1 Computer Networks An Introduction to Computer Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Wirless Networks

1

An Introduction to

Computer NetworksComputer Networks

University of TehranDept. of EE and Computer Engineering

By:Dr. Nasser Yazdani

Lecture 6: Wirless NetworksWirless Networks

Univ. of TehranIntroduction to computer

Network 2

OutlineOutline

Why wireless Networks What is special on wireless

networks Challenges Bluetooth Zigbee 802.11 802.11 mac

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.

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

Univ. of Tehran Introduction to computer Network

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1993 1994 1995 1996 1997 1998 1999 2000 2001

The Two Hottest Trends inTelecommunications Networks

Source: Ericsson Radio Systems, Inc.

Mobile TelephoneUsers

Internet Users

Millions

Year

Growth of Home wireless

Why is it so popular? Flexible Low cost Easy to deploy Support mobility

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

Ad hoc networks Collection of wireless mobile nodes dynamically

forming a temporary network without the use of any existing network infrastructure or centralized administration.

Hop-by-hop routing due to limited range of each node

Nodes may enter and leave the network Usage scenarios:

Military Disaster relief Temporary groups of participants (conferences)

Sensor networks Deployment of small, usually wireless sensor

nodes. Collect data, stream to central site Maybe have actuators

Hugely resource constrained Internet protocols have implicit

assumptions about node capabilities Power cost to transmit each bit is very high

relative to node battery lifetime Loss / etc., like other wireless Ad-hoc: Deployment is often somewhat

random

Summary Need to be connected from

everywhere and anytime. Need to be connected on

movement Need to good quality service on

those situation. Interworking with the existing

networks

Classification of Wireless Networks

Mobility: fixed wireless or mobile Analog or digital Ad hoc (decentralized) or centralized

(fixed base stations) Services: voice (isochronous) or data

(asynchronous) Ownership: public or private

Classification of Wireless Networks

Area: wide (WAN), metropolitan (MAN), local (LAN), or personal (PAN) area networks

Switched (circuit- or packet-switched) or broadcast

Low bit-rate (voice grade) or high bit-rate (video, multimedia)

Terrestrial or satellite

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

Big issues! Integration with existing data networks

sounds very difficult. It is not always possible to apply wired

networks design methods/principles here. Layering is not work very well, mostly we

need cross layer design

Wireless Differences 1 Physical layer: signals travel in open

space Subject to interference

From other sources and self (multipath) Creates interference for other

wireless devices Noisy lots of losses Channel conditions can be very

dynamic

Wireless Differences 2 Need to share airwaves rather than wire

Don’t know what hosts are involved Hosts may not be using same link

technology Interaction of multiple transmitters at

receiver Collisions, capture, interference

Use of spectrum: limited resource. Cannot “create” more capacity very easily More pressure to use spectrum efficiently

Wireless Differences 3 Mobility

Must update routing protocols to handle frequent changes

Requires hand off as mobile host moves in/out range

Changes in the channel conditions. Coarse time scale:

distance/interference/obstacles change

Other characteristics of wireless Slow

Growing Application Diversity

Relay Node

Access Point

Sensor

Wired Internet

Ad-Hoc/Sensor Networks

Collision Avoidance:Car Networks

Wireless Home Multimedia

Mesh Networks

Challenge: Diversity

New architectures must accommodate rapidly evolving technology

Must accommodate different optimization goals Power, coverage, capacity, price

INTERNETINTERNET

WirelessEdge

Network

WirelessEdge

Network

INTERNETINTERNET

20052010

WirelessEdge

Network

WirelessEdge

Network

Other Challenges Performance: Nothing is really work

well Security: It is a broadcast media Cross layer interception

TCP performance

Univ. of Tehran Computer Network 23

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 WLAN

(Heterogeneity) Easy to install and use Etc


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


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?

DSSS DSSS (Direct Sequence Spread (Direct Sequence Spread Spectrum)Spectrum)

XOR of the signal with pseudo-random number (chipping sequence) generate a signal

with a wider range of frequency: spread spectrum

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period


Radio Technology Spread Spectrum Technologies

Frequency Hopping: The sender keeps changing the carrier wave frequency at which its sending its data. Receiver must be in synch with transmitter, and know the ordering of frequencies.

