MULTIMEDIA UNIVERSITY OF KENYA
ETI 2506 - TELECOMMUNICATION SYSTEMS
 
TELECOMMUNICATION TERMINAL EQUIPMENT

CONTENTS
1.0 Introduction to Terminal Equipments
2.0 Fixed Telephone Terminals
    2.1 Dial Pulse Telephone
    2.2 MF/Touch Tone Telephone
    2.3 Introductio to Mobile Wireless Terminals
    2.4 Cordless Telephone
    2.5 Paging Systems
3.0 Cellular Phone
      3.1 Inside the Cellular Phone
      3.2 Cellular Frequency Bands
      3.3 Absolute Radio Frequency Channel Numbers (ARFCNs)
      3.4 Speech Coding and Compression in GSM
4.0 Facsmile Machine

        4.1 Introduction
        4.2 Brief History of the Facsimile Machine
        4.3 Facsimile Operation Principle
        4.4 The Facsimile Transmitter
        4.5 The facsimile Receiver


1.0 INTRODUCTION TO TERMINAL EQUIPMENTS

1.1 Terminal Equipment in Analog Communication Channels
Terminal equipment are used to transduce information to a form suitable for transmission and also to convert the transmitted information to its original form. The information to be transmitted includes words, codes, symbols, sound and pictures. During transmission, the unwanted signals (noise) are often inserted into the original message. It is therefore the  function of terminal receiver equipment to recover the information from the received signal and store or present it to the human receiver. The Figure 1 shows a basic functions of the terminal equipment.


Figure 1.1. Block diagram of analog communication system showing the functions of the terminal equipment.

1.2 Terminal Communication Equipment in Digital Communication
In a digital communication system, the signal from the source (or transducer)  must be be transformed to a suitable  form by using a source and channel encorder before being used to modulate a carrier.  At the receiver, the signal is demodulated and decoded to produce the original signal as shown in Figure 2.


Figure 1.2. Block diagram of a digital communication system showing the functions of the terminal equipment.


2.0 Fixed Telephone Terminals
Fixed telephone terminals are designed to work in the wired telephone network sometimes referred to as the Plain Old Telephone Network (POTS). The phones can be categorized according to the manner in which the called  number is transmitted to the exchange (dual pulse or multifrequency) or whether they allow limited subscriber mobility (wired or wireless). This results into three broad categories of fiexed telephones, i.e
(a) Dial Pulses telephone
(b) Touch Tone Telephone
(c) Cordless Telephone


2.1 Dial Pulse Telephone

The pulse telephone is designed to receive and transmit voice signals over a pair of wires. In addition, the phone generates dial pulses; indicates the  status of the phone (on-hook and off-hook) to the exhange; and generate audible signal by ringing a bell.  In order to perform these functions, the phone has four circuit components as follows:

(a) Ringer circuit

(b) Dialling circuit

(d) Speech circuit

(c) Status circuit

The figure below shows the complete dial pulse telephone.

Figure 2.1 Dial Pulse Telephone using a relay pulser

 

2.1.1 The Ringer Circuit

When the telephone is on-hook the hook-switch contacts are open, and only the bell is connected across the line. The exchange therefore only sees the circuit shown below.

Figure 2.2 Ringer circuit

 

Exchange sends 75v (rms) at 17 Hz to the line to ring the bell. The 2μF capacitor blocks DC .

 

2.1.2 The Dialing Circuit

When the headpiece is lifted from the cradle, the telephone exchange sees the circuit shown below. When the user dials, the rotary dial transmits number to the exchange as a sequence of pulses by repeatedly making and breaking the line with contacts marked ‘dial’.

Figure 2.3 The Dialing Circuit

The series RC network connected across the impulse contacts modifies the pulse shape, prevents arcing at the contacts and also mutes the bell.

The dial is designed to deliver 10 pulses per second. This means that it take 1 sec to send '0'. The exchange equipment is designed to accept anything between 7 and 12 pulses/sec) at a make/break ratio of 1:2 as shown in the figure below. These were the earliest form of timing in telecommunication and computer technology.

Figure 2.4 Timing in a dial pulse telephone.

 

The interpulse interval varies from one individual to another as it depended on how long the user takes to insert his/her finger on the slot and the speed at which he/she would moves the dial.

 

2.1.3 The Speech Circuit

The speech circuit is designed to move all the signal power generated in the microphone out to the line and all the signal power delivered by the line from the exchange into the receiver. The other design criteria is to minimize the power being generated by the microphone from reaching the receiver. The circuit diagram of the receiver is as shown below.

