Wireless presentation-1

Wireless LANs

• Rat’s nest of wires

• Variety of power cords and adapters

• Cables for modems, printers, scanner, mouse and keyboards

• NEED OF CONNECTING COMPUTERS

Challenges• Radio and Infrared transmissions susceptible to

noise and interference– Unreliable transmission

• Strength varies in time and space– Fading

• Finite Radio Spectrum– Shared with other systems

• Radio spectrum regulated by different bodies

Wireless LANs

• IEEE 802.11 Standards

• Non IEEE Standards like– Bluetooth, HIPERLAN, HomeRF

What is IEEE 802.11?

• IEEE:– Institute of Electrical and Electronics Engineers

• 802.11:– Family of standards set forth by the IEEE to

define the specifications for wireless LANs– Defines:

• Medium Access Control (MAC)

• Physical Layer (PHY) Specifications

IEEE 802.11 and the ISO stack

What is IEEE 802.11?

• Local, high-speed wireless connectivity for fixed, portable and moving stations – stations can be moving at pedestrian and

vehicular speeds

• Standard promises interoperability– vendors products on the same physical layer

should interoperate

• Targetted for use in– inside buildings, outdoor areas, anywhere!

IEEE 802.11

• Uses Direct Sequence spread spectrum (DSSS) technology– Frequency-Hopping spread spectrum (FHSS)

can only be used for 1 or 2Mbps in US due to FCC regulations

• Operates in unlicensed 2.4 GHz ISM band– ISM: Industrial, Scientific and Medical– ISM regulatory range:

• 2.4 GHz to 2.4835 GHz for North America

IEEE 802.11

• Supported Speeds and Distances– 1, 2, 5.5, 11 Mbps at distances of 150-2000 feet

without special antenna– Greater distances can be achieved by using

special antennas– Distance (or signal strength) greatly depends on

obstructions such as buildings and other objects– Maximum speed obtained depends on signal

strength

IEEE 802.11b

• ‘b’ in IEEE 802.11b– September 1999, 802.11b “High Rate”

amendment was ratified by the IEEE– 802.11b amendment to 802.11 only affects the

physical layer, basic architecture is the same• Added two higher speeds

– 5.5 and 11 Mbps

• More robust connectivity

• 802.11b is the current ‘favorite’ in 802.11– also known as Wi-Fi (Wireless Fidelity)

IEEE 802.11a

• “Fast Ethernet” standard of wireless LANs

• Speeds of up to 54 Mbps

• 5 GHz (U-NII band) instead of 2.4 GHz – Unlicensed National Information Infrastructure

• OFDM instead of DSSS for encoding– Orthogonal Frequency Division Multiplexing

IEEE 802.11a

• Advantages– higher speed– less RF interference than 2.4 GHz

• 2.4 GHz used by Bluetooth, cordless/cellular phones, etc.

– some interoperability, vendors currently have “dual-standard” 802.11a/b equipment

• Disadvantages– shorter range, need to increase AP density or

power 4X to compensate

IEEE 802.11g

• Another high-speed standard

• Viewed as a ‘step’ towards 802.11a

• Speeds of up to 54 Mbps– may be more like 20+ Mbps

• Still works at 2.4 GHz – not in the 5 GHz range like 802.11a

• Advantages– compatible with 802.11b– better range than 802.11a, for now

IEEE 802.11e

• Another upcoming standard for WLANs– adds quality-of-service features to MAC layer of

802.11b compatible networks• error correction

• better bandwidth management– significantly improves multimedia performance

• works around RF interference– handles interference by moving away from it

– i.e., moves to a new frequency when interference from a 2.4 GHz cordless phone is detected

IEEE 802.11 and the ISO stack

IEEE 802.11 Physical Layer

• 802.11 Physical Layer Specifications– include FHSS, DSSS, IR

• PLCP: Physical Layer Convergence Protocol– interface used by the other physical layer specs– maps data units into a suitable framing format

• PMD system: Physical Medium Dependent– defines the characteristics/method of Tx/Rx data

through a wireless medium between 2 or more stations

IEEE 802.11 Physical Layer

• Spread Spectrum– spreads the transmitted signal over a wide range

of spectrum– avoids concentrating power in a single narrow

frequency band– noise makes this necessary so that receiver can

accurately decode the transmitted signal– 2 major approaches to spread spectrum:

