Packet Transmission

1
Packet Transmission

• Computers use data grouped into packets for
  transmission
• Local Area Networks
• Wide Area Networks
• Addressing and Routing

2
Concepts of Packets

• Computer networks divide data into small blocks
  called packets
• Packets are send individually
• Often called packet networks and packet switching
  networks
• Motivation for using packets
• Sender and receiver needs to coordinate
  transmission to ensure that data arrives
  correctly
• Helps determine which blocks arrive intact and
  which do not
• Computers often share underlying connections and
  hardware
• Packet switching helps ensure fairness to access

3
Shared Resources

• The first networks
• A 5 MB file at 56 Kbps will take 12 min to
  transfer from A to D
• B C must wait
• Packet networks
• Divide data into packets of 1000 bytes each
• A sends a packet to D taking only 143 ms
• B transmits data to C
• A continues
• No long delays

4
Packets and TDM

• Time Division Multiplexing
• Many resources take turns accessing the shared
  communication resources
• All sources receive prompt service
• The source with less data finishes early

5
Packets and Frames

• Packet refers to a small block of data
• Each hardware technology uses different packet
  format
• Frame denotes packet used with specific type of
  network
• EX RS-232 mechanism
• Does not include a mechanism that allows a sender
  to signal the end of a block of characters
• Sending and receiving computers must agree on
  such details

6
Packets and Frames (Cont.)

• Network systems can choose two unused values to
  define format
• EX RS-232 can use frame delimiters
• Soh start of header
• Eot end of transmission
• Overhead is an disadvantage
• An extra, unnecessary character between blocks of
  data
• Advantageous when large delays or computer
  crashes
• Missing eot indicates sending computer crashed
• Missing soh indicates receiver missed beginning
  of frame

7
Byte Stuffing

• Data and control information must be
  distinguished
• Network system change the data slightly before it
  is sent
• Termed data stuffing
• Insert extra bits or bytes to change data
• Byte stuffing and character stuffing
• Data stuffing used with character oriented
  hardware
• Bit stuffing
• Data stuffing used with bit oriented hardware

8
Byte Stuffing (Cont)

• EX RS-232
• soh and eot must not appear in the data
• Byte stuffing reserves a third character esc
• Marks occurrences of reserved characters

9
Implementing Byte Stuffing

• Sender must scan and perform mapping before any
  data is sent
• Sender replaces characters
• Receiver looks for a combination of esc
  followed by a x, y or z
• Replaces combination by appropriate single
  characters
• Receiver is sure that soh and eot are frame
  delimiters

10
Transmission Errors

• Interference can introduce unwanted electric
  currents in wires
• Interference can cause
• The receiver to misinterpret the data
• The receiver to lose the data sent by sender
• The receiver to detect data, although sender did
  not send any data
• Termed transmission errors
• The problem of lost ,changed or spuriously
  appearing data

11
Parity

• Even or odd
• Sender and receiver must agree in which form to
  use
• Even parity the total number of 1 bits
  (including parity bit) must be even
• EX parity bit for 0100101 is 1
• Parity bit for 0101101 is 0
• Odd parity total number of 1 bits (including
  parity bit) must be odd
• EX parity bit for 0100101 is 0
• Receivers computation of parity must agree to
  senders
• Else receiver reports parity error

12
Parity Checking

• Parity check mechanism requires the sender to
  compute an additional bit, called parity bit
• RS-232 circuits uses parity check to ensure that
  each character arrives intact
• Attach parity bit to each character before
  sending
• Receiver removes the parity bit and performs the
  same operation as the sender
• Verifies the result with the value of the parity
  bit
• If one of the bits is damaged, receiver reports
  error

13
Error Detection

• Parity cannot detect error involving an even
  number of bits
• EX Two 0 bits changed to 1
• Two 1 bits changed to 0
• One 0 bit changed to 1 and vice versa
• Parity is preserved even with errors
• Alternative mechanisms used depending on
• The size of the additional information
• The computational complexity of the algorithm
• The number of bit errors that can be detected

14
Checksums

• Checksum sender treats the data as a sequence
  of binary integers and computes their sum
• Carry bits, if any, are added into the final sum
• Advantages size and ease of computation and
  cost of transmission
• Disadvantages cannot detect all common errors

15
Cyclic Redundancy Checks (CRC)

• CRC hardware uses
• A shift register
• An exclusive or (xor) unit
• To compute a CRC
• Values in shift registers initialized to 0
• Bits of message shifted once at a time
• One bit of message applied at input
• All shift register perform shift operation

Shift registers
X-or unit
16
CRC (Cont.)

• Shift registers contain CRC after entire message
  has been shifted
• Receiver uses identical hardware and compares CRC
• To simplify checking CRC
• Append and additional 16 bits of zeroes to
  message
• Receiver computes CRC over incoming message plus
  incoming CRC
• If no errors,value should be zero
• Uses a polynomial expressed as a power of X
• P( X ) X 16 X 12 X 5 1

17
Burst Errors

• CRC is especially useful with
• Vertical errors
• Burst errors
• Vertical errors appear in a vertical column when
  characters are arranged in rows
• EX Damaged character oriented I/O device
• Burst errors involve changes to a small set of
  bits near a single location
• Caused by electric interference from lighting ,
  electric motor, etc.

