1. Multiple Access Protocols Chapter 6 of Hiroshi Harada Book
Khurram Masood

2. Introduction Multiple access control channels
Each node is attached to a transmitter/receiver which communicates via a channel shared by other nodes
Transmission from any node is received by other nodes
Shared Multiple
Access Control
Channel to BS
Node 4
Node 3
Node 2
Node 1 …
Node N

3. Introduction (Cont’d)
Multiple access issues
If more than one node transmit at a time on the control channel to BS, a collision occurs
How to determine which node can transmit to BS?
Multiple access protocols
Solving multiple access issues
Different types:
Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision
Collision-free protocols (e.g., a bit-map protocol and binary countdown) ensure that a collision can never occur.

4. Packet Communication System Configuration
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5. Classification of Multiple Access Protocols
Contention-based
Conflict-free
Random access
Collision resolution
ALOHA,
CSMA,
BTMA,
ISMA, etc
TREE,
WINDOW, etc
FDMA,
TDMA,
CDMA,
Token Bus,
DQDB, etc
BTMA: Busy Tone Multiple Access
ISMA: Internet Streaming Media Alliance
DQDB: Distributed Queue Dual Bus

6. Contention Protocols ALOHA Slotted ALOHA
Developed in the 1970s for a packet radio network by Hawaii University.
Whenever a station has a data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision.
Slotted ALOHA
Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.

7. Contention Protocols (Cont’d)
CSMA (Carrier Sense Multiple Access)
Improvement: Start transmission only if no transmission is ongoing
CSMA/CD (CSMA with Collision Detection)
Improvement: Stop ongoing transmission if a collision is detected
CSMA/CA (CSMA with Collision Avoidance)
Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit
CSMA/CA with ACK
CSMA/CA with RTS/CTS

8. ALOHA
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9. ALOHA
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10. Collision mechanism in ALOHA
Waiting a random time
Node 1 Packet
Node 2 Packet
Retransmission
Retransmission 1 2 3 3 2
Time
Collision
Node 3 Packet
Collision mechanism in ALOHA

11. Collisions in (Pure) ALOHA

12. Throughput of ALOHA
The probability that n packets arrive in two packets time is given by n ( ) ! n
(2G) P e 2 G - =
where G is traffic load.
The probability P(0) that a packet is successfully received without collision is calculated by letting n=0 in the above equation. We get
We can calculate throughput S with a traffic load G as follows:
The Maximum throughput of ALOHA is

13. Unslotted ALOHA
Unslotted ALOHA (a.k.a. Pure ALOHA) was the precursor to slotted ALOHA.
In Pure ALOHA, each node transmits a new packet immediately upon receiving, rather than waiting for a slot boundary.
If a packet is involved in a collision, it is retransmitted after a random delay.

14. Unslotted ALOHA (cont.)
A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame
Vulnerable period has the length of 2 frame times

15. Unslotted ALOHA (cont.)
Since arrivals are independent, Psucc=e-2G
Since attempted transmissions occur at rate G(n), the throughput = Ge-2G
The MAX throughput of a Pure ALOHA system = 1/(2e), achieved when G=0.5.
If λ is very small and the mean retx time is very large, the system can be expected to run for long periods w/o major backlog buildup.
The main adv. of pure ALOHA is that it can be used with variable-length packets.

16. Collision mechanism in slotted ALOHA
Node 1 Packet
Nodes 2 & 3 Packets
Retransmission
Retransmission 1
2&3 2 3
Time
Collision
Slot
Collision mechanism in slotted ALOHA

17. Slotted ALOHA The basic idea:
Each unbacklogged node simply transmit a newly arriving packet in the first slot after packet arrival.
Slotted ALOHA risks occasional collisions but achieves very small delay if collisions are rare.
Contrast to TDM systems, which avoids collisions at the expense of large delays.

18. Collisions in S-ALOHA

19. Slotted ALOHA (cont.)
When a collision occurs, each node sending one of the colliding packets discovers the collision at the end of the slot and becomes backlogged.
Such nodes wait for some random number of slots before retransmitting.

20. Slotted ALOHA (cont.)
The MAX departure rate occurs at G=1 and is 1/e ≈
If G<1, too many idle slots are generated.
If G>1, too many collisions are generated.

21. Throughput of Slotted ALOHA
The probability of no collision is given by
The throughput S is
The Maximum throughput of slotted ALOHA is

22. Throughput
0.368 S
Slotted Aloha
0.184
Aloha G

23. Comparison of ALOHA and S-ALOHA

24. CSMA
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25. CSMA
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26. CSMA
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27. CSMA (Carrier Sense Multiple Access)
Max throughput achievable by slotted ALOHA is
CSMA gives improved throughput compared to Aloha protocols.
Listens to the channel before transmitting a packet (avoid avoidable collisions).

28. Collision Mechanism in CSMA
Node 5 sense
Node 1 Packet
Node 2 Packet
Delay
Node 3 Packet 1 2 3 4 5
Time
Delay
Collision
Node 4 sense

29. Kinds of CSMA Unslotted Nonpersistent CSMA Slotted Nonpersistent CSMA
Unslotted persistent CSMA
Persistent CSMA
Slotted persistent CSMA
1-persistent CSMA
p-persistent CSMA

30. Non persistent CSMA
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31. Slotted non persistent ISMA
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32. Computer Simulation Configuration
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33. Layout of access point and user terminals
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34. Simulation and results
Pure ALOHA
Slotted ALOHA
Non persistent CSMA
Non persistent ISMA
Results
Throughput
Average delay time
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35. Pure ALOHA
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36. Pure ALOHA
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37. Slotted ALOHA
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38. Slotted ALOHA
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39. Non persistent CSMA
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40. Non persistent CSMA
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41. Non persistent ISMA
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42. Non persistent ISMA
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