Special Topics on Wireless Ad-hoc Networks

1
Special Topics on Wireless Ad-hoc Networks
Lecture 12 Wireless 802.11

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

2
Covered topic

• How wireless LAN, 802.11 works
• References
• Chapter 3 of the book
• Wireless Medium Access control protocols a
  survey
• MACAW A Media Access Protocol for Wireless
  LANs
• SSCH Slotted Seeded Channel Hopping for Capacity
  
• ECHOS Enhanced Capacity 802.11 Hotspots
• Idle Sense An Optimal Access Method for High
  Throughput and Fairness in Rate Diverse Wireless
  LANS
• A wireless MAC protocol Using Implicit Pipelining

3
Outlines

• Why wireless LAN
• 802.11
• 802.11 MAC
• Some improvement
• Performance Analysis.

4
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.

5
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

6
The Two Hottest Trends inTelecommunications
Networks
Millions
Mobile Telephone Users
Internet Users
Year
Source Ericsson Radio Systems, Inc.
7
Growth of Home wireless
8
Why is it so popular?

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

9
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

10
What is special on wireless?

• Channel characteristics
• Half-Duplex
• Location dependency
• Very noisy channel, fading effects, etc.,
• Resource limitation
• Bandwidth
• Frequency
• Battery, power.
• Wireless problems are usually optimization
  problems.

11
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

12
Big issues!

• Integration with existing data networks sounds
  very difficult.
• It is not always possible to apply wired networks
  design methods/principles here.

13
Problems

• Host mobility is not considered in the initial
  Internet design.
• There is a hierarchal design in Internet. How Ad
  hoc wireless networks can be handled
• A layered design. Layer should be independent of
  each other. It is not work at all in wireless
• TCP
• Battery shortages
• Etc,.

14
High availbility requirements

• No QoS assumed from below
• Reasonable but non-zero loss rates
• Whats minimum recovery time?
• 1 RTT
• But conservative assumptions end-to-end
• TCP RTO - min(1s)!
• Interconnect independent networks
• Federation makes things hard
• My network is good. Is yours? Is the one in the
  middle?
• Scale
• Routing convergence times, etc.

15
Growing Application Diversity
Collision AvoidanceCar Networks
Mesh Networks
Wired Internet
Access Point
Sensor
Relay Node
Ad-Hoc/Sensor Networks
Wireless Home Multimedia
16
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

17
Spectrum Scarcity

• Interference and unpredictable behavior
• Need better management/diagnosis tools
• Lack of isolation between deployments
• Cross-domain and cross-technology

Why is my 802.11 not working?
18
Other Challenges

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

19
Ideal Wireless Area network?

• Wish List
• High speed (Efficiency)
• Low cost
• No use/minimal use of the mobile equipment
  battery
• Can work in the presence of other WLANs
  (Heterogeneity)
• Easy to install and use
• Etc

20
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

21
Wireless LAN Design Alternatives

• Design Choices
• Physical Layer diffused Infrared (IR) or Radio
  Frequency (RF)?
• Radio Technology Direct-Sequence or
  Frequency-Hopping?
• Which frequency range to use?
• Which MAC protocol to use.
• Peer-Peer architecture or Base-Station approach?

Univ. of Tehran
Computer Network
21
22
Wireless Standards
23
Distance vs. Data Rate
24
WiFi

• Almost all wireless LANs now are IEEE 802.11
  based
• Competing technologies, e.g., HiperLAN cant
  compete on volume and cost
• 802.11 is also known as WiFi Wireless
  Fidelity
• Fidelity Compatibility between wireless
  equipment from different manufacturers
• WiFi Alliance is a non-profit organization that
  does
• the compatibility testing (WiFi.org)

25
Architectures

• Distributed wireless Networks also called Ad-hoc
  networks
• Centralized wireless Networks also called last
  hop networks. They are extension to wired
  networks.

26
Centralized Wlan
Ad Hoc
Laptop
Laptop
Server
DS
Pager
Laptop
PDA
Laptop
27
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.

28
Starting an IBSS

• One station is configured to be initiating
  station, and is given a service set ID (SSID)
• Starter sends beacons
• Other stations in the IBSS will search the medium
  for a service set with SSID that matches their
  desired SSID and act on the beacons and obtain
  the information needed to communicate
• There can be more stations configured as
  starter.

