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.
Manchester encoding
The digital transmission technique used in the physical layer of Ethernet LANs.
medium
The matter or substance that carries the voice or data transmission. For example, the medium can be copper (wires), glass (fiber-optic cables), or air (microwave or satellite).
microwave transmission
Microwave transmission is an extremely high-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points. As its name implies, a microwave signal is an extremely short wavelength, thus the word micro-wave. Microwave radio transmissions perform the same functions as cables.
modem
A contraction of the words modulator-demodulator. A modem is a device for performing necessary signal transformation between terminal devices and communication circuits. Modems are used in pairs, one at either end of the communication circuit.
multipoint / multipoint circuit
A line or circuit interconnecting several stations/nodes in a sequential fashion.
multiplexing (MUX)
The subdivision of a transmission circuit into two or more separate circuits. This can be achieved by splitting the frequency range of the circuit into narrow frequency bands (frequency division multiplexing) or by assigning a given circuit successively to several different users at different times (time division multiplexing).
parallel transmission
Describes the way the internal transfer of binary data takes place within a computer. It may be transmitted as a parallel word, but it is converted to a serial or bit-by-bit data stream for transmission.
phase
the direction in which the wave begins. Phase is measured in the number of degrees (°).
phase modulation (PM)
A form of modulation in which the phase of the carrier is varied in accordance with the instantaneous value of the modulating signal. Phase modulation has two related techniques. Phase shift keying (PSK) uses a 180° change in phase to indicate a change in the binary value (0 or 1), differential phase shift keying (DPSK) uses a 180° change in phase every time a bit is transmitted; otherwise, the phase remains the same.
phase shift keying (PSK)
Phase shift keying (PSK) uses a 180° change in phase to indicate a change in the binary value (0 or 1)
physical circuit
the actual wire—used to connect two devices
plain old telephone service (POTS)
enables voice communication between any two telephones within its network.
point-to-point circuit
Denoting a circuit or line that has only two terminals. A link. An example is a single computer connected to a mainframe.
polarity
plus or minus measurements (for electrical current)
pulse code modulation (PCM)
Representation of a speech signal by sampling at a regular rate and converting each sample to a binary number. In PCM, the information signals are sampled at regular intervals and a series of pulses in coded form are transmitted, representing the amplitude of the information signal at that time.
quadrature amplitude modulation (QAM)
QAM involves splitting the symbol into eight different phases (3 bits) and two different amplitudes (1 bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent 4 bits, while 256-QAM sends 8 bits per symbol. 64-QAM and 256-QAM are commonly used in digital TV services and cable modem Internet services.
quantizing error
The difference between the PAM signal and the original voice signal. The original signal has a smooth flow, but the PAM signal has jagged “steps.”
radio transmission
One of the most commonly used forms of wireless media is radio; when people used the term wireless, they usually mean radio transmission. When you connect your laptop into the network wirelessly, you are using radio transmission. Radio data transmission uses the same basic principles as standard radio transmission. Each device or computer on the network has a radio receiver/transmitter that uses a specific frequency range that does not interfere with commercial radio stations. The transmitters are very low power, designed to transmit a signal only a short distance, and are often built into portable computers or handheld devices such as phones and personal digital assistants.
retrain / turnaround time
The time required to reverse the direction of transmission from send to receive or vice versa on a half-duplex circuit.
satellite transmission
is similar to microwave transmission, except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space (geosynchronous satellite).
serial transmission
- Transmitting bits one at a time and in sequence. 2. The sequential or consecutive occurrence of two or more related activities in a single device or circuit.
simplex transmission
A circuit capable of transmission in one direction only. Contrast with full duplex and half-duplex.
