Wireless Networking & Mobile Computingnadeem/classes/cs752-S12/s12/... · 2012-02-01 · • PLCP header • 8-bit signal or data rate (DR) indicates how fast data will be transmitted

Wireless Networking & Mobile Computing

CS 752/852 - Spring 2012

Tamer Nadeem Dept. of Computer Science

Lec #4: Medium Access Control - II

Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

IEEE 802.11 Standards


IEEE 802.11 MAC

• Very popular wireless MAC protocol

• Two Architectures

IEEE 802.11

Medium Access Control (PCF+DCF)

FHSS DSSS Infrared OFDM

MA

C

PH

Y

SSID

BSSID

Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 4

802.11 PHY Sublayers

• Physical layer convergence protocol (PLCP)

• Provides common interface for MAC

• Offers carrier sense status & CCA (Clear channel assesment)

• Performs channel synchronization / training

• Physical medium dependent sublayer (PMD)

• Functions based on underlying channel quality and characteristics

• E.g., Takes care of the wireless encoding


PLCP

• PLCP has two structures.

• All 802.11b systems have to support Long preamble.

• Short preamble option is provided to improve efficiency when transmitting

voice, VoIP, streaming video.

• PLCP Frame format

• PLCP preamble

• SFD: start frame delimiter

• PLCP header

• 8-bit signal or data rate (DR) indicates how fast data will be transmitted

• 8-bit service field reserved for future

• 16-bit length field indicating the length of the ensuing MAC PDU (MAC sublayer’s

Protocol Data Unit)

• 16-bit Cyclic Redundancy Code


PLCP (802.11b)

long

preamble

192us

short

preamble

96us

(VoIP, video)


• The IEEE 802.11 channelization scheme.

• The 2.4-GHz band is broken down into 11 in USA. However, at most there

is 3 non-overlapping channels.

802.11 Channels


802.11 Channels


IEEE 802.11 MAC

• Two modes:

• DCF (distributed coordination function)

• PCF (point coordination function)

• IEEE 802.11 DCF is based on CSMA/CA

• Physical Carrier Sense

• Explicit ACK from receiver (for unicast transmission)

• RTS/CTS reservation frames (Virtual Carrier Sensing)

• Retry Counters

• Different Timing Intervals for priorities


• RTS-CTS used for frames longer than a Threshold

• RTS-CTS overhead not efficient for short frames

• Some environments may not find RTS-CTS useful, e.g. many

infrastructure networks

• Threshold variable can be tuned

• Virtual carrier sensing

• Duration field in all frames, including RTS and CTS, monitored by

every station

• Duration field used to construct a network access vector (NAV)

• Inhibits transmission, even if no carrier detected

IEEE 802.11 DCF Basics – RTS/CTS & Virtual Carrier Sense


• Counter and timer for each frame

• Short or long retry counter

• Lifetime timer

• Retry counter

• Incremented for each transmission attempt

• Use of short versus long retry counter based on Threshold variable

• Threshold limit

• ShortRetryLimit for short retry counter ‰

• LongRetryLimit for long retry counter ‰

• If threshold exceeded, frame is discarded and upper

layer is notified via MAC interface

IEEE 802.11 DCF Basics – Retry Counters


• Timing intervals are defined that control a station’s access to the

medium

• Slot time (SlotTime)

• Specific value depends on PMD layer

• Derived from propagation delay, transmitter delay, etc. (20micro-sec for

DSSS and 50 for FHSS)

• Basic unit of time for MAC, e.g. for backoff time is a multiple of slot time

• Short Inter-Frame Space (SIFS)

• Shortest interval -- SIF < SlotTime e.g. 10 microsec for FHSS

• Used for highest priority access to the medium, e.g., for ACK and CTS

• Allows Data-ACK and RTS-CST to be atomic transactions

IEEE 802.11 DCF Basics – Timing Intervals


• Priority (or PCF) Inter-Frame Space (PIFS)

• PIFS = SIFS + SlotTime

• Used for Point Coordination Function (PCF) access to the medium

• Allows priority based access to the medium after ACKs but before

contention based access

• Distributed (or DCF) Inter-Frame Space (DIFS)

