1. Chapter 6 Multiple Radio Access
Prof. Chih-Cheng Tseng
EE of NIU
Chih-Cheng Tseng

2. Introduction (1/2) Multiple access control channels
Each Mobile Station (MS) has a transmitter/receiver that communicates with others via a shared channel.
Transmission from any MS is received by other MSs.
Shared Multiple
Access Medium
MS 4
MS 3
MS 2
MS 1
Node N …
EE of NIU
Chih-Cheng Tseng

3. Introduction (2/2) Multiple access issues Multiple access protocols
If more than one MS transmit at the same time, a collision occurs.
How to determine which MS can transmit?
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. (will be discussed in Chapter 7)
EE of NIU
Chih-Cheng Tseng

4. Channel Sharing Techniques
Static Channelization
Dynamic Medium Access Control
The channel assignment is done in a pre-specified way and does not change with time.
Medium - Sharing Techniques
Scheduling
Random Access
The channel is allocated as needed and changes with time.
EE of NIU
Chih-Cheng Tseng

5. Classification of Multiple Access Protocols for a Shared Channel
Conflict-free
Contention-based
(will be discussed in Chap 7)
Random access
Collision resolution
FDMA,
TDMA,
CDMA,
Token Bus,
DQDB, etc.
DQDB: Distributed Queue Dual Bus
ALOHA,
CSMA,
BTMA,
ISMA, etc.
TREE,
WINDOW, etc.
BTMA: Busy Tone Multiple Access
ISMA: Idle Signal Multiple Access
EE of NIU
Chih-Cheng Tseng

6. Contention-Based Protocols --- ALOHA Family
Pure ALOHA
Developed in the 1970s for a packet radio network by Hawaii University
Whenever a terminal (MS) has data, it transmits.
Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station.
If there is a collision, sender retransmits after some random time.
Slotted ALOHA
Time is slotted and a packet can only be transmitted at the beginning of a slot.
Slotted the time reduces the collision duration.
EE of NIU
Chih-Cheng Tseng

7. Collision mechanism in Pure ALOHA
Node 1 Packet
Waiting a random time 2
Node 2 Packet 2
Retransmission
Retransmission 1
Collision 3
Node 3 Packet 3
Time
t-T t
t+T
Collision mechanism in Pure ALOHA
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Chih-Cheng Tseng

8. Throughput of Pure ALOHA
The probability of successful transmission Ps is the probability that no other packet is scheduled in an interval of length 2T (vulnerable period)
where g is the packet rate of the traffic.
The throughput Sth of pure ALOHA as
Defining G= gT to normalize offered load, we have
Differentiating Sth with respect to G and equating to zero gives
The maximum throughput of pure ALOHA is obtained when G=1/2 and is
EE of NIU
Chih-Cheng Tseng

9. Collision mechanism in slotted ALOHA
Node 1 Packet
Nodes 2 & 3 Packets
Retransmission 3 2
Retransmission 1 2 3
Time
Slot
Collision
Collision mechanism in slotted ALOHA
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Chih-Cheng Tseng

10. Throughput of Slotted ALOHA
The probability of successful transmission Ps is the probability no other packet is scheduled in an interval of length T
where g is the packet rate of the traffic
The throughput Sth of slotted ALOHA as Sth=gTe-gT.
Defining G=gT to normalize offered load, we have Sth=Ge-G.
Differentiating Sth with respect to G and equating to zero gives
The maximum throughput of slotted ALOHA is obtained when G=1 and is
EE of NIU
Chih-Cheng Tseng

11. Throughput S: Throughput G=gT Slotted ALOHA Pure ALOHA 0.368 0.184 8 62
0.5
0.4
0.3
0.2
0.1
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Chih-Cheng Tseng

12. Contention-based Protocols --- CSMA Family
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
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Chih-Cheng Tseng

