Modulation & More

Signal Propagation Electric current cannot be transmitted on copper over long distances because the signal gets weaker as it travels over a distance. This degradation in the signal is referred to as signal loss. This loss is due to the internal resistance of the copper wire.

Signal Propagation (cont.) However, an electrical signal which oscillates at a fixed frequency will be able to propagate farther than a non- oscillating signal. Signals can be modulated in varying ways: amplitude modulation (AM), frequency modulation (FM), and phase- shift modulation.

Creation of an AM Signal Here is a continually oscillating signal. This is referred to as the carrier signal. Here is the bit pattern we wish to send on the carrier signal. The resultant signal after amplitude modulation. The carrier is reduced to 2/3 to encode a 1, and to 1/3 to encode a 0.

Creation of a Phase-Modulated Signal Here is the carrier signal. The bit pattern we wish to send on the carrier signal. The resultant phase- modulated signal.

Pros/Cons of AM Pros: Simple to modulate Simple to demodulate. Easy to “see” what’s happening with the waveform. OK for audio transmission. Cons: Bit encoding requires at least one full cycle of the carrier signal.

Pros/Cons of FM Pros: Simple to modulate. Simple to demodulate. Less audible distortion than AM – finer tuning capabilities. Pretty good for audio transmission. Cons: But encoding requires at least one full cycle of the carrier signal.

Pros/Cons of Phase-Shift Mod. Pros: Very fast! Multiple bit encoding possible in one cycle of the carrier signal. Great for data transmission! Cons: Difficult to “see” what’s happening. Not useful for audio transmission.

Modems Modem is really an acronym. It means modulator/demodulator. Why do we use modems? We can encode more data in a modulated signal than in an unmodulated signal. Plus, a modulated signal can propagate longer distances.

Modems (cont.) Here is a logical connection between two modems. The middle “stuff” can be a variety of connection types.

The “Middle Stuff” In the previous example, the modems connected via a leased line. A leased line is a dedicated connection which does not transit the telephone company’s circuit-switched network. It can be considered a point-to-point circuit between two locations.

The “Middle Stuff” (cont.) We’re not going to discuss it now, but the “middle stuff” can consist of a Plain- Old-Telephone-Service (POTS), ISDN, DSL, Frame Relay, etc. Lots of stuff, right? It’ll all come in time.

A Dialup Connection Here, the “middle stuff” is a POTS network.

An Important Note Computers don’t necessarily care what medium the modems communicate on. That is left up to the modem hardware. Computers usually communicate via RS-232 to the modems (both are serial devices, remember). All the computer has to know is how to send and receive bits on the serial port. The modem does the rest.

Modulation & Multiplexing Multiple signals can be transmitted on a single medium if the carrier signals use noticeably different frequencies. One usage of multiplexing is in cable TV reception. All channel frequencies are sent on the cable, but the descrambler (demux) only interprets one at a time. Three types of multiplexing exist: Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) Time Division Multiplexing (TDM)

FDM FDM is used when multiple, independent signals traverse a single medium. FDM can be used with a copper or fiber medium.

FDM (cont.) FDM allows for real simultaneous communication over a shared medium. FDM is quite effective as long as each carrier frequency is not: A multiple of another carrier frequency or Too close to another

WDM Wave Division Multiplexing (WDM) is really FDM, since a wave is a fixed frequency. WDM is used when discussing optical circuits. Optical FDM uses multiple frequencies of light (colors, if you will) for communication.

TDM Time Division Multiplexing (TDM) is a strict alternation form of multiplexing. Each source/receiver pair accesses the shared medium for some small time, and then releases it. This continues in a round-robin fashion. ASIDE: TDM was heavily used in early operating systems (UNIX), but it was called “timesharing.”