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Data Communication


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

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Title: Data Communication

Data Communication
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
  • Light - infra-red through air, laser through
    fibre-optic cable
  • Electromagnetic radiation - radio, microwave,

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

  • 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
  • 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.

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

Diagram of Simplified Communication Model
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

Simplified Data Communications Model
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
  • Transmission code - an agreed language to be
  • 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.

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.

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.

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
  • 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

Data Transmission Terminology
  • Transmission may be simplex, half-duplex or
  • 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).

Data Transmission Terminology
  • In a direct link, (or data link), a transmission
  • 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.

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

Data Encoding
  • Data are transmitted using electromagnetic
  • 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
  • The latter allows normal computer communications
    using widely available telephone lines.
  • This is achieved using Modems.

Analog vs. Digital Signal
Data Encoding
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.

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.

Data Transmission Data and Signals
Data vs. Signal
Data Transmission Treatment of Signals
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
  • Bit synchronisation depends on how the signal is

Transmission Synchronisation
  • In serial transmission there are two standard
    ways of achieving character and block
  • Asynchronous Transmission or Character
  • 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

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
  • The bit rate is constant for the whole
    transmission of the block I.e. the time interval
    between characters is fixed.

Asynchronous vs. Synchronous Transmission
Data Transmission Modes
  • Computer based communications always use Digital
  • 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.

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
  • These conversions are done in the computer
    interface to the network

Transmission efficiency
  • Extra bits (start and stop bits) and characters
    (synchronisation and block delimiters) are needed
    to implement asynchronous and synchronous
  • 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 efficiency
  • Transmission efficiency (useful data/total bits
  • 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

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
  • 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.

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

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

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

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.

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)

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

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

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
  • Also used to refer to the internal routing part
    of a network.
  • IMP Interface Message Processor, hardware
    connecting host to network.

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

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
  • Internet the global internet

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.

Interconnection of Networks
  • Networks of low capacity may be connected
    together via a backbone (network of high
  • 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
  • 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.

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
  • Transmission from A to C might go via B
  • Often multiple routes a fundamental question is
    which to use?

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.

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.

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

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
  • The stations in a store-and-forward topology
    network are connected by independent
    point-to-point transmission lines.

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.

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.

Bus Topology
Ring Topology
Star Topology
Hub Topology
  • The hub is derivative of the bus and ring
  • 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.

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

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

Communications System
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
  • The communications system provides the services
    required by the applications to communicate.
    These services are outlined on the next slide.

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

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

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

Communication System Architecture
  • The ISO Open Systems Interconnection (OSI) model
    has 7 layers
  • The top 3 layers are user or application
  • 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.

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

Communication System Architecture
Communication System Architecture
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.

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

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

Circuit Switched Multiplexing
  • Multiplexing combining information channels
    onto a common transmission medium.
  • FDM (Frequency Division Multiplexing)
  • Frequency spectrum of link is shared among
  • 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

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

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

Packet switching
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

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

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

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

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

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

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.

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.

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).

Basic Types of Networks
  • Yet another way to classify

Basic Types
  • Peer-to-peer
  • Does not require dedicated resource (dedicated
  • 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
  • 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
  • Include (connect) both peer-to-peer and
    server-based networks
  • May consist of multiple protocol stacks and
    network architectures

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 (or whatever)

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.

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

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

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

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

Simplified File Transfer Architecture
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

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

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

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

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

  • Different levels of entity use different
  • MAC address Identifies the NIC and set by
  • 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

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
  • IP addresses may be in one of five network
  • 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.

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

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
  • All servers cache query results to reduce need
    for repeat queries in the near future.

Name Resolution in DNS
  • Each computer has a name resolver routine
  • gethostbyname in UNIX
  • Each resolver knows the name of a local DNS
  • 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

How the DNS works
Protocol Architectures and Networks
Protocols in Simplified Architecture
Protocol Data Units (PDU)
  • At each layer, protocols are used to communicate
  • Control information is added to user data at each
  • 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

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

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

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

  • 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

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
  • Changes in one layer should not require changes
    in other layers

OSI Layers
The OSI Environment
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

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

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

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

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

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

TCP/IP Protocol Architecture Model
Protocol Data Units in TCP/IP