Direct-Sequence: The receiver listens to a set of frequencies at the same time. The subset of frequencies that actually contain data from the sender is determined by spreading code, which both the sender and receiver must know. This subset of frequencies changes during transmission.

Non-Spread Spectrum requires licensing


Wireless StandardsWireless Standards


Distance vs. Data RateDistance vs. Data Rate


Bluetooth Goals

Ad-hoc wireless connectivity for everything!

Original goal Low-cost replacement for annoying wire

between cellphone and headset Result: Two modes of operation

Point to point (serial wire replacement) Point to multipoint (ad-hoc networking)


Bluetooth devices Cellphones Headsets PDAs Laptops Two-way pagers Pads, tabs, etc…


Bluetooth design Specs Started with Ericsson's Bluetooth Project in 1994 ! Named after Danish king Herald Blatand (AD 940-

981) who was fond of blueberries Radio-frequency communication between cell

phones over short distances Intel, IBM, Nokia, Toshiba, and Ericsson formed

Bluetooth SIG in May 1998 Version 1.0A of the specification came out in late

1999. IEEE 802.15.1 approved in early 2002 is based on

Bluetooth Key Features:

Lower Power: 10 μA in standby, 50 mA while transmitting Cheap: $5 per device Small: 9 mm2 single chips


Bluetooth design Specs Frequency Range: 2402 - 2480 MHz (total 79

MHz band) 23 MHz in some countries, e.g., Spain Data Rate:1 Mbps (Nominal) 720 kbps (User) Channel Bandwidth:1 MHz Range: Up to 10 m can be extended further RF hopping: 1600 times/s => 625 μs/hop Security: Challenge/Response Authentication.

128b Encryption TX Output Power:

Class 1: 20 dBm Max. (0.1W) – 100m Class 2: 4 dBm (2.5 mW) Class 3: 0 dBm (1mW) – 10m


Piconet Piconet is formed by a master and many

slaves Up to 7 active slaves. Slaves can only transmit

when requested by master Up to 255 Parked slaves

Active slaves are polled by master for transmission

Each station gets a 8-bit parked address => 255 parked slaves/piconet

The parked station can join in 2ms. Other stations can join in more time. A device can participate in multiple

piconets => complex schedule


Bluetooth Operational States


Bluetooth Operational States (Cont)

Standby: Initial state Inquiry: Master sends an inquiry packet. Slaves

scan for inquiries and respond with their address and clock after a random delay (CSMA/CA)

Page: Master in page state invites devices to join the piconet. Page message is sent in 3 consecutive slots (3 frequencies). Slave enters page response state and sends page response including its device access code.

Master informs slave about its clock and address so that slave can participate in piconet. Slave computes the clock offset.

Connected: A short 3-bit logical address is assigned Transmit:


Bluetooth Packet Format

Packets can be up to five slots long. 2745 bits. Access codes:

Channel access code identifies the piconet Device access code for paging requests and response Inquiry access code to discover units

Header: member address (3b), type code (4b), flow control, ack/nack (1b), sequence number, and header error check (8b) 8b Header is encoded using 1/3 rate FEC resulting in 54b

Synchronous traffic has periodic reserved slots. Other slots can be allocated for asynchronous

traffic54b 0-2754bAccess

CodeBaseband/link Control Header Data

Payload

72b


Bluetooth Energy Management

Three inactive states: Hold: No ACL. SCO (Sync data) continues. Node

can do something else: scan, page, inquire Sniff: Low-power mode. Slave listens only after

fixed sniff intervals. Park: Very Low-power mode. Gives up its 3-bit

active member address and gets an 8-bit parked member address.

Packets for parked stations are broadcast to 3-bit zero address.