• FHSS: Frequency Hopping Spread Spectrum

• DSSS: Direct Sequence Spread Spectrum

IEEE 802.11 Data Link Layer

• 2 Sublayers– Logical Link Control (LLC)– Media Access Control (MAC)

• 802.11 uses the same 802.2 LLC– same 48-bit addressing as other 802 LANs

• MAC address is 6 bytes or 48 bits

– allows for simple bridging to wired networks

• MAC sublayer is unique in 802.11

IEEE 802.11 MAC Sublayer

• MAC: Regulates access to the medium

• Wired IEEE 802 LANs use CSMA/CD

• 802.11 uses CSMA/CA

• CSMA: carrier sense multiple access– CD: with collision detection– CA: with collision avoidance

• Collision detection is not possible in 802.11– near/far problem: can’t transmit and “hear” a

collision at the same time

IEEE 802.11 MAC Sublayer

• CSMA/CA avoids collisions by explicit packet acknowledgment (ACK)– station wishing to transmit first senses the medium– if no activity detected, station waits an additional,

random amount of time then transmits if the medium is still free

– ACK packet is sent by receiving station to confirm the data packet arrived intact

– collision assumed if sending station doesn’t get ACK, data is retransmitted after a random time

IEEE 802.11 MAC Sublayer

• Other unique features in 802.11– IFS: Inter Frame Space

• time interval between frames

– Handling hidden stations (hidden-node problem)• virtual carrier sense

– Power management functions– Data security (MAC address, WEP)

• WEP: Wired Equivalent Privacy

– Multirate support– Fragmentation / Defragmentation

Coordination Functions of MAC

• Determine when a station in a BSS is allowed to transmit and when it may be able to receive PDUs over the wireless medium

• Distributed Coordination Function (DCF)– Provides support for asynchronous data transfer of

MAC SDUs on a best effort basis– Contention Mode for all station

Coordination Functions of MAC

• Point Coordination Function– Optional and sits on top of DCF– May be implemented by an AP– Connection-oriented time bound transfer of MAC

SDUs– Contention and contenion-free periods– Medium usage controlled by AP (synchronization

and timing)

DCF• Basic access method• Contention services for fair service to all stations• All stations required to support DCF• Based on CSMA-CA protocol

– All stations obliged to remain quiet for a certain minimum period after a transmission has been completed called the interframe space (IFS)

– High priority frames: SIFS– PCF Priority access to the medium: PIFS– DCF Interframe Space: DIFS

• Transmit data and management MPDUs

IEEE 802.11: A Closer Look

IEEE 802.11 Frame Types• Three types of frames

– Control• RTS, CTS, ACK, Contention-Free (CF), PS-Poll• Used for handshaking and for positive acknowledgements during

the data transfer

– Management• Probe request/response• Station Association and Disassociation with the AP• Timing and Synchronization• Authentication / deauthentication• Announcement traffic indication message (ATIM)

– sent after each frame

– Data• Transmission of data

CSMA-CA operation• A station is allowed to transmit an initial MAC PDU

under DCF if the station detects the medium idle for a period DIFS or greater. If the station detects medium busy, then it must calculate a random backoff time to schedule a reattempt. A station that has scheduled a reattempt monitors the medium and decrements a counter each time an idle contention slot expires. The station is allowed to transmit when its backoff timer expires during the contention period.

• Idle period after a DIFS period called contention window (CW)

Handshaking in CSMA-CA

• Required when there is hidden station problem. If a station A wants to send data frame to station B, station A first sends a request-to-send (RTS) frame. If station B receives the RTS frame, then B issues a clear-to-send (CTS) frame. All stations within range of B receive CTS frame and are aware that station A has been given permission to send, so they remain quiet while station A proceeds with its data frame transmission. If the data frame arrives without error, station B responds with an ACK. If two stations send RTS frames at the same time and they collide at B then the stations must execute a backoff to schedule a later attempt.

