1 cs 6910 – pervasive computing spring 2007 section 6 (ch.6): multiple radio access prof. leszek...

1

CS 6910 – Pervasive ComputingSpring 2007

Section 6 (Ch.6):

Multiple Radio Access

Prof. Leszek LilienDepartment of Computer Science

Western Michigan University

Slides based on publisher’s slides for 1st and 2nd edition of: Introduction to Wireless and Mobile Systems by Agrawal & Zeng

© 2003, 2006, Dharma P. Agrawal and Qing-An Zeng. All rights reserved.

Some original slides were modified by L. Lilien, who strived to make such modifications clearly visible. Some slides were added by L. Lilien, and are © 2006-2007 by Leszek T. Lilien.

Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request.

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 2

Chapter 6 Multiple Radio Access


Outline

Introduction Contention Protocols ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance)


6.1. Introduction

© 2007 by Leszek T. Lilien

Recall Large # of traffic

channels on each BS Bec. traffic channels used by

1 MS exclusively for call duration

Small # of controlchannels on each BS

Bec. control channelsshared by many MSsfor short periods

Too expensive/inefficient to assign control channnel for call duration

MSs compete for these few shared control channels For call setup, etc.

BS

Forward

(downlin

k) contro

l channel

MS

Reverse (

uplink) c

ontrol c

hannel

Forward

(downlin

k) traff

ic channel

Reverse (

uplink) tr

affic

channel


Introduction – cont. 1 Multiple access control channels

Each MS attached to a transmitter/receiver Communicates with its BS via a control channel

(CC) CC shared by other MSs

=> “multiple access” CC Transmission from any MS is received by all MSs

within its radio range

Shared MultipleAccess Medium

MS 4

MS 3

MS 2

MS 1…

MS n…


Multiple access issue If more than one MS transmits at a time on a CC

to BS, a collision occurs on this CC How to determine which MS can transmit to BS?

Multiple (radio) access protocols (a.k.a. MAC sublayer protocols)

Solve multiple access issues Specify how shared control channels are to be

accessed by M

Introduction – cont. 2

(Modified by LTL)

Shared MultipleAccess Medium

MS 4

MS 3

MS 2

MS 1

…

MS n

…


Medium (Channel) Sharing Techniques

Medium- sharing

Techniques

Static Channelization

Dynamic Medium

Access Control

Scheduled

Random Access

Static channelization (static medium access control) – channel assigned in a pre-specified way and not changed

Dynamic channelization (dynamic medium access control) - channel allocated as needed and not changes with time

(Modified by LTL)


Recall ISO Open Systems Interconnection Reference Model for networks Communication subnetwork = 3 lowest layers of

OSI Network layer (NET) Data link layer (DLL) Physical layer (PHY)

6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols



Comm subnet in wired LANs, MANs, PRNs, PANsLAN = local area network / MAN = metropolitan area network PAN = personal area network / PRN = paceket radio network

Classic (wire!): point-to-point full-duplex channels

Comm subnet in wireless systems Broadcast half-duplex channels

Broadcast over airwaves - not point-to-point Radio transmission goes from xmitter to rcvr - not full-duplex

Can emulate point-to-point xmission in wireless systems

In order to use solutions (e.g., protocols) from wired netwroks over wireless

Note: broadcast possible in some wired net topologies

E.g. bus, ring, ...

6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols – cont. 1



Q: How to emulate point-to-point xmission in wireless systems?

