1. Packet Switching
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2. Outline Packet-switching Principles Switching Techniques
Comparison of Circuit Switching and Packet Switching
Simple Routing Schemes
Congestion Control
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3. Circuit Switching
Circuit switching designed for voice or constant rate services
Resources are dedicated to a particular call for the call duration
In data traffic, some of the time, no data are sent
Data rate is pre-arranged
Both ends must operate at the same rate
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4. Packet Switching Data transmitted in small packets
Typically 1k to 2k octets
Longer messages are split into series of packets
Each packet contains a data field plus some control info
Control information
Routing (addressing) info
Packets are received, stored briefly (buffered) and passed on to the next node
Store and forward
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5. Advantages Line efficiency Data rate conversion
Single node to node link can be shared by many packets on a on-demand basis, improve efficiency and reliability
Packets are queued and transmitted as fast as possible
Data rate conversion
Each station connects to the local node at its own speed
Nodes buffer data and send to the station
When the network is busy, packet accepting and delivery rates may be reduced
Priority services can be implemented
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6. Packets
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7. Use of Packets
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8. Example of Packet Routing
A Packet Switching Network
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9. Packet Size
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10. Outline Packet-switching Principles Switching Techniques
Comparison of Circuit Switching and Packet Switching
Simple Routing Schemes
Congestion Control
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11. Switching Technique Station breaks long message into packets
Packets are sent one at a time to the network
Packets are handled in two ways:
Datagram
Virtual circuit
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12. Datagram Mode Each packet is treated independently
Packets can take any route
Packets may arrive out of order
Packets may go missing
Up to the receiver to re-order the packets and to recover from missing packets
IP uses datagram
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13. Datagram Mode
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14. Virtual Circuit Mode
Preplanned route decided at connection establishment
Call request and call accept packets establish the connection (handshake)
Each packet contains a virtual circuit identifier instead of destination address
No routing decisions required for each packet
Clear request to terminate circuit
Not a dedicated path, links are shared by different packets
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15. Virtual Circuit Mode
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16. X.25 Use of Virtual Circuits
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17. Public Switched Data Networks
An example of packet switching network
A network for data communications over a wide area
Also operated by telephone companies
Based on X.25 standard from ITU-T, specifying an interface between a host system and a packet switching network
low speed, up to 64kb/s
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18. Virtual Circuits vs Datagram
Network provides sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node will lose all circuits through that node
Datagram
No call setup phase
Better for low number of data packets
More flexible
Routing can be used to avoid congested parts of the network
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19. Outline Packet-switching Principles Switching Techniques
Comparison of Circuit Switching and Packet Switching
Simple Routing Schemes
Congestion Control
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20. Delays
Propagation delay: time for a signal to propagate from one node to the next
Transmission time: time for a transmitter to send out a block of data, transmission capacity dependent
Node delay: the time for a node to process and to switch the data
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21. Circuit Switching vs Packet Switching
Event Timing for Circuit Switching and Packet Switching
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22. Circuit Switching vs Packet Switching (Cont’d)
Circuit switching: connection set-up is required before data transfer
Virtual circuit packet switching:connection set-up is needed before data transfer
Datagram packet switching: no set-up is required
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23. EE3900 Data Communications and LANs Packet Switching Slide 23

24. Outline Packet-switching Principles Switching Techniques
Comparison of Circuit Switching and Packet Switching
Simple Routing Schemes
Congestion Control
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25. Introduction The need for routing: e.g. Getting from A to F D B F A E
Path 1: A->B->D->F
Path 2: A->B->E->F
Path 3: A->B->C->E->F
Path 4: A->C->E->F
Path 5: A->C->E->B->D->F
Path 6: A->C->B->D->F
Path 7: A->C->B->E->F
Which one?
