COMMUNICATION TECHNOLOGIES

Archive for October, 2009

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.

A Cellular telephone system includes all the basic components necessary for cellular communications. A wireless ratio network covering a set of geographical areas inside of which mobile two way radio units, such as cellular or PCS telephones, can communicate. The radio network is defined by a set of radio-frequency transceivers located within each of the cells. The locations of these radio-frequency transceivers are called base stations. A base station serves as central control for all users within that cell.

Mobile units communicate directly with the base stations, and the base stations communicate directly with a mobile telephone switching office. An MTSO controls channel assignment, call processing, call setup, and termination, which includes signaling, switching, supervision, and allocating, radio-frequency channels. The MTSO provides a centralized administration and maintenance point for the entire network and interfaces with the public telephone network over wireline voice trunks and data links. MTSOs are equivalent to class 4 toll offices, except smaller. Local loops do not terminate in MTSOs are connected to the SS7 signaling network, which allows cellular telephones to operate outside their service area.

Base stations can be improving the transmission quality, but they cannot increase the channel capacity with in the fixed band width of the network. Base stations are distributed over the area of system coverage and are managed and controlled by a site computerized cell site controller that handles all cell site control and switching functions. Base stations communicate not only directly with mobile units through the airways using control channels but also directly with the MTSO over dedicated data control links. To complicate the issue, an MTSO is known by several different names, depending on the manufacturer and the system configuration. Mobile telephone switching office MTSO is the name given by bell telephone laboratories, electronic mobile xchange by Motorola, AEX by ericcson, NEAX by NEC, and switching mobile center and master mobile center by novatel. In PCS networks, the mobile switching center is called the MCS.

Each geographical area or cell generally accommodates many different user channels simultaneously. The number of user channels depends on the accessing technique used. With in a cell, each radio-frequency channel can support up to 20 mobile telephone users at one time. Channel maybe statically or dynamically assigned channels are assigned a mobile unit for the duration of call, whereas dynamically assigned channels are assigned a mobile unit when it is being used. With both static and dynamic assignments, mobile units can be assigned any available radio channel.

Electronic Telephones:

Although 500 and 2500 type telephone sets still work with the public telephone network, they are becoming increasingly more difficult to find. Most modern day telephones have replaced the mechanical functions performed in the old telephone sets with electronics. Electronic telephones use integrated circuit technology to perform many of the basic telephone functions as well as a myriad of new and, and in many cases, nonessential functions. The refinement of microprocessors has also led to the development of multiple-line, full-feature telephones that permit automatic control of the telephone set’s features, including telephone number storage, automatic dialing, redialing, CallerID.

However, no matter how many new gadgets are included in the new telephone sets, they still have to interface with the telephone network in much the same manner as telephones did a century ago. For atypical electronic telephone comprised of one multifunctional integrated-circuit chip, a microprocessor chip, a touch-tone keypad, a speaker, a microphone, and a handful of discrete devices. The major components included in the multifunctional integrated circuit chip are DTMF tone generator, MPU microprocessor unit interface circuitry, random access memory, tone ringer circuit, speech network, and a line voltage regulator.

Paging Systems:

Paging systems are simplex wireless communications system designed to alert subscribers of awaiting messages. Paging transmitters relay signals and messages from wire-line and cellular telephones to subscribers carrying potable receivers. The infrastructure used with paging systems is somewhat difference than the one used for cellular telephone system. This is because standard paging systems are one way, with signals transmitted from the paging system to portable pager and never in the reverse direction. To contact a person carrying a pager, simply dial the telephone number assigned that person’s potable pager.

The paging company receives the call and responds with a query requesting the telephone number you wish the paged person to call. After the number is entered, a terminating signal is appended to the number, which is usually the # sign. The caller then hangs up. The paging system converts the telephone number to digital code and transmits it in the form of a digitally encoded signal over a wireless communications system.

Wider Bandwidth and Greater Information Capacity:

Optical fibers have greater information capacity than metallic cables because of the inherently wider bandwidths available with optical frequencies. Optical fibers are available with bandwidths up to a several thousand gigahertz. The primary electrical constants in metallic cables cause them to act like low-pass filters, which limit their transmission frequencies, bandwidth, bit rate, and information-carrying capacity. Modern optical fiber communications systems are capable of transmitting several gigabits per second over hundreds of miles, allowing literally millions of individual voice and data channels to be combined propagated over one optical fiber cable.

