Data Link Layer
Data Link Layer consists of two sub-layers
Logical Link Control (LLC) Layer
Medium Access Control (MAC) Layer
We may not have a dedicated link (channel) for communication
Upper layer (LLC) is responsible for
Data link control (flow or error control)
Lower layer (MAC) is responsible for
Medium access resolution Multiple Access to a Link
Multicast link or Broadcast link
Nodes are connected with single link
We need multiple access protocol To coordinate
access to the link
Two devices do not send data at the same time
Two devices do not interrupt each other
A device do not monopolize the link Multiple Access to a Link
Multicast link or Broadcast link
Nodes are connected with single link
We need multiple access protocol To coordinate
access to the link
Two devices do not send data at the same time
Two devices do not interrupt each other
A device do not monopolize the link ALOHA Protocol
Random access evolved from ALOHA Protocol
It uses a simple procedure called ‘Multiple Access (MA)’
MA is further improved by forces the station to sense the medium before transmitting
Called Carrier Sense Multiple Access (CSMA)
CSMA evolved into two methods:
CSMA/CD: Carrier Sense Multiple Access / Collision Detection
Tells the station what to do when a collision is detected
CSMA/CA: Carrier Sense Multiple Access / Collision Avoidance
Tries to avoid the collision

1. Data Communication
and Computer Networks
[EEE 314]
Mubashir Husain RehmaniMubashir Husain Rehmani
Assistant Professor
Department of Electrical Engineering
COMSATS Institute of Information Technology,
Wah Cantt, Pakistan

2. Wired LAN
Ethernet

3. Data Link
Layer

4. Position of Data Link Layer

5. LLC and MAC

6. IEEE standards for LAN

7. Data Link Layer
Data Link Layer consists of two sub-layers
Logical Link Control (LLC) Layer
Medium Access Control (MAC) Layer
We may not have a dedicated link (channel) forWe may not have a dedicated link (channel) for
communication
Upper layer (LLC) is responsible for
Data link control (flow or error control)
Lower layer (MAC) is responsible for
Medium access resolution

8. Multiple Access to a Link
Multicast link or Broadcast link
Nodes are connected with single link
We need multiple access protocol
To coordinate access to the linkTo coordinate access to the link
Two devices do not send data at the same time
Two devices do not interrupt each other
A device do not monopolize the link

9. Multiple
Access

10. Multiple Access Protocols

11. Random Access
Also called ‘contention methods’
No device is superior to another device
No station permits another station to send data
At each instance, when a station has data to send, it willAt each instance, when a station has data to send, it will
make a decision by itself on whether or not to send
This decision is based on the state of the medium
Why we call Random Access or Contention Methods?
There is no scheduled time for a station to transmit
Transmission is random among the stations
No rules say which station should send next
Stations compete with each other

12. Random Access
In Random Access, each station has a right to send
However, if more than one station tries to send, collision
will occur
In order to avoid this collision problem, each station will
answer these questions
When can the station access the medium?
What can the station do if the medium is busy?
How can the station determine the success or failure of the
transmission?
What can the station do if there is an access conflict?

13. ALOHA Protocol
Random access evolved from ALOHA Protocol
It uses a simple procedure called ‘Multiple Access (MA)’
MA is further improved by forces the station to sense theMA is further improved by forces the station to sense the
medium before transmitting
Called Carrier Sense Multiple Access (CSMA)
CSMA evolved into two methods:
CSMA/CD: Carrier Sense Multiple Access / Collision Detection
Tells the station what to do when a collision is detected
CSMA/CA: Carrier Sense Multiple Access / Collision Avoidance
Tries to avoid the collision

14. Evolution of Random Access
Methods

15. ALOHA Protocol
Developed at University of Hawaii in early1970
Original ALOHA protocol is called as Pure ALOHA
Slotted ALOHA was invented to improve the performance of PureSlotted ALOHA was invented to improve the performance of Pure
ALOHA

16. ALOHA Network

17. Pure ALOHA

18. Slotted ALOHA

19. Some Questions
A pure ALOHA network transmits 200-bit frames on a shared
channel of 200 kbps. What is the requirement to make this frame
collision-free?
A pure ALOHA network transmits 200-bit frames on a shared
channel of 200 kbps. What is the throughput if the system (all
stations together) produces
1000 frames per second
500 frames per second
250 frames per second
A slotted ALOHA network transmits 200-bit frames using a shared
channel with a 200-kbps bandwidth. Find the throughput if the
system (all stations together) produces
1000 frames per second
500 frames per second
250 frames per second

21. Random Access
MA
CSMA
CSMA/CD
CSMA/CA

22. Collision in CSMA

23. Persistence Strategies

24. CSMA/CD Procedure

25. CSMA/CA Procedure

26. Controlled Access
• Stations consult each other to find which station has the
right to send
• A station can not send unless it has been authorized by
other stations
Reservation: make reservation before sending data
Polling: Select and Poll functions are used
Token Passing: Network is organized in logical ring
other stations

27. Select

28. Poll

29. Token Passing Network

30. Token Passing Procedure

31. Channelization
Channelization is a multiple-access method in which the
available bandwidth of a link is shared in time, frequency, or
through code, between different stations.
FDMA
TDMA
CDMA

32. In FDMA, the bandwidth is divided into
channels.

33. In TDMA, the bandwidth is just one
channel that is timeshared.

34. In CDMA, one channel carries all
transmissions simultaneously.

37. Project 802

38. Project 802

44. Ethernet Address in
Hexadecimal Notation

46. A Network with and without a
Bridge

47. Collision Domain

48. Switched Ethernet

49. Fast Ethernet