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Lecture 13 and 14: WLAN

Anders Västberg

[email protected] 08-790 44 55

Slides are a selection from the slides from

chapter 12,13, and 14 from:http://williamstallings.com/Wireless/Wireless2e.html

mailto:[email protected]

mailto:[email protected]

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Mobile IP Uses

• Enable computers to maintain Internet connectivitywhile moving from one Internet attachment point toanother

• Mobile – user's point of attachment changesdynamically and all connections are automaticallymaintained despite the change

• Nomadic - user's Internet connection is terminated

each time the user moves and a new connection isinitiated when the user dials back in – New, temporary IP address is assigned

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Operation of Mobile IP

• Mobil node is assigned to a particular network – home network

• IP address on home network is static – home address

• Mobile node can move to another network – foreignnetwork • Mobile node registers with network node on foreign

network – foreign agent

• Mobile node gives care-of address to agent on homenetwork – home agent

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Capabilities of Mobile IP

• Discovery – mobile node uses discoveryprocedure to identify prospective home andforeign agents

• Registration – mobile node uses anauthenticated registration procedure to informhome agent of its care-of address

• Tunneling – used to forward IP datagramsfrom a home address to a care-of address

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Agent Solicitation

• Foreign agents are expected to issue agentadvertisement messages periodically

• If a mobile node needs agent informationimmediately, it can issue ICMP routersolicitation message – Any agent receiving this message will then issue

an agent advertisement

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Move Detection

• Mobile node may move from one network to anotherdue to some handoff mechanism without IP levelbeing aware – Agent discovery process is intended to enable the agent to

detect such a move

• Algorithms to detect move: – Use of lifetime field – mobile node uses lifetime field as a

timer for agent advertisements – Use of network prefix – mobile node checks if any newly

received agent advertisement messages are on the samenetwork as the node's current care-of address

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Co-Located Addresses

• If mobile node moves to a network that has noforeign agents, or all foreign agents are busy, it canact as its own foreign agent

• Mobile agent uses co-located care-of address – IP address obtained by mobile node associated with mobilenode's current network interface

• Means to acquire co-located address:

– Temporary IP address through an Internet service, such asDHCP – May be owned by the mobile node as a long-term address

for use while visiting a given foreign network

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Registration Process

• Mobile node sends registration request toforeign agent requesting forwarding service

• Foreign agent relays request to home agent• Home agent accepts or denies request and

sends registration reply to foreign agent

• Foreign agent relays reply to mobile node

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Registration OperationMessages

• Registration request message – Fields = type, S, B, D, M, V, G, lifetime, home

address, home agent, care-of-address,identification, extensions

• Registration reply message – Fields = type, code, lifetime, home address, home

agent, identification, extensions

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Registration ProcedureSecurity

• Mobile IP designed to resist attacks – Node pretending to be a foreign agent sends registration

request to a home agent to divert mobile node traffic toitself

– Agent replays old registration messages to cut mobile nodefrom network

• For message authentication, registration request andreply contain authentication extension – Fields = type, length, security parameter index (SPI),

authenticator

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Tunneling

• Home agent intercepts IP datagrams sent tomobile node's home address – Home agent informs other nodes on home network

that datagrams to mobile node should be deliveredto home agent

• Datagrams forwarded to care-of address via

tunneling – Datagram encapsulated in outer IP datagram

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Mobile IP EncapsulationOptions

• IP-within-IP – entire IP datagram becomespayload in new IP datagram – Original, inner IP header unchanged except TTL

decremented by 1

– Outer header is a full IP header• Minimal encapsulation – new header is inserted

between original IP header and original IP payload – Original IP header modified to form new outer IP

header• Generic routing encapsulation (GRE) – developed

prior to development of Mobile IP

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Wireless LAN Applications

• LAN Extension• Cross-building interconnect

• Nomadic Access• Ad hoc networking

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Wireless LAN

Configurations

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LAN Extension

• Wireless LAN linked into a wired LAN onsame premises – Wired LAN

• Backbone• Support servers and stationary workstations

– Wireless LAN• Stations in large open areas

• Manufacturing plants, stock exchange trading floors,and warehouses

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Cross-Building Interconnect

• Connect LANs in nearby buildings – Wired or wireless LANs

• Point-to-point wireless link is used• Devices connected are typically bridges or

routers

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Nomadic Access

• Wireless link between LAN hub and mobiledata terminal equipped with antenna – Laptop computer or notepad computer

