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School of Business
Eastern Illinois University
Wide Area Networks
(Week 11, Thursday 3/22/2007)
© Abdou Illia, Spring 2007
Learning Objectives
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Distinguish between LAN and WAN
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Distinguish between
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Circuit Switching Networks
Datagram Packet Switching Networks
Virtual Packet Switching Networks
Understand routing decisions
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LAN and WAN
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LAN = a communication network that interconnects
networking devices within a small geographic area using
broadcast system.
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WAN = a communication network that interconnects
networks and networking devices within a wide geographic
area using Point-to-Point system.
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A Point-to-Point system:
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Many connections between individual pairs of stations
A packet may have to visit one or more intermediate stations
Multiple routes are possible; so routing decisions have to be made
Q: What are the main differences between a LAN and a WAN?
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Wide Area Networks basics
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WANs follows a mesh topology
LAN
Subnet
Subnet
Subnet
Subnet
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Only neighbor devices are connected to each other
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To transmit across the mesh, data has to be passed along a
route from node to node
Wide Area Networks basics
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A station is a device that interfaces a user to a network
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A station could be:
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a computer (for Data networks)
a telephone (For Voice networks)
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Wide Area Networks basics
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Data is transferred from node to node through the network
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A Node is a transfer point for passing data through the
network
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A Node is often a computer, a router, or a telephone switch
Q: What is the difference between a station and a node?
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Wide Area Networks basics
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The subnet is the underlying physical connection of nodes
and communication lines that transfer data from one
location to another.
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A Subnet is a collection of nodes and different types of
transmission media
Q: How does the subnet differ from the network?
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Types of Subnets
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Based on the way data is transferred from one end
of the subnet to the other:
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Circuit Switching Subnet
Packet Switching Subnet
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Circuit Switching Subnet
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Traditionally used for Voice networks
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A subnet in which a dedicated circuit is established between
sender and receiver and all data passes over this circuit.
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Reserved Capacity (Circuit Switching)
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Circuit capacity is reserved during duration of
each communication
At each switch
Reserved
On each trunk line
Capacity
Reserved
Capacity
Circuit
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Reserved Capacity
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Nothing like congestion on the Internet
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Reserved Circuit Capacity is Expensive
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Pay for it whether you use it or not
Good for voice, because conversations are fairly
constant
Bad for data, because most data transmission is bursty;
e.g., in World Wide Web, download, then stare at screen
for a long time until next download
Q: What are the main characteristics of Circuit Switching networks?
Packet Switching Subnet
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Usually used for Data networks
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A subnet in which all data messages are transmitted
using fixed-sized packages, called packets.
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Two types of Packet Switching Subnets:
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Datagram Packet Switching Subnets
Virtual-Circuit Packet Switching Subnets
Datagram Packet Switching Subnet
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In all types of Packet Switching Subnets, large messages
are broken into small pieces called packets
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Packets are short (averaging a few hundred bytes) because most
nodes (routers) handle short messages more efficiently
Message
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Packets
Multiplexing
– Packets from many conversations are mixed
(multiplexed) over each trunk line
Packet Switching
Multiplexing on
Trunk Line
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Datagram Packet Switching Subnets
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As each packet arrives at a node a routing decision is made:
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Which route the packet will follow next ?
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This dynamic routing decision allows flexibility should the
network experience congestion or failure
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But, a node has to examine each individual packet and
determine the next path
B
Router A B?
D?
D
Packet
C?
C
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Virtual-Circuit Packet Switching Subnets
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Amount of data broken into n packets
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A virtual circuit (temporary path through the network) is
determined
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Note: The virtual circuit is not dedicated (not reserved)
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All n packets transmitted through the virtual circuit
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When transfer completed, virtual circuit dissolved
Q:
Routing decisions
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Many possible routes exist for forwarding a packet.
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How does a router decide which line to transmit on?
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Routing decisions
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A subnet can be viewed as weighted network graph
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A weight is associated to each line
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Weight can be:
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Cost of using the transmission line
Time delay for transmitting data
Size of the queue
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Routing decisions
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Different algorithms (or techniques) for selecting a
route through a network
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Common algorithms/techniques:
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Dijkstra’s least cost algorithm
Flooding technique
Dijkstra’s least cost algorithm
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Calculate the costs for using each route for a given node
Determine the route that minimize the sum of the costs
Calculation performed on a periodic basis or when
changes happened (connection or node failure for
example)
Routing decisions
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Flooding Routing
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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
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Traffic grows very quickly when every node floods the
packet
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To limit uncontrolled growth, each packet has a hop
count. Every time a packet hops, its hop count is
incremented. When a packet’s hop count equals a global
hop limit, the packet is discarded
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Summary Questions

How does the subnet differ from the network?
Answer: The network include: the subnet(s), the stations, the OS & other
application software, and the other networking devices & transmission
medium needed to connect the stations to the subnet
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What is the difference between a station and a node?
Answer: A station is the device that interfaces a user to the network. A node is
a transfer point for passing data through the network. A node can be a
computer, a router, or a telephone switch.

What are the main characteristics of Circuit Switching networks?
Answer: In Circuit Switching networks: (1) a dedicated circuit is established
between sender and receiver, (2) circuit capacity is reserved during the
duration of each communication, at each node (switch) and on each
transmission line; (3) no routing decisions are necessary since circuit is
dedicated.
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Summary Questions

What are the main characteristics of Virtual-Circuit Packet
Switching networks?
Answer: (1) Data sent in packets, (2) all packets follow the
same virtual circuit, (3) the virtual circuit may be shared
with packets from other conversions, (4) no routing
decisions except the first one that creates the circuit.

Name some criteria that routing decisions are based on
Answer: see slide #17
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Name two routing algorithms/techniques.
Answer: see slide #18
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Bridge versus Router
Layers
Role
Bridge 2-layer device
Interconnecting LANs or segments of same LAN
Router 3-layer device
-Interconnecting LANs to WANs, and nodes in WANs
-Making routing decisions
Ethernet
Ethernet
Token Ring
Ethernet
Token Ring
Token Ring
Transparent bridge
Source-routing bridge