29
ESS topology

• connectivity between multiple BSSs, They use a
  common DS

30
Base-Station Approach Advantages over Peer-Peer

• No hidden terminal base station hears all mobile
  terminals, are relays their information to ever
  mobile terminal in cell.
• Higher transmission range
• Easy expansion
• Better approach to security
• Problem?
• Point of failure,
• Feasibility?

31
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.

32
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
33
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

34
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
35
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.

36
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

37
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.

38
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

39
Frame type and subtypes

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

40
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.)

41
Frame type and subtypes, Cont..

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

42
802.11 Management Operations

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

43
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

44
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

45
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.

46
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.

47
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
48
SIFS/DIFS

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

49
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

50
SIFS and DIFS
DATA1
ACK1
backoff
RTS
DIFS
SIFS
SIFS
51
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

52
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
53
A Plausible Solution

• RTS/CTS at highest power, and DATA/ACK at
  smallest necessary power level
• A cannot sense Cs data transmission, and may
  transmit DATA to some other host
• This DATA will interfere at C
• This situation unlikely if DATA transmitted at
  highest power level
• Interference range sensing range

Data sensed
B
C
D
A
Data
RTS
Ack
Interference range
54
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.

55
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.

56
802.11n

• Trend HDTV and flat screens are taking off Media
  Center Extenders from Linksys and other vendors
• Application HDTV and streaming video (over
  longer distances than permitted by 802.15.3
  WPANs)
• 11n Next Generation of 802.11
• At least 100 Mbps at MAC user layer ? 200 Mbps
  at PHY ? 4x to 5x faster than 11a/g
• (802.11a/g have 54 Mbps over the air and 25 Mbps
  to user)
• Pre-11n products already available
• Task Group n (TGn) setup Sept 2003
• Expected Completion March 2007

57
802.11n

• Uses multiple input multiple output antenna
  (MIMO)
• Data rate and range are enhanced by using spatial
  multiplexing (N antenna pairs) plus antenna
  diversity occupies one WLAN channel, and in
  compliance with 802.11
• Backwards compatible with 802.11 a,b,g
• One access point supports both standard WLAN and
  MIMO devices

58
MAC A Simple Classification
Wireless MAC
Centralized
Distributed
On Demand MACs, Polling
Guaranteed or controlled access
Random access
Our focus
SDMA, FDMA, TDMA, Polling
59
Reservation/Polling MAC Protocol

• Works only with AP
• Fair and slow. First-in-First-Out
• Wireless station send a request.
• All requests are queued.
• Wireless stations are polled in the same order
  that the requests have arrive.
• All data reception is acknowledged.

60
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

61
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

62
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

63
MACA Solution for Hidden Terminal Problem

• In order everybody to avoid send we need to
  reserved the media.
• Reservation can be done by handshaking first,
  sending data and finally acknowledgement.
• To be fair to others, reservation is done for one
  packet delivery.
• During reservation other nodes stay silent
• To do this, sender includes during in handshaking
  and others record it in their Network Allocation
  Vector (NAV)
• Upon ending transmission, everybody can contend
  to the media to send.

64
MACA Solution for Hidden Terminal Problem Karn90

• When node A wants to send a packet to node B,
  node A first sends a Request-to-Send (RTS) to A
• On receiving RTS, node A responds by sending
  Clear-to-Send (CTS), provided node A is able to
  receive the packet
• When a node (such as C) overhears a CTS, it keeps
  quiet for the duration of the transfer
• Transfer duration is included in RTS and CTS both

65
IEEE 802.11
RTS Request-to-Send
RTS
C
F
A
B
E
D
66
IEEE 802.11
RTS Request-to-Send
RTS
C
F
A
B
E
D
NAV 10
NAV remaining duration to keep quiet
67
IEEE 802.11
CTS Clear-to-Send
CTS
C
F
A
B
E
D
68
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
69
IEEE 802.11
DATA
C
F
A
B
E
D
70
IEEE 802.11
Reserved area
ACK
C
F
A
B
E
D
71
IEEE 802.11
DATA
C
F
A
B
E
D
72
Backoff Interval

