Post on 05-Jan-2016
Wireless Communication Network Lab.
Chapter 6Multiple Radio Access
Chih-Cheng Tseng 1EE of NIU
Prof. Chih-Cheng Tseng
tsengcc@niu.edu.tw
http://wcnlab.niu.edu.tw
Wireless Communication Network Lab. 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.
Introduction (1/2)
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Shared MultipleAccess Medium
MS 4
MS 3
MS 2
MS 1 Node N…
Wireless Communication Network Lab.
Introduction (2/2)
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Multiple access issues• 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)
Wireless Communication Network Lab.
Channel Sharing Techniques
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Medium - Sharing
Techniques
Static Channelization
Dynamic Medium Access
Control
Scheduling
Random Access
The channel assignment is done in a pre-specified way and does not change with time.
The channel is allocated as needed and changes with time.
Wireless Communication Network Lab.
Classification of Multiple Access Protocols for a Shared Channel
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Multiple Access Protocols
Conflict-freeContention-based
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 AccessISMA: Idle Signal Multiple Access
(will be discussed in Chap 7)
Wireless Communication Network Lab.
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.
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Wireless Communication Network Lab.
Pure ALOHA
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1Time
Node 1 Packet
2
Retransmission
Collision mechanism in Pure ALOHA
3
Retransmission
2
Node 2 Packet
Waiting a random time
Collision
3
Node 3 Packet
tt-T t+T
Wireless Communication Network Lab.
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
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2(no collision) gTsP P e
2 .gTthS gTe
2 22 0.G GthdSGe e
dG
,max
10.184.
2thSe
2 .GthS Ge
Wireless Communication Network Lab.
Slotted ALOHA
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1 Time2
Retransmission Retransmission
3
Slot
Node 1 Packet
2
Collision
Nodes 2 & 3 Packets
Collision mechanism in slotted ALOHA
3
Wireless Communication Network Lab.
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
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gTs eP
0.G GthdSGe e
dG
,max
10.368.thS
e
Wireless Communication Network Lab.
Throughput
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Slotted ALOHA
Pure ALOHA
0.368
0.184
G=gT
S:
Th
rou
ghp
ut
86420
0.5
0.4
0.3
0.2
0.1
0
Wireless Communication Network Lab.
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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Wireless Communication Network Lab.
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)
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Wireless Communication Network Lab.
Collision Mechanism in CSMA
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Time4
Collision due to hidden
terminal problem
5
MS 4 senses
Delay for MS 4
Delay for MS 5
MS 5 senseMS 1 Packet
MS 2 Packet
2 3
MS 3 Packet
1
Wireless Communication Network Lab.
Hidden Terminal Problem
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A B C
Radio transmission range
R R R
A and C are hidden with respect to each other
Wireless Communication Network Lab.
B C
Radio Transmission range
R R R
D
R
A
Exposed Terminal Problem
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Transmission at B forces C (Exposed) to stop transmission to D
Wireless Communication Network Lab.
Types of CSMA Protocols
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CSMA
Unslotted Nonpersistent CSMA
Slotted Nonpersistent CSMA
Unslotted persistent CSMA
Slotted persistent CSMA
Nonpersistent CSMA
Persistent CSMA
1-persistent CSMA
p-persistent CSMA
Wireless Communication Network Lab.
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.
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Wireless Communication Network Lab. 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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Throughput of Nonpersistent CSMA
(1 2 )
G
th G
GeS
G e
(1 )
G
th G
GeS
e
Wireless Communication Network Lab.
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:
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(1 2 )
(1 )
1 (1 / 2)
(1 2 ) (1 ) (1 )
G
th G G
G G G G G eS
G e G e
(1 )
(1 )
(1 )
(1 )(1 )
G G
th G G
G e eS
e e
Wireless Communication Network Lab.
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.
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Wireless Communication Network Lab.
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.
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Wireless Communication Network Lab.
