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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

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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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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802.11 Architecture

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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

  • 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

  • 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

  • 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

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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 Spectrum

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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

  • 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

  • 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

  • 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 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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802.11 frame priorities

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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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•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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Time-critical service via PCF

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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)

  • 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

  • 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

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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 in 802.11

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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 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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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

  • 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

  • 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)

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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

  • 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

  • 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

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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 – typically 5 times)

  • When a node successfully completes a data transfer, it restores cw to CWmin

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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

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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

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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

  • 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

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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


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]

  • 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

  • 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.

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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.

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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

  • Throughput

  • Goodput G – sum of the end-to-end throughput in WLAN

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Results - UDP

Goodput performance compare for UCP pair scenario

Fairness performance compare

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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

  • Hybrid Coordination Function (HCF): TGe (Group E) proposes HCF to provide QoS for real-time applications

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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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