Systems Architecture, Fifth Edition

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Chapter Goals

• Compare and contrast bus, ring, and star network
  topologies
• Describe packet routing across local and wide
  area networks
• Describe the CSMA/CD media access control
  protocol
• Describe network hardware devices, including
  network interface units, routers, and switches

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Chapter Goals (continued)

• Describe the OSI network model, the TCP/IP
  protocol suite, and IEEE network standards

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Network Topology

• Spatial organization of network devices, physical
  routing of network cabling, and flow of messages
  from one network node to another
• Can be physical or logical
• Three types star, bus, ring differentiated by
• Length and routing of network cable
• Type of node connections
• Data transfer performance
• Susceptibility of network to failure

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Network Topologies
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The strengths of two different topologies can be
combined by using one topology for physical
layout and another for message routing.
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Media Access Control

• Uses a protocol that specifies rules for
  accessing a shared transmission medium
• Carrier Sense Multiple Access/Collision Detection
  (CSMA/CD) Used by Ethernet
• Commonly used in bus networks to detect and
  recover from collisions
• Token passing MAC protocol
• Used by ring network topologies

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CSMA/CD Protocol

• Process
• Node listens for an idle state, transmits a
  packet, and listens for a collision
• If a collision is detected, node retransmits
  after waiting a random waiting period
• Primary advantage simplicity
• Primary disadvantage potentially inefficient use
  of data transfer capacity

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Addressing and Routing

• How messages sent by end nodes find their way
  through transmission lines and central nodes to
  their ultimate destination
• Local area networks (LANs)
• Interconnected to form WANs
• Wide area networks (WANs)

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Addressing Ethernet Address

• What is an Ethernet Address? Ethernet address,
  also called MAC address, is a 48-bit number used
  to uniquely identify each computer in a network.
  The address is usually written in hexadecimal
  form (e.g. 006097981E6B).

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Addressing Ethernet Address

• Physical Address, MAC Address or Ethernet Address
  are the same.
• Check by
• Click Start-gtRun-gtcmd
• Type IPCONFIG /ALL and then press Enter
• Physical Address is shown

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Addressing Ethernet Address

• Ethernet LAN
• Source computer sends message in a FRAME
• Ethernet is Broadcast so all devices connected
  to the hub, receive the FRAME
• Only the computer that has the correct source
  address accepts the FRAME

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Connecting two LAN segments

• Bridges decide to forward (or not) Frames from
  one segment to another
• Routing tables are maintained in the bridge
• Bridges always forward broadcast Frames

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LANs can be interconnected by using network
devices hubs, brdges, switches, routers.
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Network Hardware
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Hubs

• Connect nodes to form a LAN
• Most are Ethernet devices
• Combine separate point-to-point connections
  between nodes and the hub into a single shared
  transmission medium by repeating all incoming
  packets to every connection point
• Low-cost alternative for home and small office
  networks

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Bridges

• Connect two networks or network segments and copy
  packets between them
• Look at source addresses and update internal
  tables of node addresses on each network segment
• Common uses
• Construct a virtual LAN from two separate LANs
• Divide a network into segments in order to
  minimize congestion

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Routers

• Intelligently route and forward packets among two
  or more networks
• Forward packets based on information other than
  destination address
• Build internal map of the network constantly
  scan it to monitor traffic patterns and network
  node changes

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Switches

• High-speed devices that create virtual LANs on a
  per-packet basis
• Each input connection is treated as a separate
  LAN
• Dramatically increase network performance
• Connection decisions made by hardware are based
  only on destination address
• Each virtual LAN has only one sending and one
  receiving node eliminates congestion

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Network Symbols
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OSI Network Layers

• Open System Interconnection (OSI) model
• ISO conceptual model that divides network
  architecture into seven layers
• Each layer uses services of layer below and is
  unaware of other layers implementations
• Uses
• General model of networks
• Framework for comparing networks
• Architectural roadmap that enhances
  interoperability among network architectures and
  products

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Application Layer

• Network service request and response
• Contains programs that make and respond to
  high-level requests for network services
• End-user network utilities
• Network services embedded in the OS
• Network service providers

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Presentation Layer

• Converts and formats data
• Ensures correct interpretation of transmitted
  data
• Encryption and decryption
• Compression and decompression
• Converting data between EBCDIC and ASCII
• Font substitution
• Primarily used by applications that format data
  for user display

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Session Layer

• Negotiates and implements high-level protocol
  parameters (timeout, half or full duplex,
  synchronization, quality of service)
• Establishes and manages communication sessions
• Monitors communication to detect and resolve
  problems that arise once protocol has been
  established

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Transport Layer

• Formats messages into packets suitable for
  transmission over the network
• Places messages within a packet data area and
  adds header/trailer information (network
  addresses, error detection data, packet
  sequencing data)
• Gives packets to network layer for delivery
• Examines packets for errors requests
  retransmission if necessary (when receiving
  packets)

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Network Layer

• Routes packets to their proper destination
• Those within central node interact with one
  another to exchange routing information and
  update internal routing tables

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Data Link Layer

• Transmits packets and bits
• Interface between network software and hardware

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Physical Layer

• Transmits bit streams
• Where communication between devices actually
  takes place
• Includes hardware devices that encode and decode
  bit streams and the transmission lines that
  transport them

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TCP/IP

• The core Internet protocol suite
• Delivers most services associated with the
  Internet
• File transfer via FTP
• Remote login via Telnet protocol
• Electronic mail distribution via SMTP
• Access to Web pages via HTTP
• Predates and corresponds poorly to OSI model

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Internet Protocol

• Provides connectionless packet transport across
  LANs and WANs
• Translates datagrams into format suitable for
  transport by physical network
• IP layer can divide datagram into smaller units
  and transmit them individually
• Attaches header information to each unit,
  including its sequence in the datagram

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IP (continued)

• Assumes datagram will traverse multiple networks
  via nodes called gateways
• Determines transmission routes via related
  protocols (ICMP, RIP)
• IP nodes
• Identified by unique 32-bit address
  (nnn.nnn.nnn.nnn)
• Periodically exchange routing information to keep
  tables current

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Only the IP layer is implemented within the
gateways.
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Transmission Control Protocol

• Provides connection-oriented packet transport to
  higher-level Internet service protocols,
  including HTTP, FTP, and Telnet
• Provides framework to check for lost messages
  explicitly establishes connection with intended
  recipient before transmitting messages
• Performs connection management functions
  (verifying receipt, verifying data integrity,
  controlling message flow, securing message
  content)

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TCP (continued)

• Sender and recipient TCP layers maintain
  information about one another (message routes,
  errors encountered, transmission delays, status
  of ongoing data transfers)
• Uses positive acknowledgment protocol to ensure
  data delivery
• Establishes connections through a port and an
  socket

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Network Standards

• IEEE 802 standards
• Describe network hardware, transmission media,
  transmission methods, and protocols
• Help ensure compatibility among products from
  competing vendors
• Developed by committees whose membership is drawn
  from industry, government, and academia
• Ethernet standard (802.3) - very successful

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Summary

• Network topology
• Addressing and routing
• Media access control
• Network hardware
• OSI network layers
• Network standards