1. Chapter 5
Data Link
Layer

2. OBJECTIVES After reading this chapter, the reader should be able to:
Understand the functions of the data link layer.
Understand the concept of the hop-to-hop delivery compared to
host-to-host delivery and application-to-application delivery.
Understand the concept of access method and define different
access methods used in LANs and WANs .
Understand how error control is handled at the data link layer.
Understand the addressing mechanism used in the data link
layer and how network layer addresses are mapped to data
layer addresses.

3. 5.1
DUTIES OF THE
DATA LINK LAYER

4. Data link layer in the Internet model
Figure 5-1
Data link layer in the Internet model

5. Data-link layer duties
Figure 5-2
Data-link layer duties

6. 5.2
HOP-TO-HOP
DELIVERY

7. Figure 5-3
Hop-to-hop delivery
Transport layer :application program to application program
Network layer : host to host delivery (source to destination)
Datalink layer : hop to hop

8. 5.3
PACKETIZING

9. packetizing
Different protocols have different names for the packet at the data link layer
Most LANs (ethernet) refer to the packet as frames.
ATM WAN refers to a packet as a cell
A header and trailer are usually added to a packet received from the network layer
Header : length of the data, addressing information…
Trailer : error detection

10. 5.4
ADDRESSING

11. Technical Focus: Addresses in Local Area Networks
The physical address for most computers on local area networks
is imprinted on the network card that is installed in the computer.
If the user or network manager changes the network card
(because of a failure, for example), the physical address
of the computer is changed. In most cases, changing
the network card requires reconfiguration of the computer.

12. Addressing
The data link layer addresses are called physical addresses or MAC addresses and are used to find the address of the next hop in the hop to hop delivery
The physical address used by a LAN is totally different from that used by a WAN
A LAN address is 6 bytes (48 bits) long
A WAN address is usually longer

13. ARP (address resolution protocol)
Mapping the logical address (IP address) to the physical address (MAC address) dynamically.
Anytime a host or a router needs to find the physical address of another host or router on its network, it sends an ARP query packet.
The packet includes the physical and IP addresses of the sender and the IP address of the receiver.
Because the sender does not know the physical address of the receiver, this query is broadcast over the network.

14. Figure 5-4
ARP operation

15. 5.5
ERROR CONTROL

16. Error control In data communication, errors are inevitable
Using better equipment and more reliable transmission media may reduce the frequency of occurrence. But it can never eliminate errors.
Detect and correct errors

17. Note:
Data can be corrupted during transmission. For reliable communication, errors must be prevented, or detected and corrected.

18. Source of errors White noise : Impulse noise Crosstalk
due to the heat; is constant by nature
Impulse noise
It is a surprise signal that suddenly affects the medium
Can not be predicted
Effect of impulse noise depends on the speed of data transmission
Crosstalk
It is the effect of one medium on another

19. Source of errors Echo Jitter Attenuation Distortion
We have echo when a sending device receives some of the energy it has sent
Jitter
It is the result of a change in the signal when it passes through an electronic device
Attenuation
A signal may become too weak if it travels a lon distance
Distortion
Harmonics are combined at the receiver, the signal is not exactly the one sent by sender

20. In a single-bit error, only 1 bit in the data unit has changed.
Note:
In a single-bit error, only 1 bit in the data unit has changed.

21. Single-bit error ASCII LF : line feed ASCII STX : start of text
Figure 5-5
Single-bit error
ASCII LF : line feed
ASCII STX : start of text

22. Note:
A burst error means that two or more bits in the data unit have changed.

23. Burst error of length five
Figure 5-6
Burst error of length five
The errors not necessarily occur in consecutive bits.
The length of the burst is measured from the first corrupted bit to the last corrupted bit
Some bits in between may not have been corrupted

24. Error prevention White noise : reduce the ambient temperture
Crosstalk : twist or shield the cable
Attenuation : repeaters
Errors are inevitable, for accuracy we need to detect and correct errors

25. Note:
Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

26. Figure 5-7
Redundancy

27. Detection methods cyclic redundancy check Vertical redundancy check
Figure 5-8
Detection methods
cyclic redundancy check
Vertical redundancy check
Longitudinal redundancy check

28. Note:
In vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even.
Most common and last expensive mechanism for error detection.
Often called parity check.
A redundant bit (parity bit) is added to make the number of 1s even.

29. Even parity VRC concept
Figure 5-9
Even parity VRC concept

30. Longitudinal redundancy check
Figure 5-10
LRC
Longitudinal redundancy check
New column
Calculate the parity bit for each column and create a new column

31. Note:
In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant row of bits is added to the whole block.

32. Cyclic redundancy check
Figure 5-11
CRC
Cyclic redundancy check
Most powerful redundancy check.
Based on binary division rather than binary addition.
CRC remainder is appended to the end of the data.