DIFS

DIFS

Next Frame

Contention Window

SIFS

PIFS

Busy Medium

Defer Access Wait forreattempt time

Time

Basic CSMA-CA operation

IEEE 802.11 Topologies

• Three basic topologies for WLANs– IBSS: Independent Basic Service Set– BSS: Basic Service Set– ESS: Extended Service Set

• Independent of type of PHY chosen

IEEE 802.11 IBSS

• IBSS: Independent Basic Service Set– Peer-to-peer or ad-hoc network– Wireless stations communicate directly with one

another– Generally are not connected to a larger network– No Access Point (AP)

IEEE 802.11 BSS

• BSS: Basic Service Set– Infrastructure mode– An AP connects clients to a wired network

BSS

• Defined as group of stations that coordinate their access to the medium under a given instance of the medium access control

• Area covered by BSS called Basic Service Area (BSA)– Analogous to a cell in cellular network– Upto a diameter of tens of meters

BSS and Adhoc Wireless Network

• Adhoc Network consists of group of stations within range of each other

• Typically temporal in nature

• Can be formed spontaneously anywhere

• Disbanded after a limited period of time

IEEE 802.11 ESS

• ESS: Extended Service Set– A set of BSSs interconnected by a distribution

system– Consists of overlapping BSSs (each with an AP)

• DS connects APs together, almost always Ethernet

• ESS allows clients to seamlessly roam between APs

Access Point

• Similar in functionality to base station in a cellular system

• ESS can also provide gateway access for wireless users into a wired network such as Internet– Such access accomplished via a device called portal

• Infrastructure network refers to combination of BSSs, a distribution system and portals

Access Points (APs)

• To join an infrastructure BSS, a station must select an AP and establish an association with it

• This establishes mapping between station and the AP

• Station can then send and receive data messages via the AP

• Reassociation and Dissociation services

Access Points (APs)

• Usually connects wireless and wired networks– if not wired

• acts as an extension point

• Creation of ESS by overlapping AP coverage

– allows roaming operation– APs should be on different channels

Access Points (APs)

• Capacity and Bandwidth– Possible to keep these higher by using these

techniques• Reducing size of coverage areas

• Reducing client-to-AP ratio

• Using bandwidth aggregation– AP-to-client ratio

– load balancing

Access Points (APs)

• Roaming– More than 1 AP provides signals to a single

client– Client is responsible for choosing the best AP

• first, signal strength. second, network utilization.

– When signal in use degrades, client tries to find another AP

• if found, tries to authenticate and associate

Access Points (APs)

• Configuration– Management usually done via

• HTTP, Telnet, SNMP, serial interface

– Configuring Security Settings• SSID: Service Set Identifier

• WEP: Wired Equivalent Privacy

• EAP: Extensible Authentication Protocol

– Configuring Network Settings• DHCP: Dynamic Host Configuration Protocol

• NAT: Network Address Translation

Access Points (APs)

• How to setup a secure access point– Enable WEP or EAP– Change SSID and disable broadcast– Change the management password of your AP

• some have 2: read-only as well as read-write

– Use MAC address filtering– Consider not using DHCP

• instead use fixed IP addresses for wireless NICs

– Consider other mechanisms for privacy• PPTP, VPN, SSL, SSH

IEEE 802.11 Security

• Authentication– Open system– Shared key

• Authorization– MAC address

• Privacy– WEP: Wired Equivalent Privacy

Overview of the Bluetooth technology

• Bluetooth must be able to:• Recognize any other Bluetooth device in radio range• Permit easy connection of these devices• Be aware of the device types• Support service discovery• Support connectivity aware applications

• Examples of Bluetooth uses:• Briefcase email: access email while the PC is still in the

briefcase; when PC receives an email, you are notified thru the mobile phone. Use the mobile phone to browse the email.

• Cordless desktop: connect your desktop/laptop cordlessly to printers, scanner, keyboard, mouse, etc.

Bluetooth

• Bluetooth radio modules operate in the unlicensed ISM band centered at at 2.45GHz. RF channels:2420+k MHZ, k=0..78.

• Bluetooth devices within 10m of each other can share up to 720kbps of capacity

• Projected cost for a Bluetooth chip is ~$5. Plus its low power consumption, means you could literally place one anywhere.

• Can operate on both circuit and packet switching modes, providing both synchronous and asynchronous data services

• It is intended to support an open-ended list of applications, including data, audio, graphics and even video.

Bluetooth

Bluetooth Architecture

• Up to 8 devices can communicate in a small network, called piconet.• 10 piconets can coexist in the same coverage range of the Bluetooth

radio.• Each piconet has 1 MASTER and the rest serve as SLAVES.

SLAVES within a piconet only have links to the MASTER.• Multi-hop communication is obtained thru the scatternet.

• Does not address routing, most network functions are pushed into the link layer

• Does not support multi-hop multicasting

• Does not address how to cope with mobility !

