Data Communication - PowerPoint PPT Presentation

Loading...

PPT – Data Communication PowerPoint presentation | free to download - id: e3e69-ZDc1Z



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Data Communication

Description:

Two wire -'telegraph wires' seen in old films. ... Curvature of earth, or mountains, or buildings. Data Transmission Terminology ... – PowerPoint PPT presentation

Number of Views:212
Avg rating:3.0/5.0
Slides: 129
Provided by: sergeysa
Learn more at: http://www.isy.vcu.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Data Communication


1
Data Communication
2
What is data communication?
  • Data communications deals with the transmission
    of signals in a reliable and efficient manner
  • Ultimately, its about transmitting data (i.e.,
    bits) across some physical transmission medium
  • Electricity - copper wire, twisted pair, undersea
    cable
  • Light - infra-red through air, laser through
    fibre-optic cable
  • Electromagnetic radiation - radio, microwave,
    satellite

3
What is understood by the term communication?
  • The term communication is defined as the act of
    disseminating information.
  • It presupposes that
  • there is information to disseminate
  • the desire or requirement to disseminate exists
  • there is an agency to send/transmit information
  • there is a means of encoding information
  • there is a medium to carry the information
  • there is a recipient to receive the information
  • the recipient is capable of understanding the
    information received

4
Communication
  • Let us generalize the process just described
  • In any communication between two entities the
    following properties are required
  • Modulation
  • Signal compatibility
  • Signal strength
  • Data rate
  • Protocol
  • Demodulation
  • In a face-to-face conversation between two
    individuals following takes place
  • Conversion of brain waves into speech.
  • Agreement of both individuals on which vocabulary
    to use.
  • Agreement of both individuals on volume level at
    which both can be heard comfortably.
  • Agreement of both individuals on the rate of
    talking at which each can understand the others
    speech.
  • Agreement of both individuals on the rules used
    to decide when to speak and when to listen, i.e.
    how the flow of information is managed.
  • Conversion of the audio signals into brain waves.

5
Communication Model
  • Source
  • generates data to be transmitted
  • Transmitter
  • Converts data into transmittable signals
  • Transmission System
  • Carries data
  • Receiver
  • Converts received signal into data
  • Destination
  • Takes incoming data

6
Diagram of Simplified Communication Model
7
Key Communications Tasks
  • Transmission System Utilization
  • Interfacing
  • Signal Generation
  • Synchronization
  • Exchange Management
  • Error detection and correction
  • Addressing and routing
  • Recovery
  • Message formatting
  • Security
  • Network Management

8
Simplified Data Communications Model
9
Basic Elements of a Communication System
  • In any communication between two entities the
    following 10 elements can be identified
  • A Sender.
  • A Receiver.
  • Addressing, to identify where the Receiver is.
  • Protocol a set of co-operation rules to achieve
    communication.
  • Transmission code - an agreed language to be
    used.
  • Transmission rate - the speed at which what is
    being communicated is being sent.
  • Transmission synchronisation - how to recognise
    what is being communicated.
  • Transmission medium.
  • Error detection and correction.
  • Transmission efficiency - how much overhead must
    be added to manage the transmission.

10
Transmission Media
  • Two wire -telegraph wires seen in old films.
  • Simplest arrangement, with two wires, separated
    by air.
  • Can pick up interference, and suffer crosstalk.
  • Only reliable for low data rates.
  • Twisted Pair - currently used for domestic phones
  • Two insulated wires twisted together.
  • Any interference affects both wires equally.
  • May also have an additional protective screen of
    metallic foil shielded twisted pair.
  • Suitable for short distance medium speed links.
  • Suffers from skin effect, leading to higher
    resistance at higher data rates.
  • Skin effect HF signals carried only on skin
    of wire, in effect reducing the area of the wire
    from a solid wire to a tube of the same diameter.

