Wireless Networks & MAC

Wireless Data Networks Experiencing a tremendous growth over the last decade or so Increasing mobile work force, luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data

Wireless Network Types … Satellite networks e.g. Iridium (66 satellites), Qualcomm’s Globalstar (48 satellites) Wireless WANs/MANs e.g. CDPD, GPRS, Ricochet Wireless LANs e.g. Georgia Tech’s LAWN Wireless PANs e.g. Bluetooth Ad-hoc networks e.g. Emergency relief, military Sensor networks

Wireless Local Area Networks Probably the most widely used of the different classes of wireless data networks Characterized by small coverage areas (~200m), but relatively high bandwidths (upto 50Mbps currently) Examples include IEEE 802.11 networks, Bluetooth networks, and Infrared networks

WLAN Topology Static host/Router Distribution Network Access Point Mobile Stations

Wireless WANs Large coverage areas of upto a few miles radius For example, Metricom’s Ricochet covers the whole of the Atlanta metropolitan area Support significantly lower bandwidths than their LAN counterparts (upto a few hundred kilobits per second) Examples: CDPD, Mobitex/RAM, Ricochet

WAN Topology

WWAN Generations 1G (Past) 2G (Past/Present) 2.5G (Present) AMPS, TACS: No data 2G (Past/Present) IS-136, GSM: <10Kbps circuit switched data 2.5G (Present) GSM-GPRS, GPRS-136: <100Kbps packet switched 3G (Immediate Future) IMT-2000: <2Mbps packet switched 4G (Future) 20-40 Mbps!!

Satellite Networks Till recently satellite networks used only for fixed earth stations to communicate (with satellites being geo-stationary) With the deployment of LEO (low earth orbit satellites), using satellite networks for mobile device communication has become a reality Offer few tens of kilobits per second upstream and a few megabits per second downstream

Satellite Networks (contd.) Wide Area coverage of the earth's surface Long transmission delays Broadcast transmission Large Channel Bandwidth Transmission costs independent of distance

Ad-hoc Networks Multi-hop wireless networks Infrastructureless Typically used in military applications (where there is no infrastructure), or disaster relief (where infrastructure has been destroyed) Mobile stations double-up as forwarders/routers Can use existing WLAN technology (e.g. IEEE 802.11 supports a Distributed Coordination Function (DCF) mode of operation)

Ad-hoc Networks (contd.) Typical data rates (on a per-link basis) same as WLANs (~10Mbps) End-to-end data rates can be significantly smaller (depending on network size, diameter of network, etc.) Very different network environment (highly dynamic, routers also mobile!, etc.)

Wireless PANs Wireless personal area networks Example: Bluetooth Primarily meant for networking personal devices (music systems, speakers, microwaves, refrigerators, etc.) Lower data rates and transmission ranges (hence low power)

Sensor Networks Network of sensing devices (sensors) Applications include smart-concrete, smart-dust, etc. Useful for sensing in inaccessible locations Very low powered, resource-constrained devices Similar to ad-hoc networks with more severe constraints and a many-to-one topology

Issues in Wireless Networks Low bandwidths 10Mbps WLANs, 100Kbps WWANs, 1Mbps WPANs, 200Kbps WMANs High ER (error rates – ~10% PER) Location dependent channel characteristics Mobility and consequent “hand-offs” Unique shared environment Focus of this lecture: Wireless LANs

Medium Access in WLANs Shared medium Medium Access Control (MAC) decides which station gets access to wireless channel Why not CSMA/CD? Why not CSMA?

CSMA/CD in WLANs? Most (if not all) radios are half-duplex. Hence, listening while transmitting is not possible. Possible way out? Collision might not occur at sender (collision at receiver might not be detected by sender!)

Key Problems in WLANs Hidden Terminal Problem Exposed Terminal Problem C D A B C B WILL NOT TRANSMIT! COLLISION AT B!

CSMA with Collision Avoidance Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Control packet transmissions precede data packet transmissions to facilitate collision avoidance 4-way (RTS, CTS, Data, ACK) exchange for every data packet transmission

CSMA/CA (Contd.) A B C C knows B is listening RTS A B C CTS C knows B is listening to A. Will not attempt to transmit to B. Data ACK Hidden Terminal Problem Solved through RTS-CTS exchange!

CSMA/CA (Contd.) Can there be collisions? Control packet collisions (C transmitting RTS at the same time as A) C does not register B’s CTS C moves into B’s range after B’s CTS

CSMA/CA Algorithm Sense channel (CS) If busy Else Back-off to try again later Else Send RTS If CTS not received Send Data If ACK not received Next packet processing

CSMA/CA Algorithm (Contd.) Maintain a value CW (ContentionWindow) If Busy, Wait till channel is idle. Then choose a random number between 0 and CW and start a back-off timer for proportional amount of time (Why?). If transmissions within back-off amount of time, freeze back-off timer and start it once channel becomes idle again (Why?) If Collisions (Control or Data) Binary exponential increase (doubling) of CW (Why?)

CSMA/CA Algorithm (Contd.) IEEE 802.11 standard for WLAN MAC based on CSMA/CA Algorithm described thus far is IEEE 802.11 in DCF (distributed coordination function) mode IEEE 802.11 also supports Point Coordination Function (PCF) Mode where a centralized base-station (or access-point) coordinates medium access

Other Types of Wireless Networks Wide-area, Metropolitan-area, and Satellite Networks centralized channel allocation by base-station based on TDMA, FDMA, or CDMA Personal Area Networks Bluetooth

Recap Wireless network types WLANs WWANs WPANs Satellite Networks Ad-hoc Networks Sensor Networks WLAN MAC

Puzzle In C, what is the output of the following code: main() { int a[5] = {0, 1, 2, 3, 4}; a[2] && printf(“%d %d”, a[3], 3[a]++); }