• The MASTER node is the bottleneck

• No. of nodes in piconet is limited

• Does not address power-saving methods done at upper layers, above the link-layer

Bluetooth Limitations

Any kind of network simulation

Including mobile and wireless network simulation

NS : Network Simulator

OutlinesUse NS to simulate wireless network

Extend NS to support mobile and wireless application: Internal implementation

ns-2 overview•Collection of various protocols at multiple layers

TCP(reno, tahoe, vegas, sack)

MAC(802.11, 802.3, TDMA)

Ad-hoc Routing (DSDV, DSR, AODV, TORA)

Sensor Network (diffusion, gaf)

Multicast protocols, Satellite protocols, and many others

•Codes are contributed from multiple research communities

Good: Large set of simulation modules

Bad: Level of support and documentation varies

•The source code and documentation is currently maintained by VINT project at ISI

Introduction•ns-2 is an discrete event driven simulation

Physical activities are translated to events

Events are queued and processed in the order of their scheduled occurrences

Time progresses as the events are processed

1 2

Time: 1.5 sec Time: 1.7 sec

Time: 1.8 secTime: 2.0 sec

TX Pkt Event @ 1.5sec

Node 1 Module

Node 2 Module

TX Pkt Event @ 1.5sec

RX Pkt Event @ 1.7sec

RX Pkt Event @ 1.7sec

TX Ack Event @ 1.8sec

TX Ack Event @ 1.8sec

RX Ack Event @ 2.0sec

RX Ack Event @ 2.0sec

Event Queue

Simulation Finished!

Event Driven Simulation

Network Components inside a mobile nodeLink Layer

ARP

Interface Queue

Mac Layer: IEEE 802.11

Network Interface

Radio Propagation Model

Friss-space attenuation(1/ ) at near distance

Two ray Ground (1/ ) at far distance

2r2r

Mobile Node Modules

•AgentResponsible for packet generations and receptions

Can think of it as an Application layer

CBR(Constant Bit Rate), TCP, Sink, FTP, etc.

•RTagent(DSDV, TORA, AODV) or DSRAd-hoc network routing protocols

Configure multi hop routes for packets

•LL (Link Layer) Runs data link protocols

Fragmentation and reassembly of packet

Runs Address Resolution Protocol(ARP) to resolve IP address to MAC address conversions

Mobile Node Modules (Continued)

•IFq (Interface Queue)PriQueue is implemented to give priority to routing protocol packets

Supports filter to remove packets destined to specific address

•Mac Layer IEEE 802.11 protocol is implemented

Uses RTS/CTS/DATA/ACK pattern for all unicast pkts and DATA for broadcast pkts

Mobile Node Modules (Continued)

•NetIF (Network Interfaces)Hardware interface used by mobilenode to access the channel

Simulates signal integrity, collision, tx error

Mark each transmitted packet with transmission power, wavelength etc.

•Radio Propagation Model Uses Friss-space attenuation(1/r2) at near distance and Two ray ground (1/r4) at far distance

Decides whether the packet can be received by the mobilenode with given distance, transmit power and wavelength

Implements Omni Directional Antenna module which has unity gain for all direction

Wireless Simulation in ns-2 (Mobile Node Diagram - DSDV)

Agent(Src/ Sink)

Port

Demux

LL

I Fq

MAC

NetIFRadio

PropagationModel

Channel

ARP

Addr

Demux

RTagent(DSDV)

Agent(Src/ Sink)

Port

Demux

DSR

LL

IFq

MAC

NetIFRadio

PropagationModel

Channel

ARP

Wireless Simulation in ns-2 (Mobile Node Diagram - DSR)

Abstract the real mobile world into your simulationNode

Packets

Wireless channel and channel access

Forwarding and routing

Radio propagation model

Trace/Visualization

Event scheduler to make everything running

Implementing mobile node by Extending “standard” NS node

Classifier:Forwarding

Agent: Protocol Entity

Node Entry

Node

ARP

Radio Propagation Model

MobileNode

LL

MAC

PHY

LL

CHANNEL

LL

MAC

LL:Link layer object

IFQ:Interface queue

MAC:Mac object

PHY PHY:Net interface

Routing

ReferencesAnand Trivedi’s IEEE 802.11 Page

– http://alpha.fdu.edu/~anandt/introduction.html

• IEEE 802.11 Working Group Page– http://www.ieee802.org/11/– Can download the 802 standards here for FREE– Has links to all the latest 802.11 developments

• O’Reilly– http://oreilly.wirelessdevnet.com/

• http://wireless.telerama.com

THAT’S ALL