COMMUNICATION TECHNOLOGIES

Archive for the ‘Data Communication’ Category

In the transmitter section of a modem digital signals are encoded onto an analog carrier. The digital signals modulate the carrier, producing digital modulated analog signals that are capable of being transported through the analog communication media. The most common modems available are ones intended to be used to interface DTE through a serial interface standard voice-band telephone lines and provide reliable data transmissions rates from 36 bps to 56 bps. These types’ modems sometimes called telephone loop modems or POT modems, as they are connected to the telephone company through the same local loops that are used for voice telephone circuits.
Modem is in essence a transparent repeater that converts electrical signals received in digital from to electrical signals in analog from and vice versa. A modem is transparent, as it does not interpret or change the information contained in the data. It is a repeater, as it not destination for data it simply repeats or retransmits data

To achieve high signal-to-noise ratio, delta modulation must use oversampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist rate. Derived forms of delta modulation are continuously variable slope delta modulation, delta-sigma modulation, and differential modulation The Differential Pulse Code Modulation is the super set of DM. Rather than quantizing the absolute value of the input analog waveform, delta modulation quantizes the difference between the current and the previous step, The modulator is made by a quantize which converts the difference between the input signal and the average of the previous steps. In its simplest form, the quantize can be realized with a comparator referenced to 0 (two levels quantize), whose output is 1 or 0 if the input signal is positive or negative. The demodulator is simply an integrator whose output rises or falls with each 1 or 0 received. The integrator itself constitutes a low-pass filter.

A data communications network is any system of computers, computer terminal, or computer peripheral equipment used to transmit and / or receive information between two or more locations. Network architectures outline the products and services necessary for the individual components within a data communications network to operate together. Networks and network protocols fall into three general classifications: current, legacy, and legendary. Current networks include the most modern and sophisticated networks and protocols available. If a network or protocol becomes a legacy, no one really wants to use it, but for some reason it just will not go away. When an antiquated network or protocol finally disappears, it becomes legendary.
In general terms, computer networks can be classified in two different ways: broadcast and point to point. With broadcast networks, all stations and devices on the network share a single communications channel. Data are propagated through the network in relatively short messages sometimes called frames, blocks or packets.

The earliest forms of data communications were binary in nature. Telegraph is the very type of electronic communication in use. In this system a telegraph hand key acting as a switch, connects to a remote receiver or sounder. The sounder is a magnetic coil. Telegraphy is an important system of transmitting / receiving messages from one place to another. Different characters of a message are represented by different combination of signal elements termed as telegraph code. The telegraph transmitter converts the message into coded form of electrical current, which is then sent through the line. At the receiving end, this message is decoded. Ion telegraphy, the transmitting teletype writer produces a set of coded pulses when a given key is tapped.
In the receiving mode, the same machine prints out the appropriate letter, when a code received. Telegraphic speed is expressed in terms of baud, which may be defined as number of word transmitted / received per second.

The earliest forms of data communications were binary in nature. Telegraph is the very type of electronic communication in use. In this system a telegraph hand key acting as a switch, connects to a remote receiver or sounder. The sounder is a magnetic coil. Telegraphy is an important system of transmitting / receiving messages from one place to another. Different characters of a message are represented by different combination of signal elements termed as telegraph code. The telegraph transmitter converts the message into coded form of electrical current, which is then sent through the line. At the receiving end, this message is decoded. Ion telegraphy, the transmitting teletype writer produces a set of coded pulses when a given key is tapped.

In the receiving mode, the same machine prints out the appropriate letter, when a code received. Telegraphic speed is expressed in terms of baud, which may be defined as number of words transmitted / received per second. By increasing the speed of operation, the number of communication channels may be increased. The telegraph signals may be transmitted in different ways, duplex system: in duplex system, two signals can be transmitted one in each direction over a single transmission line. Diplex system: in diplex system, two signals can be transmitted through a same transmission line simultaneously and in same direction. Quadruplex system: in Quadruplex system four signals can be transmitted over the line simultaneously two in each direction by the two stations.

To achieve high signal-to-noise ratio, delta modulation must use oversampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist rate. Derived forms of delta modulation are continuously variable slope delta modulation, delta-sigma modulation, and differential modulation The Differential Pulse Code Modulation is the super set of DM. Rather than quantizing the absolute value of the input analog waveform, delta modulation quantizes the difference between the current and the previous step, The modulator is made by a quantize which converts the difference between the input signal and the average of the previous steps. In its simplest form, the quantize can be realized with a comparator referenced to 0 (two levels quantize), whose output is 1 or 0 if the input signal is positive or negative. The demodulator is simply an integrator whose output rises or falls with each 1 or 0 received. The integrator itself constitutes a low-pass filter.

The pulse code modulation can transmit all the bits which are used to code sample. Hence signaling rate and transmission channel bandwidth are large in PCM. To overcome this problem delta modulation is used. Delta modulation transmits only one bit per sample. That is the present sample value is compared with the previous sample value and the indication, whether the amplitude is increased or decreased is sent. Input signal is approximated to step signal by the delta modulator. The step size is fixed. The difference between the input signal and the staircase approximated signal confined to two levels. If the difference is positive, then the approximated signal is increased by one step. If the difference is negative is reduced by one.

When the step is reduced, 0 is transmitted and if the step is increased, 1 is transmitted. Thus for each sample, only one binary bit is transmitted. Now let us discuss the advantages and disadvantage of delta modulation. The advantages of delta modulation are as follows. The delta modulation transmits only one bit for one sample. Thus the signaling rate and transmission channel bandwidth is quite small for delta modulation. The transmitter and receiver implementation is very much simple for delta modulation. There is no analog to digital converter involved in delta modulation. The disadvantages of delta modulation are slope overload distortion and granular noise. These are the two drawbacks of delta modulation.

