CS3502: Computer Communications and Networks

1
CS3502 Computer Communications and Networks

• Geoffrey Xie

2
Data Link Control

• To accomplish reliable, efficient communication
  between two adjacent machines
• Dealing with stream of bits

Point to point communication channel (bits could
be inverted during transmission)
bit stream 101010001..
Machine A(transmitter)
Machine B(receiver)
3
Data Link Functions

• Perform tasks to achieve reliability
• error detection and control
• making sure that receiver will get the original
  bit sequence
• flow control
• making sure that transmitter will not overwhelm
  receiver by sending bits too fast
• Incur delays
• processing delays
• communication delays
• e.g., transmitter waiting for acknowledgements
  from receiver

4
Framing

• To facilitate error detection and correction, it
  is more efficient to partition a bit stream into
  frames
• Asynchronous transmission
• fixed-length frames (called characters) e.g.,
  ASCII characters
• start and stop bits used to mark each character
  at the physical level
• advantage(s)
• simple encoding (e.g., based on voltage levels)
  is used
• problems
• stringent timing requirement at the physical
  level when character size is large
• large communication overhead when character size
  is small

5
Framing (contd)

• Synchronous Transmission
• Large variable-length frames (called blocks) are
  used
• no start and stop bits
• how to determine the beginning and end of a
  block?
• preamble and postamble bit patterns e.g.,
  01111110
• what if the bit stream contains the
  preamble/postamble bit pattern?
• bit stuffing .01111110 gt 011111010...
• Block size cant be too large
• receiver buffer limitation
• the larger the frame size, the more likely the
  frame contains bit errors

6
Flow Control
Geoffrey Xie Assuming no transmission errors

• Stop-and-Wait
• Acknowledgement based
• will not send next frame until obtaining (from
  receiver) acknowledgement on correct reception of
  previous frame
• Problems
• only one frame can be in transit at any time
• very inefficient, especially for a high speed link

7
Flow Control (contd)

• Sliding Window
• Allow multiple frames in transit at the same time
• sequence numbers are used to keep track which
  frames have been successfully received
• Maximum window size
• the maximum number of frames that can be transit
  at the same time
• cannot exceed
• the amount of buffer space at the receiver
• the range of sequence numbers

8
Error Detection

• Types of errors (for both data and
  acknowledgement frames)
• lost frame
• damaged frame, i.e., containing bit error(s)
• Definitions
• frame size F
• probability of a single bit error Pb
• Probability that a frame arrives with no bit
  errors P1
• Probability that a frame arrives with undetected
  bit error(s) P2
• Probability that a frame arrives with only
  detected error(s) P3
• P1 (1 - Pb)F

9
Error Detection (contd)

• P2 could be too high with no error detection
• for example, when Pb 10-6 and F 1000, we
  have P2 1 - P1 10-3
• Goal of error detection
• reduce P2 for a fixed Pb
• General approach of error detection
• add error detection code (bits) to each frame
  (see Figure 6.5 of text)

10
Error Detection (contd)

• Parity Check
• Appending a parity bit to the end of each frame
• even or odd parity (regarding number of 1s in
  frame)
• Cannot detect even number of errors
• P2 Prthere are even number of bit errors in
  frame
• P3 1 - P1 - P2

11
Error Detection (contd)

• Extended Parity Check (Hamming Code)
• Multiple parity bits are used
• in each position (of frame) corresponding to a
  power of 2 1, 2, 4, 8,
• rest are data bits p1p2d3p4d5d6d7p8...
• Can detect and correct single bit error
• building the following matrix to determine p bits
  given d bits(transmitter choosing p values so
  that each row has even parity) p1
  d3 d5 d7 ... p2 d3 d6 d7 ...
  p4 d5 d6 d7 ... ...