Immunity to Crosstalk and Static Interference:

Optical fiber cables are immune to crosstalk because glass and plastic fibers are nonconductors of electrical current. Therefore, fiber cables are not surrounded by a changing magnetic field, which is the primary cause of cross between metallic conductors located physically close to each other. Because optical fiber cables are nonconductors of electrical current, they are immune to static noise due to electromagnetic interference caused by lightning, electric motors, relays, fluorescent lights, and other electrical noise sources. Fiber cables do not radiate electromagnetic energy.

Environmental Immunity and Safety Convenience:

Optical fiber cables are most resistant to environmental extremes than metallic cables. Optical cables also operate over a eider temperature range and are less affected by corrosive liquids and gases. Optical fiber cables are safer and easier to install and maintain than metallic cables. Because glass and plastic fibers are nonconductors, there are no electrical currents or voltages associated with them. Optical fibers can be used around volatile liquids and gasses with out worrying about their causing explosions or fires.

Lower Transmission Loss and Security:

Optical fibers have considerably less signal loss than their metallic counterparts. Optical fibers are currently being manufactured with as little as a few-tenths-of-a-decibel loss per kilometer. Consequently, optical regenerators and amplifiers can be spaced considerably farther apart than with metallic transmission lines. Optical fiber cables are more secure than metallic cables. It is virtually impossible to tap into a fiber cable without the user’s knowledge; optical cables cannot be detected with metal doctors unless they are reinforced with steel for strength.

The feed mechanism in a parabolic antenna actually radiates the electromagnetic energy and, the therefore, is often called the primary antenna. The feed mechanism is of primary importance because its function is to radiate the energy toward the reflector. An ideal feed mechanism should direct all the energy toward the parabolic reflector and have no shadow effect. In practice, this is impossible to accomplish, although if care is taken when designing the feed mechanism, most of the energy can be radiated in the proper direction, and the shadow effect can be minimized.

Center Feed:

The primary antenna is placed at the focus. Energy radiated toward the reflector is reflected outward in a concentrated beam. However, energy not reflected by the paraboloid spreads in all directions and has the tendency of disrupting the overall radiation pattern. The spherical reflector redirects such emissions back toward the parabolic reflector, where they are reflected in the proper direction. Although the additional spherical reflector helps concentrate more energy in the desires direction, it also has a tendency to block some of the initial reflections. Consequently, the good it accomplishes is somewhat offset by its own shadow effect, and its overall performance is only marginally better than without the additional spherical reflector.

Horn Feed:

With a horn-feed mechanism, the primary antenna is a small horn antenna rather than a simple diploes or dipole array. The horn is simply a flared piece of waveguide material that is placed at the focus and radiates a somewhat directional pattern toward the parabolic reflector. When a propagating electromagnetic field reaches the mouth of the horn, it continues to propagate in the same general direction, except that, in accordance with Huygens’s principle, it spreads laterally, and the wavefront eventually becomes spherical.

Cassegrain Feed:

The Cassegrain feed is named after an 18th- century astronomer and evolved directly from astronomical optical telescopes. The primary radiating source is located in or just behind a small opening at the vertex of the paraboloid rather than at the focus. The primary antenna is aimed at a small secondary reflector located between the vertex and the focus. The rays emitted from the primary antenna are reflected from the Cassegrain subreflector and then illuminate the main parabolic reflector just as if they had originated at the focus. The rays are colliminated by the parabolic reflector in the same way as with the center and horn feed mechanisms. The subreflector must have a hyperboloidal curvature to reflect the rays from the primary antenna in such a way as to function as a virtual source at the paraboloidal focus.

The Public Telephone Network:

The public telephone network accommodates two types of subscribers: public and private. Subscribers to the private sector are customers who lease equipment, transmission media and services from telephone companies on a permanent basis. The leased circuits are designed and configured for their use only and are often referred to as private-line circuits or dedicated circuits. For example, large bank do not wish to share their communication network with other users, but it is not cost effective for them to construct their own networks. Therefore, banks lease equipment and facilities from public telephone companies and essentially operate a private telephone or data network within the PTN. The public telephone companies are sometimes called service provider, as they lease equipment and provide services to other private companies, organizations, and government agencies.