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26. Routing Routing Complex, crucial aspect of packet switched networks
Performance used for selection of route: Minimum hop, Least cost
Characteristics required:
Correctness, Simplicity, Robustness, Stability
Fairness, Optimality, Efficiency
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27. Routing Attributes Routing information gathering
Information source: local, adjacent nodes, all nodes
Update time: never (fixed routing), regular update (adaptive routing)
Routing path selection/calculation
Time: per packet, connection set-up
Place: distributed nodes (distributed routing), centralized node (central routing), source node (source routing)
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28. Fixed Routing
Single permanent route for each source-destination pair of nodes
Determine routes using a least cost algorithm
the link cost is based on expected traffic or capacity, but not on any dynamic variable such as instant traffic volume
each node needs only to store the next forwarding address for each destination
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29. Cost of Routes in a Packet-Switched Network
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30. Fixed Routing Tables
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31. Least-Cost Path What is meant by least cost?
It can be number of hops, physical distances, costs, delay, link capacity, current loading …
Or an arbitrary function of the above parameters,
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32. Flooding
A packet is sent by the source node to every one of its neighbors
At each intermediate node, an incoming packet is retransmitted on all outgoing links except for the incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be discarded
Advantage: no network information is required
Disadvantage: unlimited amount of traffic will be generated unless something is done
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33. Flooding Example
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34. Solution for Flooding
Method 1: each node remembers the identity of those packets it has already transmitted. When duplicate copies of the packet return, they are discarded.
Method 2: Each packet contains a hop count field. Each time a node passes on a packet, it decrements the count by one. When the count reaches zero, the packet is discarded.
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35. Properties of Flooding
All possible routes between source and destination are tried. Therefore, a packet will always get through if there is a connection between source and destination
Because all routes are tried, at least one copy of the packet will arrive at the destination using a minimum-hop route; can be used to set up VC
All nodes that are directly or indirectly connected to the source node are visited; useful to distribute information (routing)
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36. Random Routing
A node selects only one outgoing path for transmission of an incoming packet
The outgoing link is chosen at random, round robin or probability based; excluding the link on which the packet arrived
Like flooding, no network information is required
Random routing carries less traffic than flooding, but higher than optimum load
Route is typically not least cost nor minimum hop
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37. Adaptive Routing
The routing decisions change as conditions on the network change, such as node failure or link congestion
Information about the state of the network are required and exchanged among the nodes
The higher the amount and the more frequent information is exchanged (higher network overhead), the better decisions can be made
React too quickly can cause oscillation, too slowly will not be adaptive
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38. Advantages of Adaptive Routing
improve performance, as seen by the network user
By-pass link congestions and node failures
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39. Drawbacks of Adaptive Routing
The routing decision is more complex; hence, increase node processing
Adaptive strategies depend on status information, collected at one location but used in another; therefore, the traffic loading on the network increases
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40. Isolated Adaptive Routing
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41. Outline Packet-switching Principles Switching Techniques
Comparison of Circuit Switching and Packet Switching
Simple Routing Schemes
Congestion Control
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42. Congestion
As packets arrive, they are stored in the input buffer of the corresponding link
The node sends each incoming packet to the appropriate output buffer
If packets arrive too fast for the node to process, or faster than the outgoing speed; packets will arrive for which no memory is available and packets will be lost.
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43. Causes of Congestion
Insufficient router memory, slow processors (at routers), low-bandwidth links, etc.
Normal Traffic
Congestion
Router
incoming links
outgoing links
Router
incoming links
outgoing links
Packets are dropped due to lack of buffers.
Can we solve congestion by using huge amount of buffers?
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44. Effects of Congestion
Throughput: As the offered load increases, throughput first increases, then drop and finally approaches to zero (Why? Any solution?)
Delay: the average delay grows without bound as the load approaches the capacity of the system.
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45. Congestion Introduction The effect of network congestion:
During congestion, the more you send, the less the network can deliver.
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46. The effect of congestion.
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47. General Strategies for Congestion Control
discard any incoming packets for which there is no available buffer space
exercise flow control over its neighbors (ask them to reduce sending rate), so that the traffic flow remains manageable. In fact, the traffic on the entire network needs to be managed.
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48. Congestion Control Algorithms
General Principles
Open-loop congestion control algorithms
Prevent congestion from occurring in the first place.
Close-loop congestion control algorithms
Monitor state of the network
Feedback state information to the senders
Adjust system operation to correct the problem
Specific algorithms will be discussed in TCP
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