Introduction to Metropolitan Area Networks and Wide Area Networks

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 10
• Introduction to Metropolitan Area Networks and
  Wide Area Networks

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This time/last time

• Focus has been on LANs
• Now MANs and WANs
• Want to connect WANs
• Internet
• Telephony
• Cell

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Data Communications and Computer Networks
Chapter 10

Introduction As we have seen, a local area
network covers a room, a building or a campus. A
metropolitan area network (MAN) covers a city or
a region of a city. A wide area network (WAN)
covers multiple cities, states, countries, and
even the solar system.
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Metropolitan Area Network Basics

MANs borrow technologies from LANs and WANs. MANs
support high-speed disaster recovery
systems real-time transaction backup
systems interconnections between corporate data
centers and internet service providers, and
government, business, medicine, and education
high-speed interconnections. Almost exclusively
fiber optic systems
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Metropolitan Area Network Basics

MANs have very high transfer speeds MANs can
recover from network faults very quickly
(failover time) MANs are very often a ring
topology (not a star-wired ring) Some MANs can be
provisioned dynamically
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Failover

• Failover is a backup operational mode in which
  the functions of a system component (such as a
  processor, server, network, or database, for
  example) are assumed by secondary system
  components when the primary component becomes
  unavailable through either failure or scheduled
  down time.
• Used to make systems more fault-tolerant,
  integral part of mission-critical systems that
  must be constantly available. The procedure
  involves automatically offloading tasks to a
  standby system component so that the procedure is
  as seamless as possible to the end user.
• Failover can apply to any aspect of a system
  network component or system of components, such
  as a connection path, storage device, or Web
  server.

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Data Communications and Computer Networks
Chapter 10

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Remember SONET

• Synchronous Time Division Multiplexing
• Three types popular today
• T-1 multiplexing (the classic)
• ISDN multiplexing
• SONET (Synchronous Optical NETwork)

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SONET versus Ethernet MANs

• Most MANs are SONET networks built of multiple
  rings (for failover purposes)
• SONET is well-proven but complex, fairly
  expensive, and cannot be provisioned dynamically.
• SONET is based upon T-1 rates and does not fit
  nicely into 1 Mbps, 10 Mbps, 100 Mbps, 1000 Mbps
  chunks, like Ethernet systems do.
• Ethernet MANs
• Well understood, scale well and best technology
  to carry IP traffic (internet)
• Have high failover times (slow recovery to
  failure)
• Growing in popularity

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SONET systems are comprised of multiple rings

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The Ethernet MAN Topology

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Wide Area Network Basics

WANs used to be characterized with slow, noisy
lines. Today WANs are very high speed with very
low error rates. WANs often follow a mesh
topology.
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Characteristics of WANs

• Similarities to LANs
• Interconnect computers.
• Use some form of media for the interconnection.
• Support network applications.
• Differences to LANs
• Include both data networks, such as the Internet,
  and voice networks, such as telephone systems.
• Interconnect more workstations, so that any one
  workstation can transfer data to any other
  workstation.
• Cover large geographic distances, including the
  earth.
• Subnet
• Originally routers and lines, now also means a
  section of network addressing

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Why WANs?
Federal Express package routing system. American
Airlines reservation system. Amazon.com. Visa
International payment process system. Any
application system that is based on the Internet.
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Data Communications and Computer Networks
Chapter 10

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Wide Area Network Basics

A station is a device that interfaces a user to a
network. A node is a device that allows one or
more stations to access the physical network and
is a transfer point for passing information
through a network. A node is often a computer, a
router, or a telephone switch. The subnet (old
terminology) or physical network is the
underlying connection of nodes and
telecommunication links.
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Data Communications and Computer Networks
Chapter 10

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Types of Network Subnets

• Categorize network by the way it transfers
  information from one node to another physical
  entity
• Circuit switched
• Packet switched
• Broadcast

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Circuit Switched Network

Circuit switched network - a network in which a
dedicated circuit is established between sender
and receiver and all data passes over this
circuit. The telephone system is a common
example. The connection is dedicated until one
party or another terminates the connection.
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Data Communications and Computer Networks
Chapter 10

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Packet Switched Network
Packet switched network - a network in which all
data messages are transmitted using fixed-sized
packages, called packets. More efficient use of a
telecommunications line since packets from
multiple sources can share the medium. One form
of packet switched network is the datagram. With
a datagram, each packet is on its own and may
follow its own path. Virtual circuit packet
switched network create a logical path through
the subnet and all packets from one connection
follow this path. Virtual path is not physically
real, but acts like a circuit. It exists only in
software of the routers. Not used in todays
internet.

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Packets vs frames Packet is a more general
term. Messages transmitted over the
network Packets for messages at the OSI network
layer (routing). Frames for messages at the OSI
data link layer. Datagrams are packets built such
that each packet is on its own and may follow its
own path.

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Switching Comparison Packet switching
transmission of messages by dividing the message
up into packets and transmitting it over many
lines. The message is reconstructed at the end.
Ex. internet Circuit switching transmission of
messages over a dedicated line all packets go
other this line.