12
Error Detection (contd)

• Cyclic Redundancy Check (CRC)
• n-bit frame check sequence (FCS) appended to
  k-bit frame (M) so that new frame is divisible by
  predetermined (n1)-bit value (P)
• modulator 2 arithmetic (addition/subtraction is
  equivalent to XOR operation)
• Suitable for hardware implementation
• Example
• M 1010001101 (k10 bits), P 110101 (6 bits),
  n 52n M 101000110100000R the remainder of
  (2n M / 110101) 01110(2n M R) is
  transmitted

13
Error Detection (contd)

• Internet Checksum
• Adds up all the words and append the sum to
  frame
• Software implementation (at higher layers)
• Weaker than CRC in detecting errors
• e.g., will not detect reordering of words

14
Error Control

• Based on Retransmission also called Automatic
  Repeat reQuest (ARQ)
• Triggered by negative acknowledgement (NACK)
• Triggered by timeout

15
Error Control (contd)

• Stop-and-Wait ARQ
• Avoid acceptance of duplicate frames
• alternatively label data frames with 0 or 1
  (Figure 6.8 of text)

16
Error Control (contd)

• Go-back-N ARQ
• Work with sliding window flow control
• receiver discarding multiple frames ( see
  Figure 6.9 of text, ignoring the RR (P bit 1)
  message )
• transmitter re-transmit multiple frames

17
Error Control (contd)

• Selective Reject (or Selective Repeat)
• More efficient in terms of link usage
• only retransmit lost or damaged frames (see
  reference book 1)

18
Comparison of Link Protocols

• Focus on link utilization performance
• Assume
• protocol processing time is negligible
• time to transmit an ACK or NACK message is
  negligible
• frame size is constant (L bits)
• Define
• tframe time to transmit a frame, i.e, send out
  all the bits
• (L / R) where R is the data rate of the link
• tprop link propagation time
• constant a tprop / tframe

19
Performance of Link Protocols

• Stop-and-Wait
• Flow control alone
• U 1 / (1 2a)
• With ARQ
• U (1 - P) / (1 2a) where P is the
  probability that a single frame is in error

20
Performance (contd)

• Sliding Window (window size being N)
• Flow control alone
• U 1 when N gt 2a 1, N / (1
  2a) otherwise
• With ARQ
• Go-back-NU (1 - P) / (1 2aP) when N gt 2a
  1, N(1 - P) / (2a 1)(1 - P NP)
  otherwise
• Selective Reject U (1 - P) when N gt 2a 1,
  N(1 - P) / (2a 1) otherwise

21
Local Area Network

• Packet broadcasting network using a share
  transmission medium

Host 2
Host 1
Shared medium
Host 3
..
Host N
22
LAN Architecture

• IEEE 802 Reference Model (Figure 12.1 of text)
• Medium Access Control (MAC)
• govern access to shared medium
• framing, addressing, and error detection
• Logical Link Control
• flow and error control
• logically separated from MAC, i.e., several MAC
  options may be provided for the same LLC

23
LAN Topologies

• Wire Based Topologies
• Bus
• each host tapping into a linear transmission
  medium
• terminators at the end of bust to remove old
  signals
• Tree
• branching cable with no closed loops
• Ring
• host attached to a repeater
• data circulated around in one direction
• Star
• all hosts connected directly to a common central
  node (star coupler)
• either broadcasting or switching

24
Medium Access Control

• Goal
• resolve contention while achieving high network
  utilization
• Where to put control?
• centralized
• distributed
• How to control?
• synchronous techniques
• e.g., TDM
• not good because of bursty data generation by
  host
• asynchronous techniques
• round robin
• reservation
• contention

25
Ethernet

• Carrier-Sense Multiple Access with Collision
  Detection (CSMA/CD)
• IEEE 802.2 MAC standard
• carrier-sense
• sender listening to channel and transmit only if
  detecting an idle channel
• Successor of ALOHA protocols
• collisions allowed
• better efficiency
• maximum achievable utilization for ALOHA is only
  about 18

26
Aloha Protocol

• Users allowed to transmit whenever they have data
  
• not a carrier sense protocol
• colliding frames are useless and will be
  discarded
• Collision happens even when the first bit of a
  new frame overlaps with just the last bit of the
  frame in such a case, both frames have to be
  retransmitted
• Each sender waits a random amount of time before
  retry upon collision
• Performance
• U G e-2G where G average number of
  transmission attempts per frame time
• maximum value of U is 1/2e 0.18 when G 0.5

