1. Physical Layer Flashcards
ADPCM
Adaptive differential pulse code modulation (ADPCM) is the alternative used by IM and many other applications that provide voice services over lower-speed digital circuits.
ADPCM works in much the same way as PCM. It samples incoming voice signals 8,000 times per second and calculates the same 8-bit amplitude value as PCM.
However, instead of transmitting the 8-bit value, it transmits the difference between the 8-bit value in the last time interval and the current 8-bit value (i.e., how the amplitude has changed from one time period to another). Because analog voice signals change slowly, these changes can be adequately represented by using only 4 bits. This means that ADPCM can be used on digital circuits that provide only 32 Kbps (4 bits per sample × 8, 000 samples per second = 32, 000 bps).
ASCII
American Standard Code for Information Exchange
AM
Amplitude Modulation
ASK
Amplitude Shift Keying
Amplitude
Amplitude = height = volume/loudness (in decibels)
Analog Transmission
Transmission of a continuously variable signal as opposed to a discrete on/off signal. The traditional way of transmitting a telephone or voice signal is analog.
Attenuation
As a signal travels through a circuit, it gradually attenuates or loses power. Expressed in decibels, attenuation is the difference between the transmitted and received power caused by loss of signal strength through the equipment, communication circuits, or other devices.
bandwidth
The difference between the highest and lowest frequencies in a band. For example, a voice-grade circuit has a 4,000-Hz bandwidth. In common usage, bandwidth refers to circuit capacity; when people say they need more bandwidth, they need a higher transmission speed.
baud rate
unit of signaling speed
bipolar
A method of digital transmission in which binary 0 is sent as a negative pulse and binary 1 is sent as a positive pulse.
bit
unit of information; 1. An abbreviation of the term binary digit. 2. A single pulse in a group of pulses. 3. A unit of information capacity.
bit rate
The rate at which bits are transmitted over a communication path, normally expressed in bits per second (bps). The bit rate should not be confused with the data signaling rate (baud), which measures the rate of signal changes being transmitted. See also bps.
bits per second (bps)
Bits per second. The basic unit of data communication rate measurement. Usually refers to rate of information bits transmitted.
carrier wave
Carrier wave: the basic wave transmitted through a circuit
An analog signal at some fixed amplitude and frequency that then is combined with an information-bearing signal to produce an intelligent output signal suitable for transmission of meaningful information. Also called carrier wave or carrier frequency
byte
A small group of data bits that is handled as a unit. In most cases, it is an 8-bit byte and it is known as a character.
character
A member of a set of elements used for the organization, control, or representation of data. Characters may be letters, digits, punctuation marks, or other symbols. Also called a byte.
circuit configuration
The actual or practical layout of a network that takes into account its software, hardware, and cabling. Configurations may be multidrop, point-to-point, LANs, and so on. By contrast, a topology is the geometric layout (ring, bus, star) of the configuration. Topologies are the building blocks of configurations. Compare with topology.
circuit
The path over which the voice, data, or image transmission travels. Circuits can be twisted-wire pairs, coaxial cables, fiber-optic cables, microwave transmissions, and so forth. Compare with channel, line, and link.
coaxial cable
An insulated wire that runs through the middle of a cable. A second braided wire surrounds the insulation of the inner wire like a sheath. Used on LANs for transmitting messages between devices
codec
A codec translates analog voice data into digital data for transmission over computer networks. Two codecs are needed—one at the sender’s end and one at the receiver’s end.
coding scheme / code
A transformation or representation of information in a different form according to some set of preestablished conventions. See also ASCII and EBCDIC.
data compression
The actual or practical layout of a network that takes into account its software, hardware, and cabling. Configurations may be multidrop, point-to-point, LANs, and so on. By contrast, a topology is the geometric layout (ring, bus, star) of the configuration. Topologies are the building blocks of configurations. Compare with topology.
data rate
number of bits transmitted per second.
Data rate = number of bits x max symbol rate
The data rate (or bit rate) is calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate. As we discussed in the previous section, the number of bits per symbol depends on the modulation technique (e.g., QAM sends 4 bits per symbol).
dedicated circuit
A leased communication circuit that goes from your site to some other location. It is a clear, unbroken communication path that is yours to use 24 hours per day, 7 days per week. Also called a private circuit or leased circuit.
digital subscriber line / DSL
A data link layer technology that provides high-speed (“broadband”) communication over traditional telephone lines. A DSL modem is used to provide three channels: a traditional voice channel, an upstream channel for communicating from the client to the ISP (often at speeds of 64–640 Kbps), and a downstream channel for communicating from the ISP to the client (often at speeds of 640 Kbps–6 Mbps).
fiber-optic cable
A transmission medium that uses glass or plastic cable instead of copper wires.
fiber-optics
A transmission technology in which modulated visible lightwave signals containing information are sent down hair-thin plastic or glass fibers and demodulated back into electrical signals at the other end by a special light-sensitive receiver.
frequency division multiplexing (FDM)
FDM can be described as dividing the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously. The circuit is divided into a series of separate channels, each transmitting on a different frequency, much like a series of different radio or TV stations. All signals exist in the media at the same time, but because they are on different frequencies, they do not interfere with each other.
frequency shift keying (FM = FSK)
FM (also called frequency shift keying [FSK]) is a modulation technique whereby each 0 or 1 is represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does not vary. One frequency (i.e., a certain number of waves per second) is the symbol defined to be a 1, and a different frequency (a different number of waves per second) is the symbol defined to be a 0.
frequency
The rate at which a current alternates, measured in Hertz, kilohertz, megahertz, and so forth. Other units of measure are cycles, kilocycles, or megacycles; hertz and cycles per second are synonymous.
full-duplex transmission
The capability of transmission in both directions at one time. Contrast with half-duplex and simplex.
guided media
Guided media are those in which the message flows through a physical medium such as a twisted pair wire, coaxial cable, or fiber-optic cable; the medium “guides” the signal
half-duplex transmission
A circuit that permits transmission of a signal in two directions but not at the same time. Contrast with full duplex and simplex.
ISO 8859
A second commonly used coding scheme is ISO 8859, which is standardized by the International Standards Organization. ISO 8859 is an 8-bit code that includes the ASCII codes plus non-English letters used by many European languages
Hertz
Same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second.
kiloHertz
Kilohertz; 1 KHz is equal to 1,000 cycles per second in a frequency.
Kilohertz; 1 KHz is equal to 1,000 cycles per second in a frequency.
V.44, the ISO standard for data compression, uses Lempel–Ziv encoding. As a message is being transmitted, Lempel–Ziv encoding builds a dictionary of two-, three-, and four-character combinations that occur in the message. Anytime the same character pattern reoccurs in the message, the index to the dictionary entry is transmitted rather than sending the actual data. The reduction provided by V.44 compression depends on the actual data sent but usually averages about 6:1 (i.e., almost six times as much data can be sent per second using V.44 as without it).
local loop
The part of a communication circuit between the subscriber’s equipment and the equipment in the local central office.
logical circuit
to how fast data can be sent through the connection.
malware
Malware is any software intentionally designed to cause damage to a computer, server, client, or computer network. A wide variety of malware types exist, including computer viruses, worms, Trojan horses, ransomware, spyware, adware, rogue software, and scareware.
