Lecture 13 and 14: WLAN

Anders Västberg

vastberg@kth.se

08-790 44 55

Slides are a selection from the slides from chapter 12,13, and 14 from:http://williamstallings.com/Wireless/Wireless2e.html

Mobile IP Uses

• Enable computers to maintain Internet connectivity while moving from one Internet attachment point to another

• Mobile – user's point of attachment changes dynamically and all connections are automatically maintained despite the change

• Nomadic - user's Internet connection is terminated each time the user moves and a new connection is initiated when the user dials back in– New, temporary IP address is assigned

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 – foreign

network• Mobile node registers with network node on foreign

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

network – home agent

Capabilities of Mobile IP

• Discovery – mobile node uses discovery procedure to identify prospective home and foreign agents

• Registration – mobile node uses an authenticated registration procedure to inform home agent of its care-of address

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

Discovery

• Mobile node is responsible for ongoing discovery process– Must determine if it is attached to its home network or a

foreign network

• Transition from home network to foreign network can occur at any time without notification to the network layer

• Mobile node listens for agent advertisement messages– Compares network portion of the router's IP address with

the network portion of home address

Agent Solicitation

• Foreign agents are expected to issue agent advertisement messages periodically

• If a mobile node needs agent information immediately, it can issue ICMP router solicitation message– Any agent receiving this message will then issue

an agent advertisement

Move Detection

• Mobile node may move from one network to another due to some handoff mechanism without IP level being 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 same network as the node's current care-of address

Co-Located Addresses

• If mobile node moves to a network that has no foreign agents, or all foreign agents are busy, it can act as its own foreign agent

• Mobile agent uses co-located care-of address– IP address obtained by mobile node associated with mobile

node's current network interface

• Means to acquire co-located address:– Temporary IP address through an Internet service, such as

DHCP– May be owned by the mobile node as a long-term address

for use while visiting a given foreign network

Registration Process

• Mobile node sends registration request to foreign 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

Registration Operation Messages

• 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

Registration Procedure Security

• 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 to itself

– Agent replays old registration messages to cut mobile node from network

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

authenticator

Types of Authentication Extensions

• Mobile-home – provides for authentication of registration messages between mobile node and home agent; must be present

• Mobile-foreign – may be present when a security association exists between mobile node and foreign agent

• Foreign-home – may be present when a security association exists between foreign agent and home agent

Tunneling

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

that datagrams to mobile node should be delivered to home agent

• Datagrams forwarded to care-of address via tunneling– Datagram encapsulated in outer IP datagram

Mobile IP Encapsulation Options

• IP-within-IP – entire IP datagram becomes payload 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

Wireless LAN Applications

• LAN Extension

• Cross-building interconnect

• Nomadic Access

• Ad hoc networking

Wireless LAN Configurations

LAN Extension

• Wireless LAN linked into a wired LAN on same premises– Wired LAN

• Backbone• Support servers and stationary workstations

– Wireless LAN• Stations in large open areas• Manufacturing plants, stock exchange trading floors,

and warehouses

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

Nomadic Access

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

• Uses:– Transfer data from portable computer to office

server– Extended environment such as campus

Ad Hoc Networking

• Temporary peer-to-peer network set up to meet immediate need

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

meeting; employees link computers in a temporary network for duration of meeting

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

Wireless LAN Categories

• Infrared (IR) LANs

• Spread spectrum LANs

• Narrowband microwave

Strengths of Infrared Over Microwave 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

Drawbacks of Infrared Medium

• Indoor environments experience infrared background radiation– Sunlight and indoor lighting– Ambient radiation appears as noise in an infrared

receiver– Transmitters of higher power required

• Limited by concerns of eye safety and excessive power consumption

– Limits range

Spread Spectrum LAN Configuration

• 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

IEEE 802 Protocol Layers

Protocol Architecture

• Functions of physical layer:– Encoding/decoding of signals– Preamble generation/removal (for synchronization)– Bit transmission/reception– Includes specification of the transmission medium

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 address recognition 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 and

error control

Separation of LLC and MAC

• The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control

• For the same LLC, several MAC options may be provided

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

Logical Link Control

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

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

layer

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

Differences between LLC and HDLC

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

• LLC supports unacknowledged connectionless service (type 1 operation)

• LLC supports acknowledged connectionless service (type 3 operation)

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

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

IEEE 802.11 Services

Distribution of Messages Within 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

Transition Types Based On Mobility

• 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 BSS

within another ESS

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

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

IEEE 802.11 Medium Access Control

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

Reliable Data Delivery

• More efficient to deal with errors at the MAC level than 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

Access Control

Medium Access Control Logic

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

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

MAC Frame Format

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

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

Frame Control Fields

• Power management – 1 if transmitting station is in sleep 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

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

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

Management Frame Subtypes

• Association request

• Association response

• Reassociation request

• Reassociation response

• Probe request

• Probe response

• Beacon

Management Frame Subtypes

• Announcement traffic indication message

• Dissociation

• Authentication

• Deauthentication

Wired Equivalent Privacy

Authentication

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

• Shared Key authentication– Shared Key assures authentication

Physical Media Defined by Original 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 802.11a and IEEE 802.11b

• 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 802.11g and IEEE 802.11n

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

• IEEE 802.11n: Even higher data rates.