Transcript Wireless LAN

An Introduction
to
Computer Networks
Lecture 8: Wirless Networks
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
1
Outline
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Why wireless Networks
What is special on wireless networks
Challenges
Bluetooth
Zigbee
802.11
802.11 mac
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Introduction to computer Network
2
Why wireless networks?
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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)
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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
The Two Hottest Trends in
Telecommunications Networks
700
600
Millions
Mobile Telephone
Users
500
400
Internet Users
300
200
100
0
1993 1994 1995 1996 1997 1998 1999 2000 2001
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Year
Source: Ericsson Radio Systems, Inc.
Introduction to computer Network
Growth of Home wireless
Why is it so popular?
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Flexible
Low cost
Easy to deploy
Support mobility
Applications ?
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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
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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:
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Military
Disaster relief
Temporary groups of participants (conferences)
Sensor networks
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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
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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
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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
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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?
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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!
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Integration with existing data networks
sounds very difficult.
It is not always possible to apply wired
networks design methods/principles here.
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Layering is not work very well, mostly we need
cross layer design
Wireless Differences 1
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Physical layer: signals travel in open
space
Subject to interference
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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
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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
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Collisions, capture, interference
Use of spectrum: limited resource.
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Cannot “create” more capacity very easily
More pressure to use spectrum efficiently
Wireless Differences 3
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Mobility
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Must update routing protocols to handle
frequent changes
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Changes in the channel conditions.
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Requires hand off as mobile host moves in/out
range
Coarse time scale: distance/interference/obstacles
change
Other characteristics of wireless
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Slow
Growing Application Diversity
Collision Avoidance:
Car Networks
Mesh Networks
Wired Internet
Access
Point
Sensor
Relay Node
Ad-Hoc/Sensor
Networks
Wireless Home
Multimedia
Challenge: Diversity
Wireless
Edge
Network
INTERNET
INTERNET
Wireless
Edge
Network
2005
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2010
New architectures must accommodate rapidly
evolving technology
Must accommodate different optimization goals
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Power, coverage, capacity, price
Other Challenges
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Performance: Nothing is really work well
Security: It is a broadcast media
Cross layer interception
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TCP performance
Ideal Wireless Area network?
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Wish List
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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
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Wireless LAN Design Goals
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Wireless LAN Design Goals
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Portable product: Different countries have
different regulations concerning RF band
usage.
Low power consumption
License free operation
Multiple networks should co-exist
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Wireless LAN Design
Alternatives
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Design Choices
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Physical Layer: diffused Infrared (IR) or Radio
Frequency (RF)?
Radio Technology: Direct-Sequence or FrequencyHopping?
Which frequency range to use?
Which MAC protocol to use.
Peer-Peer architecture or Base-Station approach?
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DSSS
(Direct Sequence Spread Spectrum)
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XOR of the signal with
pseudo-random
number (chipping
sequence)
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generate a signal with
a wider range of
frequency: spread
spectrum
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
=
resulting
signal
01101011001010
tb: bit period
tc: chip period
Radio Technology
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Spread Spectrum Technologies
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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
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Wireless Standards
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Distance vs. Data Rate
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Bluetooth
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Goals
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Original goal
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Ad-hoc wireless connectivity for everything!
Low-cost replacement for annoying wire
between cellphone and headset
Result: Two modes of operation
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Point to point (serial wire replacement)
Point to multipoint (ad-hoc networking)
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Bluetooth devices
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Cellphones
Headsets
PDAs
Laptops
Two-way pagers
Pads, tabs, etc…
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Bluetooth design Specs
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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:
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Lower Power: 10 μA in standby, 50 mA while transmitting
Cheap: $5 per device
Small: 9 mm2 single chips
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Bluetooth design Specs
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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:
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Class 1: 20 dBm Max. (0.1W) – 100m
Class 2: 4 dBm (2.5 mW)
Class 3: 0 dBm (1mW) – 10m
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Piconet
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Piconet is formed by a master and many slaves
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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
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Bluetooth Operational States
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Bluetooth Operational States
(Cont)
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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:
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Bluetooth Packet Format
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Packets can be up to five slots long. 2745 bits.
Access codes:
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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 traffic
54b
0-2754b
72b
Access Code
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Baseband/link Control Header
Computer Network
Data Payload
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Bluetooth Energy
Management
Three inactive states:
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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
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Bluetooth Protocol Stack
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RF = Frequency hopping GFSK modulation
Baseband: Frequency hop selection, connection,
MAC
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Baseband Layer
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Each device has a 48-bit IEEE MAC address 3 parts:
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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)
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Bluetooth Protocol Stack
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Logical Link Control and Adaptation Protocol (L2CAP)
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Host Controller Interface
RFCOMM Layer:
