1. G64INC Introduction to Network Communications
Ho Sooi Hock
Wide Area Networks

2. Outline Overview of WANs & Switching Packet Forwarding
Datagram
Virtual Circuit
WAN Hierarchical Addressing
Dynamic Routing Computation
Link State
Distance Vector
Performance Characteristics

3. Wide Area Networks
A WAN (Wide Area Network) spans a large geographical area, often across a state, country or continent
Most WANs are constructed using the following two distinct components: transmission lines and switching elements
WAN topologies, normally consisting of point-to-point links between switches, are chosen to accommodate expected traffic and to provide network redundancy
As WANs are used to connect hosts owned by different organisations and users, internetworking of heterogeneous networking technologies is expected
WANs can be classified according to ownership of the above two components
public, private or virtual private network (VPN)

4. Network Ownership Private network Public network
owned by the people who use it
usually LANs combined with private switches and leased lines or virtual circuits
Public network
owned and operated by an independent service provider to provide potential communications between any pair of subscribers
Virtual private network
combines the features of private and public networks.
uses public network for connectivity but ensures privacy of network traffic for an organisation
connections, called tunnels, are used as point to point link between sites with data encryption

5. Switching Technologies
Taxonomy of switched networks

6. Packet vs. Circuit Switching
In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase.
Circuit switching is wasteful for data communications as exchange of short burst of data is involved
Packet switching takes advantage of data burstiness by allocating resources on demand.
Benefits of packet switching are:
interleaving bursts from many users to maximise the use of the shared network
helps in detection and recovery of transmission errors
gives fair access for a shared connection between many computers
improves transmission time

7. Packet Switching Concepts
Store and forward
each switch receives packets, queues them in its memory and then sends them out when possible (i.e. when the trunk is available)
many computers can send packets simultaneously
Packet switched networks use a form of statistical TDM

8. Packet Forwarding Methods
Connectionless (Datagram)
adds a destination and sequence number to each packet
individual packets can follow different routes
packets reassembled at destination (by using their sequence numbers)
simpler implementation with less overhead & delay
Connection Oriented (Virtual Circuit (VC))
establishes an end-to-end circuit between the sender and receiver (before the packets are sent)
all packets for that transmission take the same route over the virtual circuit established
same physical circuit can carry many VCs
reliable service, ease of accounting, problem notification & isolation

9. WAN Addressing
A WAN technology defines a frame format and assigns addresses to its attached networking devices
Addressing is hierarchical, e.g.
first part identifies site or location
second part identifies host or computer at the site

10. Next Hop Forwarding
Packet switch needs not have complete information of the whole network. Only knowledge of how to reach the next switch (next hop) is held in the routing table – reduces memory requirement
From the destination address, a packet switch determines whether it is for local ports or another packet switch – allows fast and efficient routing decisions
Source independence
next hop to destination does not depend on source of packets
Routing table in a datagram network

11. Routing Table - Examples
Hierarchical addresses simplify routing
smaller routing table
more efficient look up

12. Virtual Circuit Networks
A virtual circuit network is a cross between a circuit switched and a datagram network. It has some characteristics of both.
all packets belonging to the same source and destination pair travel the same path; but packets may arrive at the destination with different delays if resource allocation is on demand.
Source-to-destination data transfer in a virtual-circuit network

13. Addresses and Connection Identifiers
Connection identifiers may be used instead of full addresses
For example, ATM:
424 bits packets (53 octets)
160 bits for addresses
24 bits for connection identifiers
connection identifier local significant only
shorter as compared to the full address

14. Permanent and Switched Connections
Permanent connection
established manually
held in non-volatile memory - persistent
Switched connection
established when needed

15. WANs and Graphs
A WAN can be modelled as a graph
nodes and edges

16. Default Route
A routing table can be further simplified by including (at most) one default route

17. Routing in a WAN
Large WANs use interior (backbone) switches and exterior (edge) switches
Their routing tables must guarantee the following:
universal routing i.e., there must be a next hop for every possible destination
optimal routes i.e., the next hop must point to the “shortest path” to the destination
Popular routing protocols

18. Routing Table Computation
Manual computation is not feasible for large networks, automatic generation is the solution
Static routing
program computes and installs routes when a switch boots; these routes do not change
Dynamic routing
program builds an initial table when the switch boots; this is altered as network condition changes

19. Link State Routing
Each switch shares its knowledge of its neighbourhood with every other switches in the network. The algorithm works as follows:
Instead of sending its entire routing table, a switch sends information about its neighbourhood only.
Each switch sends this information to every other switch on the network, not just its neighbours. Every switch that receives the information packet sends copies to all its neighbours. Finally, every switch will receive a copy of the same information.
Each switch sends out information about the neighbours when there is a change in the network.
Each switch then applies Djikstra’s algorithm to its link state database

