ch 6. wireless lans myungchul kim [email protected]
TRANSCRIPT
SESSION: Wireless LANs
•Wireless LAN Overview •Wireless Network Technologies (infrared, spread spectrum, microwave)•IEEE 802.11 Overview •802 MAC•802 Physical Layer•Mobile Adhoc Networks
Wireless LANs• First generation of products at about 1-2 Mbps
– Lucent’s WaveLAN, RadioLAN, etc.
– factor of 10 less bandwidth than current Ethernet
• Next generation of products at 10-11 Mbps– factor of 10 less bandwidth than 100 Mbps Ethernet
• IEEE 802.11 standard
• Important niche and enterprise applications (e.g. hospitals)
• Increasing horizontal market interest
• Forecast: Total worldwide wireless LAN market revenues: $305.4M (1998) to $1.63B by 2005 -- Frost & Sullivan.
Wireless LAN (Cell size 10 Meters to 100 Meters)
Wireless LANs
Wired LAN
Access Point
Wireless LAN Applications• LAN Extension: Wireless LAN linked into a wired LAN on same
premises– Wired LAN for Backbone– Wireless LAN (Stations in large open areas)
• Cross-building interconnect: Connect LANs in nearby buildings– Point-to-point wireless (Devices connected are typically bridges or routers)
• Nomadic Access: Wireless link between LAN hub and mobile data terminal equipped with antenna
• Ad hoc networking: Temporary peer-to-peer network set up to meet immediate need– Example: link computers in a temporary network for duration of meeting
Wireless LAN Configurations
Wireless LAN1
(peer-to-peer)
Wired
LAN1
= Wireless LAN Adapter
Wireless connection
Wireless connection
Wireless LAN2
(peer-to-peer)
Access Pointas a repeater
Wireless LAN3
(Master/slave)
Access Point
Access Point
WirelessLAN-LANBridge
Wired
LAN2
Personal AreaNetwork (PAN)
Wireless LAN Requirements• Throughput - more work completed per unit time • Number of nodes - hundreds across cells• Connection to backbone LAN - for corporate support• Service area - 100+ meters• Battery power consumption - sleep when not in use• Transmission robustness and security- reliable transmission, and maintain
security • Collocated network operation - minimize interference between
neighboring networks• License-free operation - better to operate without licensed frequencies • Handoff/roaming - MAC protocol should support smooth handoffs• Dynamic configuration - MAC addressing should support automatic
addition and deletion of addresses
LAN Stack
Upper Layers
Logical Link Control(IEEE 802.2)
Media Access Control (MAC)
IEEE802.3 Carrier Sense (Ethernet)
IEEE802.4 Token Bus
IEEE802.5 Token Ring
IEEE802.11Wireless OSI
Layer 1(Physical)
OSILayer 2(Data Link)
Physical
OSI Layers3-7
OSI StackLAN Stack
IEEE 802 Standards 802.1: High Level Interface 802.2: Logical Link Control 802.3: CSMA/CD Networks 802.4: Token Bus Networks 802.5: Token Ring Networks 802.6: Metropolitan Area Networks 802.7: Broadband Networks 802.8: Fiber Optic Networks 802.9: Integrated Data and Voice Networks 802.10 Virtual LANs 802.11 Wireless LANs 802.12 Communication media\ 802.14 Data transport over traditional cable TV network 802.15; personal area networks 802.16 Wireless Local Loops
Wireless LAN Technologies
Wireless LAN
Communication technologies
• Infra red
• Spread spectrum
• Narrowband
= Wireless LAN Adapter
Wireless connection
Wireless connection
Access Pointas a connector
Wired LAN
Wireless LAN
Communication technologies
• Infra red
• Spread spectrum
• Narrowband
Access Pointas a repeater
Roaming support
Wireless LAN Cell Wireless LAN Cell
Wireless LAN Cell
X
YZ
X, Y, Z are access points for the wireless LANs
Handoffs and roaming is accomplished through access points(not very smooth)
A Sample Wireless School
Wireless LAN Cell
Wireless LAN Cell
Wireless LAN Cell
CentrexRouter
Link to
Public Ethernet
T1orDSL
X
Y Z
LAN Server
Wired Ethernet LAN
