香港浸会大学：《Data Communications and Networking》课程教学资源（PPT讲稿）Chapter 10 Circuit Switching and Packet Switching

Data communications and Networking Chapter 10 Circuit Switching and Packet Switching References Book Chapter 10.1, 10.2, 10.5 Data and Computer Communications, 8th edition By william Stallings

1 Data Communications and Networking Chapter 10 Circuit Switching and Packet Switching References: Book Chapter 10.1 , 10.2, 10.5 Data and Computer Communications, 8th edition By William Stallings

Overview Networks are used to interconnect many devices We have checked with local area Networks Now, wide area networks Since the invention of the telephone, circuit switching has been the dominant technology for voice communications Since 1970, packet switching has evolved substantially for digital data communications. It was designed to provide a more efficient faclity than circuit switching for bursty data traffic. Two types of packet switching Datagram(such as todays Internet Virtual circuit such as Frame Relay aTm)

2 Overview • Networks are used to interconnect many devices. • We have checked with Local Area Networks. • Now, wide area networks — Since the invention of the telephone, circuit switching has been the dominant technology for voice communications. — Since 1970, packet switching has evolved substantially for digital data communications. It was designed to provide a more efficient facility than circuit switching for bursty data traffic. • Two types of packet switching: – Datagram (such as today’s Internet) – Virtual circuit (such as Frame Relay, ATM)

Switched communications Networks Long distance transmission between stations(called end devices") is typically done over a network of switching nodes Switching nodes do not concern with content of data Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination(the end device) a collection of nodes and connections forms a communications network In a switched communications network data entering the network from a station are routed to the destination by being switched from node to node

3 Switched Communications Networks • Long distance transmission between stations (called “end devices”) is typically done over a network of switching nodes. • Switching nodes do not concern with content of data. Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination (the end device). • A collection of nodes and connections forms a communications network. • In a switched communications network, data entering the network from a station are routed to the destination by being switched from node to node

Simple Switching Network Mainframe Personal computer 2 Server 3 Personal 5 Personal computer Switching E 4 Node 量 compute 7 F

4 Simple Switching Network

Switching Nodes Nodes may connect to other nodes or to some stations Network is usually partially connected -However some redundant connections are desirable for reliability Two different switching technologies Circuit switching -Packet switching

5 Switching Nodes • Nodes may connect to other nodes, or to some stations. • Network is usually partially connected —However, some redundant connections are desirable for reliability • Two different switching technologies —Circuit switching —Packet switching

Circuit Switching · Circuit switching There is a dedicated communication path between two stations (end-to-end) The path is a connected sequence of links between network nodes On each physical link, a logical channel is dedicated to the connection Communication via circuit switching has three phases: Circuit establishment (link by link) Routing resource allocation( dm or TDm) Data transfer Circuit disconnect Deallocate the dedicated resources The switches must know how to find the route to the destination and how to allocate bandwidth channel)to establish a connection 6

6 Circuit Switching • Circuit switching: — There is a dedicated communication path between two stations (end-to-end) — The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. • Communication via circuit switching has three phases: — Circuit establishment (link by link) • Routing & resource allocation (FDM or TDM) — Data transfer — Circuit disconnect • Deallocate the dedicated resources • The switches must know how to find the route to the destination and how to allocate bandwidth (channel) to establish a connection

Circuit Switching Properties Inefficiency Channel capacity is dedicated for the whole duration of a connection If no data, capacity is wasted Delay Long initial delay: circuit establishment takes time Low data delay: after the circuit establishment, information is transmitted at a fixed data rate with no delay other than the propagation delay. the delay at each node is negligible. Developed for voice traffic(public telephone network) but can also applied to data traffic. For voice connections the resulting circuit will enjoy a high percentage of utilization because most of the time one party or the other is talking. But how about data connections?

7 Circuit Switching Properties • Inefficiency — Channel capacity is dedicated for the whole duration of a connection — If no data, capacity is wasted • Delay — Long initial delay: circuit establishment takes time — Low data delay: after the circuit establishment, information is transmitted at a fixed data rate with no delay other than the propagation delay. The delay at each node is negligible. • Developed for voice traffic (public telephone network) but can also applied to data traffic. — For voice connections, the resulting circuit will enjoy a high percentage of utilization because most of the time one party or the other is talking. — But how about data connections?

Public circuit switched Network Long-distance Long-distance office office oo0oo End Office oo0oo oooooo End Office Subscriber Loop Connecting Trunk Intercity Trunk Connecting Trunk Subscribers. the devices that attach to the network Digital PBX Subscriber loop the link between the subscriber and the network Exchanges: the switching centers in the network End office: the switching center that directly supports subscribers Trunks: the branches between exchanges. They carry multiple voice-frequency circuits using either Fdm or synchronous TDM 8

8 Public Circuit Switched Network Subscribers: the devices that attach to the network. Subscriber loop: the link between the subscriber and the network. Exchanges: the switching centers in the network. End office: the switching center that directly supports subscribers. Trunks: the branches between exchanges. They carry multiple voice-frequency circuits using either FDM or synchronous TDM

Packet Switching Principles Problem of circuit switching -designed for voice service -Resources dedicated to a particular call -For data transmission much of the time the connection is idle(say web browsing) -Data rate is fixed Both ends must operate at the same rate during the entire period of connection Packet switching is designed to address these problems

9 Packet Switching Principles • Problem of circuit switching —designed for voice service —Resources dedicated to a particular call —For data transmission, much of the time the connection is idle (say, web browsing) —Data rate is fixed • Both ends must operate at the same rate during the entire period of connection • Packet switching is designed to address these problems

Basic Operation Data are transmitted in short packets Typically at the order of 1000 bytes Longer messages are split into series of packets Each packet contains a portion of user data plus some control Control info contains at least Routing (addressing info, so as to be routed to the intended destination -Recall the content of an ip header store and forward On each switching node packets are received, stored briefly (buffered)and passed on to the next node. 10

10 Basic Operation • Data are transmitted in short packets — Typically at the order of 1000 bytes — Longer messages are split into series of packets — Each packet contains a portion of user data plus some control info • Control info contains at least — Routing (addressing) info, so as to be routed to the intended destination — Recall the content of an IP header! • store and forward — On each switching node, packets are received, stored briefly (buffered) and passed on to the next node