• To give everybody a chance, each node for
  transmitting a packet, choose a backoff interval
  in the range 0,cw
• cw is contention window
• Count down the backoff interval when medium is
  idle
• Count-down is suspended if medium becomes busy
• When backoff interval reaches 0, transmit RTS

73
DCF Example
B1 and B2 are backoff intervals at nodes 1 and 2
cw 31
74
Backoff Interval

• backoff intervals is a part of MAC overhead
• large cw leads to large backoff and larger
  overhead
• small cw leads to a larger number of collisions
• A lot of work has been to reduce this overhead
  but still no a solid sloution.
• IEEE 802.11 DCF contention window cw is chosen
  dynamically depending on collision occurrence

75
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
76
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.

77
Receive-Initiated Mechanism

• In most protocols, sender initiates a transfer
• Alternatively, a receiver may send a
• Ready-To-Receive (RTR) message to a sender
  requesting it to being a packet transfer
• Sender node on receiving the RTR transmits data
• How does a receiver determine when to poll a
  sender with RTR?
• Based on history, and prediction of traffic from
  the sender

78
Reliability

• Wireless links are prone to errors. High packet
  loss rate detrimental to transport-layer
  performance.
• Mechanisms needed to reduce packet loss rate
  experienced by upper layers
• When node B receives a data packet from node A,
  node B sends an Acknowledgement (Ack). This
  approach adopted in many protocols
• If node A fails to receive an Ack, it will
  retransmit the packet

79
Fairness Issue

• Assume that initially, A and B both choose a
  backoff interval in range 0,31 but their RTSs
  collide
• Nodes A and B then choose from range 0,63
• Node A chooses 4 slots and B choose 60 slots
• After A transmits a packet, it next chooses from
  range 0,31
• It is possible that A may transmit several
  packets before B transmits its first packet

A
B
Two flows
C
D
80
MACAW Solution for Fairness

• When a node transmits a packet, it appends the cw
  value to the packet, all nodes hearing that cw
  value use it for their future transmission
  attempts
• Since cw is an indication of the level of
  congestion in the vicinity of a specific receiver
  node, MACAW proposes maintaining cw independently
  for each receiver
• Using per-receiver cw is particularly useful in
  multi-hop environments, since congestion level at
  different receivers can be very different

81
Wireless Capacity

• Wireless channel is inefficient due to
• MAC backoff procedure
• RTS/CTS mechanism
• Frequency interference.
• Possible solutions
• Use better backoff mechanisms.
• Exploit more physical resources more spectrum
  Cell mechanism
• Exploit diversity, use different frequencies.
• Parallel control with data

82
Improve Spatial ReusePower/Rate Control
Transmit Spatial Power Rate
reuse High High Low Low
Low High
A
B
C
D
83
Exploit Infrastructure

• Infrastructure provides a tunnel to forward
  packets

infrastructure
BS1
BS2
B
C
D
E
A
Z
Ad hoc connectivity
X
84
Exploit Antennas

• Diversity antenna
• Steered beam directional antenna

B
C
A
D
85
Directional Antennas
Not possible using Omni
B
D
S
C
A
86
Pipelining two stages

• Two stage pipeline
• Random backoff and RTS/CTS handshake
• Data transmission and ACK
• Total pipelining Resolve contention completely
  in stage 1

87
Next solution

• Partitioning channel dynamically in order to
  better utilize it.
• Different from direction antenna, it is done on
  the link layer and dynamically.

88
SSCH Slotted Seeded Channel Hopping Overview

• A dynamic assignment algorithm
• divides the time into equal sized slots (e.g. 10
  ms) and switches each radio across multiple
  orthogonal channels on the boundary of slots in a
  distributed manner
• Main aspect of SSCH
• channel scheduling
• self-computation of tentative schedule
• communication of schedules
• synchronization with other nodes

89
Channel Scheduling -Self-Computation

• Each node use (channel, seed) pairs to represent
  its tentative schedule for the next slot
• Seed 1 , number of channels -1 Initialized
  randomly
• Focus on the simple case of using one pair
• Update rule
• new channel (old channel seed)
  mod (number of channels)

1
0
2
1
0
2
1
0
A Seed 2
0
1
2
0
1
2
0
1
B Seed 1
Example 3 channels, 2 seeds
90
The ECHOS Solution ?

