1. TCP & Wireless Networks

2. Puzzle
10 bags with 1000 coins each. 9 bags have only regular coins (each regular coin weighs 1 unit). 1 bag has only defective coins (each defective coin weighs 1.01 units)
Using a spring balance, how many weighings do you need to find the defective bag?

3. Exam 1
On Thu, January 31st, 2008
Covers all topics till end of lecture today
Worth 10 points of your final grade
(note change! Mid-term 2 will be worth 20% of final grade)
Sample exam on the class website
10 questions that will require short answers

4. TCP Flavors TCP-Tahoe TCP-Reno TCP-newReno TCP-Vegas, TCP-SACK
W=1 adaptation on congestion
TCP-Reno
W=W/2 adaptation on fast retransmit, W=1 on timeout
TCP-newReno
TCP-Reno + intelligent fast recovery
TCP-Vegas, TCP-SACK

5. TCP Tahoe Slow-start Congestion control upon time-out or DUP-ACKs
When the sender receives 3 duplicate ACKs for the same sequence number, sender infers a loss
Congestion window reduced to 1 and slow-start performed again
Simple
Congestion control too aggressive

6. TCP Reno Tahoe + Fast re-transmit
Packet loss detected both through timeouts, and through DUP-ACKs
Sender reduces window by half, the ssthresh is set to half of current window, and congestion avoidance is performed (window increases only by 1 every round-trip time)
Fast recovery ensures that pipe does not become empty
Window cut-down to 1 (and subsequent slow-start) performed only on time-out

7. TCP New-Reno TCP-Reno with more intelligence during fast recovery
In TCP-Reno, the first partial ACK will bring the sender out of the fast recovery phase
Results in timeouts when there are multiple losses
In TCP New-Reno, partial ACK is taken as an indication of another lost packet (which is immediately retransmitted).
Sender comes out of fast recovery only after all outstanding packets (at the time of first loss) are ACKed

8. TCP SACK
TCP (Tahoe, Reno, and New-Reno) uses cumulative acknowledgements
When there are multiple losses, TCP Reno and New-Reno can retransmit only one lost packet per round-trip time
What about TCP-Tahoe?
SACK enables receiver to give more information to sender about received packets allowing sender to recover from multiple-packet losses faster

9. TCP SACK (Example) Assume packets 5-25 are transmitted
Let packets 5, 12, and 18 be lost
Receiver sends back a CACK=5, and SACK=(6-11,13-17,19-25)
Sender knows that packets 5, 12, and 18 are lost and retransmits them immediately

10. Other TCP flavors TCP Vegas TCP FACK
Uses round-trip time as an early-congestion-feedback mechanism
Reduces losses
TCP FACK
Intelligently uses TCP SACK information to optimize the fast recovery mechanism further

11. User Datagram Protocol (UDP)
Simpler cousin of TCP
No reliability, sequencing, congestion control, flow control, or connection management!
Serves solely as a labeling mechanism for demultiplexing at the receiver end
Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols)
Additional intelligence built at the application layer if needed

12. UDP Header Src Port Dst Port Length: length of header + data (min = 8)
Checksum

13. Recap TCP Connection management Reliability Flow control
Congestion control
TCP flavors
UDP

14. 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

15. 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

16. 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

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

18. 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

19. WAN Topology

20. 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: <2Mbps packet switched
4G (Future)
20-40 Mbps!!

21. 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

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

23. 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)

24. 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.)

25. 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)

26. 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

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

28. 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

29. 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

30. 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)

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

32. 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

33. 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

34. 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

35. 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

36. 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!

37. 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

38. 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

39. 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?)

40. 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)

41. 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?)