southern methodist university fall 2003 eets 8316/ntu cc745-n wireless networks

#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU

Southern Methodist University Fall 2003

EETS 8316/NTU CC745-NWireless Networks

Lecture 10: Wireless LAN

Instructor: Jila Serajemail: jseraj@engr.smu.edu

http://www.engr.smu.edu/~jseraj/tel: 214-505-6303

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Session Outline

Wireless LAN

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Wireless LAN

Wish List—High speed

—Low cost

—No use/minimal use of the mobile equipment battery

—Can work in the presence of other WLAN

—Easy to install and use

—Etc

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Wireless LAN Architecture

Server

PDA Laptop

LaptopLaptop

Laptop

Access Point Access Point

Ad Hoc

Pager

DS

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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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What Is Hidden Node?

A CB

A can hear BC can hear BA can not hear CC can not hear A sending data

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LBT MAC Protocol

LBT= Listen Before You Talk

—Based on CSMA-CA

—First send Ready To Send (RTS) to the receiving node

—Receiving node send a Continue To Send (CTS) message, takes care of hidden node.

—Data transmission starts after RTS/CTS.

—Data is acknowledged on the MAC level. Counteract error caused by RF environment.

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Integrated CSMA/TDMA MAC Protocol

Supports guaranteed bandwidth traffic and random access traffic

The bandwidth is divided into a random part and a reserved part.

Random part is LBT, reserved part

During high traffic all bandwidth can be used for reserved traffic (like wireless telephony)

H1 Reserved-1 H2 Reserved-2 H3 LBT

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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.

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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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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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Bridge Functions

Speed conversion between different devices, results in buffering.

Frame format adaptation between different incompatible LANs

Adding or deleting fields in the frame to convert between different LAN standards

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Routing

Building routing tables can be done as

—Source tree, keeps track where other nodes are and the best way of reaching them. When sending a packet the route is also determined. It must be done in each node and is heavy.

—Spanning tree, is built iteratively, each bridge advertises it identity and all other bridges it knows and how many hops it takes to get there. Then each bridge follows a specific algorithm to calculate how get to each bridge with least hop.

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Bridge Functions, Routing

Create a routing table for sending packets

Listen to all packets being sent.

Find out which nodes are in which sub-network by analyzing the source address. Store that data in a routing table.

If a packet is addressed to a known node, only repeat the data on that sub-network, otherwise repeat it on all networks.

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Bridge Functions, Routing, Cont…

Age the entries after a timer value has expired since the last communication

If the timer is too long, we might send data to a node that might have left the sub-network or is turned off or even gone to coverage area of another access point.

If the timer is too short, we remove the user too early and repeat the packet destined to it in all sub-networks.

Other functions of a bridge, buffering for speed conversion, changing frame format between LANs.

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Mobility Management

AP has three components—WLAN interface

—Backbone LAN interface

—MAC layer bridge functionBackbone Network

Access PointAccess Point

Access Point

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Mobility Management, Cont..

A node can associate when it enters the coverage area of an AP

A node can disassociate when power down or leaving the service area

It shall re-associate when it handoffs to another AP.

AP bridge function keeps track of all nodes associated with it.

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WLAN Addressing

In wireline LAN, each node has an IP address that is associated with its physical location

When a device can move from one location to another, the association between the physical location and IP address no longer holds

The solution is presented in mobile IP

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Mobile IP Principals

Internet is a large network and introducing a new function, e. g. Mobile IP can not be disruptive.

Constraints of mobile IP are

—Mobility should be at network layer

—No impact on higher levels

—No impact on the nodes not directly involved in the mobile IP function

—Uninterrupted operation for mobile devices

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Mobile IP Principals, Cont…

The principle is very simple, use c/o addressing

For each mobile device, we associate a Home IP address associated with a Home Network.

The new LAN is called the Visiting Network

The software that takes care of mobility in each server (router) is called agent.

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Mobile IP Principals, Cont…

Two types of agents, Home Agent and Visiting Agent.

Whenever the mobile device connects to a new network, a c/o address is given to it by the Visiting Agent.

This c/o address is reported to the Home Agent.

All packets addressed to the mobile device are addressed to its Home Address, and thus sent to its Home Network.

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Mobile IP Principals, Cont…

Upon reception of the packet, the Home Agent recognized the address belonging to a mobile device.

Home Agents looks up the c/o address in its table.

The packet is then wrapped in a new packet with the c/o address on it, called encapsulation

C/o address causes the packet to be forwarded to the Visiting Agent.

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Mobile IP Principals, Cont…

Visiting Agent recognizes the received address as the c/o address, unwrap the packet; called de-capsulation; and send it to its intended receiver.