11
Transmission Media
  • Coaxial cable - commonly seen on TV aerial leads
  • Single central wire, separated from woven outer
    conductor by plastic insulation.
  • Not prone to interference.
  • Can support medium to high data rates.
  • Optical Fibre
  • Similar to coaxial cable in appearance
  • Uses single strand of glass as core, with light
    shield around it.
  • Immune to electrical interference , and difficult
    to eavesdrop
  • Often used in industrial or other electrically
    noisy environments.
  • Capable of high data rates
  • Mechanically weaker than electrical wires, and
    difficult to join.

12
Transmission Media
  • Microwaves -ultra high frequency radio waves
  • Line of sight from sender to receiver.
  • No need for wires, so good across rivers, or main
    roads
  • Extremely high data rates
  • Satellite microwaves
  • Mainly through space so long lines of sight.
  • Little human interference, but affected by
    extreme solar activity.
  • Terrestrial microwaves
  • Need repeater stations if lines of sight short
  • Curvature of earth, or mountains, or buildings

13
Data Transmission Terminology
  • Transmission may be simplex, half-duplex or
    duplex.
  • Simplex in one direction only.
  • Half-duplex in both directions, but only in one
    direction at any time.
  • Full-duplex in both directions simultaneously,
    if required.
  • Transmission media may be guided or unguided.
  • Guided the medium is bounded and the
    transmission contained within it (e.g.
    fibre-optic or electrical cable)
  • Unguided the medium is unbounded (e.g. radio
    waves in the air, or in space).

14
Data Transmission Terminology
  • In a direct link, (or data link), a transmission
    path
  • Propagates signals directly from transmitter
    (sender) to receiver
  • With no intermediate devices.
  • except amplifiers (or repeaters) to increase
    signal strength.
  • In guided transmission media
  • A configuration is point-to-point if it provides
    a direct link between two devices, and those are
    the only two devices sharing the medium.
  • A configuration is multipoint, if more than two
    devices share the same medium.

15
Data Transmission Terminology
16
Data Encoding
  • Encoding means changing how data are represented.
  • This can be for convenience
  • Morse code alphabet used in early radio
    transmissions.
  • Encoding to hide the meaning of data is
    encryption.
  • Computer data are represented in an encoded form
    for storage or transmission within and between
    computers.
  • The most common codes used to store digital data
    are
  • ASCII (American Standards Committee for
    Information Interchange)
  • EBCDIC (Extended Binary Coded Decimal Interchange
    Code)

17
Data Encoding
  • Data are transmitted using electromagnetic
    signals.
  • Data exists in analogue or digital forms.
  • Analogue or digital data can be encoded using
    either analogue or digital signals.
  • For example digital data can be transmitted using
    analogue signals.
  • The telephone network traditionally used analogue
    signals to represent voices.
  • The telephone network was well-established when
    transmission of digital computer data became
    necessary.
  • The latter allows normal computer communications
    using widely available telephone lines.
  • This is achieved using Modems.

18
Analog vs. Digital Signal
19
Data Encoding
20
Signalling Technologies
  • Baseband is the transmission of digital signals
    without modulation.
  • In a baseband communication network, digital
    signals (0s and 1s) are put onto the medium as
    voltage pulses.
  • The entire bandwidth is consumed by the signal.
  • Broadband uses coaxial cable to provide data
    transfer by means of analogue signals.
  • The bandwidth is divided in different frequency
    bands or channels.
  • In a broadband communication network involving
    computers, digital signals are passed onto the
    medium through a modem and transmitted over one
    of the channels. So, several different
    communication networks can be implemented over
    the same medium.

21
Signalling Technologies
  • Analogue transmission is used to mean the
    transmission of analogue signals without regard
    to their content.
  • Digital transmission, on the other hand, is used
    to mean the content of the signal.

