Wide Area Networks

Wide Area Networks

School of BusinessEastern Illinois University

© Abdou Illia, Spring 2007

(Week 11, Thursday 3/22/2007)

2Learning Objectives

Distinguish between LAN and WAN

Distinguish between – Circuit Switching Networks– Datagram Packet Switching Networks – Virtual Packet Switching Networks

Understand routing decisions

3LAN and WAN

LAN = a communication network that interconnects networking devices within a small geographic area using broadcast system.

WAN = a communication network that interconnects networks and networking devices within a wide geographic area using Point-to-Point system.

A Point-to-Point system:– 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?

4Wide Area Networks basics

WANs follows a mesh topology

Only neighbor devices are connected to each other

To transmit across the mesh, data has to be passed along a route from node to node

Subnet

Subnet

Subnet

Subnet

LAN


A station is a device that interfaces a user to a network

A station could be:– a computer (for Data networks)– a telephone (For Voice networks)


Data is transferred from node to node through the network

A Node is a transfer point for passing data through the network

A Node is often a computer, a router, or a telephone switch

Q: What is the difference between a station and a node?


The subnet is the underlying physical connection of nodes and communication lines that transfer data from one location to another.

A Subnet is a collection of nodes and different types of transmission media

Q: How does the subnet differ from the network?

8Types of Subnets

Based on the way data is transferred from one end of the subnet to the other:– Circuit Switching Subnet– Packet Switching Subnet

9Circuit Switching Subnet

Traditionally used for Voice networks

A subnet in which a dedicated circuit is established between sender and receiver and all data passes over this circuit.

10Reserved Capacity (Circuit Switching)

– Circuit capacity is reserved during duration of each communication

– At each switch– On each trunk line

Circuit

ReservedCapacity

ReservedCapacity

11Reserved Capacity

Nothing like congestion on the Internet

Reserved Circuit Capacity is Expensive– 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?

12Packet Switching Subnet

Usually used for Data networks

A subnet in which all data messages are transmitted using fixed-sized packages, called packets.

Two types of Packet Switching Subnets:– Datagram Packet Switching Subnets– Virtual-Circuit Packet Switching Subnets

13Datagram Packet Switching Subnet

In all types of Packet Switching Subnets, large messages are broken into small pieces called packets

– Packets are short (averaging a few hundred bytes) because most nodes (routers) handle short messages more efficiently

Multiplexing onTrunk LinePacket Switching

Multiplexing– Packets from many conversations are mixed

(multiplexed) over each trunk line

Message Packets

14Datagram Packet Switching Subnets

As each packet arrives at a node a routing decision is made:– Which route the packet will follow next ?

This dynamic routing decision allows flexibility should the network experience congestion or failure

But, a node has to examine each individual packet and determine the next path

Router A B?

D?

C?

B

C

DPacket

15Virtual-Circuit Packet Switching Subnets Amount of data broken into n packets

A virtual circuit (temporary path through the network) is determined

Note: The virtual circuit is not dedicated (not reserved)

All n packets transmitted through the virtual circuit

When transfer completed, virtual circuit dissolved

Q:

16Routing decisions

Many possible routes exist for forwarding a packet.

How does a router decide which line to transmit on?

17Routing decisions

A subnet can be viewed as weighted network graph

A weight is associated to each line

Weight can be:– Cost of using the transmission line– Time delay for transmitting data– Size of the queue

18Routing decisions

Different algorithms (or techniques) for selecting a route through a network

Common algorithms/techniques:– Dijkstra’s least cost algorithm– Flooding technique

Dijkstra’s least cost algorithm– 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)

19Routing decisions

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 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

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.

21

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 Name two routing algorithms/techniques.

Answer: see slide #18

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Bridge versus Router

Layers

Bridge 2-layer device

Role

Interconnecting LANs or segments of same LAN

Router 3-layer device-Interconnecting LANs to WANs, and nodes in WANs-Making routing decisions

Ethernet EthernetEthernet Token Ring

Token Ring Token Ring

Transparent bridge

Source-routing bridge

Documents

Wide Area Networks