This Lecture. BUS Computer Facilities Network Management. Switching Network. Simple Switching Network

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Priscilla Gaines
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1 This Lecture BUS0 - Computer Facilities Network Management Switching networks Circuit switching Packet switching gram approach Virtual circuit approach Routing in switching networks Faculty of Information Technology Monash University Faculty of Information Technology Faculty of Information Technology Switching Network Simple Switching Network Long distance transmission is typically done over a network of switched nodes. More recently used in local area networks (LAN) as well. is transferred from one station to another station through intermediate node(s). Nodes are not concerned with the content of the data. End devices are stations: Computer, terminal, phone, etc. routed by being switched from node to node. Personal computer B Personal computer A Mainframe C Switching Node 7 Server E D Personal computer Faculty of Information Technology Personal computer Faculty of Information Technology F

2 Switching Network Circuit Switching Nodes may connect to other nodes only, or to stations and other nodes. Network is usually partially connected. Some redundant connections are desirable for reliability. There are two different switching technologies: Circuit Switching. Packet Switching. Dedicated communication path between two end stations. Connection oriented network. Involves three phases: Circuit establishment. transfer. Circuit disconnection. Connection path is established before data transfer. Faculty of Information Technology Channel capacity is allocated during connection establishment. Must have switching capacity and channel capacity to establish connection. Must have intelligence to determine routing. Faculty of Information Technology Circuit Switching Circuit Switching The three phases in for data transmission: Circuit establishment. Request for a connection is sent to the network node. Node passes the request to the next node (determined by the routing algorithm used) until the request reached the intended station. Determine if destination is free and if it is, construct path. transfer. Once the connection is established, data can be transferred. Transmission channel is dedicated for the two stations, regardless whether any data is being transferred. The channel is free only when the circuit is disconnected. Circuit disconnection. Signals are generated to terminate circuit and release the path. 7 Faculty of Information Technology Connection is maintained during the transmission. No delay at the nodes. Network is transparent to user. Inefficient: Channel capacity is dedicated for the duration of the connection. If no data, capacity wasted. Set up (connection) takes time. Once connected, transfer is transparent. Example: Public Switched Telephone Network (PSTN), PBX. Faculty of Information Technology

3 Public Circuit Switched Network Blocking or Non-blocking Subscriber Loop End Office Connecting Trunk Long distance office Intercity Trunk Long distance office Connecting Trunk End Office Blocking: May be unable to connect two stations because all the paths in the network are in use. For example: Used on voice systems where call duration is short. Non-blocking: Permits all stations to connect (in pairs) at once. Used for some data connections. Digital PBX 9 Faculty of Information Technology 0 Faculty of Information Technology Circuit Switching Packet Switching Circuit switching network is not suitable for data communication: In a typical user/host data connection, much of the time the line is idle. End devices must transmit and receive data at the same rate, but a network interconnects a variety of host computers and workstations. With heavy traffic, blocking can occur. Priorities can not be set. Need a suitable alternative. Faculty of Information Technology Circuit switching was designed for voice. Packet switching was designed for data. Transmitted in small packets containing user data and control info: User data may be part of a larger message. Control info includes routing (addressing) info. Packets are received, stored briefly (buffered) and past on to the next node. There is no dedicated channel. Two techniques to route packets through the network: gram. Virtual Circuit. Faculty of Information Technology

4 Packet Switching Packet Switching: gram Each packet is treated independently. Each packet has the full destination address. control information (packet header) packet Application data Packet Switching Network Each packet may take different route to the destination depending on the traffic. Reordering of packets at the destination may be required. Each packet is called a datagram. Used in connectionless service. Faculty of Information Technology Faculty of Information Technology Packet Switching: Virtual Circuit Packet Switching: gram Approach (a) (b) Logical connection is first established between the source and destination prior to data transfer. Call Request packet (from address, destination address, virtual circuit identifier). Call Accept packet (virtual circuit identifier). Each packet follows the same path to destination. anim (c) Packets arrive in order. Clear Request Packet. (d) End-to-end error and flow control techniques can be used. Suitable for data transmission over long periods. Faculty of Information Technology (e) Not a dedicated path. Faculty of Information Technology

5 Virtual Circuit vs gram (a) Transmission over extended period of time will benefit from using virtual circuit: Network provides packets sequencing and error control service. Packet Switching: Virtual-Circuit Approach (b) (c) Nodes do not need to make routing decision, may speed up transmission. Short transmission will benefit from datagram: No call setup overhead (d) Adapt better to the changes in network traffic than virtual circuit (no planned route). If a node has failed, packets can still be delivered through alternate nodes by avoiding the failed node. 7 Faculty of Information Technology (e) Faculty of Information Technology Effect of Packet Size on Transmission Time Packet vs Circuit Switching X a b Example shows a virtual circuit from X to Y through nodes a and b. Each node receives the entire packet before transmitting to the next node. Larger packet size can increase transmission time. However header overhead must also be considered. Y Time (a) packet message (b) packet message (c) packet message (d) 0 packet message Header X a b Y X a b Y X a b Y Packet Message is broken up into packets. Channel is not dedicated to a connection. High Line efficiency for non-voice communication. Two stations communicating do not need to be in the same data rate. During busy traffic, packets will not be blocked, but delivery delay time will increase. Priorities can be used. Packets are stored at the nodes. Dynamic use of bandwidth. Overhead bits in each packet. Circuit Message is sent on its entirely. Dedicated channel. Low line efficiency for non-voice communication. Two station communicating needs to be in the same data rate. During busy traffic, calls are blocked until free channel is found. Priorities cannot be used. Message is not stored at any nodes. Fixed bandwidth transmission. No overhead bits after call setup. X a b Y 9 Faculty of Information Technology 0 Faculty of Information Technology

