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IT Infrastructure > L3 The Physical Layer > Flashcards

L3 The Physical Layer Flashcards

0
Q

Describe a circuit and the differences between physical circuits and logical circuits.

A

A circuit interconnects two data link entities.
The physical circuit is the actual mechanical or electrical interconnection such as a wire, optical cable, physical check.
The logical circuit is an interconnection abstracted in terms of its characteristics such as bandwidth, modulation, reach, and symbol rate

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1
Q

Describe the physical layer

A

The physical layer is concerned with the physical interface connection between devices and transmission of bits (1&0) across the link. It can be mechanical, electrical, optical, or functional.

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2
Q

Describe analog data transmission

A

Analog data transmission uses a signal that is continuous in time.
The signal can have any value between defined upper and lower bounds. Signal shape may be arbitrary

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3
Q

Describe digital data transmission

A

Digital data transmission uses a signal that has limited defined values. Values represent either zero or one. The signal shape is a series of pulses.

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4
Q

What are the advantages of digital data transmission over analog data transmission?

A
  • Fewer errors (only 0s & 1s transmitted easier to detect and correct errors).
  • Higher transmission rate
  • More efficient use of transmission media
  • more secure (digital data is easier to encrypt)
  • enables convergence (voice, video, data in single form is easier to integrate)
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5
Q

What are the two fundamental circuit configurations?

A

Point-to-point and multipoint

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6
Q

Describe point-to-point circuit configuration

A

A point-to-point connection (also called dedicated circuits - direct client to server) is required only for very high producers or consumers of data.
Communications consume most of the capacity of the link.
It is too expensive to be practical solution for any deployment of appreciable scale
Common connection type in modern wired networks. The physical link is used only by two devices.

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7
Q

Describe multipoint circuit configuration

A

There are multiple hosts on the network issue of which consumed a portion of the available medium capacity.
Much cheaper than point-to-point
Only a single host can use the medium at one time.

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8
Q

What are the three types of the data flow transmission characterizations?

A
  1. Symplex transmission: Data flows in one direction only (radio, TV)
  2. Half duplex transmission: data flows in both directions, one direction at a time. (Walkie-talkie)
  3. Full-duplex transmission: data flows in both directions simultaneously with no turnaround time.
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9
Q

Define turnaround time (also called retrain time or reclocking time)

A

The amount of time communication takes to switch between sending and receiving data.

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10
Q

True or false, in any modern circuit, the capacity (bandwidth) of the link is not divided when the link is operated in full-duplex mode.

A

True

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11
Q

What is multiplexing?

A

Multiplexing means to break one high-speed physical communication circuit into several lower speed logical circuits so that many different devices can simultaneously to use it but still “think” that they have their own separate circuits.

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12
Q

Draw the multiplexing diagram

A

Multiple clients lead to mux distributed over a high bandwidth link to another mux where it is partitioned to multiple servers.

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13
Q

What are the four types of multiplexing?

A
  1. Frequency division multiplexing
  2. Time division multiplexing (synchronous)
  3. Statistical time division multiplex thing
  4. Wavelength division duplexing
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14
Q

True or false, multiplexing is often done in multiples of 4.

A

True

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15
Q

What are the benefits of multiplexing?

A

The primary benefit of multiplexing to save money by reducing the amount of cable or the number of network circuits that must be installed.

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16
Q

True or false, frequency division multiplexing can be described as dividing the circuit HORIZONTALLY so that many signals can travel a single communication circuit simultaneously. All signals exist in the media at the same time.

A

True

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17
Q

Describe frequency division multiplex (FDM)

A

With the frequency division multiplexing the total bandwidth of the median is divided into smaller bands. Each channel is allocated the bandwidth of a single band. Guardbands are needed to prevent interchannel interference. Some examples include cable TV and ADSL

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18
Q

What does mux stand for?

A

Multiplexing

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19
Q

Describe Synchronous time division multiplex (STDM)

A

Medium capacity divided into equal length time slots. Each device takes over the entire medium while it is transmitting
Each device gets the same length of time to transmit on each slot
Slot allocated even if device has no data to transmit
This technique is synchronous
Each end must be perfectly time-aligned

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20
Q

Is the synchronous time division multiplex more efficient than the frequency division multiplex?

A

Yes, the synchronous time division multiplex does not need guard bands which makes it a more efficient use of resources; however, a time slot is still allocated to a device even if it is not in use.

