WAN Technologies and Routing

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WAN Technologies and Routing

• Prof. Martins
• Department of Computer Science and Computer
  Information Systems

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Goals

• In this chapter you will learn about the basic
  components used to build a packet switching
  system that can span a large area.

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Large Networks Wide Areas

• Local Area Network (LAN) can span a single
  building or campus
• A Metropolitan Area Network (MAN) can span a
  single city
• A Wide Area Network (WAN) can span sites in
  multiple cities, countries or continents.

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Large Networks Wide Areas

• A bridged LAN is not considered a wide area
  technology
• Bandwidth limitations prevent a bridged LAN from
  serving arbitrarily many computers at many sites.
• Scalability The key issue that separates LAN
  technologies from WAN

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Large Networks Wide Areas

• A WAN must be able to grow as needed to connect
  many sites spread across large geographic
  distances, with many computers at each site.
• A WAN should be able to connect all the
  computers in a large corporation that has offices
  or factories at dozens of locations spread
  across thousands of miles.

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Large Networks Wide Areas

• A WAN does not merely connect to many computers
  at many sites it must provide sufficient
  capacity to permit the computers to communicate
  simmultaneously.

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Packet Switches
A WAN is constructed from many switches to which
individual computers connect.
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Packet Switches

• Each packet switch is a small computer that has a
  processor and memory as well as I/O devices used
  to send and receive packets.
• Almost every form of point-to-point communication
  has been used to build a WAN (including leased
  lines, optical fibers, microwaves, and satellite
  channels).

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Forming a WAN
A small WAN formed by interconnecting packet
switches. Connections between packet switches
usually operate at a higher speed than
connections to individual computers.
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Forming a WAN

• A set of packet switches are interconnected to
  form a WAN.
• A WAN need not be symmetric the
  interconnections among switches are chosen
• to accommodate expected traffic, and
• provide redundancy in case of failure.

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We can summarize

• A packet switch is the basic building block of
  Wide Are Networks.
• A WAN is formed by interconnecting a set of
  packet switches, and then connecting computers.
• Additional switches or interconnections can be
  added to increase the capacity of the WAN.

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Store and Forward

• A WAN permit many computers to send packets
  simultaneously (unlike a LAN).
• The fundamental paradigm store and forward.
• Packets arriving at a switch are placed in a
  queue until the switch can forward them on toward
  their destination.
• The technique allows a packet switch to buffer a
  short burst of packets that arrive simultaneously.

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Physical Addressing in a WAN
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Physical Addressing in a WAN

• Many WANs use a hierarchical addressing scheme
  that makes forwarding more efficient.
• The simplest scheme partitions an address into
  two parts
• The first identifies a packet switch
• The second identifies a computer attached to that
  packet switch.

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Next-Hop Forwarding
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Next-Hop Forwarding

• A packet switch must choose an outgoing path over
  which to forward each packet.
• A packet switch does not keep complete
  information about how to reach all possible
  destinations.
• A switch has information about the next place
  (hop) to send a packet so the packet will
  eventually reach its destination.
• This is called next-hop-forwarding.

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Hierarchical Addresses
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Routing in A WAN

• Packet switches must have a routing table and
  both types must forward packets.
• Value in the table must guarantee
• Universal routing The routing table in a switch
  must contain a next-hop route for each possible
  destination
• Optimal routes The next-hop value must point to
  the shortest path to the destination.

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Routing in A WAN
The easiest way to think about routing in a WAN
is to imagine a graph that models the network.
Each node in the graph corresponds to a packet
switch, and each edge represents a connection
between the corresponding packet switches.
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Routing in a WAN
The routing table for each node in the graph. The
next-hop field in an entry contains a pair (u,v)
to denote the edge in the graph from node u to
node v.
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Default Routes

• A default route entry replaces a long list of
  entries that have the same next-hop value.
• Only one default value is allowed in any routing
  table.
• The entry has lower priority than other entries.
• If the forwarding mechanism does not find an
  explicit entry for a given destination, it uses
  the default entry.

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Default Routes
Revised version of the routing tables in figure
13.7. An asterisk in the column labeled
destination denotes a default route.
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Shortest Path Computation
Figure 13.9 A graph with weights assigned to
edges. The shortest path between nodes 4 and 5
is shown darkened. The distance along the path
is 19, the sum of the weights on the edges.
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Other routing algorithms

• Distributed Route Computation
• Distance Vector Routing
• Link State Routing (SPF)

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Example WAN Technologies

• ARPANET
• X.25
• Frame relay
• SMDS
• ATM

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ARPANET

• One of the first packet switched WANs
• Developed by ARPA for battlefield conditions
• By current standards ARPANET was slow.
• The project left a legacy of concepts, algorithms
  and terminology still in use.

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x.25

• X25 networks are more popular in Europe than in
  the USA.
• Invented before personal computers became popular
• Early X25 networks were engineered to connect
  ASCII terminals
• The technology is expensive for the performance
  it delivers.

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Frame Relay

• Originally designed to bridge LAN segments
• Designed to accept and deliver blocks of data (up
  to 8K octets of data)
• Designers envisioned Frame Relay running at
  speeds between 4 and 100Mbps.
• In practice, many subscribers choose to use 1.5
  Mbps or 56Kbps connections.

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SMDS

• Switched Multi-megabit Data Service
• High-speed WAN data service offered by many
  long-distance carriers
• It is designed to carry data
• Operate at the highest speeds (faster than Frame
  Relay).

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ATM

• Asynchronous Transfer Mode (ATM)
• Came from the telecommunications industry
• Designed to
• handle conventional telephone voice traffic as
  well as data traffic
• To serve both as a LAN and WAN