A: Modify OSI for point-to-point xmission in wireless systems Specifically, split DLL into 2 sublayers:

Logical link control (LLC)sublayer (upper sublayer)

Medium access control(MAC) sublayer (lower sublayer)

MAC sublayer protocols = multiple access protocols = multiple radio access protocols (see above)

(Modified by LTL)


6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols – cont. 1

DLL

NET


Types of multiple access protocols (cf. next slide): Contention-based protocols - resolve a collision

after it occurs Execute a collision resolution protocol after each

collision

Collision-free protocols (a.k.a. conflict free p.) - ensure that a collision never occurs

E.g., a bit-map protocol and binary countdown protocol

Types of Multiple Access Protocols

(Modified by LTL)


Classification of Multiple Access Protocols

Random access protocols – upon collision, retrans-mit after a ran-dom delay

Collision resolution protocols - upon collision, retransmit according to a more sophisti-cated method

Multiple-access protocols

Contention-based Collision-free

Random access Collision resolution

FDMA

TDMA

CDMA

Token Bus

DQDB

etc.

ALOHA

CSMA

BTMA,

ISMA

etc.

TREE

WINDOWetc.

DQDB: Distributed Queue Dual Bus BTMA: Busy Tone Multiple AccessISMA: Idle Signal Multiple Access

ALOHA and CSMA protocols will be discussed in this section.


6.3. Contention-Based Protocols

Major categories of contention-based protocols1) ALOHA (a.k.a. Pure ALOHA)2) Slotted ALOHA3) CSMA (Carrier Sense Multiple Access)

... discussed in turn below ...



6.3.1. ALOHA (a.k.a. Pure ALOHA) Developed in the 1970s for a packet radio network by

University of Hawaii To interconnect islands’ campuses / Single-hop network

Principles: Sender S may xmit a packet at any time (hoping no

collision will occur) S finds out whether transmission was successful or

experienced a collision by listening for an ACK broadcast from the destination station

Successful if an ACK arrrives within a time-out period T If no ACK within T, S assumes that there was a collision

=> packet was lost after colliding with pckt of another station

If there was a collision, S retransmits after some random time

Analogy: 2 people entering a doorway simultaneously Collision can occur

(Modified by LTL)


6.3.1. ALOHA – cont.

Collision in Pure ALOHA (can last up to 2T)

1 2 3 3 2Time

Collision

Retransmission Retransmission

MS 1 packetWait for a random time

MS 2 packetMS 3 packet

t - T t t + T T

Recall ALOHA principles: Sender S may xmit a packet at any time S waits for an ACK broadcast from the destination station If there was a collision, S retransmits after some random time

Example: Packet 2 from MS2 & Packet 3 from MS3 collide MS3 retransmits Packet 3 pretty soon (random wait) MS2 retransmits Packet 2 much later (random wait)

Note: Collision can block channel for period up to 2T

(Modified by LTL)


** OPTIONAL ** Throughput of ALOHA

n

!n

(2G)nP

e 2G

• T – packet length (duration)• Probability P(n) that n packets arrive in time 2T

-- If 2 packets of length T arrive in time > 2T, they didn’t collide

where G is normalized offerred traffic load for the channel

GeP 20

• Probability P(0) that a packet is successfully received (without collision) - calculated by letting n=0 in P(n)

GeGPGS 20

• Throughput S

184.02

1max

eS

• Maximum throughput for ALOHA (occurs for G = ½ - cf. p. 129)

Analogy: two people sufficiently distant from each other do not collide in a doorway

(Modified by LTL)


Improvements over Pure ALOHA: Time is slotted

Slot length = packet length (duration) = T Packet transmission can start only at the

beginning of a slot This reduces collision duration

Pure ALOHA: If Packet 1 is almost finished when it collides with just-starting Packet 2, collision can lasts for nearly T+T = 2T (details: Slide +2)

Slotted ALOHA: 2 Packets can collide only when both are just starting => collision can last at most T (details: Slide +4)

(Modified by LTL)

6.3.2. Slotted ALOHA


6.3.2. Slotted ALOHA – cont.

Collision in Slotted ALOHA (can last max. T)

1 2&3 2Time

Collision

Retransmission Retransmission

3

Slots

MS 1 packet

MSs 2 & 3 packets Wait for a random time

Recall Slotted ALOHA improvements: Time is slotted / Packet can only start at the beginning of a slot This reduces collision duration

Collision can block channel for max. T Example: Packet 2 from MS2 & Packet 3 from MS3 collide

