2Topics discussed in this section: 5-2 ANALOG AND DIGITALAnalog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.Topics discussed in this section:Amplitude Modulation Frequency Modulation Phase Modulation
14Packet Switching Vs Circuit Switching Packet-switched and circuit-switched networks use two different technologies for sending messages and data from one point to another.Each have their advantages and disadvantages depending on what you are trying to do.
15Packet SwitchingIn packet-based networks, the message gets broken into small data packets.These packets seek out the most efficient route to travel as circuits become available.Each packet may go a different route from the others.
16Packet SwitchingEach packet is sent with a ‘header address’ for final destinationThe header address describes the sequence so that the packets are put back into the correct order.One packet contains details of how many packets should be arriving.If a packet fails to arrive, the recipient computer sends a message back to the computer asking for the missing packet to be resent.
17Packet SwitchingDifference between circuit switching and packet switching:Packet SwitchingMessage is broken up into segments (packets).Each packet carries the identification of the intended recipient, data used to assist in data correction and the position of the packet in the sequence.Each packet is treated individually by the switching centre and may be sent to the destination by a totally different route to all the others.
18Packet Switching Packet Switching Advantages: Security Bandwidth used to full potentialDevices of different speeds can communicateNot affected by line failure (rediverts signal)Availability – do not have to wait for a direct connection to become availableDuring a crisis or disaster, when the public telephone network might stop working, s and texts can still be sent via packet switching
19Packet Switching Disadvantages Under heavy use there can be a delay Data packets can get lost or become corruptedProtocols are needed for a reliable transferNot so good for some types data streams e.g real-time video streams can lose frames due to the way packets arrive out of sequence.
20Circuit SwitchingCircuit switching was designed in 1878 in order to send telephone calls down a dedicated channel.This channel remained open and in use throughout the whole call and could not be used by any other data or phone calls.
21Circuit Switching There are three phases in circuit switching: EstablishTransferDisconnectThe telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.
22Circuit SwitchingIn modern circuit-switched networks, electronic signals pass through several switches before a connection is established.During a call, no other network traffic can use those switches.The resources remain dedicated to the circuit during the entire data transfer and the entire message follows the same path.Circuit switching can be analogue or digital
23Circuit SwitchingWith the expanded use of the Internet for voice and video, analysts predict a gradual shift away from circuit-switched networks.A circuit-switched network is excellent for data that needs a constant link from end-to-end. For example real-time video.
24Circuit Switching Advantages: Circuit is dedicated to the call – no interference, no sharingGuaranteed the full bandwidth for the duration of the callGuaranteed Quality of Service
25Circuit Switching Disadvantages: Inefficient – the equipment may be unused for a lot of the call, if no data is being sent, the dedicated line still remains openTakes a relatively long time to set up the circuitDuring a crisis or disaster, the network may become unstable or unavailable.It was primarily developed for voice traffic rather than data traffic.
269-1 TELEPHONE NETWORKTelephone networks use circuit switching. The telephone network had its beginnings in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice.
31DIAL-UP MODEMSTraditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility.
35Figure 9.8 The V.32 and V.32bis constellation and bandwidth
36Figure 9.9 Uploading and downloading in 56K modems
37Topics discussed in this section: 9-3 DIGITAL SUBSCRIBER LINEAfter traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops.Topics discussed in this section:ADSLADSL LiteHDSLSDSL VDSL
38NoteADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses.
39Note ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line.