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CS 453 Computer Networks

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CS 453Computer Networks

Lecture 7

Layer 1 – Physical Layer

- 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

- 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

- 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

- 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

- 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.

- 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?

- 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

- 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

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

From: Tanenbaum (2003) pg. 119

- 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

- 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…

- 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?

From: Tanenbaum (2003)

- 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

- 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

- 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

- 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)

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

- So, if the baud rate of our modems are 2400 baud…
- How do we get data rates of 4800 bps, 9600 bps,…?

- Remember that we talked about encoding 1 bit per sample…
- Can we do more than one bit?
- If so, how?

- 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?

- 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

- Constellation Diagrams for PSK and QPSK

90

180

0

270

QPSK

Binary PSK

From: Tanenbaum (2003) pg. 128

- So how do we get higher data rates?
- Can we take these modulation techniques further?
- How?

- 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

- 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?

- 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?

From: Tanenbaum (2003) pg. 129

- 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 = ?