1. Wireless Networks & MAC
Lecture 4

2. 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

3. Some Facts
By 2005, more than 1/3rd of internet users will have internet connectivity through a wireless enabled device (750 million users)!!! (Source: Intermarket group)
By the year 2010 the number of wireless data subscribers will hit 1B!!

4. 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

5. 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 networks, Bluetooth networks, and Infrared networks

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

7. 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

8. WAN Topology

9. 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!!

10. 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

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

12. 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 supports a Distributed Coordination Function (DCF) mode of operation)

13. 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.)

14. 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)

15. 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

16. 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

17. 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?

18. 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!)

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

20. 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

21. 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!

22. 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

23. 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

24. 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?)

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

26. 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

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

28. Puzzle Muddy children problem N kids playing in the mud
Only the foreheads of K kids get dirty
A kid does not know if his/her forehead is dirty
One of the parents comes and asks all “dirty” kids to step forward. He keeps asking till all the dirty kids step forward.
How many times does the parent need to ask before kids step forward (all kids are honest, smart, and obedient)