LectA..ppt - 04/06/05 CDA 6505 Network Architecture and Client/Server Computing Wireless LANs by Zornitza Genova Prodanoff

Wireless LAN Applications LAN Extension Cross-building interconnect Nomadic Access Ad hoc networking 02

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

Multiple-cell Wireless LAN

05 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

06 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

07 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

08 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

09 Wireless LAN Categories Infrared (IR) LANs Spread spectrum LANs Narrowband microwave

010 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

011 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

012 IR Data Transmission Techniques Directed Beam Infrared Ominidirectional Diffused

013 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

014 Ominidirectional Single base station within 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

015 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

016 Spread Spectrum LAN Configuration Multiple-cell arrangement (Figure 13.2) 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

017 Spread Spectrum LAN Configuration Hub topology –Mounted on the ceiling and connected to backbone –May control access –May act as multiport repeater –Automatic handoff of mobile stations –Stations in cell either: Transmit to / receive from hub only Broadcast using omnidirectional antenna

018 Narrowband Microwave LANs Use of a microwave radio frequency band for signal transmission Relatively narrow bandwidth Licensed Unlicensed

019 Licensed Narrowband RF Licensed within specific geographic areas to avoid potential interference Motorola licenses in 18-GHz range –Covers all metropolitan areas –Can assure that independent LANs in nearby locations don’t interfere –Encrypted transmissions prevent eavesdropping

020 Unlicensed Narrowband RF RadioLAN introduced narrowband wireless LAN in 1995 –Uses unlicensed ISM spectrum –Used at low power (0.5 watts or less) –Operates at 10 Mbps in the 5.8-GHz band –Range = 50 m to 100 m

IEEE 802 Protocol Layers

022 IEEE Wireless 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

023 IEEE Wireless LAN 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

024 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

025 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

026 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

027 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

028 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)

029 IEEE 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 Services

031 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 LAN and station on integrated IEEE 802.x LAN

032 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

033 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

034 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

035 IEEE Medium Access Control MAC layer covers three functional areas: –Reliable data delivery –Access control –Security

036 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

039 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

040 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

042 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

043 Frame Control Fields Protocol version – 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

044 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

045 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

046 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

047 Management Frame Subtypes Association request Association response Reassociation request Reassociation response Probe request Probe response Beacon

048 Management Frame Subtypes Announcement traffic indication message Dissociation Authentication Deauthentication

Wired Equivalent Privacy

050 Authentication Open system authentication –Exchange of identities, no security benefits Shared Key authentication –Shared Key assures authentication

051 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

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