Although most of us take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. If you want to talk to someone, all you have to do is pick up the phone and dial a few digits. You are instantly connected to that person, and you can have a two-way conversation.
The telephone network extends worldwide, so you can reach nearly anyone on the planet. When you compare that to the state of the world just 100 years ago, when it might have taken several weeks to get a one-way written message to someone, you realize just how amazing the telephone is!
Major Components of a Telephone Network
Local loop is a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. Local loop has a bandwidth of 4000 Hz (4KHz) when used for voice. For a long distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3000 Hz (3Hz). All the frequencies of the voice below 400 Hz and above 3400 Hz are eliminated. (howstuffworks.com)
Trunks are transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or satellite links.
Switching office is a component of a telephone network that handles the connection of several local loops or trunks and allows a connection between a different subscribers. The switch may operate in local, regional, national or international level. It also avoids a permanent link between two subscribers.
ILEC vs. CLEC
Incumbent Local Exchange Carriers (ILEC) is the only carrier that provides intra-LATA services before 1996 (before the Telecommunications Act of 1996). This was found out to be a monopoly and it also owns cabling system and so it opens opportunity for other local exchange carriers inside a LATA.
Competitive Local Exchange Carriers (CLEC) is the new carriers that can provide services after the Telecommunications Act of 1996. It was agreed that the ILECs would continue to provide their main services, and the CLECs would provide the other services such as mobile telephone service, toll calls inside a LATA, and so on. (For more info about the ILEC vs. CLEC visit howstuffworks.com)
Points of Presence (POP)
In a telephone network POP is the interface point that lets carriers interact with each other. let us say that in intra-LATA services, it can be provided by several LECs (may it be one ILEC and possibly more than one CLEC). Also in the inter-LATA services, it can be provided by several IXCs (interexchange carriers). Each IXCs that wants to provide inter-LATA services in LATA must have POP in that LATA. The LECs that provides services inside the LATA must provide connections so that every suscriber can have access to all POPs.
Rotary Dialing vs. Touch-tone Dialing
Rotary dialing or pulse dialing is the thing in the past, in which a digital signal was sent to the end office for each number dialed. This type of dialing was prone to errors due to inconsistency of humans during the dialing process.
Touch-tone dialing is the technique use nowadays. instead of sending a digital signal, the user sens two small bursts of analog signals, called the dual tone. The frequency of the signals sent depends on the row and the column of the pressed pad.
800 Service vs. 900 Service
800 Service is used if a subscriber (normally an organization) needs to provide free connections for other subscribers (normally customers). It can request an 800 service also 888, 877, 866 because 800 numbers are already exhausted. The call in this case is free for the caller, but it is paid by the callee. An organization uses this service to encourage the subscribers to call. The rate is less expensive than a normal long distance call.
900 Service are like 800/888 services, but the call is paid by the caller and is normally much more expensive than a normal long-distance call. Is is more expensive because the carrier charges two fees; the first is the long-distance toll, and the second is the fee paid to the callee for each call. This service is used by an organization that needs to charge their customers for its services (eg. a software company may charge a customer for technical support).
Digital Service Unit (DSU)
It is used in a switched/56 service where it changes the rate of the digital data created by the subscriber's device to 56 Kbps and encodes them in the format used by the service provider.
Sources: Data Communications and Networking 3rd ed. by Behrouz A. Foruozan
www.howstuffworks.com
Tuesday, July 27, 2010
Friday, July 23, 2010
Tranmission Media
To start with, there are two classes of transmission media: the guided media and the unguided media. Let me first discuss the guided media, these are those that provide a channel from one device to the other. There are three categories of guided media; the twisted-pair cable, coaxial cable, and the fiber-optic cable.
Guided Media
Twisted-pair Cable
Twisted-pair cable consists of two conductors twisted together with its own plastic as insulation. The twisting helps to reduce the interference (noise) and crosstalk. One of the wires is used to carry signals to the reciever, and the other is used only as a ground reference. The receiver uses the difference between the two levels It comes in two forms: unshielded and the shielded. Unshielded TP is the most common type of telecommunication medium in use today. The range is suitable for transmitting both data and video. Advantages of UTP are its cost and ease of use. UTP is cheap, flexible, and easy to install.
