Chapter 5: The Network Layer

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Chapter 5 The Network Layer
ElEn 475 - Computer Networks
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Learning Objectives

• Explain why networking standards are needed
• Describe the types of networking standards
• Explain the function of commonly used network
  devices such as MAUs, multiplexers, repeaters,
  bridges, routers, brouters, hubs, and gateways

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Learning Objectives

• Determine when and how to use those devices
• Describe what OSI layer(s) correspond to the
  function of each device
  
• Explain the function of ATM switching
• Describe how VLANs are an important tool for
  managing large networks

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Network Layer

• Determines the network path on which to route
  frames
  
• Helps reduce network congestion
• Establishes virtual circuits
• Routes frames to other networks, resequencing
  packet transmissions when needed

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Network Layer Definitions

• A router is a physical device that contains
  software to enable frames formatted on one
  network to reach a different network in a format
  that the second network understands.
• A virtual circuit is a logical communi-cation
  path established by the OSI network layer for
  sending and receiving data.

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Signal Transmission Definitions

• Carrier sense is the process of checking a
  communication medium, such as cable, for a
  specific voltage or signal level that indicates
  the presence of a data-carrying signal.
• A collision occurs when two or more packets are
  detected at the same time on an Ethernet network.

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Network Addressing

• Ethernet addressing is accomplished through the
  OSI MAC (media access control) sublayer within
  the data link layer.
  
• The MAC sublayer uses an address associated with
  the NIC to direct encapsulated data to the data
  link layer of the receiving node.

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Multistation Access Unit

• A multistation access unit (MAU) is a central hub
  that links token ring nodes into a topology that
  physically resembles a star but in which packets
  are transferred in a logical ring pattern.

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Multistation Access Unit
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TCP/IP

• Transmission Control Protocol/Internet Protocol
  (TCP/IP) is a protocol particularly well suited
  for medium and large networks.
  
• TCP performs extensive error checking to ensure
  that data is delivered successfully
  
• IP consists of rules for packaging data and for
  ensuring it reaches the correct destination
  address

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TCP/IP

• A terminal is a device consisting of a monitor
  and a keyboard to communicate with a host
  computer that runs programs. It does not have a
  processor to use for running programs locally.

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TCP/IP

• Static addressing is an IP addressing method that
  requires the network administrator to manually
  assign and set up a unique network address on
  each workstation connected to a network.
• Dynamic addressing is a method in which an IP
  address is assigned to a workstation without the
  need for the network administrator to hard-code
  it in the workstations network setup.

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TCP/IP

• Dynamic Host Configuration Protocol (DHCP)
  provides a way for a server to automatically
  assign an IP address to a workstation on its
  network.
• The Windows Internet Naming Service (WINS)
  enables the server to convert workstation names
  to IP addresses for Internet communications.

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TCP/IP

• A subnet mask is a method to show which part of
  the IP address uniquely identifies the network
  and which part uniquely identifies the
  workstation.

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X.25

• X.25 is a packet-switching protocol for
  connecting remote networks at speeds up to 64
  Kbps.

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Switching Techniques

• Circuit switching is a network communication
  technique that uses a dedicated channel to
  transmit information between two modes.
  
• Message switching sends data from point to point,
  with each intermediate node storing the data,
  waiting for a free transmission channel, and
  forwarding the data to the next point until the
  destination is reached.

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Switching Techniques

• Packet switching establishes a logical channel
  between two transmitting nodes but uses several
  different paths of transmission to continually
  find the best routes to the destination.

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X.25 Transmission Modes

• A switched virtual circuit is a communication
  channel that is established for only as long as
  the communication session lasts.
  
• In a permanent virtual circuit the communica-tion
  channel stays connected at all times.
  
• An X.25 datagram does not use a particular
  communication channel. Data arrives at different
  times because each datagram may follow a
  different route to the destination.