MS2 retransmits Packet 2 pretty soon (random wait) MS3 retransmits Packet 3 much later (random wait)

Note: Shows again that collision can block channel for period T

(Modified by LTL)


** OPTIONAL ** Throughput of Slotted ALOHA

GeP 0

• Probability of no collision

GeGPGS 0

• Throughput S

368.01

max e

S

• Maximum throughput for slotted ALOHA (occurs for G = 1 - cf. p. 130)


Comparison of Throughputsfor ALOHA and Slotted ALOHA

Slotted Aloha

Aloha

0.368

0.184

Traffic load G

Thr

ough

put S

th

(Modified by LTL)

G86420

0.5

0.4

0.3

0.2

0.1

0

Max. throughput for ALOHA (A) Sth = 0.184 at G = ½

Max. throughput for Slotted ALOHA (SA) Sth = 0.368 at G = 1

Notice that Sth for SA is exactly 2 x bigger than Sth for A

Hypothesis: bec. max. duration of a collision for SA (=2T) is 2 x smaller than for A (=T)


6.3.2-B. CSMA (Carrier Sense Multiple Access) Group of Protocols

Max. throughputs for Pure & Slotted ALOHA are 0.184 & 0.368

CSMA protocols give better throughput than both Aloha protocols By avoiding collisions

Basic improvement in all CSMA protocols over ALOHA protocols: Listen to the channel (“sense” for the presence of the “carrier”) before transmitting a packet Don’t transmit if channel busy

Avoids some collisions - but not all See Fig - next slide


Perspective on MAC & Access Methods

Medium Access Control (MAC) Different nodes must gain access to

the shared medium in a controlledfashion (otherwise there will becollisions)

Access methods FDMA - assigning channels in frequency domain TDMA - assigning time slots in time domain CDMA - assigning code sequences in code

domain CSMA - assigning transmission opportunities in

time domain on a statistical basis

© 2006, Michael Hall, Helsinki Univ. of Technology

DLL

NET


Basic Collision Mechanism in CSMA Protocols

Some collisions avoided Packet 4 was not sent so it did not collide with Packet 2 Packet 5 was not sent so it did not collide with Packet 3

Collisions can still occur if sensing occurs nearly simulta-neously Packet 6 collided with Packet 7

7

Time1 2 3

Collision

6

MS 4 senses,avoids collision

Delay for MS 4

MS 5 senses,avoids collision

MS 5 Delay

MS 1 senses, sends packet


MS 3 senses,sends packet



(Modified by LTL)


1) Basic CSMA, often just CSMA (Carrier Sense Multiple Access)

2) CSMA/CD (CSMA with Collision Detection)

3) CSMA/CA (CSMA with Collision Avoidance)

Types of CSMA Protocols



(Basic) CSMA improvement over ALOHAs:Sender S starts transmission only if no transmission is ongoing

6.3.3 (Basic) CSMA Protocol



Types of CSMA Protocols

CSMA

Nonpersistent CSMA

Persistent CSMA

Unslotted nonpersistent CSMA

Unslotted persistent CSMA

Slotted nonpersistent 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 then go to Step 1 Note:

Random backoff reduces probability of collisions Wasted 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

Note: There will always be a collision if two or more nodes

wait for idle medium Usually senders stop transmission attempts after a few

unsuccessful tries


Nonpersistent/x-persistent CSMA Protocols – cont.

p-persistent CSMA Protocol: Slotted time required.

Step 1: If the medium is idle, transmit with probability p, or delay till next time slot with probability (1-p)

Step 2: If transmission was delayed by one time slot, go to Step 1

Step 3: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1

Note: P-persistent CSMA is a good tradeoff between

nonpersistent and 1-persistent CSMA


How to Select Probability p ?