27
Slotted Aloha Protocol

• Time divided up into discrete intervals
• Users must transmit at beginning of a time slot
• require clock synchronization
• Performance
• U G e-G
• maximum of U is 1/e .368 when G 1

28
CSMA/CD

• Senders abort transmissions as soon as the detect
  a collision
• bandwidth not wasted on transmitting garbled
  frames
• frame size should be large so that collision
  detection happens way before end of transmission
• require collision detection
• sender compares received signals with ones that
  it transmitted

29
CSMA/CD Rules

• When sending a frame

Listen to channel
yes
no
transmit
Is channel idle ?
Listen to channel
collision?
no
yes
Transmit jamming signal
Perform random wait
30
Token Ring

• IEEE 802.5 standard
• collision avoidance based on token passing
  (Figure 13.5 of text)
• host starting transmission only after it gets
  hold of token and not releasing token until
  transmission is successful
• token-holding timer to prevent one host from
  monopolizing token
• performance similar to round robin under heavy
  load
• when majority of stations have data to send
• token maintenance must be reliable
• to prevent loss or duplication of token
• one host acting as a monitor

31
Fiber Distributed Data Interface (FDDI)

• Similar to IEEE 802.5
• Designed for high data rate (e.g., 100 Mbps)
• token frame separate from data frames
• data frames are ready to go in buffer
• early token release
• host to release token as soon as it completes
  data transmission, not waiting for the
  transmitted frame to loop back

32
FDDI Capacity Allocation

• Each host guaranteed a minimum period of
  transmission time each time it gets hold of
  token
• Definitions
• Target Token-Rotation Time TTRT
• Synchronous Allocation time for host i SAi
• constant propagation time for circuit of ring
  DMax
• time required to transmit token frame
  TokenTime
• time to transmit maximum length frame FMax
• Allocation condition
• DMax FMax TokenTime S SAi lt TTRT

33
FDDI Rules

• Each host maintains
• Token-rotation timer TRT initialized to TTRT
  and count down
• Token-holding timer THT
• Late counter LC initialized to 0 used to
  detect error
• Upon receiving token
• 1. THT lt TRT LC lt 02. TRT lt
  TTRT3. enable TRT4. transmit synchronous
  frames for up to SAi time
• 5. enable THT and transmit asynchronous frames
  until THT 0

34
Performance of CSMA/CD and Token Ring

• Appendix 13.B of text
• CSMA/CD
• U 1 / ( 1 2a(1-A)/A )
• where A ( 1 - 1/N )N-1 with N being the total
  number of hosts
• U 1 / ( 1 3.44a ) for large N
• Token ring
• U 1 / ( 1 a/N ) when a lt1, 1 / ( a(a
  1/N) ) otherwise

35
Bridging
LAN A
. . .
A Bridge
Data units MAC frames
LAN B
. . .
36
A 4-Port Bridge
LAN 1
Port 1
LAN 4
LAN 2
Port 2
Port 4
Port 3
LAN 3
37
Bridge Operations

• Frame forwarding between different LANs
• routing
• forward every frame to appropriate LAN
• participation in MAC control of each LAN
  connected
• behave just like a host in each LAN
• frame format conversion

38
Routing with Bridges

• Fixed (static) routing
• routing table preloaded into each bridge
• is simple and requires minimal processing
• not flexible
• Spanning tree routing
• routing tables being automatically built and
  updated

39
Spanning Tree Routing

• Initial setup
• flooding
• Frame forwarding (Figure 14.8 of text)
• each bridge maintains a filtering database
  record format (DA, Output-Port, Timer)
• Address Learning
• use source addresses
• assuming a tree network topology
• build a spanning tree first for an arbitrary
  topology

Making sure the route is fresh
40
Wide Area Networks (WANs)
Data units packets
Network node
Sender host/station
Receiver host/station
. . .
41
Switched Networks

• Circuit switching (legacy telephony)
• dedicated communication path set up between the
  stations
• path consisted of a sequence of links between
  network nodes
• one logical channel dedicated to the connection
  at each link
• e.g., each link uses Time Division Multiplexing
  to operate such channels
• Packet switching (current technology)
• data are transmitted in short packets
• each packet stored and forwarded by a sequences
  of network nodes
• different packets may go through different
  sequences of nodes
• much more efficient for bursty data traffic