• DIFS = SIFS + 2×SlotTime

• Used for Distributed Control Function (DCF) access to the medium

• Results in lower priority access than using SIFS or PIFS

• Extended Inter-Frame Space (EIFS)

• EIFS = SIFS + (8×ACK) + PreambleLength + PLCPHeaderLength + DIFS

• Used in the event that the MAC receives a frame with an error

• Provides an opportunity for a fast retransmit of the error frame

• In summary …

• SIFS < SlotTime < PIFS < DIFS << EIFS

IEEE 802.11 DCF Basics – Timing Intervals


• When a sender has a data to transmit, it picks a random wait period.

The wait period is decremented if the channel is idle

• When this period expires, the node tries to acquire the channel by

sending a RTS packet

• The Receiving node (destination) responds with a CTS packet

indicating that its ready to receive the data

• The sender then completes the packet transmission

• If the packet is received without errors, the destination node responds

with an ACK

• If an ACK is not received, the packet is assumed to be lost and the

packet is retransmitted

802.11 DCF Mode Principles


• If RTS fails, the node attempts to resolve the collision by doubling the

wait period. (This is known as binary exponential back-off (BEB)).

• Station trying to send an ACK is given preference over a station that

is acquiring a channel (Different waiting intervals are specified)

• A node needs to sense channel for Distributed Inter- Frame Space

(DIFS) interval before making an RTS attempt and a Short Inter

Frame Space (SIFS) interval before sending an ACK packet

802.11 DCF Mode Principles


DIFS

RTS

CTS

DATA

ACK

NAV (CTS)

NAV (RTS)

SIFS SIFS SIFS

B

A

C

Contention

Window

DIFS

802.11 DCF Mode

RTS

Deferred CW

A B

C

D

D


• Because SIFS is shorter than the DIFS interval, the station sending an

ACK attempts transmission before a station sending a data packet

• In addition to physical channel sensing, virtual carrier sensing is

achieved due to NAV (Network allocation vector) field in the packet

• NAV indicates the duration of current transmission

• Nodes listening to RTS, or CTS messages back off NAV amount of

time before sensing the channel again

• Several papers describe this protocol and even suggest

enhancements.

802.11 DCF Mode Notes


• Control Frame:

• RTS, CTS, ACK

• Data Frame

• Management Frame:

• Beacon

• Probe Req, Probe Resp

• Assoc Req, Assoc Resp

• Reassoc Req, Reassoc Resp

• Disassociation

• Authentication

• Deauthentication

802.11 Frames Type


• Ver - The Protocol Version number is always 0

• Type - indicates whether the frame is a

Management, Control or Data frame.

802.11 Data Frame Format

• Subtype - describe the detail

of the frame type.

• To DS - set if the frame is to

be sent by the AP to the

Distribution System

• From DS - set if the frame

is from the Distribution

System

• More Frag - set if this

frame is a fragment of a

bigger frame and there are

more fragments to follow.

• Retry - set if this frame is a retransmission, maybe through the loss of an ACK



• Power Mgmt - indicates what power mode ('save' or 'active') the station is to be

in once the frame has been sent

• More Data - set by the AP to indicate that more frames are destined to a

particular station that may be in power save mode. These frames will be buffered

at the AP ready for the station should it decide to become 'active'.

• WEP - set if WEP is being used to encrypt the body of the frame

• Duration & ID - In Power save poll messages this is the station ID, whereas in

all other frames this is the duration used when calculating the NAV

• Address 1 - The recipient station address on the BSS. If To DS is set, this is the

AP address; if From DS is set then this is the station address

• Address 2 - The transmitter station address on the BSS. If From DS is set, this

is the AP address; if To DS is set then this is the station address

• Address 3 - If Address 1 contains the destination address then Address 3 will

contain the source address. Similarly, if Address 2 contains the source address

then Address 3 will contain the destination address.



• Address 4 - If a Wireless Distribution System (WDS) is being used (with AP to

AP communication), then Address 1 will contain the receiving AP address;

Address 2 will contain the transmitting AP address; Address 3 will contain the

destination station address and Address 4 the source station address.

