wireless communication network lab. chapter 6 multiple radio access chih-cheng tseng1ee of niu prof....

Wireless Communication Network Lab.

Chapter 6Multiple Radio Access

Chih-Cheng Tseng 1EE of NIU

Prof. Chih-Cheng Tseng

[email protected]

http://wcnlab.niu.edu.tw

mailto:[email protected]

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)

EE of NIU Chih-Cheng Tseng 2

Shared MultipleAccess Medium

MS 4

MS 3

MS 2

MS 1 Node N…


Introduction (2/2)


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)


Channel Sharing Techniques


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.


Classification of Multiple Access Protocols for a Shared Channel


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)


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.



Pure ALOHA


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


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


2(no collision) gTsP P e

2 .gTthS gTe

2 22 0.G GthdSGe e

dG

,max

10.184.

2thSe

2 .GthS Ge


Slotted ALOHA


1 Time2

Retransmission Retransmission

3

Slot

Node 1 Packet

2

Collision

Nodes 2 & 3 Packets

Collision mechanism in slotted ALOHA

3


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


gTs eP

0.G GthdSGe e

dG

,max

10.368.thS

e


Throughput


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


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



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)



Collision Mechanism in CSMA


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


Hidden Terminal Problem


A B C

Radio transmission range

R R R

A and C are hidden with respect to each other


B C

Radio Transmission range

R R R

D

R

A

Exposed Terminal Problem


Transmission at B forces C (Exposed) to stop transmission to D


Types of CSMA Protocols


CSMA

Unslotted Nonpersistent CSMA

Slotted Nonpersistent CSMA

Unslotted persistent CSMA

Slotted persistent CSMA

Nonpersistent CSMA

Persistent CSMA

1-persistent CSMA

p-persistent CSMA


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.


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


Throughput of Nonpersistent CSMA

(1 2 )

G

th G

GeS

G e

(1 )

G

th G

GeS

e


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:


(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


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.



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.



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


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


Throughput of p-persistent CSMA Protocol (2/2)


(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


Throughput


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


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.


CSMA/CD (CSMA with Collision Detection) (2/3)



CSMA/CD in Ethernet (2/2)


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


Throughput of Slotted Nonpersistent CSMA/CD


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


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.


The Basic Mechanism of CSMA/CA


Contention window

Medium Busy

Defer access

Backoff after defer

Slot

Time

DIFS – Distributed Inter Frame Spacing

DIFSDIFS

Next Frame


Example of Backoff Mechanism (1/2)


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


Example of Backoff Mechanism (2/2)


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


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



CSMA/CA/ACK


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


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



CSMA/CA with RTS/CTS


DIFS

RTS

Other MSs

Source MS

Destination MS

DIFS

SIFS

Defer access

Contention window

Backoff

Next Frame

CTS

SIFS

SIFS

ACK

TimeData


RTS/CTS


CTS

Data

ACK

MS A MS B

RTSPropagation delay

This helps avoid hidden terminal problem in ad hoc networks


Homework

P6.2P6.5P6.8P6.11P6.17


wireless communication network lab. chapter 6 multiple radio access chih-cheng tseng1ee of niu prof....

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