The Electronic Industries Association (EIA) has developed standards to grade UTP.
Category 1. The basic twisted-pair cabling used in telephone systems. This level of quality is fine for voice but inadequate for data transmission.
Category 2. This category is suitable for voice and data transmission of up to 2Mbps.
Category 3.This category is suitable for data transmission of up to 10 Mbps. It is now the standard cable for most telephone systems.
Category 4. This category is suitable for data transmission of up to 20 Mbps.
Category 5. This category is suitable for data transmission of up to 100 Mbps. (categories 6 and 7 are still in the draft stage)
The most common UTP connector is RJ45 (RJ stands for Registered Jack). The RJ45 is a keyed connector, meaning the connector can be inserted in only one way.
Shielded twisted-pair cable (STP) has a metal foil or braided-mesh covering that enhances each pair of insulated conductors. The metal casing prevents the penetration of electromagnetic noise. Materials and manufacturing requirements make STP more expensive than UTP but less susceptible to noise.
Twisted-pair cables are used in telephones lines to provide voice and data channels. It is also used within buildings for private branch exchange (PBX). The DSL lines that are used by the telephone companies to provide high data rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. Local area networks, such as 10Base-T and 100Base-T, also used UTP cables.
Its cheapness and easy installation are the only advantages. But its transmission is limited in distance where in analog signal an amplifier is needed for every 5-6 kilometers and that for digital signal it needs a repeater for every 2-3 kilometers. Aside from that it has limited bandwidth just about 1MHz and limited data rate 100MHz. and the big downside of it all is that it is susceptible to interference and noise.
Coaxial Cable
Coaxial cable carries signals of higher frequency ranges than twisted-pair cable. Instead of having two wires, coax cable has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. The outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
It is categorized by their radio government (RG) ratings. The RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing. These are the coaxial cable standards: RG-8, RG-9, RG-11 are used in thick Ethernet, RG-58 Used in thin Ethernet, RG-59 Used for cable TV.
The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. There are three types: the BNC connector is used in TV set, the BNC T connector is used in Ethernet networks, and the BNC terminator used to prevent the reflection of the signal.
Coaxial cable is used in cable TV networks, and some traditional Ethernet LANs like 10Base-2, or 10-Base5. Being a versatile medium that it can carry as much 10,000 voice calls simultaneously.
Fiber-optic Cable
Fiber-optic cable is made of glass or plastic and transmit signal in the form of light. Light, a form of electromagnetic energy, travels at 300,000 Kilometers/second (186,000 miles/second), in a vacuum. The speed of the light depends on the density of the medium through which it is traveling (the higher density, the slower the speed).
Optical fibers use reflection to guide light through a channel.
A glass or core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being into it. Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.
There are two basic types of fiber: multimode fiber and single-mode fiber. Multimode fiber is best designed for short transmission distances, and is suited for use in LAN systems and video surveillance. Single-mode fiber is best designed for longer transmission distances, making it suitable for long-distance telephony and multichannel television broadcast systems.
Current technology supports two modes for propagating light along optical channels, each requiring fiber with different physical characteristics: Multimode and Single Mode. In multimode, multiple beams from a light source move through the core in different paths and can be implemented in two forms: step-index or graded index.
In multimode step-index fiber, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and cladding. At the interface there is an abrupt change to a lower density that alters the angle of the beam’s motion.
While in a multimode graded-index fiber the density is highest at the center of the core and decreases gradually to its lowest at the edge.
In single mode propagation it uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal.
Fiber-optic cable has three different kinds of connectors, the subscriber channel (SC) connector is used in cable TV that uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices that uses a bayonet locking system and is more reliable than SC. And lastly is the MT-RJ is a new connector with the same size as RJ45.
Advantages
1. Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable.
2. Less signal attenuation. The distance of transmission is significantly greater than that of other guided media. A signal can run for 50 kilometers without requiring regeneration.
3. Immunity to electromagnetic interference. EM noise cannot affect fiber-optic cables.
4. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.
