IT-101 Section 001 Introduction to Information Technology Lecture #10

IT-101 Section 001 Introduction to Information Technology Lecture #10

Overview Chapter 13 The Telephone System: Wired and Wireless Analog Telephone system Digital telephone system Cellular telephone system

Introduction Now that we have learned what information is, how to represent it, and how to convert it from analog to digital form, we can now learn the techniques and systems used to transmit this information The oldest system used to transmit information globally is considered to be the telephone system We will specifically learn about the analog and digital telephone system, as well as the cellular telephone system

The Analog Telephone System The analog system was the first telephone system established worldwide. Currently, telephone systems in a lot of countries are still completely analog. In time however, these systems will become redundant as the world switches to digital telephony The components of a telephone system include: Microphone Receiver Transmission System Switching and signaling system

Telephone system components Signaling and switching system Transmission system receiver microphone

Microphone Converts the vibrations in the air into an electrical signal Receiver Converts the received electrical signal into sound waves (the reverse action of a microphone) e.g.: loudspeaker Transmission system Conveys the information representing the audio signal from the microphone to the receiver Signaling and switching system Determines and makes appropriate connections among the pieces of the transmission system to create a path from the transmitter to the receiver

The Digital Telephone System While the description of the analog telephone system provides an accurate overview of the principles of current telephone systems, it is a fact that most telephone calls today are really digital telephone calls In a digital telephone system, the two ends of the call are analog, and the middle section is digital. Conversions from analog to digital (A/D), and back to analog (D/A), are made in such a way that it is essentially impossible to determine that they were made at all Although the analog telephone system is gradually being converted to digital, the input and output of the system still remains analog because the eventual use is for humans that are able to process analog information

At present, most telephone calls are analog from the telephone at home to the first switching office, so the A/D and D/A conversion is made at this office In the future, as telephone systems become all digital, this conversion from A/D and from D/A will be made within the telephone set at home The A/D conversion process was explained in the previous lectures- The voice signal- an analog waveform was sampled at a sampling frequency, and quantized to a number of levels. These values were then assigned binary codes to complete the conversion process from analog to digital The D/A process was also explained briefly. The bits were decoded into their quantized values, and a waveform similar to the original analog waveform was obtained

For voice, remember that the standard sampling frequency is 8000Hz The standard number of quantization levels for audio signals is 256, requiring 8 bits So, the bit rate for a digital telephone call is: 8,000x8 64,000 bits per second (64 Kbps) 1/s Hz bits This is the bit rate that would reach the central office if the A/D conversion was being done inside the telephone at home Since many calls arrive at the central office, they can all be combined, and switched to another center to be routed to the destination Combining many channels and sending them simultaneously through a single transmission line is called multiplexing. We will learn about this in a later chapter

One advantage of digital transmission, is that after digitization, all types of information are in the form of bits, so a single system, such as a telephone system can be used to carry telephone calls, internet data or any other data at a suitable bit rate

The Cellular Telephone System The cellular telephone system is different from the previous systems that we learned, because the major transmission medium is air instead of wires (between the mobile unit and the base station) as in the analog and digital telephone systems In a cellular system, the signal from a mobile unit (cell phone) to a base station is transmitted by radio waves through the air, instead of through metallic wires However, the signal from the base station is sent to a mobile switching center and possibly to a telephone central office through electrical wires where it is switched to the appropriate destination The antenna at the base station converts the radio waves to electrical signals and circuits in the base station send the signal to the appropriate mobile switching center

The Cellular Concept Base station

An example cellular telephone syste

How cellular telephone systems work The area (a city, or a part of town) is divided into a number of cells (typically 2 to 10 miles in size, but can be smaller for more crowded areas) and a base station is positioned within each cell If a user (mobile phone) is within a particular cell, the call is handled by the corresponding base station within that cell The base station transmits the signal to the mobile switching center (MTSO) which switches the signal to another base station, or to a Public Switched Telephone Network (PSTN), depending on the destination of the call: whether another mobile unit or a regular telephone As a user moves from one cell to another, the call is “handed over” to the base station of the other cell-This is called handoff The handover is (hopefully) transparent to the user

MTSO Mobile Telephone Switching Office PSTN Public Switched Telephone Network

The mobile unit and the base station in a cell communicate at a certain frequency The signal from the mobile unit arrives at the antenna of the base station and is converted into an electrical signal Base station antenna (3 sector) 1/3rd of cell is covered by each sector of antenna

Every cell uses a different set of frequencies So how does the phone “know” what frequency to be on? A Cell-Site Controller handles this process When a cell phone is turned on, it registers with the network and guards a control frequency When a call is placed, the phone requests a frequency (really two) be assigned When a call is received, the call is set up over the control channel (find the right phone, tell it what frequencies to use, connect the call)

Comments for next class Finish chapter 13 Generations of cellular systems Satellite telephones Chapter 16 Radio-Frequency and Satellite Systems -Satellite systems -GPS