22
Data Transmission Data and Signals
23
Data vs. Signal
24
Data Transmission Treatment of Signals
25
Transmission Synchronisation
  • Synchronisation is essential for transmitter and
    receiver to understand each other.
  • In serial transmission the following types of
    synchronisation are required
  • Bit synchronisation - how to detect each bit.
  • Byte or character synchronisation - how to group
    the bits to make a character or byte.
  • Block synchronisation - how to group the
    characters/bytes to make a block (a frame or a
    packet)
  • Bit synchronisation depends on how the signal is
    encoded

26
Transmission Synchronisation
  • In serial transmission there are two standard
    ways of achieving character and block
    synchronisation
  • Asynchronous Transmission or Character
    Synchronisation
  • The time interval between characters is random.
  • Each character is synchronised by the use of a
    start bit, and either one or two stop bits.
  • The bit rate is constant on a per character basis

27
Transmission Synchronisation
  • Synchronous Transmission or Block Synchronisation
  • Each block is synchronised by the use of a number
    of synchronisation characters that are
    transmitted first
  • These are followed by a start of block character,
    which is followed by the data block, and
    transmission is finished with an end of block
    character.
  • The bit rate is constant for the whole
    transmission of the block I.e. the time interval
    between characters is fixed.

28
Asynchronous vs. Synchronous Transmission
29
Data Transmission Modes
  • Computer based communications always use Digital
    Transmission,
  • What is transmitted is digital data, using either
    an analogue or digital signal.
  • Normally, the digital data are recovered and
    repeated at intermediate points to reduce the
    effects of noise.
  • Irrespective of the type of communications
    facility being used, in most applications data
    are transmitted between computers in a bit-serial
    mode,more commonly known as serial transmission.

30
Data Transmission Modes
  • Within a computer, data are transferred in a
    word-parallel mode, most commonly known as
    parallel transmission.
  • In computer communications is necessary to
    perform a parallel-to-serial conversion, in the
    transmitter, serial-to-parallel conversion in the
    receiver.
  • These conversions are done in the computer
    interface to the network

31
Transmission efficiency
  • Extra bits (start and stop bits) and characters
    (synchronisation and block delimiters) are needed
    to implement asynchronous and synchronous
    transmission.
  • These add nothing to the content of the message,
    but must be included in what is sent.
  • They reduce the overall information capacity of
    the transmission
  • They reduce the overall efficiency of the
    transmission.

32
Transmission efficiency
  • Transmission efficiency (useful data/total bits
    transmitted)100
  • For example for asynchronous transmission of
    8-bit characters with 1 start and 1 stop bit, we
    have to send 10 bits for each character
  • Transmission efficiency (8/10)100 80
  • Effective Data Rate (Transmission
    Efficiency/100)Capacity

33
Transmission Codes
  • Symbolic data/information must be encoded in a
    format suitable for transmission.
  • Normally, the codes used for transmission are
    similar to the codes used to store the
    information.
  • The most common code is ASCII
  • ASCII is a 7-bit code, permitting 128 different
    symbols to be encoded.
  • The second most commonly used code is EBCDIC
  • EBCDIC is an 8-bit code enabling 256 different
    symbols to be encoded.

34
Networking
35
Data Communication vs. Networking
  • Communication Two Nodes. Mostly EE issues.
  • Networking Two or more nodes. More issues,
    e.g., routing

36
Distributed Systems vs. Networks
  • Distributed Systems
  • Users are unaware of underlying structure.
  • Mostly operating systems issues.
  • Nodes are generally under one organizations
    control.
  • Networks
  • Users specify the location of resources.
  • Nodes are autonomous.

37
Networking
  • Point to point communication not usually
    practical
  • Devices are too far apart
  • Large set of devices would need impractical
    number of connections
  • Solution is a communications network

38
What are computer networks?
  • Networking deals with the technology
    architecture of the communications networks used
    to interconnect communicating devices.
  • Computer network is a collection of autonomous
    computers interconnected by a single technology.
  • The Internet is not a single network but a
    network of networks.

39
Types of Networks
  • Point to point vs. Broadcast
  • Circuit switched vs. packet switched
  • Local Area Networks (LAN)
  • vs.
  • Metropolitan Area Networks (MAN)
  • vs.
  • Wide Area Networks (WAN)

40
Communications Networks
  • A Communications Network is a set of
    interconnected devices that provide data
    transmission facilities between user's end points.

