1. Data Communication Networks
Lec 16 and 17

2. Multiple Access
Assumed that there is available dedicated link between sender and receiver.
Assumptions is not always true.
If we use our cellular phone to connect to another cellular phone, the channel is not dedicated , a person a few feet away from us use the same channel to talk to her friend.

3. Multiple access Data link layer is divided into two sub layers.
The upper sublayer is responsible for data link control
And lower sublayer is responsible for resolving access to the shared media.
If channel is dedicated , we do not need the lower sublayer.

4. Multiple access

5. Multiple access
The upper sublayer is responsible for flow and error control is called logical link control(LLC).
The lower sublayer is responsible for multiple access resolution us called media access control(MAC)layer.
When stations are connected and use a common link, called a multipoint , we need multiple-access protocol to coordinate access to link.
Rules of speaking.

6. Multiple-access protocols

7. Random access protocols
In RS or contention methods, no station is superior to another, none assign control.
No permit, does not permit another station to send.
The station has to use procedure defined by protocol to make decision on whether or not to send.
Idle or busy.

8. Random access protocols
No scheduled time for stations to transmit, called random access.
No rules specify which station to send next, stations compete with one another to access the medium, called contention methods.
Every one has right, if more than one stations tries to send , there is access conflict –collision.
Frames will be destroyed or modified.

9. Random access

10. Random access The random access methods evolve from ALOHA.
Method improved with the addition of , that forces the station to sense the medium before transmitting , called carrier sense multiple access.
CSMA/CD, tells the station what to do when a collision is detected
CSMA/CA, tries to avoid the collision.

11. ALOHA
Earliest, random access method, developed by university of Hawaii in 1970.
Designed for radio(wireless)LAN , can be used on any shared medium.

12. Pure ALOHA
Each station sends a frame whenever it has a frame to send .
However, since there is only one channel to share , possibility of collision between frames from different stations.
ALOHA relies on acknowledgements from the receiver.
If acknowledgement does not arrive after time-out period , the station assume that frame is destroyed and resend the frame.

13. ALOHA
The protocol dictates that when time-out period passes , each station waits a random amount of time before resending its frame.
This randomness will avoid more collisions , called back off time.
It again uses second method to prevent congestion with transmitted frames, after maximum number of retransmission attempts, a station give up and try again.

14. CSMA The station sense the medium before trying to use it.
First listen to the medium before sending it.
“sense before transmit” or “listen before talk”
Reduce the possibility , cannot eliminate it.
Because of propagation delay; when a station sends a frame , it takes time for a first bit to reach every station and for every station to sense it.

15. CSMA

16. CSMA
A station may sense the medium and finds it idle, only because the first bit sent by another station has not been received.

17. Vulnerable time It is propagation time.
Time needed for a signal to propagate from one end of the medium to the other.

18. Vulnerable time in CSMA

19. Flow charts

20. Carrier sense Multiple Access with Collision Detection (CSMA/CD)
Augments the algorithm to handle collision.
The station monitors the medium and sends a frame to see if the transmission was successful.
If collision , frame is send again.

21. CSMA/CD

22. CSMA/CD
At time t 1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t2, station C has not yet sensed the first bit sent by A.
Station C executes its persistence procedure and starts sending the bits in its frame, which propagate both to the left and to the right.
The collision occurs sometime after time t2' Station C detects a collision at time t3 when it receives the first bit of A's frame.
Station C immediately (or after a short time, but we assume immediately) aborts transmission.

23. CSMA/CD When a host has a packet to transmit:
1. Carrier Sense: Check that the line is quiet before transmitting.
2. Collision Detection: Detect collision as soon as possible. If a collision is detected, stop transmitting; wait a random time, then return to step 1.

24. Collision and abortion in CSMA/CD

25. Carrier sense Multiple Access with Collision Avoidance (CSMA/CA)
Use 3 strategies
interframe space
Contention window
Acknowledgement

26. IFS
Collisions are avoided by deferring transmission even if the channel is found idle.
Wait for a period of time called IFS.
Even though the channel may appear idle a distant station may have already started transmitting after the IFS time the channel is still idle, the station can send.

27. IFS
In CSMA/CA, the IFS can also be used to define the priority of a station or a frame.

28. Contention Window The amount of time divided into slots.
The station that is ready to send chooses a random number of slots as its wait time.
Uses backoff strategy
This means that it is set to one slot the first time and doubles each time the station can not detect an idle channel after the IFS time.
Similar to p-persistent method expect that the random outcome defines the number of slots taken by the waiting station.

29. Contention Method
station needs to sense the channel after each time slot.
However, if the station finds the channel busy, it does not restart the process; it just stops the timer and restarts it when the channel is sensed as idle.
This gives priority to the station with the longest waiting time.

30. it stops the timer and restarts it when the channel becomes idle.
In CSMA/CA, if the station finds the channel busy, it does not restart the timer of the contention window;
it stops the timer and restarts it when the channel becomes idle.

31. Acknowledgements
Positive ACK guarantees that the receiver has received the frame.