1. Wireless Networks & MAC

2. Puzzle
How can a toss be called over the phone (without requiring trust)?

3. List of Projects (Testbeds)
ns2 (****)
C & Network programming, networked apps
New transport layer research problem
PlanetLab (****)^
C & Network programming, kernel programming
Split flow networking vs. Confluent flow networking
Sensor Network Test-bed (***)
C programming, network systems experience
Real time sensor networks
Wireless VoIP (***)
C/Java programming, Network programming
Improving call capacity of wireless VoIP
Heterogeneous WiFi n/b/g/a (**)
Network systems experience, some programming
Comparing different technologies

4. Project Assignments Wireless VoIP Aaron Sproul and Patrick hamilton
James Michaels and Jonathan Levitt
Stan Komsky and Scott Travis*
Sensor networks
Craig Young and Chris Theodoris
Michael Sabot and Ben Heruska
PlanetLab
Cheng-lin and Ruhull
Felipe Santos
Heterogeneous Wifi
Kevin sinha and Rigoberto

5. Wireless VoIP Build wireless testbed for VoIP
Demonstrate simple VoIP calls over wireless testbed
Monitor performance with increasing number of VoIP calls (loss-rate, subjective opinion)
Use more sophisticated metrics (ITU R-factor, MOS) and monitor VoIP call performance
Build monitoring and display tools to spoof packets and present metrics online
Related sophisticated metrics to simple metrics

6. Wireless Sensor Networks
Understand TinyOS
Understand Mica motes
Set-up sensor network topology. Monitor light/sound/etc. and collect information through multi-hop wireless network and display through surge application
Identify current report rate
Tune report rate for optimal performance and identify insights

7. Heterogeneous WiFi Compare 802.11 b/g/a/n Set-up testbed
Install performance monitoring tools and perform data transfers (CBR, data) and monitor performance
Throughput, delay, loss-rate
Multiple conditions – indoor/outdoor, range, data-rates, # of clients, 3D vs. 2D, etc.
Present insights

8. Planet Lab Set-up planet lab topology
Perform simple path analysis (traceroute)
Analyze path diversity and present insights
Perform sophisticated path analysis (pathchar)
Apply insights to benefits of overlay routing, and identify performance improvements

9. Wireless VoIP* WLAN VoIP vs. mesh VoIP Set-up WLAN VoIP
Perform analysis (call capacity)
Set-up wireless mesh VoIP
Performance analysis (call capacity)
Compare and contrast WLAN vs. mesh networks in terms of VoIP performance

10. New Project Timeline Abstract Preliminary presentations
Due Thursday, October 5th
Preliminary presentations
November 7th
Final presentation
December 5th and 7th

11. 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

12. Some Facts
By end of 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!!

13. 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

14. 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

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

16. 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

17. WAN Topology

18. 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!!

19. 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

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

21. 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)

22. 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.)

23. 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)

24. 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

25. 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

26. 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?

27. 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!)

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

29. 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

30. 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!

31. 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

32. 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

33. 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?)

34. 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

35. 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

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

37. Puzzle In C, what is the output of the following code: main() {
int k=8;
printf(“%d %d”, k, k++); }