1. ECE 6610: Wireless Networks
Raghupathy Sivakumar

2. Guest Lectures
Professor Sivakumar traveling for a lecture at the WISARD (Wireless Systems: Advanced Research and Development) 2009 conference
Guest lectures on January 6th and 8th

3. e-Handouts (online) Class information Schedule
0th assignment (does not need to be turned in)

4. Goals
Fundamental concepts in wireless network protocols and algorithms
Impact of wireless networks on the different layers of the network protocol stack
Theoretical and practical exposure to problems and solutions in wireless network protocols

5. Requirements * See 0th assignment Knowledge of the OSI protocol stack*
Knowledge of the TCP/IP protocol suite and associated protocols*
Experience in programming for assignments & project
* See 0th assignment

6. Administrative Information
Raghupathy Sivakumar
Room 5164, Centergy
Office hours: By appointment
Class website TA
TBD

7. Policies
Slides: Lecture slides available on website by 12noon on the day of class
Lecture slides are NOT comprehensive – students are expected to attend classes to “fill in” information
Classes: Miss classes at your own risk. Professor and TAs will not be responsible for any information you might not have because of a missed class.
Cheating: Zero tolerance policy toward cheating. You will receive an F for the entire class if you are found cheating in any exam or assignment.
Disturbance: No talking in class and disturbing other students when lecture is being delivered. Please turn OFF your cellphones before sitting down for lectures. You will be asked to leave the classroom if you violate this policy.

8. Textbook No textbook Primary material: Suggested reading
Class discussions + slides
Suggested reading
Supplementary reading to be suggested during lectures

9. Grading Exams Assignments Project Other 2 midterms (2 x 15% = 30%)
1 final (1 x 35% = 35%)
Assignments
3 programming assignments (3 x 6% = 18%)
Project
1 programming mini-project (1 x 12% = 12%)
Other
Class participation (1 x 5% = 5%)
Active participation required
Need to turn in a participation report at the end of the semester (make notes of interactions in class)

10. Programming Assignments
Students to complete assignments in groups of 5*
18% of your final grade
All assignments will be based on the ns2 network simulator
* Subject to change

11. Project 1 mini-project 18% of your final grade
Based on the ns2 network simulator*
Project will be defined for you
Deliverables: Preliminary report, demonstration and results, final report/presentation
* A few test-bed project options available

12. Groups: Regular Students
Form groups of five
Try to choose students with complimentary skills
Send groups to TA (one per group w/ subject: ECE 6610 Groups) by end of day Thursday, 1/15

13. Distance Learning Students
Will work on theory assignments as opposed to programming assignments
Deadlines one week after those for regular students

14. Schedule All deadlines for distance learning students will
be one week after the regular deadlines

15. References, Reading Material, and Lecture Notes
Most references – research papers in the concerned area. Available online.
Follow lecture slides and class discussions closely

16. OSI Model

17. Communication Networks
Broadcast networks
Each station can hear every other station in the network (fully connected network)
Switched networks
Stations interconnected through a (non-fully) connected mesh
Packet switched vs. Circuit switched
What kind of a network is the Internet?

18. Communication Protocols
Rules used in communication
Monolithic vs. Layered
Protocol data units – used to exchange information between peer layers of protocol stack
Examples of communication protocols?

19. OSI
Open Systems Interconnection: ISO (International Standards Organization)’s standard for communication protocols
Also referred to as the OSI reference model or simply the OSI model
7 layer protocol stack

20. OSI Protocol Stack Physical Data link Network Transport Session
Presentation
Application

21. OSI (contd.)
Physical: mechanical/electrical rules for transferring bits
Data link:
flow control
error detection
error recovery

22. OSI (contd.) Network Transport Routing Congestion control
Quality of service
Transport
End-to-end communication of data
Reliability
Sequencing
Flow control

23. OSI (contd.) Session Presentation Application
Application specific functionality
Still, generic to multiple applications (e.g. security)
Presentation
Data formatting
Application

24. TCP/IP Protocol Suite Differences between OSI and TCP/IP? 5 layers:
Physical
Data link/MAC (ARP, SLIP)
Network (IP, ICMP, IGMP)
Transport (TCP, UDP)
Application (http, ftp, telnet, smtp)

