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Special Topics on Wireless Ad-hoc Networks. Lecture 12: Wireless 802.11. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Covered topic. How wireless LAN, 802.11 works References Chapter 3 of the book “Wireless Medium Access control protocols” a survey

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Special topics on wireless ad hoc networks

Special Topicson Wireless Ad-hoc Networks

Lecture 12: Wireless 802.11

University of Tehran

Dept. of EE and Computer Engineering

By:

Dr. Nasser Yazdani

Computer Network


Covered topic

Covered topic

  • How wireless LAN, 802.11 works

  • References

    • Chapter 3 of the book

    • “Wireless Medium Access control protocols” a survey

    • “MACAW: A Media Access Protocol for Wireless LAN’s”

    • SSCH: Slotted Seeded Channel Hopping for Capacity …

    • ECHOS: Enhanced Capacity 802.11 Hotspots

    • Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANS

    • A wireless MAC protocol Using Implicit Pipelining

Computer Network


Outlines

Outlines

  • Why wireless LAN

  • 802.11

  • 802.11 MAC

  • Some improvement

  • Performance Analysis.

Computer Network


Why wireless networks

Why wireless networks?

  • Mobility: to support mobile applications

  • Costs: reductions in infrastructure and operating costs: no cabling or cable replacement

  • Special situations: No cabling is possible or it is very expensive.

  • Reduce downtime: Moisture or hazards may cut connections.

Wireless Ad hoc/Sensor Networks


Why wireless networks cont

Why wireless networks? (cont)

  • Rapidly growing market attests to public need for mobility and uninterrupted access

  • Consumers are used to the flexibility and will demand instantaneous, uninterrupted, fast access regardless of the application.

  • Consumers and businesses are willing to pay for it

Wireless Ad hoc/Sensor Networks


The two hottest trends in telecommunications networks

The Two Hottest Trends inTelecommunications Networks

Millions

Mobile Telephone

Users

Internet Users

Year

Wireless Ad hoc/Sensor Networks

Source: Ericsson Radio Systems, Inc.


Growth of home wireless

Growth of Home wireless

Wireless Ad hoc/Sensor Networks


Why is it so popular

Why is it so popular?

  • Flexible

  • Low cost

  • Easy to deploy

  • Support mobility

Wireless Ad hoc/Sensor Networks


Applications

Applications ?

  • Ubiquitous, Pervasive computing or nomadic access.

  • Ad hoc networking: Where it is difficult or impossible to set infrastructure.

  • LAN extensions: Robots or industrial equipment communicate each others. Sensor network where elements are two many and they can not be wired!.

  • Sensor Networks: for monitoring, controlling, e

Wireless Ad hoc/Sensor Networks


What is special on wireless

What is special on wireless?

  • Channel characteristics

    • Half-Duplex

    • Location dependency

    • Very noisy channel, fading effects, etc.,

  • Resource limitation

    • Bandwidth

    • Frequency

    • Battery, power.

  • Wireless problems are usually optimization problems.

Wireless Ad hoc/Sensor Networks


What is special on wireless1

What is special on wireless?

  • Mobility in the network elements

  • Very diverse applications/devices.

  • Connectivity and coverage (internetworking) is a problem.

  • Maintaining quality of service over very unreliable links

  • Security (privacy, authentication,...) is very serious here. Broadcast media.

  • Cost efficiency

Wireless Ad hoc/Sensor Networks


Big issues

Big issues!

  • Integration with existing data networks sounds very difficult.

  • It is not always possible to apply wired networks design methods/principles here.

Wireless Ad hoc/Sensor Networks


Problems

Problems

  • Host mobility is not considered in the initial Internet design.

  • There is a hierarchal design in Internet. How Ad hoc wireless networks can be handled

  • A layered design. Layer should be independent of each other. It is not work at all in wireless

    • TCP

    • Battery shortages;

    • Etc,.

Wireless Ad hoc/Sensor Networks


High availbility requirements

High availbility requirements

  • No QoS assumed from below

  • Reasonable but non-zero loss rates

    • What’s minimum recovery time?

      • 1 RTT

    • But conservative assumptions end-to-end

      • TCP RTO - min(1s)!

  • Interconnect independent networks

    • Federation makes things hard:

      • My network is good. Is yours? Is the one in the middle?

    • Scale

      • Routing convergence times, etc.

