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Multiple Radio Access. Chapter 6. Introduction Contention Protocols ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance). Outline. Multiple access control channels

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Chapter 6

Multiple Radio Access

Chapter 6


Outline

Introduction

Contention Protocols

ALOHA

Slotted ALOHA

CSMA (Carrier Sense Multiple Access)

CSMA/CD (CSMA with Collision Detection)

CSMA/CA (CSMA with Collision Avoidance)

Outline


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

Introduction

Node 3

Node 4

Node 2

Shared Multiple

Access Medium

Node 1

Node N


Introduction cont d

Multiple access issues

If more than one node transmit at a time on the channel, a collision occurs

How to determine which node can transmit at a specific time?

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.

Introduction (Cont’d)


Channel sharing techniques

Channel Sharing Techniques

Static Channelization

Channel Sharing Techniques

Scheduling

Dynamic Medium Access Control

Random Access


Classification of multiple access protocols

Classification of Multiple Access Protocols

Multiple access protocols

Contention-based

Conflict-free

Collision resolution

Random access

TREE,

WINDOW, etc

ALOHA,

CSMA,

BTMA,

ISMA, etc

FDMA,

TDMA,

CDMA,

Token Bus,

DQDB, etc

BTMA: Busy Tone Multiple Access

ISMA: Idle signal Multiple Access

DQDB: Distributed Queue Dual Bus


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.

Contention Protocols


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

Contention Protocols (Cont’d)


Aloha

ALOHA

Waiting a random time

Node 1 Packet

Node 2 Packet

Retransmission

Retransmission

1

2

3

3

2

Time

Collision

Node 3 Packet

Collision mechanism in ALOHA


Throughput of aloha

(2G)

e

-

2

G

(

)

=

P

n

n

!

Throughput of ALOHA

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

n

where G is traffic load. (number of packets arrive in one packet time.)

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

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

  • The Maximum throughput of ALOHA is


Slotted aloha

Slotted ALOHA

Node 1 Packet

Nodes 2 & 3 Packets

Retransmission

Retransmission

1

2&3

2

3

Time

Collision

Slot

Collision mechanism in slotted ALOHA


Throughput of slotted aloha

Throughput of Slotted ALOHA

  • The probability of no collision is given by

  • The throughput S is

  • The Maximum throughput of slotted ALOHA is


Throughput

Throughput

0.368

Slotted Aloha

S

0.184

Aloha

G


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).

CSMA (Carrier Sense Multiple Access)


Collision mechanism in csma

Collision Mechanism in CSMA

Node 5 sense

Node 1 Packet

Node 2 Packet

Delay

Node 3 Packet

1

2

3

4

5

Time

Delay

Collision

Node 4 sense


Kinds of csma

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


Nonpersistent x persistent csma protocols

Nonpersistent CSMA Protocol:

Step 1: If the medium is idle, transmit immediately

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

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)

(At the time when the medium first become free, collisions may occur with a high probability.)

Nonpersistent/x-persistent CSMA Protocols


Nonpersistent x persistent csma protocols1

p-persistent CSMA Protocol:

Step 1: If the medium is idle, transmit with probability p, and delay for worst case propagation delay for one packet with probability (1-p)

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

Nonpersistent/x-persistent CSMA Protocols


How to select probability p

Assume that N nodes have a packet to send 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, where N is the maximum number of nodes that can be active at a time

How to Select Probability p ?


Throughput1

Throughput

0.01-persistent CSMA

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Nonpersistent CSMA

0.1-persistent CSMA

0.5-persistent CSMA

S

1-persistent CSMA

Slotted Aloha

Aloha

0 1 2 3 4 5 6 7 8 9

G


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

Step 4:Wait a random amount of time and repeats the same algorithm

CSMA/CD (CSMA with Collision Detection)


Csma cd cont d

CSMA/CD (Cont’d)

T0

A begins transmission

A

B

T0+-

B begins transmission

A

B

T0+

B detects collision

A

B

T0+2 -

A detects collision just before end of transmission

A

B

Time

(collision detection time is 2 times end-to-end propagation delay)

( is the propagation time)


Wireless communication needs to avoid collisions

Collision detection (in CSMA/CD) is not easy in wireless communication

Wireless communication needs to avoid collisions

Medium free

Transmit a packet

Transmit a packet

The medium is Noisy, signal strength is weak.


The spirit of csma ca

Can sense the medium ( to check if it’s busy)

Avoid collisions

If medium is busy, back off for a random time interval (count down) ; when the interval expires, sense again

Use random back-off interval to avoid new collision. It’s still possible that a colision occurs. Use binary exponential back-off.

The spirit of CSMA/CA


Basics of csma ca

Elements of CSMA/CA: IFS-interframe spacing, CW- contention window, back-off counter

Basics of CSMA/CA


Chapter 6 4043534

1

2

3

4


Example

A transmits; B,C,D request to transmit,but the medium is busy.

B,C,D get back-off intervals, B’s interval is the longest, D’s is the second; C’s is the shortest; in the next CW, C counts down to zero first; C transmits and others wait and freeze their counters.

In the next CW, D counts to zero first; D transmits and others wait.

E counts to zero; E transmits then B transmits.

Example


Chapter 6 4043534

To allow three contention methods (functions), three interframe spacing lengths is designed in CSMA/CA.

SIFS (shortest IFS) <PIFS (Point coordination function IFS)< DIFS (Distributed coordination function IFS)

The shorter the IFS length, the higher the priority

The basic contention method in 802.11 is DCF-Distributed Coordination Function, CSMA/CA

IFS


Chapter 6 4043534

PCF (Point Coordination Function)is for time critical application such as real time application, voice, multimedia

Priority-based access, A point coordinator (PC) controls the PCF. The PC is always located in an AP (access point)


Chapter 6 4043534

SIFS is designed for ACK and for solving the hidden terminal problem

Used in the RTS (request to send), CTS (clear to send) protocol


Csma ca with ack

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 with ACK


Csma ca ack

CSMA/CA/ACK

DIFS

Time

Data

Source

SIFS

ACK

Destination

DIFS

Contention window

Next Frame

Other

Defer access

Backoff after defer

SIFS – Short Inter Frame Spacing


Hidden terminal problem

Hidden terminal problem

  • A and C cannot see each other, but all want to communicate with B. B’s messages are corrupted.

C

A

B


Csma ca with rts cts

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


Csma ca with rts cts cont d

CSMA/CA with RTS/CTS (Cont’d)

DIFS

SIFS

Time

Data

Source

RTS

SIFS

SIFS

CTS

ACK

Destination

DIFS

Contention window

Next Frame

Other

Backoff after defer

Defer access


Rts cts

RTS/CTS

Node A

Node B

RTS

Propagation delay

CTS

Data

ACK


Exposed terminal problem

Exposed terminal problem

R

C

D

A

B

  • A, B can communicate; B and C can communicate; C and D can communicate; A cannot hear C,D.

  • When B wants to send a message to A, it sends an RTS which delays C’s message to D (which does not interfere with the data reception of A).

  • The exposed terminal problem is created by the RTS/CTS protocol and leads to lower network performance


Csma ca csma with collision avoidance

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 terminals during this time period (backoff time), the terminal freezes its counter.

It resumes count down after other terminals finish transmission + DIFS. The terminal can start its transmission when the counter reaches to zero.

CSMA/CA (CSMA with collision Avoidance)


Handoff procedure in ieee 802 11

Handoff procedure in IEEE 802.11


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