Computer Networks: The Physical Layer Terms 2.1 The Theoretical Basis for Data Communication 2.2 Transmission Media 2.3 Wireless Transmission 2.4 The Telephone System 2.5 Narrowband ISDN 2.6 Broadband ISDN and ATM

2.2 Transmission Media
TERM	DEFINITION
Magnetic Media	Material on which to store data; used in diskettes and magnetic tape. Magnetic media is a common way to transport data quickly.
twisted pair	The oldest and still most common transmission medium. Two insulated copper wires. Twisting the wires reduces electrical interference from nearby wires. It also counters the antenna effect caused by parallel wires.
Category 3 "Cat 3"	Type of twisted pair wire used in computing. It is gently twisted together, and was popular before Cat 5 wiring came into popularity.
wiring closet	Centralized place on each floor in which most of the wiring for the floor originates.
Category 5 "Cat 5"	Came into popularity in the late 80's. Due to its greater number of twists per centimeter, it is more suitable for carrying a signal long distances. It is popularly used for high-speed data connections.
UTP Unshielded Twisted Pair	General category into which Cat 3 and Cat 5 wires fall.
coaxial cable "coax"	Its superior shielding allows it to span greater distances than UTPs. It consists of stiff copper wire surrounded by insulation., which is encased in a conducting mesh, which is covered by a plastic sheath. It has a high transmission speed combined with immunity to noise. To the right is an diagram of a coaxial cable from Novell's Network Primer.
broadband	Either a cable with a bandwidth of 4kHz or a cable network using analog transmission.
head-end	In a dual cable system, there are two cables which only transmit data in one direction each (opposite to each other). At one end of the network, data is collected from one line for retransmission on the line going the opposite direction. The end of the network responsible for the data collection and retransmission is called the head-end.
subsplit	When two cables are not used, but there is a need for simulation of a dual cable system (see above), bandwidth on a single cable can be split up, with one portion representing one cable, and the other portion representing the second cable. Splitting the frequencies so that the lower frequencies are used for one purpose and the higher for another is called a subsplit system.
midsplit	Similar system to subsplit, except that there is a gap between the two available bands. This gap is based on historical allocation of bandwidth by the FCC for broadcasting.
mode	Characteristic of a light wave; associated with the reflection of the wave through the silica fiber.
multimode fiber	fiber that can carry more than one light wave at once; different waves bounce off the silica edges in order to propagate along the length of the fiber.
single mode fiber	Can only carry one light wave at a time, and the light must travel in a straight line.
dispersion	When light pulses travel over a fiber line, they spread out as they go. This is called dispersion.
solitons	A pulse of light that is shaped in such a way that dispersion effects are nearly obliterated, allowing light to travel long distances without loss of information.
active repeater	In fiber optics, an active repeater converts the light signal to electricity, and then regenerates it at its original strength before retransmitting it as light.
passive star	Construction used to broadcast messages on a fiber network. All incoming signals are combined and they illuminate the combined output fibers. The more fibers that are connected for output, the weaker the light signal sent on each fiber.

2.3 Wireless Transmission
TERM	DEFINITION
frequency f	the number of oscillations per second of an electromagnetic (EM) wave Light and electricity travel in the form of waves. Waves are periodic in that they repeat themselves, so the frequency of a wave is just the number of times the wave repeats itself in one second.
Hertz Hz	The unit of measure of frequency. The name comes from the German physicist who first produced electromagnetic waves.
wavelength	The distance between two consecutive maxima or minima. Just like it sounds, wavelength is the measure of the length of a wave. It is designated by the Greek letter (lambda)
speed of light c	The speed that electromagnetic waves travel in a vacuum approximately 3 x 108 m/sec. Electromagnetic waves do not travel as fast through a medium as they do through a vacuum.
spread spectrum	A pattern for EM transmissions which is popular in the military for its ability to avoid jamming. The transmitter hops from frequency to frequency across a wide frequency band.
multipath fading	When microwaves diverge over a distance and end up interfering with each other, effectively jamming itself.

