatic Repeat reQuest)

28. Link Utilization

29. Link Utilization Example:
50-kbps satellite channel with a 500-msec round-trip propagation delay
Size of frames : 1000-bit
The time which first frame has been completely sent:
The time which first frame fully arrived:
The time which first acknowledgement arrived back:
t = 20 msec
t = 270 msec
t = 520
Sender was blocked 500/520 or 96% of the time. In other words, only 4% of the available bandwidth was used
Solution : Pipelining

30. Sliding Window Protocols
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. Sliding Window Protocols (2)

32. Sliding Window Enhancements
Receiver can acknowledge frames without permitting further transmission (Receive Not Ready)
Must send a normal acknowledge to resume
If duplex, use piggybacking
If no data to send, use acknowledgement frame
If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)
bandwidth-delay product of the link : BD

33. Go Back N Based on sliding window
If no error, ACK as usual with next frame expected
Use window to control number of outstanding frames
If error, reply with NACK
Discard that frame and all future frames until error frame received correctly
Transmitter must go back and retransmit that frame and all subsequent frames

34. Go Back N - Damaged Frame
Receiver detects error in frame i
Receiver sends NACK-i
Transmitter gets NACK-i
Transmitter retransmits frame i and all subsequent

35. Go Back N - Lost Frame (1) Frame i lost Transmitter sends i+1
Receiver gets frame i+1 out of sequence
Receiver send NACK i
Transmitter goes back to frame i and retransmits

36. Go Back N - Lost Frame (2) Frame i lost and no additional frame sent
Receiver gets nothing and returns neither acknowledgement nor NACK
Transmitter times out and sends acknowledgement frame with P bit set to 1
Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )
Transmitter then retransmits frame i

37. Go Back N - Damaged Acknowledgement
Receiver gets frame i and send acknowledgement (i+1) which is lost
Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame i
If transmitter times out, it sends acknowledgement with P bit set as before
This can be repeated a number of times before a reset procedure is initiated

38. Go Back N - Damaged NACK ?

39. Go Back N - Diagram
RR : Ready Receive
Ack
REJ : Reject
Nack

40. Go Back N
The maximum number of frames that may be outstanding at any instant is not the same as the size of the sequence number space
The sender sends frames 0 through 7.
A piggybacked acknowledgement for 7 comes back to the sender.
The sender sends another eight frames, again with sequence numbers 0 through 7.
Now another piggybacked acknowledgement for frame 7 comes in.
Did all eight frames belonging to the second batch arrive successfully, or did all eight get lost?

41. Selective Repeat
Also called Selective Reject or selective retransmission
Only rejected frames are retransmitted
Subsequent frames are accepted by the receiver and buffered
Minimizes retransmission
Receiver must maintain large enough buffer
More complex logic in transmitter

42. Selective Repeat - Diagram

43. ? Selective Repeat Limitation of windows size:
For K-bit sequence number ?

44. Example Data Link Protocols
HDLC – High-Level Data Link Control
The Data Link Layer in the Internet

45. High-Level Data Link Control
Bit-oriented
Using bit stuffing techniques for redundancy
Address use for detecting command/response in P2P line
Sending just flags -> idle P2P line

46. High-Level Data Link Control (2)
Control field of
(a) An information frame.
(b) A supervisory frame.
(c) An unnumbered frame.

47. High-Level Data Link Control (3) Information frame
Using sliding window with 3-bit Seq number
Next : Use for piggybacking
P/F : Poll/Final

48. High-Level Data Link Control (4) Supervisory frame
Type 0 : Receive Ready (RR)
Type 1 : Reject
Next : First Frame which has received with error
Type 2 : Receive Not Ready (RNR)
Next : First Frame which has received with error.
Stop sending
Send RR or Reject for start
Type 3 : Selective Reject

49. High-Level Data Link Control (5) Unnumbered frame
Use for control
Use for Sending data in Unreliable connection-less services
5 bit for Type
DIS Connect (DISC)
Set Normal Response Mode (SNRM)
FRaMe Reject (FRMR) : Checksum is OK
Number of bit is less than 32
Wrong Ack …

50. The Data Link Layer in the Internet
A home personal computer acting as an internet host.

51. PPP – Point to Point Protocol
PPP Features
Separate packets (Framing), error detection
Link Control Protocol (LCP)
bringing lines up, testing them, negotiating options, and bringing them down again gracefully when they are no longer needed.
Network Control Protocol (NCP)
negotiate network-layer options in a way that is independent of the network layer protocol to be used

52. PPP – Point to Point Protocol
PPP and HDLC
PPP is character oriented
PPP uses byte stuffing

53. PPP – Point to Point Protocol
is broadcast
Control field indicates an unnumbered frame
LCP can omit Address and Control
Protocol field
Codes starting with a 0 : IPv4, IPv6, IPX, AppleTalk, …
Codes starting with a 1 : PPP configuration protocols (LCP, NCP)
default length of Payload is 1500 bytes
Padding may follow the payload if it is needed

54. PPP – Point to Point Protocol (2)
A simplified phase diagram for bring a line up and down.

55. ADSL and ATM ?