• Sequence Control - contains the Fragment Number and Sequence

Number that define the main frame and the number of fragments in the frame

• Frame Body - contains the actual data e.g. IP datagrams and can be up to 2312

octets in size

• CRC - 32-bit Cyclic Redundancy Check on the whole 802.11 frame.


802.11 Control Frame Format


802.11 Contention Window

• Random number selected from [0,cw]

• If transmission was successful, set CW = CWmin

• If transmission fails (i.e., no ACK), CW =

min{2(CW+1)-1, CWmax}

• Small value for cw

• Less wasted idle slots time

• Large number of collisions with multiple senders

(two or more stations reach zero at once)

• Optimal CW for known number of contenders & know packet size

• Computed by minimizing expected time wastage (by both collisions and

empty slots)

• Tricky to implement because number of contenders is difficult to estimate

and can be VERY dynamic

• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios

• Enhance/New adaptive CW scheme


802.11 Fragmentation

t

SIFS

DIFS

data

ACK1

other

stations

receiver

sender frag1

DIFS

contention

RTS

CTS SIFS SIFS

NAV (RTS) NAV (CTS)

NAV (frag1) NAV (ACK1)

SIFS ACK2

frag2

SIFS


Physical Carrier Sense Mechanisms

• Energy detection threshold

• Monitors channel during “idle” times between packets to measure floor noise

• Energy levels above this floor noise by a threshold trigger carrier sense

• DSSS correlation threshold

• Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal

• Triggers carrier sense if the correlation peak is above a threshold

• More sensitive than energy detection (but only works for 802.11 transmissions)

• High BER disrupts transmission but not detection

• Carrier can be sensed at lower levels than

packets can be received

• Results in larger carrier sense range than transmission range

• More than double the range in NS2 802.11 simulations

Receive Range

Carrier Sense Range


• RTS/CTS & Carrier Sense • When RTS/CTS is useful?

• Should Carrier Sensing replace RTS/CTS?

• Interference Range vs. Carrier Sense Range • How effective CSMA carrier sense?

• BER & Date rate and Transmission Range (data rate affect the SNR

threshold and hence the transmission range but not the physical CS)

• Contention Window Size

• Is ACK necessary? • MACA said no ACKs. Let TCP recover from losses

On 802.11 Issues

The search for the best MAC protocol is still on. However, 802.11 is being optimized too.

Thus, wireless MAC research still alive


On RTS/CTS & Carrier Sense

• Does RTS/CTS (Virtual CS) solve hidden terminals ?

• Assuming carrier sensing zone = communication zone

C F

A B

E

D

CTS RTS

E does not receive CTS successfully Can later initiate transmission that

interferes with D

Hidden terminal problem remains



• Hidden Terminal: How about increasing Physical Carrier Sense range ??

• E will defer on sensing carrier no collision !!!

C B D Data

A

E

CTS

RTS F



• Exposed Terminal: B should be able to transmit to A

• Carrier sensing makes the situation worse

C A B

E

D

CTS

RTS



• 802.11 does not solve HT/ET completely

• Only alleviates the problem through RTS/CTS and recommends larger CS zone

• Large CS zone aggravates exposed terminals

• Spatial reuse reduces A tradeoff

• RTS/CTS packets also consume bandwidth

• Moreover, backing off mechanism is also wasteful

• Carrier sense relies on channel measurements at the sender to infer the probability of reception at the receiver!

• Project Idea: • Evaluation of the benefits and drawbacks of carrier sense

• Scheme to intelligently choose a Carrier sensing threshold

• Evaluate tracking correlation between channel conditions at the

sender and at the receiver.


On Contention Window Size

• Optimal CW for known number of

contenders & know packet size

• Computed by minimizing expected time wastage (by

both collisions and empty slots)

• Tricky to implement because number of contenders is

difficult to estimate and can be VERY dynamic

• 802.11 adaptive scheme is unfair • Under contention, unlucky nodes will use larger cw than lucky nodes (due to

straight reset after a success)

• Lucky nodes may be able to transmit several packets while unlucky nodes are

counting down for access

• Fair schemes should use same cw for all contending nodes

• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios

• Enhance/New adaptive CW scheme


• 802.11 physical layer (e.g., Direct Sequence Spread Spectrum (DSSS)

used in 802.11b)

Capture effect: two transmissions received by the same receiver, the

signals of the stronger transmission will capture the receiver radio, and

signals of the weaker transmission will be rejected as noise.