18
Frame Format and Error Detection

• Networks usually associate error detection with
  each frame
• If no characters are lost , byte stuffing of CRC
  is not required
• If CRC is not byte stuffed, a single character
  loss causes the receiver to discard two frames
• Individual standards specify whether CRC is
  computed on the message or the encoded frame

19
Local Area Network (LAN)

• Most networks are local i.e. the network fits
  inside a building or a single room
• Permits multiple computers to share resources
• Exa printer accessed by two computers in a
  network
• No separate modems and cables
• Computers must take turns using the shared medium

20
Direct Point to Point Communication

• Point to point network or mesh network
• Each communication channel connects and is
  availabel to two computers
• Advantages
• Independent installation facilitates use of
  appropriate hardware
• Connected computers decide how to communicate
• Easy to enforce security and privacy
• Disadvantages
• Must provide a separate communication
• channel for each pair of computers
• Number of connections grows quickly
• as the size of set increases

21
Direct Point to Point Communication (Cont.)

• Number of connections needed for N computers is
  (N2 N) /2
• Adding Nth computer requires N-1 connections
• Expenses are high because many connections follow
  same physical path
• Ex In fig., 6 connections pass between two
  locations
• If one computer is added to location1 , number of
  connection become 9

22
Shared Communication Channels

• LAN developed during the late 1960s and early
  1970s
• Consists of a single shared medium
• Computers take turns using the medium to send
  packets
• Reduces cost
• Shared network used for only local commnucation
• Large geographic separation introduces longer
  delays
• Shred network with long delays are inefficient
• Providing high bandwidth communication channel
  over long distances is expensive

23
Locality of Reference

• LANs now connect more computers than any other
  type of network
• Locality of reference computer communication
  follows two patterns
• Temporal locality of reference a computer is
  more likely to communicate with the same set of
  computer repeatedly
• Physical locality of reference a computer tends
  to communicate with computers that are physically
  nearby

24
LAN Topologies

• Star topology - All computers attach to a
  central point
• The center of the star network often called hub
• Hub accepts and delivers data
• In practice, star networks seldom have a
  symmetric shape
• A hub often resides in a location separate from
  the computers attached to it

25
LAN Topologies (Cont.)

• Ring topology- arranges for computers to be
  connected in a closed loop
• A cable connects first computer to second,another
  cable connects second to third and so on
• A cable connects the final computer to the first
• Refers to logical connection not physical
  orientation

26
LAN Topologies (Cont.)

• Bus topology- Consists of a single ,long cable
  to which computers attach
• Any computer can send data to any computer
• Coordination is necessary to ensure that only one
  computer sends a single at any time

27
Why Multiple Topologies?

• Each topology has advantages and disadvantages
• Advantages
• Ring makes it easy to coordinate access and
  detect operation
• Star protects network from damage by single wire
• Bus requires fewer wires than star
• Disadvantages
• Entire ring network is disabled if one of the
  cables is cut
• Bus network is disabled if main wire is damaged

28
Ethernet

• Widely used network topology that employs bus
  topology
• Invented at Xerox corporations Palo Alto
  Research center in early 1970s
• Consists of a single coaxial cable, called the
  ether, to which multiple computers connect
• Ethernet coaxial cable also termed segment
• Length limited to 500 m , minimum separation
  between pairs is 3 m

29
Ethernet Operation

• Original Ethernet hardware operated at 10 Mbps
• Fast Ethernet operates at 100 Mbps
• Gigabit Ethernet operates at 1000 Mbps or 1 Gbps
• The Ethernet standard specifies all details
• Multiple computers share access to a single
  medium
• Sending computer has exclusive use of the entire
  cable

30
Carrier Sense on Multiple Access(CSMA) Networks

• Ethernet network does not have a centralized
  controller
• Ethernet employs CSMA to coordinate transmission
  among multiple attached computers
• CSMA - Idea of using the presence of a signal to
  determine when to transmit
• Uses electrical activity on the cable to
  determine status
• Signals informally called a carrier
• If no carrier present , transmit
• Carrier present, must wait for the sender to
  finish
• Technically , Carrier Sense is checking for a
  carrier wave

31
Collision Detect

• CSMA cannot prevent all possible conflicts
• Two computers send a frame at the same time
  finding the cable idle
• Interference between two signals is called a
  collision
• No hardware damage but produces garbled value
• Ethernet standards require sending station to
  monitor signals
• Technically termed as collision detect
• Ethernet mechanism known as Carrier Sense
  Multiple Access with Collision Detect

32
Back Off With CSMA/CD

• While detecting collisions , CSMA/CD recovers
  from them
• To avoid multiple collisions , each computer
  delays retransmission
• Computer choose random delay, between 0 and
  maximum delay , d
• If choice of delay is nearly same , collisions
  occur
• Random delay doubled at each successive
  collisions 0-d , 0-2d, 0-4d ,
• Binary exponential back off
• Doubling the range of random delay after each
  collision

33
Wireless LAN

• Uses antenna to broadcast RF signals
• Data send at 2 Mbps using 900 Mhz frequency
• All computers configured to the same frequency
• Transmitters use low power
• Enough power to travel a short distance
• Metallic obstructions can block the signal
• Cannot use CSMA/CD mechanism