Wireless Ad hoc/Sensor Networks


Growing application diversity

Growing Application Diversity

Collision Avoidance:Car Networks

Mesh Networks

Wired Internet

Access

Point

Sensor

Relay Node

Ad-Hoc/Sensor Networks

Wireless Home Multimedia

Wireless Ad hoc/Sensor Networks


Challenge diversity

Challenge: Diversity

  • New architectures must accommodate rapidly evolving technology

  • Must accommodate different optimization goals

    • Power, coverage, capacity, price

Wireless

Edge Network

INTERNET

INTERNET

Wireless

Edge Network

2005

2010

Wireless Ad hoc/Sensor Networks


Spectrum scarcity

Spectrum Scarcity

  • Interference and unpredictable behavior

    • Need better management/diagnosis tools

  • Lack of isolation between deployments

    • Cross-domain and cross-technology

Why is my 802.11 not working?

Wireless Ad hoc/Sensor Networks


Other challenges

Other Challenges

  • Performance: Nothing is really work well

  • Security: It is a broadcast media

  • Cross layer interception

    • TCP performance

Wireless Ad hoc/Sensor Networks


Ideal wireless area network

Ideal Wireless Area network?

  • Wish List

    • High speed (Efficiency)

    • Low cost

    • No use/minimal use of the mobile equipment battery

    • Can work in the presence of other WLANs (Heterogeneity)

    • Easy to install and use

    • Etc

Computer Network


Wireless lan design goals

Wireless LAN Design Goals

  • Wireless LAN Design Goals

    • Portable product: Different countries have different regulations concerning RF band usage.

    • Low power consumption

    • License free operation

    • Multiple networks should co-exist

Computer Network


Wireless lan design alternatives

Wireless LAN Design Alternatives

Design Choices

Physical Layer: diffused Infrared (IR) or Radio Frequency (RF)?

Radio Technology: Direct-Sequence or Frequency-Hopping?

Which frequency range to use?

Which MAC protocol to use.

Peer-Peer architecture or Base-Station approach?

Univ. of Tehran

Computer Network

21


Wireless standards

Wireless Standards

Computer Network


Distance vs data rate

Distance vs. Data Rate

Computer Network


Special topics on wireless ad hoc networks

WiFi

  • Almost all wireless LANs now are IEEE 802.11 based

  • Competing technologies, e.g., HiperLAN can’t compete on volume and cost

  • 802.11 is also known as WiFi = “Wireless Fidelity”

  • Fidelity = Compatibility between wireless equipment from different manufacturers

  • WiFi Alliance is a non-profit organization that does

  • the compatibility testing (WiFi.org)

Computer Network


Architectures

Architectures

  • Distributed wireless Networks: also called Ad-hoc networks

  • Centralized wireless Networks: also called last hop networks. They are extension to wired networks.

Computer Network


Centralized wlan

Access Point

Access Point

Centralized Wlan

Ad Hoc

Laptop

Laptop

Server

DS

Pager

Laptop

PDA

Laptop

Computer Network


Ieee 802 11 topology

IEEE 802.11 Topology

  • Independent basic service set (IBSS) networks (Ad-hoc)

  • Basic service set (BSS), associated node with an AP

  • Extended service set (ESS) BSS networks

  • Distribution system (DS) as an element that interconnects BSSs within the ESS via APs.

Computer Network


Starting an ibss

Starting an IBSS

  • One station is configured to be “initiating station,” and is given a service set ID (SSID);

  • Starter sends beacons;

  • Other stations in the IBSS will search the medium for a service set with SSID that matches their desired SSID and act on the beacons and obtain the information needed to communicate;

  • There can be more stations configured as “starter.”

Computer Network


Ess topology

ESS topology

  • connectivity between multiple BSSs, They use a common DS

Computer Network


Base station approach advantages over peer peer

Base-Station Approach Advantages over Peer-Peer

  • No hidden terminal: base station hears all mobile terminals, are relays their information to ever mobile terminal in cell.

  • Higher transmission range

  • Easy expansion

  • Better approach to security

  • Problem?

    • Point of failure,

    • Feasibility?

Computer Network


802 11 logical architecture

802.11 Logical Architecture

  • PLCP: Physical Layer Convergence Procedure

  • PMD: Physical Medium Dependent

  • MAC provides asynchronous, connectionless service

  • Single MAC and one of multiple PHYs like DSSS, OFDM, IR

  • and FHSS.

