The Telephone Network

The telephone network began in the late 1800s which was referred to as plain old telephone system (POTS). It was orignally analog, but with the advancement in computer technology the network started to carry data as well as voice in 1980's. It is now both digital and analog.

Major Components

The telephone network is made of three major components - the local loops, the trunks and the switching office.

The local loop connects the subscriber to the nearest end office (or local central office) through a twisted-pair cable. It has a bandwidth of 4000Hz.

The trunk is a transmission media that connects switching offices. It handles a lot of connections through multiplexing. these are usually optical fibers or satellite links.

The switching office establishes a connection between two subscribers. The connection between two subscribers is not permanent and will only be made upon request. Connections are limited by the total bandwidth of a transmission media therefore having permanent idle lines would limit the services of the network.

LATA

The local telephone network is referred to as Local Access Transport Areas (LATA). LATAs are made up of multiple local loops connected to a tandem office. Services of Comon Carriers (telephone companies) within a LATA are called intra-LATA services. These carriers are refered to as Local Exchange Carriers (LEC). LEC has to types. The Incumbent Local Exchange Carriers (ILEC) is the original company that set up the LATA. To avoid cost for new cabling, Competitive Local Exchange Carriers (CLEC) were allowed to use the LATA of the ILEC for their own services.

Services between LATAs (Inter-LATA services) are handled by Interexchange Carriers (IXC), commonly referred to as long-distance companies. LECs are also allowed to become IXCs. To allow multiple IXCs to use a LATA, a Point of Presence (POP) switching office is created for each IXC.A caller, who needs to connect to a receiver in another LATA, first connects to an end switch then, either directly or through a tandem office, to a POP of the caller's choice. The call then goes from the POP in the caller's LATA to the POP of the same IXC in the reciever's LATA then down to the switching offices and finally to the telephone of the receiver.

Signals

Voice communication used analog signals in the past, but is now moving to digital signals. On the other hand, dialing started with digital signals (rotary) and is now moving to analog signals (touch-tome).

Services

A common carrier may provide Analog and/or Digital services.

Analog Services

Analog Switched Service is the familiar dial-up service most often encountered when a home telehpone is used. Analog Switched services include multiple optional services. Local Call service is normally provided for a flat monthly rate. Toll call services (calls that need to pass through the tandem offices) are charged per call. 800 services are free for the caller and is charged to the callee. 800 services are usually used by companies to encourage customers to call. 900 services are charged to the caller and is normally more expensive than toll call services. This is because the carrier charges two fees - first is charge of the carrier, second the charge of the callee, for example a software company that charges for technical support.

Analog Leased Service offers customers the opportunity to lease a dedicated line that is permanently connected to another customer. Although calls still pass through the switching office, no dialing is needed.

Digital Services

Switched/56 Service is the digital version of the analog switched service. It allows a data rate of up to 56 Kbps. Sender and reciever must both subscribe to this service for connection to happen. Because the line is already digital, the users do not need a modem. All they need to use is a Digital Service Unit (DSU) which changes the rate of the digital data sent by a subscriber to 56Kbps and encodes it in a format used by the carrier.

Digital Data Service is the digital version of an analog leased line with a data rate of up to 64Kbps.

Forouzan, B. (2003). Data Communications and Networking. New York: McGraw Hill.

Wireless Communication and the Electromagnetic Spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. As the frequency of an electromagnetic wave increases its wavelength decreases. In any frequency, electromagnetic waves travel through a vacuum at the speed of light. When traveling through a medium the speed of electromagnetic waves decrease.

Data communication frequencies

1. Twisted Pair (0 - 100Mhz)
   
2. Coaxial Cable (1Khz - 1Ghz)
   
3. AM Radio (530Khz - 1600Khz)
   
4. FM Radio (88Mhz - 108hz)
   
5. Terrestrial and Satellite / Microwave (1Ghz - 1Thz)
6. Infrared (1Thz - 100Thz)
   
7. Optical Fiber (100Thz - 1Phz)

Unguided Media

Unguided Media transmit electromagnetic waves without using a solid conductor. Some authors say that air or water is unguided media's media. However it should be noted that Electromagnetic waves do not require any media to propagate and can travel even through vacuum. Wireless Communication would be a better term.


Wireless Data Communication frequencies

1. AM Radio (530Khz - 1600Khz)
   
2. FM Radio (88Mhz - 108hz)
   
3. Terrestrial and Satellite (1Ghz - 100Ghz)
4. Infrared (1Thz - 100Thz)

Propagation Methods

1. Ground Propagation - radio waves travel through the lowest portion of the atmosphere following the curvature of the planet.
2. Sky Propagation - high frequency radio waves radiate upward into the ionosphere where they are reflected back to the earth.
3. Line-of-sight Propagation - very high frequency signals are transmitted in straight lines directly from antenna to antenna.

