Overview

1
Overview
2
Network Goals

• resource sharing
• high reliability
• saving money
• communication

3
Application of Networks

• access to remote programs
• access to remote databases
• value-added communication facilities

4
Network Structure

• hosts -- machines intended for running user
  programs
• hosts are connected by a communication subnet
  (transmission lines and switching elements)
• switching elements -- specialized computers used
  to connect two or more transmission lines

5
Network Structure

• IMP (Interface Message Processors) -- switching
  element must choose an outgoing line to forward
  them (packet switch)
• two types of communication subnets
• point-to-point channels
• broadcast channels

6
Point to Point Channels

• network contains numerous cables, each one
  connecting a pair of IMPs
• if two IMPs are not directly connected, they will
  have to pass through other IMPs
• point to point, store and forward, packet
  switched subnets

7
Broadcast Channels

• single communication channel that is shared by
  all the machines in the network
• packets sent by one machine are received by all
  the others (an address field in the packet
  specifies for whom it is intended)
• multicasting -- transmission to a subset of the
  machines

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commitment -- making time when there is none
9
Broadcast Channels

• Static allocation of channel -- divide time into
  discrete intervals and run a round robin allowing
  each machine to broadcast only when its time is
  up
• dynamic allocation of channel (on
  demand)centralized (there is a bus arbitration
  unit) or decentralized (each machine must decide
  for itself whether to transmit or not

10
Network Architectures

• to reduce complexity, most networks are organized
  as a series of layers or levels (protocol
  hierarchy)
• the purpose of each layer is to offer services to
  the higher layers, shielding the upper layers
  from the details of how the services are actually
  implemented

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Protocols
12
Network Architectures

• between each pair of layers is an interface which
  defines primitive operations and services the
  lower layer offers to the upper layer
• each layer perform a specific collection of
  well-understood functions
• well-defined interfaces makes it easier to
  replace the implementation of one layer

13
Design Issues

• every layer must have a mechanism for connection
  establishment and termination
• rules for data transfer
• simplex communication
• half-duplex
• full-duplex
• error control (error-detecting and
  error-correcting)

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Design Issues

• preserving the order of messages/data packets
  sent
• how to keep a fast sender from swamping a slow
  receiver
• inability of processes to accept long messages
  (disassemble, transmit, assemble)
• multiplexing and demultiplexing
• routing

15
OSI Reference Model

• Open Systems Interconnection
• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

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Physical Layer

• transmitting raw bits over a communication
  channel
• design issue make sure that when 1 bit is sent,
  it is received as 1 bit, not as 0 bit
• mechanical and electrical problems

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Data Link Layer

• take a raw transmission facility and transform it
  into a line that appears free of transmission
  errors to the network layer
• break the input data into data frames
• create and recognize frame boundaries
• solve the problems due to damaged, lost or
  duplicate frames

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Network Layer

• controlling the operation of the subnet
• how packets are routed from source to destination
• static routing tables
• determined at the start of each conversation
• highly dynamic
• controlling network congestion
• accounting (packets sent/received)

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Transport Layer

• accept data from the session layer, split it to
  smaller pieces, pass these to the network layer,
  and ensure that the pieces all arrive correctly
  at the other end
• creates a distinct network connection for each
  transport connection required
• end-to-end
• take care of establishing and terminating the
  connection

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Session Layer

• allows users on different machines to establish
  sessions between them
• token management
• synchronization

22
Presentation Layer

• concerned with the syntax and semantics of the
  information transmitted
• managing abstract data structures and converting
  from program data representation to network
  standard representation
• data compression
• cryptography

23
Application Layer

• contains a variety of protocols that are commonly
  needed
• addressing software incompatibilities (ie.
  terminal emulation, file systems)

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Data Transmission
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TO LOVE IS THE LAW OF LIFE

• The first law of life is to love. Not to love is
  to lose life. We need to love more than we need
  to be loved. To preserve life, we need to give
  it to something or somebody. For the selfish
  man, the world grows smaller and smaller until he
  lives alone. For the man who gives and loves,
  the world grows larger with his touch.

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Services (terminologies)

• entities
• peer entities
• service provider / service user
• SAP (service access points)
• interface

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Services (terminologies)

• IDU (interface data unit)
• SDU (service data unit)
• ICI (interface control information)
• PDU (protocol data unit)

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Services

• Connection-Oriented
• telephone system
• Connectionless
• postal mail system
• DATAGRAM service (unreliable -- no acks)
• acknowledged datagram service
• request-reply service

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Service Primitives

• request -- an entity wants the service to do some
  work
• indication -- an entity is to be informed about
  an event
• response -- an entity wants to respond to an
  event
• confirm -- an entity is to be informed about its
  request

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Service Primitives

• connect.request -- establish a connection or get
  data
• connect.indication -- the peer entity gets this
  signal
• connect.response -- the response of the peer
  entity (accept/reject)
• connect.confirm -- the entity receives the
  response

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Relationship of Services to Protocols

• service -- a set of primitives (operations) that
  a layer provides to the layer above it
• protocol -- set of rules governing the format and
  meaning of the frames, packets, or messages that
  are exchanged by the peer entities within a layer