41
Simplified Network Model
42
Objectives of Networking
  • To share and exchange data between systems
  • To share expensive resources
  • To facilitate communication among humans and
    machines

43
Some terminology
  • Host a machine on the network
  • End system/end point a machine on the edge of
    the network, rather than an internal
    (switching) node
  • Subnet sub network, a subset of the whole
    network
  • Also used to refer to the internal routing part
    of a network.
  • IMP Interface Message Processor, hardware
    connecting host to network.

44
Some terminology
  • Packet we often break messages into many
    chunks, sent separately. The chunks are called
    packets.
  • Size of packet and how its treated depends on
    network protocol in use.
  • A packet might get split up further by another
    protocol.
  • Some protocols (e.g. IP) use varying size
    packets in others (e.g. ATM) theyre fixed.
    Small fixed-size packets are called cells

45
Some terminology
  • internetworking act of connecting multiple
    networks together to form a larger network
  • Fun issues include how to route and address
    across multiple heterogeneous networks
  • internet a network thus produced
  • Also the name of a common protocol for doing this
    (IP)
  • Internet the global internet

46
Network sizes
  • Computer Networks can be classified by the area
    they cover
  • PAN Personal Area Network very small
  • LAN Local Area Network room/building/campus
  • MAN Metropolitan Area Network city, region
  • WAN Wide Area Network country/continent.

47
Interconnection of Networks
  • Networks of low capacity may be connected
    together via a backbone (network of high
    capacity)
  • LANs and WANs can be interconnected via T1 or T3
    digital leased lines
  • According to the protocols involved, networks
    interconnection is achieved using one or several
    of the following devices
  • Bridge a computer or device that links two
    similar LANs based on the same protocol.
  • Router a communication computer that connects
    different types of networks using different
    protocols.
  • B-router or Bridge/Router a single device that
    combines both the functions of bridge and router.
  • Gateway a network device that connects two
    different systems, using direct and systematic
    translation between protocols.

48
Broadcast vs. Point-to-point
  • Broadcast Networks
  • A single communication channel shared by all
    machines on a network
  • Multicast simultaneous transmission to a subset.
  • Point-to-point networks
  • Many connections between individual pairs of
    machines
  • Transmission from A to C might go via B
  • Often multiple routes a fundamental question is
    which to use?

49
Local Area Networks
  • A Local Area Network (LAN) is a computer network
    intended to link computers and associated devices
    within a small geographical area.
  • The linking distances are relatively short, with
    cable lengths rarely exceeding 5 kilometres.
  • The linked computers may include large computers,
    word processors, or desktop computers.
  • Associated devices include computer terminals,
    printers, plotters, scanners, etc.

50
Local Area Networks
  • LANs normally offer much higher data transmission
    rates than WANs.
  • This difference is apparent in the network
    oriented protocols only.
  • At application level, LANs provide the sharing of
    resources like programs, files, printers,
    plotters, scanners, etc.

51
LAN Topologies
  • LAN topology is one of the issues that must be
    considered when selecting LAN technology.
  • It defines the interconnection of stations to
    form the network.
  • LAN topologies are classified as
  • Broadcast topology
  • Store-and-forward topology

52
LAN Topologies
  • Broadcast topology
  • This implies that all stations are connected to
    a common transmission medium.
  • Store-and-forward topology
  • A complete message or packet is received into a
    buffer in the memory of an intermediate station
  • It is then re-transmitted on the route to its
    destination.
  • The stations in a store-and-forward topology
    network are connected by independent
    point-to-point transmission lines.

53
LAN Topologies
  • The topology of a LAN is important because it
    influences the following features of the network
  • expansion cost
  • the incremental cost of adding another station to
    an existing network.
  • reconfiguration capabilities
  • the ease of modifying the topology to deal with a
    failed node or component.
  • reliability
  • The extent of dependency on a single component
    for network operation.

