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Outline. Wireless introduction Wireless cellular (GSM, CDMA, UMTS) Wireless LANs, MAC layer IEEE 802.11 Bluetooth Wireless Ad hoc networks routing: proactive routing, on-demand routing, scalable routing, geo-routing wireless Ad hoc multicast TCP in ad hoc networks

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outline

Outline

Wireless introduction

Wireless cellular (GSM, CDMA, UMTS)

Wireless LANs, MAC layer

IEEE 802.11

Bluetooth

Wireless Ad hoc networks

routing: proactive routing, on-demand routing, scalable routing, geo-routing

wireless Ad hoc multicast

TCP in ad hoc networks

QoS, adaptive voice/video apps

Sensor networks

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wireless protocol layers

Application Processing

Application Setup

Application

RTP Wrapper

RCTP

Transport Wrapper

TCP/UDP Control

Transport

RSVP

IP Wrapper

IP

IP/Mobile IP

Routing

VC

Handle

Flow

Control

Routing

Clustering

Packet Store/Forward

Network

Link Layer

Packet Store/Forward

Ack/Flow Control

Clustering

MAC Layer

Frame Wrapper

RTS/CTS

CS/Radio Setup

Frame Processing

Radio Status/Setup

Radio

Propagation Model

Mobility

Channel

Wireless Protocol Layers

Control Plane

Data Plane

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ieee 802 11 standard
IEEE 802.11 Standard

Why we study this standard:

  • overall architecture
  • MAC layer spec.
    • channel access
    • mobility support
  • physical layer spec.
    • direct sequence
    • frequency hopping

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802 11 features
802.11 Features
  • CSMA/CA based MAC protocol

- DCF (Distributed Coordination Function)

  • support for both time-critical - PCF( Point Coordination Function) and non-critical traffic (DCF)
  • support multiple priority levels
  • spread spectrum technology (no licensing)
  • power management allows a node to doze off

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802 11 protocol entities
802.11 Protocol Entities
  • MAC entity
    • basic access mechanism
    • fragmentation & encryption
  • MAC layer management entity
    • synchronization
    • power management
    • roaming
  • Physical layer convergence protocol (PLCP)
    • PHY-specific, common PHY SAP support
    • provides carrier sense
  • Physical medium dependent sublayer (PMD)
    • modulation & coding
  • PHY layer management
    • channel tuning & PHY MIB

MAC

Sublayer

MAC layer

Management

PLCP

sublayer

PHY layer

Management

PMD sublayer

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phy spec
PHY spec
  • Infrared PHY (No products !)
    • diffuse infrared
    • 1 and 2Mbps
  • Radio PHY
    • Frequency hopping PHY
    • Direct Sequence PHY
    • CCA (clear channel assessment) - how to sense a channel is clear:
      • energy level is above a threshold
      • can detect a signal
      • use both

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frequency hopping spread spectrum
Frequency Hopping Spread Spectrum
  • Pseudo-random frequency hopping
  • 2.4Ghz ISM band, 1-2Mbps; 2GFSK (2 level Gaussian frequency shift keying), 4GFSK; hop over 79 channels
  • spreads the power over a wide spectrum -> spread spectrum
  • narrowband interference cannot jam
  • developed initially for military

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direct sequence spread spectrum1
Direct Sequence Spread Spectrum
  • Spreading factor = code bits/data bit, 10-100 commercial (min 10 by FCC)
  • Signal bandwidth>10*data bandwidth
  • code sequence synchronization
  • correlation between codes -> interference: orthogonal
  • 2.4Ghz band, 1,2Mbps; DBPSK(differential binary phase shift keying), DQPSK(differential quadrature phase shift keying); 11 chip barker sequence

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mac layer
MAC Layer
  • Media Access Control protocol: coordination and scheduling of transmissions among competing neighbors
  • Goals:
    • low latency
    • good channel utilization
    • best effort + real time support
  • MAC layer clustering: aggregation of nodes in a cluster (= cell) for MAC enhancement; different from network layer clustering/ partitioning such as used for routing

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basic mac features
Basic MAC Features
  • DCF: Carrier sense multiple access with collision avoidance (CSMA/CA) based
    • based on carrier sense function in PHY called Clear Channel Assessment (CCA)
    • CSMA/CA+ACK for unicast frames, with MAC level recovery
    • parameterized use of RTS/CTS to protect against hidden nodes
    • frame formats to support both infrastructure and ad-hoc networks
  • PCF (option, not been widely implemented)
    • centralized, polling based
    • restricted to infrastructure network

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csma ca ack 4 way handshake
CSMA/CA+ACK: 4-way handshake
  • MAC headers format differs per type
    • control frames: RTS, CTS, ACK
    • management frames, e.g. beacon, probe/probe response, (re)-association request/response,
    • data frames

