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Hardware Building Blocks and Encoding
COM211 Communications and NetworksCDA CollegePelekanou OlgaEmail: [email protected]/site/info-com-technology-ll/
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PerformanceBandwidth: number of bits per time unit.
We can talk about bandwidth at the physical level, but we can also talk about logical process-to-process bandwidth.
Latency: time taken for a message to travel from one end of the network to the other.
1
(a)
1
(b)
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LatencyQueueTransmitnPropagatioLatency
a vaccum in /103.0
cablea in /102.3
fibera in /102.0
light of Speed8
8
8
sm
sm
sm
Bandwidth/SizeTransmit
light of Speed / DistancenPropagatio
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Delay and Bandwidth
Bandwidth
Delay
This product is analogous to the volume of a pipe or the number of bits it holds. It corresponds to how many bits the sender must transmit before the first bit arrives at the receiver.
Delay may be thought of as one-way latency or round-trip time (RTT) depending on the context.
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ThroughputimeTransfer t / sizeTransfer Throughput
sizeTransfer h)1/Bandwidt(RTT imeTransfer t (effective end-to-end throughput)
We often think of throughput as measured performance. Implementation inefficiencies may cause the achievable bit rate to be less than the bandwidth for which the networks was designed.
Throughput and Transfer Time
Example: A user fetches a 1-MB file across a 1-G pbs network with a round-trip time of 100 ms. Compute the transfer time.
msmsmsRTTmeTransferTi 1088100810)10
1( 6
9
MbpsmsMbpshroughputEffectiveT 1.74108/1
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Shannon’s Theorem
Real communication have some measure of noise. This theorem tells us the limits to a channel’s capacity (in bits per second) in the presence of noise. Shannon’s theorem uses the notion of signal-to-noise ratio (S/N), which is usually expressed in decibels (dB):
)/(log10 10 NSdB In a typical analog system, e.g. analog telephone system, dB = 30, which gives30 = 10 * log10(S/N) S/N = 1000 for a typical analog system, including plain oldtelephone systems
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Shannon’s Theorem
))/(1(log2 NSBC
Kbps30)10001(log3000 2 C
Question: How come you get more than this with your modem?
Shannon’s Theorem:
C: achievable channel rate (bps)B: channel bandwidth
For POTS, bandwidth is 3000 Hz (upper limit of 3300 Hz and lower limit of 300 Hz), S/N = 1000
Information: Frequency audible to human ears: 20-20KHz
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Jitter
Network
Interpacket gap
Packetsource
Packetsink
1234 1234
Jitter is a variation (somewhat random) of the latency from packet to packet. Jitter is most often observed when packets traverse multiple hops from source to destination.
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Building Blocks
Networks nodes/ End Devices Links
Dedicated cablesLeased linesLast-mile linksWireless
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Network Node/End Device
Memory: getting larger and larger as we can see the last years, but never enough!
Processor: Moore’s law still holds for speed On a typical networked application, one must
keep in mind the computation to communication ratio.
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Links and Signals
Links: Twisted pair, coax, optical fiber, the ether; half-duplex or full-duplex.
Signals: Waveforms that travel on some medium
TT
f1
(frequency)
(period)
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Frequency & WavelengthWavelength: the distance between a pair of adjacentMaxima or minima of a wave, denoted as λ.
λ *f = c, c is the speed of light in a given medium.
Example: take c = 300 M meters/second,f = 100 M Hz, its wavelength λ = 3 meters
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Spectrum
Radio Infrared UVMicrowave
f(Hz)
FM
Coax
Satellite
TV
AM Terrestrial microwave
Fiber optics
X ray
100
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016
102 106 108 1010 1012 1014 1016 1018 1020 1022 1024104
Gamma ray
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Encoding – NRZ (Non Return to Zero)
Signals that maintain constant voltage levels with no signal transitions (non return to a zero voltage level) during a bit interval.
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4B/5B Encoding
Insert extra bits into the stream to break up long sequences of 0s and 1s. Doesn’t allow more than one leading 0 and no more than two trailing 0s.
4 bits 5 bitsf
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4B/5B Encoding4-bit Data Symbol 5-bit Code
0000 11110
0001 01001
0010 10100
0011 10101
0100 01010
0101 01011
0110 01110
0111 01111
1000 10010
1001 10011
1010 10110
1011 10111
1100 11010
1101 11011
1110 11100
1111 11101
16 codes are “left over” and some can be used for purposes other than encoding data. For instance:
11111 = idle line
00000 = dead line
00100 = halt
7 codes violate the “one leading 0, two trailing 0s rule”.
322
1625
4