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Protocol multiplexing
Segmentation and reassembly
Controls peak bandwidth, latency, and delay variation
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
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Bluetooth Protocol Stack
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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).
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ZigBee
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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:
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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
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ZigBee
Two types of devices:
Full Function Devices (FFD) for network routing and
link coordination
Reduced Function Devices (RFD): Simple send/receive
devices
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LAN standard
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Early Experiences
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IBM Switzerland,Late 1970
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HP Labs, Palo Alto, 1980
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Factories and manufacturing floors
Diffused IR technology
Could not get 1 Mbps
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
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Architectures
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Distributed wireless Networks: also called
Ad-hoc networks
Centralized wireless Networks: also called
last hop networks. They are extension to
wired networks.
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Wireless LAN Architecture
Ad Hoc
Laptop
Server
Laptop
DS
Access Point
Access Point
Pager
PDA
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Laptop
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Access Point Functions
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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
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Medium Access Control
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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.
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MAC: A Simple Classification
Wireless
MAC
Centralized
Distributed
On Demand MACs, Polling
Guaranteed
or
controlled
access
Random
access
Our focus
SDMA, FDMA, TDMA, Polling
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Wireless MAC issues
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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.
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Hidden nodes
Exposed nodes
Capture: when a receiver can cleanly receive data from
two sources simultaneously, the farther one sounds a
noise.
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Performance Metrics
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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
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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
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Power Management
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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
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Sleep
Awake
Transmit
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Power Management, Cont…
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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.
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802.11 Features
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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.
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IEEE 802.11 Topology
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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.
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ESS topology
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connectivity between multiple BSSs, They use a
common DS
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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.
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802.11 MAC Frame Format
Bytes
32
Preamble
34~2346
6
MPDU
PLCP
header
MAC Header
Frame Duration Addr 1 Addr 2 Addr 3 Sequence Address 4 User
Control
Control
Data
Bytes 2
2
6
6
2
6
6
CRC
4
Encrypted to WEP
Bits 2
2
Protocol
Version
4
1
1
1
Type Sub type To From
DS DS
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Fragment
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802.11 MAC Frame Format
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Address Fields contains
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Source address
Destination address
AP address
Transmitting station address
DS = Distribution System
User Data, up to 2304 bytes long
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Special Frames: ACK, RTS,
CTS
bytes
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Acknowledgement
2
2
6
Frame
Receiver
Duration
Control
Address
ACK
4
CRC
bytes
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Request To SendRTS
2
2
6
6
Frame
Receiver Transmitter
Duration
Control
Address Address
bytes
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Clear To Send
CTS
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
4
CRC
IEEE 802.11 LLC Layer
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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
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IEEE 802.11 LLC Layer, Cont..
Destination Source
SAP
SAP
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Control Data
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
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IEEE 802.11 LLC <-> MAC
Primitives
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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
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Reception of packets
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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
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Frame type and subtypes
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Three type of frames
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Management
Control
Asynchronous data
Each type has subtypes
Control
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RTS
CTS
ACK
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Frame type and subtypes,
Cont..
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Management
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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.)
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Frame type and subtypes,
Cont..
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Management…
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TIM (Traffic Indication Map) indicates traffic
to a dozing node
dissociation
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802.11 Management
Operations
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Scanning
Association/Reassociation
Time synchronization
Power management
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Scanning in 802.11
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Goal: find networks in the area
Passive scanning
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Not require transmission
Move to each channel, and listen for Beacon
frames
Active scanning
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Require transmission
Move to each channel, and send Probe
Request frames to solicit Probe Responses
from a network
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Association in 802.11
1: Association request
2: Association response
3: Data traffic
AP
Client
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Reassociation in 802.11
1: Reassociation request
3: Reassociation response
5: Send buffered frames
Client
6: Data traffic
New AP
2: verify
previous
association
Old AP
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4: send
buffered
74
frames
Time Synchronization in
802.11
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Timing synchronization function (TSF)
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AP controls timing in infrastructure networks
All stations maintain a local timer
TSF keeps timer from all stations in sync
Periodic Beacons convey timing
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Beacons are sent at well known intervals
Timestamp from Beacons used to calibrate
local clocks
Local TSF timer mitigates loss of Beacons
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Power Management in 802.11
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A station is in one of the three states
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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
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Authentication
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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.
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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.
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Computer Network
78
802.11 Frame Priorities
Busy
DIFS
PIFS
SIFS
content
window
Frame transmission
Time

