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

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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

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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.

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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

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Diagram of Simplified Communication Model
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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

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Simplified Data Communications Model
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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.

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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.

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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.

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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

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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).

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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.

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Data Transmission Terminology
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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)

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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.

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Analog vs. Digital Signal
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Data Encoding
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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.

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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.

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Data Transmission Data and Signals
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Data vs. Signal
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Data Transmission Treatment of Signals
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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

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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

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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.

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Asynchronous vs. Synchronous Transmission
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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.

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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

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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.

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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

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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.

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Networking
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Data Communication vs. Networking

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

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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.

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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

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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.

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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)

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Communications Networks

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

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Simplified Network Model
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Objectives of Networking

• To share and exchange data between systems
• To share expensive resources
• To facilitate communication among humans and
  machines

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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.

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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

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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

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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.

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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.

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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?

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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.

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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.

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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

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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.

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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.

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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.

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Bus Topology
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Ring Topology
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Star Topology
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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.

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Hub Topology Network with and without hub
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Network topologies

• Tree
• Corresponding to an organisational hierarchy?
• Internal nodes maybe bottlenecks.

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Network topologies

• Graph
• Generalisation of a tree
• Cycles allowed
• Complete graph (Mesh)
• Dedicated link fromevery node to everyother
  node
• Rapidly becomes prohibitively expensive

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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.

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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

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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.

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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.

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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.

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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

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Communication System Architecture
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Communication System Architecture
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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.

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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)

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Circuit Switching
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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.

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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

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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.

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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

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Packet switching
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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.

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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.

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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

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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

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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

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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).

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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

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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

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Simplified File Transfer Architecture
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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 ArchitectureNetwork 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