Network Operations & administration CS 4592

Lecture 10

Instructor: Ibrahim Tariq

5.2

5-2 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 ModulationFrequency ModulationPhase 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: BAM = 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: BFM = 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 SwitchingPacket 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, e-mails 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 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.

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• 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 SwitchingDisadvantages:

• 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.

9.26

9-1 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

Endoffices

Local loop

Trunk

Tandemoffices Regional offices

Trunk

• • •

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

Modemstands for modulator/demodulator.

Note

9.34

Modulation/demodulation

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

9.37

9-3 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.

ADSLADSL LiteHDSLSDSLVDSL

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 ratebased on the condition of

the local loop line.

Note

9.40

Discrete multitone technique

9.41

Figure 9.11 Bandwidth division in ADSL

9.42

ADSL modem

9.43

DSLAM

9.44

Summary of DSL technologies