Local Area Networks: Internetworking

1
Data Communications and Computer Networks A
Business Users Approach Third Edition

• Chapter 8
• Local Area Networks Internetworking

2
Objectives

• List the reasons for interconnecting multiple
  local area network segments and interconnecting
  local area networks and wide area networks
• Identify the functions and purposes of the
  various interconnection devices that have been
  used over time
• Define a hub and describe the situations in which
  a hub is used
• Describe how a transparent bridge learns

3
Objectives (continued)

• Describe what a switch is and how it differs from
  other devices
• Describe the types of situations in which using a
  switch is advantageous
• Describe what a router is and how it differs from
  other devices
• Describe the types of situations in which a
  router is used

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Introduction

• Sometimes necessary to connect local area network
  to another LAN or to wide area network
• Local area network to local area network
  connections often performed with bridge-like
  device
• Local area network to wide area network
  connections usually performed with router
• Switch - can be used to interconnect segments of
  a local area network

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Why Segment or Internetwork Local Area
Networks?

• To separate / connect one corporate division with
  another
• To connect two LANs with different protocols
• To connect a LAN to the Internet
• To break a LAN into segments to relieve traffic
  congestion
• To provide a security wall between two different
  types of users

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Hubs

• Hub - interconnects two or more workstations into
  a local area network
• When workstation transmits to a hub
• Hub immediately resends data frame out to all
  connecting links
• Hub can be managed or unmanaged
• Managed hub possesses enough processing power
  that it can be managed from a remote location

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Bridges

• A bridge (or bridge-like device)
• Can be used to connect two similar LANs, such as
  two CSMA/CD LANs
• Can also be used to connect two closely similar
  LANs, such as a CSMA/CD LAN and a token ring LAN
• Examines destination address in a frame
• Either forwards this frame onto next LAN or does
  not
• Examines source address in frame
• Places this address in a routing table to be used
  for future routing decisions

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Bridges (continued)

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Transparent Bridge

• Does not need programming
• Observes all traffic ? builds routing tables
• This observation is called backward learning
• Each bridge has two connections (ports)
• Routing table associated with each port

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Transparent Bridge (continued)

• Bridge
• Observes each frame that arrives at port
• Extracts the source address from the frame
• Places that address in the ports routing table
• Transparent bridge is found with CSMA/CD LANs

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Transparent Bridge (continued)

12
Transparent Bridge (continued)

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Transparent Bridge (continued)

• Transparent bridge can also convert one frame
  format to another
• Note that some people / manufacturers call this
  kind of bridge a gateway or router
• Bridge
• Removes headers and trailers from one frame
  format
• Inserts (encapsulates) headers and trailers for
  second frame format

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Remote Bridge

• Capable of passing data frame from one local area
  network to another when
• Two LANs are separated by long distance
• There is a wide area network connecting the two
  LANs
• Remote bridge takes the frame before it leaves
  the first LAN
• Encapsulates the WAN headers and trailers
• When packet arrives at destination remote bridge
  ? that bridge removes the WAN headers and
  trailers leaving original frame

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Remote Bridge (continued)

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Switches

• Combination of hub and bridge
• Can interconnect two or more workstations
• Like bridge, it observes traffic flow and learns
• When a frame arrives at a switch, the switch
• Examines destination address
• Forwards frame out the one necessary connection
• Workstations that connect to
• Hub ? on shared segment
• Switch ? on switched segment

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Switches (continued)

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Switches (continued)

• Backplane of a switch is fast enough to support
  multiple data transfers at one time
• A switch that employs cut-through architecture is
  passing on the frame before the entire frame has
  arrived at the switch

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Switches (continued)

• Multiple workstations connected to a switch use
  dedicated segments
• Very efficient way to isolate heavy users from
  the network
• A switch can allow simultaneous access to
  multiple servers, or multiple simultaneous
  connections to a single server

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Switches (continued)

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Isolating Traffic Patterns and Providing
Multiple Access

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Isolating Traffic Patterns and Providing
Multiple Access (continued)

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Isolating Traffic Patterns and Providing
Multiple Access (continued)

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Isolating Traffic Patterns and Providing
Multiple Access (continued)

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Full-Duplex Switches

• Allows for simultaneous transmission and
  reception of data to and from a workstation
• This full duplex connection helps eliminate
  collisions
• To support a full duplex connection to a switch,
  at least two pairs of wires are necessary
• One for the receive operation
• One for the transmit operation
• Most people install four pairs today, so wiring
  is not problem

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Full-Duplex Switches (continued)

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Network Servers

• Network servers provide necessary storage for LAN
  software
• Usually focal point for network operating system
• Increasingly, network servers are functioning as
  bridges, switches, and routers
• By adding appropriate card, a server can assume
  multiple functions

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Routers

• Router - device that connects a LAN to a WAN or a
  WAN to a WAN
• Router
• Accepts outgoing packet
• Removes any LAN headers and trailers
• Encapsulates necessary WAN headers and trailers
• Because router has to make wide area network
  routing decisions ? router has to dig down into
  the network layer of the packet to retrieve
  network destination address

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Routers (continued)

• Routers are often called layer 3 devices
• Operate at the third layer, or OSI network layer,
  of the packet
• Often incorporate firewall functions

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Routers (continued)

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LAN Internetworking in Action A Small
Office Revisited

• Recall In Action example from Chapter Seven
• A small office with 20 workstations in one room
  and 15 workstations in another room were
  connected to a server via 100BaseTX
• One hub was kept in a closet near the 20
  workstations while a second hub was near the
  server

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LAN Internetworking in Action A Small
Office Revisited (continued)

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LAN Internetworking in Action A Small
Office Revisited (continued)

• Now Hannah wants to connect the LAN to the
  Internet
• She adds a router next to the server and connects
  it to the hub
• She connects the router to a high-speed telephone
  line such as a T-1 service
• She will also have to program the router to
  perform IP addressing and firewall functions

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LAN Internetworking in Action A Small
Office Revisited (continued)

35
LAN Internetworking in Action A Small
Office Revisited (continued)

• Now network usage is so high that Hannah must
  consider segmenting the network
• She decides to install a database server near the
  original server and replace both hubs with
  switches

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LAN Internetworking in Action A Small
Office Revisited (continued)

37
Summary

• Interconnecting multiple LAN segments as well as
  LANs and WANs
• Interconnection devices
• Hubs
• Transparent bridges
• Switches
• Routers