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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






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 = ?