•X, Y, Z are access points for the wireless Ethernet LANs
A B
C D
•A, B, C, D arestudent laptops
IEEE 802.11
IEEE 802.11 Wireless
IEEE 802.2 Logical Link Control
Upper Layers
IEEE 802.11 Wireless
IEEE 802.2 Logical Link Control
Upper Layers
Local Bridge(Access Point)
MACLayer_______
PhysicalLayer
Wireless LAN Categories• Spread spectrum LANs• Infrared (IR) LANs• Narrowband microwave
• Carrier current LANs (psuedo wireless LANs)– Does not require installation of network cables– Uses power cables and a powerline modem– Can be used to carry 1 to 2 Mbps data– Example: Radioshack Master Console to control coffee machine,
lamps, heating systems–
Spread Spectrum LANs• 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
• Hub topology– Mounted on the ceiling, connected to backbone– May control access and/or multiport repeater– Automatic handoff of mobile stations– Stations in cell either:
• Transmit to / receive from hub only• Broadcast using omnidirectional antenna
Infrared Over Microwave RadioStrengths:• 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: Indoor environments experience infrared background radiation– Sunlight and indoor lighting– Ambient radiation appears as noise – Transmitters of higher power required (safety)
IR Data Transmission Techniques• Directed Beam Infrared
– Used to create point-to-point links– Range depends on emitted power and degree of focusing– Focused IR data link can have range of kilometers
• Cross-building interconnect between bridges or routers• Ominidirectional
– Single base station in line of sight of all other stations on LAN– Station typically mounted on ceiling– Base station acts as a multiport repeater
• Ceiling transmitter broadcasts signal received by IR transceivers• IR transceivers transmit with directional beam aimed at ceiling base unit
• Diffused: All IR transmitters focused and aimed at a point on diffusely reflecting ceiling– IR radiation strikes ceiling: Reradiated omnidirectionally
• Picked up by all receivers
Narrowband Microwave LANs• Use of a microwave radio frequency band for signal transmission
• Relatively narrow bandwidth
• Licensed within specific geographic areas to avoid potential interference
– Motorola - 600 licenses in 18-GHz range
• Covers all metropolitan areas
• Can assure that LANs in nearby locations don’t interfere
• Encrypted transmissions prevent eavesdropping
• Unlicensed: Uses unlicensed ISM spectrum
– RadioLAN narrowband wireless LAN in 1995
• Low power (0.5 watts or less), Range = 50 m to 100 m
• Operates at 10 Mbps in the 5.8-GHz band
IEEE 802.11 Wireless LAN Standard
General LAN Protocol Architecture• Functions of physical layer:
– Encoding/decoding of signals– Preamble generation/removal (for synchronization)– Bit transmission/reception– Includes specification of the transmission medium
• 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
IEEE 802.11 Architecture
• Stations• Access point (AP)• Basic service set (BSS)
– Stations competing for access to shared wireless medium– Isolated or connected to backbone DS through AP
• Distribution system (DS) - interconnects several BSSs• Extended service set (ESS)
– Two or more basic service sets interconnected by DS
A Sample Wireless School -- IEEE 802.11 Terms
Wireless LAN Cell (BSS)
Wireless LAN Cell (BSS)
Wireless LAN Cell (BSS)
CentrexRouter
Link to
Public Ethernet
T1orDSL
X
Y Z
LAN Server
Wired Ethernet LAN(DS)•X, Y, Z are
access points for the wireless Ethernet LANs
A B
C D
•A, B, C, D arelaptops (Stations)
Extended service set (ESS)
802.11 Stack
Logical Link Control
Distributed Coordination Function(DCF)
MAC Layer
2.4 GHzfrequency hoppingspread
spectrum1 Mbps2 Mbps
Infraredomni-
directional1 Mbps2 Mbps
5-GhzOrthogonal
FDM6,9,12
18,24,36,48, 54 Mbps
PhysicalLayer