Computer Network


802 11 mac frame format

MAC Header

Frame

Control

Duration

Addr 1

Addr 3

Sequence

Control

Address 4

User

Data

CRC

Addr 2

Protocol

Version

Type

Sub type

To

DS

From

DS

Last

Fragment

Retry

Power

Mgt

EP

RSVD

802.11 MAC Frame Format

Bytes

34~2346

32

6

6

2

6

4

6

2

2

6

Bytes

Encrypted to WEP

Bits

2

1

4

2

1

1

Computer Network


802 11 mac frame format1

802.11 MAC Frame Format

  • Address Fields contains

    • Source address

    • Destination address

    • AP address

    • Transmitting station address

  • DS = Distribution System

  • User Data, up to 2304 bytes long

Computer Network


Special frames ack rts cts

Special Frames: ACK, RTS, CTS

bytes

2

2

6

4

  • Acknowledgement

  • Request To Send

  • Clear To Send

Frame

Control

Duration

Receiver

Address

CRC

ACK

bytes

2

2

6

6

4

Frame

Control

Duration

Receiver

Address

Transmitter

Address

CRC

RTS

bytes

2

2

6

4

Frame

Control

Duration

Receiver

Address

CRC

CTS


802 11 features

802.11 Features

  • Power management: NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. Put to sleep, etc.

  • Bandwidth: To compress data

  • Security:

  • Addressing: destination address does not always correspond to location.

Computer Network


Power management

Power Management

  • Battery life of mobile computers/PDAs are very short. Need to save

  • The additional usage for wireless should be minimal

  • Wireless stations have three states

    • Sleep

    • Awake

    • Transmit

Computer Network


Power management cont

Power Management, Cont…

  • AP knows the power management of each node

  • AP buffers packets to the sleeping nodes

  • AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when?

  • The node is awake only when it is sending data, receiving data or listening to TDIM.

Computer Network


Ieee 802 11 llc layer

IEEE 802.11 LLC Layer

  • Provides three type of service for exchanging data between (mobile) devices connected to the same LAN

    • Acknowledged connectionless

    • Un-acknowledged connectionless, useful for broadcasting or multicasting.

    • Connection oriented

  • Higher layers expect error free transmission

Computer Network


Frame type and subtypes

Frame type and subtypes

  • Three type of frames

    • Management

    • Control

    • Asynchronous data

  • Each type has subtypes

  • Control

    • RTS

    • CTS

    • ACK

Computer Network


Frame type and subtypes cont

Frame type and subtypes, Cont..

  • Management

    • Association request/ response

    • Re-association request/ response: transfer from AP to another.

    • Probe request/ response

    • privacy request/ response: encrypting content

    • Authentication: to establish identity

    • Beacon (Time stamp, beacon interval, channels sync info, etc.)

Computer Network


Frame type and subtypes cont1

Frame type and subtypes, Cont..

  • Management…

    • TIM (Traffic Indication Map) indicates traffic to a dozing node

    • dissociation

Computer Network


802 11 management operations

802.11 Management Operations

  • Scanning

  • Association/Reassociation

  • Time synchronization

  • Power management

Computer Network


Scanning in 802 11

Scanning in 802.11

  • Goal: find networks in the area

  • Passive scanning

    • Not require transmission

    • Move to each channel, and listen for Beacon frames

  • Active scanning

    • Require transmission

    • Move to each channel, and send Probe Request frames to solicit Probe Responses from a network

Computer Network


Time synchronization in 802 11

Time Synchronization in 802.11

  • Timing synchronization function (TSF)

    • AP controls timing in infrastructure networks

    • All stations maintain a local timer

    • TSF keeps timer from all stations in sync

  • Periodic Beacons convey timing

    • Beacons are sent at well known intervals

    • Timestamp from Beacons used to calibrate local clocks

    • Local TSF timer mitigates loss of Beacons

Computer Network


Authentication

Authentication

  • Three levels of authentication

    • Open: AP does not challenge the identity of the node.

    • Password: upon association, the AP demands a password from the node.

    • Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key.

Computer Network


Inter frame spacing

Inter Frame Spacing

  • SIFS = Short inter frame space = dependent on PHY

  • PIFS = point coordination function (PCF) inter frame space = SIFS + slot time

  • DIFS = distributed coordination function (DCF) inter frame space = PIFS + slot time

  • The back-off timer is expressed in terms of number of time slots.

