Transcript Hardware Building Blocks and Encoding

Hardware Building
Blocks and Encoding
COM211 Communications and Networks
CDA College
Pelekanou Olga
Email: [email protected]
www.cdacollege.ac.cy/site/info-com-technology-ll/
Performance
Bandwidth: number of bits per time unit.
(a)
1
(b)
1
We can talk about bandwidth at the physical
level, but we can also talk about logical processto-process bandwidth.
Latency: time taken for a message to travel from
one end of the network to the other.
Latency
Latency  Propagatio n  Transmit  Queue
Propagatio n  Distance / Speed of light
Transmit  Size / Bandwidth
 2.0  108 m / s in a fiber

Speed of light   2.3  108 m / s in a cable
3.0  108 m / s in a vaccum

Delay and Bandwidth
Delay
Bandw idth
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.
Throughput
Throughput  Transfer size / Transfer t ime
(effective end-to-end throughput)
Transfer t ime  RTT  (1/Bandwidt h)  Transfer size
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.
1
TransferTime  RTT  ( 9 ) 106  8  100ms  8ms  108ms
10
EffectiveThroughput  1Mbps / 108ms  74.1Mbps
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):
dB  10  log 10 (S / N )
In a typical analog system, e.g. analog telephone system,
dB = 30, which gives
30 = 10 * log10(S/N)
S/N = 1000 for a typical analog system, including plain old
telephone systems
Shannon’s Theorem
Shannon’s Theorem:
C  B log 2 (1  (S / N ))
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
C  3000 log 2 (1  1000)  30Kbps
Question: How come you get more than this with your modem?
Information: Frequency audible to human ears: 20-20KHz
Jitter
Interpacket gap
4
Packet
source
3
2
1
4
Network
3
2
1
Packet
sink
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
cables
 Leased lines
 Last-mile links
 Wireless
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
T (period)
1
f 
T
(frequency)
Frequency & Wavelength
Wavelength: the distance between a pair of adjacent
Maxima 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
f(Hz) 100
102
104
106
Radio
104
105
106
107
108
1010
1012
1014
Microw ave
Infrared
108
1010
109
1011
1016
UV
1012
Satellite
Coax
AM
FM
TV
Terrestrial microw ave
1018
1020
X ray
1013
1014
1022
1024
Gamma ray
1015
Fiber optics
1016
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
f
5 bits
4B/5B Encoding
4-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
11111 = idle line
1010
10110
00000 = dead line
1011
10111
1100
11010
1101
11011
1110
11100
1111
11101
24  16
25  32
16 codes are “left over” and some
can be used for purposes other than
encoding data. For instance:
00100 = halt
7 codes violate the “one leading 0,
two trailing 0s rule”.