Test 1 Study Guide Flashcards
What are the characteristics of a data communications system?
Delivery: The system must deliver data to the correct destination.
Accuracy: The system must transfer the data accurately, unaltered.
Timeliness: The data must arrive in a timely manner.
Jitter: There must be little jitter, that is, variations in packet arrival time.
What is simplex communication?
Simplex communication is when one “station” can send and the other can receive, but not the other way around. This is very rare nowadays, but it might be like how a mouse works with a computer.
What is half-duplex communication?
Half-duplex communication is when each station can send and receive, but not at the same time.
What is full duplex communication?
Full duplex communication is when both stations can send and receive at the same time.
What are the criteria for defining a network?
Performance, reliability, and security.
What are the main topologies?
Mesh topology
Star topology
Bus topology
Ring topology
What is a mesh topology?
In a mesh topology, ever device has a dedicated point-to-point link to every other device.
What is the star topology?
The star topology is where each device needs only one link and one I/O port to connect it to any others, generally using a hub.
What is the bus topology?
The bus topology uses one central line as a “backbone”, and then has nodes branching off of it using drop lines and taps.
What is the ring topology?
The ring topology has it so that each device connects to only two other devices, using dedicated point-to-point systems.
What are the layers of the TCP/IP model? Give an example of each.
Application - HTTP/FTP Transport - TCP/UDP Network - The Internet, Internet Protocol (IP), grouping of routers. DCHP Data Link - Routers, Link Layer Switch Physical - Cabling
What are the layers of the OSI model?
Application Presentation Session Transport Network Data Link Physical
How does the OSI model relate to the TCP/IP model?
In TCP/IP, the top three layers of OSI are combined; that is, application, presentation, and session in OSI are all combined into application in TCP/IP.
What is the difference between a reliable vs. unreliable connection?
A reliable connection’s protocol has error-correcting capabilities, as well as being able to retransmit information as needed. A reliable connection generally guarantees that there will be no delay, damage, or loss of packets, and that they will be delivered in order.
What is retransmission? What layer does it take place at?
Retransmission is the resending of packets which have been damaged or lost. This happens at the Transport layer.
What is unpacking?
Unpacking (and packing) occur between each layer in a protocol. As a packet moves up through protocols, each protocol puts its header on it and wraps the packet, and as it comes back down, it gets unwrapped.
Explain the difference between a digital and an analog signal?
An analog signal is contiguous, that is, it can be zoomed in on infinitely.
What do the horizontal and vertical axises represent on a time domain plot?
Horizontal represents time, and vertical represents signal amplitude.
What is frequency?
Frequency is the number of occurrences of a repeating event per unit of time.
What is amplitude?
Amplitude is the measure of the change of a repeating event over a single period.
What is phase?
Phase is the position of the waveform relative to time 0.
What is attenuation?
Attenuation is a loss of energy, caused by resistance in the transmission medium, which generates heat, which increases resistance.
What is peak amplitude?
Peak amplitude is the absolute value of a signal’s highest intensity.
What is period?
Period refers to the amount of time, in seconds, that a signal needs to complete one cycle.
What is frequency? What unit is it commonly expressed in?
Frequency is the number of periods in one second. It is generally measured in hertz (Hz).
What is distortion?
Distortion is whenever a signal’s shape changes. It generally is caused when composite signals have different frequencies, which may cause a difference when they arrive at the final destination. This can then cause a difference in phase.
What is noise?
Noise is when you receive alterations to the signal due to either heat (causing random motion of electrons), noise from electronics and motors, which act as a sending antenna, forcing the medium to act as a receiving antenna, and crosstalk, which is the result of wires being too close to one another. There is also impulse noise, which is due to a spike of excess power.
What is the Nyquist bit rate and how is it used?
The Nyquist bit rate formula defines the theoretical maximum bit rate.
BitRate = 2 x Bandwidth x log2L
Where L is the number of signal levels used to represent data.
What is the Shannon capacity, and how is it used?
The Shannon capacity is a formula that is used to determine the theoretical highest data rate for a noisy channel.
Capacity = Bandwidth x log2(1 + SNR)
SNR is the “signal-to-noise” ratio, and capacity is in bits-per-second.
What is line coding?
Line coding is the process of converting digital data to digital signals, that is, a series of bits to a digital signal.
What is block coding?
Block coding is a method for creating inherent error detecting and redundancy by taking a block of “m” bits and increasing it to “n” bits. The additional bits allow for prevention of sequential 0s and 1s, and control sequences.
What is scrambling?
Scrambling is a method to provide synchronization without increasing the number of bits. This is done at the same time as encoding.
Scrambling works by creating “syntax” of sorts with how ones and zeros are represented, and then signaling specific events, such as a large number of zeroes, by violating those rules in a specific way.
How would you calculate how many redundant bit combinations there are in a particular block coding form?