Computer Network


802 11 frame priorities

802.11 Frame Priorities

  • Short interframe space (SIFS)

    • For highest priority frames (e.g., RTS/CTS, ACK)

  • PCF interframe space (PIFS)

    • Used by PCF during contention free operation

  • DCF interframe space (DIFS)

    • Minimum medium idle time for contention-based services

DIFS

PIFS

contentwindow

Frame transmission

Busy

SIFS

Time

Computer Network


Sifs difs

SIFS/DIFS

SIFS makes RTS/CTS/Data/ACK atomic

Example:Slot Time = 1, CW = 5, DIFS=3, PIFS=2, SIFS=1,

Computer Network


Priorities in 802 11

Priorities in 802.11

  • CTS and ACK have priority over RTS

    After channel becomes idle

  • If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle

  • If a node wants to send RTS, it waits for DIFS > SIFS

Computer Network


Sifs and difs

SIFS and DIFS

DATA1

ACK1

backoff

RTS

DIFS

SIFS

SIFS

Computer Network


Energy conservation

Energy Conservation

  • Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest

  • Two approaches to reduce energy consumption

    • Power save: Turn off wireless interface when desirable

    • Power control: Reduce transmit power

Computer Network


Power control with 802 11

Power Control with 802.11

  • Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver

  • A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission

  • B’s transmission to A at high power interferes with reception of ACK at C

A

B

C

D

Computer Network


A plausible solution

A Plausible Solution

  • RTS/CTS at highest power, and DATA/ACK at smallest necessary power level

  • A cannot sense C’s data transmission, and may transmit DATA to some other host

  • This DATA will interfere at C

  • This situation unlikely if DATA transmitted at highest power level

    • Interference range ~ sensing range

Data sensed

A

B

C

D

Data

RTS

Ack

Interference range

Computer Network


02 11 activities ieee

02.11 Activities IEEE

  • 802.11c: Bridge Operation (Completed. Added to IEEE 802.1D)

  • 802.11d: Global Harmonization (PHYs for other countries. Published as IEEE Std 802.11d-2001)

  • 802.11e: Quality of Service. IEEE Std 802.11e-2005

  • 802.11f: Inter-Access Point Protocol (Published as IEEE Std Std 802.11F-2003)

  • 802.11h: Dynamic Frequency Selection and transmit power control to satisfy 5GHz band operation in Europe. Published as IEEE Std 802.11h-2003

  • 802.11i: MAC Enhancements for Enhanced Security. Published as IEEE Std 802.11i-2004

  • 802.11j: 4.9-5 GHz operation in Japan. IEEE Std 802.11j-2004

  • 802.11k: Radio Resource Measurement interface to higher layers. Active.

Computer Network


02 11 activities ieee1

02.11 Activities IEEE

  • 802.11m: Maintenance. Correct editorial and technical issues in 802.11a/b/d/g/h. Active.

  • 802.11n: Enhancements for higher throughput (100+ Mbps). Active.

  • 802.11p: Inter-vehicle and vehicle-road side communication at 5.8GHz. Active.

  • 802.11r: Fast Roaming. Started July 2003. Active.

  • 802.11s: ESS Mesh Networks. Active.

  • 802.11T: Wireless Performance Metrics. Active.

  • 802.11u: Inter-working with External Networks. Active.

  • 802.11v: Wireless Network Management enhancements for interface to upper layers. Extension to 80211.k. Active.

  • Study Group ADS: Management frame security. Active

  • Standing Committee Wireless Next Generation WNG: Globalization jointly w ETSI-BRAN and MMAC. Active.

Computer Network


802 11n

802.11n

  • Trend: HDTV and flat screens are taking off Media Center Extenders from Linksys and other vendors

  • Application: HDTV and streaming video (over longer distances than permitted by 802.15.3 WPANs)

  • 11n = Next Generation of 802.11

  • At least 100 Mbps at MAC user layer ⇒ 200+ Mbps at PHY ⇒ 4x to 5x faster than 11a/g

  • (802.11a/g have 54 Mbps over the air and 25 Mbps to user)

  • Pre-11n products already available

  • Task Group n (TGn) setup: Sept 2003

  • Expected Completion: March 2007

Computer Network


802 11n1

802.11n

  • Uses multiple input multiple output antenna (MIMO)

  • Data rate and range are enhanced by using spatial multiplexing (N antenna pairs) plus antenna diversity occupies one WLAN channel, and in compliance with 802.11

  • Backwards compatible with 802.11 a,b,g

  • One access point supports both standard WLAN and MIMO devices

Computer Network


Mac a simple classification

MAC: A Simple Classification

Wireless

MAC

Centralized

Distributed

On Demand MACs, Polling

Guaranteed

or

controlled

access

Random

access

Our focus

SDMA, FDMA, TDMA, Polling

Computer Network


Reservation polling mac protocol

Reservation/Polling MAC Protocol

  • Works only with AP

  • Fair and slow. First-in-First-Out

  • Wireless station send a request.