54
LAN Topologies
  • As well as
  • software complexity
  • the complexity of the protocols required to
    achieve communications.
  • performance
  • The effectiveness of the LAN in terms of
    throughput, or delays in transmission.
  • broadcast capabilities
  • how difficult it is to broadcast in the LAN, i.e.
    to transmit a single message which is received by
    all other stations in the network.

55
Bus Topology
56
Ring Topology
57
Star Topology
58
Hub Topology
  • The hub is derivative of the bus and ring
    topologies
  • It has the appearance of the star topology, with
    a central hub in place of the central node.
  • The hub is simply the bus or ring wiring
    collapsed into a central unit.
  • Unlike the central node in the star topology, the
    hub does not perform any switching. The hub
    simply consists of a set of repeaters.
  • Many modern networks are implemented using hubs
    for convenience.
  • Care is needed when deciding what topology is
    being used in a real network.

59
Hub Topology Network with and without hub
60
Network topologies
  • Tree
  • Corresponding to an organisational hierarchy?
  • Internal nodes may be bottlenecks.

61
Network topologies
  • Graph
  • Generalisation of a tree
  • Cycles allowed
  • Complete graph (Mesh)
  • Dedicated link from every node to every other
    node
  • Rapidly becomes prohibitively expensive

62
Communications System
63
Communications System
  • A communications system is the combination of
    network hardware and communications system
    software that supports the communications between
    user-oriented processes running in remote
    computers.
  • The communications system provides the services
    required by the applications to communicate.
    These services are outlined on the next slide.

64
Communications System
  • Communication System Functions
  • Naming and Addressing of entities.
  • Segmenting and reassembly of messages
  • Blocking of messages
  • Connection or session control
  • Error control
  • Congestion and flow control
  • Synchronisation
  • Priority

65
Communication System Architecture
  • The user-oriented layers
  • The application offers services to users through
    a set of rules or steps for accessing web-sites
    or sending e-mails.
  • Some applications operate on different types of
    user-interface. A means of converting alphabets
    and screen formats may be needed
  • Some applications require a session of activity
    with a definite set-up and closedown of the
    session (e.g. logon and logoff)
  • The transport layer provides an end-to-end
    virtual channel between the source and
    destination.

66
Communication System Architecture
  • The system-oriented layers
  • Implement the connections between nodes that make
    a machine part of a communications network
  • The network layer is responsible for routing
    between nodes
  • The Data link and Physical layers provide the
    means of moving packages of data between pairs of
    nodes.

67
Communication System Architecture
  • The ISO Open Systems Interconnection (OSI) model
    has 7 layers
  • The top 3 layers are user or application
    oriented.
  • The bottom 3 layers are system-oriented.
  • The middle layer, transport, acts as a broker
    between the basic services provided by the
    network and the needs of the users
  • Each layer can be thought of as talking
    directly to its peer on another machine.
  • A user of a web-browser holds a conversation
    with a remote web-site
  • Only at the physical layer does direct
    communication take place, using signals.

68
Communication System Architecture
  • The TCP/IP model has 4 layers
  • The top layers is the application.
  • The bottom 2 layers are system-oriented.
  • The middle layer, transport, acts as a broker
    between the basic services provided by the
    network and the needs of the users.
  • Although the model is simpler than OSI it
    recognises the same purpose and requirements.
  • The transport level protocols are TCP and UDP
  • The network level protocol is usually IP
  • The data link and physical level protocols are
    specific to the network

69
Communication System Architecture
70
Communication System Architecture
71
Circuit Switching vs. Packet Switching
  • Fundamental question how to move bits from one
    host to another, via n others?
  • Two key approaches (opposed)
  • Circuit switching
  • Establish fixed-bandwidth circuit use it
  • Packet switching
  • Split messages into packets, send separately
  • Trend is very much towards packet switching.