Assume: N - # nodes that can be active at a time

(i.e., ready to send a packet) => Np - the expected number of nodes that will

attempt to transmit once the medium becomes idle Np ≥ 1 => a collision is expected to occur

=> to avoid collisions,network must make sure that Np < 1

(Modified by LTL)


Throughput for Different ALOHA and CSMA Protocols with =0.01

0 1 2 3 4 5 6 7 8 9

Traffic Load G

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Thr

ough

put S

th

AlohaSlotted Aloha

1-persistent CSMA

0.5-persistent CSMA

0.1-persistent CSMA

0.01-persistent CSMA

Nonpersistent CSMA

= / T (p. 132)

- propagation delayT – packet transmission time

Recall:p-persistent: transmit with probability p if medium idle

Observe:p = 0.01 is best in this Fig. (others suffer more collisions reducing thruput => “don’t hurry too much”)(Modified by

LTL)


6.3.4. CSMA/CD(CSMA with Collision Detection)

In Basic CSMA: If 2 terminals begin sending packets at the same time, each will transmit its complete packet Even if collision is taking place

Problem: Wasting medium for an entire packet duration

Solution principle (main CSMA/CD idea): Backoff immediately after a collision

Modified by LTL


6.3.4. CSMA/CD (CSMA with Collision Detection) – cont.

Solution: CSMA/CD (CD = collision detection)Step 1: If the medium is idle, then transmit Step 2: If the medium is busy, continue to listen until the channel is idle, then transmit Step 3: When a collision is detected during transmission,

cease xmitting immediatelyStep 4: Wait a random amount of time & go to Step 1

Improvement over Basic CSMA:Able to terminate xmission as soon as collision detected Prevents wasting whole time slot (slot = packet

duration)

Modified by LTL


Use of Jamming Signal in CSMA/CD

Collision Mechanisms in CSMA/CD1) Without a jamming signal2) With a jamming signal

Jamming signal facilitates collision detection by other terminals

Note: Different timing assumptions in the textbook (pp. 135-

136)



1) Collision Mechanism in CSMA/CD Without Jamming Signal

A B

τ – propa-gation delaycd – time to detect collis. ≈ 0

T0

A begins transmission at T0 when channel is idle [will reach B at T0+]

A B

B begins transmission just before A’s trans-mission reaches B

Time

T0+ τ -

A B

T0 + τ + τcd

A B

B detects collision cd after A’s transmission reaches B & stops its xmission immediately

(Modified by LTL)

A detects collision cd after B’s xmission reaches A[at (T0+ +cd) ++ ( - ( +cd)) + cd ≈ T0+2 +cd for ≈0

≈ T0+2τ +cd

=> Transmission period: = 2+ cd

gap ~

distance ~

distance ~ cd

length ~ + cd distance ~ cd

distance ~ - ( + cd )


A B

T0

A begins transmission at T0 when channel is idle (will reach B at T0+)

A B

B begins transmission just bef. A’s trans-mission reaches B

Time

T0+ τ -

A B

T0 + τ + τcd

A B

T0+2τ+ 2τcd

B detects collision cd after A’s transmission reaches B & starts sen-ding jamming signal (not shown yet) to warn other terminals about collision

(Modified by LTL)

B finishes sending jam. sig. (red) that lasted crA detects jam. sig. cd after it reaches A [at (T0++cd+cr) + (-cr+cd) = T0+2+2cd]

T0+τ+τcd+τcr

A B

=> Transmission period: = 2+ 2cd

2) Collision Mechanism in CSMA/CD With Jamming Signal

τ – prop. delaycd – time to detect collis. ≈ 0cr – duration of jamming signal – long enough to ensure that all terminals detect collision (p.136/1)

gap ~

distance ~

distance ~ cd

length ~ cr

length ~ + cd

distance ~ - cr + cd

length ~ cd

distance ~ - cr


T – packet xmission time G = gT – normalized offered load - propagation delay= / T - normalized time unit’ = / T, where is propagation path loss slope