25. Medium Access Control

26. Overview Medium Access Control Scheduling
ALOHA, slotted-ALOHA, CSMA, CSMA/CD
Scheduling
FCFS, Priority scheduling
Fair queuing, Weighted fair queuing
Round robin, Weighted round robin
Deficit round robin
Class based queuing

27. Medium Access Control
When multiple stations share a common channel, the protocol that determines which station gets access to the shared channel
Key characteristics based on which MAC protocols are evaluated: utilization and fairness

28. ALOHA ALOHA When a station wants to transmit, it transmits …
Collisions detected at a higher layer and retransmissions done as required
Simple logic
Highly inefficient at large loads. Maximum utilization of 18% at a mean load of 0.5 transmissions/slot

29. Slotted ALOHA
Stations can transmit only at the beginning of pre-determined slots
Reduces the “vulnerable period” for collisions
More efficient
Maximum utilization of 36% at a mean load of 1 transmission/slot

30. Carrier Sense Multiple Access
Station wishing to transmit senses channel. If channel idle, transmits. Else, backs-off and tries later
Carrier sensing hardware required
More efficient than both versions of ALOHA
3 flavors of CSMA

31. CSMA (contd.) 1-persistent CSMA p-persistent CSMA non-persistent CSMA
On finding channel busy, station continues listening and transmits when channel becomes idle
p-persistent CSMA
On finding channel idle, station transmits with a probability of p, backs-off and tries again when channel is busy
non-persistent CSMA
On finding channel busy, station backs-off for a random amount of time and tries later

32. CSMA/CD
In CSMA, when there is a collision of two transmissions, it is detected only after the entire frames have been transmitted … irrespective of when the collision occurs
CSMA/CD includes a collision detection mechanism that can detect collisions even as stations are transmitting
Transmitting stations terminate transmissions upon collision detection and try later

33. CSMA/CD (Contd.)
When will the performance of CSMA/CD be the same as that of CSMA?
IEEE Ethernet LAN Standard
802.3u – fast ethernet
802.3ab – gigabit ethernet
802.3ae – 10G ethernet
Full-duplex Ethernet

34. Other MAC Schemes Collision free protocols
Bit map protocol
Binary countdown protocol
Limited contention protocols
Adaptive tree walk protocol

35. Recap Class goals and overview Grading and other administrative stuff
Communication networks
OSI Protocol Stack

36. Scheduling

37. Scheduling
When a station gets to transmit, the protocol that determines which packet gets to be transmitted
Fairness the primary consideration
Weighted fairness … an extension
Default scheduling in today’s Internet?

38. Scheduling Policies … FCFS Priority
Packets queued on a first-come-first-served basis
No fairness
Priority
Multiple queues with different priorities
Packets belonging to queue k served only when queues with higher priorities are empty

39. Scheduling (Contd.) Generalized Processor Sharing (GPS)
Ideal fair queuing approach based on a fluid flow system
Complex, idealistic
Packetized GPS (WFQ)
Packetized version of GPS (practical)
Finish times in a correponding GPS system used for prioritizing packets

40. Scheduling - WFQ 3 flows A, B, C Weights: A(1), B(2), C(3)
Assume same packet sizes 8 7
Complex overhead due to
computation of finishing times
Simpler approach? 5 4 6 3 2 1
Tx Schedule: C B C C B A C B …

41. Scheduling (Contd.) Weighted Round Robin A Fixed Tx Schedule:
Simpler approximation of WFQ
Assumes constant packet sizes
Can cause unfair delay A
Fixed Tx Schedule:
C C C B B A A B C

42. Scheduling (Contd.) WRR with WFQ Spread
For each flow k with weight wk generate wk elements of the form (1/wk,k), (2/wk,k), …, (wk/wk,k)
Sort all such elements from all competing flows in lexicographic order
Extracting the second element of each pair from the resulting sorted list gives the WRR with WFQ spread