Short interframe space (SIFS)


PCF interframe space (PIFS)


For highest priority frames (e.g., RTS/CTS, ACK)
Used by PCF during contention free operation
DCF interframe space (DIFS)

Minimum medium idle time for contention-based
services
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Computer Network
79
SIFS/DIFS
SIFS makes RTS/CTS/Data/ACK atomic
Example: Slot Time = 1, CW = 5, DIFS=3, PIFS=2,
SIFS=1,
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Computer Network
80
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

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Computer Network
81
SIFS and DIFS
DATA1
ACK1
SIFS DIFS
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backoff
RTS
SIFS
Computer Network
82
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
Univ. of Tehran
Computer Network
83
Power Control with 802.11

Transmit RTS/CTS/DATA/ACK at least
power level needed to communicate
with the receiver
A


B
C
D
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
Univ. of Tehran
Computer Network
84
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.
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Computer Network
85
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.
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Computer Network
86
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
Univ. of Tehran
Computer Network
87
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
A
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B
Computer Network
C
88
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
A
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B
Computer Network
C
89
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
A

B
C
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
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Computer Network
90
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
Univ. of Tehran
C
D
E
Computer Network
F
91
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
NAV = 10
NAV = remaining duration to keep quiet
Univ. of Tehran
Computer Network
92
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
Univ. of Tehran
C
D
E
Computer Network
F
93
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
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Computer Network
94
IEEE 802.11
DATA
A
B
Univ. of Tehran
C
D
E
Computer Network
F
95
IEEE 802.11
Reserved area
ACK
A
Univ. of Tehran
B
C
D
Computer Network
E
F
96
IEEE 802.11
Carrier sense
range
Interference
range
DATA
A
B
C
D
E
F
Transmit range
Univ. of Tehran
Computer Network
97
IEEE 802.11
ACK
A
B
Univ. of Tehran
C
D
E
Computer Network
F
98
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
Univ. of Tehran
Computer Network
99
Backoff Interval

When transmitting a packet, choose a
backoff interval in the range [0,cw]


Count down the backoff interval when
medium is idle


cw is contention window
Count-down is suspended if medium becomes
busy
When backoff interval reaches 0, transmit
RTS
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Computer Network
100
DCF Example
B1 = 25
B1 = 5
wait
data
data
B2 = 20
cw = 31
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wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
Computer Network
101
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)
Univ. of Tehran
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102
Binary Exponential Backoff in
DCF

When a node fails to receive CTS in
response to its RTS, it increases the
contention window


When a node successfully completes a data
transfer, it restores cw to Cwmin


cw is doubled (up to an upper bound)
cw follows a sawtooth curve
802.11 has large room for improvement
Random
backoff
Univ. of Tehran
RTS/CTS
Data Transmission/ACK
Computer Network
103
Related Standards Activities

IEEE 802.11


Hiperlan/2


http://www.etsi.org/technicalactiv/hiperlan2.htm
BlueTooth


http://grouper.ieee.org/groups/802/11/
http://www.bluetooth.com
IETF manet (Mobile Ad-hoc Networks) working
group

http://www.ietf.org/html.charters/manet-charter.html
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