Figure 2.5 The speech circuit

 

2.1.4 The Status Circuit

The status circuit consists of two relays that indicate to the telephone exchange the status of the telephone by ensuring that the phone presents high impedance to the exchange when on-hook and low impendance when off-hook. In the electromechanical exchanges, the line relay would operate whenever the subscriber lifted the hand-set as shown below.

Figure 2.6 Operation of the status circuit relays (see figure 2.1) signals the exchange that the subsriber wishes to make a telephone call.

 


2.2 TOUCH TONE/MF TELEPHONE

DTMF (Dual tone multi frequency) as the name suggests uses a combination of two sine wave tones to represent a key. These tones are called row and column frequencies as they correspond to the layout of a telephone keypad.

A generator in the DTMF phone generates a sinusoidal tone which is mixture of the row and column frequencies. The row frequencies are low group frequencies while the column frequencies belong to high group frequencies. This prevents misinterpretation of the harmonics.

The frequencies for DTMF are so chosen that none have a harmonic relationship with the others and that mixing the frequencies would not produce sum or product frequencies that could mimic another valid tone. The high-group frequencies (the column tones) are slightly louder than the low-group to compensate for the high-frequency roll off of voice audio systems.


Figure 2.7 Multifrequency (Touch Tone) Telephone.

 

The frequency components can be seen by connecting a DTMF phone to a telephone line and a spectrum analyzer. By pressing any of the keys the spectrum of the signal can be used to identify the frequencies as shown below.

Figure 2.8 Spectrum of the MF telephone Signal as observed from a spectrum analyser.

 


 

2.3 INTRODUCTION TO Mobile Wireless Terminals

 

2.3.1 Classification of Wireless Communication Terminals

 

Wireless Terminals can be classified into three categories as follows.

 

1. Simplex radio systems utilize simplex channels i.e., the communication is unidirectional. The base station can communicate with a terminal. But, a terminal cannot communicate with the Base Station. A pager is an example of a simplex radio system.

 

2. Half duplex radio systems that use half duplex radio channels allow for non-simultaneous bidirectional communication. A walkie-talkie which uses `push to talk' and `release to listen' is an example of a half duplex system. Half duplex systems can have both the transmitter and receiver operating on the same frequency.

 

 

Figure 2.9 Walkie-Takie

 

3.Full duplex radio systems allow two way simultaneous communications. Both the users can communicate to each other simultaneously. This can be done by providing two simultaneous but separate channels to both the users.  The cellular Mobile telephone is an example of a full-duplex radio systems. Full duplex systems generally require the transmitter and the receiver to operate on different frequencies.

 

Figure 2.10. Full-duplex radio system

 

2.3.2 Personal Communication Services

 

Personal Communication Services (PCS)  refers to a wide variety of wireless access and personal mobility services provided through a small terminal with the goal of facilitating communication at any time, any location and any form. There are basically two types of Personal Communication Services, i.e

1. Low Tier Telecommunication Systems which include Cordless Telephone 1 and 2 (CT1 and CT2), Digital Enhanced Cordless Telephone (DECT), Personal Access Communication Systems (PACS), Personal Handy Phone System (PHS), etc.

 

Figure 2.11. Example of Low-tier Personal Communication Service

 

2.High-tier cellular communication systems which include Advanced Mobile Phone Service (AMPS) and the Global System for Mobile Communication (GSM), etc.

 

Figure 2.12. High-tier Personal Communication Service

 


2.4 CORDLESS TELEPHONE

2.4.1 INTRODUCTION

A cordless  telephone uses  a  radio link  instead of a physical  cord to connect the handset to the base station. Therefore both the  mobile  handset and the  base  station  have  radio  transceivers.  The  first generation  of cordless  telephones  was  analogue  techniques and  is more commonly referred to as CT1 (Cordless Telephone 1st Generation).

 

CT1 was  followed by a digital system known as the 2nd Generation, i.e CT2.  Both CT1 and CT2 rely upon the base station to connect  the mobile handset to the Pupblic Switched Telephone Network (PSTN) as shown below.

Figure 2.11 Operating Principle of Cordless telephones. Each terminal has an uplink and downlink frequency.

 

In recent years, a new standard referred to as DECT was introduced that uses digital cellular technology to provide a mobile telephone service to large Private Automatic Telephone Exchanges (PABXs).