• AP CST algorithm (CST- Carrier Sense Thersh.)
• Dynamically adjusts the CST in order to allow
  more flows to co-exist in the same channel in
  current 802.11 architectures.
• RNC SC algorithm
• Allows each cell or AP access to all available
  channels.
• RNC algorithm executes in a centralized radio
  network controller
• Uses one channel as primary the other two as
  secondary channels
• Allows to improve Hotspot performance beyond
  AP-CST.

91
Abilities of the Algorithms

• Dynamically allocate channels to stations
• Flexibly adopts parameters such as CST and/or
  transmit power
• THE CLAIM !
• Performance of 802.11-based hotspots can be
  improved by both these algorithms by up to 195
  per-cell and 70 overall.

92
Observations on Carrier Sensing in 802.11

• Qualnet simulator
• transmission at 2Mbps
• with a CST of -93dBm
• transmit power of 15dBm
• How to calculate the ranges?

93
Range Calculation

• Suppose T T are two transmitters at distance
  dt di from the receiver.
• T is the interferer to the transmission from T.
• Then,
• SNR at the receiver is assuming that
• both the transmitters transmit with the same
  power
• Strength of the received signal falls off as
• Where,
• K is a suitable constant
• is the transmission power
• d is the distance from the signal source
• For successful reception, the requirement is that
  the SNR be above a threshold
• This yields the requirement

Range
94
Observation 1
How to chose the optimum value of CST ? - Dynamic
95
Idle Sense Access Method

• GOALS
• Optimize Throughput
• Dynamically Adapt to Physical Channel Conditions
• Equal Time Shares for hosts with different bit
  rate
• Short-term fairness and Minimize Delay

96
Channel Contention
--Idle Slot (No carrier) --Idle slots are shorter
Two-host contention modeled as a stochastic
process with three states
97
Channel Contention

• Host have always packets to send
• Host can hear each others

• Pe Attempt p. for a slot per node
• Pt Successful Tx p. for a given slot
• Pc Collision p. for a given slot
• Pi Slot Idle p.
• __
• ni No. of consecutive idle slots
• between two trans/colission

98
Channel Contention
All host have the same CW and trans/col are like
the wait interval
Approximate Pe 5
Throughput Function
Cost Function
Sd ave. frame size,
5Bianchi,Fratta Oliveri, Performance
Analysis of 802.11 CSMA/CA Medium Access Control
Protocol, Proc of PIMRC1996
99
Channel Contention
of stations
Cost function w.r.t Contention window (for
different numbers of hosts)
100
Channel Contention
Replace values in cost function and put first
derivative to zero gives

• ?Optimal value of CW increases with N
• ?Cost function less sensitive to variations in CW
• Optimal values obtained by limiting N to 8

101
Channel Contention

• Idle Sense
• If mean (ni) exceeds this optimal value
• -gt too much time spent waiting in idle slots
• If mean (ni) less than the optimal value
• -gtexcessive collisions
• Nlt8 specific root of cost derivative

102
Principles of Idle Sense

• Each host estimates ni and uses it to compute its
  CW
• If N is known, we can determine optimal ni from
  predetermined optimal values
• If N is not known, a best estimate is used for
  nitarget

103
Principles of Idle Sense

• Control Algorithm
• AIMD Additive Increase, Multiplicative Decrease
• Using Pe2/CW

104
Performance Analysis
DIFS distributed interframe space ( decide if it
is idle) SIFS short interframe space ( shorter
than DIFS) NAV Network Allocation Vector (
contains info about packet length being Tx)
SIFS
BO 3
BO 5
BO 7
A
RTS
DATA
RTS
DIFS
DIFS
DIFS
collision
CTS
ACK
B
BO 4
BO 8
BO 5
BUSY
NAV (RTS)
RTS
DIFS
DIFS
DIFS
C
NAV(CTS)
105
CRITICAL ASSUMPTIONS

• Ideal Channel conditions and finite number of
  terminals
• Ideal Channel conditions include (No Hidden
  Terminals, No Channel Capture)
• Constant independent collision probability P
  for each transmitted packet
• System is in Overload Condition (Every station is
  always ready to Transmit a Packet)