20. Computing the Shortest Path
WAN modelled as nodes and edges in a graph
Dijkstra’s algorithm – find the distance from a source node to each of the other nodes in a graph
Run this for each switch and create next hop routing table as part of the process
“Distance” represented by weights (costs) on edges
indication of the desirability of sending traffic over that link, e.g. hop count, capacity, delay and traffic
problem is to find the lowest cost (shortest) path between any two nodes

21. Dijkstra’s Algorithm
Data structure to store information about the graph (nodes and edges)
Integer labels for the nodes [1, 2, ….n]
Three data structures:
current distance to each node - array - D[1, 2,…n]
next hop for each node - array - R[1, 2…n]
set of remaining nodes - linked list - S
Weight (i,j) function (infinity if no edge)

22. Given:
a graph with nonnegative weight assigned to each edge and a
designated source node
Compute:
the shortest distance from the source node to each other node
and a next hop routing table
Method:
Initialise the sets S to contain all nodes except the source
Initialise D so the D[u] = weight(source, u) if there is an edge
from the source to u and infinity otherwise
Initialise R so that R[v] = v if there is an edge from the source
to v and zero otherwise

23. while ( set S is not empty ) {
choose a node u from S such that D[u] is minimum
if ( D[u] is infinity ) {
error: no path exists to nodes in S; quit }
delete u from set S
for each node v such that (u,v) is an edge {
if ( v still in S ) {
c = D[u] + weight (u,v);
if ( c < D[v] {
D[v] = c;
R[v] = R[u];

24. Distance Vector Routing
Each switch periodically shares its knowledge about the entire network with its neighbours. The algorithm works as follows:
Each switch collects information about the network and sends this knowledge to its neighbours. At the outset, this information may be sparse. How much it knows, however is unimportant. It sends whatever it has.
It send its knowledge to only those switches that have direct links. This information is received and used by each neighbouring switch to update its knowledge about the network.
The information sharing happens at regular intervals, e.g. every 30 secs and it occurs whether or not the network has changed since the last exchange.

25. Distance Vector Algorithm
Given:
a local routing table, a weight for each link that connects to
another switch and an incoming routing message
Compute:
an updated routing table
Method:
maintain a distance field in each routing table entry
initialise routing table with a single entry that has the
destination equal to the local packet switch, the next hop
unused and the distance set to zero

26. repeat forever {
wait for the next routing message to arrive over the network; let
the sender be switch N
for each entry in the message {
let V be the destination in the entry and D the distance
compute C as D plus the weight of the link over which the message arrived
examine and update the local routing table {
if ( no route exists to V ) {
add an entry to the local table for V with next hop N
and distance C
} else if ( a route exists with next hop N) {
replace existing distance with C
} else if ( route exists with distance > C )
change next hop to N and distance to C }

27. Distance Vector Routing
Initialization of tables in distance vector routing

28. Distance Vector Routing
Updating in distance vector routing

29. Distance Vector Routing
Routing tables after two updates

30. Performance Characteristics
Delay
time for one bit to travel across the network
may vary with location on the network
interested in maximum, average and variation
results from propagation delays, switching delays, access delays and queuing delays
Throughput
rate at which data can be sent in bits per second (bps) into a network
hardware bandwidth vs. effective throughput

31. Relating Delay to Utilization
Delay and throughput are related due to congestion (Queuing Theory) D0
D =
( 1 – U )
D0 = delay when idle, U = utilisation, D = current delay

32. Transmission and Propagation
Performance of media can be measured in bandwidth, propagation speed, propagation time
Throughput: How fast data can pass through a point in the medium
Propagation speed: The distance a signal (or a bit) can travel through a medium in one second

33. Delay-Bandwidth Product
the volume of data that can be present on the network, i.e. in transit at any one time
for good performance, the receiver’s window must be at least as large as the bandwidth-delay product
Jitter
delay variation between packets
important for time critical applications
jitter must be kept small to ensure quality

34. Example WAN Technologies
Read Comer’s book chapter on WAN Technologies and Dynamic Routing
ARPANET
X.25
Frame Relay
SMDS
ATM

35. Summary
WAN can span arbitrary distances and interconnect arbitrarily many computers
WAN uses packet switches and point to point connections
Packet switches use routing tables and store-and-forward operation to deliver packets to destination
Hierarchical addressing and default routing are used for efficient table lookup
Graph algorithms can be used to compute routing tables
Many WAN technologies exist

36. Acknowledgements
Most lecture slides used in this presentation are adopted from the same module taught in Nottingham, UK Campus, with addition of diagrams from the reference texts by Douglas E. Comer and A. Forouzan.