Packet Switching EE3900 Data Communications and LANs Packet Switching Slide 1

Outline Packet-switching Principles Switching Techniques Comparison of Circuit Switching and Packet Switching Simple Routing Schemes Congestion Control EE3900 Data Communications and LANs Packet Switching Slide 2

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 EE3900 Data Communications and LANs Packet Switching Slide 3

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 EE3900 Data Communications and LANs Packet Switching Slide 4

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 EE3900 Data Communications and LANs Packet Switching Slide 5

Packets EE3900 Data Communications and LANs Packet Switching Slide 6

Use of Packets EE3900 Data Communications and LANs Packet Switching Slide 7

Example of Packet Routing A Packet Switching Network EE3900 Data Communications and LANs Packet Switching Slide 8

Packet Size EE3900 Data Communications and LANs Packet Switching Slide 9

Outline Packet-switching Principles Switching Techniques Comparison of Circuit Switching and Packet Switching Simple Routing Schemes Congestion Control EE3900 Data Communications and LANs Packet Switching Slide 10

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 EE3900 Data Communications and LANs Packet Switching Slide 11

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 EE3900 Data Communications and LANs Packet Switching Slide 12

Datagram Mode EE3900 Data Communications and LANs Packet Switching Slide 13

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 EE3900 Data Communications and LANs Packet Switching Slide 14

Virtual Circuit Mode EE3900 Data Communications and LANs Packet Switching Slide 15

X.25 Use of Virtual Circuits EE3900 Data Communications and LANs Packet Switching Slide 16

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 EE3900 Data Communications and LANs Packet Switching Slide 17

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 EE3900 Data Communications and LANs Packet Switching Slide 18

Outline Packet-switching Principles Switching Techniques Comparison of Circuit Switching and Packet Switching Simple Routing Schemes Congestion Control EE3900 Data Communications and LANs Packet Switching Slide 19

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 EE3900 Data Communications and LANs Packet Switching Slide 20

Circuit Switching vs Packet Switching Event Timing for Circuit Switching and Packet Switching EE3900 Data Communications and LANs Packet Switching Slide 21

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 EE3900 Data Communications and LANs Packet Switching Slide 22

EE3900 Data Communications and LANs Packet Switching Slide 23

Outline Packet-switching Principles Switching Techniques Comparison of Circuit Switching and Packet Switching Simple Routing Schemes Congestion Control EE3900 Data Communications and LANs Packet Switching Slide 24

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? EE3900 Data Communications and LANs Packet Switching Slide 25

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 EE3900 Data Communications and LANs Packet Switching Slide 26

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) EE3900 Data Communications and LANs Packet Switching Slide 27

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 EE3900 Data Communications and LANs Packet Switching Slide 28

Cost of Routes in a Packet-Switched Network EE3900 Data Communications and LANs Packet Switching Slide 29

Fixed Routing Tables EE3900 Data Communications and LANs Packet Switching Slide 30

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, EE3900 Data Communications and LANs Packet Switching Slide 31

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 EE3900 Data Communications and LANs Packet Switching Slide 32

Flooding Example EE3900 Data Communications and LANs Packet Switching Slide 33

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. EE3900 Data Communications and LANs Packet Switching Slide 34

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) EE3900 Data Communications and LANs Packet Switching Slide 35

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 EE3900 Data Communications and LANs Packet Switching Slide 36

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 EE3900 Data Communications and LANs Packet Switching Slide 37

Advantages of Adaptive Routing improve performance, as seen by the network user By-pass link congestions and node failures EE3900 Data Communications and LANs Packet Switching Slide 38

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 EE3900 Data Communications and LANs Packet Switching Slide 39

Isolated Adaptive Routing EE3900 Data Communications and LANs Packet Switching Slide 40

Outline Packet-switching Principles Switching Techniques Comparison of Circuit Switching and Packet Switching Simple Routing Schemes Congestion Control EE3900 Data Communications and LANs Packet Switching Slide 41

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. EE3900 Data Communications and LANs Packet Switching Slide 42

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? EE3900 Data Communications and LANs Packet Switching Slide 43

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. EE3900 Data Communications and LANs Packet Switching Slide 44

Congestion Introduction The effect of network congestion: During congestion, the more you send, the less the network can deliver. EE3900 Data Communications and LANs Packet Switching Slide 45

The effect of congestion. EE3900 Data Communications and LANs Packet Switching Slide 46

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. EE3900 Data Communications and LANs Packet Switching Slide 47

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 EE3900 Data Communications and LANs Packet Switching Slide 48