Baseband Transmission Format:

Baseband transmissions formats are defined as transmission formats that use digital signaling, in addition, base band formats use the transmission medium as a single-channel device. Only one station can transmit at a time, and all stations must transmit and receive the same types of signals. Baseband transmission formats time-division multiplex signals onto transmissions medium. All the station can be use the media but only one at a time. The entire frequency spectrum is used by whichever is presently transmitting. With base band format, transmissions are bidirectional. A signal inserted at any point on the transmissions medium propagates in both directions to the ends, where it as absorbed. Digital signaling requires a bus topology because digital signal cannot be easily propagated through the splitters and joiners necessary in a tree bus topology.

Broadband Transmissions Format:

Broadband transmission formats use the connecting media as a multichannel device. Each channel occupies a different frequency hand within the total allocated bandwidth. Consequently, each channel can contain different modulation and encoding schemes and operate at different transmissions rates. A broad band network permits voice, digital data, and video to be transmitted simultaneously over the same transmission medium. However, broad band systems are unidirectional and require RF modems, amplifiers, and more complicated transceivers than base band systems. Baseband systems are more prevalent. The layout for a baseband system is much less complex than broadband systems and, therefore easier and less expensive to implement. The primary disadvantages of baseband are its limited capacity and length.

Token Passing:

Token passing is a network access method used primarily with LAN configured in a ring topology using either baseboard or broadband transmission formats. When using token passing access. Nodes do not contend for the right to transmit data. With token passing, a specific packet of data called a token is calculated around the ring from station to station, always in the same direction. The token is generated by a designated station known as the active monitor .before a station is allowed to transmit; it must first posses the token. Each station in turn acquires the token and examines the data frame to determine if it is carrying a packet addressed to it. If the frame contains a packet with the receiving station’s address, it copies the packet into memory, appends any messages it has to send to the token, and then relinquishes the token by retransmitting all data packets and the token to the next node on the network. With token passing, each station has equal access to the transmission medium. As with CDMA/CD each transmitted packet contains source and designation address fields.

International Standards Organization ISO:

Created in 1946, the International standards organization ISO is the International organization for standardization on a wide range of subjects. The ISO is a voluntary, non-treaty organization whose membership is comprised mainly of members from the standards committees of various governments throughout the world. The ISO creates the sets of rules and standards for graphics and documents exchange and provides models for equipment and system compatibility, quality enhancement, improved productivity, and reduced costs. The ISO is responsible for endorsing and coordinating the work of the other standards organizations. The member body of the ISO from the United States is the American national standards institute.

International Telecommunications Union:

The International Telecommunications Union- telecommunications sector, formerly the committee consultative international de telegraphy ET telephony, is one of four permanent parts of the international telecommunications union based in Geneva, Switzerland, membership in the ITU-T consists of government authorities and representatives from many countries. The ITU-T is now the standards organizations for the United Nations and developes the recommended sets of rules and standard for telephone and data communications.

IEEE:

The Institute of Electrical And Electronics Engineers (IEEE) is an international professional organization founded in the United States and is comprised of electronics, computer, and communications engineers. The IEEE is currently the world’s largest professional society with over 200,000 members. The IEEE works closely with ANSI to develop communications and information processing standards with the underlying goal of advancing theory, creativity, and product quality in any field associated with electrical engineering.

American National Standards Institute:

The International Telecommunications Union is the official standards agencies for the United States and is the U.S. voting representative for the ISO. ANSI membership is comprised of people from professional societies, industry associations, governmental and regulatory bodies, and consumer groups.

Telecommunications Industry Association:

The Telecommunications Industry Association TIA is the leading trade association in the communications and information technology. The TIA facilities business development opportunities and a competitive market place through market development, trade promotion, trade shows, domestic and international advocacy, and standards development. The TIA represents manufactures of communications and information technology products and service providers for the global market place through its core competencies.

Data Communication:

A data communications network is any system of computers, computer terminal, or computer peripheral equipment used to transmit and / or receive information between two or more locations. Network architectures outline the products and services necessary for the individual components within a data communications network to operate together. Networks and network protocols fall into three general classifications: current, legacy, and legendary. Current networks include the most modern and sophisticated networks and protocols available. If a network or protocol becomes a legacy, no one really wants to use it, but for some reason it just will not go away. When an antiquated network or protocol finally disappears, it becomes legendary.

In general terms, computer networks can be classified in two different ways: broadcast and point to point. With broadcast networks, all stations and devices on the network share a single communications channel. Data are propagated through the network in relatively short messages sometimes called frames, blocks or packets. Many or all subscribers of the network receive transmitted messages; each message contains an address that identifies specifically which subscriber is intended to receive the message. When messages are intended for all subscribers on the network, it is called broadcasting, and when messages are intended for a specific group of subscribers, it is called multicasting.

Protocols:

Computer networks communicate using protocols, which define the procedures that systems involved in the communications process will use. Numerous protocols are used today to provide networking capabilities, such as how much data can be sent, how it will be addressed, and what procedure will be used to ensure that there are no undetected errors. Protocols are arrangements between people or process. In essence, a protocol is set of customs, rules, or regulations dealing with formality or precedence, such as diplomatic or military protocol. Each functional layer is network is responsible for providing a set of rules, called Protocols that perform a specific function within the network. Datacommunications protocols are sets of rules governing the orderly exchange of data within the network or a portion of the network, whereas network architecture is a set of layers and protocols that govern the operation of the network.