43. WRR with WFQ Spread A (1) (1,A) B (2) (1/2,B), (1,B) C (3)
(1/3,C), (2/3,C), (1,C)
Lexicographically Sorted: (1/3,C), (1/2,B), (2/3,C), (1,A), (1,B), (1,C)
Schedule: C B C A B C

44. Other scheduling policies…
Deficit Round Robin (DRR)
Handles varying size packets
Frame of N bits split among the competing flows in the ratio of their weights
Flows get to transmit HOL packet only when they have accumulated packet-length number of bits
Class based queuing (CBQ)
General scheduling and link-share scheduling
Greater flexibility to control scheduling policy

45. Internet Protocol (IP)

46. TCP/IP Protocol Suite Physical layer Data-link layer – ARP, RARP, SLIP
Network layer – IP, ICMP, IGMP, BootP
Transport layer _ TCP, UDP, RTP
Application layer – http, smtp, ftp

47. Internet Protocol (IP)
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing

48. Addressing
Need unique identifier for every host in the Internet (analogous to postal address)
IP addresses are 32 bits long
Hierarchical addressing scheme
Conceptually …
IPaddress =(NetworkAddress,HostAddress)

49. Address Classes Class A Class B Class C 0 netId hostId
7 bits 24 bits
netId hostId
14 bits 16 bits
netId hostId
21 bits 8 bits

50. IP Address Classes (contd.)
Two more classes
1110 : multicast addressing
1111 : reserved
Significance of address classes?
Why this conceptual form?

51. Addresses and Hosts
Since netId is encoded into IP address, each host will have a unique IP address for each of its network connections
Hence, IP addresses refer to network connections and not hosts
Why will hosts have multiple network connections?

52. Special Addresses hostId of 0 : network address
hostId of all 1’s: directed broadcast
All 1’s : limited broadcast
netId of 0 : this network
Loopback :
Dotted decimal notation: IP addresses are written as four
decimal integers separated by decimal points, where each
integer gives the value of one octet of the IP address.

53. Exceptions to Addressing (norm today)
Subnetting
Splitting hostId into subnetId and hostId
Achieved using subnet masks
Useful for?
Supernetting (Classless Inter-domain Routing or CIDR)
Combining multiple lower class address ranges into one range
Achieved using 32 bit masks and max prefix routing

54. Examples Subnetting Supernetting 192.168.1.0/24 – class C network
/26 and /26 – 2 subnetworks with upto 62 stations each!
Supernetting
/24 and /24 – 2 class C networks
/23 – 1 super network with upto 126 stations!!

55. Weaknesses Mobility Switching address classes
Notion of host vs. IP address

56. IP Routing Direct Indirect
If source and destination hosts are connected directly
Still need to perform IP address to physical address translation. Why?
Indirect
Table driven routing
Each entry: (NetId, RouterId)
Default router
Host-specific routes

57. IP Routing Algorithm RouteDatagram(Datagram, RoutingTable)
Extract destination IP address, D, from the datagram and compute the netID N
If N matches any directly connected network address deliver datagram to destination D over that network
Else if the table contains a host-specific route for D, send datagram to next-hop specified in table
Else if the table contains a route for network N send datagram to next-hop specified in table
Else if the table contains a default route send datagram to the default router specified in table
Else declare a routing error

58. Routing Protocols Interior Gateway Protocol (IGP)
Within an autonomous domain
RIP (distance vector protocol), OSPF (link state protocol)
Exterior Gateway Protocol (EGP)
Across autonomous domains
BGP (border gateway protocol)

59. IP Fragmentation
The physical network layers of different networks in the Internet might have different maximum transmission units
The IP layer performs fragmentation when the next network has a smaller MTU than the current network
MTU = MTU=500
IP fragmentation

60. IP Reassembly Fragmented packets need to be put together
Where does reassembly occur?
What are the trade-offs?

61. Multiplexing Web Email MP3 Web Email MP3 TCP UDP TCP UDP IP IP
IP datagrams
IP datagrams

62. IP Header Used for conveying information to peer IP layers Source
Destn
Application
Application
Transport
Router
Router
Transport IP IP IP IP
DataLink
DataLink
DataLink
DataLink
Physical
Physical
Physical
Physical

