Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 1
Chapter 6
Multiple Radio Access
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 2
Outline
Introduction
Multiple Radio Access Protocols
Contention-based Protocols
Pure ALOHA
Slotted ALOHA
CSMA (Carrier Sense Multiple Access)
CSMA/CD (CSMA with Collision Detection)
CSMA/CA (CSMA with Collision Avoidance)
Summary
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 3
Introduction
Multiple access control channels Each Mobile Station (MS) is attached to a
transmitter / receiver which communicates via a channel shared by other nodes
Transmission from any MS is received by other MSs
Shared Multiple
Access Medium
MS 4
MS 3
MS 2
MS 1 Node N …
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 4
Introduction (Cont’d)
Multiple access issues
If more than one MS transmit at a time on the control channel to BS, a collision occurs
How to determine which MS 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 ensure that a collision can never occur
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 5
Channel Sharing Techniques
Medium -
Sharing
Techniques
Static
Channelization
Dynamic
Medium Access
Control
Scheduling
Random
Access
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 6
Classification of Multiple Access Protocols
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.
DQDB: Distributed Queue Dual Bus
BTMA: Busy Tone Multiple Access
ISMA: Internet Streaming Media Alliance
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 7
Contention-based Protocols
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
Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 8
Pure ALOHA
1 Time
Node 1
Packet
2
Retransmission
Collision mechanism in ALOHA
3
Retransmission
2
Node 2
Packet
Waiting a random time
Collision
3
Node 3
Packet
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Throughput of Pure ALOHA
gT
s
e
collisionnoPP
2
)_(
where g is the packet rate of the traffic
gT
th gTeS 2
• The throughput Sth of pure Aloha as:
• Defining G= gT to normalize offered load, we have
184.02
1max
eS
• The Maximum throughput of ALOHA is
G
th GeS 2
02 22 GGth eGedG
dS• Differentiating Sth with respect to G and equating to zero gives
• The probability of successful transmission Ps is the probability no
other packet is scheduled in an interval of length 2T
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Slotted ALOHA
1 Time
2
Retransmission Retransmission
3
Slot
Node 1
Packet
2
Collision
Nodes 2 & 3
Packets
Collision mechanism in slotted ALOHA
3
No
transmission
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 11
Throughput of Slotted ALOHA
gT
s eP
n
• 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
gT
th gTeS
• The throughput Sth of pure Aloha as:
• Defining G= gT to normalize offered load, we have
368.01
max e
S
• The Maximum throughput of ALOHA is
G
th GeS
0 GGth eGedG
dS• Differentiating Sth with respect to G and equating to zero gives
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 12
Throughput
Slotted Aloha
Aloha
0.368
0.184
G=gT
S:
Th
rou
gh
pu
t
8 6 4 2 0
0.5
0.4
0.3
0.2
0.1
0
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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
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CSMA (Carrier Sense Multiple Access)
Max 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)
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Collision Mechanism in CSMA
Time 4
Collision
5
MS 4 senses
Delay for MS 4
Delay for MS 5
MS 5 sense MS 1 Packet
MS 2 Packet
2 3
MS 3 Packet
1
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Kinds of CSMA
CSMA
Unslotted Nonpersistent
CSMA
Slotted Nonpersistent CSMA
Unslotted persistent CSMA
Slotted persistent CSMA
Nonpersistent
CSMA (no wait)
Persistent CSMA
(wait)
1-persistent
CSMA
p-persistent
CSMA
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Nonpersistent CSMA Protocols
Nonpersistent CSMA Protocol:
Step 1: If the medium is idle, transmit immediately (same as p = 1)
Step 2: If the medium is busy, wait a random amount of time and repeat Step 1
Random backoff reduces probability of collisions
Waste idle time if the backoff time is too long
G
T
theG
GeS
)21(
2
For unslotted nonpersistent CSMA, the throughput is given by:
For slotted nonpersistent CSMA, the throughput is given by:
)1(
2
G
T
the
GeS
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 18
1-persistent CSMA Protocols
1-persistent CSMA Protocol:
Step 1: If the medium is idle, transmit immediately
Step 2: If the medium is busy, continue to listen until medium becomes idle, and then transmit immediately
There will always be a collision if two nodes want to retransmit (usually you stop transmission attempts after few tries)
)1(
)21(
)1()1()21(
)2/1(1
GG
G
theGeG
eGGGGGS
For unslotted 1-persistent CSMA, the throughput is given by:
For slotted 1-persistent CSMA, the throughput is given by:
)1(
)1(
)1)(1(
)1(
GG
GG
thee
eeGS
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p-persistent CSMA Protocols
p-persistent CSMA Protocol (time is slotted):
Step 1: If the medium is idle, transmit with probability p, or delay the transmission with probability (1 - p) until the next slot