 

2.4.1  Cordless Telephone 1st Generation (CT1)

 

CT1 was a first generation cordless telephone using analogue technology and operating up to  a maximum range of about 200 metres between handset and base station. Introduced in 1983. CT1 operated on two separate frequency bands for uplink and down link as shown below.

Figure 2.12 Transmission parameters of Cordless Telephone Generation 1 (CT1)

 

CHANNEL BASE STATION TO USER TERMINAL USER TERMINAL TO BASE STATION

1

1642.00 kHz

47.45625 MHz

2

1662.00 kHz

47.46875 MHz

3

1682.00 kHz

47.48125 MHz

4

1702.00 kHz

47.49375 MHz

5

1722.00 kHz

47.50625 MHz

6

1742.00 kHz

47.51875 MHz

7

1762.00 kHz

47.53125 MHz

8

1782.00 kHz

47.54375 MHz

 

The CT1 system had many disadvantages which are listed below.

(a)The quality of the received speech was not good.

(b)There was no encryption. As a result, transmissions could be received by an AM or Shortwave sound broadcast radio receiver

(c) Only eight radio frequency channels could be allocated. This limited the use in large offices.

(d) A telephone had no ability to search for a free channel and hence it could easily be blocked off by another cordless telephone that has been set to use the same channel.

(e) The range is limited to about 200m.

 

2.4.2  Cordless Telephone 2nd Generation (CT2)

CT2 was developed in the early 1980 and adopted in 1983. It uses a digital speech path in any one of  forty 100 kHz  wide RF  channels in the frequency band  864-868 MHz. Each handset has up to 11 unique identity codes loaded in at manufacture. Figure 2.13 shows the frequency plan.

 

Figure 2.13. Frequency plan of Cordless Telephone Generation 2 (CT2)

 

Each base station is programmed to recognize up to eight separate handset  identities that it is able to deal  with  simultaneously.  This provides a  PABX  function with little  risk of  privacy invasion. 

The modulation method that is employed is two-level FSK is as illustrated in Figure 2.4:

(a) frequency deviations of 14.4 to 25.2 kHz above the carrier frequency representing binary 1, and

(b) frequency deviation of between 14.4 to 25.2 kHz below the carrier frequency indicating binary 0.

 

Figure 2.14. FSK modulation in Cordless Telephone Generation 2 (CT2) for the first RF channel, i.e fc = 864

 

2.4.3 CT2 Modulation

 

In CT2 system,  speech signals in either direction of transmission are sampled and coded at 32 kbit/s. Both the user terminal and the base station package the digital data into 1ms data bursts. In the air interface, the uplink and downlink signals therefore appear to be interleaved  into composite data streams of 72 kbit/s as illustrated in Figure 2.15.

Figure 2.15 Modulation Scheme used in 2nd Generation Cordless Phones (CT2)

 

2.4.4 Incoming Calls in CT2

 

When an incoming call is detected by the base station, it scans the 40 radio frequency channels to find a free one that has an adequate signal-to-noise ratio. The base station then transmits a call signal over the selected channel.

Periodically the handset moves out of its SLEEP state into its SCAN state, in which it scans the r.f. channels.

When the call signal is detected on one of the r.f. channels, the handset remains on that frequency and achieves bit synchronization with the base station.

The handset then checks that the call is for it (not for some other handset) ; if so , burst synchronization is obtained to establish a link to the base station. The ringer of the handset then rings until the call is answered (picked by the user), when speech can commence.

 

2.4.5 Outgoing Calls in CT2

 

When a handset wishes to make a call, the CALL button is pressed. This action causes the handset to scan the 40  channels to find a free one with adequate signal-to-noise ratio. The handset then signals the base station over the selected channel.

 

The base station is continually scanning all the 40 radio frequency  channels, and so it rapidly detects the call from the handset. Synchronization between handset and base station is established and then the base station seizes a line to the local telephone exchange or PABX. Dialling tone is then returned to the caller.

 


2.5 PAGING SYSTEMS

Paging systems started off as very-low-power systems giving effective coverage over a limited area, e.g several buildings of a major hospital complex. Paging system users carried a receiver half the size of a paperback book which was clipped on their belts. When staff were required in for instance an  operating theatre a coded call went out from the central transmitter. This operated a buzzer in the pager of the called staff.