33. Error correction
Once the data are detected, the receiver has 3 choice :
Simply discard the data
Discard the data but request a retransmission
Correct the data
Data correction is select when the data packet is very small and we need the accurcy but can not wait for retransmission
Very difficult to achieve if there are more than a few bits in error.

34. Error correction using retransmission
If the sender has not received positive news in due time, the frame is retransmitted.
The frame is damage :
The receiver send a negative acknowledgement
The receiver does not send an acknowledgement
The acknowledgement is lost
There are two protocols for retransmission :
Stop and wait automatic repeat request
Sliding window automatic repeat request

35. Stop and wait ARQ Only the fate of one frame at any moment is unknown
Figure 5-12
Stop and wait ARQ
Only the fate of one frame at any moment is unknown
After sending a frame the sender stops and waits before the next
frame is sent

36. Technical Focus: Procedure for Stop-And-Wait ARQ
The sending device keeps a copy of the last frame transmitted until it receives an acknowledgment for that frame
Both data frames and ACK frames are
numbered 0 and 1 alternately. A data 0 frame is acknowledged
by an ACK 1 frame.
If an error is discovered in a data frame, a negative
acknowledgment (NAK) frame is returned.
If an expected acknowledgment is not received within an
allotted time period, the sender assumes that the last data frame was lost in transit and sends it again.

37. Figure 5-13
Sliding-window ARQ

38. Technical Focus: Procedure for Sliding-Window ARQ
The sending device keeps copies of all transmitted frames until
they have been acknowledged.
In addition to ACK frames, a receiver can return a NAK frame
if the data have been received damaged. The NAK frame
tells the sender to retransmit a damaged frame.
Like stop-and-wait ARQ, the sending device in sliding-window
ARQ is equipped with a timer to enable it to handle lost
acknowledgments.

39. 5.6
FLOW CONTROL

40. Note:
Flow control refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment.
Buffer : the rate of transmission is faster than check and
process , for this reason, each receiving device has a block
of memory called buffer, reserved for storing incoming data
until they are processed.
If the buffer begins to fill up, the receiver must tell the sender
to halt transmission until it is once again able to receive.

41. 5.1
MEDIUM ACCESS
CONTROL

42. Figure 5-14
Medium access methods

43. Select The select procedure is used whenever the primary has
Figure 5-15
Select
Know if
the secondary
is ready to
accept the data
The select procedure is used whenever the primary has
something to sent.
The select procedure has priority over the poll procedure

44. Figure 5-16
Poll

45. Note:
The poll/select method is mostly used in time-sharing systems when a central computer is used to control other computers.

46. Token passing
In the token passing method, the permit is passed from one computer to another.
Stations are arranged around a physical or logical ring
Permit is a small frame called token.
If a station needs to send data, it waits for the token and captures it.
After capturing the token, the station send one or more frames.

47. Figure 5-17
Token passing network

48. Random access
The station checks to make sure the medium is idle.(carrier sense)
If the medium is idle, the station can send data.
There is still a potential for collision.to detect collision and send the data again, the station needs to continue monitoring the medium.

49. CSMA/CD Carrier sense multiple access with collision detection.
Check the level of energy in the medium to tell the idleness of the medium.
Frame from a distant station might be undetectable
If the stations discovered a collision(a very high level of energy), they know that their frames have been destroyed
To minimize the risk of collision, two stations each wait a random amount of time

50. CSMA/CA
Collision is avoided by sending a special frame to tell other stations how long it is usuing the medium before sending data.
If it receives no objection from any other station, it means that it can send data

51. Note:
Token passing is used mostly by local area networks (LANs). We discuss LANs in Chapter 9.

52. CSMA/CD is used in the Ethernet LAN discussed in Chapter 9.
Note:
CSMA/CD is used in the Ethernet LAN discussed in Chapter 9.

53. CSMA/CA is used in the wireless LANs discussed in Chapter 10.
Note:
CSMA/CA is used in the wireless LANs discussed in Chapter 10.

54. 5.8
DATA LINK
PROTOCOLS

55. Note:
A data link protocol is a set of specifications used to implement the data link layer.

56. Note:
In a character-oriented protocol, the frame is interpreted as a series of characters. In a bit-oriented protocol, the frame or packet is interpreted as a series of bits.

57. Note:
All bit-oriented protocols are related to high-level data link control (HDLC), a bit-oriented protocol.