Wireless Transmission Frequencies

Radio Wave

Although there is no clear cut division between radio waves and microwaves, electromagnetic frequencies between 3Khz and 1Ghz. Behavior of the waves, rather than frequencies, is a better criterion for classification. Radio waves are mostly omnidirectional and propagated using an omnidirectional antenna. When an antenna transmits radio waves, they are propagated in all directions. This means that sending and receiving antennas do not need to be aligned. Radio waves are used for multi-casting, in which there is one sender but many receivers such as AM and RM radio, television, maritime radio, cordless phones and paging. Radio waves can penetrate walls. Radio waves travel to long distances. The disadvantage of omnidirectionality is that the signal is susceptible to interference by another signal of the same frequency. The entire band is regulated by authorities and any part of the band requires permission.

Microwaves

Microwaves have frequencies between 1 and 300 GHz. Microwaves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. This means that even if two pairs of antennas transmit microwaves of the same frequency, they will not interfere with each other as long as the pairs are not aligned. Microwave propagation needs to be line-of-sight. The microwave band is relatively wide. Therefore wider sub-bands can be assigned and a high data rate for each sub-band is possible. Though microwaves travel shorter distances relative to radio waves. Microwaves use unidirectional antennas such as a parabolic dish and a horn antenna. They are used in cellular phones, satellite networks and wireless LANs.

Infrared

Infrared signals, with frequencies from 300Ghz to 4Thz, can be used for short-range communication. Infrared signals, having high frequencies never pass through walls. This is advantageous when communication systems are separated by physical walls. However we can not use infrared outside a building because the sun's rays contain infrared waves that interfere with the communication.

Fiber-optic Cables and the Nature of Light

One of the most significant technological breakthroughs in data transmission has been the creation of the fiber-optic cable. It allows data rate of up to 1600Gbs using WDM. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available. The fiber-optic cable transmits electromagnetic signals known as visible light.

A fiber-optic cable has a cylindrical shape and consists of three concentric sections: the core, the cladding and the jacket. The core is made up of dense plastic or glass and it is here that the light travels. The cladding is made up of less dense material which reflects the light signals back to the core instead of absorbing it. The jacket surrounds one or more cladded cores to protect against environmental dangers.

Propagation Modes

Before we discuss propagation modes, we need to discuss how light travels through a medium. Light travels through a straight line as long as it is moving through a single uniform substance. When a light enters another substance it changes direction. If the angle of incidence (the angle the light makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts to the substance of lower density. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects and travels again in the denser substance.

Multimode

Multimode fiber-optics transmits multiple light signals. How the beam travel depends on the density composition of the core.

Multimode step-index

The core is evenly dense. The light travels straight until it reaches the interface of the core and cladding. The abrupt change causes it to be reflected back to the core. Step-index refers to the abrupt change of direction of the light. There are multiple paths with varying lengths, therefore varying transmission time which limits the data rate. This is the least expensive propagation mode and its commonly used for local area networks.

Multimode graded-index.

The core is densest at the center and the density decreases gradually towards the edge of the core. This varying density allows the light to bend. This allows focusing the signals more efficiently than step-index. This propagation mode is moderately expensive compared to the other two modes and is commonly used for telephone lines.

Single-mode

Single mode uses step-index fiber and a highly focused light source that limits the light beam to a small range of angles close to the horizontal. The core itself has a small diameter and lower density making the path of the light almost horizontal. The propagation time for all signals are almost equal. Delays are negligible. This is the most expensive propagation mode for fiber-optics commonly used for long distance telecom lines.

Fiber Sizes

Optical fibers are defined by the ration of the diameter of their core to the diameter of their cladding.

Type	Core	Cladding	Mode
50/125	50	125	Multimode, graded-index
62.5/125	62.5	125	Multimode, graded-index
100/125	100	125	Multimode, graded-index
7/125	7	125	Single-mode

Connectors

Subscriber Connectors (SC) is used in cable TV. The Straight-Tip (ST) is used for networking devices. It is more reliable than SC. MT-RJ connector has the same size as the RJ45.

Advantages

1. Very high bandwidth and data rate
2. Less signal attenuation
3. Immunity to electromagnetic interference
4. Resistance to corrosion
5. Light weight
6. Difficult to tap

Disadvantages

1. Installation/maintenance
2. Unidirectional
3. Cost

Coaxial Cable

Like a twisted pair, the coaxial cable consists of two conductors. However, its structure permits a wider range of frequencies. The coaxial cable consists of a hollow outer cylindrical conductor (solid or braided) which surrounds a single conductor (solid or stranded). The outer and inner conductors are separated by insulating strings or a solid dielectric material.

Categories

Coaxial cables are categorized by their radio government ratings (RG). Each 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. Each category is adapted to a particular use.