• Uses: – Transfer data from portable computer to office

server

– Extended environment such as campus

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Ad Hoc Networking

• Temporary peer-to-peer network set up to meetimmediate need

• Example: – Group of employees with laptops convene for a

meeting; employees link computers in a temporarynetwork for duration of meeting

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Wireless LAN Requirements

• Throughput• Number of nodes• Connection to backbone LAN

• Service area• Battery power consumption• Transmission robustness and security• Collocated network operation• License-free operation• Handoff/roaming• Dynamic configuration

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Wireless LAN Categories

• Infrared (IR) LANs• Spread spectrum LANs

• Narrowband microwave

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Strengths of Infrared OverMicrowave Radio

• Spectrum for infrared virtually unlimited – Possibility of high data rates

• Infrared spectrum unregulated

• Equipment inexpensive and simple• Reflected by light-colored objects

– Ceiling reflection for entire room coverage

• Doesn’t penetrate walls – More easily secured against eavesdropping – Less interference between different rooms

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Drawbacks of InfraredMedium

• Indoor environments experience infraredbackground radiation – Sunlight and indoor lighting

– Ambient radiation appears as noise in an infraredreceiver

– Transmitters of higher power required• Limited by concerns of eye safety and excessive power

consumption – Limits range

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Spread Spectrum LANConfiguration

• Multiple-cell arrangement• Within a cell, either peer-to-peer or hub

• Peer-to-peer topology – No hub – Access controlled with MAC algorithm

• CSMA

– Appropriate for ad hoc LANs

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IEEE 802 Protocol Layers

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Protocol Architecture

• Functions of medium access control (MAC) layer: – On transmission, assemble data into a frame with address

and error detection fields

– On reception, disassemble frame and perform addressrecognition and error detection – Govern access to the LAN transmission medium

• Functions of logical link control (LLC) Layer:

– Provide an interface to higher layers and perform flow anderror control

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Separation of LLC and MAC

• The logic required to manage access to ashared-access medium not found in traditionallayer 2 data link control

• For the same LLC, several MAC options maybe provided

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MAC Frame Format

• MAC control – Contains Mac protocol information

• Destination MAC address – Destination physical attachment point

• Source MAC address – Source physical attachment point

• CRC – Cyclic redundancy check

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Logical Link Control

• Characteristics of LLC not shared by othercontrol protocols: – Must support multiaccess, shared-medium nature

of the link – Relieved of some details of link access by MAC

layer

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LLC Services

• Unacknowledged connectionless service – No flow- and error-control mechanisms – Data delivery not guaranteed

• Connection-mode service – Logical connection set up between two users – Flow- and error-control provided

• Acknowledged connectionless service

– Cross between previous two – Datagrams acknowledged – No prior logical setup

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Differences between LLCand HDLC

• LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation)

• LLC supports unacknowledged connectionlessservice (type 1 operation)

• LLC supports acknowledged connectionlessservice (type 3 operation)

• LLC permits multiplexing by the use of LLCservice access points (LSAPs)

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IEEE 802.11 Architecture

• Distribution system (DS)• Access point (AP)• Basic service set (BSS)

– Stations competing for access to shared wireless medium – Isolated or connected to backbone DS through AP

• Extended service set (ESS)

– Two or more basic service sets interconnected by DS

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IEEE 802.11 Services

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Distribution of MessagesWithin a DS

• Distribution service – Used to exchange MAC frames from station in one

BSS to station in another BSS

• Integration service – Transfer of data between station on IEEE 802.11

LAN and station on integrated IEEE 802.x LAN

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Transition Types Based OnMobility

• No transition – Stationary or moves only within BSS

• BSS transition – Station moving from one BSS to another BSS in

same ESS

• ESS transition

– Station moving from BSS in one ESS to BSSwithin another ESS

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Association-Related Services

• Association – Establishes initial association between station and AP

• Reassociation

– Enables transfer of association from one AP to another,allowing station to move from one BSS to another