34
CSMA/CA

• Wireless LANs use Carrier Sense Multiple Access
  with Collision Avoidance
• Operation
• Computer 1 transmits brief control message
• Computer 2 receives and responds
• Computer 1 receives response and begins
  transmission
• Control message collide
• Sending station apply random
• back-off before retransmission

35
Local Talk

• A LAN that employs bus topology
• Invented by apple computer corporation
• Designed for apple Macintosh which includes all
  required hardware
• Uses a version of CSMA/CD
• Disadvantages
• Lower bandwidth (230.4 kbps)
• Distance limitations
• Advantages
• Almost free
• Easy to install
• Available on many computers

36
IBM Token Ring

• LANs employing ring topology use token passing
  mechanism
• Token Ring operates as a single shared medium
• A special, short message called token coordinates
  use of the ring
• A token permits transmission of one frame

37
IBM Token Ring (Cont.)

• One token exists on the ring t any time
• Each computer sends one frame before passing
  token
• Token cycles around when no data to send
• Time taken is brief (milliseconds) because
• Token is small
• Handled by ring hardware , not CPU
• IMB Token Ring is best known token passing
  network
• Operates at 16 million bps
• Used with computers from IBM , other vendors and
  printers

38
Fiber distributed data Interconnect (FDDI)

• Token Ring technology
• Transmission rate of 100 million bps
• Uses optical fibers to interconnect computers
• Contains two complete rings ( counter rotating)
  to overcome failures
• Self healing network
• Hardware detects
• a catastrophic failure
• and recovers
• automatically

39
Asynchronous Transfer Mode (ATM)

• A star topology developed by telephone companies
• One or more interconnected switches form a
  central hub to which all computers attach
• Designed to provide high bandwidth
• ATM switch operates at 155 Mbps or faster
• Each connection uses a pair of optical fibers

40
Hardware Addressing

• Any signal sent across a shared network reaches
  all attached stations
• Each station on the LAN is assigned a unique
  numeric value
• Called physical/hardware/media access address
• Sender includes hardware address of intended
  recipients
• Each frame begins with a header consisting of
• Destination address fields
• Source address fields
• Network interface hardware examines address
  fields in frames
• Accepts only those frames where destinations
  address matches stations address

41
LAN Hardware

• Handles details of sending and receiving frames
• Operates without using the stations CPU
• Uses physical addressing to prevent receiving all
  packets

42
Addressing Schemes

• Static addressing scheme
• Hardware manufacturers assign unique physical
  address
• Address does not change unless hardware is
  replaced
• Easy to use and permanent
• Configurable addressing scheme
• Mechanism to set a physical address
• Used by most network administrators because
• Address are permanent
• No large addresses because unique only to a
  single network
• Interface can be replaced without changing
  computers physical address

43
Addressing Schemes (Cont.)

• Dynamic addressing scheme
• Mechanism that automatically assigns a physical
  address to the station when the station first
  boots
• Tries random numbers until a unique address is
  found
• Advantages
• No need for manufacturers to coordinate in
  assigning addresses
• Allows each address to be smaller
• Uniqueness is only important within a single LAN
• Disadvantages
• Lack of permanence
• Potential conflict

44
Broadcasting

• Refers to transmissions available to a large
  audience
• All stations receive a copy of the signal each
  time a frame is transmitted
• To make broadcasting efficient, most LANs use
  broadcast address
• Hardware interface recognizes both the special
  broadcast address and the stations physical
  address
• A frame with either of the two addresses is
  accepted and delivered to the computers
  operating system
• Ex Finding a printer by its name

45
Multicasting

• Broadcasting is extremely inefficient because
• Processing and discarding a frame requires
  computational resources
• Multicasting operates like broadcasting
• Single copy of the frame travels across the
  network
• All network interfaces receives a copy
• Interface hardware must be programmed with
  specifications
• Accepts or rejects frames according to the
  specifications

46
Multicast Addressing

• Some addresses reserved for multicast
• Interface is programmed to recognize only the
  computers address and the broadcast address
• Application wishing to receive multicast frames
  must inform interface
• Multicast address must be chosen for an
  application
• Application must be configured to use the address
• Passes multicast address to the interface
• Interface adds the address to the set it
  recognizes

47
Identifying Packets Contents

• The address does not specify what the packet
  contains
• Each frame contains additional information
  specifying the type of the contents
• Two methods used to identify contents of the
  frame
• Explicit frame type
• Network hardware designers specify how type
  information is included in the frame
• Different values used to identify various frame
  types
• Also called self identifying frame
• Implicit frame type
• Frame carries only data
• Sender and receiver must agree on the contents of
  the frame

48
Frame Headers Format

• Frame format is defined by LAN technology
• Most LAN technologies define a frame consisting
  of two parts
• Frame header
• Contains information such as source and
  destination addresses
• Data area or payload
• Contains the information being sent
• All frames have same header size but different
  data area

49
Ethernet Frame Format

• Begins with a header with three fields
• 64-bit preamble contains alternating 1s and 0s
  for synchronization
• First two fields contains physical address
• Ethernet uses 48-bit static addressing scheme
• Third field contains16-bit frame type
• Ethernet types have been standardized

