An Introduction to Computer Networks

1
An Introduction to Computer Networks
Lecture 8 Wirless Networks

• University of Tehran
• Dept. of EE and Computer Engineering
• By
• Dr. Nasser Yazdani

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Outline

• Why wireless Networks
• What is special on wireless networks
• Challenges
• Bluetooth
• Zigbee
• 802.11
• 802.11 mac

3
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.

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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

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The Two Hottest Trends inTelecommunications
Networks
Millions
Mobile Telephone Users
Internet Users
Year
Source Ericsson Radio Systems, Inc.
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Growth of Home wireless
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Why is it so popular?

• Flexible
• Low cost
• Easy to deploy
• Support mobility

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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

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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)

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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

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Wireless Applications
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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

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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

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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

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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

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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

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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

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Wireless Differences 2

• Need to share airwaves rather than wire
• Dont 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

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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

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Growing Application Diversity
Collision AvoidanceCar Networks
Mesh Networks
Wired Internet
Access Point
Sensor
Relay Node
Ad-Hoc/Sensor Networks
Wireless Home Multimedia
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Challenge Diversity
Wireless Edge Network
INTERNET
INTERNET
Wireless Edge Network
2005
2010

• New architectures must accommodate rapidly
  evolving technology
• Must accommodate different optimization goals
• Power, coverage, capacity, price

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Other Challenges

• Performance Nothing is really work well
• Security It is a broadcast media
• Cross layer interception
• TCP performance

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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

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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

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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?

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DSSS (Direct Sequence Spread Spectrum)

• XOR of the signal with pseudo-random number
  (chipping sequence)
• generate a signal with a wider range of
  frequency spread spectrum

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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

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Wireless Standards
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Distance vs. Data Rate
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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)

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Bluetooth devices

• Cellphones
• Headsets
• PDAs
• Laptops
• Two-way pagers
• Pads, tabs, etc

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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

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Bluetooth design Specs

• Frequency Range 2402 - 2480 MHz (total 79 MHz
  band) 23 MHz in some countries, e.g., Spain
• Data Rate1 Mbps (Nominal) 720 kbps (User)
• Channel Bandwidth1 MHz
• Range Up to 10 m can be extended further
• RF hopping 1600 times/s gt 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

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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 gt 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 gt
  complex schedule

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Bluetooth Operational States
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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

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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
  traffic
• 54b 0-2754b

72b
Access Code Baseband/link Control Header Data Payload
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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
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Bluetooth Protocol Stack

• RF Frequency hopping GFSK modulation
• Baseband Frequency hop selection, connection,
  MAC

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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
• UAPNAP 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

• 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

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Bluetooth Protocol Stack

• 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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802.11 LAN Architectures

• 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
Laptop
Server
DS
Pager
Laptop
PDA
Laptop
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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

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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

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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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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.

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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

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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.

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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.

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ESS topology

• 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

342346
32
6
Preamble PLCP header MPDU
6
2
6
6
4
2
2
6
Bytes
Encrypted to WEP
Bits
2
1
2
4
1
1
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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

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Special Frames ACK, RTS, CTS
bytes
2
2
6
4
Frame Control
Duration
Receiver Address
CRC

• Acknowledgement
• Request To Send
• Clear To Send

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
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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

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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

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IEEE 802.11 LLC lt-gt 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

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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

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Frame type and subtypes

• Three type of frames
• Management
• Control
• Asynchronous data
• Each type has subtypes
• Control
• RTS
• CTS
• ACK

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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.)

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Frame type and subtypes, Cont..

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

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802.11 Management Operations

• Scanning
• Association/Reassociation
• Time synchronization
• Power management

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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

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Association in 802.11
1 Association request
2 Association response
AP
3 Data traffic
Client
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Reassociation in 802.11
1 Reassociation request
New AP
3 Reassociation response
5 Send buffered frames
2 verifypreviousassociation
Client
6 Data traffic
Old AP
4 send buffered frames
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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

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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.

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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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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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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.

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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

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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

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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

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IEEE 802.11
RTS Request-to-Send
RTS
C
F
A
B
E
D
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IEEE 802.11
RTS Request-to-Send
RTS
C
F
A
B
E
D
NAV 10
NAV remaining duration to keep quiet
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IEEE 802.11
CTS Clear-to-Send
CTS
C
F
A
B
E
D
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IEEE 802.11

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

CTS Clear-to-Send
CTS
C
F
A
B
E
D
NAV 8
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IEEE 802.11
DATA
C
F
A
B
E
D
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IEEE 802.11
Reserved area
ACK
C
F
A
B
E
D
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IEEE 802.11
DATA
C
F
A
B
E
D
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IEEE 802.11
ACK
C
F
A
B
E
D
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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

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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

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DCF Example
B1 and B2 are backoff intervals at nodes 1 and 2
cw 31
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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)

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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/ACK
RTS/CTS
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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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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
Univ. of Tehran
Computer Network
90
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SIFS/DIFS

• SIFS makes RTS/CTS/Data/ACK atomic
• Example Slot Time 1, CW 5, DIFS3, PIFS2,
  SIFS1,

Univ. of Tehran
Computer Network
91
92
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 gt
  SIFS

Univ. of Tehran
Computer Network
92
93
SIFS and DIFS
DATA1
ACK1
backoff
RTS
DIFS
SIFS
SIFS
Univ. of Tehran
Computer Network
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94
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
94
95
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 Ds data transmission
• Bs transmission to A at high power interferes
  with reception of ACK at C

B
C
D
A
Univ. of Tehran
Computer Network
95
96
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.ht
  ml

Univ. of Tehran
Computer Network
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