13. CSMA (Carrier Sense Multiple Access)
Maximal throughput achievable by slotted ALOHA is 0.368
CSMA gives improved throughput compared to ALOHA protocols
Listens to the channel before transmitting a packet (avoid avoidable collisions)
EE of NIU
Chih-Cheng Tseng

14. Collision Mechanism in CSMA
Delay for MS 5
MS 5 sense
MS 1 Packet
MS 2 Packet 2 3
MS 3 Packet 1
Collision due to hidden terminal problem 5 4
Time
MS 4 senses
Delay for MS 4
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Chih-Cheng Tseng

15. Hidden Terminal ProblemC
Radio transmission range R
A and C are hidden with respect to each other
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Chih-Cheng Tseng

16. Exposed Terminal ProblemC
Radio Transmission range R D A
Transmission at B forces C (Exposed) to stop transmission to D
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17. Types of CSMA Protocols
Unslotted Nonpersistent CSMA
Slotted Nonpersistent CSMA
Unslotted persistent CSMA
Slotted persistent CSMA
Nonpersistent CSMA
Persistent CSMA
1-persistent CSMA
p-persistent CSMA
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Chih-Cheng Tseng

18. Nonpersistent CSMA Protocols
The MS senses the medium first whenever it has a packet to send
If the medium is idle, transmits immediately.
If the medium is busy, waits a random amount of time and senses the medium again.
If collision occurs, waits a random amount of time and starts all over again.
Random backoff reduces probability of collisions.
Waste idle time if the backoff time is too long.
EE of NIU
Chih-Cheng Tseng

19. Throughput of Nonpersistent CSMA
For unslotted nonpersistent CSMA, the throughput is
For slotted nonpersistent CSMA, the throughput is
G is the offered traffic rate
T is the packet transmission time
t is the propagation delay
a=t/T is the normalized time unit
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Chih-Cheng Tseng

20. 1-persistent CSMA Protocols
The MS senses the medium first whenever it wants to send
If the medium is idle, the MS transmits immediately. (This is the reason why it is called 1-persistent.)
If the medium is busy, the MS keeps listening until medium becomes idle, and then transmits immediately.
There will always be a collision if more than one node have ready packets and wait for the channel to become idle.
For unslotted 1-persistent CSMA, the throughput is given by:
For slotted 1-persistent CSMA, the throughput is given by:
EE of NIU
Chih-Cheng Tseng

21. p-persistent CSMA Protocols
In this protocol, the time is slotted.
The size of slot is the contention period, i.e., the round trip propagation time.
The MS senses the medium first whenever it has a packet to send
If the medium is idle, the MS transmits with probability p, or defer transmission by one time slot with probability (1-p).
If the medium is busy, the MS waits until the next slot and checks the medium again.
If a collision occurs, the MS waits for a random amount of time and starts all over again.
A good tradeoff between nonpersistent (p=0) and 1-persistent (p=1) CSMA.
EE of NIU
Chih-Cheng Tseng

22. How to Select The Probability p?
Assume that N is the number of nodes have a packet to send at a time and the medium is busy.
Then, Np is the expected number of nodes that will attempt to transmit once the medium becomes idle.
If Np > 1, then a collision is expected to occur.
Therefore, network must make sure that Np ≤ 1 to avoid collision.
EE of NIU
Chih-Cheng Tseng

23. Throughput of p-persistent CSMA Protocol (1/2)
Throughput S as:
where G is the offered traffic rate, a=t/T=propagation delay/packet transmission time and
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Chih-Cheng Tseng

24. Throughput of p-persistent CSMA Protocol (2/2)
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25. Throughput S: Throughput 0.01-persistent CSMA Nonpersistent CSMA
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.01-persistent CSMA
Nonpersistent CSMA
0.1-persistent CSMA
0.5-persistent CSMA
S: Throughput
1-persistent CSMA
Slotted ALOHA
ALOHA
Traffic Load G
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26. CSMA/CD (CSMA with Collision Detection) (1/3)
In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place)
Wasting medium for an entire packet time
CSMA/CD
Step 1: If the medium is idle, transmit.
Step 2: If the medium is busy, continue to listen until the channel is idle then transmit .
Step 3: If a collision is detected during transmission, ceases the transmission (detection is impossible by wireless devices) and waits a random amount of time and repeats the same algorithm.
EE of NIU
Chih-Cheng Tseng