TimeMS A’s frame

MSs B & C sense the medium

MS B resenses the medium and transmits its

frame

MS B’s frame

Backoff -delay for B Backoff -

delay for C

MS C resenses the medium but defers to MS B

6.3.5. CSMA/CA(CSMA with Collision Avoidance)

A basic collision avoidance (CA) scheme CSMA/CA rule: Backoff before collision (to avoid collisions)

(Modified by LTL)


When medium idle for a period ≥ DIFS => can xmit immediately DIFS = Distributed InterFrame Space

In 802.11b networks, DIFS = 50 μs


6.3.5. CSMA/CA (CSMA with Collision Avoidance) – cont. 1

DIFS

->

(Modified by LTL)


Types of CSMA/CA

Types of CSMA/CA1) Basic CSMA/CA2) CSMA/CA with ACK3) CSMA/CA with RTS and CTS

... discussed in turn ...



1) Basic CSMA/CA Terminal T senses the medium before xmission1) If medium is busy, T defers access until the end of current xmission - (b1) in

Fig.2) If medium is idle or once it becomes idle:

T defers access for time period DIFS - (a) and (b2) in Fig. T picks a random backoff period - (c) in Fig. As long as medium idle during this backoff period, T keeps counting down Whenever medium busy during this backoff period, T freezes its counter After medium becomes idle again:

T defers access for the DIFS period T resumes countdown

T can start transmission when backoff counter = 0

(Modified by LTL)

≤ DIFS

Medium BusyMedium Busy

DIFS Contention window (cf. next slide)

Deferred access (b)- deferred till idle (b1)

+ deferred for DIFS after

became idle (b2)

Rand. backoff period (c)(after deferments)

Slot (cf. next slide)

Time

Could not start xmission (a)

since medium becamebusy in ≤ DIFS

b1 b2


1) Basic CSMA/CA – cont. 1

All except red text repeated (in larger font) Terminal T senses the medium before xmission1) If medium is busy, T defers access until the end of

current xmission2) If medium is idle or once it becomes idle:

T defers access for time period DIFS T picks a random backoff period

Backoff counter - initialized with the value of backoff period As long as medium idle during this backoff period,

T keeps counting down = decreases its backoff counter

I.e., count down as long as medium idle, stop countdown when medium busy again

Whenever medium busy during this backoff period, T freezes its counter

After medium becomes idle again: T defers access for the DIFS period T resumes countdown

T can start transmission when backoff counter = 0(Modified by LTL)



Fragment of the CSMA/CA protocol: As long as medium idle during this backoff period, T

keeps counting down = decreases its backoff counter I.e., count down as long as medium idle, stop

countdown when medium busy again

(Modified by LTL)

≤ DIFS

Medium BusyMedium Busy

DIFS Contention window

Deferred access (b)- deferred till idle (b1)

+ deferred for DIFS after

became idle (b2)

Rand. backoff period (c)(after deferments)

Slot

Time

Could not start xmission (a)

since medium becamebusy in ≤ DIFS

b1 b2

Idle – starts countdown

Busy – stops countdown

Use of slots: Backoff counter is decreased by 1 each time medium is idle for 1 time slot interval


Contention Window (CW) Recall: Backoff counter is decreased by 1 each time medium is detected to be idle for

an interval of one time slot

=> backoff period = backoff number (bn) = # of idle slots within CW to wait before being allowed to xmit a frame

Random bn value is chosen based on CW size CW_size: 31 - 1023 slots (=> bn can be from 31 to 1032) bn value: uniform distribution over 0 … CW_size

If transmission of a frame unsuccessful & frame to be re-transmitted => CW_size is doubled before retran. (up to 1023)

© 2007 by Leszek T. Lilien© 2006, Michael Hall, Helsinki Univ. of Technology (sl.19) (Modified by LTL)



Selection of Random Backoff Value

Recall: If transmission of a frame unsuccessful & frame allowed to be retransmitted =>=> before each retransm. CW_size is doubled (up to 1023)

