CSE 453: Wireless Networks

Lecture 6:

Introduction to Wireless LANs

Fall 2014

Outline

Introduction

Overview of 802.11 Networks Wireless LAN Architecture

Wireless LAN standards

Media Access Problem

The IEEE 802.11 MAC standard MAC Sublayer

Access Procedures

DCF and PCF in detail

DCF Performance

Frame Aggregation

Radio Propagation and Interference Receiver Sensitivity, Capture Threshold

Interference Models

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Overview of 802.11

• IEEE 802

– a series of specification for wireless LAN technologies.

– Focused on the lowest two layers of OSI model.

• MAC component– Set of rules to determine how to access the medium and send

data

• Physical Component– Details of transmission and reception

The IEEE 802 Family and its Relationship with OSI

Wireless LAN Pros and Cons

Pros Flexibility – Place your device anywhere in you house

Easy Setup – No cable, connectors.

Cost – Initially higher, afterwards no cost for new wiring.

Cons Speed – Lower compared to wired. 100Mbps can be achieved

using 802.11n hardware.

Radio Interference – Cordless phones, microwaves, and ham radios interfere the communication

Distance – Signal strength degrades exponentially over distance

Security – Radio signal broadcasts in all directions, anyone within range can tune in.

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WLAN Architecture

Infrastructure Wireless stations communicate with a Central Coordinator

named as Access Point (AP) The coverage area of an AP is referred to as Basic Service

Set (BSS) Coverage area of other Access Points is called Extended

Service Set (ESS)

Ad-hoc Distributed Coordination (Local)

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Infrastructure Architecture

APs are used to coordinate BSS

STAs communicate with AP within BSS

AP to AP communication through Wired Network

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Ad-hoc Architecture

Distributed Coordination by each STA within their vicinities

No AP present, Hop-by-Hop communication

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Components of 802.11 LANs

• Components of IEEE 802.11 LANs are– Stations are computing devices with

wireless network interfaces. Typically, the stations are battery-powered laptop and handheld computers, but they could also be fixed workstations.

– Access points perform the wireless-to-wired bridging function, although they have a number of other functions.

– Distribution system (DS) is the mechanism by which APs exchange frames with one another and with wired networks.

– Wireless medium is used for signal transmission. Several physical layers are defined and the architecture allows multiple physical layers to be developed to support the 802.11 MAC.

802.11 LANs Building blocks - BSS

• Basic Service Set (BSS)– a set of stations that

communicate with one another.

• Two types of BSS1.Independent BSS (IBSS)

• When all of the stations in the BSS are mobile stations and there is no connection to a wired network

• called ad hoc networks

802.11 LANs Building blocks - BSS

2. Infrastructure BSS

– all of the mobile stations in the BSS communicate with the

– The AP provides both the connection to the wired LAN and the local relay function for the BSS.

802.11 LANs Building blocks - ESS

• Extended Service Set (ESS) – is a set of infrastructure BSSs, where the APs communicate among

themselves • to forward traffic from one BSS to another • to facilitate the roaming of mobile stations between the BSSs.

– The APs perform this communication via the DS. – ESS is the highest-level abstraction supported by 802.11 networks. – Roaming between different ESSs is not supported by IEEE 802.11 and

must be supported by a higher-level protocol

IEEE 802.11 Network Services

• 9 services defined by the 802.11 architecture.

• Divided into two groups

– station services

• implemented in every 802.11 station, including AP stations.

– distribution services

• provided between BSSs – may be implemented in an AP or in another special-purpose

device attached to the distribution system.

IEEE 802.11 Network Services

• Distribution– Service used by stations to exchange MAC frames when the

frame must traverse the DS to get from a station in one BSS to a station in another BSS.

• Integration– Frame delivery to an IEEE 802 LAN outside the wireless network.

• Association– Used to establish a logical connection between a mobile station

and an AP. This connection is necessary in order for the DS to know where and how to deliver data to the mobile station.

• Reassociation– Enables an established association to be transferred from one

AP to another, allowing a mobile station to move from one BSS to another.

IEEE 802.11 Network Services

• Disassociation– Removes the wireless station from the network.

• Authentication– Establishes identity prior to establishing association.

• Deauthentication– Used to terminate authentication, and by extension,

association.

• Privacy– Provides protection against eavesdropping.

• MSDU delivery– Delivers data to the recipient.

Wireless LAN Standards

802.11a Speed 54Mbs Frequency used is 5 gigahertz (GHz)

802.11b Speed 11Mbs Frequency used is 2.4 gigahertz (GHz)

802.11g Speed 54Mbs Frequency used is 2.4 gigahertz (GHz)

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Wireless LAN Standards

New and Future Technology

Multiple-Input Multiple-Output (MIMO) Allows present technology to achieve greater throughput with the present standards

802.11n going to be the new standard for LAN: Throughput may reach 540 Mbps (PHY)

10 times faster than 802.11a or 802.11g

100 Mbps at MAC Service Access Point (SAP)

A better operating distance

Shall include the MIMO technology

802.11s going to be the new standard for Mesh Networks:

All features up to the 802.11n

With Layer–2 Routing (Router to Router)

With Deterministic Access to the channel (MDA)

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Wireless Nodes/Stations18

STAs either can transmit or receive at any instant

As the receiving circuit is inactive when transmit, STAs cannot detect collisions from the transmitting end (CSMA/CD is not possible)

Basic Access idea is: Transmit packets if and only if there is no signal in the channel.

Wireless Tx-Rx19

One STA acts asTransmitter, one or moreSTAs receives

Transmitter injects theelectro-magnetic signalinto the wireless channel

The signal follows radiopropagation rules andattenuates over distance

If the signal strength at areceiver is high enough toreceive and decode, thepacket is received by thereceiver.

Wireless Tx-Rx 20

Signal Level at Receiver Radiated (Transmitted) Signal Strength

Channel Quality (Attenuation Factor)

Distance between the Transmitting and Receiving STAs

Receiver Sensitivity The minimum level of the signal strength

for receiving a packet

Receiver cannot receive and extract a packet if the received signal is below this level

Collision (Simple Illustration)21

Receiver R is supposed to receive signal from T as the received signal is above the threshold

Receiver R can/cannot receive from T’ depending on signal strength

When they arrive simultaneously at R, the combined level is also above the sensitivity, however, R cannot receive either

Recommended Reading

• The Wireless Networks: The Definite Guide, Mathew Gast, Chapter 2.

• Wireless Networks: Local and Ad Hoc Networks, Ivan Marsic, Chapter 4.