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frame format
Frame Format

Frame Control Field

  • Addressing: Address 1 Address 2 Address 3 Address 4
    • Ad hoc: DA SA BSSID -
    • From AP: DA BSSID SA -
    • To AP: BSSID SA DA -
    • AP to AP: RA TA DA SA

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csma ca ack explained
CSMA/CA+ACK explained
  • Reduce collision probability where mostly needed
  • defer access based on carrier sense
    • CCA from PHY and virtual carrier sense state
  • direct access when medium is sensed free
  • longer than DIFS, otherwise defer and backoff
  • receiver of directed frames to return ACK when
  • CRC correct

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slide18
•Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector(NAV)

•Defer on either NAV or “CCA” indicating Medium Busy

•Use of RTS/CTS is optimal but must be implemented

•Use is controlled by a RTS

-Threshold parameter per station

-To limit overhead for short frames

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pcf access procedure
PCF Access Procedure
  • Point Coordinator (PC) senses the medium at the beginning of each CFP
  • PC in Access Point transmits a beacon containing “CF parameter set element” when idle > PIFS
  • each station presets its NAV to the CFPMaxDuration from the CF Parameter Set Element in beacons from the PC

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pcf access procedure cont
PCF Access Procedure(cont)
  • after a SIFS period, PC sends one of the following: a data frame, CF-Poll frame, Data+CF-Poll frame, CF-end frame (when no traffic buffered & no polls to send at the PC)
  • PC maintains a polling list to select stations that are eligible to receive CF-Polls during contention-free periods.
  • A CF-Pollable station always responds to a CF-Poll: if no data from the station, responds with a Null Frame or a CF-ACK (no data) frame (when ACK is required);
  • “piggyback” ACK or Poll in the data frame whenever possible

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further details
Further details
  • Alternating Contention free and contention operations under PCF control
  • NAV prevents contention traffic until reset by the last PCF transfer -> variable length contention free period per interval
  • both PCF and DCF defer to each other causing PCF burst start variations
  • CF-burst by polling bit in CF-down frame
  • immediate response by station on a CF_Poll

CS219

synchronization in 802 11
Synchronization in 802.11
  • All stations maintain a local timer
  • Timing synchronization function (TSF)
    • keeps timers from all stations in synch
    • AP controls timing in infrastructure networks
  • timing conveyed by periodic beacons
    • beacons contain timestamp for the entire BSS
    • timestamp from beacons to calibrate local clocks
    • not required to hear every beacon to stay in synch
  • used for power management
    • beacons sent at well known intervals
    • all station timers in BSS are synchronized

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roaming approach
Roaming Approach
  • Station decides that link to its current AP is poor
  • station uses scanning function to find another AP
  • station sends Reassociation Request to new AP
  • if Reassociation Response is successful
    • then station has roamed to the new AP
    • else station scans for another AP
  • if AP accepts Reassociation Request
    • AP indicates Reassociation to the Distribution System
    • Distribution System information is updated
    • normally old AP is notified thru distributation system

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scanning
Scanning
  • Scanning required for many functions
    • finding and joining a network
    • finding a new AP while roaming
    • initializing an ad hoc network
  • 802.11 MAC uses a common mechanism
    • Passive scanning
      • by listening for Beacons
    • Active Scanning
      • probe + response

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active scanning
Steps to Association:

Station sends Probe

APs send Probe Response

Station selects best AP:

Station sends Association Request to select AP

AP sends Association Response

Active scanning

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power management
Power Management
  • A station can be in one of three states:

- Transmitter on

- Receiver only on

- Dozing: Both transmitter and receivers off

  • Access point (AP) buffers traffic for dozing stations
  • AP announces which stations have frames buffered. Traffic indication map included in each beacon. All multicasts/broadcasts are buffered.
  • Dozing stations wake up to listen to the beacon. If there is data waiting for it, the station sends a poll frame to get the data.

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congestion avoidance ieee 802 11 dcf
Congestion Avoidance:IEEE 802.11 DCF
  • Before transmitting a packet, randomly choose a backoff interval in the range [0,cw]
    • cw is the contention window
  • Direct access when medium is sensed free longer than DIFS, otherwise defer and backoff
  • “Count down” the backoff interval when medium is idle
    • Count-down is suspended if medium becomes busy
  • When backoff interval reaches 0, transmit packet (or RTS)

CS219

dcf example count down

B1 = 25

B1 = 5

wait

data

data

wait

B2 = 10

B2 = 20

B2 = 15

DCF Example (count down)

Let cw = 31

B1 and B2 are backoff intervals

at nodes 1 and 2

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congestion avoidance
Congestion Avoidance
  • The time spent counting down backoff intervals contributes to MAC overhead
  • Choosing a large cwleads to large backoff intervals and can result in larger overhead
  • Choosing a small cw leads to a larger number of collisions (more likely that two nodes count down to 0 simultaneously)

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congestion control
Congestion Control
  • Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed
  • IEEE 802.11 DCF: Congestion control achieved by dynamically adjusting the contention window cw