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X.25 Packet Switching

• Data terminal equipment (DTE) consists of
  terminals, workstations, servers, and host
  computers that operate on a packet-switching
  network.
• Data communications equipment (DCE) consists of
  network devices that perform packet switching.

continued
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X.25 Packet Switching

• A packet assembler/disassembler (PAD) is a device
  that converts data from a format used by a DCE to
  one that can be placed on an X.25 communications
  network, and it translates data received in an
  X.25 communications format to a format that can
  be read by a DCE.

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X.25 Network
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X.25 Communication Layers
Relationship of X.25 communication layers and the
OSI model
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Frame Relay

• Frame relay is a communications protocol that
  relies on packet switching and virtual circuit
  technology to transmit data packets.
  
• It does not incorporate extensive error checking,
  assuming that intermediate nodes will perform
  that task.

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Switching and Virtual Circuits

• Frame relay uses multiple virtual circuits over a
  single cable medium.
  
• Permanent
• A continuously available path between two nodes
• Switched
• Based on the need to establish a transmission
  session

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Cell Relay

• Cell relay is a communications protocol that uses
  large fixed-length cells to transmit voice,
  video, and data.
  
• A cell is a large fixed-length data-carrying unit
  primarily consisting of a header with
  transmission control information and a large
  payload section that contains data.

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ATM

• Asynchronous Transfer Mode (ATM)
• Uses multiple channels and switching to send
  voice, video, and data transmissions on the same
  network
  
• Stresses efficient, quality of service (QOS),
  high-capacity data transport
  
• Provides an international communications standard

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Acceptance of ATM

• ATM handles data, voice, and video
  transmissions.
  
• Because there is flexibility in geographic
  distance, it can be used for LAN and WAN
  communications.
  
• It can accommodate high-speed communications.
• It can provide high-speed communications between
  Ethernet, token ring, Fast Ethernet, FDDI, and
  other kinds of networks.

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ATM Cell
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ATM and OSI Layered Communications
OSI network layer
ATM layer 3 Responsible for physical and logical
routes and for constructing voice, video, and da
ta
into cell payload
OSI data link layer
ATM layer 2 Responsiible for cell construction
and
point-to-point connectivity
OSI physical layer
ATM layer 1 Responsible for physical
connectivity via
electrical transport interface
Relationship of ATM layered communications to the
OSI model
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Advantages of ATM Switching

• ATM switching enables data to be transmitted at
  access speeds appropriate to the type of data
  sent.
  
• It permits use of higher bandwidths.
• Each ATM connection has its own dedicated
  bandwidth.
  
• Connection processes are more clearly defined
  because they are handled by the switch from point
  to point.

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Multistation Access Units (MAUs)

• Connect workstations in a logical ring through a
  physical star topology
  
• Move the token and packets around the ring
• Amplify data signals
• Connect in a daisy-chained manner to expand a
  token ring network
  
• Provide orderly movement of data

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Multistation Access Units (MAUs)

• A passive hub connects nodes in a star topology,
  performing no signal enhance-ment as the packet
  moves from one node to the next through the hub.
  
• An active hub connects nodes in a star topology,
  regenerating, retiming, and amplifying the data
  signal each time it passes through the hub.

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MAU with Eight Connections
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MAUs Connected Using the RI and RO Ports
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To Draw the Network Diagram

• Drawing the token ring network in Microsoft Paint

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Multiplexers

• A multiplexer is a switch that divides a
  communication medium into multiple channels so
  several nodes can communicate at the same time.
  
• A signal that is multiplexed must be
  demultiplexed at the other end.

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Multiplexers
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Multiplexers

• Work at the OSI physical level, switching from
  channel to channel using one of three physical
  methods
  
• Time division multiple access (TDMA)
• Frequency division multiple access (FDMA)
• Statistical multiple access

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Multiplexers

• TDMA enables multiple devices to communi-cate
  over the same communications medium by creating
  time slots in which each device transmits.
  
• FDMA creates separate channels on one
  communication medium by establishing different
  frequencies for each channel.
  
