Multiple Access Protocols
Chapter 6 of Hiroshi Harada Book
Khurram Masood
200806100
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
Node 3
Node 1
Shared Multiple
Access Control
Channel to BS
Node 4
…
Node 2
Node N
2
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.
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Packet Communication System Configuration
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Classification of Multiple Access Protocols
Multiple access protocols
Contention-based
Random access
Conflict-free
Collision resolution
ALOHA,
TREE,
FDMA,
CSMA,
WINDOW,
etc
TDMA,
BTMA,
ISMA,
etc
BTMA: Busy Tone Multiple Access
ISMA: Internet Streaming Media Alliance
CDMA,
Token Bus,
DQDB, etc
DQDB: Distributed Queue Dual Bus
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Contention Protocols
• 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.
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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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ALOHA
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ALOHA
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ALOHA
Waiting a random time
Node 1 Packet
Node 2 Packet
Retransmission
1
2
3
3
Retransmission
2
Time
Collision
Node 3 Packet
Collision mechanism in ALOHA
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Collisions in (Pure) ALOHA
Station 1
1.1
1.2
1.3
Transmission
Time
(F)
2.1
2.2
Station 2
3.1
3.2
Station 3
Complete
Collision
Broadcast
channel
Partial
Collision
Throughput of ALOHA
• The probability that n packets arrive in two packets time is given by
P(n ) =
n
(2G) e-2G
n!
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
P(0) = e-2G
• We can calculate throughput S with a traffic load G as follows:
S = G  P(0) = G  e-2G
• The Maximum throughput of ALOHA is
S max =
1
 0.184
2e
12
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.
Unslotted ALOHA (cont.)
Frame which collides
with start of red frame
Frame which collides
with end of red frame
Frame
t0-F
t0
Vulnerable
Period of red frame
•
•
t0+F
Time
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
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.
Slotted ALOHA
Node 1 Packet
Nodes 2 & 3 Packets
Retransmission
1
2&3
2
Retransmission
3
Time
Slot
Collision
Collision mechanism in slotted ALOHA
16
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.
Collisions in S-ALOHA
Station 1
1.1
1.3
1.2
Transmission
Delay
2.1
2.2
Station 2
3.2
3.1
Station 3
Complete
Collision
Broadcast
channel
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.
Slotted ALOHA (cont.)
• The MAX departure rate occurs at G=1 and is 1/e ≈
0.368.
• If G<1, too many idle slots are generated.
• If G>1, too many collisions are generated.
Throughput of Slotted ALOHA
• The probability of no collision is given by
P(0) = e-G
• The throughput S is
S = G  P(0) = G  e-G
• The Maximum throughput of slotted ALOHA is
S max =
1
 0.368
e
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Throughput
0.5
0.4
0.368
0.3
S
Slotted Aloha
0.2
0.1
00
0.184
Aloha
2
4
G
6
8
G
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Comparison of ALOHA and S-ALOHA
0.5
Throughput (ALOHA)
Ideal (no collisions): R
0.4
Slotted ALOHA: Re -R
0.3
0.2
Pure ALOHA: Re-2R
0.1
0
0
0.5
1
1.5
R
2
2.5
3
CSMA
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CSMA
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CSMA
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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
Node 5 sense
Node 1 Packet
Node 2 Packet
Node 3 Packet
1
2
Delay
3
4
5
Time
Delay
Collision
Node 4 sense
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Kinds of CSMA
Unslotted Nonpersistent CSMA
Nonpersistent CSMA
Slotted Nonpersistent CSMA
CSMA
Unslotted persistent CSMA
Persistent CSMA
Slotted persistent CSMA
1-persistent CSMA
p-persistent CSMA
Non persistent CSMA
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Slotted non persistent ISMA
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Computer Simulation Configuration
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Layout of access point and user terminals
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Simulation and results
• Simulation
–
–
–
–
Pure ALOHA
Slotted ALOHA
Non persistent CSMA
Non persistent ISMA
• Results
– Throughput
– Average delay time
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Pure ALOHA
Throughput of Pure ALOHA system
0.25
Throughput
0.2
0.15
0.1
0.05
0
without capture effect
with capture effect
theory
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Traffic(Simulation result)
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Pure ALOHA
Average Delay time of Pure ALOHA system
3000
Average Delay time(packet)
without capture effect
with capture effect
2500
2000
1500
1000
500
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Traffic(Simulation result)
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Slotted ALOHA
Throughput of Slotted ALOHA system
0.5
0.45
Throughput
0.4
0.35
0.3
0.25
0.2
0.15
without capture effect
with capture effect
theory
0.1
0.05
0
0.5
1
1.5
2
2.5
3
3.5
4
Traffic(Simulation result)
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Slotted ALOHA
Average Delay time of Slotted ALOHA system
Average Delay time(packet)
1400
without capture effect
with capture effect
1200
1000
800
600
400
200
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Traffic(Simulation result)
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Non persistent CSMA
Throughput of np CSMA system
Throughput
1.5
without capture effect (a=0.01)
with capture effect (a=0.01)
theory
without capture effect (a=0.1)
with capture effect (a=0.1)
theory
1
0.5
0
-2
10
-1
10
0
10
1
10
2
10
3
10
Traffic(Simulation result)
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Non persistent CSMA
Average Delay time of np CSMA system
Average Delay time(packet)
900
800
700
without capture effect (a=0.01)
with capture effect (a=0.01)
without capture effect (a=0.1)
with capture effect (a=0.1)
600
500
400
300
200
100
-1
10
0
10
1
10
2
10
Traffic(Simulation result)
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Non persistent ISMA
Throughput of Slotted np ISMA system
0.9
0.8
Throughput
0.7
0.6
0.5
0.4
without capture effect (d=0.01)
with capture effect (d=0.01)
theory
without capture effect (d=0.1)
with capture effect (d=0.1)
theory
0.3
0.2
0.1
0
-2
10
-1
10
0
10
1
10
2
10
3
10
Traffic(Simulation result)
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Non persistent ISMA
Average Delay time of Slotted ISMA system
Average Delay time(packet)
450
400
350
without capture effect (d=0.01)
with capture effect (d=0.01)
without capture effect (d=0.1)
with capture effect (d=0.1)
300
250
200
150
100
50
-2
10
-1
10
0
10
1
10
2
10
3
10
Traffic(Simulation result)
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