1. Wireless Data Networks

2. Puzzle Consider the C code-snippet: What is the output of the program?
main() {
int a[5] = {0, 1, 2, 3, 4};
2[a] && printf(“%d %d”, 3[a], 3[a]++); }
What is the output of the program?
Compile time error
Run-time error
Other (what?)

3. Puzzle (Solution) 4, 3 Why? Function calls, jumps, stack
Arrays vs. pointer arithmetic
Short-circuit evaluation

4. Wireless Data Networks
Experiencing a tremendous growth over the last decade or so
US wireless data revenue in the $15-20B range in 2007
Increasing mobile work force, luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data

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

6. 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, close to 150Mbps with n)
Examples include IEEE networks, Bluetooth networks, and Infrared networks

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

8. Wireless WANs Large coverage areas of upto a few miles radius
Support significantly lower bandwidths than their LAN counterparts (upto a few hundred kilobits per second)
Examples: CDPD, RAM, GPRS, EDGE, EV-DO, HSDPA

9. WAN Topology

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

11. 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

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

13. 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)

14. 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.)

15. 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)

16. 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

17. Wireless MAC Channel partitioned approaches
FDMA, TDMA, CDMA
Random multiple access schemes
ALOHA, slotted-ALOHA
CSMA
CSMA/CA

18. Wireless MAC CSMA as wireless MAC?
Hidden and exposed terminal problems make the use of CSMA an inefficient technique
Several protocols proposed in related literature – MACA, MACAW, FAMA
IEEE standard for wireless MAC

19. Hidden Terminal Problem
Collision A B C
A talks to B
C senses the channel
C does not hear A’s transmission (out of range)
C talks to B
Signals from A and B collide

20. Exposed Terminal Problem
Not
possible A B C D
B talks to A
C wants to talk to D
C senses channel and finds it to be busy
C stays quiet (when it could have ideally transmitted)

21. Hidden and Exposed Terminal Problems
Hidden Terminal
More collisions
Wastage of resources
Exposed Terminal
Underutilization of channel
Lower effective throughput

22. IEEE
The standard provides MAC and PHY functionality for wireless connectivity of fixed, portable and moving stations moving at pedestrian and vehicular speeds within a local area.
Specific features of the standard include the following:
Support of asynchronous and time-bounded delivery service
Continuity of service within extended areas via a Distribution System, such as Ethernet.
Accommodation of transmission rates of 1, 2 and 10 Mbps
Support of most market applications
Multicast (including broadcast) services
Network management services
Registration and authentication services

23. IEEE 802.11 MAC Layer Primary operations Wireless medium access
Accessing the wireless medium
Joining the network
Providing authentication and privacy
Wireless medium access
Distributed Coordination Function (DCF) mode
Point Coordination Function (PCF) mode

24. IEEE 802.11 MAC (contd.) DCF PCF
CSMA/CA – A contention based protocol
PCF
Contention-free access protocol usable on infrastructure network configurations containing a controller called a point coordinator within the access points
Both the DCF and PCF can operate concurrently within the same BSS to provide alternative contention and contention-free periods

25. 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

26. 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!

27. 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

28. 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

29. CSMA/CA Algorithm (Contd.)
Maintain a value CW (Contention-Window)
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?)

30. IEEE 802.11 MAC Frame Format Overall structure:
Frame control (2 octets)
Duration/ID (2 octets)
Address 1 (6 octets)
Address 2 (6 octets)
Address 3 (6 octets)
Sequence control (2 octets)
Address 4 (6 octets)
Frame body ( octets)
FCS (4 octets)

31. Puzzle Two great mathematicians S & P
S knows the sum of two positive integers (> 1) x and y
P knows the product of x and y
S calls P and says “You cannot find the two numbers”
P replies “I know the two numbers”
S responds “I know the two numbers too”
What are the two numbers?!!
Hint: 1 < x < 100, 1 < y < 100