Instruments and Local Loops:

An instrument is any device used to originate and terminate calls and to transmit and receive signals into and out of the telephone network, such as a 2500-type set, a cordless telephone, or a data modem. The instrument is often referred to as station equipment and the location of the instrument as the station. A subscriber is the operator or user of the instrument. If you a home telephone, you are a subscriber. The local loop is simply the dedicated cable used to connect an instrument at a subscriber’s station to the closest telephone office. In the United States alone, there are several hundred million miles of cable for local subscriber loops. Every one who subscribes to the PTN is connected to the closest telephone office through a local loop.

Trunk Circuits and Exchanges:

A trunk circuit is similar to a local loop except trunk circuits are used to interconnect two telephone offices. The primary difference between a local loop and a trunk is that a local loop is permanently associated with a particular station, whereas a trunk is a common-usage connection. A trunk circuit can be as a pair of copper wires twisted together or as sophisticated as an optical fiber cable. An exchange is a central location where subscribes are interconnected, either temporarily or on a permanent basis. Telephone companies switching machines are located in exchanges. Switching machines are programmable matrices that provide temporary signal paths between two subscribes.

The North American Telephone Numbering plan was established to provide a telephone numbering system for the United States, Mexico, and Canada that would allow any subscriber in North America to direct dial virtually any other subscriber without the assistance of an operator. The network is often referred to as the direct distance dialing network. Prior to the establishment of the NANP, placing a long distance telephone call began by calling the long distance operator and having her manually connect you to a trunk circuit to the city you wished to call. Any telephone number outside the caller’s immediate areas was considered a long-distance call.

North American is now divided into numbering plan areas with NPA assigned a unique three-digit number called an area code. Each NPA is further subdivided into smaller service areas each with its own three-digit number called an exchange code. Initially, each service area had only one central telephone switching office and one prefix. However, today a switching office can be assigned several exchange codes, depending on user density and the size of the area the office services. Each subscriber to a central office prefix is assigned a four-digit extension number. The three- digit area code represents the first three digits of a 10-digit telephone number, the three-digit prefix represents the next three digits, and the four-digit extension represents the last four digits of the telephone number. Initially, within the North American telephone numbering plan area, if a digit could be any value from 0 through 9, the variable x designated it.

If a digit could be any value from 2 through 9, the variable N designated it. The popularity of cellular telephone had dramatically increased the demand for telephone numbers. North America ran out of NPA area codes, so the requirement that the second digit be a 1 or 0 was dropped. This was made possible because by 1995 there were very few electromagnetic switching machines in use in North America, and with the advent of ss7 signaling networks, telephone numbers no longer had to be transported over voice switching paths. The international telecommunications union has adopted an international numbering plan that adds a prefix in front of the area code, which outside North America is called a city code.

Open-wire Transmission Lines:

Open-wire transmission lines consist simply of two parallel wires closely spaced and separated by air. Non conductive spacers are placed at periodic intervals not only for support but also to keep the distance between the conductors constant. The distance between the two conductors is generally between 2 inches and 6 inches. The dielectric is simply the air between and around the two conductors in which the TEM wave propagates. The only real advantage of this type of transmission line is its simple construction. Because there is no shielding, radiation losses are high, and the cable is susceptible to picking up signals through mutual induction, which produces crosstalk. Crosstalk occurs when a signal on one cable interferes with a signal on adjacent cable. The primary use of open-wire transmission lines is in standard voice-graded telephone applications.

Twisted-pair Transmission Lines:

A twisted-pair transmission line is formed by twisting two insulated conductors around each other. Twisted pairs are often stranded in units, and the units are then cabled into cores containing up to 3000 pairs of wire. The cores are then covered with various types of sheaths forming cables. Neighboring pairs are sometimes twisted with different pitches to reduce the effected of electromagnetic interference and radio frequency interference from external sources, such as fluorescent lights, power cables, motors, relays, and transformers. Twisting the wires also reduces crosstalk between cable pairs.

