1. Packet Switching
Around 1970, research began on a new form of architecture for long distance communications: Packet Switching.

2. Introduction
Packet Switching refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

3. Packet Switching Application
Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as messages and Web pages.

4. Recall Circuit Switching
Call Set-up
Data Transfer
Call disconnect

5. Features of Circuit Switching
Resources are allocated for the call throughout the network.
Calls may be blocked if the resources are not available.
Circuit switching is connection oriented.
Circuit Switching originated due to need for voice communications.

6. Circuit Switching for Data
When Circuit Switching networks started to be used for data communications it became clear that:
In circuit switching resources dedicated to a particular call whereas data traffic is bursty so most of the time allocated resources would be unutilized.
Data rate is fixed in circuit switching so Both ends must operate at the same rate - whereas there is asymmetry in the data rate required for each communicating party for data communication needs.

7. Packet Switching Packet Switching started in the 1970s.
ARPANET that became Internet
In the beginning most people did not believe it would work
The basic technology of packet switching is fundamentally the same today as it was in the early 1970’s networks
Packet switching remains one of the few technologies for effective long-distance data communications.

8. Packet Switching Networks are Switched Networks

9. Packet Switching Networks are Switched Networks

10. Packet Switching Operation
Data are transmitted in short packets. Typically an upper bound on packet size is 1000 octets.
If a station has a longer message to send it breaks it up into a series of small packets. Each packet now contains part of the user's data and some control information.
The control information should at least contain:
Destination Address
Source Address
Store and forward - Packets are received, stored briefly (buffered) and past on to the next node

11. Packet Switching Operation

12. Use of Packets

13. Advantages Line efficiency
Single node to node link can be shared by many packets over time
Packets queued and transmitted as fast as possible
Data rate conversion
Each station connects to the local node at its own speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy
Delivery may slow down
Priorities can be used

14. Switching Technique - Virtual Circuits and Datagrams
Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways
Datagram
Virtual circuit

15. Datagram Packet Switching
In datagram approach each packet is treated independently with no reference to packets that have gone before. No connection is set up.
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and recover from missing packets
More processing time per packet per node
Robust in the face of link or node failures.

16. Packet Switching Datagram Approach

17. Virtual Circuit Packet Switching
In the Virtual Circuit approach a pre-planned route is established before any packets are sent.
There is a call set up before the exchange of data (handshake).
All packets follow the same route and therefore arrive in sequence.
Each packet contains a virtual circuit identifier instead of destination address
More set up time
No routing decisions required for each packet - Less routing or processing time
Susceptible to data loss in the face of link or node failure
Clear request to drop circuit
Not a dedicated path

18. Packet Switching Virtual Circuit Approach

19. One Station Can Have Many Virtual Circuit Connections

20. Virtual Circuits vs. Datagram
Network can provide sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node looses all circuits through that node
Datagram
No call setup phase
Better if few packets
More flexible
Routing can be used to avoid congested parts of the network

21. Packet switching - datagrams or virtual circuits
Interface between station and network node
Connection oriented
Station requests logical connection (virtual circuit)
All packets identified as belonging to that connection & sequentially numbered
Network delivers packets in sequence
External virtual circuit service
e.g. X.25
Different from internal virtual circuit operation
Connectionless
Packets handled independently
External datagram service
Different from internal datagram operation

22. Combinations (1) External virtual circuit, internal virtual circuit
Dedicated route through network
External virtual circuit, internal datagram
Network handles each packet separately
Different packets for the same external virtual circuit may take different internal routes
Network buffers at destination node for re-ordering

23. Combinations (2) External datagram, internal datagram
Packets treated independently by both network and user
External datagram, internal virtual circuit
External user does not see any connections
External user sends one packet at a time
Network sets up logical connections

24. External Virtual Circuit and Datagram Operation

25. Internal Virtual Circuit and Datagram Operation

26. External and Internal Operation - ED/IVC

27. External and Internal Operation - ED/ID

28. External and Internal Operation - EVC/IVC

29. External and Internal Operation - EVC/ID

30. Packet Size

31. Circuit vs. Packet Switching
Performance
Propagation delay
Transmission time
Node delay

32. Comparison with Circuit Switching - Event Timing

33. Comparison with Circuit Switching

34. Routing Complex, crucial aspect of packet switched networks
Characteristics required
Correctness
Simplicity
Robustness
Stability
Fairness
Optimality
Efficiency