CS219

binary exponential backoff in dcf
Binary Exponential Backoff in DCF
  • When a node fails to receive CTS in response to its RTS, it increases the contention window
    • cw is doubled (up to an upper bound – typically 5 times)
  • When a node successfully completes a data transfer, it restores cw to CWmin

CS219

mild algorithm in macaw bharghavan94sigcomm
MILD Algorithm in MACAW[Bharghavan94Sigcomm]
  • When a node fails to receive CTS in response to its RTS, it multiplies cw by 1.5
    • Less aggressive than 802.11, which multiplies by 2
  • When a node successfully completes a transfer, it reduces cw by 1
    • More conservative than 802.11, where cw is restored to Cwmin
      • 802.11 reduces cw much faster than it increases it
    • MACAW: cw reduction slower than the increase
      • Exponential Increase Linear Decrease
  • MACAW can avoid wild oscillations of cw when congestion is high

CS219

fairness issue

A

B

C

D

Fairness Issue
  • Many definitions of fairness plausible
  • Simplest definition: All nodes should receive equalbandwidth
  • Observation: unfairness occurs when one node has backed off much more than some other node

Two flows

CS219

fairness issue1

A

B

C

D

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

Two flows

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macaw solution for fairness
MACAW Solution for Fairness
  • When a node transmits a packet, it appends its current cw value to the packet
  • All nodes hearing that cw value use it for their future transmission attempts
  • The effect is to reset all competing nodes to the same ground rule

CS219

distributed fair scheduling dfs vaidya mobicom00
Distributed Fair Scheduling (DFS) [Vaidya Mobicom00]
  • A fully distributed algorithm for achieving weighted fair queueing: Assign a weight to each node
  • Goal: bandwidth used by each node should be proportional to the weight assigned to the node
  • Chooses backoff intervals proportional to

(packet size / weight)

  • DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm
  • Works well on a LAN

CS219

distributed fair scheduling dfs

B1 = 10

B1 = 5

B1 = 15

wait

wait

Collision !

data

data

B2 = 5

B2 = 5

B2 = 5

Distributed Fair Scheduling (DFS)

B1 = 15 (DFS actually picks a random valuewith mean 15)

B2 = 5 (DFS picks a value with mean 5)

Weight of node 1 = 1

Weight of node 2 = 3

Assume equal

packet size

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performance improvement for 802 11 based wireless networks l zhang icc06
Performance Improvement for 802.11-based Wireless Networks [L. Zhang ICC06]
  • Problem with WLANs
    • Every packet need the AP to forward
    • The AP has the same priority with wireless stations to access the wireless channel
  • Motivation
    • Make the AP with higher priority
    • The AP send a frame immediately after receiving a frame from the WS

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action for the ap
Action for the AP
  • The AP must be involved in any communication.
    • If the AP is the receiver, it will set its backoff time counter to be zero
  • the AP should obtain the channel immediately and send the data, since its backoff time counter is zero.
  • As all wireless stations has increased their backoff time counter by one after the communication, there is no collision.
  • As a result, the AP can send one frame, after any wireless station sending a frame. It will not be the bottleneck anymore.

CS219

action for wireless stations
Action for Wireless Stations
  • In backoff procedure, the backoff counter is
    • decremented while the medium is sensed idle,
    • frozen when a transmission is detected on the channel.
      • increased by one If the sender is one of other wireless stations (except when the backoff counter is already at its maximum)
    • reactivated when the channel is sensed idle again
    • The station transmits a frame when the backoff counter reaches zero.

CS219

slide43

Model: a discrete-time Markov chainfor two-dimensional process {s (t), b (t)} s (t) - stochastic process - backoff stage b (t) - stochastic process - backoff-time counterq - probability that at least one station transmits

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goodput analysis
Goodput Analysis
  • Throughput
  • Goodput G – sum of the end-to-end throughput in WLAN

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results udp
Results - UDP

Goodput performance compare for UCP pair scenario

Fairness performance compare

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mac enhancements for qos ieee 802 11e
MAC Enhancements for QoS: IEEE 802.11e
  • The major enhancement of 802.11e
    • Traffic differentiation
    • Concept of transmission opportunity (TXOP)
    • Enhanced DCF (contention-based)
    • HCF(Hybrid Coordination Function) controlled channel access (contention free)
    • Burst ACK (optional)
    • Direct link protocol (DLP)

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ieee 802 11e mac architecture
IEEE 802.11e MAC Architecture
  • Hybrid Coordination Function (HCF): TGe (Group E) proposes HCF to provide QoS for real-time applications

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hcf introduction
HCF - Introduction
  • HCF combines functions from the DCF and PCF with enhanced QoS-specific mechanisms
  • HCF consists of
    • Enhance DCF (EDCF) for contention-based access: provides differentiated access to the WM (Wireless Mobility) for 8 priorities for non-AP STAs (stations)
    • Controlled Access for contention-free access

CS219

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