5. Lightweight. Fiber-optic cables are much lighter than copper cables.
6. More immune to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antennas that can be easily tapped.
Disadvantages
1. Installation/maintenance. Fiber-optic is a relatively new technology. I/M need expertise that is not yet available everywhere.
2. Unidirectional. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed.
3. Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified. Also, a laser light source can cost thousands of dollars, compared to hundreds of dollars for electrical signal generators.
4. Fragility. Glass fiber is more easily broken than wire, making it less useful for applications where hardware portability is required.
Unguided Media
Unguided media transport electromagnetic waves without using a physical conductor. Signals are broadcast though air or water, and thus are available to anyone who has a device capable of receiving them. The EM spectrum covers frequencies from 3 Hz (ELF) to gamma rays (30 ZHz) and beyond (cosmic rays). But only frequencies ranging from 3 KHz to 900 THz are used for wireless communication.
Propagation of Radio Waves
Radio technology considers the earth as surrounded by two layers of atmosphere: the troposphere and the ionosphere.
The troposphere is the portion of the atmosphere extending outward approximately 30 miles from the earth's surface. The troposphere contains what we generally think of as air. Clouds, wind, temperature variations, and weather in general occur in the troposphere.
The ionosphere is the layer of the atmosphere above the troposphere but below space.
Unguided signals can travel from the source to destination in several ways. There is ground propagation, sky propagation, and line-of-sight propagation.
In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. The distance depends on the power in the signal.
In Sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. This type of transmission allows for greater distances with lower power output.
In Line-of-Sight Propagation, very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by the curvature of the earth.
Radio Waves
Radio wave frequencies are between 3 KHz to 1 GHz, and uses omnidirectional antenna. Omniderectional antenna propagates signal in all direction. This means that the sending and receiving antennas do not have to be aligned. But it has disadvantage too, it is susceptible to interference wherein a radio wave transmitted by one antenna may be interfered by another antenna that may send signals using the same frequency or band.
Radio waves are used for multicast communications, such as radio (AM and FM radio), maritime radio, television, cordless phones and paging systems.
Microwaves
Frequencies between 1 and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. Its advantage is that a pair of antennas can be aligned without interfering with another pair of aligned antennas.
The propagation of microwave is line-of-sight. The problem with this propagation is that towers that are far apart from each other need to be very tall. And the curvature of the earth as well as other blocking obstacles does not allow two short towers to communicate. For long distance communication, repeaters are often needed. Another disadvantage is that very high frequency microwaves cannot penetrate walls.
In a unidirectional antenna, there are two types: the parabolic dish and the horn. A parabolic dish antenna is based on the geometry of the parabola. Every line parallel to the line of symmetry reflects off the curve at angles such that all the lines intersect in a common point called focus. The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point.
A horn antenna on the other hand looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem.
There is another type of microwave transmission with the use of satellite relay. It requires geo-stationary orbit with the height of 35,784km to match the earth’s rotation. It has uplink that receives transmission on one frequency and a downlink that transmits on a second frequency. It Operates on a number of frequency bands known as transponders. It can operate in two ways: a. Point to point- Ground station to satellite to ground station
b. Multipoint (Broadcast link)- Ground station to satellite to multiple receiving stations
Microwaves are used in unicast communication such as cellular telephones, satellite networks, and wireless LANs.
Infrared Waves
Infrared signals with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 700 nm), can be used for short-range communication. Having high frequencies cannot penetrate walls. This characteristic prevents interference between one system and another; a short-range communication cannot be affected by another system in the next room. The same characteristic makes infrared signals useless for long range communication. Infrared waves cannot be used outside a building because the sun’s rays contained infrared waves can interfere with the communication.
The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate. The infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established a standard for using these signals for communication between devices such as the keyboard, mice, PCs, and printers
Infrared signals defined by the IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission occurs.
SOURCES: Data Communications and Networking 3rd ed. by Behrouz Forouzan
Data and Computer Communications, Sixth Edition by William Stallings
Guided Media
Twisted-pair Cable
Twisted-pair cable consists of two conductors twisted together with its own plastic as insulation. The twisting helps to reduce the interference (noise) and crosstalk. One of the wires is used to carry signals to the reciever, and the other is used only as a ground reference. The receiver uses the difference between the two levels It comes in two forms: unshielded and the shielded. Unshielded TP is the most common type of telecommunication medium in use today. The range is suitable for transmitting both data and video. Advantages of UTP are its cost and ease of use. UTP is cheap, flexible, and easy to install.