 

2.4.1 Paging System Standard

Pagers use the ITU-R paging code 1 , also known as POCSAG ( Post Office Code Standardization Advisory Group ), which is the internationally agreed standard for radio paging. POCSAG has a system capability of addressing many pagers and operates in the frequency band 138-174 MHz with a 25 kHz channel spacing . The modulation used is NRZ FSK with a ±4.5 kHz shift on the carrier. The high frequency represents a 0 and the low frequency a 1. The transmission rate can vary from 512, 1200, or 2400 baud.

 

3.0 THE CELLULAR PHONE

 

3.1 Inside the Cellular Phone

On a "complexity per cubic inch" scale, cellular phones are some of the most complex devices people use on a daily basis. A simple cell phones costing Kshs. 2,000 has a CPU that can process millions of calculations per second. Apart from handling the mobile communication protocols, the CPU compress and decompress the voice stream from the standard 64kbps (i.e 4KHz signal sampled at 8KHZ with each sample coded at 8 bits/sample) and compressed to 13 Kbps.

Figure 3.1. Hardware components in a  Mobile Phone

A simple GSM cellular  phone  contains the following parts:

1. Input/Output Devices as follows:

   (a) A microphone

   (b) A speaker

   (c) A keyboard

   (d) A liquid crystal display (LCD)

2. Analog-to-Digital and Digital-to-analog conversion chips

3. The Digital Signal Processor (DSP)

4. ROM and Flash Memory

5. RF Amplifier

6. An antenna

7. A battery

8. SIM Card

 

 

1. I/O Devices convert speech to electrical signals and vice versa. They also allow the user to enter data  and to view it. Cell phones have such tiny speakers and microphones that it is incredible how well most of them reproduce sound. The keyboard and the display have undergone many enhancements to handle advanced cellular phone services. The display, for instance,  has grown considerably in size in order to offer new services such as  built-in phone directories, calculator, games and  Web browsing.

 

2. The Analog-to-digital and digital-to-analog conversion  chip translates the outgoing audio signal from analog to digital and the incoming signal from digital back to analog.

3. The digital signal processor (DSP) is a highly customized microprocessor designed to perform signal-manipulation calculations at high speed. It handles all of the housekeeping chores for the keyboard and display, deals with command and control signaling with the base station; coordinates the rest of the functions on the board; and performs signal compression and decompression.

Figure 3.2. Photograph of an early generation Digital Signal Processor (DSP) in a cellular phone.

 

4. The ROM and Flash memory chips provide storage for the phone's operating system and customizable features, such as the phone directory.

 

5. The radio frequency (RF) and power section handles power management and recharging. It also peforms  carrier generation and modulation of  the   Radio Frequency  channels; and  amplification of signals to and from the antenna.

 

 7. The antenna transmits and receives the RF signal to and from the Base Transceiver Station (BTS).

8. The battery provides eletrical power to all the electronic modules in the phone. It usually is connected to advanced charging and power management circuitry which ensure that the phone operates for a long period after charging.

9. Subsriber Identity Module 

 

 

3.2 Cellular Frequency Bands

 GSM phones send and receive data over radio waves at around 850, 900, 1800 or 1900 megahertz depending on the region as shown in Figure 3.3.

 

 

 Figure 3.3. GSM radio frequency bands

 

In the US, Canada, Central and parts of South America the frequency used is around 850Hz and  1900 MHz. A lot of mobile phones are designed to work in other countries and are either "dual band", meaning they work on 900 MHz and 1800 MHz networks, or "tri-band", meaning they can work on 1900 MHz networks as well.

 

3.3 Absolute Radio Frequency Channel Numbers (ARFCNs)

GSM used frequency division multiplexing and time division  to transmit many voice channels on a bandpass frequency of 200KHz.  Since the operation of the cellular system is digital, each transmission channel is referenced as number and named Absolute Radio Frequency Channel Number (ARFCN). The ARFCN is a number that describes a pair of frequencies each 200KHz, one uplink and one downlink.

The uplink and downlink frequencies have a specific offset that varies for each band. The offset is the frequency separation of the uplink from the downlink.In the 900 MHz band, the offset is 45MHz. Every time the ARFCN increases, the uplink will increase by 200 khz and the downlink also increases by 200 khz.  Figure 4 shows the uplink and downlink frequencies for ARFCN 1.

 



Figure 3.4.
Uplink and Downlink frequency for ARFCN 1.

Since cellular phones are designed to work globally i.e in any GSM band, the ARFCNs were developed to uniquely identify a channel in any band. For instance, GSM 900 has ARFCNs in the range 1-124 while the 1800MHz band has ARFCNs in the range 512-885 as shown in table 1.