72
Circuit Switching
  • Resources along a path are reserved for duration
    of communication.
  • Buffers, link bandwidth, CPU time, etc.
  • All nodes on path genuinely maintain connection
    state information
  • All data in a some communication is sent on the
    same circuit, through same nodes
  • Classic example PSTN (Public Switched Telephone
    Network)

73
Circuit Switching
74
Circuit Switching
  • Each circuit has a fixed bandwidth for its
    lifetime.
  • Channels typically split into n equal bandwidth
    circuits.
  • Pro Makes QoS (Quality of Service) guarantees
    easy to achieve
  • Con Wasteful during silent periods.
  • Data transmission tends to be bursty.

75
Circuit Switched Multiplexing
  • Multiplexing combining information channels
    onto a common transmission medium.
  • FDM (Frequency Division Multiplexing)
  • Frequency spectrum of link is shared among
    circuits
  • Typically, each of n circuits gets 1/n
  • e.g. PSTN bandwidth divided in 4KHz bands
  • TDM (Time Division Multiplexing)
  • Time divided into fixed size chunks
  • Each circuit gets a portion of the total time

76
Packet Switching
  • No prior reservation of resources
  • Each packet transmitted separately
  • Nodes dont maintain connection state
    information
  • Each packet dealt with individually
  • Two packets might take different paths
  • Classic example the Internet.

77
Packet Switching
  • Con QoS harder to do, can only really make best
    effort promises
  • IPv6 addresses this somewhat complex
  • Pros more efficient use of bandwidth, no hard
    limit to number of comms.
  • Ideally graceful degradation curves
  • What happens when queues fill? Delays and,
    ultimately, packet loss.
  • Store-and-forward (on routers)
  • Read entire packet in, then send it out

78
Packet switching
79
Delay Loss in Packet Switching
  • Processing delay
  • Time to examine packet decide where to send it
    maybe also some error checking
  • Queuing delay
  • Delay while packet is queued depends on size of
    queue, ie traffic levels
  • Transmission delay
  • Time taken for node to push out packet
  • Depends on size of packet speed of outbound
    link.

80
Delay Loss in Packet Switching
  • Propagation delay
  • Time taken for packet to propagate across link to
    next node
  • Depends on speed of physical medium and distance
    to next node
  • Packet loss
  • Happens when things get too busy, queues
    overflow, nodes cant keep up
  • End-to-end delay
  • Total delay on transmission between two end
    points.

81
Frame Relay
  • Packet switching systems have large overheads to
    compensate for errors
  • Modern systems are more reliable
  • Errors can be caught in the end system
  • Most overhead for error control is stripped out

82
Asynchronous Transfer Mode
  • ATM
  • Evolution of frame relay
  • Little overhead for error control
  • Fixed packet (called cell) length
  • Anything from 10Mbps to Gbps
  • Constant data rate using packet switching
    technique

83
Virtual circuits vs. datagram networks
  • We can, in fact, simulate circuit switching on
    packet switched networks
  • Virtual Circuits being the result
  • Otherwise, its a datagram network
  • Datagram another word for packet
  • Choice has huge impact on routing
  • At IP level, Internet is a datagram network

84
Virtual Circuit Networks
  • Packets carry VC identifier
  • Hosts have table mapping VCIDs to outbound
    connections
  • Setting up involves both ends and every host in
    between
  • Every packet follows the same path
  • Requires complex state maintenance protocols.

85
Datagram Networks
  • Packets carry destination address
  • Host has (more complex) table to help it decide
    where to send next.
  • Table at a given host can change over lifetime of
    a communication
  • Packets really can take different paths
  • No connection state information maintained
    (except maybe at ends)
  • Almost all of the Internet.

86
Connection-oriented vs. Connectionless Services
  • Characterises end-to-end communication services
    available to end users.
  • Connection-oriented
  • Application must establish connection to other
    end before sending any actual data
  • Each packet then sent via that connection.
  • Allows delivery guarantees.
  • Connectionless
  • Application just sends each packet individually
  • Thus, must know destination address every time
    you send a packet.
  • No guarantee of delivery, generally.