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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0 0
0 0 0
(1 ) (1 )( , , )
(1 )[ (1 ) 1 ]
Gs s
th G
e P PS G p
e t t
1 1 1 10 0
( ) ( ) 1 1
n ns s n s s n n n
n n n n
P P n P P n t t t t
Wireless Communication Network Lab.
Throughput of p-persistent CSMA Protocol (2/2)
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(1 )
1
0
,1
,
[(1 ) ], 0
!
(1 )( ) Pr{ }
1 (1 )
, 1!(1 )
Pr{ }
( )where Pr{ } , Pr{ } [1 (1 ) ] ,
( )!
and is the Kronecker delta.
l
nG
n
l
s nl n
n g
n g
n nk
l nkg n
n n l nk
i j
Ge n
n
lp pP n L l
p
g en
n e
t t k
kgL l e t k p l n
l n
Wireless Communication Network Lab.
Throughput
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0 1 2 3 4 5 6 7 8 9
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
S:
Th
rou
ghp
ut
ALOHASlotted ALOHA
1-persistent CSMA
0.5-persistent CSMA
0.1-persistent CSMA
0.01-persistent CSMA
Nonpersistent CSMA
Traffic Load G
Wireless Communication Network Lab.
CSMA/CD (CSMA with Collision Detection) (1/3)
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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.
Wireless Communication Network Lab.
CSMA/CD (CSMA with Collision Detection) (2/3)
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Wireless Communication Network Lab.
CSMA/CD in Ethernet (2/2)
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A B(t is the propagation time)
T0 A begins transmission
Time
A BB begins transmission
T0+t-
T0+ttcd
A BB detects collision
A B
A detects collision just before end of transmission
T0+2t+tcd+
tcr-
cd: time required for a terminal to detect a collision detectioncr: time required to send the jamming signal to achieve the consensus reinforcement procedure
Wireless Communication Network Lab.
Throughput of Slotted Nonpersistent CSMA/CD
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0
0.2
0.4
0.6
0.8
1
1.2
0.001 0.01 0.1 1 10 100 1000Traffic load G
S: T
hro
ughp
ut
α = 1
α = 0.1
α = t /T= 0
α = 0.01
Wireless Communication Network Lab.
CSMA/CA (CSMA with Collision Avoidance) for Wireless Devices
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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.
Wireless Communication Network Lab.
The Basic Mechanism of CSMA/CA
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Contention window
Medium Busy
Defer access
Backoff after defer
Slot
Time
DIFS – Distributed Inter Frame Spacing
DIFSDIFS
Next Frame
Wireless Communication Network Lab.
Example of Backoff Mechanism (1/2)
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DIFS DIFS DIFS
Frame
Frame
Frame
Frame
100
160
60
100
40
60
Backoff Time
Frame Issued
MS 1
MS 2
MS 3
MS 4
Wireless Communication Network Lab.
Example of Backoff Mechanism (2/2)
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DIFS DIFS DIFS
Frame
Frame
Frame
Frame
120
120
60
100
60 120
MS 1
MS 2
MS 3
MS 4
MS 5160
60Frame
100
40Frame
80
60
DIFS
Wireless Communication Network Lab.
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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Wireless Communication Network Lab.
CSMA/CA/ACK
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Next Frame
DIFS
ACK
Other MSs
Source MS
BS
DIFS
SIFS
Data
Defer access
Contention window
Backoff after defer
SIFS – Short Inter Frame Spacing
Time
Wireless Communication Network Lab.
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
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Wireless Communication Network Lab.
CSMA/CA with RTS/CTS
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DIFS
RTS
Other MSs
Source MS
Destination MS
DIFS
SIFS
Defer access
Contention window
Backoff
Next Frame
CTS
SIFS
SIFS
ACK
TimeData
Wireless Communication Network Lab.
RTS/CTS
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CTS
Data
ACK
MS A MS B
RTSPropagation delay
This helps avoid hidden terminal problem in ad hoc networks
Wireless Communication Network Lab.
Homework
P6.2P6.5P6.8P6.11P6.17
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