Wireless & Mobile Networking - Old Dominion Universitynadeem/classes/cs752-S11/s11/material/Lec-03_M… · Wireless & Mobile Networking CS 752/852 - Spring 2011 Tamer Nadeem ... The

Wireless & Mobile Networking

CS 752/852 - Spring 2011

Tamer Nadeem Dept. of Computer Science

Lec #3: Medium Access Control - I

Page 2 Spring 2011 CS 752/852 - Wireless and Mobile Networking

• Main Task of the data link layer:

• Provide error-free transmission over a link

Data Link Layer (DLL)


• Framing

• The DLL translates the physical layer's raw bit stream into discrete units (messages)

called frames. How can the receiver recognize the start and end of a frame?

• Flow Control

• Flow control deals with throttling the speed of the sender to match that of the

receiver. Usually, this is a dynamic process, as the receiving speed depends on such

changing factors as the load, and availability of buffer space.

DLL Services


• Link Management

• Allocating buffer space, control blocks, agreeing on the maximum message size, etc.

Synchronize and initialize send and receive sequence numbers with its peer at the

other end of the communications channel

• Error Control

• Error control is concerned with insuring that all frames are eventually delivered

(possibly in order) to a destination. Three items are required: Acknowledgments,

Timers, Sequence Numbers

• Error Detection and Correction

• line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as

lightning, and the telephone repairman). Error Detecting Codes: Include enough

redundancy bits to detect errors and use ACKs and retransmissions to recover from

the errors. Error Correcting Codes: Include enough redundancy to detect and correct

errors. CRC Checksums

DLL Services


• LANs first began to emerge as potential

business tools in the late 1970s

• IEEE launched Project 802 (1980,

February) to define certain LAN

standards.

• Project 802 defined network standards for

the physical components of a network (the

interface card and the cabling)

• Define the ways NICs access and transfer

data over physical media. These include

connecting, maintaining, and disconnecting

network devices.

• The IEEE 802 standards incorporated the

specifications in the bottom two OSI

layers, the physical layer and the data-link

layer.

DLL = LLC + MAC


• More detail was needed at the data-link layer, the 802 standards

committee divided the data-link layer into two sublayers.

DLL = LLC + MAC

• Logical Link Control (LLC) Sublayer: Manages

data-link communication: establishing and

terminating links, controlling frame traffic,

sequencing frames, and acknowledging frames.

• Media Access Control (MAC) Sublayer:

Communicates directly with the NIC to provide

shared access to the physical layer: Managing

media access, delimiting frames, checking frame

errors, and recognizing frame addresses.


DLL = LLC + MAC


DLL = LLC + MAC

Wireless LAN Standards

(IEEE 802.11)


Medium Access Control (MAC)


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

Shared Channel

Node 4

Node 3

Node 2

Node 1 …

Node N

Page 11 Spring 2011 CS 752/852 - Wireless and Mobile Networking 11

The Channel Access Problem

• Multiple nodes share a channel

• Pairwise communication desired

• Simultaneous communication not possible

• MAC Protocols

• Suggests a scheme to schedule communication

• Maximize number of communications

• Ensure fairness among all transmitters

A C B


The Trivial Solution

• Transmit and pray

• Plenty of collisions --> poor throughput at high load

A C B

collision


Classification of MAC Protocols

Single Channel and Sender Initiated Protocols

• Contention-free MAC

• TDMA, FDMA, CDMA: Divides channel by time, frequency, or code

• More applicable to static networks and/or networks with centralized control

• Contention-based MAC

• Single Channel vs.

Multi-Channels

• Sender Initiated vs.

Receiver Initiated


Carrier Sense Multiple Access

(CSMA)


CSMA

• Listen before you talk

• Carrier sense multiple access (CSMA)

• Defer transmission when signal on channel

• Advantages

• Fairly simple to implement

• Functional scheme that works

• Disadvantages

• Can not recover from a collision

(inefficient waste of medium time)

A C B

Don’t

transmit

Can collisions still occur?