Category	Impedance	Use
RG-59	75 W	Cable TV
RG-58	50 W	Thin Ethernet
RG-11	50 W	Thick Ethernet

Each type of coaxial cable has a characteristic impedance depending on its dimensions and materials used, which is the ratio of the voltage to the current in the cable. In order to prevent reflections at the destination end of the cable from causing standing waves, any equipment the cable is attached to must present an impedance equal to the characteristic impedance (called 'matching'). Thus the equipment "appears" electrically similar to a continuation of the cable, preventing reflections.

Connectors

The BNC (Bayonet Neill-Concelman) connector is used to connect the cable to a device. The BNC T connector is used to branch out another coaxial cable. The BNC terminator is used at the end of the cable to prevent reflection of the signal.

Advantages

1. Superior bandwidth and data rate compare to twisted pair (350Mhz – 500Mbps)
2. Shielded against crosstalk and external noise without the need for twists

Disadvantages

1. Susceptible to thermal noise and intermodulation noise
2. Rigid and heavy Cable
3. Higher attenuation compared to twisted pairs.

Twisted Pair Cable

The twisted pair consists of two insulated copper wires arranged in a spiral pattern. The pair acts as a single communication link. Normally, a number of these twisted pair cables are wrapped together in a protective sheath. One of the wires is used to carry signals to the receiver and the other is only used to capture the amount of noise. The receiver will then calculate the difference between the two wires. Assuming the wires received the same amount of noise, once the receiver calculates the difference, the receiver would be able to recreate the original signal.

Wire A = original signal + noise

Wire B = noise

original signal = Wire A – Wire B

Twisted pair is normally used in the local loop of the telephone line to provide voice and data channels.

Types

There are two types of twisted pair cables – the Shielded Twisted Pair (STP) and the Unshielded Twisted Pair (UTP). As the name suggests, STP has a metal shield which protects it from noise. This metal shield is absent in the UTP. The STP was created by IBM. Although the shield provides additional protection from noise the cable is bulkier and more expensive. The STP is seldom used outside IBM.

Categories

The Electronic Industries Association (EIA) has developed standards to classify unshielded twisted pair cables into seven categories. Categories are determined by cable quality. Each category is best used with a particular technology.

Category	Bandwidth	Data Rate	Digital/Analog	Use
1	very low	<>	Analog	Telephone
2	<>	2 Mbps	Analog/digital	T-1 lines
3	16 MHz	10 Mbps	Digital	LANs
4	20 MHz	20 Mbps	Digital	LANs
5	100 MHz	100 Mbps	Digital	LANs
6 (draft)	200 MHz	200 Mbps	Digital	LANs
7 (draft)	600 MHz	600 Mbps	Digital	LANs

Why are the wires twisted?

The wires are twisted to reduce noise. Because the wires are twisted, they are affected by noise almost equally. Suppose in one twist one wire is close to the source of noise, in the next twist, the other wire would be closer to the source of noise and so forth. The noise in one wire will cancel out the noise in the other wire. This means that the receiver who calculates the difference between the two receives almost no noise.

Wire A = original signal + noise A

Wire B = noise B

original signal = Wire A – Wire B if an only if noise A = noise B

Connectors

The standard connector for twisted pairs are Registered Jacks (RJ). Some of the common RJs are RJ45 and RJ11.

Advantages

1. Low-cost for local networking
2. Wires are thin and flexible

Disadvantages

1. Limited distance due to strong attenuation
2. The twist loosens over time which makes a difference in the noise received by the wires.
3. Limited bandwidth and data rate (250Khz – 4Mbps)
4. A lot of repeaters or amplifiers are needed for long distance use.

Transmission Media and Electromagnetic Signals

The transmission media is considered lower than the physical layer and is directly controlled by it. Transmission media is said to be at layer zero. Computers and other devices send data using electromagnetic signals which travel through the media.

Electromagnetic signals are a combination of electric and magnetic fields which oscillate perpendicular to one another. Electromagnetic signals or waves are named according to frequency – radio waves, infrared, visible light, microwaves, ultraviolet light, X rays, gamma rays and cosmic rays. All electromagnetic waves of all frequencies comprise the Electromagnetic Spectrum. Not all portions of the electromagnetic spectrum are usable for telecommunication. It should be noted that all electromagnetic signals can travel through a vacuum. In other words, these waves do not require media. Electromagnetic signals also travel the fastest in a vacuum at exactly 299,792,458 m/s which is known as the speed of light.

The speed of electromagnetic waves that travel through a medium is described by the refractive index (RI). RI = velocity of light in a vacuum / velocity of light in medium

Data communication authors classify transmission media into two types – guided and unguided media. Guided media force electromagnetic waves to travel through a conductor by encasing the conductor with an insulator. Guided media include twisted-pair cable, coaxial cable and fiber-optic cable. Unguided media transport signals without conductors and are normally broadcasted through air. It should be noted that ‘unguided media’ does not mean that the signals sent through them cannot be directed. Electromagnetic waves used in unguided media are microwaves, radio waves and infrared.

Transmission through unguided media is also referred to as wireless communication. Wireless communication may also refer to electromagnetic signals that are transmitted through vacuum.