35. Routing Performance Criteria
Used for selection of route
Minimum hop
Least cost
Using some algorithm
Delay
Throughput

36. Routing Decision Time and Place
Packet basis
virtual circuit basis
Place
Distributed
Made by each node
Centralized
Source (originating node)

37. Network Information Source
Routing decisions usually based on knowledge of network (not always)
Distributed routing
Nodes use local knowledge
May collect info from adjacent nodes
May collect info from all nodes on a potential route
Central routing
Collect info from all nodes

38. Network Information Update Timing
When is network info held by nodes updated
Fixed - never updated
Adaptive - regular updates

39. Routing Strategies
Fixed
Flooding
Random
Adaptive

40. Fixed Routing
Single permanent route for each source to destination pair
Determine routes using a least cost algorithm (appendix 10A)
Route fixed, at least until a change in network topology

41. Fixed Routing Tables

42. Flooding No network info required
Packet sent by node to every neighbor
Incoming packets retransmitted on every link except incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be discarded
Nodes can remember packets already forwarded to keep network load in bounds
Can include a hop count in packets

43. Flooding Example

44. Properties of Flooding
All possible routes are tried
Very robust
At least one packet will have taken minimum hop count route
Can be used to set up virtual circuit
All nodes are visited
Useful to distribute information (e.g. routing)

45. Random Routing
Node selects one outgoing path for retransmission of incoming packet
Selection can be random or round robin
Can select outgoing path based on probability calculation
No network info needed
Route is typically not least cost nor minimum hop

46. Adaptive Routing Used by almost all packet switching networks
Routing decisions change as conditions on the network change
Failure
Congestion
Requires info about network
Decisions more complex
Tradeoff between quality of network info and overhead
Reacting too quickly can cause oscillation
Too slowly to be relevant

47. Adaptive Routing - Advantages
Improved performance
Aid congestion control (See chapter 12)
Complex system
May not realize theoretical benefits

48. Classification Based on information sources Local (isolated)
Route to outgoing link with shortest queue
Can include bias for each destination
Rarely used - do not make use of easily available info
Adjacent nodes
All nodes

49. ARPANET Routing Strategies(1)
First Generation
1969
Distributed adaptive
Estimated delay as performance criterion
Bellman-Ford algorithm
Node exchanges delay vector with neighbors
Update routing table based on incoming info
Doesn't consider line speed, just queue length
Queue length not a good measurement of delay
Responds slowly to congestion

50. ARPANET Routing Strategies(2)
Second Generation
1979
Uses delay as performance criterion
Delay measured directly
Uses Dijkstra’s algorithm
Good under light and medium loads
Under heavy loads, little correlation between reported delays and those experienced

51. ARPANET Routing Strategies(3)
Third Generation
1987
Link cost calculations changed
Measure average delay over last 10 seconds
Normalize based on current value and previous results

52. Costing of Routes
Who determines the link costs? And how?

53. Dijkstra’s Algorithm Define: N = set of all nodes in the network
s = source node
M = set of nodes so far incorporated by the algorithm
dij = link cost from node i to node j; dii = 0 and dij =  if the two nodes are not directly connected; dij  0 if the two nodes are directly connected
Dn = cost of the least cost path from node s to node n that is currently known to the algorithm

54. Dijkstra’s Algorithm Steps
1. Initialize: this is done once in the beginning
M = {s} i.e. set of nodes incorporated is only the source node
Dn = dsn for n  s i.e. initial path costs to neighboring nodes are simply link costs
2. Find the neighboring node not in M that has the least cost path from node s and incorporate that node into M:
Find w  M such that Dw = min Dj (j M)
Add w to M

55. Dijkstra’s Algorithm 3. Update least-cost paths:
Dn = min[Dn, Dw+dwn] for all n  M
If the latter term is the minimum, the path from s to n is now the path from s to w concatenated with the link from w to n

57. Switching vs Routing Switching path set up at connection time
simple table look up
table maintainance via signaling
no out of sequence delivery
lost path may lose connection
much faster than pure routing
link decision made ahead of time, and resources allocated then
Routing
can work as connectionless
complex routing algorithm
table maintainance via protocol
out of sequence delivery likely
robust: no connections lost
significant processing delay
output link decision based on packet header contents - at every node