2.4 The Telephone System
TERM	DEFINITION
Public Switched Telephone Network (PSTN)	The currently existing network used for telephone communication. It was designed specifically for voice, and is not well suited for use by computers for transmitting data to one another.
end office or local central office	The place to which your telephone lines connect to receive the telephone service. The distance from the telephone to the end office is usually small (around 1 to 10 km).
local loop	The connection between the telephone and the end office is called the local loop.
toll offices	Switching centers that connect several end offices.
tandem offices	Similar to a toll office, except it connects end offices that are within the same local area.
toll connecting trunks	The hardware (e.g., wire) which connects the end office to the toll offices.
intertoll trunks or interoffice trunks	High bandwidth connections which connect two toll offices.
Bell Operating Companies (BOCs)	Former divisions of AT&T which provided local telephone service. The BOCs were separated from AT&T in 1984 by a court order.
Modified Final Judgment (MFJ)	Document containing the details of the "divestiture" of AT&T (the splitting up of AT&T into several smaller companies instead of one large company (which had the ability to behave like a monopoly).
Local Access and Transport Area (LATA)	Logical division of the United States. A LATA is approximately the size of an area code.
Local Exchange Carrier (LEC)	Company that provides local telephone service within a LATA.
IntereXchange Carrier (IXC)	Communication between LATAs is handled by an IXC (e.g., AT&T, MCI, and Sprint).
Point of Presence (POP)	When a IXC wished to provide service to a LATA, it has to establish a POP within the LATA, connecting to each end office (either directly or indirectly) in that LATA.
attenuation	When a signal loses energy as it propagates, it is called attenuation. Through copper wires and fiber optics, the signal diminishes logarithmically in proportion to distance.
delay distortion	Since signals are sent in Fourier components, and since these components travel at different speeds, one component can interfere with another, causing delay distortion.
noise	When external energy interferes with a transmitted signal, it is called noise.
Thermal noise	noise that is caused by the motions of the electrons on the wire.
cross talk	caused by inductive coupling of two wires that are close to each other.
modem	Actually short for a "modulator-demodulator", the modem translates between the digital computer and the analog phone line.
sine wave carrier	Signal (shaped like a sine wave) which is used to allow the DC phone system to carry AC signals.
amplitude modulation	Bits 0 and 1 are differentiated by different amplitudes (achieved by varying the voltage levels).
frequency modulation frequency shift keying (FSK)	Bits 0 and 1 are differentiated by changes in the frequency of a signal.
phase modulation	shifting the carrier wave a certain amount at determined intervals. This allows more than one bit to be sent at a time.
Quadrature Amplitude Modulation (QAM)	Combination of modulation techniques designed to allow more than one bit to be transmitted per signal change. Phase and Amplitude are both changeable in order to allow a large number of combinations (and bit sequences) to be sent in one signal.
constellation patterns	Diagrams of valid combinations of phase and amplitude changes. Basically, it is a diagram of the QAM possibilities.
V.32 bis	14,400 bps modem which uses a constellation pattern with 64 points.
V.34 bis	28,800 bps modems which use constellation patterns.
trellis coding	The coding of points on a constellation pattern in such a way that the change of detecting errors is maximized.
MNP 5	Popular compression scheme on modems
V.42 bis	Popular compression scheme on fax modems; uses the Ziv-Lempel Compression Algorithm.
echo suppressor	a device that suppresses echoes on a line when human speech is detected. This allows a two-way line to be one-way from the direction of the sender to the receiver, depending on which end is sending at the time. Does not allow full-duplex communication.
full-duplex	A system that allows two transmissions at once is full duplex. In such a system, the system A can transmit to system B at the same time as system B transmits to system A without interfering with one another.
half-duplex	Transmission is possible in two directions (from system A to system B *or* from system B to system A) but only one system may transmit at a time.
in-band signaling	Placing a control signal inside the band accessible to the user; used to override echo suppressors.
echo cancellers	Devices that calculate the echo and cancel it by creating a reverse echo. It allows full-duplex communication.
RS-232-C	Standard for computer communication via modem.
Data Terminal Equipment (DTE)	The computer (in a communication between the computer and modem)
Data Circuit-Terminating Equipment (DCE)	The modem (in a communication between the computer and modem)
null modem	A device used to connect two computers with the RS-232-C modem protocol, and allowing them to connect without an actual modem device present.
RS-449	The new standard in modem communications (as opposed to RS-232-C)
Fiber to the Home (FTTH)	The idea that fiber optical cable will replace *ALL* of the current copper wiring for communications (all the way to the home)
Fiber to the Curb (FTTC)	The idea that fiber optical cable will replace the current copper wiring for communications from the end office to the curb, but actual fiber will not be run to the household--copper will still connect the home to the curbside fiber cable.
Frequency Division Multiplexing (FDM)	A way to multiplex over a wire in which certain frequencies are allotted only for use by one user/station; i.e., a user gets allotted a fraction of the bandwidth all the time.
Time Division Multiplexing (TDM)	A way to multiplex over a wire in which certain time slots are allotted for use by only one user at a time; i.e., a user gets to use the entire bandwidth of the wire, but only for small, nonconsecutive timeperiods.
multiplex	Allowing more than one user to use a wire at a time.
Wavelength Division Multiplexing (WDM)	The fiber optical counterpart to FDM. Instead of dividing out portions of the bandwidth based on the frequency, WDM divides the bandwidth based on wavelength.
codec	short for coder-decoder; device that digitizes analog signals (and vice versa).
Pulse Code Modulation (PCM)	"The heart of the modern telephone system." It uses a codec to sample 8000 samples per second, because Nyquist's theorem says that is sufficient to obtain all the data on a 4-kHz channel.
SONET	Synchronous Optical NETwork. American standard in optical networking. It it synchronous in that it sends a frame of 810 bytes every 125 microseconds, whether or not there is actual data to send. ATM, in contrast, is asynchronous in that it only sends cells when there is actual information.
Synchronous Digital Hierarchy (SDH)	Similar to SONET; differs slightly, but in minor ways.
Synchronous Payload Envelope (SPE)	The user data inside a SONET frame. The SPE can be anywhere in the frame, except the overhead bytes (first three columns of a SONET frame).
tributaries	the data streams that are multiplexed together. Several tributaries merge to form a larger river of data.
photonic sublayer	One of the four sublayers of SONET's physical layer. "It specifies the physical properties of the light and fiber to be used."
circuit switching	Used widely in the current telephone system, circuit switching is creating a direct path over copper from one user to another. This path is unchanged during the entire duration of the call.
message switching	No copper connection is established from end to end (i.e., from sender to receiver); instead, the connection is from sender to switching office, from switching office to switching office, and from switching office to receiver. In this method, the entire message is stored at each stop along the way before being forwarded to the next switching office. Because there is no limit on message size, the delays can be considerable on large messages because each switch will have to wait for the entire message to arrive before passing it on.
packet switching	Packets are fixed-size messages, and allow the switching station to begin forwarding the first packet of a message before the second has completely arrived. This contrasts to the message switching plan in that the entire message does not have to be received by one switching office before some of that message can be passed on the the next one.
store-and-forward	Waiting for each message or packet to arrive at a switching office in its entirety before forwarding it on the next location.

