Khurram Masood 200806100 Multiple Access Protocols Chapter 6 of Hiroshi Harada Book

Khurram MasoodKhurram Masood200806100200806100

Multiple Access Protocols

Chapter 6 of Hiroshi Harada Book

2

IntroductionIntroduction

• 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 MultipleAccess Control Channel to BS

Node 4

Node 3

Node 2

Node 1…

Node N

3

Introduction (Cont’d)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.

Packet Communication System ConfigurationPacket Communication System Configuration

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5

Classification of Multiple Access ProtocolsClassification of Multiple Access Protocols

Multiple access protocols

Contention-based Conflict-free

Random access Collision resolution

FDMA,

TDMA,

CDMA,

Token Bus,

DQDB, etc

ALOHA,

CSMA,

BTMA,

ISMA, etc

TREE,

WINDOW, etc

DQDB: Distributed Queue Dual Bus

BTMA: Busy Tone Multiple AccessISMA: Internet Streaming Media Alliance

6

Contention ProtocolsContention 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.

7

Contention Protocols (Cont’d)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

ALOHAALOHA

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ALOHAALOHA

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10

ALOHAALOHA

1 2 3 3 2Time

Collision

Retransmission Retransmission

Node 1 Packet

Collision mechanism in ALOHA

Waiting a random time

Node 2 Packet

Node 3 Packet

Collisions in (Pure) ALOHACollisions in (Pure) ALOHA

1.1 1.2

TransmissionTime

(F)

Station 1

2.1Station 2

3.1 3.2Station 3

Broadcastchannel

2.2

1.3

CompleteCollision

PartialCollision

12

Throughput of ALOHAThroughput of ALOHA

n

!n

(2G)nP

e 2G

• The probability that n packets arrive in two packets time is given by

where G is traffic load.

GeP 20

• The probability P(0) that a packet is successfully received without collision is calculated by letting n=0 in the above equation. We get

GeGPGS 20

• We can calculate throughput S with a traffic load G as follows:

184.02

1max

eS• The Maximum throughput of ALOHA is

Unslotted ALOHAUnslotted 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.)Unslotted ALOHA (cont.)

Frame which collideswith start of red frame

Frame

t0-F t0 t0+F

VulnerablePeriod of red frame

Time

Frame which collideswith end of red frame

• 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.)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

Slotted ALOHASlotted ALOHA

1 2&3 2Time

Collision

Retransmission Retransmission

3

Slot

Node 1 Packet

Nodes 2 & 3 Packets

Collision mechanism in 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-ALOHACollisions in S-ALOHA

1.1 1.2

TransmissionDelay

Station 1

2.1Station 2

3.1 3.2

Station 3

Broadcastchannel

2.2

1.3

CompleteCollision

Slotted ALOHA (cont.)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.)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.

21

Throughput of Slotted ALOHAThroughput of Slotted ALOHA

GeP 0

• The probability of no collision is given by

GeGPGS 0

• The throughput S is

368.01

max e

S

• The Maximum throughput of slotted ALOHA is

22

ThroughputThroughput

G86420

0.5

0.4

0.3

0.2

0.1

0

Slotted Aloha

Aloha

0.368

0.184

G

S

Comparison of ALOHA and S-ALOHAComparison of ALOHA and S-ALOHA

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

R

Thr

ough

put

(ALO

HA

)

Slotted ALOHA: Re-R

Pure ALOHA: Re-2R

Ideal (no collisions): R

CSMACSMA

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CSMACSMA

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CSMACSMA

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27

CSMA (Carrier Sense Multiple Access)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).

28

Collision Mechanism in CSMACollision Mechanism in CSMA

1 2 3Time

Collision

4

Node 4 sense

Delay

5

Node 5 sense

Delay

Node 1 PacketNode 2 Packet

Node 3 Packet

Kinds of CSMAKinds of CSMA

CSMA

Nonpersistent CSMA

Persistent CSMA

Unslotted Nonpersistent CSMA

Unslotted persistent CSMA

Slotted Nonpersistent CSMA

Slotted persistent CSMA

1-persistent CSMA

p-persistent CSMA

Non persistent CSMANon persistent CSMA

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Slotted non persistent ISMASlotted non persistent ISMA

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Computer Simulation ConfigurationComputer Simulation Configuration

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Layout of access point and user terminalsLayout of access point and user terminals

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Simulation and resultsSimulation and results

• Simulation– Pure ALOHA– Slotted ALOHA– Non persistent CSMA– Non persistent ISMA

• Results– Throughput– Average delay time

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Pure ALOHAPure ALOHA

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.05

0.1

0.15

0.2

0.25

Traffic(Simulation result)

Th

rou

gh

pu

tThroughput of Pure ALOHA system

without capture effect

with capture effecttheory

Pure ALOHAPure ALOHA

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

500

1000

1500

2000

2500

3000


Ave

rag

e D

ela

y tim

e(p

ack

et)

Average Delay time of Pure ALOHA system


with capture effect


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0 0.5 1 1.5 2 2.5 3 3.5 40.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5


Th

rou

gh

pu

tThroughput of Slotted ALOHA system


with capture effecttheory


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0 0.5 1 1.5 2 2.5 3 3.5 40

200

400

600

800

1000

1200

1400


Ave

rag

e D

ela

y tim

e(p

ack

et)

Average Delay time of Slotted ALOHA system


with capture effect


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10-2

10-1

100

101

102

103

0

0.5

1

1.5


Th

rou

gh

pu

tThroughput of np CSMA system

without capture effect (a=0.01)

with capture effect (a=0.01)

theorywithout capture effect (a=0.1)


theory


04/21/23 40

10-1

100

101

102

100

200

300

400

500

600

700

800

900


Ave

rag

e D

ela

y tim

e(p

ack

et)

Average Delay time of np CSMA system





Non persistent ISMANon persistent ISMA

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10-2

10-1

100

101

102

103

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Th

rou

gh

pu

tThroughput of Slotted np ISMA system

without capture effect (d=0.01)

with capture effect (d=0.01)

theorywithout capture effect (d=0.1)


theory

Non persistent ISMANon persistent ISMA

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10-2

10-1

100

101

102

103

50

100

150

200

250

300

350

400

450


Ave

rag

e D

ela

y tim

e(p

ack

et)

Average Delay time of Slotted ISMA system





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Khurram Masood 200806100 Multiple Access Protocols Chapter 6 of Hiroshi Harada Book