CS 453
Computer Networks
Lecture 7
Layer 1 – Physical Layer
Physical Layer - Layer 1
Real Networks for Real People
Recall that we said Layer 1 is about
moving bits
So we look at ways to move bits from one
place to another without being concerned
with higher level communications issues
That means that we have to have some
medium to move those bits from one place
to another
Physical Layer - Layer 1
Remember the earlier discussion about
physically connecting a set of n computers…
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If n = 2, no problem – 1 wire
If n = 3, no problem – 2 wires
If n = 5, ok - 10 wires
If n = 6, well – 15 wires
Its getting out of control
… so as our intended network gets bigger it gets
increasingly impractical to directly connect all
pairs of computers
Physical Layer - Layer 1
So, when computer networking was
getting off the ground…
…we needed a communication medium
infrastructure that would not require us to
pull wire from every computer to every
other computer…
…this is especially important for
connections over distances
Physical Layer - Layer 1
The ideal solution to this problem would be
to find an infrastructure that is already in
place…
And it just so happened that there was
one…
PSTN – The Public Switched Telephone
Network
Physical Layer - Layer 1
PSTN
30 or so years ago the PSTN was almost
exclusively the only infrastructure for
computer networking…
… and we could not imagine that that
would ever change much.
Today, the PSTN has a much smaller role
in the computer networking world, but…
It still has an important role…
… and will for the foreseeable future.
Physical Layer - Layer 1
PSTN
When the telephone was invented, in the late
1800s, it was point to point device…
To talk to your neighbor you had to string a wire
from your phone to your neighbor’s phone, if
your neighbor had a phone.
If two neighbors had phones, then each
neighbor had to have a wire running from their
phone to each other phone-owning neighbor’s
phone… and …
Does this seem familiar?
Physical Layer - Layer 1
PSTN
The immediate solution was to put is
switchboards (switches) and …
Each phone in the neighborhood was
connected to a neighborhood switch, so
Each home only had to run one wire.
A call, by the way, involved calling the
switch operator and being manually
connected to the receiving phones circuit
Physical Layer - Layer 1
PSTN
This worked pretty well as long as you wanted to
call a neighbor, but….
What if you wanted call a friend in a different
neighborhood?
To solve this telephone companies created trunk
circuits to connect switches
So a call to your friend might involve going from
you to a switch, then to another switch, then to
another switch, then your friend
Physical Layer - Layer 1
PSTN
(a) all possible neighbors, (b) through a switchboard, (c)
interconnected switches
From: Tanenbaum (2003) pg. 119
Physical Layer - Layer 1
PSTN
Lines or circuits interconnecting switches are
called trunks
Trunks are higher bandwidth
A lot of work has been invested in making trunks
yet higher bandwidth
The connection from the customer/home to the
switch is called the local loop
The local loop in almost all cases is twisted pair
(cat3 these days) copper cable
Physical Layer - Layer 1
PSTN
Trunks have improved tremendously over the
years, but…
The local loop has remained roughly the same
for about 100 years.
Recall that local loops terminate at the switch in
a 3100 Hz low pass filter.
So we have bandwidth of about 3000 Hz on the
local loop…
And remember at layer 1 we are trying to move
bits…
Physical Layer - Layer 1
PSTN
So how do we move bits across the PSTN?
In particular, how do we move bits across the
local loop?
Answer:
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Use a 1000 Hz – 2000 Hz sine wave carrier, and
Modulate our data on top of that carrier…
And, of demodulate the signal on the other end
…How do we modulate the data signal?
Physical Layer - Layer 1
PSTN
From: Tanenbaum (2003)
Physical Layer - Layer 1
PSTN
Types of modulation
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Amplitude modulation – binary 0 and 1
encode with different amplitudes
Frequency modulations – frequency shift
keying (FSK) – encode the data by shifting
between two frequencies (tones)
Phase modulation – Phase Shift Keying (PSK)
– encode the data by shifting the phase of the
sine wave 0 or 180 degrees with changes in
the data stream
Physical Layer - Layer 1
PSTN
Remember that our local loop only has about
3000 Hz of bandwidth
Remember Nyquist’s theorem – so we can, at
max, sample the signal 6000 samples per
second (assuming clean signals)
But the signal is not necessarily clean, so most
modems sample at 2400 samples per second
… this ought to leave you pondering some
things
Physical Layer - Layer 1
PSTN
OK, lets take a definition break…
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Bandwidth – refers to the range of frequencies that
will propagate through a medium with little attenuation
– measured in Hertz
Baud – refers to a sampling of a signal
Baud rate – is the rate of sampling a signal ( not the
same a data rate) - samples/second
Symbol – the information encoded in one sample
Bit rate (or data rate) – is the speed in which
information travel through a medium
Physical Layer - Layer 1
PSTN
More definitions
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So, for simple binary (1 bit) encoding…
bit rate = Baud rate
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But, more generally…
Bit rate = baud rate * bits per symbol (i.e. bits per
sample)
Physical Layer - Layer 1
PSTN
So, if the baud rate of our modems are
2400 baud…
How do we get data rates of 4800 bps,
9600 bps,…?
Physical Layer - Layer 1
PSTN
Remember that we talked about encoding
1 bit per sample…
Can we do more than one bit?
If so, how?
Physical Layer - Layer 1
PSTN
PSK –
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We said we can shift phase 0 or 180 degrees
…that gives us 1 bit
What if we used phase shifts of 45, 135, 225
and 315 degrees?
…how many bits could we encode?
Physical Layer - Layer 1
PSTN
PSK
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So with 4 possible phase shifts…
We double the number of bits per sample
(bits per baud)
Now our bit rate doubles our baud rate
…so what is our data rate
4800 bps
… called QPSK – Quadrature Phase Shift
Keying
Physical Layer - Layer 1
PSTN
Constellation Diagrams for PSK and QPSK
90
180
0
270
Binary PSK
QPSK
From: Tanenbaum
(2003) pg. 128
Physical Layer - Layer 1
PSTN
So how do we get higher data rates?
Can we take these modulation techniques
further?
How?
Physical Layer - Layer 1
PSTN
How about combining modulation techniques…
Suppose you combine QPSK with 4 level
amplitude modulation…
How many discrete states would you get in one
sample?
4 phase shifts X 4 amplitude level = 16 states?
How many bits can you encode using this
combined technique?
QAM-16 – Quadrature Amplitude Modulation 16
Physical Layer - Layer 1
PSTN
So with QAM-16
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How many bits can you encode per baud?
What bit rate can you get at 2400 baud?
Can you take this idea further?
QAM-64
How many bits/baud?
Bit rate?
Physical Layer - Layer 1
PSTN
Can you go further?
Yes, but the quality of the signal depends on the
modem’s ability to resolve phase shift levels and
amplitude levels.
Noise makes this different
TCM – Trellis Coded Modulation - using a bit for
parity
V.32 modems – 32 Constellation points
4 data bits + 1 parity bit
Data rate?
Physical Layer - Layer 1
PSTN
From: Tanenbaum (2003) pg. 129
Physical Layer - Layer 1
PSTN
V.32bis modems
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Bit – 6+1 bits = 14,400 bps
QAM-128
V.34
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12 data bits/baud = 28,800
V.34bis
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14 data bits/baud = ?