1. Wireless Data Networks

2. Puzzle You have a chain consisting of 63 inter-linked gold links
You have to stay at a motel where the charge per day is 1 gold link
You do not trust the motel manager and neither does he trust you
What is the minimum number of links you need to break in order to stay for 63 days?
Clue: The manager will not sell the links till you check out

3. Wireless Data Networks
Experiencing a tremendous growth over the last decade or so
US wireless data revenue in the $30-35B range in 2008
Increasing mobile work force, luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data

4. 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 mesh networks
e. g. Athens mesh network
Wireless PANs
e.g. Bluetooth headsets
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 54Mbps currently, close to 150Mbps with n)
Examples include IEEE networks, Bluetooth networks, and the erstwhile 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
Support significantly lower bandwidths than their LAN counterparts (upto a few hundred kilobits per second)
Examples: CDPD, RAM, GPRS, EDGE, EV-DO, HSDPA

8. WAN Topology

9. 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!!

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
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 (~54Mbps)
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 mesh networks
Wireless backhaul as opposed to the traditional wired backhaul
Multi-hop wireless routing across multiple wireless routers possible
One or more gateways are connected back to the backbone
Typical backhaul rates one in three to one in seven

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 Topology Independent Basic Service Set (IBSS) Networks
Stand-alone BSS that has no backbone infrastructure and consists of at-least two wireless stations
Often referred to as an ad-hoc network
Applications include single room, sale floor, hospital wing

24. Infrastructure BSS Use of an access point
Access points used for all communication including for communication between nodes in the same service area
Association with a specific access point required for the mobile node to be served

25. IEEE 802.11 Topology (contd.) Extended Service Set (ESS) Networks
Large coverage networks of arbitrary size and complexity
Consists of multiple cells interconnected by access points and a distribution system, such as Ethernet
Consist of multiple Infrastructure BSS’

26. IEEE 802.11 Logical Architecture
The logical architecture of the standard that applies to each station consists of a single MAC and one of multiple PHYs
Frequency hopping PHY
Direct sequence PHY
Infrared light PHY
OFDM PHY
MAC uses CSMA/CA (carrier sense multiple access with collision avoidance)

27. 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

28. 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

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 (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?)

34. Carrier Sensing and Network Allocation Vector
Both physical carrier sensing and virtual carrier sensing used in
If either function indicates that the medium is busy, treats the channel to be busy
Virtual carrier sensing is provided by the NAV (Network Allocation Vector)

35. NAV
Most frames carry a duration field which is used to reserve the medium for a fixed time period
Tx sets the NAV to the time for which it expects to use the medium
Other stations start counting down from NAV to 0
When NAV > 0, medium is busy

36. Illustration SIFS Sender RTS DATA SIFS SIFS Receiver CTS ACK NAV RTS

37. Interframe Spacing 802.11 uses 4 different interframe spacings
Interframe spacing plays a large role in coordinating access to the transmission medium
Varying interframe spacings create different priority levels for different types of traffic!

38. Types of IFS SIFS DIFS Short interframe space
Used for highest priority transmissions – RTS/CTS frames and ACKs
DIFS
DCF interframe space
Minimum idle time for contention-based services (> SIFS)

39. Types (contd.) PIFS EIFS PCF interframe space
Minimum idle time for contention-free service (>SIFS, <DIFS)
EIFS
Extended interframe space
Used when there is an error in transmission

40. Power Saving Mode (PS)
stations can maximize battery life by shutting down the radio transceiver and sleeping periodically
During sleeping periods, access points buffer any data for sleeping stations
The data is announced by subsequent beacon frames
To retrieve buffered frames, newly awakened stations use PS-poll frames
Access point can choose to respond immediately with data or promise to delivery it later

41. 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)

42. 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?!!