The Electronic Industries Association (EIA) has developed standards to grade UTP.
Category 1. The basic twisted-pair cabling used in telephone systems. This level of quality is fine for voice but inadequate for data transmission.
Category 2. This category is suitable for voice and data transmission of up to 2Mbps.
Category 3.This category is suitable for data transmission of up to 10 Mbps. It is now the standard cable for most telephone systems.
Category 4. This category is suitable for data transmission of up to 20 Mbps.
Category 5. This category is suitable for data transmission of up to 100 Mbps. (categories 6 and 7 are still in the draft stage)
The most common UTP connector is RJ45 (RJ stands for Registered Jack). The RJ45 is a keyed connector, meaning the connector can be inserted in only one way.
Shielded twisted-pair cable (STP) has a metal foil or braided-mesh covering that enhances each pair of insulated conductors. The metal casing prevents the penetration of electromagnetic noise. Materials and manufacturing requirements make STP more expensive than UTP but less susceptible to noise.
Twisted-pair cables are used in telephones lines to provide voice and data channels. It is also used within buildings for private branch exchange (PBX). The DSL lines that are used by the telephone companies to provide high data rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. Local area networks, such as 10Base-T and 100Base-T, also used UTP cables.
Its cheapness and easy installation are the only advantages. But its transmission is limited in distance where in analog signal an amplifier is needed for every 5-6 kilometers and that for digital signal it needs a repeater for every 2-3 kilometers. Aside from that it has limited bandwidth just about 1MHz and limited data rate 100MHz. and the big downside of it all is that it is susceptible to interference and noise.
Coaxial Cable
Coaxial cable carries signals of higher frequency ranges than twisted-pair cable. Instead of having two wires, coax cable has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. The outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
It is categorized by their radio government (RG) ratings. The RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing. These are the coaxial cable standards: RG-8, RG-9, RG-11 are used in thick Ethernet, RG-58 Used in thin Ethernet, RG-59 Used for cable TV.
The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. There are three types: the BNC connector is used in TV set, the BNC T connector is used in Ethernet networks, and the BNC terminator used to prevent the reflection of the signal.
Coaxial cable is used in cable TV networks, and some traditional Ethernet LANs like 10Base-2, or 10-Base5. Being a versatile medium that it can carry as much 10,000 voice calls simultaneously.
Fiber-optic Cable
Fiber-optic cable is made of glass or plastic and transmit signal in the form of light. Light, a form of electromagnetic energy, travels at 300,000 Kilometers/second (186,000 miles/second), in a vacuum. The speed of the light depends on the density of the medium through which it is traveling (the higher density, the slower the speed).
Optical fibers use reflection to guide light through a channel.
A glass or core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being into it. Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.
There are two basic types of fiber: multimode fiber and single-mode fiber. Multimode fiber is best designed for short transmission distances, and is suited for use in LAN systems and video surveillance. Single-mode fiber is best designed for longer transmission distances, making it suitable for long-distance telephony and multichannel television broadcast systems.
Current technology supports two modes for propagating light along optical channels, each requiring fiber with different physical characteristics: Multimode and Single Mode. In multimode, multiple beams from a light source move through the core in different paths and can be implemented in two forms: step-index or graded index.
In multimode step-index fiber, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and cladding. At the interface there is an abrupt change to a lower density that alters the angle of the beam’s motion.
While in a multimode graded-index fiber the density is highest at the center of the core and decreases gradually to its lowest at the edge.
In single mode propagation it uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal.
Fiber-optic cable has three different kinds of connectors, the subscriber channel (SC) connector is used in cable TV that uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices that uses a bayonet locking system and is more reliable than SC. And lastly is the MT-RJ is a new connector with the same size as RJ45.
Advantages
1. Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable.
2. Less signal attenuation. The distance of transmission is significantly greater than that of other guided media. A signal can run for 50 kilometers without requiring regeneration.