50
Ethernet Frame Format(Cont.)

• DIX standard specifies the values used in the
  header fields and their meanings

51
Networks Without Self Identifying Frames

• Some technologies do not include type field
• Type of data is specified by two approaches
• To use a single format of data
• To use first few octets of the data field to
  store type information

52
Type Information Standard

• IEEE standard includes a field to specify
  standards organization and individual field types
• Known as Logical Link Control(LLC) Sub Network
  Attachment Point(SNAP)
• LLS specifies that a type field follows
• SNAP contains two fields
• Organizationally Unique Identifier (OUI)
  identifying organization
• Second contains a type value defined by
  organization
• LLC/SNAP type field makes it possible to
  broadcast frames

53
Network Analyzer

• A device used to determine how well a network
  system is performing
• Most analyzers are portable flexible
• Consists of a standard portable computer and LAN
  interface
• User configures parameters used by analyzers
• Network interface hardware is in promiscuous mode
  i.e. accepts all frames
• Can be used to debug problems on a network
• Network analyzers can be configured for specific
  analysis

54
Network Interface Hardware

• Networks operates at a much higher speed than a
  CPU
• Network adapter card/ network Interface Card
  (NIC)
• Connects computer to a network and handles all
  details of packet transmission and reception

55
NIC

• Operates independent of the CPU
• Handles the details of accessing the medium and
  transmitting bits
• Ex Receiving a packet
• CPU allocates buffer space in memory
• Instructs NIC to read incoming packets
• NIC copies, verifies and checks the frame
• If address matches, NIC stores a copy
• Interrupts the CPU

56
Thick Ethernet Wiring

• Informally called thick wire Ethernet or Thicknet
• Consists of a large coaxial cable
• Digital hardware
• NIC handles digital aspects including error
  detection and address recognition
• Analog hardware
• Transceiver handles analog signals
• Must for each computer
• Attaches directly to the Ethernet cable
• A separate called Attachment Unit Interface(AUI)
  connects the transceiver and the NIC

57
Thick Ethernet Wiring (Cont.)

• AUI cables contains many wires
• Two for data
• One each for providing power to and controlling
  transceiver
• Cable terminated by a terminator
• It is a resistor connecting center wire in a
  cable to the shield
• Prevents reflection of the signal from the end

58
Connection Multiplexing

• Connection multiplexor allows multiple computers
  to attach to a single transceiver
• Provides exactly the same signal as a transceiver
• Cable from each computer connects to a port on
  multiplexor
• A single AUI cable connects the multiplexor to
  the Ethernet

59
Thin Ethernet Wiring

• Informally called thin wire Ethernet or Thinnet
• Uses a thinner, more flexible coaxial cable
• Advantages
• Costs less to install and operate
• No external transceivers are needed
• Uses BNC connectors instead of AUI cable
• Both thick and thin cables are coaxial, requires
  termination and use the bus topology

60
Twisted Pair Ethernet

• Formally called 10 Base T
• Also twisted pair Ethernet or simply TP Ethernet
• An electronic device called an
• Ethernet hub serves as a
• center of the network
• Connection from NIC to
• the hub uses twisted pair
• wiring with RJ-45
• connectors

61
Office Wiring Schemes
Thicknet
Thinnet
10Base-T
62
Topology Paradox

• Network technology can use a variety of wiring
  schemes
• Technology determines logical topology
• Wiring scheme determines the physical topology
• Physical topology can be different can be
  different from logical topology
• Ex A twisted pair Ethernet forms a star but
  functions like a bus

63
NIC and Wiring Schemes

• Network interface supports multiple wiring
  schemes
• A single Ethernet NIC has three connectors
• Can use only one wiring scheme at a time
• Wiring can be changed without changing NIC

64
Other Network Technologies

• Different technologies accommodate a variety of
  wiring scheme
• Ex The original Local Talk uses transceivers
  like thicknet
• Uses point-to-point connection between pairs of
  transceivers
• Although Local Talk is a bus technology it
  sometimes uses hub technology

65
LAN Design

• Distance limitation is a fundamental point
• LANs use a shared communication media
• CSMA/CD or Token passing is used to guarantee
  fair access to medium
• LAN is designed with a fixed maximum cable length
  to minimize delays
• An electrical signal gradually becomes weaker as
  it travels along a copper wire
• This puts a limitation on the maximum length of
  the wire allowed

66
Fiber Optic Extensions

• LAN extension mechanisms insert additional
  hardware components that can relay signals across
  longer distances
• Ex Optical fibers and a pair of fiber modems
• Fiber has low density and high bandwidth
• Provides a connection between a computer and a
  distant Ethernet
• Inserted between the network interface on a
  computer and a remote transceiver

67
Repeaters

• An analog electronic device that continuously
  monitors signals on each cable
• Used to extend LAN
• Connects two Ethernet cables called segments
• When it senses a signal on one cable, it
  transmits an amplified copy on another
• A repeater can double the effective length
• Any pair of computers on the extended LAN can
  communicate

Repeater
68
Repeaters(Cont.)

