# CS 453 Computer Networks - PowerPoint PPT Presentation

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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

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## 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…

• 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?

• 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 = ?