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© 2006, Monash University, Australia CSE4884 Network Design and Management Lecturer: Dr Carlo Kopp, MIEEE, MAIAA, PEng Lecture 3 Switched vs Packet Network Concepts
© 2006, Monash University, Australia Circuit vs Packet Switch Network Concepts Historical evolution of network technologies Circuit switched networks analogue vs digital Structure of circuit switched networks Topology of circuit switched networks Design issues in circuit switched networks Packet Switch networks Catenet model Structure of packet switched networks Topology of packet switched networks Hybrid networks Futures issues – VoIP, fibre, wireless, ad hoc WAN, MAN, SAHN, LAN and other networks.
© 2006, Monash University, Australia Historical evolution of network technologies Messenger relays, optical signalling relay chains Telegraph relays – manual operators Telephone networks with human operators Telephone networks with electrical stepper and crossbar switches. Packet switching networks with minicomputers used as routers. Digital telephone switches, ISDN, then ATM networks. Modern microprocessor based routers and digital packet switches. Further reading: http://telephonetribute.com/switches_survey_chapter_6. html http://telephonetribute.com/switches_survey_chapter_6. html
© 2006, Monash University, Australia Stepper Switch and Crossbar Switch UCLA Image Telephone Tribute Image
© 2006, Monash University, Australia Evolutionary Issues There have always been three primary drivers in the evolution of networking technology: 1. Speed of switching 2. Volume/Cost of switching equipment 3. Reliability of switching equipment Solenoid driven electromechanical switches were an important step, replacing human operators. Electronic switching using digital logic chips was the next important step, reducing cost, improving speed and reliability. Future evolution? Faster and more dense digital switching, using Silicon or optical hardware.
© 2006, Monash University, Australia Circuit switched networks analogue vs digital What is an analogue signal? Analogue signals are such signals, where the instantaneous magnitude of a variable is used to represent a sound or image, or other physical effect. In networking, electrical voltage is usually employed to carry analogue signals. The advantage of analogue hardware is that it is usually very simple. The disadvantage is that analogue signals are usually susceptible to noise and interfering signals. Analogue hardware is usually ‘hardwired’ to perform a particular task and cannot be easily altered for another.
© 2006, Monash University, Australia Analogue Representation of Human Voice Time [ms] Magnitude/Amplitude [dB]
© 2006, Monash University, Australia Structure of circuit switched networks Individual subscriber phones are connected to switches via two pair electrical cables. Each switch can form connections to other local subscriber phones, or to other switches at other physical locations. Telephone numbers are used to address a specific subscriber when calling. In automatic switches, the subscriber dials a number using pulse or tone systems, the switch inteprets these numbers, and forms the connection, either to a local subscriber or subscriber at a different switch. Prefix numbers are usually associated with specific switches in geographical areas.
© 2006, Monash University, Australia Circuit Switched Telephony
© 2006, Monash University, Australia Circuit Switched System
© 2006, Monash University, Australia Topology of circuit switched networks Circuit switched systems are typically designed to minimise the cost of cabling runs, by minimising distance, and to minimise the number of switches. Geographical location of subscribers constrains the location of switches, which are typically located to minimise the total cable length to all subscribers. Each suburb might have one or more switches. Switches are then interconnected with bundles of cables (legacy) or high speed cables (today). In metropolitan areas, large central switches in the CBD area are usually used to route connections between suburban switches, and to switches in other cities, including globally located sites.
© 2006, Monash University, Australia Design issues in circuit switched networks Regardless of the technology used in circuit switches systems – analogue, digital, multiplexed etc – the key issue is that for every connection between a pair of subscribers, a consecutive chain of circuits much be allocated and interconnected for the duration of the connection. Design in circuit switched networks revolves around determining what is the proper sizing for the network to ensure that subscribers can get connections when they need them. Sizing is determined by the capacity of the switches and the capacity of the links (cables) between switches. If capacity is undersized, heavy or peak loads will saturate the network.
