1. Physical Layer Flashcards

Question

digital subscriber line / DSL

Answer 1

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).

Answer 2

A transmission medium that uses glass or plastic cable instead of copper wires.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

The capability of transmission in both directions at one time. Contrast with half-duplex and simplex.

Answer 8

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

Answer 9

A circuit that permits transmission of a signal in two directions but not at the same time. Contrast with full duplex and simplex.

Answer 10

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

Answer 11

Same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second.

Answer 12

Kilohertz; 1 KHz is equal to 1,000 cycles per second in a frequency.

Answer 13

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).

Answer 14

The part of a communication circuit between the subscriber’s equipment and the equipment in the local central office.

Answer 15

to how fast data can be sent through the connection.

Answer 16

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.

Answer 17

The digital transmission technique used in the physical layer of Ethernet LANs.

Answer 18

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).

Answer 19

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.

Answer 20

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.

Answer 21

A line or circuit interconnecting several stations/nodes in a sequential fashion.

Answer 22

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).

Answer 23

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.

Answer 24

the direction in which the wave begins. Phase is measured in the number of degrees (°).

Answer 25

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.

Answer 26

Phase shift keying (PSK) uses a 180° change in phase to indicate a change in the binary value (0 or 1)

Answer 27

the actual wire—used to connect two devices

Answer 28

enables voice communication between any two telephones within its network.

Answer 29

Denoting a circuit or line that has only two terminals. A link. An example is a single computer connected to a mainframe.

Answer 30

plus or minus measurements (for electrical current)

Answer 31

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.

Answer 32

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.

Answer 33

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.”

Answer 34

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.

Answer 35

The time required to reverse the direction of transmission from send to receive or vice versa on a half-duplex circuit.

Answer 36

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).

Answer 37

1. 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.

Answer 38

A circuit capable of transmission in one direction only. Contrast with full duplex and half-duplex.

Answer 39

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.

Answer 40

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

Answer 41

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

Answer 42

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.

Answer 43

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.

Answer 44

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.

Answer 45

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.

Answer 46

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.

Answer 47

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.

Answer 48

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

Answer 49

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.

Answer 50

Wireless media are those in which the message is broadcast through the air, such as microwave or satellite.

Answer 51

movement / flow of electrons (from positive to negative)

Answer 52

travels first in one direction and then in the other

Answer 53

travels only in one direction

Answer 54

electrical pressure - the amount of electrical force pushing electrons through a circuit

Answer 55

Ampers (amps)

Answer 56

fundamental unit of electrical power; it is a rate unit not a quantity. W = V x Amps

Answer 57

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

Answer 58

``` 0 volts (no current) = 0 +5 volts = 1 the voltage is always positive or negative (like a DC current). ```

Answer 59

+5 volts = 1 −5 volts = 0 0 volts = never returns 1 and 0 vary from +V to -V (like AC current).

Answer 60

+5 volts = 1 −5 volts = 0 0 volts = always returns after each bit

Answer 61

+5 volts / −5 volts = 1 0 volts = 0 AMI is used on T1 and T3 circuits

Answer 62

high to low = 0 low to high = 1 Ethernet (which is used in LAN)

Answer 63

analog (wave based) | digital (binary)

Answer 64

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.

Answer 65

Point-to-point (wired) | Multipoint (wireless)

Answer 66

Simplex (one direction) Half-duplex (both directions, one at a time) Full duplex (both directions, simultaneously)

Answer 67

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

Answer 68

Guided (physical medium (TP Twisted-Pair, coax, fiber optics) Wireless / radiated (through the air, like microwave, satellite, radio, infrared)

Answer 69

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)

Answer 70

Parallel: multiple bits transmitted simultaneously Serial: bits are transferred sequentially, one at a time

Answer 71

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

Answer 72

Modulation is the modification of the carrier's wave fundamental characteristic in order to encode info

Answer 73

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

Answer 74

1. transmission rate | 2. data compression (V.44, Lempel-Ziv encoding, 6:1)

Answer 75

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

Answer 76

Transmits phone conversations over digital network Digital phones with built-in codecs, thanks to which the phone can be directly connected to LAN

Answer 77

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

Answer 78

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.

1. Physical Layer Flashcards

(102 cards)