1. CS 453 Computer Networks Lecture 7 Layer 1 – Physical Layer

2. 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

3. 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 = 2, no problem – 1 wire If n = 3, no problem – 2 wires If n = 3, no problem – 2 wires If n = 5, ok - 10 wires If n = 5, ok - 10 wires If n = 6, well – 15 wires If n = 6, well – 15 wires Its getting out of control Its getting out of control … so as our intended network gets bigger it gets increasingly impractical to directly connect all pairs of computers

4. 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

5. 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

6. 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.

7. 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?

8. 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

9. 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

10. Physical Layer - Layer 1 PSTN (a) all possible neighbors, (b) through a switchboard, (c) interconnected switches From: Tanenbaum (2003) pg. 119

11. 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

12. 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…

13. 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: Use a 1000 Hz – 2000 Hz sine wave carrier, and Use a 1000 Hz – 2000 Hz sine wave carrier, and Modulate our data on top of that carrier… Modulate our data on top of that carrier… And, of demodulate the signal on the other end And, of demodulate the signal on the other end …How do we modulate the data signal?

14. Physical Layer - Layer 1 PSTN From: Tanenbaum (2003)

15. Physical Layer - Layer 1 PSTN Types of modulation Amplitude modulation – binary 0 and 1 encode with different amplitudes 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) 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 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

16. 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

17. Physical Layer - Layer 1 PSTN 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 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 – refers to a sampling of a signal Baud rate – is the rate of sampling a signal ( not the same a data rate) - samples/second Baud rate – is the rate of sampling a signal ( not the same a data rate) - samples/second Symbol – the information encoded in one sample Symbol – the information encoded in one sample Bit rate (or data rate) – is the speed in which information travel through a medium Bit rate (or data rate) – is the speed in which information travel through a medium

18. Physical Layer - Layer 1 PSTN More definitions So, for simple binary (1 bit) encoding… So, for simple binary (1 bit) encoding… bit rate = Baud rate But, more generally… But, more generally… Bit rate = baud rate * bits per symbol (i.e. bits per sample)

19. 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,…?

20. 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?

21. Physical Layer - Layer 1 PSTN PSK – We said we can shift phase 0 or 180 degrees We said we can shift phase 0 or 180 degrees …that gives us 1 bit …that gives us 1 bit What if we used phase shifts of 45, 135, 225 and 315 degrees? What if we used phase shifts of 45, 135, 225 and 315 degrees? …how many bits could we encode? …how many bits could we encode?

22. Physical Layer - Layer 1 PSTN PSK So with 4 possible phase shifts… So with 4 possible phase shifts… We double the number of bits per sample (bits per baud) We double the number of bits per sample (bits per baud) Now our bit rate doubles our baud rate Now our bit rate doubles our baud rate …so what is our data rate …so what is our data rate 4800 bps 4800 bps … called QPSK – Quadrature Phase Shift Keying … called QPSK – Quadrature Phase Shift Keying

23. Physical Layer - Layer 1 PSTN Constellation Diagrams for PSK and QPSK QPSK 90 180 270 0 Binary PSK From: Tanenbaum (2003) pg. 128

24. Physical Layer - Layer 1 PSTN So how do we get higher data rates? Can we take these modulation techniques further? How?

25. 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

26. Physical Layer - Layer 1 PSTN So with QAM-16 How many bits can you encode per baud? How many bits can you encode per baud? What bit rate can you get at 2400 baud? What bit rate can you get at 2400 baud? Can you take this idea further? Can you take this idea further? QAM-64 QAM-64 How many bits/baud? How many bits/baud? Bit rate? Bit rate?

27. 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?

28. Physical Layer - Layer 1 PSTN From: Tanenbaum (2003) pg. 129

29. Physical Layer - Layer 1 PSTN V.32bis modems Bit – 6+1 bits = 14,400 bps Bit – 6+1 bits = 14,400 bps QAM-128 QAM-128V.34 12 data bits/baud = 28,800 12 data bits/baud = 28,800V.34bis 14 data bits/baud = ? 14 data bits/baud = ?