• Each repeater and segment along the path increase
  delay
• Ethernet standards limits that no more than four
  repeaters separate any pairs of stations
• The connection can be extended by using fiber
  modems and Fiber Optic Intra Repeater Link (
  FOIRL)
• Along with valid transmissions, the repeaters
  propagates a collision or electrical interference

69
Bridges

• An electronic device that connects and extends
  two LAN segments
• Handles complete frames and uses same network
  interface as a conventional computer
• Helps isolate problems by forwarding only
  complete and correct frames
• Any pair of computers can communicate on extended
  LAN

Bridge
70
Frame Filtering

• A typical bridge consists of a conventional
  computer with a CPU, memory and two network
  interfaces
• A bridge performs frame filtering
• Does not forward a frame
• unless necessary
• Uses physical address to
• determine whether to forward a frame
• Called adaptive or learning bridges because they
  learn the locations of computers automatically
• Uses source address to list computers

71
Bridged Networks

• Bridged networks running for a long time
  restricts frames to the fewest segments necessary
• Propagation principle
• In the steady state, a bridge forwards each frame
  only as far as necessary
• Permits communication on separate segments at the
  same time(parallelism)
• To optimize performance, a set of computers that
  interact frequently should be attached to the
  same segment

72
Bridging Between Buildings

• An optical fiber and pair of fiber modems are
  used to extend one of the connections between a
  bridge and a LAN segment
• The use of a bridge has following advantages
• Single fiber connection makes it less expensive
• Individual computer can be added or removed
  without installing or changing the wiring
• Communication in buildings is independent

73
Bridges Across Longer Distances

• Involves a long distance point-to-point
  connection and special bridge hardware
• Leased serial line used because it is less
  expensive
• Leased satellite channel used for communication
  across an arbitrary distance
• Bridge hardware has
• two main functions
• Filtering frames
• Buffering

74
Cycle Of Bridges

• A bridge network can span many segments
• Not all bridges allowed to broadcast frames
• A cycle of bridges causes infinite number of
  frames

Eight segment bridged network
Bridges connected in a cycle
75
Distributed Spanning Tree(DST)

• To prevent infinite loops, a bridged network
  cannot allow
• All bridges forwarding all frames
• A cycle of bridged segments
• To prevent loops, bridges configure themselves
  automatically
• When a bridge first boots, it communicates with
  other bridges
• Computes Distributed Spanning Tree algorithm
• To decide which bridges will not forward frames
• DST prevents bridges from introducing a cycle
• After DST completes, bridges are arranged in a
  form of a tree

76
Switching

• A switched LAN consists
• of a single electronic
• device that transfers frame among many
  computers
• A switch simulates a bridged LAN with one
  computer per segment
• Consists of multiple ports each attached to a
  computer
• One-half of the computers can send data at the
  same time

• A switched LAN consists
• of a single electronic
• device that transfers frame among many
  computers
• A switch simulates a bridged LAN with one
  computer per segment
• Consists of multiple ports each attached to a
  computer
• One-half of the computers can send data at the
  same time

77
Switches And Hubs

• Switches cost more per connection than a hub
  because it provides higher aggregate data rates
• Combination is used to reduce cost
• A hub connects to each port on switch
• Each computer connects to one of the hub
• Each hub appears to be a single LAN segment
• Switch makes it appears that bridges connect all
  segments
• Communication can occur in parallel

78
Digital Telephony

• Digitization is performed by an analog-to-digital
  converter(A-to-D converter
• Takes analog input a signal
• Samples the signal regularly
• Computes a corresponding value at time of the
  sample
• Known as Pulse Code Modulation (PCM)
• Samples once every 125 µ sec and converts into an
  integer between 0 and 255

79
Synchronous Communication

• Telephone industry have devise complex digital
  communication systems
• Voice system use synchronous or clocked
  technology
• Most data networks use asynchronous technology
• Data moves at a precise rate in synchronous
  network
• Network does not slow down as traffic increases
• Telephone systems transmits additional
  information along with digitized data to ensure
  continuous transmission

80
Digital Circuits and DSU/CSU

• leased digital circuits from common carriers form
  the fundamental building blocks for long distance
  computer networks
• Standards differ between computer and telephone
  industry
• Data Service Unit/Channel Service Unit (DSU/CSU)
• Hardware needed to interface a computer to a
  digital circuit
• CSU portion
• Handles line termination and diagnostics
• Helps in installing and testing circuits
• Uses bit stuffing
• DSU portion
• Translates data between
• two digital formats

81
Telephone Standards
82
DS Standards

• A single voice channel requires 64 Kbps(8000 8
  bit samples/sec)
• Digital circuits are classified according to a
  set of telephone standards
• Most popular circuit types in North America
• T1 and T3
• Digital signal level standards or DS standards
• Specify how to multiplex phone calls onto a
  single connection
• 28 T1 circuits can be multiplexed over single T3
  circuit

83
Lower Capacity Circuits

• T1 circuit is too expensive
• Fractional T1 circuits
• Capacity much less than 1.544 Mbps
• Most popular fractional T1 rate is 56 Kbps
• Time Division Multiplexing (TDM)
• Concept of subdividing T1 circuits