106
Mathematical Model for Dynamics of DCF

• s(t) stochastic process representing back off
  stage (0, . , m) of a given station
• b(t) stochastic process representing back off
  time counter (k, k-1,,1,0) of a station
• Bi-dimensional Process s(t), b(t) with state
  space (i, k)
• and W CWmin (minimum
  contention window length)
• p Conditional Collision Probability seen by a
  packet being transmitted (const and indep)

• -1
• 2
• 3
• 4

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

• Eq 1 Once Back off Counting Starts, Counting
  has to decrement with Probability 1
• Eq 2 Counter hits zero at t, Tx is a success,
  s(t1) 0, b(t1) k (uniform distribution in
  0)
• Eq 3 Counter hits zero at t, Tx is a collision,
  s(t1) i, b(t1) k (uniform distribution in
  i)
• Eq 4 Counter is zero at t, Tx is a collision
  but s(t) m, s(t1) m, b(t1) k, no new CW
• t Probability that station
  transmits a packet (remember SLOTTED ALOHA)
• n number of stations
• What can we do now ?
• STATE TRANSITION DIAGRAM OF THE CHAIN BASED ON
  ABOVE TRANSITION MATRIX
• STEADY STATE ANALYSIS OF THE CHAIN TO FIND
  SOLUTION TO
• THROUGHPUT ANALYSIS OF RTS/CTS and Basic Access
  SCHEMES

107
State Transition Diagram for the Chain
S
C
108
Throughput Analysis Based on Model

• S Fraction of Time channel is used to
  successfully transmit payload bits
• As an outside observer, see a random slot and
  observe what is happening
• Probability, Exactly 1 TX Occurring on the
  channel is successful given someone transmits

• Hybrid Scheme also possible.
• Packet Length may vary and throughput may relate
  itself to packet size distribution mean
• Ts, Tc, s,P are constant for model verification
  constant, and determined by standard
• Maximizing throughput over probabilities which
  are in terms of t, we get S is max when

109
IMPORTANT RESULTS

• For Sufficiently Large n, Smax is practically
  independent of no. of stations in wireless
  network
• Maximum throughput achievable by BAS is very
  close to RTS/CTS mechanism
• RTS/CTS scheme throughput is less insensitive
  to transmission probability t
• RTS/CTS scheme is network size independent for W
  lt 64 values. Basic Mechanism throughput
  increases but significantly decreases with
  network size
• Key to these results RTS/CTS mechanism reduces
  the time spent during a collision, and it becomes
  more effective than Basic Access when W and n
  increases the collision probability
• RTS/CTS even more effective when packet length
  are longer
• SEE PERFORMANCE EVALUATION NEXT

110
PERFORMANCE EVALUATION

• Performance is based on following Parameters
• Network size n
• Transmission probability t
• Initial contention window size CWmin
• Maximum Backoff stage m
• Packet size

111
Performance Evaluation Network Size

• Basic access strongly depends on it
• n Throughput (except W 128)
• RTS/CTS not depends on it much

112
Performance evaluation transmission probability
Both decrease dramatically when n is large, but
the basic access is more sententive
Basic access
RTS/CTS
113
Performance evaluation CWmin

• Basic Access increases when station CWmin gets
  closer to 64, decreases as n increases
• RTS/CTS is almost independent on CWmin and n when
  CWminlt64

RTS/CTS
Basic access
114
Performance Evaluation Maximum Backoff stage
Almost no effect when m gt 5
115
Performance Evaluation Packet Size
RTS/CTS is effective when packet size increases
116
CONCLUSION

• Giuseppe Bianchi, Performance Analysis of the
  IEEE 802.11 Distributed Coordination Function,
  IEEE Journal on
• selected areas in Communications, Vol. 18, No. 3,
  March 2000
• Contributions of the referenced Paper
• Proposed analytical model
• Accurate verified by comparison with simulations
  
• Simple
• Account for all exponential backoff details
• Evaluate basic and RTS/CTS access schemes
• Performance evaluation on saturation throughput
• Other Remarks
• Model lacks in considering non-ideal channel
  conditions (like hidden terminals, interfering
  stations, or multiple access points)
• It can be extended towards study of throughput
  for different classes of customer with different
  access priorities
• Only considers saturation throughput (overload
  conditions)