Chapter 15 Wireless LANs

IEEE 802 Standards Working Groups WiMAX, an acronym that stands for Worldwide Interoperability for Microwave Access(802.16) WiFi (Wireless Fidelity) : 802.11 TAG(Technical Advisory Grooup)

Architecture Physical Layer MAC Layer Addressing Mechanism Introduction Architecture Physical Layer MAC Layer Addressing Mechanism

15.1 IEEE 802.11 - Architecture Basic Service Set (BSS) Extended Service Set (ESS)

Basic Service Set made up of stationary or mobile wireless stations, and possibly a central base station (access point) BSS without an AP is stand-alone, cannot send data to other BSSs

ESS – Extended Service Set Made up of two or more BSSs with APs – connected through a distribution system (usually a wired LAN)

Station Types No-transition mobility – either stationary or moving only inside a BSS BSS-transition mobility – can move from one BSS to another, confined to one ESS ESS-transition mobility – can move from one ESS to another

Physical layer specifications 802.11 defines the specs for the conversion of bits to a signal

FHSS – Frequency hopping spread spectrum sender sends on one carrier frequency for a short amount of time, hops to another frequency for the same time, then another, and so on, eventually the cycle is repeated makes it difficult for unauthorized person to make sense of data 2.4 GHz industrial scientific, and medical (ISM) band (2.4 ~ 2.48Ghz) – 79 subbands of 1MHz modulation is FSK (1 or 2 bits/buad, two-level or four-level) : date rate of 1 or 2 Mbps

Frequency-Hopping example

DSSS – direct sequence spread spectrum Each bit sent is replaced by a chip code (similar to CDMA, but this is at the physical layer, not a multiple-access data link layer method) 2.4 GHz ISM band Modulation is PSK (BPSK or QPSK)

802.11 802.11 Infrared Two capacities 1 Mbps or 2 Mbps. Range is 10 to 20 meters and cannot penetrate walls. Does not work outdoors. 802.11 FHSS (Frequency Hopping Spread Spectrum) The main issue is multipath fading. 79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band. Same pseudo-random number generator used by all stations. Dwell time: min. time on channel before hopping (400msec).

801.11a,b,g 802.11a OFDM – orthogonal frequency division multiplexing 5-GHz ISM band 48 subbands for sending 48 groups of bits at a time, 4 subbands for control information PSK for 18Mbps and QAM for 54 Mbps 802.11b HR-DSSS – high-rate direct sequence spread spectrum 2.4 GHz band similar to DSSS except for encoding, complementary code keying (CCK), encodes 4 or 8 bits to one CCK symbol modulation BPSK and QPSK 4 data rates (1, 2, 5.5, and 11 Mbps) : DSSS for 1, 2 Mbps, BPSK for 5.5, QPSK for 11 Mbps. 802.11b and 11a are incompatible!!

801.11a,b,g (cont’d) 802.11g OFDM 2.4 GHz band 54-Mbps data rate Is backward compatible with 802.11b.

MAC layers in IEEE 802.11 standard PCF : Point Coordination Function DCF : Distributed Coordination Function

CSMA/CA Wireless LAN cannot implement CSMA/CD: collision detection implies the ability to send and receive at the same time, which implies costly stations and increased bandwidth requirements collision may not be detected because of a hidden terminal distance could cause signal fading which could prevent a station from hearing a collision

Hidden and Exposed Terminals (a) The hidden station problem (b) The exposed station problem

Another Example for Exposed Terminal Problem

CSMA/CA flowchart

CSMA/CA and NAV DIFS – distributed interframe space NAV – network allocation vector SIFS – short interframe space collision only occurs during RTS – request to send handshake period (RTS, CTS) CTS – clear to send

Frame format FC : frame control

Subfields in FC (frame control) field Explanation Version The current version is 0. Type Type of information: management (00), control (01), or data (10). Subtype Defines the subtype of each type (see ). To DS Defined later. From DS More flag When set to 1, means more fragments. Retry When set to 1, means retransmitted frame. Pwr mgt When set to 1, means station is in power management mode. More data When set to 1, means station has more data to send. WEP Wired equivalent privacy. When set to 1, means encryption implemented. Rsvd Reserved.

Control frames

Values of subfields in control frames type field : 01 Subtype Meaning 1011 Request to send (RTS) 1100 Clear to send (CTS) 1101 Acknowledgment (ACK)

Subfields in FC field To DS From DS Address 1 Address 2 Address 3 Destination station Source station BSS ID N/A 1 Sending AP Receiving AP

Addressing mechanism: case 1 To DS = 0 and From DS = 0 Frame is not going to a distribution system Frame is going from one station in a BSS to another ACK sent to original sender

Addressing mechanism: case 2 To DS = 0 and From DS = 1 Frame is coming from a distribution system Frame is coming from an AP and going to a station ACK sent to AP

Addressing mechanism: case 3 To DS = 1 and From DS = 0 Frame is going to a distribution system Frame is coming from a station to an AP ACK sent to original station address 3 contains final destination of frame

Addressing mechanism: case 4 To DS = 1 and From DS = 1 Distribution system is wireless Frame is coming from an AP going to another AP

Architecture Radio Layer Baseband Layer L2CAP Layer Other Upper Layers 15.2 Bluetooth Architecture Radio Layer Baseband Layer L2CAP Layer Other Upper Layers

Architecture - Two Types of Networks Piconets Scatternet

Piconet up to eight stations, one is the master, rest are slaves slaves synchronize their clocks and hopping sequence with master only one master communication between master and slave can be 1-1 or 1-many can have up to eight additional slaves in parked state slave in parked state is synchronized, but cannot communicate

Scatternet scatternet – combined piconets slave in one piconet can be master in another

Bluetooth layers

Radio Layers roughly equivalent to the physical layer of the Internet model 2.4-GHz ISM band FHSS – frequency hopping spread spectrum modulation – sophisticated version of FSK, called GFSK (FSK w/ Gaussian bandwidth filtering)

Baseband Layer roughly equivalent to MAC sublayer in LANs access method is TDMA TDD-TDMA (time division duplexing TDMA) – a kind of half duplex communication single slave communication – master uses even number slots, slave uses odd numbered multiple slave communication – master uses even number slots, slave sends in the next odd-numbered slot if the packet in the previous slot was addressed to it

Single-slave Communication

Multiple-slave communication

Physical Links SCO – synchronous connection oriented link – avoiding latency is more important than integrity – damaged packet is never retransmitted ACL – asynchronous connectionless link – data integrity is more important than avoiding latency – if packet is damaged, it is retransmitted

Frame Format Types

L2CAP data packet format Logical link control and adaptation protocol – roughly equivalent to the LLC sublayer in LANs used for data exchange on an ACL link specific duties: multiplexing, segmentation and reassembly, quality of service, and group management

Questions !