ANALOG AND DIGITAL Analog-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. Amplitude Modulation Frequency Modulation Phase Modulation Topics discussed in this section:
5.3 Types of analog-to-analog modulation
5.4 Amplitude modulation
5 Amplitude Modulation
5.6 The total bandwidth required for AM can be determined from the bandwidth of the audio signal: B AM = 2B. Note
5.7 AM band allocation
5.8 The total bandwidth required for FM can be determined from the bandwidth of the audio signal: B FM = 2(1 + β)B. Note
5.9 Frequency modulation
10 Frequency Modulation
5.11 FM band allocation
5.12 Phase modulation
13 Phase Modulation
Packet 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.
Packet Switching In 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.
Packet Switching Each packet is sent with a header address for final destination The 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.
Packet Switching Difference between circuit switching and packet switching: – Packet Switching Message 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.
Packet Switching – Advantages: Security Bandwidth used to full potential Devices of different speeds can communicate Not affected by line failure (rediverts signal) Availability – do not have to wait for a direct connection to become available During a crisis or disaster, when the public telephone network might stop working, s and texts can still be sent via packet switching
Packet Switching Disadvantages Under heavy use there can be a delay Data packets can get lost or become corrupted Protocols are needed for a reliable transfer Not 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.
Circuit Switching Circuit switching was de signed 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.
Circuit Switching There are three phases in circuit switching: – Establish – Transfer – Disconnect The telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.
Circuit Switching In 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
Circuit Switching With 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.
Circuit Switching – Advantages: Circuit is dedicated to the call – no interference, no sharing Guaranteed the full bandwidth for the duration of the call Guaranteed Quality of Service
Circuit 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 open Takes a relatively long time to set up the circuit During a crisis or disaster, the network may become unstable or unavailable. It was primarily developed for voice traffic rather than data traffic.
TELEPHONE NETWORK Telephone 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.
9.27 A telephone system
9.28 The tasks of data transfer and signaling are separated in modern telephone networks: data transfer is done by one network, signaling by another. Note
9.29 Data transfer and signaling networks
9.30 Layers in SS7
9.31 DIAL-UP MODEMS Traditional 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.
9.32 Telephone line bandwidth
9.33 Modem stands for modulator/demodulator. Note
9.35 Figure 9.8 The V.32 and V.32bis constellation and bandwidth
9.36 Figure 9.9 Uploading and downloading in 56K modems
DIGITAL SUBSCRIBER LINE After 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. ADSL ADSL Lite HDSL SDSL VDSL Topics discussed in this section:
9.38 ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. Note
9.39 ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line. Note