This activity is called tunneling, referring to the idea creating a tunnel between the Home Network and Visiting Network and sending all data to that mobile device on that tunnel.

Several tunnels can be created between two networks

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Mobility Management in WLAN

Mobile IP principles are used to take care of mobility in the wireless LAN.

Every wireless device has an address in its Home LAN, and gets a c/o address in the Visiting LAN.

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IEEE 802.11 WLAN, History

1997 IEEE 802.11 working group developed standard for inter-working wireless LAN products for 1 and 2 Mbps data rates in 2.4 GHz ISM (industrial, scientific, and medical) band (2400-2483 MHz)

Required that mobile station should communicate with any wired or mobile station transparently (802.11 should appear like any other 802 LAN above MAC layer), so 802.11 MAC layer attempts to hide nature of wireless layer (eg, responsible for data retransmission)

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802.11 WLAN History, Cont..

1999 IEEE 802.11a amendment for 5 GHz band operation and 802.11b amendment to support up to 11 Mbps data rate at 24 GHz

MAC sub layer uses CSMA/CA (carrier sense multiple access with collision avoidance)

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802.11 Architecture

MAC Layer

Physical Layer ConvergenceProcedure (PLCP)

Physical Medium Dependent(PMD) sub layer

MAC provides asynchronous, connectionless service

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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

—Probe request/ response

—privacy request/ response

—Beacon (Time stamp, beacon interval, TDIM period, TDIM count, channels sync info, ESS ID, TIM broadcast indicator)

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Frame type and subtypes, Cont..

Management…

—TIM (Traffic Indication Map) indicates traffic to a dozing node

—dissociation

—Authentication

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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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802.11 MAC Frame Format

FrameControl

Duration Addr 1

ProtocolVersion

Type Sub type To DS

FromDS

RetryLastFragment

RSVDEPPower Mgt

CRCSequenceControl

User Data

Address 4Addr 2 Addr 3

MAC Header

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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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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

Destination SAP

Source SAP

DataControl

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IEEE 802.11 LLC <-> 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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HIPERLAN

1995 ETSI technical group RES 10 (Radio Equipment and Systems) developed HIPERLAN/1 wireless LAN standards using 5 channels in 5.15-5.3 GHz frequency range—Technical group BRAN (Broadband Radio

Access Network) is standardizing HIPERLAN/2 for wireless ATM

—ETSI URL for Hiperlan information http://www.etsi.org/frameset/home.htm?/technicalactiv/Hiperlan/hiperlan2.htm

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HIPERLAN Characteristics

HIPERLANs with same radio frequencies might overlap

—Stations have unique node identifiers (NID)

—Stations belonging to same HIPERLAN share a common HIPERLAN identifier (HID)

—Stations of different HIPERLANs using same frequencies cause interference and reduce data transmission capacity of each HIPERLAN

—Packets with different HIDs are rejected to avoid confusion of data

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HIPERLAN Protocol Layers

Data link layer = logical link control (LLC) sub layer + MAC sub layer + channel access control (CAC) sub layer

data link

physical

LLC

MAC

network

CAC

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HIPERLAN Protocol Layers, Cont..

MAC sub layer:—Keeps track of HIPERLAN addresses (HID

+ NID) in overlapping HIPERLANs—Provides lookup service between network

names and HIDs—Converts IEEE-style MAC addresses to

HIPERLAN addresses—Provides encryption of data for security

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HIPERLAN Protocol Layers, Cont..

MAC sub layer:—Provides “multi hop routing” – certain

stations can perform store-and-forwarding of frames

—Recognizes user priority indication (for time-sensitive frames)

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HIPERLAN Protocol Layers, Cont..

CAC sub layer:—Non-preemptive priority multiple access

(NPMA) gives high priority traffic preference over low priority

—Stations gain access to channel through channel access cycles consisting of 4 phases:

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HIPERLAN Protocol Layers, Cont…

CAC is designed to give each station (of same priority) equal chance to access the channel —First stations with highest priority data are

chosen. The rest will back off until all higher priority data is transmitted.

—Stations with the same priority level data, compete according to a given rule to choose “survivors”

—Survivors wait a random number of time slots and then listen to see if the channel is idle

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HIPERLAN Protocol Layers, Cont…

—If the channel is idle then it starts transmitting.

—Those who could not transmit wait until next period

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Reading assignment

Mobile Data and Wireless LAN technologies, Riffat Dayem, Chapters 4, 6 and 8.

Wirless LAN, Jim Geier, Part I chapter 3, Part II chapter 4

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3G

http://www.3gpp.org/