Wireless Personal Communications Systems – CSE5807 Lecture: 06 Stephen Giles and Satha K. Sathananthan School of Computer Science and Software Engineering Monash University Australia These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .

Wireless LAN Applications LAN Extension: Wireless LAN linked into a wired LAN on same premises. Cross-building interconnect: Connecting wired or wireless LANs by point-to-point wireless link. 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.

Wireless LAN AP WAN Backbone LAN Router

Wireless LAN Ad hoc networks Infrastructure networks WAN LAN Access Point LAN WAN

Wireless LAN Requirements Throughput Number of nodes Connection to backbone LAN Service area Battery power consumption Transmission robustness and security License-free operation Handoff/roaming Dynamic configuration

Wireless LAN Categories Microwave Radio Infrared (IR) Directed Omini-directional Diffused Spread spectrum Narrowband

Infrared Data Transmission Techniques Directed Beam Infrared: Used to create point-to-point links. Range depends on emitted power and degree of focusing. Focused infrared data link can have range of kilometers. Cross-building interconnect between bridges or routers. Ominidirectional: Single base station within line of sight of all other stations on LAN. Base station broadcasts signal that can be received by infrared transceivers. Infrared transceivers transmit with directional beam aimed at base station. Diffused: All infrared transmitters focused and aimed at a point on diffusely reflecting ceiling Infrared radiation strikes ceiling and reradiated omnidirectionally. Picked up by all receivers.

Infrared Transmission Advantages: Spectrum for infrared virtually unlimited => 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. Disadvantages: Indoor environments experience infrared background radiation. 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. Hub topology: Mounted on the ceiling and connected to backbone. May control access and act as multiport repeater. Automatic handoff of mobile stations. Stations in cell either: Transmit to / receive from hub only.

Narrowband Microwave LAN Use of a microwave radio frequency band for signal transmission. Relatively narrow bandwidth. Licensed: Licensed within specific geographic areas to avoid potential interference. Motorola - 600 licenses in 18-GHz range Encrypted transmissions prevent eavesdropping. Unlicensed: RadioLAN introduced narrowband wireless LAN in 1995. Uses unlicensed ISM spectrum => Operates at 10 Mbps in the 5.8-GHz band Used at low power (0.5 watts or less). Range = 50 m to 100 m.

License-Free Bands No permission required for bandwidth usage. No licensing cost. Limit on power of transmission exists. Potential interference is high. Industrial Scientific Medical (ISM) bands: 900 MHz ISM Band 902 MHz – 928 MHz => Used in wireless home phones and wireless camera systems. 2.4 GHz ISM Band 2.4 GHz – 2.5 GHz => Used by IEEE802.11, IEEE802.11b and IEEE802.11g devices. 5.8 GHz ISM Band 5.725 GHz– 5.875 GHz Unlicensed national Information Infrastructure (UNNI) Bands: Lower Band 5.15 GHz – 5.25 GHz Middle Band 5.25 GHz – 5.35 GHz Upper Band 5.725 GHz – 5.825 GHz

WLAN Organizations Institute of Electrical and Electronic Engineers (IEEE). European Telecommunications Standards Institute (ETSI) The Wi-Fi Alliance Wireless LAN Association (WLANA) Infrared Data Association (IrDA)

WLAN Standards IEEE802.11 family of standards. HiperLAN HomeRF IEEE802.11a IEEE802.11b IEEE802.11g HiperLAN HiperLAN 1 HiperLAN 2 HomeRF

Other IEEE802.11 Standards IEEE802.11c IEEE802.11e IEEE802.11f Define MAC procedure for the bridge operation. IEEE802.11e Enhance current 802.11 MAC to expand support for applications with QoS requirements. IEEE802.11f Define procedure for Inter Access Point Protocol (IAPP). IEEE802.11i Enhance the 802.11 MAC to enhance security and authentication mechanisms.

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 and topology.

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.

IEEE 802.11 Services

MAC Frame Format MAC control Destination MAC address Contains MAC protocol information. Destination MAC address Source MAC address Data Cyclic Redundancy Check (CRC)

Logical Link Control Properties not shared by other control protocols: Must support multi-access, 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

IEEE 802.11 Architecture Access point (AP): Basic service set (BSS) : MAC protocol by a central coordination function. Basic service set (BSS) : Stations competing for access to shared wireless medium. Isolated or connected to backbone distribution system (DS) through AP. Distribution system (DS): Can be a switch, a wired network or a wireless network. Extended service set (ESS): Two or more basic service sets interconnected by DS.

IEEE 802.11 Architecture AP WAN Backbone LAN Router BSS BSS BSS ESS

IEEE 802.11 Services Service Provider Used to support Association Distribution System MSDU delivery Authentication Station/AP LAN access and security Deauthentication Disassociation Distribution Integration Privacy Reassociation MSDU – MAC Service Data Unit

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.

Association-Related Services Before DS can deliver data to or accept data from a station, that station must be associated. Three transition types: 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 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. Deauthentication 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 Distributed Foundation Wireless MAC (DFWMAC). Distributed Access: Attractive for ad hoc network and bursty traffic. Handled by lower sublayer of the MAC layer, Distributed Coordination Function (DCF). Use CSMA. Ordinary asynchronous traffic directly uses DCF. DCF includes a set of delays to provide priority scheme. Optional Centralized Access: Point Coordination Function (PCF) provides contention free service. PCF is built on top of DCF.

Access Control

Medium Access Control Logic

Interframe Space (IFS) Values Short IFS (SIFS): Shortest IFS Used for immediate response actions. Acknowledgment (ACK) Clear to send (CTS) Poll response Point coordination function IFS (PIFS): Midlength IFS. Used by centralized controller in PCF scheme when using polls. Takes precedence over normal contention traffic. Distributed coordination function IFS (DIFS): Longest IFS Used as minimum delay of asynchronous frames contending for access Used for all ordinary asynchronous traffic.

Communication Process Beacon Contention-Free Period (PCF Mode) Contention Period (DCF Mode) Superframe PIFS DIFS Contention Period time Stations in DCF mode contend for access here. AP seizes control of medium here.

MAC Frame Format

Control Frames Data Frames Power save – poll (PS-Poll) Request to send (RTS) Clear to send (CTS) Acknowledgment Contention-free (CF)-end CF-end + CF-ack Data Frames Data-carrying frames Data Data + CF-Ack Data + CF-Poll Data + CF-Ack + CF-Poll

Management Frame Beacon Probe request Probe response Authentication Deauthentication Association request Association response Reassociation request Reassociation response Dissociation Announcement traffic indication message

Required Reading Reference W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2000. >> Chapter 13 & 14 Reference K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.