87
Caution dont get confused
  • Circuit-switched vs. packet switched
  • Concerns how packets are routed
  • Distinction made in core of network
  • Mainly at Network layer (see later).
  • Connection-oriented vs. connectionless
  • Concerns how packets are sent/received
  • Distinction made at edge of network
  • Mainly at Transport layer (see later).

88
Basic Types of Networks
  • Yet another way to classify

89
Basic Types
  • Peer-to-peer
  • Does not require dedicated resource (dedicated
    server)
  • Any host can share its resources
  • Typically less expensive, easier to work with
  • Less secure, support fewer users (10 or fewer),
    experience more problems with file system
    management
  • Server-based
  • Configuration of nodes, certain of which are
    dedicated to providing resources (servers)
  • Offer (better) user security
  • Dedicated servers can be expensive, may require a
    full-time network administrator
  • Enterprise network (which combines the two)
  • Provide connectivity among all nodes in an
    organization
  • Include (connect) both peer-to-peer and
    server-based networks
  • May consist of multiple protocol stacks and
    network architectures

90
Client/Server Networks
  • Client/Server is a networking model mainly
    applicable at the Application layer
  • Concerns the roles of end systems
  • Client system requesting some service
  • Server system providing some service.
  • Ubiquitous example HTTP
  • Client is your web browser
  • Server is www.isy.vcu.edu (or whatever)

91
Peer Networks
  • Not all applications use Client/Server model
  • Often, all parties have equal status
  • In some sense theyre all clients and servers.
  • Although sometimes have distinguished nodes
    providing certain services.

92
Protocols
  • Used for communications between entities in a
    system
  • Must speak the same language
  • Entities
  • User applications
  • e-mail facilities
  • terminals
  • Systems
  • Computer
  • Terminal
  • Remote sensor

93
Key Elements of a Protocol
  • Syntax
  • Data formats
  • Signal levels
  • Semantics
  • Control information
  • Error handling
  • Timing
  • Speed matching
  • Sequencing
  • Protocols define format, order of messages sent
    and received among network entities, and actions
    taken on messages transmission, receipt

94
In Summary, a protocol is ....
  • An agreement about communication between two or
    more entities
  • It specifies
  • Format of messages
  • Meaning of messages
  • Rules for exchange
  • Procedures for handling problems

95
Protocol Architecture
  • Task of communication broken up into modules
  • For example file transfer could use three modules
  • File transfer application
  • Communication service module
  • Network access module

96
Simplified File Transfer Architecture
97
A Three Layer Model
  • At the Top
  • User Oriented layer-Application Layer
  • In the Middle
  • Transport Layer
  • At the Bottom
  • System Oriented Layer - Network Access Layer

98
Network Access Layer
  • Exchange of data between the computer and the
    network
  • Sending computer provides address of destination
  • May invoke levels of service
  • Dependent on type of network used (LAN, packet
    switched etc.)

99
Transport Layer
  • Reliable data exchange
  • Independent of network being used
  • Independent of application

100
Application Layer
  • Support for different user applications
  • e.g. e-mail, file transfer

101
Addressing Requirements
  • Two levels of addressing required
  • Each computer needs unique network address
  • Each application on a (multi-tasking) computer
    needs a unique address within the computer
  • The service access point or SAP
  • The port on TCP/IP stacks

102
Addressing
  • Different levels of entity use different
    addresses.
  • MAC address Identifies the NIC and set by
    manufacturer.
  • Used by Physical and Data Link layer
  • IP address Identifies a computer in a network.
  • Used by the Network layer
  • Socket Identifies a process (running program).
  • Used by the Transport layer
  • Application level addresses vary
  • One example is the Uniform Resource Locator (URL)
    used by WWW applications

103
IP Addresses
  • IP Internet Protocol
  • Each IP address is 32 bits long
  • An IP address has a network part and host part
  • The former identifies a specific network and the
    latter a specific computer, or host, on that
    network.
  • IP addresses may be in one of five network
    classes
  • Class A Used for a small number of networks,
    each with many hosts.
  • Class B Used for a larger number of networks,
    each with a medium number of hosts
  • Class C Used for a large number of networks,
    each with only a few hosts
  • Classes D and E are for special purposes.