2.5 Narrowband ISDN
TERM	DEFINITION
Integrated Services Digital Network (ISDN)	fully digital, circuit switched telephone system that is designed to accommodate both voice and data services.
digital bit pipe	A bi-directional logical pipe which will pass bits on from source to destination without concern about whether they are voice or data.
NT1	Device placed between the user and the nearest end office; allows ISDN connection.
NT2 or Private Branch eXchange (PBX)	a device placed between the end user (with a larger/different demand than the home user--e.g., businesses), which can provide a variety of ISDN services.
Plain Old Telephone Service (POTS)	The telephone system we currently use. ISDN aims to replace the POTS with its digital voice systems.
Narrowband ISDN (N-ISDN)	ISDN services on 64-kbps channels.

2.6 Broadband ISDN and ATM
TERM	DEFINITION
Broadband ISDN (B-ISDN)	more recent design for a digital network (ISDN) on which data can travel at rates of 155Mbps. B-ISDN is based on ATM technology.
permanent virtual circuits	Virtual circuit that remains in place for extended periods of time (not related to session time).
switched virtual circuits	Virtual circuit that remains in place for short periods of time, typically related to the session time (built for a session, destroyed after it is over).
physical medium dependent	sublayer of ATM. This layer provides a uniform interface for ATM regardless of what the physical medium lies under the system.
switching fabric	The system that takes input from ATM lines, processes them, and prepares to forward them to their proper destination.
head-of-line blocking	If a questionable cell tries to come into an ATM switch, it will be stalled, effectively stalling the cells behind it unfairly. This is called head-of-line blocking.
knockout switch	To solve the head-of-line blocking problem, queuing on the output side has been proposed. The knockout switch does just this, by simulating a single output queue by having several output queues that are activated on a round-robin type basis.
Batcher-banyan switch	Like the knockout switch in that it is synchronous and accepts a set of cells per cycle; but it is much more complicated that the knockout switch.

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