Frame 2

Frame 1

Received Frame

Frame 2 Frame 1

Received Frame

• Simple and widely accepted model:

• Capturing stronger signal ≠ Capturing stronger frame

On Interference Range vs. Carrier Sense Range


Inefficiency:

• Interference Range:

R

I 1 2

I

C

d

• Power path loss model:

• Capture model:

Given:

R=250m, C=550, l =2, α=5




• Project Idea: • How to estimate interference range (distance)

• Propagation Delay?

• Interference Aware MAC Scheme


On Transmission Date rate

Floor Noise Data Rate

Received Power

Channel Bandwidth

• Bit error (p) for BPSK and

QPSK :

SNR


On Transmission Date rate


On ACKnowledgment

• APs typically backlogged with traffic

• Persistent traffic possibility of optimization

• Use implicit ACK optimization

• Piggyback the CTS with ACK for previous dialog

802.11

Implicit

ACK

Gain


On ACKnowledgment

• The optimization timeline

T R

RTS

CTS

Data

ACK

RTS

CTS

Data

ACK

T R

RTS

CTS

Data

RTS

CTS +ACK

Data

T R

RTS

CTS

Data

Poll +ACK

Data

RTS

CTS +ACK

Ba

cko

ff

Ba

cko

ff

Ba

cko

ff

Ba

cko

ff

Poll +ACK

Data

Ba

cko

ff

Ba

cko

ff

802.11 Implicit ACK Hybrid Channel Access


Performance Analysis of the

IEEE 802.11 Distributed

Coordination Function (Giuseppe Bianchi)


802.11 DCF Throughput Analysis (Bianchi)

• Objective:

• Analytical Evaluation of Saturation Throughput

• Assumptions:

• Fixed number of stations having packet for transmission

• Each packet collide with constant and independent probability

• Model bi-dimensional process {s(t) , b(t)} with discrete-

time Markov chain

• Analysis divided into two parts:

• Study the behavior of single station with a Markov model

• Study the events that occur within a generic slot time & expressed

throughput for both Basic & RTS/CTS access method


Markov Chain Model


• Closed form solution for Markov chain

Markov Chain Model

• Stationary Probability


• In general τ depends on conditional collision

probability p

Markov Chain Model

• Probability τ that a station transmits in randomly

chosen slot time

• When m =0 no exponential backoff is considered

probability τ results independent of p


Throughput Analysis

• Normalized system throughput S

• Probability of transmission Ptr

• Probability of successful transmission Ps


Normalized system throughput

Throughput Analysis

Specify Ts and Tc to compute throughput for DCF access mechanism


• Considering System via Basic Access mechanism

• Packet header H = PHYhrd +MAChrd

• Propagation delay δ

Throughput Analysis


• Packet transmission via RTS/CTS Access

mechanism

Throughput Analysis


Model Validation

• Compared analytical results with that obtained by

means of simulation

• Analytical model extremely accurate

• Analytical results (lines) coincide with simulation results

(symbols) in both Basic Access & RTS/CTS cases

Saturation throughput

analysis vs. simulation


Performance Evaluation

Saturation throughput vs. initial window

size for Basic Access mechanism

• Greater the network size lower is the throughput for basic access


• Throughput of Basic Access mechanism depends

on W

• W depends on number of terminals

• High value of W gives

excellent throughput

performance



size for Basic Access mechanism


• Throughput obtained with RTS/CTS mechanism

• Independent of value of W



size for RTS/CTS mechanism


• Number of transmissions per packet increases as W

reduces & network size n increases.


Average number of transmissions

per packet


Questions

Documents

Wireless Networking & Mobile Computingnadeem/classes/cs752-S12/s12/... · 2012-02-01 · • PLCP header • 8-bit signal or data rate (DR) indicates how fast data will be transmitted