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Networking Clip 4 SwitchingSchool of Computer Science & CommunicationsPh. Janson, W. Zwaenepoel

Information, Computing & CommunicationICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonThis video clip is part of the E.P.F.L. introductory course on Information, Computing, and Communication.This is the 4th one in a set of video clips on computer communication & networking.

1OutlineComputer communication basicsClip 1 Protocols & messagesClip 2 Protocol layersClip 3 Protocol encapsulationClip 4 Switching

Internet basicsClip 5 Internet topology & interfacesClip 6 Internet addressing & routingClip 7 Internet route calculationClip 8 Internet protocols

Computer network paradigmsClip 9 Network paradigmsIntro clipPrevious clipNext clip

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonIt explains the fundamental difference between switching methods in old telephone networks and in modern computer networks.2Layer 3: SwitchingThe case of telephonyInformation throughput / flow is quasi constant over timetimethroughput

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonIn the case of old telephone networks the amount of information to be transmitted, i.e. the bandwidth required for such transmission is fairly constant.Communicating speakers at either end of a conversation speak at about the same speed, so that encoding their speech into some electronic signal to send across a network requires a fairly constant bandwidth across that network.3One builds and reserves a dedicated electronic connection between speakersThe connection is busy during the entire conversation time, incl. silencesLayer 3: Circuit switchingThe case of telephonycircuit avec reserved capacity CCtimethroughput

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonIn such a case it makes sense to establish and to maintain a dedicated, fixed-bandwidth connection between communicating parties.That fixed-bandwidth connection is efficiently utilized and remains busy for the entire duration of the call, even during brief silences.4Layer 3: Circuit switchingThe case of telephony

SDM = space-division multiplexing of physical circuits (analog telephony)TDM = time-division multiplexing of physical circuits (digital / mobile phones)FDM = frequency-division multiplexing of physical circuits (radio, TV)In all casesthe reservation of the circuit blocks it (the line is busy)

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonIn former telephone switches, the connection of a phone conversation through the network was done through circuit switching.Electrical wires from the calling party to the called party were effectively connected to one another end-on-end, building a circuit through the intermediate switches.During the whole connection both calling and called lines as well as all intermediate lines were busy.This was called space-division multiplexing (SDM) because spacially distinct intermediate lines could be used over time by different subscribers.Something similar happens in radio and TV broadcasting, albeit not over physical circuits but through the air or antenna cables.The radio and TV signals of several senders are broadcast simultaneously but in different frequency bands, which is called frequency-division multiplexing.In modern phone networks physical wires or wireless connections are multiplexed over time between several conversations, which is called time-division multiplexing (TDM).But in all cases the established connection builds a circuit, which is busy for the entire conversation or broadcasting time.

5Layer 3: SwitchingThe case of computer networksInformation flow is asymmetrical, jerky, with variable-intensity bursts separated by silencestimethroughput

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonThis is very different in computer communication because computer communications are very bursty and asymmetrical, and include many potentially large silences.When a user sitting at her computer surfs the web, she typically types just a few characters and then gets a load of information from the server she querried.Then there is silence until she has digested that and comes back with another query.

Even when two computers communicate with one another, the amount of exchanged information can be very asymmetrical and bursty.And the sending and receiving of messages can be separated by eminently variable silences.

6Layer 3: Packet switchingThe case of computer networksReserving high-throughput circuits would be inefficientReserving low-throughput circuits would be slowOne does not reserve any circuits; instead one sends information in high-speed packetsLines and switches remain free for other packets during silencesWhich throughout to consider?

In any case it is inefficienttimethroughput

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonIt would thus be very difficult and inefficient to dedicate end-to-end circuits for two computers to communicate.Allocating a large bandwidth circuit would be inefficient due to the waste during silences.But allocating a narrow bandwidth circuit would be hopelessly slow for the computers (and their waiting users).Thus one does not reserve circuits but one transmits messages in bursts, called packets, over wide-band cables and radio waves.And those physical cables and radio waves, as well as the switches (routers) remain available to transmit other packets during silences.This is called packet-switching.7Payer 3: Packet switchingThe case of computer networks

BADBBCABCBDBCCModelled after the postal instead of the telephone system=> Packets can get lostor arrive out of order

Protocols (e.g. retransmission) are there to accommodate such hiccoughsand it even works for telephony (Skype)

ICC Module 3 Lesson 4 Networking# / 8 2015 Ph. JansonPacket switching works like mail or parcel sorting in postal centers.Instead of a path being reserved between a sender and a receiver, a sender can more or less send whatever it likes whenever it likes to the nearest switching / sorting center.The center will then do its best efforts to convey the packet (mail or parcel) as soon as possible to the next nearest switching / sorting center and so on until the packet (mail or parcel) reaches its final destination.

For that to work, every packet (mail or parcel) needs to carry a destination address (which we already know they do from our protocol discussion). As suggested by this drawing, a packet (black) may be delivered to C out of order after another packet (yellow) that was sent before the black one.It is even possible that a packet (mail or parcel) be completely lost as happens occasionally in postal systems.

In view of the capacity and experience with modern computer packet switching networks, it turns out that their cost and performance is so much superior to those of circuit-switched networksthat almost all telephone traffic today is switched over packet networks anyway.Old telephone trunk lines and local phone lines remain circuit-switched and thus blocking.But most such lines are now at least time-multiplexed if not packet-switched, so that multiple conversations are carried simultaneously on the same wires.

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