27. CSMA/CD (CSMA with Collision Detection) (2/3)
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28. CSMA/CD in Ethernet (2/2)
cd: time required for a terminal to detect a collision detection
cr: time required to send the jamming signal to achieve the consensus reinforcement procedure A B
(t is the propagation time) T0
A begins transmission
Time
B begins transmission
T0+t-
T0+t+tcd
B detects collision
A detects collision just before end of transmission
T0+2t+tcd+tcr- 
EE of NIU
Chih-Cheng Tseng

29. Throughput of Slotted Nonpersistent CSMA/CD
0.2
0.4
0.6
0.8 1
1.2
0.001
0.01
0.1 10
100
1000
Traffic load G
S: Throughput
α = 1
α = 0.1
α = t /T= 0
α = 0.01
EE of NIU
Chih-Cheng Tseng

30. CSMA/CA (CSMA with Collision Avoidance) for Wireless Devices
Each terminal ready to transmit senses the medium.
If medium is busy, it waits until the end of current transmission.
It again waits for an additional predetermined time period DIFS (Distributed Inter Frame Space).
Then, picks up a random number of slots (the initial value of backoff counter) within a contention window to wait before transmitting its frame.
If there are transmissions by other MSs during this time period (backoff time), the MS freezes its counter.
After the channel has been free longer than DIFS, the MS resumes count down. The MS starts its transmission when the counter reaches to zero.
EE of NIU
Chih-Cheng Tseng

31. The Basic Mechanism of CSMA/CA
Contention window
Medium Busy
Defer access
DIFS
DIFS
Backoff after defer
Slot
Next Frame
Time
DIFS – Distributed Inter Frame Spacing
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Chih-Cheng Tseng

32. Example of Backoff Mechanism (1/2)
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Chih-Cheng Tseng

33. Example of Backoff Mechanism (2/2)
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Chih-Cheng Tseng

34. CSMA/CA with ACK for Ad Hoc Networks
Immediate Acknowledgements from receiver upon reception of data frame without any need for sensing the medium
ACK frame transmitted after time interval SIFS (Short Inter-Frame Space) (SIFS < DIFS)
Receiver transmits ACK without sensing the medium
If ACK is lost, retransmission done
EE of NIU
Chih-Cheng Tseng

35. CSMA/CA/ACK Data ACK DIFS Time SIFS Source MS BS DIFS
Contention window
Backoff after defer
Next Frame
Other MSs
Defer access
SIFS – Short Inter Frame Spacing
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Chih-Cheng Tseng

36. CSMA/CA with RTS/CTS for Ad Hoc Networks
Transmitter sends an RTS (request to send) after medium has been idle for time interval more than DIFS
Receiver responds with CTS (clear to send) after medium has been idle for SIFS
Then Data is exchanged
RTS/CTS is used for reserving channel for data transmission so that the collision can only occur in control message
EE of NIU
Chih-Cheng Tseng

37. CSMA/CA with RTS/CTS Next Frame Time Source MS Destination MS
DIFS
SIFS
SIFS
Time
CTS
Source MS
RTS
Data
SIFS
ACK
Destination MS
DIFS
Contention window
Backoff
Next Frame
Other MSs
Defer access
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Chih-Cheng Tseng

38. RTS/CTS MS A MS B RTS CTS Data ACK
Propagation delay
CTS
Data
This helps avoid hidden terminal problem in ad hoc networks
ACK
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Chih-Cheng Tseng

39. Homework
P6.2
P6.5
P6.8
P6.11
P6.17
EE of NIU
Chih-Cheng Tseng