The Physical Layer

Scope: How signals are used to transfer bits over a link. i.e, how analog signals are converted to digital signals, and vise versa.

Coding and Modulation

A modem (modulator-demodulator) converts digital signals to analog signals, and vise versa.

A simple coding

A high positive voltage for 1, and a low negative voltage for 0. This is called NRZ(Non-Return-to-Zero). Each time interval (symbol) is like a sample point.

Problems?

Only 1 bit/symbol. Can use more than just 2 voltage levels to get more bits/symbol. To get N bits/symbol, need 2^n voltage levels. There is a tradeoff between encoding efficiency and the sensitivity to noise.

There are many other practical coding schemes, all of which are driven by engineering considerations.

Clock Recovery

Reciever needs requent signal transitions to decode bits. Several possible designs, including Manchester Coding and Scrambling.

A simple solution is to alternate between positive/negative, and zero voltages. This is return to zero (RZ) coding.

    0       1        1      1       0
+V |        ___     ___     ___
   |   |   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |   |   |
0  |   |___|   |___|   |___|   |___|   |
   |   |   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |   |   |
-V |___|   |   |   |   |   |   |   |___|

Better Solution

• Can map arbitrary bit patterns to eachother (as long as you don't decrease the number of bits to decode). Design encoding such that long runs of zero can't happen
• Can even use xor and a psuedorandom bit pattern to encode and decode to make the encoded data random looking as well, getting rid of most long runs of zero.

Modulation vs. Coding

In order to agree on the timing of data streams, AKA the start and end of a symbol being transmitted, you need to have a common clock between the two systems that are communicating.

With coding, signal is sent directly on a wire. This doesn't work well for wireless, so we use modulation. Modulation carries a signal by varying the frequency, amplitude, or phase of a carrier wave. Baseband is the original signal, and passband is the modulated signal. We can modulate a signal by varying the amplitude, frequency, or phase of a carrier wave.

Some examples:

• NRZ signal of bits
• Amplitude shift keying (zigbee)
• Frequency shift keying (bluetooth)
• Phase shift keying

WiFi for example goes all in and listens on an entire band of frequencies instead of just the binary 2 frequencies.Modern WiFi uses 256 frequency levels.

Key Points

• Everythign is analog, even digital signals.
• Digital signals are conceptually discrete, but are represented physically in a continuous medium.
• Modulating and demodulating a signal is converting between analog to digital, and vise versa.
• A coding is an agreed upon "language" for your data.

Simple Link Model

Two main parameters:

• Rate (bandwidth, capacity, speed): Number of bits per second
• Delay: Related to the time it takes to deliver a message

Additional info:

• type of cast: unicast, multicast, broadcast
• error rate

Message Latency

Latency is the time it takes for a message to travel from one end of a link to the other. It is the sum of the transmission delay (time to put bits on wire) and the propagation delay (time for bits to travel from one end of the link to the other).

Transimission Delay:
T (delay) = L (message length) / R (rate) = L/R seconds

Propagation Delay:
P (delay) = D (distance) / S (speed) = D/(2/3 * C) = 3D/2C seconds

Total Latency:
L_t = T + P = L/R + 3D/2C

Example

Broadband cross-country link:
P = 50ms, R = 10Mbps, L = 1MB

L_t = 1MB/10MBps + 50ms = .1s + .05s = .15s

Cut Through Routing