Text 2 Basic principles of Wireless Communication

Let us now discuss the basic principles of wireless radio communications. We shall mainly concentrate on the principle of amplitude modulation and demodulation.

The simplest scheme of wireless communication would be to convert the speech or music to be transmitted to electric signals using a microphone, boost up the power of the signal using amplifiers and radiate the signal in space with the air of an antenna. This would constitute the transmitter. At the receiver end, one could have a pick-up antenna feeding the speech or music signal to an amplifier and a loud speaker (below figure).

The above scheme suffers from the following drawbacks:

- EM waves in the frequency range of 20 Hz - 20 kHz (audio-frequency range) cannot be efficiently radiated and do not propagate well in space.

- Simultaneous transmission of different signals by different transmitters would lead to confusion at the receiver.

In order to solve these problems; we need to devise methods to convert or translate the audio signals to the radio-frequency range before transmission and recover the audio-frequency signals back at the receiver. Different transmitting stations can then be allotted slots in the radio-frequency range and a single receiver can then tune into these transmitters without confusion. The frequency range 500 kHz to 20 MHz is reserved for amplitude-modulated broadcast, which is the range covered by most three band transistor radios. The process of frequency translation at the transmitter is called modulation. The process of recovering the audio-signal at the receiver is called demodulation. A simplified block diagram of such a system is shown in the below figure.

Why microwaves are preferred over other waves to beam signals in a particular direction?

Microwaves are radio waves with frequencies higher than television signals. The wavelengths of microwaves are of the order of a few millimeters. We know that sound waves spread and bend around the corner of an obstacle. This is because the wavelength of sound wave is generally comparable to the size of the obstacle. Unlike a sound wave, a light wave keeps itself along a straight path. Moreover, the light waves bend by only a small amount at the corners of the obstacles. This is because the wavelength of light waves is smaller as compared to the wavelength of sound waves. Thus, lesser the wavelength of a wave, smaller is its bending at the corners of ordinary obstacles and greater the ability of the wave to follow a straight path. The wavelength of microwaves is very small as compared to the wavelength of radio waves. Therefore, microwaves are better suited to beam signals in a particular direction.

Unit 2 Telephony

2.1 Read and translate the text. Use a dictionary to help you.

Text 1 Voice over IP (VoIP)

Voice over IP is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. Other terms commonly associated with VoIP are IP telephony, Internet telephony, broadband telephony, and broadband phone service.

The term Internet telephony specifically refers to the provisioning of communications services (voice, fax, SMS, voice messaging) over the public Internet, rather than via the public switched telephone network (PSTN). The steps and principles involved in originating VoIP telephone calls are similar to traditional digital telephony and involve signaling, channel setup, digitization of the analog voice signals, and encoding. Instead of being transmitted over a circuit-switched network, however, the digital information is packetized, and transmission occurs as IP packets over a packet - switched network. Such transmission entails careful considerations about resource management different from time – division multiplexing (TDM) networks.

Early providers of voice-over-IP services offered business models and technical solutions that mirrored the architecture of the legacy telephone network. Second-generation providers, such as Skype, have built closed networks for private user bases, offering the benefit of free calls and convenience while potentially charging for access to other communication networks, such as the PSTN. This has limited the freedom of users to mix-and-match third-party hardware and software. Third-generation providers, such as Google Talk, have adopted the concept of federated VoIP—which is a departure from the architecture of the legacy networks. These solutions typically allow dynamic interconnection between users on any two domains on the Internet when a user wishes to place a call.

VoIP systems employ session control and signaling protocols to control the signaling, set-up, and teardown of calls. They transport audio streams over IP networks using special media delivery protocols that encode voice, audio, video with audio codecs, and video codecs as Digital audio by streaming media. Various codecs exist that optimize the media stream based on application requirements and network bandwidth; some implementations rely on narrowband and compressed speech, while others support high fidelity stereo codecs. Some popular codecs include μ-law and a-law versions of G.711, G.722, a popular open source voice codec known as iLBC, a codec that only uses 8 kbit/s each way called G.729, and many others.

VoIP is available on many smartphones, personal computers, and on Internet access devices. Calls and SMS text messages may be sent over 3G or Wi-Fi.

Vocabulary

2.2 Complete the vocabulary (term) log, i.e. find out definition, part of speech, translation, synonyms and antonyms if possible, decode abbreviations.

Grammar

2.3 Put the verb into the correct form, past perfect or past simple:

1. Therefore, about 2.5 billion years___________ (to pass) on the earth when life (to originate). 2. The Wright Brothers _______(to launch) their plane at Kitty Hawk before Bell___________ (to develop) his own flying machine. 3. Bell ________ (to patent) his telephone first. 4. Bell__________ (to regard) the photo phone as the greatest invention he____________ (to make). 5. By the end of the XX century a satellite of the planet Pluto____________ (to discover). 6. It_____ (to happen) before they_____________ (to connect) the telephone. 7. When we___________ (to come in) he_____________ (to dial already) the number. 8. The samples ________ to vary) in quality but were generally acceptable. 9. We__________ (to wash) our hands before we___________ (to handle) food. 10. He__________ (to show) the ticket which he__________ (to buy) earlier.

Date: 2016-03-03; view: 1360