WAN Technologies

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WAN Technologies Topologies

• Lecture 8
• October 4, 2000

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WAN Background

• WAN - Wide Area Network, spans multiple
  cities/states.
• MAN - Metropolitan Area Networks exist in a
  single city.
• LAN Single building environment.
• Multiple MANs can make a WAN, multiple LANs can
  make a MAN, etc.

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WAN Example
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MAN As A Building Block

• As I mentioned earlier, multiple MANs usually
  make up the WAN.
• Each MAN has control of its own domain, but its
  uplink to the other MANs is considered the WAN
  (backbone).
• Using Verio Bostons example

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The Interconnects

• MAN WAN circuits must terminate in some form of
  a packet switch.
• Using the UNet example from the other day, the
  packet switches were the Cisco Catalyst 6509
  switches.
• But keep in mind, the telephone company also has
  some switches in the field that handle even more
  traffic than what youre getting!

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Switch Interconnects The Cloud
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WAN Switch Functionality

• WAN switches use store and forward technology.
• The store operation occurs when the packet
  arrives the I/O hardware copies the packet,
  sticks it in memory, and signals the processor to
  forward the packet.
• The forward operation is the act of removing the
  packet from memory, and sends it to the
  appropriate interface.

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WAN Switch Functionality (cont.)

• Storing the packets also leads to a form of
  queuing for each interface.
• If the destination interface is busy, the packet
  is queued until the destination interface is
  idle, then the forward occurs.
• The store and forward paradigm allows to handle
  the maximum bandwidth of the WAN connection,
  since all data is buffered!

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Physical Addressing in the WAN Environment

• A hierarchical scheme is used with WAN
  addressing.
• The simplest form of this scheme The first part
  of the address holds the destination switch, the
  second part holds the specific machine that the
  packet is destined for on that switch.
• This is scheme is used in many WAN environments.

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WAN Addressing (cont.)
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Next-Hop Forwarding

• In order for networking to occur, each device
  must have some knowledge of the devices which it
  is connected to.
• Next-hop forwarding is a scheme where devices
  know their neighbors, but dont know the
  specifics of what is connected to each neighbor.

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The Airline Example

• Suppose a passenger is traveling from San
  Francisco to Miami. Only one flight is listed,
  with three legs Dallas, Atlanta, Miami.
• From San Francisco, his next destination is
  Dallas.
• From Dallas, his next destination is Atlanta.
• From Atlanta, his last destination is Miami.
• But all along, the LAST destination was Miami,
  even though the next hop was changing at each leg.

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Next-Hop Diagram
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Source Independence

• The next hop does not depend on the direction
  that the packet came from.
• This is referred to source independence.
• Source independence is a fundamental concept in
  data networks. It allows for low-overhead,
  efficient networks.

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Hierarchical Addressing Routing

• Heirarchical addressing is almost routing
• The act of forwarding a packet to the next
  address is dubbed routing.
• Routing uses a table format to determine the next
  hop of the communication, and since it only needs
  to inspect the first part of the address, it is
  efficient!

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A Simplified Routing Table
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Routing (cont.)

• The two part addressing scheme provides us with
  the following
• Switches along the path of the transmission need
  to inspect the first part of the hierarchical
  address.
• The last switch in the transmission must inspect
  the last part of the address.
• Really efficient network transport! Not much
  overhead.

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Routing in the WAN

• There are LOTS of routing algorithms that are
  commonly used in the WAN environment.
• RIP Router Information Protocol
• OSPF Open Shortest Path First
• BGP Border Gateway Protocol
• IGRP Interior Gateway Routing Protocol
• Im not going to focus on the different
  algorithms, but please read up and understand the
  differences between them!

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Routing in the WAN (cont.)

• The best way to visualize routing is to imagine
  how all of the networks are connected together.
• Each node in the network is a packet switch.
• Each connection between switches is a link or an
  edge.

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A Network Diagram
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Routing Table for the Example
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Default Routes

• Default routes allow for the simplification of
  routing tables.
• Since many packets would have the same routes, a
  default route would reduce the amount of work the
  router/switch would have to do.

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Simplified Routing Table
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Default Routing (cont.)

• Only one default route per device is allowed.
• The default route has lower priority to other
  entered routes.
• If a transmission does not find a valid route, it
  will send the packet down the default route.

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Determination of the Routing Table

• Two ways exist for route determination
• Static routing
• Dynamic routing
• Why have different options?

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Static Routing

• Static routing is the most straightforward of the
  two schemes.
• Pros
• Simple to visualize
• Low overhead on devices which perform routing
• Cons
• Static, inflexible

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Dynamic Routing

• Dynamic routing can be very confusing.
• Multiple types of dynamic routing exist
• OSPF
• IGRP
• BGP
• RIP
• Main disadvantage of dynamic routing difficult
  to understand.

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OSPF

• OSPF Open Shortest Path First
• Uses a system of metrics to determine route
  preference.
• The entire route preference is the sum of the
  individual metrics of the links between the
  computers.
• The routers send their routing tables out
  periodically to their neighbors.

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Common WAN Technologies

• ATM- Asynchronous Transfer Mode
• Frame Relay
• SONET

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ATM

• Data is divided into small, fixed packets called
  cells.
• Each cell is 53 octets
• 5 octets for header info
• 48 octets for data transmission
• ATM originally designed for simultaneous
  transmission of data, voice, and video.
• Quality of Service (QOS) is adjustable with
  bandwidth needs higher for video, lower for data
  and video.

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ATM (cont).

• Connections are usually 155Mbps OC-3.
• Uses the cloud concept.

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Frame Relay

• Data is divided into small, variable sized cells,
  up to 16,000 octets!
• Up to 1.544Mbps transmission rate.
• Uses cloud concept shared bandwidth with
  other connections.

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OSPF Example