Unshielded Twisted-pair:

Unshielded twisted-pair (UTP) cable consists of two copper wires where each wire is separately encapsulated in (PVC) poly vinyl chloride insulation. Because a wire can act like an antenna, the wires are twisted two or more times at varying lengths to reduce crosstalk and interference. By carefully controlling the number of twists per foot and the manner in which multiple pairs are twisted around each other, manufactures can improve the bandwidth of the cable pair significantly. Most telephone systems use UTP cable, and the majority of new buildings are prewired with UTP cable. Generally, more cable is installed than is initially needed, providing room for orderly growth. This is one of the primary reasons why UTP cable is so popular. UTP is expensive, flexible, and easy to install. It is the least expensive transmission medium, but it is also the most susceptible to external electromagnetic interference.

Balanced Transmission Lines:

With two-wire balanced lines, both conductors carry current; however, one conductor carries the signal, and the other conductor id the return path. This type of transmission is called differential, or balanced, signal transmission. The signal propagating down the wire is measured as the potential difference between the two wires. Both conductors in balanced line carry signal currents. The two currents are equal in magnitude with respect to electrical ground but travel in opposite directions. Currents that flow in the same direction are balanced wore pair are called metallic circuit currents. Currents that flow in the same direction are called longitudinal currents. Common-mode rejection ratios of 40 db to 70 db are common in balanced transmission lines. Any pair of wires can operate in the balanced mode, provided that neither wire is at ground potential.

Unbalanced Transmission Lines:

With an unbalanced transmission line, one wire is at ground potential, whereas the other wire is at signal potential. This type of transmission line is called single-ended, or unbalanced, signal transmission. With unbalanced signal transmission, the ground wire must go wherever any of the signal wires go. Sometimes this creates a problem because a length of wire has resistance, inductance, and capacitance and, therefore, a small potential difference may exist between any two points on the ground wire. Consequently, the ground wire is not a perfect reference point and is capable of having noise induced into it.

Baluns:

A circuit device used to connect a balanced transmission line to an unbalanced load is called a balun. An unbalanced transmission line, such as a coaxial cable, can be connected to a balanced load, such as an antenna, using a special transformer with an unbalanced primary and a center-tapped secondary winding. The outer conductor of an unbalanced coaxial transmission line is generally connected to ground. An ordinary transformer can be used to isolate the ground from the load. The balun must have an electrostatic shield to earth ground to minimize the effects of stray capacitances. The most common type is narrowband balun, sometimes called a choke, sleeve, or bazooka balun. A quarter- wavelength sleeve is placed around and connected to the outer conductor of a coaxial cable. Thus, one wire of the balanced pair can be connected to the sleeve without short-circuiting the signal. The second conductor is connected to the inner conductor of the coaxial cable.

Conductor Losses:

Because an electrical flow through a metallic transmission line and the line has a finite resistance, there is an inherent and unavoidable power loss. This is sometimes called conductor loss or conductor heating loss and is simply a power loss. Because resistance is distributed throughout a transmission line, conductor loss is directly proportional to the square of the line length. Also, because power dissipation is directly proportional to the square of the current, conductor loss is inversely proportional to characteristic impedance. To reduce conductor loss, simply shorten the transmission line or use a larger-diameter wire. Keep in mind, however, that changing the wire diameter also changes the characteristic impedance and, consequently, the current. Conductor loss depends somewhat on frequency because of a phenomenon called the skin effect. When current flows through an isolated round wire, the magnetic flux associated with it is in the form of concentric circles surrounding the wire core.

Dielectric Heating Losses and Radiation Losses:

A difference of potential between two conductors of a metallic transmission line causes dielectric heating. Heat is a form of energy and must be taken from the energy propagating down the line. Forair dielectric transmission lines, the heating loss negligible. However, for solid-core transmission lines, dielectric heating loss increases with frequency. If the separation between conductors in a metallic transmission line is an appreciable fraction of a wavelength, the electrostatic and electromagnetic fields that surround the conductor cause the line to act as if it were an antenna and transfer energy to any conductive material. The energy radiated is called radiation loss and depends on dielectric material, conductor spacing, and length of the transmission line.

Coupling Losses and Corona:

Coupling loss occurs whenever a connection is made to or from a transmission or when two sections of transmission line are connected together. Mechanical connections are discontinuities, which are locations where dissimilar materials meet. Discontinuities tend to heat up, radiate energy, and dissipate power. Corona is a luminous discharge that occurs between the two conductors of a transmission line when the difference of potential between them exceeds the breakdown voltage of the dielectric insulator. Generally, when corona occurs, the transmission line is destroyed.