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Sub topic : Packet Forwarding in WAN

Teaching aids : Diagrams

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Recap

In the previous topic, you have learnt about

• Wide Area Network (WAN) Architecture

• WAN features

• Applications

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Objectives

On the completion of this topic, you would be

able to understand about

• Definition of packet forwarding (or packet switching)

• WAN addressing

• WAN routing

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Definition of Packet Switching

• Packet switching refers to protocols in which messages are broken up into small packets before they are sent Each packet is transmitted individually across the net

• The packets may even follow different routes to the destination, depends on the type of packet switching Thus, each packet has header information in which enable to route the packet to its destination. At the destination the packets are reassembled into the original message

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There are two basic types of Packet Switching networks.

1. Virtual Circuit Packet Switching Networks

2. Datagram Packet Switching Networks

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Virtual Circuit Packet Switching Networks (VCP)

• An initial setup phase is used to set up a route between the

intermediate nodes for all the packets passed during the

session between the two end nodes

• In each intermediate node, an entry is registered in a table

to indicate the route for the connection that has been set up

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VCPS networks cont….

• The packets passed through this route, have short headers, containing only a virtual circuit identifier (VCI)

• Each intermediate node passes the packets according to the information that was stored in its table, in the setup phase and according to the packets header content

• In this way, packets arrive at the destination in the correct sequence

• The most common forms of Virtual Circuit networks are ATM and Frame Relay, which are commonly used for public data networks (PDN)

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Datagram Packet Switching Networks (DPS Network)

• This approach uses a different, more dynamic scheme, to determine the route through the network links

• Each packet is treated as an independent entity, and its header contains full information about the destination of the packet

• The intermediate nodes examine the header of the packet, and decide the next hop of this packet

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DPS Networks cont…..

• The shortest way to pass the packet to its destination -

protocols such as RIP/OSPF is used to determine the

shortest path to the destination

• Finding a free node to pass the packet to - in this way,

bottle necks are eliminated, since packets can reach

the destination in alternate routes. Thus, in this

method, the packets don't follow a pre-established

route, and the intermediate nodes (the routers) don't

have pre-defined knowledge of the routes that the

packets should be passed through

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DPS Networks contd..

• Packets can follow different routes to the destination

• Due to the nature of this method, the packets can reach the destination in a different order than they were sent, thus they must be sorted at the destination to form the original message

• This approach is time consuming since every router has to decide where to send each packet

• The main implementation of Datagram Switching network is the Internet which uses the IP network protocol

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Packet Switching Transfers Packets Across a Carrier Network

Fig .1

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• The basic component of a WAN is a packet switch because it moves packets from one connection to another

• It consists of special-purpose hardware with a processor, memory, and I/O connectors

• A packet switch is illustrated in Figure 1

Packet Switch

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Packet Switch

Fig .2

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Packet Switch

• High-speed I/O devices connect one switch to another

• Lower speed devices connect the switch to an individual computer

• The actual hardware depends on the WAN technology and the speed required, and most forms of point-to-point communication are used

• An interconnected set of packet switches forms the WAN

• A small WAN is illustrated in Figure 3

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Small WAN

Fig .3

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WAN Addressing

• A WAN operates in the same way as a LAN

• Each WAN technology defines the frame format to be

used and each computer connected to a switch is

allocated a physical address

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WAN addressing contd….

• Many WANs use hierarchical addressing which simplifies forwarding

• The simplest scheme divides an address into two parts, one identifying the packet switch and the other the computer connected to that switch

• This is illustrated in Figure 4

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Hierarchical Addresses

Fig .4

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• In practice, a physical address is represented as a single binary number, with some bits representing the switch and the other bits the computer

• A packet switch uses the destination address to decide how to forward the packet, i.e. which connection to use

• It does not keep complete information about all destinations, but has information about the next hop to send the packet nearer its destination

• This is called next-hop forwarding, and is illustrated in Figure 5

Hierarchical Addresses contd….

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A Small WAN Network

Fig .5

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• In fig (a), we have a small network of three packet switches

• In fig (b), we have the next-hop forwarding table for switch 2

• Note that next-hop forwarding does not depend on the packet's original source or on the path the packet has taken before arriving at the switch

• This is known as source independence and is fundamental to packet forwarding

A Small WAN Network contd…

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• The table containing the next-hop information is called a routing table and the process of forwarding a packet is known as routing

• When forwarding a packet, the packet switch only needs to examine the first part of the hierarchical address

• This means the routing table can be shortened to contain one entry per destination switch rather than one entry per destination computer

• This is illustrated in Figure 6

Routing

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Routing Table

Fig .6

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Routing Table contd….

• This is an abbreviated version of the routing table, made

possible by hierarchical addressing

• When forwarding to a local computer, the switch uses the

second part of the address to select the specific

computer

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Summary

In this class, you have learnt about

• Definition of packet forwarding or packet switching

• WAN addressing

• WAN routing

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Quiz

1. The difference between Packet Switching and

Circuit Switching is

  a)  There is no difference

. b)  In Circuit Switching the lines are dedicated to the

connection while in Packet Switching they are not

 c) Packet Switching is ideal for audio or video

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2. How do switches route arriving packets?

A) The switches route the packets to the default router

  B) According to the IP address in the packet header

  C) The switch will send the packet on the interface with the least load

  D) On the basis of information contained in each packet header

Quiz

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Frequently Asked Questions

1. Define packet switching

2. Explain the types of packet switching

3. Explain WAN addressing

4. Explain routing