  • All requests are queued.

  • Wireless stations are polled in the same order that the requests have arrive.

  • All data reception is acknowledged.

Computer Network


Ieee 802 11 wireless mac

IEEE 802.11 Wireless MAC

  • Distributed and centralized MAC components

    • Distributed Coordination Function (DCF)

    • Point Coordination Function (PCF)

  • DCF suitable for multi-hop and ad hoc networking

  • DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol

Computer Network


Ieee 802 11 dcf

A

B

C

IEEE 802.11 DCF

  • Uses RTS-CTS exchange to avoid hidden terminal problem

    • Any node overhearing a CTS cannot transmit for the duration of the transfer

  • Uses ACK to achieve reliability

  • Any node receiving the RTS cannot transmit for the duration of the transfer

    • To prevent collision with ACK when it arrives at the sender

    • When B is sending data to C, node A will keep quite

Computer Network


Hidden terminal problem

A

B

C

Hidden Terminal Problem

  • Node B can communicate with A and C both

  • A and C cannot hear each other

  • When A transmits to B, C cannot detect the transmission using the carrier sense mechanism

  • If C transmits, collision will occur at node B

Computer Network


Maca solution for hidden terminal problem

MACA Solution for Hidden Terminal Problem

  • In order everybody to avoid send we need to reserved the media.

  • Reservation can be done by handshaking first, sending data and finally acknowledgement.

  • To be fair to others, reservation is done for one packet delivery.

  • During reservation other nodes stay silent

    • To do this, sender includes during in handshaking and others record it in their Network Allocation Vector (NAV)

  • Upon ending transmission, everybody can contend to the media to send.

Computer Network


Maca solution for hidden terminal problem karn90

A

B

C

MACA Solution for Hidden Terminal Problem [Karn90]

  • When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A

  • On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet

  • When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer

    • Transfer duration is included in RTS and CTS both

Computer Network


Ieee 802 11

IEEE 802.11

RTS = Request-to-Send

RTS

A

B

C

D

E

F

Computer Network


Ieee 802 111

IEEE 802.11

RTS = Request-to-Send

RTS

A

B

C

D

E

F

NAV = 10

NAV = remaining duration to keep quiet

Computer Network


Ieee 802 112

IEEE 802.11

CTS = Clear-to-Send

CTS

A

B

C

D

E

F

Computer Network


Ieee 802 113

IEEE 802.11

  • DATA packet follows CTS. Successful data reception acknowledged using ACK.

CTS = Clear-to-Send

CTS

A

B

C

D

E

F

NAV = 8

Computer Network


Ieee 802 114

IEEE 802.11

DATA

A

B

C

D

E

F

Computer Network


Ieee 802 115

IEEE 802.11

Reserved area

ACK

A

B

C

D

E

F

Computer Network


Ieee 802 116

Interference

range

Carrier sense

range

A

F

Transmit range

IEEE 802.11

DATA

A

B

C

D

E

F

Computer Network


Backoff interval

Backoff Interval

  • To give everybody a chance, each node for transmitting a packet, choose a backoff interval in the range [0,cw]

    • cw is contention window

  • Count down the backoff interval when medium is idle

    • Count-down is suspended if medium becomes busy

  • When backoff interval reaches 0, transmit RTS

Computer Network


Dcf example

B1 = 25

B1 = 5

wait

data

data

wait

B2 = 10

B2 = 20

B2 = 15

DCF Example

B1 and B2 are backoff intervals

at nodes 1 and 2

cw = 31

Computer Network


Backoff interval1

Backoff Interval

  • backoff intervals is a part of MAC overhead

  • large cwleads to large backoff and larger overhead

  • small cw leads to a larger number of collisions

  • A lot of work has been to reduce this overhead but still no a solid sloution.

  • IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence

Computer Network


Binary exponential backoff in dcf

Binary Exponential Backoff in DCF

  • When a node fails to receive CTS in response to its RTS, it increases the contention window

    • cw is doubled (up to an upper bound)

  • When a node successfully completes a data transfer, it restores cw to Cwmin

    • cw follows a sawtooth curve

  • 802.11 has large room for improvement

Random

backoff

RTS/CTS

Data Transmission/ACK

Computer Network


Inter frame spacing1

Inter Frame Spacing

  • SIFS = Short inter frame space = dependent on PHY

  • PIFS = point coordination function (PCF) inter frame space = SIFS + slot time

  • DIFS = distributed coordination function (DCF) inter frame space = PIFS + slot time

  • The back-off timer is expressed in terms of number of time slots.