Sniff


Bluetooth Protocol Stack

RF = Frequency hopping GFSK modulation Baseband: Frequency hop selection,

connection, MAC


Baseband Layer Each device has a 48-bit IEEE MAC address 3

parts: Lower address part (LAP) – 24 bits Upper address part (UAP) – 8 bits Non-significant address part (NAP) - 16 bits

UAP+NAP = Organizationally Unique Identifier (OUI) from IEEE

LAP is used in identifying the piconet and other operations

Clock runs at 3200 cycles/sec or 312.5 μs (twice the hop rate)



Logical Link Control and Adaptation Protocol (L2CAP) Protocol multiplexing Segmentation and reassembly Controls peak bandwidth, latency, and delay variation

Host Controller Interface RFCOMM Layer:

Presents a virtual serial port Sets up a connection to another RFCOMM

Service Discovery Protocol (SDP): Each device has one SDP which acts as a server and client for service discovery messages

IrDA Interoperability protocols: Allow existing IrDA applications to work w/o changes



IrDA object Exchange (IrOBEX) and Infrared Mobile Communication (IrMC) for synchronization

Audio is carried over 64 kbps over SCO links over baseband

Telephony control specification binary (TCS-BIN) implements call control including group management (multiple extensions, call forwarding, and group calls)

Application Profiles: Set of algorithms, options, and parameters. Standard profiles: Headset, Cordless telephony, Intercom, LAN, Fax, Serial line (RS232 and USB).


ZigBee Ultra-low power, low-data rate, industrial monitoring

and control applications requiring small amounts of data, turned off most of the time (<1% duty cycle), e.g., wireless light switches, meter reading, patient monitoring

IEEE 802.15.4 Less Complex. 32kB protocol stack vs 250kB for

Bluetooth Range: 1 to 100 m, up to 65000 nodes. Tri-Band:

16 Channels at 250 kbps in 2.4GHz ISM 10 Channels at 40 kb/s in 915 MHz ISM band One Channel at 20 kb/s in European 868 MHz band

! Ref: ZigBee Alliance, http://www.ZigBee.org


ZigBee

Two types of devices:Full Function Devices (FFD) for network routing and link coordinationReduced Function Devices (RFD): Simple send/receivedevices


LAN standard


Early Experiences IBM Switzerland,Late 1970

Factories and manufacturing floors Diffused IR technology Could not get 1 Mbps

HP Labs, Palo Alto, 1980 100 Kbps DSSS around 900 Mhz CSMA as MAC Experimental licensing from FCC Frequency administration was problematic, thus

abandoned Motorola, ~1985

1.73 GHz Abandoned after FCC difficulties


Architectures

Distributed wireless Networks: also called Ad-hoc networks

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


Wireless LAN Architecture

Server

PDA Laptop

LaptopLaptop

Laptop

Access Point Access Point

Ad Hoc

Pager

DS


Access Point Functions Access point has three components

Wireless LAN interface to communicate with nodes in its service area

Wireline interface card to connect to the backbone network

MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently


Medium Access Control

Wireless channel is a shared medium

Need access control mechanism to avoid interference

MAC protocol design has been an active area of research for many years. See Survey.


MAC: A Simple Classification

WirelessMAC

Centralized Distributed

Guaranteedor

controlledaccess

RandomaccessOur focus

SDMA, FDMA, TDMA, Polling

On Demand MACs, Polling


Wireless MAC issues Half duplex operations: difficult to receive

data while sending Time varying channel: Multipath propagation,

fading Burst Channel error: BER is as high as 10-3.

We need a better strategy to overcome noises.

Location dependant carrier sensing: signal decays with path length. Hidden nodes Exposed nodes Capture: when a receiver can cleanly receive data

from two sources simultaneously, the farther one sounds a noise.


Performance Metrics Delay: ave time on the MAC queue Throughput: fraction used for data

transmission. Fairness: Not preference any node Stability: handle instantaneous loads

greater than its max. capacity. Robust against channel fading Power consumption: or power saving Support for multimedia


Wireless LAN Architecture, Cont…

Logical Link Control Layer

MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer

Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI


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


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.


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.


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.


ESS topology connectivity between multiple BSSs, They use

a common DS


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.


802.11 MAC Frame Format

FrameControl

Duration Addr 1

ProtocolVersion

Type Sub type To DS

FromDS

RetryLastFragment

RSVDEPPower Mgt

CRCSequenceControl

User Data

Address 4Addr 2 Addr 3

MAC Header

Encrypted to WEP

Preamble PLCP header

MPDU

Bytes 32 6 34~2346

22 66Bytes 26 6 4

2Bits 2 4 1 11


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

Special Frames: ACK, RTS, CTS

Acknowledgement

Request To Send

Clear To Send

FrameControl

DurationReceiverAddress

TransmitterAddress

CRC

2 2 6 6 4bytes

FrameControl


CRC

2 2 6 4bytes

FrameControl


CRC

2 2 6 4bytes

ACK

RTS

CTS


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


IEEE 802.11 LLC Layer, Cont..

