Ch 6. Wireless LANs Myungchul Kim

SESSION: Wireless LANs Wireless LAN Overview Wireless Network Technologies (infrared, spread spectrum, microwave) IEEE 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 Mbps –factor of 10 less bandwidth than 100 Mbps Ethernet IEEE 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 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 LAN2 (peer-to-peer) Access Point as a repeater Wireless LAN3 (Master/slave) Access Point Wireless LAN-LAN Bridge Wired LAN2 Personal Area Network (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 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 IEEE Wireless OSI Layer 1 (Physical) OSI Layer 2 (Data Link) Physical OSI Layers 3-7 OSI Stack LAN 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  Virtual LANs  Wireless LANs  Communication media\  Data transport over traditional cable TV network  ; personal area networks  Wireless Local Loops

Wireless LAN Technologies Wireless LAN Communication technologies Infra red Spread spectrum Narrowband = Wireless LAN Adapter Wireless connection Access Point as a connector Wired LAN Wireless LAN Communication technologies Infra red Spread spectrum Narrowband Access Point as a repeater

Roaming support Wireless LAN Cell X Y Z 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 Centrex Router Link to Public Ethernet T1 or DSL X Y Z LAN Server Wired Ethernet LAN X, Y, Z are access points for the wireless Ethernet LANs A B CD A, B, C, D are student laptops

IEEE Wireless IEEE Logical Link Control Upper Layers IEEE Wireless IEEE Logical Link Control Upper Layers Local Bridge (Access Point) MAC Layer _______ Physical Layer

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 Radio Strengths: 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 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 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 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 Terms Wireless LAN Cell (BSS) Centrex Router Link to Public Ethernet T1 or DSL X Y Z LAN Server Wired Ethernet LAN (DS) X, Y, Z are access points for the wireless Ethernet LANs A B CD A, B, C, D are laptops (Stations) Extended service set (ESS)

Stack Logical Link Control Distributed Coordination Function (DCF) MAC Layer 2.4 GHz frequency hopping spread spectrum 1 Mbps 2 Mbps Infrared omni- directional 1 Mbps 2 Mbps 5-Ghz Orthogonal FDM 6,9,12 18,24,36, 48, 54 Mbps Physical Layer 2.4 GHz direct sequence spread spectrum 1 Mbps 2 Mbps 2.4 GHz direct sequence spread spectrum 5.5 Mbps 11 Mbps IEEE IEEE a802.11b Point Coordination Function (PCF) Contention Service Contention-Free Service 2.4 GHz Orthogonal FDM Up to 54 Mbps g

Physical Media Defined by Original 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 Family IEEE a –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 b –Makes use of 2.4-GHz band –Provides data rates of 5.5 and 11 Mbps –Uses Direct sequence Spread Spectrum IEEE g –Makes use of 2.4-GHz band –Provides data rates of 54 Mbps an higher

IEEE 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 LAN2 (peer-to-peer) Access Point as a repeater Wireless LAN3 (Master/slave) Access Point Wireless LAN-LAN Bridge Wired LAN2 Personal Area Network (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 ( 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 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 –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 users not connected to the network Need special services:

Mobile Adhoc Networks Overview - 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 Overview 802 MAC 802 Physical Layer MANETs