63. IP Header (contd.) 4 bit version 4 bit hdr length 16 bit total length
TOS
16 bit identification
3 bit
flags
13 bit fragment offset
8 bit TTL
8 bit protocol
16 bit header checksum
32 bit source IP address
32 bit destination IP address
Options (if any) (maximum 40 bytes)
data

64. Recap MAC Protocols: ALOHA, slotted-ALOHA, CSMA, CSMA/CD
Scheduling: FCFS, Priority, FQ, WFQ, WRR, CBQ
Internet Protocol: Addressing, Routing, Fragmentation & Reassembly, Mux/Demux

65. Transmission Control Protocol (TCP)

66. Transmission Control Protocol (TCP)
End-to-end transport protocol
Responsible for reliability, congestion control, flow control, and sequenced delivery
Applications that use TCP: http (web), telnet, ftp (file transfer), smtp ( ), chat
Applications that don’t: multimedia (typically) – use UDP instead

67. Ports, End-points, & Connections
IP Layer
TCP
UDP
http ftp smtptelnet A1 A2 A3
Transport
Port
IP address
Protocol ID
Thus, an end-point is represented by (IP address,Port)
Ports can be re-used between transport protocols
A connection is (SRC IP address, SRC port, DST IP address, DST port)
Same end-point can be used in multiple connections

68. TCP Connection Establishment Connection Maintenance
Reliability
Congestion control
Flow control
Sequencing
Connection Termination

69. Active and Passive Open
How do applications initiate a connection?
One end (server) registers with the TCP layer instructing it to “accept” connections at a certain port
The other end (client) initiates a “connect” request which is “accept”-ed by the server

70. Connection Establishment & Termination
3-way handshake used for connection establishment
Randomly chosen sequence number is conveyed to the other end
Similar FIN, FIN+ACK exchange used for connection termination
Server does passive open
Accept connection request
Send acceptance
Start connection
Active open
Send connection
request
SYN
SYN+ACK
ACK
DATA

71. Reliability (Loss Recovery)
ack
data
Sequence Numbers
TCP uses cumulative Acknowledgments (ACKs)
Next expected in-sequence packet sequence number
Pros and cons?
Piggybacking 5 1 2 3 4 3 1 2 4
Timeout calculation
Rttavg = k*Rttavg + (1-k)*Rttsample
RTO = Rttavg + 4*Rttdeviation

72. Fundamental Mechanism
Simple stop and go protocol
Timeout based reliability (loss recovery)
ack
data
data
retx
ack
data
Multiple unacknowledged packets!
Sliding Window Protocol: ….

73. Congestion Control Alternative: Fall to W/2 and start
Slow Start
Start with W=1
For every ACK, W=W+1
Congestion Avoidance (linear increase)
For every ACK,
W = W+1/W
Congestion Control (multiplicative decrease)
ssthresh = W/2
W = 1
Alternative: Fall to W/2 and start
congestion avoidance directly

74. Why LIMD? (fairness) W=1 W=W/2 100 10 diff = 90 1 1 diff = 0
Problem? – inefficient
W=W/2
diff = 45
51 6 diff = 45
52 7 diff = 45 ..
diff = 45
diff = 23.5
diff = 23.5
diff = 11.2

75. Flow Control Prevent sender from overwhelming the receiver
Receiver in every ACK advertises the available buffer space at its end
Window calculation
MIN(congestion control window, flow control window)

76. Sequencing Starting sequence number randomly chosen during connection3 1 2 4
Byte sequence numbers
TCP receiver buffers out of order segments and reassembles them later
Starting sequence number
randomly chosen
during connection
establishment
Why?
1 given to app
2 given to app
Loss
4 buffered (not given to app)
3 & 4 given to app
4 discarded

77. TCP Segment Format 16 bit SRC Port 16 bit DST Port
32 bit sequence number
32 bit ACK number HL
resvd
flags
16 bit window size
Flags: URG, ACK,
PSH, RST, SYN,
FIN
16 bit TCP checksum
16 bit urgent pointer
Options (if any)
Data

78. 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

79. 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

80. 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

81. 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

82. 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

83. 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

84. 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

85. 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

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

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