Computer Network


Receive initiated mechanism

Receive-Initiated Mechanism

  • In most protocols, sender initiates a transfer

  • Alternatively, a receiver may send a

    Ready-To-Receive (RTR) message to a sender requesting it to being a packet transfer

  • Sender node on receiving the RTR transmits data

  • How does a receiver determine when to poll a sender with RTR?

    • Based on history, and prediction of traffic from the sender

Computer Network


Reliability

A

B

C

Reliability

  • Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance.

  • Mechanisms needed to reduce packet loss rate experienced by upper layers

  • When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). This approach adopted in many protocols

  • If node A fails to receive an Ack, it will retransmit the packet

Computer Network


Fairness issue

Fairness Issue

  • Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide

  • Nodes A and B then choose from range [0,63]

    • Node A chooses 4 slots and B choose 60 slots

    • After A transmits a packet, it next chooses from range [0,31]

    • It is possible that A may transmit several packets before B transmits its first packet

A

B

Two flows

C

D

Computer Network


Macaw solution for fairness

MACAW Solution for Fairness

  • When a node transmits a packet, it appends the cw value to the packet, all nodes hearing that cw value use it for their future transmission attempts

  • Since cw is an indication of the level of congestion in the vicinity of a specific receiver node, MACAW proposes maintaining cw independently for each receiver

  • Using per-receiver cw is particularly useful in multi-hop environments, since congestion level at different receivers can be very different

Computer Network


Wireless capacity

Wireless Capacity

  • Wireless channel is inefficient due to

    • MAC backoff procedure

    • RTS/CTS mechanism

    • Frequency interference.

  • Possible solutions:

    • Use better backoff mechanisms.

    • Exploit more physical resources: more spectrum Cell mechanism

    • Exploit diversity, use different frequencies.

    • Parallel control with data

Computer Network


Improve spatial reuse power rate control

A

B

C

D

Improve Spatial ReusePower/Rate Control

TransmitSpatial

PowerRatereuse

High High Low

Low Low High

A

B

C

D

Computer Network


Exploit infrastructure

Exploit Infrastructure

  • Infrastructure provides a tunnel to forward packets

infrastructure

BS1

BS2

B

C

D

E

A

Z

Ad hoc connectivity

X

Computer Network


Exploit antennas

B

C

A

D

Exploit Antennas

  • Diversity antenna

  • Steered beam directional antenna

B

C

A

D

Computer Network


Directional antennas

Directional Antennas

Not possible using Omni

B

D

S

C

A


Special topics on wireless ad hoc networks

Random

backoff

RTS/CTS

Data Transmission/ACK

Stage1

Stage2

Pipelining two stages

  • Two stage pipeline:

    • Random backoff and RTS/CTS handshake

    • Data transmission and ACK

  • “Total” pipelining: Resolve contention completely in stage 1

Computer Network


Next solution

Next solution

  • Partitioning channel dynamically in order to better utilize it.

  • Different from direction antenna, it is done on the link layer and dynamically.

Computer Network


Ssch slotted seeded channel hopping overview

SSCH: Slotted Seeded Channel Hopping – Overview

  • A dynamic assignment algorithm

    • divides the time into equal sized slots (e.g. 10 ms) and switches each radio across multiple orthogonal channels on the boundary of slots in a distributed manner

  • Main aspect of SSCH

    • channel scheduling

      • self-computation of tentative schedule

      • communication of schedules

      • synchronization with other nodes

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Channel scheduling self computation

Channel Scheduling -Self-Computation

  • Each node use (channel, seed) pairs to represent its tentative schedule for the next slot

  • Seed: [1 , number of channels -1] Initialized randomly

  • Focus on the simple case of using one pair

  • Update rule:

    new channel = (old channel + seed) mod (number of channels)

A: Seed = 2

1

0

2

1

0

2

1

0

B: Seed = 1

0

1

2

0

1

2

0

1

Example: 3 channels, 2 seeds

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The echos solution

The ECHOS Solution 

  • AP – CST algorithm (CST- Carrier Sense Thersh.)

    • Dynamically adjusts the CST in order to allow more flows to co-exist in the same channel in current 802.11 architectures.

  • RNC – SC algorithm

    • Allows each cell or AP access to all available channels.