• Statistical multiple access multiplexing
  allocates the communication resources according
  to what is needed for the task.

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Repeaters

• A repeater amplifies and retimes a
  packet-carrying signal so it can be sent along
  all cable segments.
  
• As used in this context, a segment of cable is
  one cable run within the IEEE specifications.

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How Repeaters Are Used

• To extend a cable segment
• To increase the number of nodes beyond the limit
  of one segment
  
• To sense a problem and shut down a cable segment
• To amplify and retime a signal (as a component in
  other network devices)

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Using a Repeaterto Extend a Network
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Repeaters

• An attached unit interface (AUI) connects coax or
  fiber-optic backbone cable to a network node,
  such as a repeater.
  
• A partitioned segment is one that has been shut
  down because a portion of the segment is
  malfunctioning.

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To Diagram the Interoffice Network

• Microsoft Paint diagram of two rooms connected by
  a repeater

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Bridges

• A bridge is a network device that connects
  different LAN segments using the same access
  method.

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How Bridges Are Used

• To extend a LAN when the maximum connection limit
  has been reached
  
• To extend a LAN beyond the length limit
• To segment LANs to reduce data traffic
  bottlenecks
  
• To prevent unauthorized access to LAN (for
  security)

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Bridged Network
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Bridges

• A network device that operates in promiscuous
  mode reads frame destination address information
  before sending a packet onto other connected
  segments of the network.

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Types of Bridges

• A local bridge connects networks in close
  proximity and is used to segment a portion of a
  network to reduce problems caused by heavy
  traffic.
• A remote bridge joins networks across the same
  city, between cities, and between states to
  create one network.

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Important Functions of a Bridge

• Learning
• Filtering
• Forwarding

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Cascaded Network Segments
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Token Ring Bridging

• Token ring bridges use source routing to forward
  packets on the network.
  
• Hops are the number of times a packet travels
  point-to-point from one network to the next.

Source route bridging
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Routers

• A router connects networks having the same or
  different access methods.
  
• It forwards packets to networks by using a
  decision-making process based on
  
• Routing table data
• Discovery of the most efficient routes
• Preprogrammed information from the network
  administrator

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How Routers Are Used

• To efficiently direct packets from one network to
  another, reducing excessive traffic
  
• To join neighboring or distant networks
• To connect dissimilar networks
• To prevent network bottlenecks by isolating
  portions of a network
  
• To secure portions of a network from intruders

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A Router Forwarding a Frame to the Right Network
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Static and Dynamic Routing

• Static routing involves control of routing
  decisions by the network administrator through
  preset routing instructions.
  
• In dynamic routing, the router constantly
• Checks the network configuration
• Automatically updates routing tables
• Makes its own decisions about how to route frames

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Routing Tables and Protocols

• Routers maintain information about node addresses
  and network status in databases.
  
• The routing table database contains the addresses
  of other routers and each end node.
  
• Routers regularly exchange information about
  network traffic, the network topology, and the
  status of network lines.
  
• Routers exchange information by using one or more
  routing protocols.

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Local and Remote Routers

• A local router joins networks in the same
  building or between buildings in close
  proximity.
  
• A firewall is software and/or hardware employed
  to restrict who has access to a network, to
  specific network segments, or to certain network
  resources (such as servers).
• A remote router joins networks across large
  geographical areas, such as between cities,
  states, and countries.

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A Local Router Connecting Networks in Adjacent
Buildings
Business building
Manufacturing building
Router
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Brouters

• A brouter, also called a multiprotocol router, is
  a network device that acts like a bridge or a
  router, depending on how it is set up to forward
  a given protocol.
• It is used on networks that operate with several
  different protocols.

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How Brouters Are Used

• For efficient packet handling on a multiprotocol
  network with some protocols that can be routed
  and some that cannot
  
• To isolate and direct network traffic to reduce
  congestion
  
• To join networks
• To secure a certain portion of a network by
  controlling who can access it

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Hubs

• A hub is a central network device that connects
  network devices in a star topology.
  