Statistical time division multiplexing (STDM, Stat Mux)
Stat mux or STDM. A time division multiplexer (TDM) that dynamically allocates communication circuit time to each of the various attached terminals, according to whether a terminal is active or inactive at a particular moment. Buffering and queuing functions also are included. See also concentrator.
switch
Switches connect more than two LAN segments that use the same data link and network protocol. They may connect the same or different types of cable. Switches typically provide ports for 4, 8, 16, or 32 separate LAN segments, and most enable all ports to be in use simultaneously, so they are faster than bridges
symbol rate
number of symbols transmitted per second (the number of times per second the signal changes on the comm circuit).
Symbol rate
◦ How often symbols are sent
◦ e.g., with a symbol rate of 64 kilohertz (kHz), a symbol is sent every 1/64,000 of
a second
time division multiplexing (TDM)
A device that combines data traffic from several low-speed communication circuits onto a single high-speed circuit. In TDM, separate time segments are assigned to each terminal. During these time segments, data may be sent without conflicting with data sent from another terminal.
twisted pair cable (TP cable)
A pair of wires used in standard telephone wiring. They are twisted to reduce interference caused by the other twisted pairs in the same cable bundle. Twisted-pair wires go from homes and offices to the telephone company end office.
unicode
Unicode is the other commonly used coding scheme. There are many different versions of Unicode. UTF-8 is an 8-bit version, which is very similar to ASCII. UTF-16, which uses 16 bits per character (i.e., 2 bytes, called a “word”), is used by Windows. By using more bits, UTF-16 can represent many more characters beyond the usual English or Latin characters, such as Cyrillic or Chinese.
unipolar
A form of digital transmission in which the voltage changes between 0 volts to represent a binary 0 and some positive value (e.g., +15 volts) to represent a binary 1. See also bipolar transmission.
USB drive
A USB flash drive is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than 30 g.
V.44
V.44 is a data compression standard that can be combined with any of the foregoing types of modems to reduce the amount of data in the transmitted signal by a factor of up to six. Thus, a V.92 modem using V.44 could provide an effective data rate of 56,00 × 6 = 336, 000 bps.
Voice over Internet Protocol (VoIP)
is commonly used to transmit phone conversations over digital networks. VoIP is a relatively new standard that uses digital telephones with built-in codecs to convert analog voice data into digital data
wavelength division multiplexing (WDM)
WDM is a version of FDM used in fiber-optic cables. When fiber-optic cables were first developed, the devices attached to them were designed to use only one color of light generated by a laser or LED.
Light has different frequencies (i.e., colors), so rather than building devices to transmit using only one color, why not send multiple signals, each in a different frequency, through the same fiber-optic cable? By simply attaching different devices that could transmit in the full spectrum of light rather than just one frequency, the capacity of the existing fiber-optic cables could be dramatically increased, with no change to the physical cables themselves.
Wireless media
Wireless media are those in which the message is broadcast through the air, such as microwave or satellite.
Current
movement / flow of electrons (from positive to negative)
AC (alternating current)
travels first in one direction and then in the other
DC (direct current)
travels only in one direction
Voltage
electrical pressure - the amount of electrical force pushing electrons through a circuit
Ampers (amps)
Ampers (amps)
Watts
fundamental unit of electrical power; it is a rate unit not a quantity. W = V x Amps
Digital transmission
Sender and receiver must agree upon:
Set of Symbols
◦ How bits are encoded as voltages or light pulses
◦ e.g., +5V might be encodes as a “1”
Symbol Rate
◦ How often symbols are sent
◦ e.g., with a symbol rate of 64 kilohertz (kHz), a symbol is sent every 1/64,000 of a second
Unipolar
0 volts (no current) = 0 +5 volts = 1 the voltage is always positive or negative (like a DC current).
Bipolar NRZ
+5 volts = 1 −5 volts = 0 0 volts = never returns
1 and 0 vary from +V to -V (like AC current).
Bipolar RZ
+5 volts = 1 −5 volts = 0 0 volts = always returns after each bit
Bipolar AMI
+5 volts / −5 volts = 1 0 volts = 0
AMI is used on T1 and T3 circuits
Manchester Encoding
high to low = 0 low to high = 1
Ethernet (which is used in LAN)
Data Types
analog (wave based)
digital (binary)
Circuit Types
Physical (connect devices and includes wires)
Logical (to how fast data can be sent through the connection)
Physical and logical can be separate or the same. Logical determines how fast data can be sent through the connection.