    • RNC algorithm executes in a centralized radio network controller

    • Uses one channel as primary & the other two as secondary channels

    • Allows to improve Hotspot performance beyond AP-CST.

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Abilities of the algorithms

Abilities of the Algorithms

  • Dynamically allocate channels to stations

  • Flexibly adopts parameters such as CST and/or transmit power

    THE CLAIM !

  • Performance of 802.11-based hotspots can be improved by both these algorithms by up to 195% per-cell and 70% overall.

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Observations on carrier sensing in 802 11

Observations on Carrier Sensing in 802.11

  • Qualnet simulator

    transmission at 2Mbps

    with a CST of -93dBm

    & transmit power of 15dBm

  • How to calculate the ranges?

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Range calculation

Range Calculation

  • Suppose T & T’ are two transmitters at distance dt & di from the receiver.

  • T’ is the interferer to the transmission from T.

  • Then,

    • SNR at the receiver is assuming that

      both the transmitters transmit with the same power

      Strength of the received signal falls off as

      Where,

      K is a suitable constant

      is the transmission power

      d is the distance from the signal source

      • For successful reception, the requirement is that the SNR be above a threshold

      • This yields the requirement:

Range

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Observation 1

Observation 1

How to chose the optimum value of CST ?

- Dynamic

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Idle sense access method

Idle Sense Access Method

GOALS

  • Optimize Throughput

  • Dynamically Adapt to Physical Channel Conditions

  • Equal Time Shares for hosts with different bit rate

  • Short-term fairness and Minimize Delay


Channel contention

Channel Contention

--Idle Slot (No carrier)

--Idle slots are shorter

Two-host contention modeled as a stochastic process with three states


Channel contention1

Channel Contention

  • Host have always packets to send

  • Host can hear each others

  • Pe : Attempt p. for a slot per node

  • Pt : Successful Tx p. for a given slot

  • Pc : Collision p. for a given slot

  • Pi : Slot Idle p.

    __

  • ni : No. of consecutive idle slots

    between two trans/colission


Channel contention2

Channel Contention

All host have the same CW and trans/col are like the wait

interval

Approximate Pe[5]

Throughput Function

Cost Function

Sd= ave. frame size,

[5]Bianchi,Fratta & Oliveri, ”Performance Analysis of 802.11 CSMA/CA Medium Access Control Protocol”, Proc of PIMRC1996


Channel contention3

Channel Contention

# of stations

Cost function w.r.t Contention window

(for different numbers of hosts)


Channel contention4

Channel Contention

Replace values in cost function and put first derivative

to zero gives:

  • Optimal value of CW increases with N

  • Cost function less sensitive to variations in CW

  • Optimal values obtained by limiting N to ∞


Channel contention5

Channel Contention

  • Idle Sense:

    If mean (ni) exceeds this optimal value:

    -> too much time spent waiting in idle slots

    If mean (ni) less than the optimal value:

    ->excessive collisions

  • N<∞: specific root of cost derivative


Principles of idle sense

Principles of Idle Sense

  • Each host estimates ni and uses it to compute its CW

  • If N is known, we can determine optimal ni from predetermined optimal values

  • If N is not known, a best estimate is used for nitarget


Principles of idle sense1

Principles of Idle Sense

Control Algorithm

  • AIMD : Additive Increase, Multiplicative Decrease

  • Using Pe=2/CW


Performance analysis

Performance Analysis

DIFS: distributed interframe space ( decide if it is idle)

SIFS: short interframe space ( shorter than DIFS)

NAV: Network Allocation Vector ( contains info about packet length being Tx)

SIFS

BO = 3

BO = 5

BO = 7

A

DIFS

RTS

DATA

DIFS

RTS

DIFS

collision

CTS

ACK

B

BO = 4

BO = 8

BO = 5

BUSY

DIFS

NAV (RTS)

DIFS

RTS

DIFS

C

NAV(CTS)

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Critical assumptions

CRITICAL ASSUMPTIONS

  • Ideal Channel conditions and finite number of terminals

  • Ideal Channel conditions include (No Hidden Terminals, No Channel Capture)

  • Constant & independent collision probability P for each transmitted packet

  • System is in Overload Condition (Every station is always ready to Transmit a Packet)

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Mathematical model for dynamics of dcf

Mathematical Model for Dynamics of DCF

  • s(t) – stochastic process representing back off stage (0, …. , m) of a given station

  • b(t) – stochastic process representing back off time counter (k, k-1,……,1,0) of a station

  • Bi-dimensional Process {s(t), b(t)} with state space (i, k)