The number of usable bit combinations of the new encoding cannot be larger than the old; simply find the size of the new (2^x), and subtract the old (2^y).
What is NRZ?
NRZ stands for “Non-Return-to-Zero”. Non-return-to-zero means that the signal does not return to zero in the middle of the bit, and means that positive voltage represents “1”, and zero represents “0”.
1”
What is NRZ-I?
NRZ-I is non-return-to-zero Invert, meaning that whether or not a bit is zero depends not on the level of the signal, but on whether or not the signal changed levels at all. In NRZ-I, it has less issue with synchronization, in that a long string of “1”s will have the signal moving constantly, making it easy to follow.
What is NRZ-L?
NRZ-L stands for non-return-to-zero Level. In this NRZ schema, the bit is determined directly by the level of the signal.
What encoding method is used for gigabit ethernet?
Four-dimensional five-level pulse amplitude modulation (4D-PAM5).
4D-PAM5 sounds complex, but simply means that it has four dimensions (four transmission mediums, wires), and has five voltage levels (-2, -1, 0, 1, 2), although one level, zero, is only used for error detection. These five (four) levels then become very similar to 8B4Q (an 8-bit word translated to a signal element with four different levels, sent over four wires, representing two bits per wire.
What is pulse-code-modulation (PCM)?
Pulse-code modification is the most common technique to change an analog signal to digital data (also known as digitization). PCM encoding has three processes.
- The analog signal is modulated.
- The sampled signal is quantized.
- The quantized values are encoded into streams of bits.
What is sampling?
Sampling occurs by taking an interval of a signal and determining the signal’s level at that point.
What is the idea sampling rate?
The ideal sampling rate was set down by Nyquist, the sampling rate must be at least twice as high as the highest frequency in the original signal.
What is quantization?
Quantization is a necessity born of the contiguous nature of the analog signal. Where a digital signal can be distilled down to finite points, an analog signal cannot, as it is infinite in nature. Quantization constrains the values in a sample to a discrete set.
What is Delta Modulation (DM)?
Delta Modulation is a simpler method to convert analog signals into digital data, compared to PCM. Delta modulation works by comparing the current signal to the previous signal, then determining the value from that. If the change is positive, a 1 is sent. If negative, a 0. This is compared to a staircase signal, which is built as a stair-stepped line, going up each time a 1 is sent, and down every time a 0 is sent.
What are the four types of digital to analog conversions?
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PSK)
Quadrature amplitude modulation (QAM)
What is shift keying?
Shift keying is the process by way a carrier signal (a high-frequency signal created as a base for information) is modulated in some way (either by modifying amplitude, frequency, or phase) in order to represent data.
What is amplitude shift keying?
This is done by varying the amplitude of the signal between two levels, although it can be done with more. Multiple amplitudes can be used to represent multiple combinations of bits.
What is frequency shift keying?
Frequency shift keying is modulating the frequency of the carrier signal in order to denote changes in value. Generally, two carrier frequencies (that is, two different set frequencies of the carrier) to denote one and zero.
What is phase shift keying?
Phase shift keying is done by modulating the phase from 180 degrees to zero. While the phase is at 180, the signal is one, and vice-versa. Phase shift keying is highly resistant to noise.
What is quadrature amplitude modulation (QAM)?
Quadrature amplitude modulation is a method of converting digital data to an analog signal by combining amplitude shift keying and phase shift keying. Where PSK has the benefit of having a resistance to noise, it’s more technically difficult to pick up in phase shifts. QAM uses two carriers, one in-phase and the other quadrature, each with different amplitude levels.
What are the three main forms of analog-to-analog conversion?
Amplitude modulation (AM) Frequency modulation (FM) Phase modulation (PM)
What is amplitude modulation? How much bandwidth is allocated to each channel?
Amplitude modulation is a form of analog-to-analog conversion, wherein the carrier signal is modulated so that its amplitude varies with the changing amplitude of the modulating signal.
The bandwidth of AM for an audio signal is generally 5 kHz, therefore, it requires a total bandwidth of 10 kHz
What is frequency modulation? How much bandwidth does an FM channel take up?
Frequency modulation is an analog-to-analog conversion, wherein the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. Amplitude and phase remain constant.
The bandwidth required for audio signal in stereo is nearly 15 kHz, but the FCC allows for 200 kHz per station (to prevent overlap).
What is phase modulation?
Phase modulation is an analog-to-analog conversion method, wherein the phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. PM is basically the same as FM.
What’s the relationship between baud rate and bit rate?
Data rate: n Signal rate (baud): s
s = n/r
Where r has been previously defined. Just remember that they’re proportional.
Define bit rate.
Bit rate is the number of data elements (0s and 1s) sent in 1 second.
Define baud rate. What else is it called?
Baud rate, also known as pulse rate, signal rate, or modulation rate, is the number of signal elements sent in 1 second.
Define multiplexing.
Multiplexing is the technology or techniques that allow multiple signals to be sent over a single data link.