Table 1.
The Fequencies, ARFCNs and Offset of the GSM bands


If the GSM band is given, then the uplink and downlink frequencies can be calculated by multiplying the ARFCN with 0.2 MHz and additing to the frequency where the band starts. Figure 4 illustrates how this can be done in the GSM 900MHz band.


Figure 3.5.
Calculating the frequency from ARFCN in the 900MHz band.

3.4 Speech Coding and Compression in GSM

Digital speech compression in GSM system  compresses the  digitized audio signal, transmits that compressed digital signal to the BTS, and decoding the compressed signal received from the BTS to recreate the original (or approximate of the original) signal.

The GSM digital speech compression process works by grouping the digital audio signals into 20 msec speech frames. These speech frames are analyzed and characterized (e.g. volume, pitch) by the speech coder. The speech coder removes redundancy in the digital signal (such as silence periods) and characterizes digital patterns that can be made by the human voice using code book tables. The code book table codes are transmitted instead of the original digitized audio signal. This results in the transmission of a 13 kbps compressed digital audio instead of the 64 kbps digitized audio signal.

Figure 3.6 shows the basic speech data compression process used for the GSM speech coder. This diagram shows that the analog voice signal is sampled 8,000 times each second and digitized into a 64 kbps digital signal. The digitized signal is grouped into 20 msec speech frames. The speech frames are analyzed and compressed into a new 13 kbps digital signal.

Figure 3.6. Speech Coding and compression in GSM networks


4.0 FACSIMILE (FAX) MACHINE

4.1 Introduction

Facsimile (FAX) is a machine that makes copies of a document over a telecommunication channel anywhere in the world at the same speed as the one the originating fax scans as illustrated in Figure 4.1.

Figure 4.1. The Principle of operation of a Fax


4.2 Brief History of the Fax

1843:  The first fax machine was invented by Scottish mechanic and inventor Alexander Bain.
1850:  A London inventor named F. C. Blakewell received a patent what he called a "copying telegraph".
1860:  A fax machine called the Pantelegraph sent the first fax between Paris and Lyon. The Pantelegraph was invented Giovanni Caselli.
1902:  Dr Arthur Korn invented an improved and practical fax, the photoelectric system.
1924: The telephotography machine (a type of fax machine) was used to send political convention photos long distance for newspaper publication. It was developed by the American Telephone & Telegraph Company (AT&T).
1926: Radio Corporation of America (RCA) invented the Radiophoto that faxed by using radio broadcasting technology.
March 4, 1955:  The first radio fax transmission was sent across the Atlantic.
1964: Xerox Corporation introduced (and patented) the first commercialized version of the modern fax machine, under the name (LDX) or Long Distance Xerography.


4.3 Facsimile Operation Principle
At the sending end, there an optical sensor which read the paper line by line.
The white and black spots that the optical sensor reads are encoded they can travel through a phone line. Usually, a modern fax machine also has a paper-feed mechanism so that it is easy to send multi-page faxes.At the receiving end, the information is decoded and sent to the printer which  marks the paper with black (or colour)  dots (or prints). Figure 4.2 shows the block diagram of this process.


Figure 4.2. Block diagram of the operation of a facsimile (fax) machine

4.4 The Fax Transmitter
When paper as graphical information is inserted into fax machine, it is scanned row–by–row.The Charge Coupled Device (CCD) converts this information into proportional analog signals which is fed to A/D converter circuit to produce a digital signal which is compressed before modulating a carrier for transmission.


Figure 4.3. Charge Coupled Device Sensor Mechanism
 
The Charge Coupled Device (CCD) is a special integrated circuit consisting of a flat, two dimensional array of small light detectors referred to as pixels as shown in Figure 8. Each pixel acts like a bucket for electrons. A CCD chip acquires data as light or electrical charge. During an exposure, each pixel fills up with electrons in proportion to the amount of light that enters it. The CCD takes this optical or electronic input and converts it into an electronic signal.The electronic signal is then processed to either produce an image or provide information.

Figure 4.4. Photograph and Organization of a Charge Coupled Device (CCD) Detector

4.5 The Fax Receiver
When the fax signal reaches receiver block through telephone line, it is demodulated using demodulator within the modem. The data is fed to digital data expansion block to recover the original data from the compressed form. The signals are fed to a printer together with the control signals such as Line Feed (LF).

 

                                                                                                                                                        ©  Prof. James Kulubi May-Aug 2015