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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

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Cs 453 computer networks

CS 453Computer Networks

Lecture 7

Layer 1 – Physical Layer


Physical layer layer 1 real networks for real people
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
Physical Layer - Layer 1

  • Remember the earlier discussion about physically connecting a set of n computers…

    • 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 11
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 12
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
Physical Layer - Layer 1PSTN

  • 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 pstn1
Physical Layer - Layer 1PSTN

  • 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 pstn2
Physical Layer - Layer 1PSTN

  • 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 pstn3
Physical Layer - Layer 1PSTN

  • 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 pstn4
Physical Layer - Layer 1PSTN

  • (a) all possible neighbors, (b) through a switchboard, (c) interconnected switches

From: Tanenbaum (2003) pg. 119


Physical layer layer 1 pstn5
Physical Layer - Layer 1PSTN

  • 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 pstn6
Physical Layer - Layer 1PSTN

  • 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 pstn7
Physical Layer - Layer 1PSTN

  • So how do we move bits across the PSTN?

  • In particular, how do we move bits across the local loop?

  • Answer:

    • 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 pstn8
Physical Layer - Layer 1PSTN

From: Tanenbaum (2003)


Physical layer layer 1 pstn9
Physical Layer - Layer 1PSTN

  • Types of modulation

    • 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 pstn10
Physical Layer - Layer 1PSTN

  • 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 pstn11
Physical Layer - Layer 1PSTN

  • OK, lets take a definition break…

    • 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 pstn12
Physical Layer - Layer 1PSTN

  • More definitions

    • So, for simple binary (1 bit) encoding…

      • bit rate = Baud rate

    • But, more generally…

      • Bit rate = baud rate * bits per symbol (i.e. bits per sample)


Physical layer layer 1 pstn13
Physical Layer - Layer 1PSTN

  • 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 pstn14
Physical Layer - Layer 1PSTN

  • Remember that we talked about encoding 1 bit per sample…

  • Can we do more than one bit?

  • If so, how?


Physical layer layer 1 pstn15
Physical Layer - Layer 1PSTN

  • PSK –

    • 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 pstn16
Physical Layer - Layer 1PSTN

  • PSK

    • 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 pstn17
Physical Layer - Layer 1PSTN

  • Constellation Diagrams for PSK and QPSK

90

180

0

270

QPSK

Binary PSK

From: Tanenbaum (2003) pg. 128


Physical layer layer 1 pstn18
Physical Layer - Layer 1PSTN

  • So how do we get higher data rates?

  • Can we take these modulation techniques further?

  • How?


Physical layer layer 1 pstn19
Physical Layer - Layer 1PSTN

  • 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 pstn20
Physical Layer - Layer 1PSTN

  • So with QAM-16

    • 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 pstn21
Physical Layer - Layer 1PSTN

  • 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 pstn22
Physical Layer - Layer 1PSTN

From: Tanenbaum (2003) pg. 129


Physical layer layer 1 pstn23
Physical Layer - Layer 1PSTN

  • V.32bis modems

    • Bit – 6+1 bits = 14,400 bps

    • QAM-128

  • V.34

    • 12 data bits/baud = 28,800

  • V.34bis

    • 14 data bits/baud = ?


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