A typical cable modem consists of the following components inside it:
A Tuner
A Modulator
A Demodulator
A MAC (media access control) device
A CPU
The Tuner The tuner connects to the CATV outlet. The tuner receives digital modulated signals and then forwards them towards the demodulator. There will be circumstances that the tuner can be attached to a diplexer, which enables the tuner to use a range of frequencies for both the downstream and upstream traffic.
Demodulator The demodulator has four main functions, these are:
QAM (Quadrature Amplitude Modulation)
A/D converter (Analog to Digital)
Error correction
MPEG (Moving Picture Experts Group) synchronizer
The QAM takes the radio-frequency signal and changes it so that the A/D converter can process it. The A/D converter then converts the signal into a digital signal of 0's and 1's. The error correct module then compares the data received to a known standard, fixing errors in transmission. The MPEG synchronizer is then used to sequence and organise data groups into their correct order.
Modulator or Burst Modulator The modulator has three main functions, these are:
To insert data that is ready for error correction
QAM (Quadrature Amplitude Modulation)
D/A conversion (digital to analog)
A modulator, sometimes known as the burst modulator, converts digital data back into analog radio frequency signals so that they are ready for transmission.
MAC (Media Access Control) The MAC acts as the interface between parts of various network protocols. Either the CPU (Central Processing Unit) of the cable modem or the CPU of the computer is used to take some pressure away from the MAC because of the complexity of its functions.
CPU (Central Processing Unit) Most cable modems have a CPU inside them which handles processing in the same way as the CPU inside hardware dial-up analog modems. The CPU is the brain that controls all of the activity inside the cable modem itself.
