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Terminologies in Telecommunication

Oum Saokosal

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Terminologies in Telecom

What is digital?Source coding for communication?Channel coding?ModulationMultiplexing

CDM -> CDMA TDM -> GSM FDM

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What is digital? (1)

Analog is one in which a base carrier’s alternating current frequency is modified in some way, such as by amplifying the strength of the signal or varying the frequency, in order to add information to the signal.

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What is digital? (2)

Digital: Electronic technology that processes data in: positive (1) and non-positive (0). Thus, data transmitted or stored with digital technology is expressed as a string of 0's and 1's. Each of these state digits is referred to as a bit. 

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Source Coding for communication

Source coding, also known as quantization or signal formatting, is a way of processing data in order to reduce redundancy or prepare it for later processing. Analog-to-digital conversion and data compression are two categories of source coding.

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Channel Coding? (1)

Channel coding refers to the forward error correction (FEC)1 code and ”bit interleaving”2 in communication and storage where the communication media or storage media is viewed as a channel.

The channel code is used to protect data sent over it for storage or retrieval even in the presence of noise (errors).

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Channel Coding? (2)

1. Forward error correction (FEC): is a system of error control for data transmission, whereby the sender adds redundant data to its messages. This allows the receiver to detect and correct errors without asking the sender for additional data.

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Channel Coding? (3)

2. Bit interleaving is a technique to keep track of the number and sequence of bits which are broken by time division multiplexing and sent across one communications channel simultaneously so that they can be quickly and efficiently reassembled into their original form upon receipt.

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Modulation

Modulation is the process where a Radio Frequency or Light Wave's amplitude, frequency, or phase is changed in order to transmit intelligence. The characteristics of the carrier wave are instantaneously varied by another "modulating" waveform. Common Methods: Amplitude Modulation Frequency Modulation Phase Modulation Pulse Modulation

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Multiplexing (1)

Multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource.

Demultiplexing is a reverse process which can extract the original channels on the receiver side.

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Multiplexing (2)

Frequency Division Multiplexing (FDM) : each signal is assigned a different frequency.

Time Division Multiplexing (TDM) : each signal is assigned a fixed time slot in a fixed rotation.

Statistical Time Division Multiplexing (STDM): time slots are assigned to signals dynamically to make better use of bandwidth.

Wavelength Division Multiplexing (WDM) : each signal is assigned a particular wavelength; used on optical fiber.

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CDM

Code Division Multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses.

Code-division multiplexing (CDM) is a technique that enables digital bit streams be transferred over an analog channel.

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CDMA (1)

Basic Idea: An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division).

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CDMA (2)

Code Division Multiple Access (CDMA) is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space.

CDMA assigns unique codes to each communication to differentiate it from others in the same spectrum.

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TDM

Time-Division Multiplexing (TDM) is a type of digital or analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel.

The GSM telephone system is an application examples of TDM.

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GSM (1)

GSM uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency.

GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band.

GSM (2)

GSM is the most popular standard for mobile phones in the world.

GSM also pioneered a low-cost (to the network carrier) alternative to voice calls, the Short message service which is now supported on other mobile standards as well.

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FDM (1)

Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel.

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FDM (2)

When FDM is used in a communications network, each input signal is sent and received at maximum speed at all times.

However, if many signals must be sent along a single long-distance line, the necessary bandwidth is large, and careful engineering is required to ensure that the system will perform properly.

In some systems, time-division multiplexing is used instead.

Thanks

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