=> Pr{ bn value is larger } is higher Since it is unlikely that several stations will choose

the same value of bn, collisions are avoided

© 2007 by Leszek T. Lilien© 2006, Michael Hall, Helsinki Univ. of Technology (sl. 20)


(Modified by LTL)


2) CSMA/CA with ACK Positive Acknowledgement (ACK) frame from receiver

upon reception of each data frame ACK sent after receipt + after time interval SIFS

SIFS = Short Interframe Space Receiver transmits ACK without sensing the medium

(cf. next) If ACK lost, retransmission of ACK

DIFS

Medium Busy

ACK

Data

OtherMS

SourceMS

DestinationMS

DIFS

SIFS

Contention window

Defer access Backoff after defer

Time

SIFS < DIFSE.g., in 802.11b networks:

SIFS = 10 μs / DIFS = 50 μs

(Modified by LTL)


When a frame is received without bit errors,receiving station B sends ACK back to vmitting station A

© 2007 by Leszek T. Lilien© 2006, Michael Hall, Helsinki Univ. of Technology

CSMA/CA with ACK – cont. 1

->

->



Receiver transmits ACK without sensing the medium It works well because medium (radio channel) is

“reserved” for ACK

Medium “reserved” during the transmission sequence:Transmitted frame + SIFS + ACK

Next frame (from any station) can be transmitted at earliest after ACK and after next DIFS period


->

->

(Modified by LTL)


Recal: SIFS < DIFS -- E.g., in 802.11b: SIFS = 10 μs / DIFS = 50 μs

When 2 stations try to access the medium at the same time, the one that has to wait for the shorter SIFS period “wins over” the one that has to wait for the longer DIFS period

“Wins over” = waits for shorter time Just bec. SIFS < DIFS

In other words, ACK frame waiting for SIFS has higher priority over Data frame waiting for DIFS

© 2007 by Leszek T. LilienFigure based on © 2006, Michael Hall, Helsinki Univ. of Technology


Now you see why medium “reserved” in quotes - not real reservation, just given an advantage to guarantee it always wins a race

(Modified by LTL)

C -> D

Data

Data

SIFSDIFS


When Station S wants to send a frame & channel busy=> S must wait a backoff time before it may to xmit frame

Reason? Next two slides…




No backoff => collision is certain Suppose that stations B and C waiting to access channel When channel becomes idle,

B & C start sending their packets at the same time=> collision!




Random backoff => Pr { collision } is reduced Contending stations generate random backoff numbers bn Backoff counters count down, starting from bn

When backoff counter reaches zero,station allowed to send its frame

All other counters stop counting until the channel becomes idle again

Example below (fig): B & C back off after deferring for DIFS

C has small bn – xmits sooner B has large bn – waits longer



(Modified by LTL)


Example for CSMA/CA with ACK

Four stations: A, B, C, D


Observe: C attempts to access medium before B does

Coincidentally, C’s backoff number is smaller

(Modified by LTL)

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 53© 2006, Michael Hall, Helsinki Univ. of Technology (sl.22)

Example for CSMA/CA with ACK – cont. 1

Observe backoff counter for B: Frozen when medium becomes busy

When C starts xmission (when C’s backoff counter becomes 0)

B resumes countdown when medium becomes idle After C completes its xmission

(Modified by LTL)

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 54© 2006, Michael Hall, Helsinki Univ. of Technology (sl. 23)

Example for CSMA/CA with ACK – cont. 2

Observe backoff counter for D: Frozen when medium becomes busy

When B starts xmission (when B’s backoff counter becomes 0)

D resumes countdown when medium becomes idle After B completes its xmission

(Modified by LTL)


3) CSMA/CA with RTS and CTS Transmitter sends an RTS (Request To Send) after

medium has been idle for time interval > DIFS Receiver responds with CTS (Clear To Send) after

medium has been idle for SIFS Data can be xmitted (after SIFS) ACK sent after SIFS

DIFS

Medium Busy

CTS

RTS

Other MS

Source MS

Destination MS

SIFS

Contention window

Defer access Backoff

SIFSData

SIFS

ACK

Time

DIFS

(Modified by LTL)