42
System model of a packet switching network node
Packets queued in buffer before their turn for
transmission
43
Routing in WAN

• Routing -¾ finding a communication path from
  source to destination
• Must be scalable
• there can be millions of hosts in a WAN
• Must be robust
• many things could go wrong in a WAN
• fixed routing alone would not work

44
Packet Switching Networks

• Two routing approaches (Figure 9.3 of text)
• connectionless (datagram)
• connection-oriented (virtual circuit)
• Performance measures
• total network delay of a packet queueing delay
  propagation delay transmission delay
• packet losses due to buffer overflow

45
Virtual Circuit

• End-to-end connection is set up before data
  transmission
• all packets for one user session carry a unique
  connection id in header
• routing table indexed by connection id
• N2-scaling problem
• How many possible connections are there for N
  stations? N (N-1)

46
Datagram Routing

• Connectionless
• each packet is treated independently by the
  network
• routing table indexed by destination address
• robust
• unreliable service
• quality of service may vary from one packet to
  another
• packets may be out of order at the receiver
• rely on transport layer to perform error control

47
Asynchronous Transfer Mode (ATM)

• Connection-oriented signaling
• virtual channel (VC) connections
• virtual path (VP) connections
• semi- permanent, user controlled or network
  controlled
• aggregation of virtual channels (Figure 11.2 of
  text)solve the N2-scaling problem with
  hierarchical routing
• Small fixed-size packets
• 5-byte header and 48-byte payload (Figure 11.4 of
  text)
• Quality of Service (QoS) support

48
A 4 x 4 ATM Switch
Input ports
Output ports
Upstream node
downstream node
Switch fabric
49
Internet Routing

• Hierarchical routing (Figure 16.10 of text)
• network partitioned into Autonomous Systems
  (ASs)
• Interior Router Protocol (IRP) passes routing
  information within an AS
• Open Shortest Path First (OSPF) algorithm
• Exterior Router Protocol (ERP) passes routing
  information between ASs
• Border Gateway Protocol (BGP)

50
Internet Protocol (IP)

• IP (version 4) header (Figure 16.7 of text)
• IP address hierarchy (Figure 16.8 of text)
• network classes A, B, C
• facilitate hierarchical routing
• IPv6 (Ipng) will use 128-bit addresses
• Subnets
• user defined address hierarchy

51
Unicast, Multicast and Anycast

• Unicast
• single destination
• Multicast
• multiple destinations
• need to minimize packet duplications
• multicast routers and IP tunneling
• Anycast
• closest one among a set of destinations

52
Transport Protocols

• Required because of unreliable services at
  network layer
• connection management
• flow control
• error detection and control

53
Transmission Control Protocol (TCP)

• Data units TCP segments
• TCP header format (Figure 17.14 of text)
• 3-way handshake for connection management
• sender and receiver exchanging SYN packets
• Sliding window flow control
• Reliable service based on acknowledgement/retransm
  ission
• Congestion control window

54
TCP 3-way handshake

• Two-way handshake is sufficient if network
  service is reliable, i.e., if there is no delayed
  duplicate of a connection request or a connection
  acknowledgement. (Figure 17.7 and 17.8 of text)
• Active/Passive open
• Each side to acknowledge explicitly the others
  SYN and sequence number (Figure 17.13 of
  text)
• receiver asking sender if the ACKed connection is
  the right one
• receiver not entering Connected state until its
  SYN is ACKed

55
TCP Socket Interface

• Transport Service Access Point (TSAP) consists of
• IP address
• port number --- application/user address
• System routines
• open/connect/close
• listen/bind
• send/receive

56
Example Socket Program
57
Application-Level Protocols

• File Transfer Protocol (FTP)
• Telnet
• HyperText Transfer Protocol (HTTP)
• used for Web documents
• Simple Mail Transfer Protocol (SMTP)
• used for email messages
• Simple Network Management Protocol (SNMP)

58
R2
R1
R3
R4