Each SAP (Service Access Point) address is 7 bits. One bit is added to it to indicate whether it is order or response.

Control has three values Information, carry user data Supervisory, for error control and flow

control Unnumbered, other type of control packet

Destination SAP

Source SAP

DataControl


IEEE 802.11 LLC <-> MAC Primitives

Four types of primitives are exchanged between LLC and MAC Layer

Request: order to perform a function

Confirm: response to Request Indication: inform an event Response: inform completion of

process began by Indication


Reception of packets AP Buffer traffic to sleeping nodes Sleeping nodes wake up to listen to

TIM (Traffic Indication Map) in the Beacon

AP send a DTIM (Delivery TIM) followed by the data for that station.

Beacon contains, time stamp, beacon interval, DTIM period, DTIM count, sync info, TIM broadcast indicator


Frame type and subtypes Three type of frames

Management Control Asynchronous data

Each type has subtypes Control

RTS CTS ACK


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.)


Frame type and subtypes, Cont..

Management… TIM (Traffic Indication Map) indicates

traffic to a dozing node dissociation


802.11 Management Operations

Scanning Association/Reassociation Time synchronization Power management


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


Association in 802.11

AP

1: Association request

2: Association response

3: Data trafficClient


Reassociation in 802.11

New AP

1: Reassociation request

3: Reassociation response

5: Send buffered frames

Old AP

2: verifypreviousassociation

4: sendbufferedframes

Client6: Data traffic


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


Power Management in 802.11

A station is in one of the three states Transmitter on Receiver on Both transmitter and receiver off (dozing)

AP buffers packets for dozing stations AP announces which stations have frames

buffered in its Beacon frames Dozing stations wake up to listen to the

beacons If there is data buffered for it, it sends a poll

frame to get the buffered data


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.


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.


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

Time

Busy SIFSPIFS

DIFS

contentwindow

Frame transmission


SIFS/DIFSSIFS makes RTS/CTS/Data/ACK atomicExample: Slot Time = 1, CW = 5, DIFS=3,

PIFS=2, SIFS=1,


Priorities in 802.11 CTS and ACK have priority over RTSAfter 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


SIFS and DIFS

DATA1 ACK1

SIFS

backoff

DIFS

RTS

SIFS


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


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

B C DA

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.


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.



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


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

quiteA B C


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


MACA Solution for Hidden Terminal Problem

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

A B C


IEEE 802.11

C FA B EDRTS

RTS = Request-to-Send


IEEE 802.11

C FA B EDRTS

RTS = Request-to-Send

NAV = 10

NAV = remaining duration to keep quiet


IEEE 802.11

C FA B EDCTS

CTS = Clear-to-Send


IEEE 802.11

C FA B EDCTS

CTS = Clear-to-Send

NAV = 8

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


IEEE 802.11

C FA B EDDATA


C FA B EDACK

IEEE 802.11

Reserved area


IEEE 802.11

C FA B EDDATA

Transmit range

Interferencerange

Carrier senserange

FA


IEEE 802.11

C FA B EDACK


CSMA/CA Carrier sense in 802.11

Physical carrier sense Virtual carrier sense using Network Allocation Vector

(NAV) NAV is updated based on overheard

RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission

Collision avoidance Nodes stay silent when carrier sensed

(physical/virtual) Backoff intervals used to reduce collision probability


Backoff Interval When 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


DCF Example

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31

B2 = 10


Backoff Interval

The time spent counting down backoff intervals is a part of MAC overhead

Choosing a large cw leads to large backoff intervals and can result in larger overhead

Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)


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

Data Transmission/ACKRTS/CTS


Related Standards Activities

IEEE 802.11 http://grouper.ieee.org/groups/802/11/

Hiperlan/2 http://www.etsi.org/technicalactiv/hiperlan2.htm

BlueTooth http://www.bluetooth.com

IETF manet (Mobile Ad-hoc Networks) working group http://www.ietf.org/html.charters/manet-charter.h

tml

Documents

1 Computer Networks An Introduction to Computer Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Wirless Networks