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Broadcast Network

Broadcast network - a network typically found in
local area networks but occasionally found in
wide area networks. A workstation transmits its
data and all other workstations connected to
the network hear the data. Only the
workstation(s) with the proper address will
accept the data.
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Connection-oriented versus Connectionless

The network structure is the underlying physical
component of a network. What about the software
or application that uses the network? What is the
logical entity of the connection? A network
application can be either connection-oriented or
connectionless.
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Connection-oriented versus Connectionless

A connection-oriented application requires both
sender and receiver to create a connection before
any data is transferred. Applications such as
large file transfers and sensitive transactions
such as banking and business are typically
connection-oriented. A connectionless application
does not create a connection first but simply
sends the data. Electronic mail is a common
example.
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Connection-oriented telephone call

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Connectionless postal system

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Logical on the physical

A connection-oriented application can operate
over both a circuit switched network or a packet
switched network. A connectionless application
can also operate over both a circuit switched
network or a packet switched network but a packet
switched network may be more efficient.
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Routing Choices

Each node in a WAN is a router that accepts an
input packet, examines the destination address,
and forwards the packet on to a particular
telecommunications line. How does a router decide
which line to transmit on? A router must select
the one transmission line that will best provide
a path to the destination and in an optimal
manner. Often many possible routes exist between
sender and receiver.
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Data Communications and Computer Networks
Chapter 10

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How Routing Works

The subnet with its nodes and telecommunication
links is essentially a weighted network
graph. The edges, or telecommunication links,
between nodes, have a cost associated with
them. The cost could be a delay cost, a queue
size cost, a limiting speed, or simply a dollar
amount for using that link.
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A Simple Example of a Network Graph

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Network Graphs

• What do we notice about network graphs?
• Connectivity
• Values on the links weighted network graph
• Transitivity

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Routing Method

• The routing method, or algorithm, chosen to move
  packets through a network should be
• Optimal, so the least cost can be found
• Fair, so all packets are treated equally
• Robust, in case link or node failures occur and
  the network has to reroute traffic.
• Not too robust so that the chosen paths do not
  oscillate too quickly between troubled spots.

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Find the minimal cost route from one node to
another. Need an algorithm to do that
eyeballing does not always work This is what a
routing algorithm does

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Forward Search Least Cost Routing
Algorithm Dijkstras least cost algorithm finds
all possible paths between two locations. By
identifying all possible paths, it also
identifies the least cost path. Cost is reflected
in value on the links. Examples? The algorithm
can be applied to determine the least cost path
between any pair of nodes. Only calculated
periodically since one has to get through the
entire net to do it.

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Flooding Routing

When a packet arrives at a node, the node sends a
copy of the packet out every link except the link
the packet arrived on. Traffic grows very quickly
when every node floods the packet. To limit
uncontrolled growth, each packet has a hop count.
Every time a packet hops, its hop count is
incremented. When a packets hop count equals a
global hop limit, the packet is discarded.
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Network with Flooding Routing Starting from Node A
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Flooding has continued and is now being done at
Nodes B, C D

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Centralized Routing

One routing table is kept at a central
node. Whenever a node needs a routing decision,
the central node is consulted. To survive central
node failure, the routing table should be kept at
a backup location. The central node should be
designed to support a high amount of traffic
consisting of routing requests.
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Data Communications and Computer Networks
Chapter 10

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Data Communications and Computer Networks
Chapter 10

Centralized Routing Problems Centralized routers
can easily fail. How does the routing table scale?
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Distributed Routing

Each node maintains its own routing table. No
central site holds a global table. Somehow each
node has to share information with other nodes so
that the individual routing tables can be
created. Possible problem with individual routing
tables holding inaccurate information.
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Data Communications and Computer Networks
Chapter 10

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Isolated Routing

Each node uses only local information to create
its own routing table. Advantage - routing
information does not have to be passed around the
network. Disadvantage - a nodes individual
routing information could be inaccurate, or out
of date.
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Adaptive Routing versus Static Routing

With adaptive routing, routing tables can change
to reflect changes in the network Static routing
does not allow the routing tables to
change. Static routing is simpler but does not
adapt to network congestion or failures.
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Network Congestion

When a network or a part of a network becomes so
saturated with data packets that packet transfer
is noticeably impeded, network congestion
occurs. Preventive measure include providing
backup nodes and links and preallocation of
resources. To handle network congestion, you can
perform buffer preallocation, choke packets, or
permit systems. Forward and backward explicit
congestion control also used
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Quality of Service (QoS)

Before making a connection, user requests how
much bandwidth is needed, or if connection needs
to be real-time Network checks to see if it can
satisfy user request If user request can be
satisfied, connection is established If a user
does not need a high bandwidth or real-time, a
simpler, cheaper connection is created
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Data Communications and Computer Networks
Chapter 10

WANs In Action Making Internet Connections Home
to Internet connection - modem and dial-up
telephone provide circuit switched subnet, while
connection through the Internet is a packet
switched subnet. The application can be either a
connection-oriented application or a
connectionless application.
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Data Communications and Computer Networks
Chapter 10

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Data Communications and Computer Networks
Chapter 10

WANs In Action Making Internet Connections A
work to Internet connection would most likely
require a broadcast subnet (LAN) with a
connection to the Internet (packet switched
subnet).
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Data Communications and Computer Networks
Chapter 10

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What we covered

• WANs and MANs
• Physical network connections
• Circuit, Packet, Broadcast switching
• Logical network connections
• Connection oriented vs connectionless
• Network graphs
• Nodes and edges (links)
• Routing algorithms

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What we covered

• Routing algorithms find the path
• Global vs decentralized
• Flooding
• Dynamic vs static