3. Immunity to electromagnetic interference. EM noise cannot affect fiber-optic cables.
4. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.
5. Lightweight. Fiber-optic cables are much lighter than copper cables.
6. More immune to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antennas that can be easily tapped.
Disadvantages
1. Installation/maintenance. Fiber-optic is a relatively new technology. I/M need expertise that is not yet available everywhere.
2. Unidirectional. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed.
3. Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified. Also, a laser light source can cost thousands of dollars, compared to hundreds of dollars for electrical signal generators.
4. Fragility. Glass fiber is more easily broken than wire, making it less useful for applications where hardware portability is required.
Unguided Media
Unguided media transport electromagnetic waves without using a physical conductor. Signals are broadcast though air or water, and thus are available to anyone who has a device capable of receiving them. The EM spectrum covers frequencies from 3 Hz (ELF) to gamma rays (30 ZHz) and beyond (cosmic rays). But only frequencies ranging from 3 KHz to 900 THz are used for wireless communication.
Propagation of Radio Waves
Radio technology considers the earth as surrounded by two layers of atmosphere: the troposphere and the ionosphere.
The troposphere is the portion of the atmosphere extending outward approximately 30 miles from the earth's surface. The troposphere contains what we generally think of as air. Clouds, wind, temperature variations, and weather in general occur in the troposphere.
The ionosphere is the layer of the atmosphere above the troposphere but below space.
Unguided signals can travel from the source to destination in several ways. There is ground propagation, sky propagation, and line-of-sight propagation.
In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. The distance depends on the power in the signal.
In Sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. This type of transmission allows for greater distances with lower power output.
In Line-of-Sight Propagation, very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by the curvature of the earth.
Radio Waves
Radio wave frequencies are between 3 KHz to 1 GHz, and uses omnidirectional antenna. Omniderectional antenna propagates signal in all direction. This means that the sending and receiving antennas do not have to be aligned. But it has disadvantage too, it is susceptible to interference wherein a radio wave transmitted by one antenna may be interfered by another antenna that may send signals using the same frequency or band.
Radio waves are used for multicast communications, such as radio (AM and FM radio), maritime radio, television, cordless phones and paging systems.
Microwaves
Frequencies between 1 and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. Its advantage is that a pair of antennas can be aligned without interfering with another pair of aligned antennas.
The propagation of microwave is line-of-sight. The problem with this propagation is that towers that are far apart from each other need to be very tall. And the curvature of the earth as well as other blocking obstacles does not allow two short towers to communicate. For long distance communication, repeaters are often needed. Another disadvantage is that very high frequency microwaves cannot penetrate walls.
In a unidirectional antenna, there are two types: the parabolic dish and the horn. A parabolic dish antenna is based on the geometry of the parabola. Every line parallel to the line of symmetry reflects off the curve at angles such that all the lines intersect in a common point called focus. The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point.
A horn antenna on the other hand looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem.
There is another type of microwave transmission with the use of satellite relay. It requires geo-stationary orbit with the height of 35,784km to match the earth’s rotation. It has uplink that receives transmission on one frequency and a downlink that transmits on a second frequency. It Operates on a number of frequency bands known as transponders. It can operate in two ways: a. Point to point- Ground station to satellite to ground station
b. Multipoint (Broadcast link)- Ground station to satellite to multiple receiving stations
Microwaves are used in unicast communication such as cellular telephones, satellite networks, and wireless LANs.
Infrared Waves
Infrared signals with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 700 nm), can be used for short-range communication. Having high frequencies cannot penetrate walls. This characteristic prevents interference between one system and another; a short-range communication cannot be affected by another system in the next room. The same characteristic makes infrared signals useless for long range communication. Infrared waves cannot be used outside a building because the sun’s rays contained infrared waves can interfere with the communication.
The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate. The infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established a standard for using these signals for communication between devices such as the keyboard, mice, PCs, and printers
Infrared signals defined by the IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission occurs.
SOURCES: Data Communications and Networking 3rd ed. by Behrouz Forouzan
Data and Computer Communications, Sixth Edition by William Stallings
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