• reduces chance of collisions

• reduces the efficiency

• increases the chance for

collisions

• 1-persistant

• p-persistant

CSMA


CSMA/CD (Collision Detection)

Collisions can still occur: Propagation delay non-zero

between transmitters

When collision: Entire packet transmission

time wasted

spatial layout of nodes

note: Role of distance & propagation delay

in determining collision probability


CSMA/CD (Collision Detection)

• Keep listening to channel

• While transmitting

• If (Transmitted_Signal != Sensed_Signal)

Sender knows it’s a Collision

ABORT


2 Observations on CSMA/CD

• Transmitter can send/listen concurrently

• If (Sensed - Transmitted = null)? Then success

• The signal is identical at Tx and Rx

• Non-dispersive

The TRANSMITTER can detect if and

when collision occurs


Unfortunately …

Both observations do not hold for wireless

Because …


Wireless Medium Access Control

A B

C D

Distance

Signal

power

CS threshold

A cannot send and listen in parallel

Signal not same at different locations


• CMSA/CA

• The CSMA/CA algorithm is based on a basic

time unit called slot σ.

• Slot duration (σ) is equal to maximum

propagation delay.

• Time space is slotted at the boundaries of σ.

• Channel access slotted CSMA can only occur

at the boundary of σ

CSMA with Collision Avoidance (CSMA/CA)

Next Frame

Slot Time

Slotting solved collisions because of propagation delays


But …

Still, More Problems to Solve


Hidden Node Collisions

A B

C D

Distance

Signal

power

CS threshold

Important: D has not heard A, but can interfere at receiver B

D is the hidden node to A


Hidden Node Collisions

A B

C D

Important: D has not heard A, but can interfere at receiver B

D is the hidden node to A

DB

D

transmitD

B

AB

A

A

B

d

PI

d

PSoI

NNoiseIceInterferen

SoIterestSignalOfInSNR

transmit

)()(

)(

DB

D

transmit

AB

A

A

B

d

PN

d

P

SNR

transmit


Exposed Node Collisions

A B

C D

Distance

Signal

power

CS threshold

Important: X has heard A, but should not defer transmission to Y

X

Y

X is the exposed terminal to A


So, how do we cope with

Hidden/Exposed Terminals?


The Emergence of MACA, MACAW, & 802.11

• Wireless MAC proved to be non-trivial

• 1992 - research by Karn (MACA)

• 1994 - research by Bhargavan (MACAW)

• Led to IEEE 802.11 committee

• The standard was ratified in 1999


• Alternative to carrier sensing, i.e. does not use CSMA

CA with Control Handshaking - (MACA)

• Multiple access with collision avoidance (MACA)

uses a three way handshake to avoid hidden

terminal problem (Karn, 90)

• When node A wants to send a packet to node B,

node A first sends a Request-to-Send (RTS) to B

• On receiving RTS, node B responds by sending

Clear-to-Send (CTS)

• All nodes within one hop of node A hear the RTS

and defer their transmissions until

corresponding CTS.

• When a node (such as D) overhears a CTS, it

keeps quiet for the duration of the transfer

• Transfer duration is included in RTS and CTS both


30

• MACA avoids the problem of hidden terminals

• A and C want to

send to B

• A sends RTS first

• C waits after receiving

CTS from B

• MACA avoids the problem of exposed terminals

• B wants to send to A, C

to another terminal

• now C does not have

to wait for it cannot

receive CTS from A

MACA examples

A B C

RTS

CTS CTS

A B C

RTS

CTS

RTS


MACA

• Limitations

• MACA does not provide ACK

• RTS-CTS approach does not always solve the hidden node problem

• Examples


MACAW (MACA for Wireless)

• RTS-CTS-DS-DATA-ACK

• RTS from A to B

• CTS from B to A

• Data Sending (DS) from A to B

• Data from A to B

• ACK from B to A

• Random wait after any successful/unsuccessful transmission

• Significantly higher throughput than MACA

• Does not completely solve hidden & exposed node problems

A B C D

RTS

CTS

Data

Ack

DS


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


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 trasnmitting

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)


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


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


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 to 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


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

• In general τ depends on conditional collision probability 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


Maximum Saturation Throughput

• τ depends on n, W, and m

• Analytical model determines maximum achievable saturation

throughput


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 & Mobile Networking - Old Dominion Universitynadeem/classes/cs752-S11/s11/material/Lec-03_M… · Wireless & Mobile Networking CS 752/852 - Spring 2011 Tamer Nadeem ... The