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22
Q

Describe statistical time division multiplex (STDM)

A

Mux only transmits data for a device when that device has data to send

  • Does not waste medium capacity with empty data time slots
  • Mux may drop traffic if statistical assumptions are violated
  • An address field must be added to the data so that the sender can be identified.
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23
Q

What are the advantages and disadvantages of statistical time division multiplex?

A

Advantages

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24
Q

Describe wavelength division multiplexing

A
  • uses light
  • A different coloured laser (lambda) Is used for each channel.
  • fiber-optic cable is used as the transmission medium (622 Mb/s)
  • different channels can have different bandwidth
25
Q

Describe dense wavelength division multiplexing

A
  • uses a large number (up to 160/ fiber-optic cable) of closely spaced channels by adding Time Division Multiplexing (TDM) to Wavelength Division Multiplexing (WDM)
  • each channel has the bandwidth of 10 GB/s
26
Q

What is inverse multiplexing?

A

Inverse multiplexing is a method of aggregating two or more low bandwidth links to allow the transmission of a high data rate stream that could not be carried across a single low bandwidth link.

27
Q

What does BONDING stand for? Describe it.

A

Bandwidth on Demand Interoperability Networking Group. BONDING splits outgoing messages from one client or host across several low-speed telephone lines and combines incoming messages from several telephone lines into one circuit so that the client or host “thinks” it has a faster circuit.
Commonly used for videoconferencing.

28
Q

What is the difference between guided and wireless communication media?

A

With GUIDED media messages flow through a physical medium such as a twisted pair wire, coaxial cable, or fiber-optic cable.
WIRELESS media are those in which the message is broadcast through the air, such as wifi, microwave or settlement satellite.

29
Q

Communication media: define medium

A

Physical mechanism by which a communication signal propagates from sender to receiver.

30
Q

Communication media: what are the two types of twisted-pair cables. Define each.

A

Unshielded twisted pair (UTP): two insulated copper wires twisted along their length. The twisting reduces the effects of electromagnetic interference. Bundles are not shielded.
Shielded twisted pair (STP): hired three seafood configurations similar to UTP. More twists per inch. Bundles are shielded. Used for higher data rate connections.
- range 100m

31
Q

Communication media: describe coaxial (COAX) cables

A

consists of an inner solid copper conductor surrounded by an outer cylindrical conductive shield.

  • very expensive compared to UTP
  • Used in early ethernet installations
  • range 500m
32
Q

Communication media: describe fiber-optic cable

A

components consist of: under jacket, protective sheathing, buffer, cladding, Core fiber.

  • The inner core diameter hurries from 9 to 62.5 microns (human hair is 30 microns).
  • Data is transmitted via a beam of IR light reflected through the core.
  • used extensively in carrier backhaul networks.
33
Q

What are the three types of fiber-optic cable? Described each.

A 
MULTIMODE STEP INDEX
- Diameter: large (50-62.5 µm)
- light source: LED
- high attenuation (Signal weakening) Limited range 500 m.
MULTIMODE GRADED INDEX
- Diameter: large (50-62.5 µm)
- light source: LED
- Medium attenuation (Signal weakening) range < 1000 m.
SINGLE-MODE
- Diameter: small (8.5 µm)
- light source: laser
- low attenuation (Signal weakening) range 50-100 km.
34
Q

Describe and draw a diagram depicting the levels of dispersion for each of the three types of fiber-optic cables

A

Multimode Step Index: light takes several paths through the core. Large dispersion /\/\/\/. Pulse out is wide and short because light does not exit at the same time.
Multimode Graded Index: Changing Refractive Index across core allows like taking a different path to exit at almost the same time. Medium dispersion. Pulse out not as round and not as short.
Single-Mode: late one path through core from end to end. Almost no dispersion.

35
Q

What are the advantages and disadvantages of fiber-optic cables?

A 
Advantages: 
- Supports very long cable runs with the repeaters.
- Very high-bandwidth
- Very light
- weather resistant
- highly secure
Disadvantages:
- expensive
- patch cables are delicate
36
Q

How odes the multimode graded index reduce the level of dispersion?

A

As the light approaches the outer edge of the fiber, it speeds up, which compensates for the slightly longer distance it must travel compared with light in the center of the fiber.