Ladder Diagram for CSMA/CA with RTS/CTS Compare this diagram to the previous one

RTS

CTS

Data

ACK

Source MS Destination MS

Propagation delay

SIFS

SIFS

Propagation delay

Propagation delay

Propagation delay

SIFS

DIFS

DIFS

Other MS

3) CSMA/CA with RTS and CTS – cont. 1a


3) CSMA/CA with RTS and CTS – cont. 1b

RTS/CTS performs channel reservation for data xmission Collision can occur at worst during sending RTS control message

Bec. SIFS (preceding CTS, Data, ACK) < DIFS (preceding other MS’s RTS)

DIFS

Medium Busy

CTS

RTS

Other MS

Source MS

Destination MS

SIFS

Contention window

Defer access Backoff

SIFSData

SIFS

ACK

Time

DIFS

(Modified by LTL)


3) CSMA/CA with RTS/CTS – cont. 1c

RTS = Request To Send / CTS = Clear To Send

RTS/CTS scheme - used as a countermeasure against the “hidden node” problem

Hidden node problem WS 1 & WS 2 can hear

AP but not each other => If WS 1 sends a packet,

WS 2 does not notice this(and vice versa)=> collision!

© 2006, Michael Hall, Helsinki Univ. of Technology (sl. 25)


RTS/CTS scheme makes reservation of the medium to avoid collisions

RTS reserves medium for time needed for xmission of: CTS + Data + ACK + 3 * SIFS

= SIFS + CTS + SIFS + Data + SIFS + ACK CTS reserves medium for time needed for xmission of:

Data + ACK + 2 * SIFS (original: “3 * SIFS [?]) = SIFS + Data + SIFS + ACK


3) CSMA/CA with RTS/CTS – cont. 1b

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 60© 2006, Michael Hall, Helsinki Univ. of Technology

3) CSMA/CA with RTS/CTS – cont. 2

Danger of collision only during RTS + SIFS (see Fig. below), because: WS 2 does not hear the RTS

frame from WS 1 to AP => WS 2 misses reservation for

CTS + Data + ACK + 3 * SIFS WS 2 can hear the CTS frame

from AP to WS 1 => WS 2 knows about reservation

for Data + ACK + 2 * SIFS


Advantages of RTS & CTS Advantage #1 of RTS/CTS:

Reduces collision probability if the data frame is very long compared to the RTS frame Does not prevent collisions during very short RTS

frames Not a big problem - they are not very likely

Prevents more likely collisions during long data frame



Advantage of RTS & CTS – cont. Advantage #2 of RTS/CTS

Avoids having long intervals with high collision danger (same Fig)

Good! Long interval w/ high collision danger should be avoided

Should be for the following reasons: Larger probability of collision Greater waste of capacity if a collision occurs and the

frame has to be retransmitted Frames shorter than a certain threshold value can be

transmitted without using RTS/CTS Because small waste of medium if collision occurs for short

frames=> low risk

Threshold parameter (dot11RTSThreshold) set in a xmitting station© 2006, Michael Hall, Helsinki Univ. of Technology (sl. 29)


1) (Basic) CSMA1a) Nonpersistent CSMA

– Slotted – Unslotted

1b) Persistent CSMASubcategories # 1 of Persistent CSMA:– Slotted– UnslottedSubcategories # 2 (orthogonal) of Persistent CSMA:

–1-persistent CSMA– p-persistent CSMA

2) CSMA/CD (CSMA with Collision Detection)

3) CSMA/CA (CSMA with Collision Avoidance)3a) Basic CSMA/CA3b) CSMA/CA with ACK3c) CSMA/CA with RTS/CTS

Summary: Full Taxonomy ofCSMA (Carrier Sense Multiple Access)

Protocols



The End of Section 6

1 cs 6910 – pervasive computing spring 2007 section 6 (ch.6): multiple radio access prof. leszek...

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