  • and W = CWmin (minimum contention window length)

  • p – Conditional Collision Probability seen by a packet being transmitted (const and indep)

  • -1

  • 2

  • 3

  • 4

Transition Probabilites P for process {s(t),b(t)}

  • Eq 1 – Once Back off Counting Starts, Counting has to decrement with Probability 1

  • Eq 2 – Counter hits zero at t, Tx is a success, s(t+1) = 0, b(t+1) = k (uniform distribution in 0)

  • Eq 3 – Counter hits zero at t, Tx is a collision, s(t+1) = i, b(t+1) = k (uniform distribution in i)

  • Eq 4 – Counter is zero at t, Tx is a collision but s(t) = m, s(t+1) = m, b(t+1) = k, no new CW

  • t – Probability that station transmits a packet (remember SLOTTED ALOHA)

  • n – number of stations

    What can we do now ?

    • STATE TRANSITION DIAGRAM OF THE CHAIN BASED ON ABOVE TRANSITION MATRIX

    • STEADY STATE ANALYSIS OF THE CHAIN TO FIND SOLUTION TO

    • THROUGHPUT ANALYSIS OF RTS/CTS and Basic Access SCHEMES

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State transition diagram for the chain

  • USE Global Balance Equationsand Sum of Probability Distributions of all states=1to solve for

State Transition Diagram for the Chain

S

C

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Throughput analysis based on model

  • Remember Slotted Aloha Stabilization ?

  • tdepends on m and W and can be changed adaptively

  • But m and W fixed because of Physical Layer Standard

  • Result – S can be significantly lower than maximum

Throughput Analysis Based on Model

  • S := Fraction of Time channel is used to successfully transmit payload bits

  • As an outside observer, see a random slot and observe what is happening

  • Probability, Exactly 1 TX Occurring on the channel is successful given someone transmits

  • Hybrid Scheme also possible.

  • Packet Length may vary and throughput may relate itself to packet size distribution mean

  • Ts, Tc, s,P are constant for model verification constant, and determined by standard

  • Maximizing throughput over probabilities which are in terms of t, we get S is max when

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Important results

IMPORTANT RESULTS

  • For Sufficiently Large n, Smax is practically independent of no. of stations in wireless network

  • Maximum throughput achievable by BAS is very close to RTS/CTS mechanism

  • RTS/CTS scheme throughput is less insensitive to transmission probability t

  • RTS/CTS scheme is network size independent for W <= 64 values. Basic Mechanism throughput increases but significantly decreases with network size

  • Key to these results – RTS/CTS mechanism reduces the time spent during a collision, and it becomes more effective than Basic Access when W and n increases the collision probability

  • RTS/CTS even more effective when packet length are longer

  • SEE PERFORMANCE EVALUATION NEXT

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Performance evaluation

PERFORMANCE EVALUATION

  • Performance is based on following Parameters

    • Network size n

    • Transmission probability t

    • Initial contention window size CWmin

    • Maximum Backoff stage m

    • Packet size

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Special topics on wireless ad hoc networks

Basic access strongly depends on it

n Throughput (except W = 128)

RTS/CTS not depends on it much

Performance Evaluation: Network Size

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Special topics on wireless ad hoc networks

Performance evaluation: transmission probability

Both decrease dramatically when n is large, but the basic access is more sententive

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Basic access

RTS/CTS


Special topics on wireless ad hoc networks

Performance evaluation: CWmin

  • Basic Access: increases when station CWmin gets closer to 64, decreases as n increases

  • RTS/CTS is almost independent on CWmin and n when CWmin<64

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RTS/CTS

Basic access


Special topics on wireless ad hoc networks

Performance Evaluation: Maximum Backoff stage

Almost no effect when m > 5

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Special topics on wireless ad hoc networks

Performance Evaluation: Packet Size

RTS/CTS is effective when packet size increases

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Conclusion

CONCLUSION

Giuseppe Bianchi, “ Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE Journal on

selected areas in Communications, Vol. 18, No. 3, March 2000

  • Contributions of the referenced Paper

    • Proposed analytical model

      • Accurate: verified by comparison with simulations

      • Simple

      • Account for all exponential backoff details

      • Evaluate basic and RTS/CTS access schemes

    • Performance evaluation on saturation throughput

  • Other Remarks

    • Model lacks in considering non-ideal channel conditions (like hidden terminals, interfering stations, or multiple access points)

    • It can be extended towards study of throughput for different classes of customer with different access priorities

    • Only considers saturation throughput (overload conditions)

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