Configuration Types
Point-to-point (wired)
Multipoint (wireless)
Data Flow Types
Simplex (one direction)
Half-duplex (both directions, one at a time)
Full duplex (both directions, simultaneously)
Multiplexing Types
Frequency / wavelength (FDM (channels, guard-bands), WDW (in fiber optic))
Time (TDM (equal), STDM (statistical)
Divides high-speed circuit into several slower (logical circuits). Cost efficient. Determines Internet speed. Inverse multiplexing
Media Types
Guided (physical medium (TP Twisted-Pair, coax, fiber optics)
Wireless / radiated (through the air, like microwave, satellite, radio, infrared)
Coding Schemes
ASCII (America Standard Code for Information Interchange). 2 types: 7-bit with 128 characters; 8-bit with 256 characters combinations
ISO 8859 8-bit code: ASCII + non-English letters (european)
Unicode: UTF-8, UTF-16 (cyrilic or chinese)
Transmission Modes
Parallel: multiple bits transmitted simultaneously
Serial: bits are transferred sequentially, one at a time
Data to Analog: Characteristics
Amplitude = height = volume/loudness (in decibels) Always 2 equal (same height) parts: positive is above 0, negative is below 0
Frequency = width/length = pitch (in hertz)
Inverse to length:
high Hz - many short waves in 1-sec interval, l
ow Hz - fewer but longer waves in 1-sec interval
Phase = direction (in degrees)
0 degrees phase wave - up and to the right
180 degrees - down and to the right
Modulation
Modulation is the modification of the carrier’s wave fundamental characteristic in order to encode info
Types of modulation
AM (Amplitude Modulation) = ASK (Amplitude Shift Keying) high (tallest wave) = 1 low (deepest wave = 0
FM (Frequency Modulation) = FSK (Frequency Shift Keying) high Hz (more waves per time period) = 1 low Hz (fewer waves) = 0
PM (Phase Modulation) = PSK (Phase Shift Keying) 0 degree (up/R) = 0 180 degree (down/R) = 1
QAM (Qaudrature Amplitute Modulation)
16QAM = PSK (8 phase/3 bits) x ASK (2 amplitude/1 bit)
16QAM = 4 bits; 16 different possible values
Quadrature amplitude modulation (QAM) is a very widely used digital modulation system for providing multiple symbols
Modem’s throughput is determined by 2 factors
- transmission rate
2. data compression (V.44, Lempel-Ziv encoding, 6:1)
Quantizing Error (analog to data)
The difference between the original analog signal and the approximated digital signal Error reduction can be done by sampling more frequently (oversampling) and using more amplitude levels in encoding
VoIP (Voice over Internet)
Transmits phone conversations over digital network Digital phones with built-in codecs, thanks to which the phone can be directly connected to LAN
Symbol
Symbol: one or more modifications to a carrier wave used to encode data
Send 1 bit by defining two different symbols (e.g., amplitudes, frequencies, etc.).
Send multiple bits by defining more than two symbols.
◦ Need more complicated information coding schemes
◦ 1 bit of information → 2 symbols
◦ 2 bits of information → 4 symbols
◦ 3 bits of information → 8 symbols
◦ n bits of information → 2n symbols
Satellite Transmission
Satellite transmission is similar to microwave transmission, except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space.
Geosynchronous means that the satellite remains stationary over one point on the earth. One disadvantage of satellite transmission is the propagation delay that occurs because the signal has to travel out into space and back to earth, a distance of many miles that even at the speed of light can be noticeable. Low earth orbit (LEO) satellites are placed in lower orbits to minimize propogation delay. Satellite transmission is sometimes also affected by raindrop attenuation when satellite transmissions are absorbed by heavy rain. It is not a major problem, but engineers need to work around it.
