Packet forwarding in WANs involves breaking messages into packets that are transmitted individually and may follow different routes to the destination. There are two types of packet switching networks: virtual circuit networks which set up dedicated routes for connections, and datagram networks which treat each packet independently. WAN addresses use a hierarchical structure to simplify forwarding, with some bits identifying the packet switch and others identifying the connected computer. Packet switches use routing tables containing next-hop forwarding information to route packets towards their destination based on the address without knowing the complete network topology.

1. Sub topic : Packet Forwarding in WAN
Teaching aids : Diagrams
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2. Recap
In the previous topic, you have learnt about
• Wide Area Network (WAN) Architecture
• WAN features
• Applications
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3. 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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4. 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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5. There are two basic types of Packet Switching
networks.
1. Virtual Circuit Packet Switching Networks
2. Datagram Packet Switching Networks
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6. 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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7. 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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8. 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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9. 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 9

10. 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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11. Packet Switching Transfers Packets Across a Carrier Network
Fig .1
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12. Packet Switch
• 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
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13. Packet Switch
Fig .2
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14. 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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15. Small WAN
Fig .3
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16. 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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17. 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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18. Hierarchical Addresses
Fig .4
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19. Hierarchical Addresses contd….
• 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
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20. A Small WAN Network
Fig .5
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21. A Small WAN Network contd…
• 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
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22. Routing
• 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
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23. Routing Table
Fig .6
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24. 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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25. Summary
In this class, you have learnt about
• Definition of packet forwarding or packet switching
• WAN addressing
• WAN routing
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26. 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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27. Quiz
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
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28. Frequently Asked Questions
1. Define packet switching
2. Explain the types of packet switching
3. Explain WAN addressing
4. Explain routing
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