84
Intermediate Capacity Digital Circuits

• Slightly more than T1 and less than T3
• Inverse multiplexing is used
• Allows one to lease multiple T1 circuits
• Multiple circuits acts like a single higher
  capacity circuit
• Inverse multiplexor is needed at each end of line
• DSU/CSU may be required if not built in inverse
  mux

85
Highest Capacity Circuits

• Also termed as trunk
• Synchronous Transport Signal (STS) standards
• Specifies details of high speed connections
• Serves connections across country or between
  countries

86
Optical Carrier

• Higher data rates associated with the STS
  standards require optical fiber
• STS referred to electrical signals
• OC refers to optical signals
• Both can be concatenated (suffix C)
• C denotes a circuit with no inverse multiplexing
• OC-3 consists of 3 OC-1 operating at 51.840 Mbps
  each
• OC-3C (STS-3C) is a single circuit operating at
  155.520 Mbps
• Single circuit is more flexible

87
Synchronous Optical NETwork (SONET)

• Used in North America
• Known as Synchronous Digital Hierarchy(SDH) in
  Europe
• Specifies details about framing, multiplexing and
  synchronization
• Size of the SONET frame depends on the bit rate
• Can be used to build a high capacity ring network
  with multiple data circuits
• Mostly used to define framing and encoding

STS-1 SONET frame
88
Local Subscriber Loop

• Termed local loop or local subscriber line
• Connection between the phone company Central
  Office and individual subscriber residence
• Uses analog signals
• Most subscribers use a telephone to dial a local
  service provider
• Voice bandwidth and signal-to-noise ratio of
  telephone lines limit the rate at which bits are
  sent

89
ISDN

• Integrated Services Digital Network
• Provides digitized voice and data over local loop
  wiring
• Uses twisted pair copper wiring
• Offers three separate digital channels
• B, B and D (2B D)
• The two B channels
• Operate at 64 Kbps each
• Carries digitized voice, data or compressed video
• The D channel
• Operates at 16 Kbps
• Intended as a control channel
• Manages or terminates a session
• Both B channels bonded as a single channel

90
Asymmetric Digital Subscriber Line

• Asymmetric service, termed ADSL
• Bit rate in one direction is much higher
• Typical users receive more information than they
  send
• ADSL provides higher bit rate downstream (to the
  subscriber) than upstream (from subscriber to the
  provider)
• Maximum downstream rate is 6.144 Mbps
• Maximum upstream rate is 640 Kbps
• Operates on local loop wiring

91
ADSL (Cont.)

• ADSL is adaptive
• Modems probe the line and agree to communicate
  using techniques to optimize line
• Uses Discrete Multitone Modulation (DMT)
• Combination of frequency
• division and inverse multiplexing
• Divides bandwidth in 286
• separate frequencies or
• sub channels
• Selects the best frequencies
• and modulation techniques

92
Other DSL Technologies

• Symmetric Digital Subscriber Line (SDSL)
• Provides symmetric rates in both direction
• Businesses prefer SDSL
• Can operate over local loops
• High-rate Digital Subscriber Line (HDSL)
• Provides DS-1 (1.544 Mbps) in two directions
• Requires two independent twisted pairs
• Able to tolerate failure
• Very-high bit rate Digital Subscriber Line(VDSL)
• Data rate of up to 52 Mbps
• Requires intermediate concentration parts

93
Cable Modem Technology

• Uses cable TV wiring
• Offers higher speed and less susceptibility to
  electromagnetic interference
• Consists of high capacity coaxial cable
• Uses broadband signaling (frequency division
  multiplexing)
• One pair of cable modems is required for each
  subscriber
• When subscribers are more, Time Division
  Multiplexing is used
• One frequency for a set of subscribers

94
Upstream Communication

• CATV is designed for downstream direction only
• Dual path approach
• Cable system handles only downstream traffic
• Upstream traffic travels across a dial-up
  telephone connection
• Needs hardware interface device to connect cable
  modem and dial-up modem
• Hybrid Fiber Coax (HFC)
• Combination of optical fibers and coaxial cables
• HFC can only be used with modified infrastructure
• Trunk lines replaced by optical fibers
• All amplifier modified to be bi-directional

95
Large Networks

• Local Area Networks (LAN)
• Spans a single building or campus
• Metropolitan Area Networks (MAN)
• Spans a single city
• Wide Area Networks (WAN)
• Spans sites in multiple cities, countries or
  continents
• WAN differs from LAN
• Must be able to grow (scalability)
• Must deliver reasonable performance to large
  sized networks
• Must provide capacity for simultaneous
  communication

96
Packet Switches

• WAN is constructed from many switches to which
  individual computers connect
• Called a packet switch
• Moves complete packet from one connection to
  another
• Consists of a small computer with processor,
  memory and I/O devices
• Two types of I/O connectors
• One operates at high speed and connects to other
  packet switches
• Second operates at low speed and connects switch
  to computers

97
Forming a WAN

• A set of packet switches are interconnected
• A switch has multiple I/O connectors
• Forms many different topologies
• Can connect multiple computers

98
Store And Forward

• WAN permits many computers to send packets
  simultaneously
• Uses store and forward switching
• A packet switch must buffer packets in memory
• The store operation
• Occurs when a packet arrives
• Copies the packet in memory
• Informs the processor
• The forward operation
• Processor examines the packet
• Determines the destination path
• Start the output device
• Buffers a short burst of packets that arrives
  simultaneously