2.4 GHzdirect
sequencespread
spectrum1 Mbps2 Mbps
2.4 GHzdirectsequencespread spectrum5.5 Mbps11 Mbps
IEEE 802.11 IEEE 802.11a 802.11b
Point Coordination Function(PCF)
ContentionService
Contention-Free Service
2.4 GHzOrthogonalFDM
Up to54 Mbps
802.11g
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 802.11 Family • 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– Makes use of 2.4-GHz band– Provides data rates of 5.5 and 11 Mbps– Uses Direct sequence Spread Spectrum
• IEEE 802.11g– Makes use of 2.4-GHz band– Provides data rates of 54 Mbps an higher
IEEE 802.11 Medium Access Control
• MAC layer covers three functional areas:– Reliable data delivery– Access control– Security
Reliable Data Delivery• MAC must handle interference, noise, fading, etc• 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
• Approaches proposed and accepted: – Distributed Coordination Function (required)
• Uses Ethernet type CSMA
• Useful in adhoc networks
– Point Coordination Function (optional)• Implemented on top of DCF
• Uses polling, typically done by an access point
• Suitable for a centralized LAN
Wireless LAN Configurations
Wireless LAN1
(peer-to-peer)
Wired
LAN1
= Wireless LAN Adapter
Wireless connection
Wireless connection
Wireless LAN2
(peer-to-peer)
Access Pointas a repeater
Wireless LAN3
(Master/slave)
Access Point
Access Point
WirelessLAN-LANBridge
Wired
LAN2
Personal AreaNetwork (PAN)
Interframe Space (IFS) DCF includes a set of delays (IFSs) with the following values: • Short IFS (SIFS)
– Shortest IFS– Used for immediate response actions (high priority)
• 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 for ordinary asynchronous frames
contending for access
• Extended IFS: Long period for errors
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
Management Frames Used to manage communications between stations and Aps with subtypes
(802.11 MIB): • Association request - from station to AP (includes security requirement)• Association response - AP to station• Reassociation request - station to AP when moved from one BSS, interactions
with old AP • Reassociation response - AP to station • Probe request - locate a BSS (similar to ping) • Probe response - • Beacon - periodic signals to identify a BSS • Authentication - exchange of authentication frames • Deauthentication - terminate authentication
Wired Equivalent Privacy Algorithm (WEP) • WEP intended to provide modest security for 802.11 LANs
• Uses an encryption algorithm based on RC4
• Uses the following steps: – Integrity algorithm creates and appends a CRC
– A pseudo random number generator (PRNG) is used to generate a ciphertext
• Authentication in 802.11– Open system authentication
• Exchange of identities, no security benefits
– Shared Secret Key authentication (secure)
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
As compared to wired LANs, wireless LANs must serve usersnot connected to the network
Need special services:
Mobile Adhoc Networks Overview
Mobile Station
Mobile Station
Mobile Station
Mobile Station
Mobile Station
Mobile Station
- Networks without any access points or fixed network infrastructure-Nodes “discover” each other when in vicinity and communicate -Bluetooth and wireless sensor networks are major examples
HiperLAN Type 2 or HiperLAN2
Wireless LAN standard developed by the European Telecommunications Standards Institute (ETSI).
• - Data rate of 54 Mbps • - High level of security• - QoS capabilities to support virtually any type
of service or application• - High and scalable capacity as the number of
users increase in the system
Summary
•Wireless LAN Overview •Wireless Network Technologies (infrared, spread spectrum, microwave)•IEEE 802.11 Overview •802 MAC•802 Physical Layer•MANETs