• Disassociation – Association termination notice from station or AP

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Access and Privacy Services

• Authentication – Establishes identity of stations to each other

• Deathentication – Invoked when existing authentication is terminated

• Privacy – Prevents message contents from being read by

unintended recipient

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IEEE 802.11 MediumAccess Control

• MAC layer covers three functional areas: – Reliable data delivery – Access control – Security

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Reliable Data Delivery

• More efficient to deal with errors at the MAC levelthan higher layer (such as TCP)

• Frame exchange protocol – Source station transmits data – Destination responds with acknowledgment (ACK) – If source doesn’t receive ACK, it retransmits frame

• Four frame exchange

– Source issues request to send (RTS) – Destination responds with clear to send (CTS) – Source transmits data – Destination responds with ACK

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Access Control

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Medium Access ControlLogic

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Interframe Space (IFS)Values

• Short IFS (SIFS) – Shortest IFS – Used for immediate response actions

• Point coordination function IFS (PIFS) – Midlength IFS – Used by centralized controller in PCF scheme when using

polls

• Distributed coordination function IFS (DIFS) – Longest IFS – Used as minimum delay of asynchronous frames

contending for access

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IFS Usage

• SIFS – Acknowledgment (ACK) – Clear to send (CTS) – Poll response

• PIFS – Used by centralized controller in issuing polls – Takes precedence over normal contention traffic

• DIFS – Used for all ordinary asynchronous traffic

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MAC Frame Format

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MAC Frame Fields

• Frame Control – frame type, control information• Duration/connection ID – channel allocation time• Addresses – context dependant, types include source

and destination• Sequence control – numbering and reassembly• Frame body – MSDU or fragment of MSDU

• Frame check sequence – 32-bit CRC

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Frame Control Fields

• Protocol version – 802.11 version• Type – control, management, or data• Subtype – identifies function of frame

• To DS – 1 if destined for DS• From DS – 1 if leaving DS• More fragments – 1 if fragments follow

• Retry – 1 if retransmission of previous frame

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Frame Control Fields

• Power management – 1 if transmitting station is insleep mode

• More data – Indicates that station has more data to

send• WEP – 1 if wired equivalent protocol is implemented• Order – 1 if any data frame is sent using the Strictly

Ordered service

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Control Frame Subtypes

• Power save – poll (PS-Poll)• Request to send (RTS)• Clear to send (CTS)• Acknowledgment• Contention-free (CF)-end

• CF-end + CF-ack

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Data Frame Subtypes

• Data-carrying frames – Data – Data + CF-Ack – Data + CF-Poll – Data + CF-Ack + CF-Poll

• Other subtypes (don’t carry user data) – Null Function – CF-Ack – CF-Poll – CF-Ack + CF-Poll

M F

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Management FrameSubtypes

• Association request• Association response• Reassociation request• Reassociation response• Probe request

• Probe response• Beacon

M F

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Management FrameSubtypes

• Announcement traffic indication message• Dissociation• Authentication• Deauthentication

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Wired Equivalent Privacy

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Authentication

• Open system authentication – Exchange of identities, no security benefits

• Shared Key authentication – Shared Key assures authentication

Ph i l M di D fi d b

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Physical Media Defined byOriginal 802.11 Standard

• Direct-sequence spread spectrum – Operating in 2.4 GHz ISM band – Data rates of 1 and 2 Mbps

• Frequency-hopping spread spectrum – Operating in 2.4 GHz ISM band – Data rates of 1 and 2 Mbps

• Infrared

– 1 and 2 Mbps – Wavelength between 850 and 950 nm

IEEE 802 11 d IEEE

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IEEE 802.11a and IEEE802.11b

• IEEE 802.11a – Makes use of 5-GHz band – Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps – Uses orthogonal frequency division multiplexing (OFDM) – Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-

QAM

• IEEE 802.11b – Provides data rates of 5.5 and 11 Mbps – Complementary code keying (CCK) modulation scheme

IEEE 802 11 d IEEE

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IEEE 802.11g and IEEE802.11n

• IEEE 802.11g: Provides data rates up to 108Mbps and is compatible with 802.11b

• IEEE 802.11n: Even higher data rates.

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