Step 2: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1
Step 3: If transmission is delayed by one time slot, continue with Step 1
A good tradeoff between nonpersistent and 1-persistent CSMA
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 20
Throughput
0 1 2 3 4 5 6 7 8 9
Traffic Load G
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
S:
Th
rou
gh
pu
t
Aloha
Slotted
Aloha
1-persistent CSMA
0.5-persistent CSMA
0.1-persistent CSMA
0.01-persistent CSMA
Nonpersistent CSMA
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CSMA/CD (CSMA with Collision Detection)
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,
cease transmitting (detection not possible by
wireless devices)
Step 4: Wait a random amount of time and repeats
the same algorithm
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 22
CSMA/CD in Ethernet (Cont’d)
A B (t is the propagation time)
T0 A begins
transmission
Time
A B
B begins
transmission
T0+t-
T0+ttcd
A B
B detects collision
A B
A detects collision just
before end of transmission
T0+2t+tcd
+tcr -
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 23
Throughput of slotted non-persistent CSMA/CD
0
0.2
0.4
0.6
0.8
1
1.2
0.001 0.01 0.1 1 10 100 1000
Traffic load G
S:
Th
rou
gh
pu
t
α = 1
α = 0.1
α = t /T= 0
α = 0.01
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 24
CSMA/CA (CSMA with collision Avoidance) for wireless devices
All terminals listen to the same medium as CSMA/CD
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
It resumes count down after other MSs finish transmission + DIFS. The MS can start its transmission when the counter reaches to zero
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 25
CSMA/CA (Cont’d) for Wireless
Devices
Time
MS A’s frame
MSs B resenses the
medium and transmits
its frame
MS B’s frame
MSs C starts
transmitting
MSs C resenses the
medium but defers to
MS B
MSs B & C sense
the medium
Delay: B Delay: C
MS C’s frame
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 26
CSMA/CA Explained
DIFS Contention window
DIFS
Medium Busy
Defer access
Next Frame
Backoff after defer
Slot
Time
DIFS – Distributed Inter Frame Spacing
Contention
window
Example of Backoff Intervals
busy
busy
busy
busy
DIFS DIFS DIFS DIFS Backoff=9 Backoff=4 Backoff=2
Backoff=5
Backoff=7 Backoff=2
Station 1
Station 2
Station 3
Station 4
Packet arrival at MAC
(1)
(2) (3)
(4)
(5)
(1) After packet arrival at MAC, station 3 senses medium free for DIFS, so it starts transmission
immediately (without backoff interval).
(2) For station 1,2, and 4, their DIFS intervals are interrupted by station 3. Thus, backoff
intervals for station 1,2, and 4, are generated randomly (i.e. 9,5, and 7, respectively).
(3) After transmission of station 2, the remaining backoff interval of station 1 is (9-5)=4.
(4) After transmission of station 2, the remaining backoff interval of station 4 is (7-5)=2.
(5) After transmission of station 4, the remaining backoff interval of station 1 is (4-2)=2.
Backoff time = CW* Random() *
Slot time
CW = starts at CWmin, and
doubles after each failure until
reaching CWmax (e.g., CWmin
= 7, CWmax = 255)
Random() = (0,1)
Slot Time = Transmitter turn on
delay + medium propagation
delay + medium busy detect
response time
Random Backoff Time
8
CWmax
CWmin 7
15
31
第二次重送
第一次重送
第三次重送 初始值
63 127
255 255
Priority Scheme in MAC
Priorities of frames are distinguished by the IFS (inter-frame spacing) between two consecutive frames.
4 IFS's:
SIFS: the highest priority
ACK, CTS, the second or subsequent MPDU of a fragmented burst, and to respond to a poll from the PCF.
PIFS (PCF-IFS): 2nd highest
by PCF to send any of the Contention Free Period frames.
DIFS (DCF-IFS): 3nd highest
by the DCF to transmit MPDUs or MMPDUs
EIFS (Extended-IFS): lowest
by the DCF to transmit a frame when previous frame was not received correctly
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 30
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
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Next Frame
CSMA/CA/ACK
DIFS
ACK
Other
MSs
Source
MS
BS
DIFS
SIFS
Data
Defer access
Contention window
Backoff after defer
SIFS – Short Inter Frame Spacing
Time
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 32
Hidden Terminal Problem
A B C
Radio transmission range
R R R
A and C are hidden with respect to each other
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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
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 34
CSMA/CA with RTS/CTS
DIFS
RTS
Other
MSs
Source
MS
Destination
MS DIFS
SIFS
Defer access
Contention window
Backoff
Next Frame
CTS
SIFS
Data
SIFS
ACK
Time
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 35
RTS/CTS
MS A MS B
Propagation delay
This helps avoid
hidden terminal
problem in Ad hoc
networks
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 36
Exposed Terminal Problem
A B C
Radio Transmission range
R R R
D
R
Transmission at B forces C (Exposed) to stop transmission to D
A
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Homework
Problems: 6.6, 6.10, 6.21 (Due Oct. 31)
Practice at home: 6.8, 6.12, 6.16
37
Quiz 1
What is the difference between fast fading and
slow fading?
38