CS 453Computer 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… • 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 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 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 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 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 1PSTN • (a) all possible neighbors, (b) through a switchboard, (c) interconnected switches From: Tanenbaum (2003) pg. 119
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 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 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 1PSTN From: Tanenbaum (2003)
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 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 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 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 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 1PSTN • Remember that we talked about encoding 1 bit per sample… • Can we do more than one bit? • If so, how?
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 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 1PSTN • Constellation Diagrams for PSK and QPSK 90 180 0 270 QPSK Binary PSK From: Tanenbaum (2003) pg. 128
Physical Layer - Layer 1PSTN • So how do we get higher data rates? • Can we take these modulation techniques further? • How?
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 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 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 1PSTN From: Tanenbaum (2003) pg. 129
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 = ?