6 Event Timing for Circuit Switching and Packet Switching Routing in Packet Switched Network Call request signal (a) Circuit switching propagation delay User data link link link processing delay Call accept signal Acknowledge ment signal Call request packet (b) Virtual circuit packet switching Pkt Pkt Pkt Nodes: Faculty of Information Technology Pkt Pkt Pkt Pkt Pkt Pkt Call accept packet Acknowledge ment packet (c) gram packet switching Pkt Pkt Pkt Pkt Pkt Pkt Pkt Pkt Pkt Key design issue for (packet) switched networks. Select a route across the network between end nodes. Characteristics required: Correctness Simplicity Robustness Stability Fairness Optimality Efficiency Route selection criteria: simplest is minimum hop but can be generalised as least cost. Faculty of Information Technology Example Packet Switched Network Decision Time and Place N N N Time for routing decision: Packet (for each datagram) or virtual circuit (during circuit establishment) basis. Fixed route or dynamically changing route. N 7 N N Place of routing decision: Distributed routing - made by each node. Centralised routing - a centralised node (single point of failure). Source routing - made by the network user. Two arrows between a pair of nodes represents a link and the number a current cost. Transmission from N to N: shortest path would be N-N-N (cost = + = 0), least cost would be N-N-N-N (cost = + + = ). Faculty of Information Technology Faculty of Information Technology

7 Network Information Source and Update Timing Routing Strategies - Fixed Routing Routing decisions usually based on knowledge of network (not always). Distributed routing: Using local knowledge, info from adjacent nodes, info from all nodes on a potential route. Central routing: Collect info from all nodes. Issue of update timing. When is network info held by nodes updated. Fixed - never updated. Adaptive - regular updates. Use a single permanent route for each source to destination pair. Determined using a least cost algorithm. Route is fixed. At least until a change in network topology. Hence cannot respond to traffic changes. Advantage: simple. Disadvantage: lack of flexibility. Faculty of Information Technology Faculty of Information Technology Fixed Routing Tables Routing Strategies - Flooding CENTRAL ROUTING DIRECTORY From Node To Node Node Directory Node Directory Node Directory Destination Next Node Destination Next Node Destination Next Node Node Directory Node Directory Node Directory Destination Next Node Destination Next Node Destination Next Node 7 Faculty of Information Technology Packet sent by node to every neighbour. Eventually multiple copies arrive at destination. No network information is required. Each packet is uniquely numbered so duplicates can be discarded. Need some way to limit incessant retransmission. Nodes can remember packets already forwarded to keep network load in bounds. Or include a hop count in packets. Faculty of Information Technology

8 Flooding Example (hop count = ) Properties of Flooding All possible routes are tried. Very robust. At least one packet will have taken the minimum hop count route. (a) First hop Can be used to set up virtual circuit. All nodes are visited. Useful to distribute information (eg. routing). (b) Second hop Disadvantage: high traffic load generated. (c) Third hop 9 Faculty of Information Technology 0 Faculty of Information Technology Routing Strategies - Random Routing Routing Strategies - Adaptive Routing Simplicity of flooding with much less load. Node selects one outgoing path for retransmission of incoming packet. Selection can be random or round robin. A refinement is to select outgoing path based on probability calculation. No network info needed. But a random route is typically neither least cost nor minimum hop. Faculty of Information Technology Used by almost all packet switching networks. Routing decisions change as conditions on the network change due to failure or congestion. Requires info about the network. Disadvantages: Decisions more complex. Tradeoff between quality of network info and overhead. Reacting too quickly can cause oscillation. Reacting too slowly means info may be irrelevant. Advantages: Improved performance. Aid congestion control. Faculty of Information Technology

9 Classification of Adaptive Routing Strategies Isolated Adaptive Routing Based on information sources. Local (isolated): Route to outgoing link with shortest queue. Can include bias for each destination. Rarely used - does not make use of available info. Adjacent nodes: Takes advantage on delay / outage info. Distributed or centralised. All nodes: Like adjacent. Node s Bias Table for Destination Next Node Bias 9 0 To To Node To To Faculty of Information Technology Faculty of Information Technology Further Reading Stallings, W. and Computer Communications, Prentice Hall. Chapters 0 and. Faculty of Information Technology

Routing in packet-switching networks

Routing in packet-switching networks Circuit switching vs. Packet switching Most of WANs based on circuit or packet switching Circuit switching designed for voice Resources dedicated to a particular call