Communication Links

Physical properties
Limitations:
- Attenuation
- Distortion
- Dispersion
- Noise
Modulation and Demodulation:
- Synchronous/Asynchronous
- Baseband/Broadband/Optical

Physical Media

Vacuum/air:
- Radio
- Microwave
- Optical
"Copper":
- twisted pair
- coaxial cable
Optic Fiber
- Multimode: Step-index or Graded-Index (GRIN)
- Single-Mode

Typical Distances/Bandwidths

Figure 7.2

Notes:

Mention log-log scale.

Mention repeaters for greater distances.

Frequencies

Medium	Min	Max
Twisted Pair	0	100 KHz
AM Radio	1 MHz	1 MHz
VHF TV	20 MHz	80 MHz
FM Radio	88 MHz	108 MHz
UHF TV	300 MHz	600 MHz
Coaxial	100 KHz	1 GHz
Cell Phones	850 MHz	1.2 GHz
Waveguides	1 GHz	100 GHz
Satellites	1 GHz	100 GHz
Optic Fiber	100 THz	100 THz

Limitations: Attenuation

Strength/Power of signal diminishes with distance
Some of the energy of the signal is dissipated as heat
Reflections cancel each other out
Shadowing (absorption)
r^{-2} decrease for un-guided EM waves
if power is too low, detection is not reliable

Limitations: Distortion

Signal has multiple frequencies
Each frequency has different attenuation and delay
Result: received signal not same as transmitted signal
Equalizer can help with differential attenuation
with sufficient distortion, signal will not be recognizable

Limitations: Dispersion

A tight pulse or burst will tend to become wider (disperse)
If data is encoded in tight pulses, and these disperse, neighboring pulses may overlap.
Product of rate R and distance L, R * L, is limited due to dispersion.

Limitations: Noise

Unpredictable variation in the signal
Thermal noise
Quantum noise
External sources of EM radiation (lightning, cosmic rays, neighboring wires/fibers, power supplies, other transmitters)

Noise and Channel Capacity

Shannon
Without noise, could transmit arbitrarily fast:
- want to send N bits/second
- line delay T seconds
- group into "words" of N * T bits
- each word is an N * T-bit integer
- convert that integer to a voltage (voltage is a real quantity, so can represent every integer as a voltage, no matter how many bits)
- because no noise, voltage is received exactly
- convert voltage to N * T-bit integer
The more noise, the slower the achievable rate
Cannot reach zero bit error rate (BER)

Asynchronous Baseband Transmission

Bipolar modulation
+V for 1, -V for 0
fixed-length words (e.g. bytes)
zero voltage when idle and between words
receiver clock must be synchronized with sender clock -- this limits word length
e.g. RS-232-C, Modems

Asynchronous Optical Transmission

On-Off-Keying (OOK)
on for 1, off for 0
fixed-length words (e.g. bytes)
must send "1" before each word, to start synchronization
again, limited word length
e.g. TV remote control

Broadband Transmission

Frequency-Shift Key	Binary Phase-Shift Key
(FSK)	(BPSK)
f0 for 1	Phase 0 for 10
f1 for 0	Phase 180 for 0

receiver can synchronize on carrier frequency, so any length transmission (e.g. synchronous)
e.g. radio teletype uses FSK

Synchronous Baseband Transmission

Manchester code (Ethernet)
(V,0) for 1, (0,V) for 0
Always at least one transition per bit
Requires twice as much bandwidth as the signal

Non-return-to-zero Inverted (NRZI)
if previous-bit's voltage is V0, -V0 for 1, V0 for 0
A transition on every 1 bit
Requires a "sufficient" number of 1 bits to keep the receiver synchronized (inserted)

Synchronous Optical Transmission

Manchester Code
4B/5B:
- used for FDDI
- use OOK (On-Off Keying)
- use selected 5-bit codewords to represent each group of 4 bits
- selected codewords have enough 0-1 or 1-0 transitions to allow receiver to synchronize
- idle symbol keeps endpoints synchronized, used when there is no data
- 6B/8B or 8B/10B use same idea