37
Q

Communication media: wireless/radiated radio is a form of _________________ that travels through space at what speed?

A

wireless radio is a form of ELECTROMAGNETIC RADIATION that travels through space at THE SPEED OF LIGHT.

38
Q

Communication media: how does wireless radio work?

A

Radio requires a transmitter and a receiver (transceiver) in each device that is modulated to carry data (0’s and 1’s)

39
Q

Give some examples of the uses of radio

A

Wireless LAN
bluetooth
Microwave
Satellite

40
Q

Microwaves are radio waves with a wavelength between ____ and ____. Microwaves require ________ and signals can be impacted by _______.

A

1 cm to 1 m. Require line of sight. weather (water can absorb microwaves)

41
Q

Communication media: describe wireless satellite communications

A

Satellite communications use microwaves. Satellites may be in geostationary orbit (GEO at 35,000 km) or in low Earth orbit (LEO under 2000 km)

42
Q

What are some of the issues with wireless satellite communications?

A
  • Satellites are very expensive to put into orbit.
  • GEO satellites have significant communication latency (.24s one way)
  • Microwave signals may be absorbed by water/weather (known as raindrop attenuation).
43
Q

When selecting a form of media which factors should be considered?

A
  • type of network(LAN OR WAN)
  • cost (based on distance, speed, and reliability, acquisition, installation and maintenance)
  • transmission distance
  • security
  • error rates
  • transmission speed
44
Q

What is a coding scheme?

A

The coding scheme is the language that computers use to represent data.

45
Q

Name the three predominant coding schemes in use today.

A
  1. American Standard Code for Information Interchange (ASCII)
    - 8 bit code. Most common.
  2. International Standards Organization (ISO)/IEC 8859-n
    - used to represent characters in languages other than English
  3. Unicode UTF-n
    - unifies other schemes. Dominant on the web
46
Q

Digital data transmission: describe the difference between parallel and serial transmission.

A

Parallel Transmission: requires a separate cable for each bit for in the data unit to be transmitted (8 bits requires 8 separate cables) all bits are sent at the same time.
Serial transmission: uses a single cable to transmit one bit or character at a time. Widely used in data communication.

47
Q

Digital data transmission: Describe signalling over wire

A

Change voltage level to represent two states (one voltage used to represent 0, and another used to represent 1).

48
Q

One of the five types of digital transmission techniques?

A
  1. unipolar
  2. Bipolar non-return to zero
  3. Bipolar return to zero
  4. Alternate Mark Inversion
  5. Manchester encoding
49
Q

Digital data transmission techniques: Draw Unipolar.

A

0V = 0, +nV = 1

50
Q

Digital data transmission techniques: Draw Bipolar non-return to zero.

A

-nV = 0, +nV = 1

The larger separation of volts makes it more difficult for the signal become corrupted

51
Q

Digital data transmission techniques: Draw Bipolar return to zero.

A

-nV = 0, +nV = 1 (returns to zero after each symbol is transmitted)

52
Q

Digital data transmission techniques: Draw Alternate Mark Inversion.

A

0V = 0, non-zeroV = 1 (always opposite polarity to the last 1 bit transmitted)

53
Q

Digital data transmission techniques: Draw Manchester encoding.

A

lowV to highV = 1, highV to lowV = 0 (Data transmission always happens the mid-bit position).

  • Voltage level does not represent any bit value.
  • error detection is easier
  • used in ethernet
54
Q

Describe Analog Data Transmission

A

Analog transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern (sine wave) much like the human voice.

55
Q

Analog Data transmission: Describe a carrier wave.

A

A carrier wave (typically a sinusoidal wave) is used to piggyback data based on wave amplitude, frequency, and phase (360 degrees in a wave).

56
Q

Analog Data transmission: Define modulation and list three types of modulation.

A

Modulation is the art of changing the carriers attributes. The three types of modulation are:

  1. amplitude modulation (AM)
  2. frequency modulation (FM)
  3. phase modulation
57
Q

Analog Data transmission: describe amplitude modulation (AM) on the wire

A

low amplitude = 0

high amplitude = 1

58
Q

Analog Data transmission: describe frequency modulation (FM) on the wire

A

low frequency = 0

high frequency = 1

59
Q

Analog Data transmission: describe Phase modulation on the wire

A

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IT Infrastructure (4 decks)

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