99
Physical Addressing

• Each computer assigned a physical address
• For efficient forwarding hierarchical addressing
  scheme is used
• Divides an address into multiple parts
• First part indicates a packet switch
• Second part identifies computer attached to that
  packet switch
• An address is represented as a single binary value

100
Next-hop Forwarding

• A packet switch uses destination address to
  forward each packet
• Next-hop forwarding
• Switch contains information about the next place
  (hop)
• Depends on the packets destination and not on
  the source
• Called source independent

101
Hierarchical Addresses To Routing

• Routing
• Process of forwarding a packet to its next hop
• Routing table
• Table used to store next-hop information
• All destination addresses have an identical first
  part
• Using only the first part helps in
• Reducing computation time
• Shortening routing table
• The final packet switch
• uses the second part

102
Routing in a WAN

• A WAN with large capacity can be build by
  increasing switching capacity
• Interior switches
• Handles load, but need not have computers
  attached
• Exterior switches
• Packet switches to which computers attach
• Both switches have routing tables
• Universal routing
• Routing table contains next-hop route for each
  possible destination
• Optimal routes
• The next-hop value points to the shortest path to
  the destination

103
Routing in a WAN (Cont.)

• A graph can model a network
• Each node corresponds to a packet switch
• Each link corresponds to a direct connection

104
Default Routes

• A graph representing a large WAN may contain many
  duplicate entries
• Default route or Default routing table
• A long list of entries having same next-hop value
  is replaced by a single entry
• Only one default entry is allowed in any routing
  table
• A default entry is present only if more than one
  destination has the same next-hop value

105
Routing Table Computation

• Static routing
• A program computes and installs routes when a
  packet switch boots, the routes do not change
• Dynamic routing
• A program builds an initial routing table and
  then alters the table as condition changes
• Static routing is simple and has low overhead
• Most networks use dynamic routing because
• Handles problems automatically
• Modifies routes to accommodate failures

106
Shortest Path Computation

• Dijkstras algorithm
• Finds the distance along a shortest path from a
  single source node to each of the other nodes in
  the graph
• A next-hop routing table is constructed during
  the computation of shortest path
• Uses weights on edges as a measure of distance
• A path with fewest number of edges may not be the
  path with least weight

The shortest path between 4 5 is shown darkened
107
Distributed Route Computation

• Each packet switch computes its routing table
  locally
• Informs the network of the result
• Sends routing information to neighbors
  periodically
• Each packet switch learns the shortest path to
  all destinations
• Produces the same next-hop routing table as
  Dijkstras algorithm
• Allows the network to adapt to a failure

108
Distance Vector Routing

• Distance-vector algorithm uses distributed route
  computation
• Each link in network is assigned a weight
• Distance to a destination is defined to be the
  sum of weights along the paths
• A packet switch periodically updates the network
• Each message contains pairs of (destination,distan
  ce)

109
Link-State Routing (SPF)

• Also called shortest path first or SPF routing
• Packet switches sends messages with status of the
  link
• Message broadcast to all switches
• Each switch collects information and builds the
  graph of the network
• Switches use Dijkstraa algorithm to produce
  routing table
• SPF algorithm can adapt to hardware failures

110
WAN Technologies

• ARPANET
• One of the first packet switched WANs
• Fast when invented, slow by current standards
• X.25
• Developed an early standard for WAN technology
• More popular in Europe
• Frame relay
• Accepts and delivers blocks of data
• Must operate at high data rates
• Switched Multi-megabit Data Service( SMDS)
• Offered by long-distance carriers
• Operates at speed faster than frame relay

111
Asynchronous Transfer Mode (ATM)

• Provides voice, video and data services across a
  wide area
• Has high data rates, low delay and low jitter
  (low variance in delay)
• Data divided into fixed sized packets called
  cells
• Each ATM cell has 53 octets
• 5 for header information and 48 for data
• A Constant Bit Rate (CBR) is specified for voice
  or video
• Uses switches as primary building blocks
• Uses optical fiber as interconnection media

112
Network Ownership

• Private networks
• Owned and used by a single company or an
  individual
• Public networks
• Owned by common carriers such as telephone
  networks
• Anyone can subscribe to the service and connect a
  computer
• LAN technology is most often used for public
  networks
• Almost all public networks are WANs
• The chief advantage with private networks is
  complete control
• Public networks are flexible and able to use
  state-of-the-art networking without maintaining
  technical expertise

113
Virtual Private Networks (VPN)

• Combines advantages of both private and public
  networks
• Allows a company with multiple sites to have
  private network
• Uses a public network as a carrier
• VPN technology restricts traffic only between the
  companys sites
• A special hardware and software system is placed
  between companys and public network
• VPN encrypts each packet before transmission
• Network manager must also configure routing

114
Service Paradigm

• Connection-oriented service
• Operates analogous to a telephone system
• Requires a pair of computers to establish a
  connection before sending data
• Either computer can choose to terminate the
  connection
• Connection-less service
• Operates analogous to a postal mail system
• Computers do not need to establish a connection
  before they can communicate
• Accepts and delivers individual frames that each
  specify a destination
• Less initial overhead