• It is also referred to as a concentrator (or
  switch), which is a device that can have multiple
  inputs and outputs all active at one time.

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Services Offered by Hubs

• Provide a central unit from which to connect
  multiple nodes into one network
  
• Permit large numbers of computers to be connected
  on single or multiple LANs
  
• Reduce network congestion through centralizing
  network design
  
• Provide multiprotocol services

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Types of Hubs

• MAUs
• 10BASE-T hubs
• 100BASE-X hubs
• Intelligent and modular hubs

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10BASE-T Hubs

• One of the simplest hubs
• Popular way to connect workgroups on small and
  large LANs
  
• Uses physical star topology to connect PCs to the
  central hub
  
• Additional hubs are added by connecting one hub
  to the next.

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Switching Hubs

• Permit significant increase in the throughput
  capability of an existing 4 Mbps, 10 Mbps, or 16
  Mbps network by taking full advantage of exiting
  bandwidth capabilities
• Allow an existing network to be separated into
  multiple smaller segments, each independent of
  the others
  
• Can be installed on LANs in a WAN where specific
  LANs are experiencing increased network traffic

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100BASE-X Hubs

• Multimedia, video, and GUI client/server
  applications have fostered the need for
  high-bandwidth, high-speed technologies.

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Intelligent and Modular Hubs

• An intelligent hub has network management and
  performance monitoring capabilities.
  
• A modular hub, also called a chassis hub,
  contains a backplane into which different modules
  can be inserted.
  
• A backplane is the main circuit board in modular
  equipment, containing slots as plug-ins for
  modular cards. It provides connections between
  the modular boards, a power source, and grounding.

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To Diagram 10BASE-T Star Topology on Two Floors

• Network diagram of 10BASE-T network segments
  connected to an intelligent hub

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Gateways

• A gateway enables communications between two
  different types of networked systems, such as
  between complex protocols or between different
  e-mail systems.

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How Gateways Are Used

• To convert commonly used protocols to a
  specialized protocol
  
• To convert message formats from one format to
  another
  
• To translate different addressing schemes
• To link a host computer to a LAN

continued
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How Gateways Are Used

• To provide terminal emulation for connections to
  a host computer
  
• To direct e-mail to the right network
  destination
  
• To connect networks with different architectures

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Systems Network Architecture (SNA)

• SNA is a layered communications protocol used by
  IBM for communications between IBM mainframe
  computers and terminals.
  
• It employs seven-layered communications that are
  similar to the OSI model, but there are
  differences in the way the services are grouped
  within the layers.

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IBM/Mainframe Connected through an SNA Gateway
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ATM Switches

• An ATM switch determines the network channel used
  to transmit an ATM cell received from a node,
  taking into account the type of information in
  the cell (voice, video, data) and the
  transmission speed needed.

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Capabilities of ATM Switches

• Provide high-speed communications on a network
  backbone
  
• Provide cell transmissions directly to the
  desktop
  
• Enable high-speed communication between network
  hubs
  
• Centralize network design for better management

continued
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Capabilities of ATM Switches

• Connect to very high speed networks, such as
  SONET
  
• Enable network design around workgroup members at
  dissimilar locations (virtual LANs)
  
• Reduce network bottlenecks through high-speed
  communications and efficient traffic management
  through workgroups

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ATM Switch
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Virtual LANs (VLANs)

• A virtual LAN (VLAN) uses switches, routers, and
  internetworking software to configure a network
  into subnetworks of logical workgroups,
  independent of the physical network topology.

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Advantages of VLANs

• VLANs enable a network to operate at the most
  efficient level it is not limited by physical
  topology.
  
• Network resources can be managed based on the
  actual work groupings of users.
  
• VLANs can be reconfigured to move a user from an
  old workgroup to new one.

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VLAN Workgroups through ATM Switching
ATM switch
ATM switch
Central ATM switch
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