© 2006, Monash University, Australia Packet Switch networks All packet switching schemes share a common high speed channel between multiple host systems. Traffic is divided into packets, each of which carries headers with unique addressing and control information. Packets are sent into the network and switches or routers along the network decode the packet headers to determine where to route the packets to. Packets are usually asynchronous in time. Quality of Service is a consideration – packets must not be lost, must not be damaged, must arrive in a timely manner. Packet switching networks use often complex protocols for internal management of the network.
© 2006, Monash University, Australia Catenet model Reference: IEN 48 - THE CATENET MODEL FOR INTERNETWORKING; Vinton Cerf DARPA/IPTO; July 1978: http://www.isi.edu/in-notes/ien/ien48.txthttp://www.isi.edu/in-notes/ien/ien48.txt DARPA: “collection of packet networks which are connected together”. Cerf: “The basic assumption regarding datagram transport within any particular network is that the datagram will be carried, embedded in one or more packets, or frames, across the network. If fragmentation and reassembly of datagrams occurs within a network it is invisible for purposes of the catenet model.” The network is transparent to the format of the data being carried.
© 2006, Monash University, Australia Structure of packet switched networks Unlike circuit switched networks where the basic structure is strongly hierarchical, and redundancy is often minimal, packet switched networks allow for more flexibility in topology. In most packet networks, hosts (computers, VoIP phones etc) are connected to a high speed local network. High speed local networks on a site are interconnected with repeaters (hubs) or switches. Local networks connect to other networks via routers, which filter local traffic from traffic intended for other sites. High speed connections are nearly always shared between multiple host systems.
© 2006, Monash University, Australia Topology of Packet Switched Networks Topology is widely variable in packet networks, and usually reflects the specific application. A national ISP will have major routers in capital cities and larger regional centres. These are interconnected, and also connected to high speed links overseas. Smaller routers are then located on user premises, such as campuses, buildings or industrial sites. Such sites will usually have high speed Local Area Networks to interconnect desktops and servers. Home subscribers historically used voiceband modems to connect to ISPs or campus/building main sites. ADSL and Cable modems have allowed faster connections to ISP or campus/building main sites.
© 2006, Monash University, Australia Topology
© 2006, Monash University, Australia LANs
© 2006, Monash University, Australia Hybrid networks The availability of technology like voiceband modems permits digital signals to be sent over analogue channels, albeit at modest or low speeds. The result is a ‘hybrid’ network, where a circuit switched analogue network might be used to provide subscriber connections to sites where their packet traffic can be routed to a high speed packet network. Dedicated high speed microwave or fibre links, separate from the circuit switch telephony network, might be used for site to site connections. Telephone networks today usually employ ATM switches at exchanges. ATM uses fixed size packets termed cells. Arguably, the current telephone network is also ‘hybrid’ as parts are packet oriented in how they carry traffic, and parts retain circuit switching.
© 2006, Monash University, Australia Futures– VoIP, Fibre, Wireless, Ad Hoc VoIP – Voice over IP is a protocol which allows voice telephony to be carried over an Internet Protocol (IP) packet network. VoIP is now displacing circuit switched telephony in the PABX market and STD/ISD market. Optical fibre links have until recently been used only for connections between switches/exchanges, and high speed LANs. Over the next decade we can expect to see optical fibres to subscriber premises. A fibre has the potential to carry Gigabits/s of data. Wireless networking will become increasingly popular. Products like the Blackberry will increasingly displace conventional mobile phones. Ad hoc – self organising – wireless networking will become increasingly popular in situations where cabled infrastructure is inadequate.
© 2006, Monash University, Australia WAN, MAN, SAHN, LAN WAN – Wide Area Network – spanning national, state, or municipal boundaries. MAN – Metropolitan Area Network – confined to a metropolital area like a CBD, using common protocols. SAHN – Suburban Ad Hoc Network – confined to a suburban extent, using wireless protocols. LAN – Local Area Network – confined to a room, floor, building or campus. The boundaries are now becoming increasing blurred between the categories. When is the network a MAN or WAN? The long term trend is for universal broadband connectivity.
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