chapter 6 multiple radio access n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Chapter 6 Multiple Radio Access PowerPoint Presentation
Download Presentation
Chapter 6 Multiple Radio Access

Loading in 2 Seconds...

play fullscreen
1 / 39

Chapter 6 Multiple Radio Access - PowerPoint PPT Presentation


  • 134 Views
  • Uploaded on

Chapter 6 Multiple Radio Access. 曾志成 國立宜蘭大學 電機工程學系 tsengcc@niu.edu.tw. Introduction (1/2). 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. MS 3.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chapter 6 Multiple Radio Access' - margot


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
chapter 6 multiple radio access

Chapter 6Multiple Radio Access

曾志成

國立宜蘭大學 電機工程學系

tsengcc@niu.edu.tw

EE of NIU

introduction 1 2
Introduction (1/2)
  • 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.

MS 3

MS 4

MS 2

Shared Multiple

Access Medium

MS 1

Node N

EE of NIU

introduction 2 2
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)

EE of NIU

channel sharing techniques

Static Channelization

Scheduling

Dynamic Medium Access Control

Random Access

Channel Sharing Techniques

The channel assignment is done in a pre-specified way and does not change with time.

Medium - Sharing Techniques

The channel is allocated as needed and changes with time.

EE of NIU

classification of multiple access protocols for a shared channel

Contention-based

Conflict-free

Collision resolution

Random access

FDMA,

TDMA,

CDMA,

Token Bus,

DQDB, etc.

TREE,

WINDOW, etc.

ALOHA,

CSMA,

BTMA,

ISMA, etc.

DQDB: Distributed Queue Dual Bus

Classification of Multiple Access Protocols for a Shared Channel

Multiple Access Protocols

  • (will be discussed in Chap 7)

BTMA: Busy Tone Multiple Access

ISMA: Idle Signal Multiple Access

EE of NIU

contention based protocols aloha family
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.

EE of NIU

pure aloha

Node 2 Packet

Retransmission

2

2

3

Collision

Node 3 Packet

Pure ALOHA

Node 1 Packet

Waiting a random time

Retransmission

1

3

Time

t

t+T

t-T

Collision mechanism in Pure ALOHA

EE of NIU

throughput of pure aloha
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

EE of NIU

slotted aloha

Retransmission

Retransmission

2

3

Slotted ALOHA

Nodes 2 & 3 Packets

Node 1 Packet

1

2

3

Time

Collision

Slot

Collision mechanism in slotted ALOHA

EE of NIU

throughput of slotted aloha
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

EE of NIU

throughput

0.5

0.4

0.368

0.3

Slotted ALOHA

S: Throughput

0.184

0.2

0.1

Pure ALOHA

0

0

2

4

6

8

G=gT

Throughput

EE of NIU

contention based protocols csma family
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

EE of NIU

csma carrier sense multiple access
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)

EE of NIU

collision mechanism in csma

MS 5 sense

MS 2 Packet

Delay for MS 5

MS 3 Packet

2

3

5

Delay for MS 4

Collision due to hidden terminal problem

MS 4 senses

Collision Mechanism in CSMA

MS 1 Packet

1

4

Time

EE of NIU

hidden terminal problem
Hidden Terminal Problem

Radio transmission range

R

R

R

A

B

C

A and C are hidden with respect to each other

EE of NIU

exposed terminal problem
Exposed Terminal Problem

Radio Transmission range

R

R

R

R

A

B

C

D

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

EE of NIU

types of csma protocols

Unslotted Nonpersistent CSMA

Nonpersistent CSMA

Slotted Nonpersistent CSMA

Unslotted persistent CSMA

Persistent CSMA

Slotted persistent CSMA

1-persistent CSMA

p-persistent CSMA

Types of CSMA Protocols

CSMA

EE of NIU

nonpersistent csma protocols
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.

EE of NIU

throughput of nonpersistent csma
Throughput of Nonpersistent CSMA
  • For unslottednonpersistent 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

EE of NIU

1 persistent csma protocols
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:

EE of NIU

p persistent csma protocols
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.

EE of NIU

how to select the probability p
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.

EE of NIU

throughput of p persistent csma protocol 1 2
Throughput ofp-persistent CSMA Protocol (1/2)
  • Throughput S as:

where G is the offered traffic rate, a=t/T=propagation delay/packet transmission time and

EE of NIU

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

1-persistent CSMA

S: Throughput

Slotted ALOHA

ALOHA

0 1 2 3 4 5 6 7 8 9

Traffic Load G

EE of NIU

csma cd csma with collision detection 1 3
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.

EE of NIU

csma cd in ethernet 2 2

T0+t-

B begins transmission

A

B

T0+t+tcd

B detects collision

A

B

T0+2t+tcd+tcr-

A detects collision just before end of transmission

A

B

CSMA/CD in Ethernet (2/2)

T0

A begins transmission

cd: time required for a terminal to detect a collision detection

cr: time required to send the jamming signal to achieve the consensus reinforcement procedure

A

B

(tis the propagation time)

Time

EE of NIU

throughput of slotted nonpersistent csma cd

1.2

α =t /T= 0

1

0.8

α = 0.01

S: Throughput

0.6

0.4

α = 0.1

α = 1

0.2

0

0.001

0.01

0.1

1

10

100

1000

Traffic load G

Throughput of Slotted Nonpersistent CSMA/CD

EE of NIU

csma ca csma with collision avoidance for wireless devices
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.

EE of NIU

the basic mechanism of csma ca

Medium Busy

Defer access

Slot

Backoff after defer

The Basic Mechanism of CSMA/CA

Contention window

DIFS

DIFS

Next Frame

Time

DIFS – Distributed Inter Frame Spacing

EE of NIU

csma ca with ack for ad hoc networks
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

EE of NIU

csma ca ack

DIFS

SIFS

DIFS

Contention window

Backoff after defer

CSMA/CA/ACK

Data

Time

Source MS

ACK

BS

Next Frame

Other MSs

Defer access

SIFS – Short Inter Frame Spacing

EE of NIU

csma ca with rts cts for ad hoc networks
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

EE of NIU

csma ca with rts cts

DIFS

SIFS

SIFS

SIFS

CTS

DIFS

Contention window

Backoff

Defer access

CSMA/CA with RTS/CTS

Time

RTS

Data

Source MS

ACK

Destination MS

Next Frame

Other MSs

EE of NIU

rts cts

RTS

Propagation delay

CTS

Data

ACK

RTS/CTS

MS A

MS B

This helps avoid hidden terminal problem in ad hoc networks

EE of NIU

homework
Homework
  • P6.2
  • P6.6
  • P6.7
  • P6.8
  • P6.11

EE of NIU