Flashcards in Quiz 1 Content (Ch. 1&2) Deck (54):
Define: Distributed Systems
a computer network made to appear as a single coherent entity
Define: Computer Networks
Autonomous computers that are often connected by a single technology
Identify the two contrasting categories of network hardware technologies.
broadcast vs point-to-point
In a short phrase, indicate the essential conclusion from theoretical work in data communication during the 1920s and 1940s
For fibre optical technology,
the benefit is?
the challenge/ limitation is
benefit -> substantial capacity (how much info you send)
challenge -> electro-optical conversion
Identify the two promary design issues for the data link layer
Error handling and flow control
Identify four generally accepted levels of networking based on scope of area
PAN (personal area network)
LAN (local area network)
MAN (metropolitan area network)
WAN (wide area network)
An agreement between communication entities or roles and conventions to follow
The two forms of communication that can be supported by protocols are
connection-oriented and connectionless
The Nyquist formula for maximum data rate depends on two aspects:
channel bandwidth and number of discrete signal levels
Whaat is a key physical difference of coaxial cable related to twisted pair?
And what is the benefit?
difference -> Shielding
Benefit -> More bandwidth
What is the main reason for the layered architecture used in networking?
To reduce complexity (details of each layer hidden from the layer above)
Identify issues that protocols in various network layers should address
- flow control
- error detection
- quality of service
- data integrity
The shannon formula for maximum data rate depends on two aspects
channel bandwidth and signal-to-noise ratio
A single-bit-correcting Hamming code has m message bits and r redundant bits. How many valid code words?
Invalid but correctable code words?
valid -> 2^m
invalid -> nx(2^m)
where n = r + m
What technique can help address the possibility of burst errors?
Interleaving (reordering of bits)
Compare the achknowledgement frame contents between Protocol 2 (simplex stop-and -wait) and Protocol 3 (PAR). Identify any difference(s) between them.
Stop-and-wait: ack frame itself is sufficient - no contents needed
PAR: need sequence number information in the ack frame because errors can occur
In Protocol 3 (PAR), how does the receiver know that an ack frame transmission sicceeded? A brief but technically pecific answer is expected for this question.
For PAR, sequence number must be 0 or 1. After reciever transmits an ack, if it recieves a message with sequenece number opposity of the last ack, that means sender has seen ack and advanced to next packet.
What is the purpose of the physical layer?
Transport bits from one machine to another
What is the most important element that makes packet delivery possible?
What did Nyguist realize (in 1942)?
That a perfect channel has a finite capacity for data transmission
What did shannon do in 1948
Carried Nyguist's idea to a noisy channel and also proved important sampline theorem
Why are twisted wire pairs used?
Cancel radiation and use differential signaling for immunity to noise
What is the crudest (most basic form) of Guided transmission media?
vehicle carrying recorded media
What is the advantage of using coaxial cables?
is shielded for higher bandwidth
It has better shielding and greater bandwidth than unshielded twisted pairs, so it can span longer distances at higher speeds
What is the advantage and disadvantage of using existing high/ low voltage power lines?
adv: data signal super emposed on the power signal (both signals use the wiring at the same time)
dis: designed to distribute power signals not data and electical properties of wiring vary from house to house so the data signals may bounce around the wiring
What are some issues related to copper vs fibre
legacy installation, cost, weight, bandwidth
WHat are the two possible signal orientations for wireless transmission?
omnidirectional transmission (ie broadcasting -> sending to all directions from source)
Narrow-beam (one direction)
What are the different frequencies that the Electromagnetic specturm transmits?
Narrow-band (vary phase/ amplitude)
Wide-Band (one signal frequency)
What does it mean that radio waves are frequency dependent? -> path loss in radio transmission
At low frequencies - radio waves pass through obstacles well but the power falls sharply with distance from the source
At high frequencies - travel in straight lines and bounce off obstacles
At all frequencies, radio waves are subject to interference from motors and other electrical equipment
What is multipath fading?
A effect (and serious proplem) that can happen to microwaves.
When waves are refracted off low-lying atmospheric layers and take slightly longer to arrive then direct waves. The delayed waves arrive out of phase with the direct waves and thus cancel out the signal.
What are infrared waves used for?
- directional, cheap, they do not pass through solid objects (means that can't interfere with systems in adjacent rooms and security against evesdropping is better than other systems)
What are communication satellites?
Narrow-area/Broad-area repeaters, with or without processing before rebroadcast
narrow-area (only hundreds of kilometers in diameter) or broad-area (covering a fraction of the earth's surface) repeaters, with or without broadcasting before rebroadcast
What are some factors related to how 'useful' a satellite is?
- the orbital period of the satellite varius depending on the radius (higher the satellie = longer the period) -> low orbital satellites pass out of view fairly quickly
- more important issue is the presence of the Van Allen belts, layers of highly charged particles trapped by the earths magnetic field. Any satellite flying within them would be destroyed very quickly
What is a satellites footprint?
satellites spatial beam illuminating about 1/3 of the earths surface
What is digital modulation?
The process of converting between bits and signals tha represent them
To send digital information we must
devise analog signals to represent bits
What is baseband transmission?
Schemes that directly convert bits into a signals
-> common for wires
Scemes that regulate the amplitude, phase or frequency of the carrier signal to convey bits
-> common for wireless and optical signals
What is multiplexing?
Channels shared by multiple signals, use single wire to carry several signals
What is the symbol rate?
The rate at which the signal changes
What is the bit rate?
Bit rate = symbol rate * # bits per symbol
Management/ regulation of the spectrum. What does everyone want? and how does the government regulate this?
Everyone wants higher data, everyone wants more specturm.
Government deides who gets what frequency:
-> Beauty contest: requires each carrier to explain why its proposal serves the public interest best
-> Lottery: holding a lottery amoung the interested companies
-> Auction: $$$
What is ISM?
free zone, everyone transmit at will
Describe laser communication links?
narrow beam, easy to step up, hard to aim
What is Frequency division multiplexing?
Takes advantage of passbad transmission to share a channel
Divides the spectrum into frequency bands, with each user having exclusive possession of some band in which to send their signal
What is time division multiplexing?
Users take turns, (round robin style) each one periodically getting the entire bandwidth for a little burst of time
What id CDMA?
Code Division Multiplexing Algorithm
Used to allow each station to transmit over the entire frequency spectrum all the time
What is Bandwidth? and describe one strategy for using limited bandwidth more efficiently
Bandwidth is a fundamental limit for how fast a modulation can run (Nyquist rate)
One strategy: use more than two signaling levels (# of bits per symbol> 1)
Why is clock recovery required?
Reciever needs to know when one symbol ends and another begins
Accurate clocks are expensive
What is Manchester encoding?
Part of clock revocery
Mix the clock signal with the data signal by XORing them, no extra line is needed
- requires double the bandwidth as NRZ
What is NRZI?
Simplify the clock recovery situation by coding a 1 as a transition and a 0 as no transition
What are balanced signals?
Have as much positive voltage as negative voltage, average to zero
No DC electrical component
Helps to provide transitions for clock recovery