104
IP Addressing Example
  • All hosts on a network have the same network
    prefix

105
User Oriented Names and DNS
  • Human users prefer names to numbers.
  • The communications system translates these names
    into IP addresses, and vice versa.
  • The translation is done using the Domain Name
    System (DNS) application.
  • This is a directory service.
  • It uses multiple levels of server to resolve
    queries as close to the point of issue as
    possible.
  • All servers cache query results to reduce need
    for repeat queries in the near future.

106
Name Resolution in DNS
  • Each computer has a name resolver routine
  • gethostbyname in UNIX
  • Each resolver knows the name of a local DNS
    server
  • Resolver sends a DNS request to the server
  • DNS server either gives the answer, forwards the
    request to another server, or gives a referral
  • Referral Next server to whom request should be
    sent

107
How the DNS works
108
Protocol Architectures and Networks
109
Protocols in Simplified Architecture
110
Protocol Data Units (PDU)
  • At each layer, protocols are used to communicate
  • Control information is added to user data at each
    layer
  • Transport layer may fragment user data
  • Each fragment has a transport header added
  • Destination SAP
  • Sequence number
  • Error detection code
  • This gives a transport protocol data unit

111
Protocol Data Units
112
Network PDU
  • Adds network header
  • network address for destination computer
  • Facilities requests

113
Operation of a Protocol Architecture
114
Standards
  • Required to allow for interoperability between
    equipment
  • Advantages
  • Ensures a large market for equipment and software
  • Allows products from different vendors to
    communicate
  • Disadvantages
  • Freeze technology
  • May be multiple standards for the same thing

115
Standardized Protocol Architectures
  • Required for devices to communicate
  • Vendors have more marketable products
  • Customers can insist on standards based equipment
  • Two standards
  • OSI Reference model
  • Never lived up to early promises
  • TCP/IP protocol suite
  • Most widely used

116
OSI
  • Open Systems Interconnection
  • Developed by the International Organization for
    Standardization (ISO)
  • Seven layers
  • A theoretical system delivered too late!
  • TCP/IP is the de facto standard

117
OSI - The Model
  • A layer model
  • Each layer performs a subset of the required
    communication functions
  • Each layer relies on the next lower layer to
    perform more primitive functions
  • Each layer provides services to the next higher
    layer
  • Changes in one layer should not require changes
    in other layers

118
OSI Layers
119
The OSI Environment
120
TCP/IP Protocol Architecture
  • Developed by the US Defense Advanced Research
    Project Agency (DARPA) for its packet switched
    network (ARPANET)
  • Used by the global Internet
  • Not official model but a working one.
  • Application layer
  • Host to host or transport layer
  • Internet layer
  • Network access layer
  • Physical layer

121
TCP/IP Protocol Architecture Physical Layer
  • Physical interface between data transmission
    device (e.g. computer) and transmission medium or
    network
  • Characteristics of transmission medium
  • Signal levels
  • Data rates
  • etc.

122
TCP/IP Protocol Architecture Network Access Layer
  • Exchange of data between end system and network
  • Destination address provision
  • Invoking services like priority

123
TCP/IP Protocol Architecture Internet Layer (IP)
  • Systems may be attached to different networks
  • Routing functions across multiple networks
  • Implemented in end systems and routers

124
TCP/IP Protocol Architecture Transport Layer
(TCP)
  • Reliable delivery of data
  • Ordering of delivery

125
TCP/IP Protocol Architecture Application Layer
  • Support for user applications
  • e.g. http

126
TCP/IP Protocol Architecture Model
127
Protocol Data Units in TCP/IP
128
OSI vs. TCP/IP
About PowerShow.com