115
Connection Types

• Permanent connection
• Dedicated wires between a pair of computers
• Is persistent and always available
• Does not require maintenance
• Always ready to accept data
• Switched connection
• Must establish a connection to communicate
• Each computer maintains physical connection to
  network
• Is flexible and general
• Permanent connections survive
• either a computer or
• a network reboot

Switched Connections
116
Examples of Service Paradigms
117
Connection Identifiers

• Connection oriented service uses abbreviated
  addresses
• A small integer used to communicate after a
  connection is established
• Ex ATM network uses 28-bit connection
  identifiers
• The computer places the identifier in each
  outgoing cell
• ATM divides connection identifiers into two parts
• 12-bit virtual path
• identifier ( VPI )
• 16-bit virtual circuit
• identifier ( VCI)

ID - Identifier
118
Network Performance Characteristics

• Delay
• Specifies how long it takes for a bit of data to
  travel across the network ( in seconds)
• Propagation delay
• Time a signal requires to travel across a wire or
  optical fiber
• Switching delay
• Delay introduced by electronic devices in network
• Access delay
• Delays caused when waiting to access a shared
  media
• Queuing delay
• Occurs in packet switched WAN because it enqueues
  packets

119
Network Performance Characteristics (Cont.)

• Throughput
• Measure of the rate at which data can be sent
  through a network
• Specified in bits per second, bps
• Throughput is measure of capacity, not speed
• Throughput and delay are related by
• D D0 / ( 1 U)
• D0 idle network delay
• U current utilization between 0 and 1
• D Effective delay
• Volume of data present on the network
• Product of delay and throughput ( T D )

120
Protocols

• Protocol
• A set of rules that specify the format of
  messages and the appropriate action required for
  each message
• Protocol software
• The software that implements such rules
• Application programs do not interact with network
  hardware
• Communication software is divided into multiple
  protocols
• Protocols are designed and developed in complete,
  cooperative sets called suites or families

121
Protocol Design

• Layering model
• Describes one way a communication problem can be
  divided into sub-pieces called layers
• ISO defined a 7-layer model

122
The Seven Layers

• Layer 1 Physical
• Corresponds to basic network hardware
• Ex RS 232
• Layer 2 Data Link
• Specifies how to organize data into frames and
  transmit over a network
• Ex Frame format and CRC
• Layer 3 Network
• Specifies how addresses are assigned and how
  packets are forwarded
• Layer 4 Transport
• Specifies how to handle details of reliable
  transfer

123
The Seven Layers (Cont.)

• Layer 5 Session
• Specifies how to establish a communication
  session with a remote system
• Ex Security details
• Layer 6 Presentation
• Specifies how to represent data
• Needed to translate from the representation on
  one computer to another
• Layer 7 Application
• Specifies how one particular application uses the
  network
• Ex specifications for an application that
  transfers files

124
Stacks Layered Software

• When protocol software sends or receives data,
  each module only communicates with the next
  highest and lowest level
• Incoming and outgoing data passes through each
  layer

125
Stacks Layered Software (Cont.)

• Vendors use the word stack to refer to protocol
  software
• Software in the given layer on the sending
  computer adds information to outgoing data
• Software in the same layer on receiving computer
  uses the additional information to process
  incoming data

Stacks are incompatible
126
Multiple, Nested Headers

• Each layer places additional information in a
  header before sending data to a lower layer
• The header corresponding to the lowest-level
  protocol occurs first

127
Scientific Basis for Layering

• Layering principle
• Layer N software on the destination computer must
  receive the exact message sent by layer N
  software on the sending computer
• Whatever transformation a protocol applies before
  sending a frame must be completely reversed when
  the frame is received

128
Techniques Protocols Use

• Sequencing for Out-of-order Delivery
• Connectionless networks often deliver packets out
  of order
• To handle this transfer protocol use sequencing
• Each packet has a sequence number
• Sequencing to Eliminate Duplicate Packets
• Malfunctioning hardware causes packet duplication
• Ex a transceiver using CSMA/CD
• Sequencing solves the problem of duplication

129
Techniques Protocols Use (Cont.)

• Retransmit ting Lost Packets
• Protocols use positive acknowledgement with
  retransmission
• Protocol software uses a timer
• Protocols bound the maximum number of
  retransmissions
• Avoiding Replay Caused by Excessive Delay
• Replay means that an old, delayed packet affects
  later communication
• A correct packet may be discarded as a duplicate
• Protocols mark each session with a unique ID

130
Techniques Protocols Use (Cont.)

• Flow Control to Prevent Data Overrun
• Data overrun A computer sends data faster than
  the destination can absorb
• Flow control techniques

4 packet Window
Stop-and-go Flow control
Sliding Window
131
Techniques Protocols Use (Cont.)

• Mechanisms to Avoid Net Congestion
• Congestion More packets arrive than can be send
• The queue grows and the effective delay increases
• Congestion collapse
• Persistent congestion causes the entire network
  to become unusable
• Protocols avoid congestion collapse by
• Arranging for packet switches to inform senders
  when congestion occurs
• Use packet loss as as estimate of congestion

Congestion prone network
Packet Switches