Data Transmission

Physical Transmission Media

Nodes can be: computers, printers, routers, terminals, copiers, vending machines, household appliances...

Connections can be: wires, glass fiber, radio, infrared, microwave, staellite, laser...

Wire (almost always copper)

Copper is the nearly universal choice for a wire since it is a good conductor, but there is a potential for interference, which is usually solved in two main ways

Glass fiber

no electrical interference (won't cause it, won't be susceptible to it)
pulse of light can be carried farther than a signal in a wire
light can encode more information than electricity
single fiber is okay, you don't need a circuit!
expensive to manufacture
may be hard to detect the location of a break
expensive to repair a break

Radio

Use RF antennas
No physical connections needed

Microwave

Can be beamed in a given direction (unlike radio)
Can't penetrate metal or even some vegetation, need TALL antennas

Infrared

Confined pretty much to a single room
No antenna needed

Satellite

Similar to RF. Transponders used to amplify signal. A single satellite will often have 6-12 transponders and share channels too.

geosynchronous - high earth orbit, 22,236 miles up over equator. Limited space in this orbit, since you need 4 to 8 degrees of separation to avoid serious interference.
low-earth-orbit - generally just a few hundred miles up so they orbit in 90 minutes or so.
low-earth-orbit-satellite-arrays - refers to placing a whole bunch of LEO satellites up there so you always have one near you at any time.

Laser

pretty similar, feature-wise, to microwave communication

Speed

Theoretical Maximums at today's state of the art

Medium	Max bit rate	Max byte rate
Analog Modems	56 Kbps	7 KB/sec
ISDN Phone Line	128 Kbps	16 KB/sec
Satellite	400 Kbps	50 KB/sec
Electric Line	1 Mbps	128 KB/sec
T1	1.544 Mbps	197 KB/sec
DSL Phone Line	6 Mbps	768 KB/sec
Cable Modem	27 Mbps	3456 KB/sec
T3	45 Mbps	5760 KB/sec
OC3/STM1	155 Mbps	19840 KB/sec
OC12	620 Mbps	79360 KB/sec
OC48/STM16	2.5 Gbps	327680 KB/sec
OC192/STM64	10 Gbps	1310720 KB/sec

The speed of a line doesn't mean your computer can download data at that rate. For example a line may be shared, like in a Cable Modem so you may only be getting, say, 1 MB/sec to your modem. Then the data has to go to your network card where it is processed and shoved to your memory, which may be slow! Also the actual data you are downloading is only part of what is travelling though the net, as you are getting a lot of packets with a lot of header and checksum information, that must be stripped and processed, taking even more time. And don't forget, upstream data rates might be much slower.

Local Communication

Can be done by sampling voltages on the media
Time between samples, detecting the beginning of a frame, and detecting the end of the frame, is part of a protocol
Example of a protocol: RS-232

Long Distance Communication

Requires carrier waves
Common ways to encode: AM, FM, Phase-shift modulation
Need modems at both ends
Multiplexing required (some strategies: FDM/WDM/DWDM, TDM/STDM/Synchronous)