hardware building blocks and encoding com211 communications and networks cda college pelekanou olga...

Hardware Building Blocks and Encoding

COM211 Communications and NetworksCDA CollegePelekanou OlgaEmail: fe_olga@mail.ruwww.cdacollege.ac.cy/site/info-com-technology-ll/

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.

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LatencyQueueTransmitnPropagatioLatency

a vaccum in /103.0

cablea in /102.3

fibera in /102.0

light of Speed8

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Bandwidth/SizeTransmit

light of Speed / DistancenPropagatio

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.

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

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MbpsmsMbpshroughputEffectiveT 1.74108/1

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

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

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.

Building Blocks

Networks nodes/ End Devices Links

Dedicated cablesLeased linesLast-mile linksWireless

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.

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)

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

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

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.

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

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”.

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