internal dial-up modems do. The cable modems tuner connects directly into the CATV outlet. For two-way data transfer, a tuner must have a two-way diplexer to break out the upstream and downstream traffic (Ostergard, 1999). The Cable Modems Demodulator receives the downstream IF signal from the tuner and, as the name implies, demodulates it. The Demodulator is composed of an A/D converter, a QAM64/256 demodulator, MPEG frame synchronization, and Reed Solomon error correction. Downstream data is framed according to the MPEG-TS (transport stream) specification. The frame format for this specification is a 188/204 byte block, with a single fixed sync byte in front of each block. The Reed-Solomon error correction algorithm reduces the block size from 204 bytes to 188 bytes, which leaves 187 bytes for MPEG header and payload (Ostergard, 1998). The upstream data traffic is modulated by the cable modems Burst Modulator. The Burst Modulator feeds the cable modems Tuner, performs Reed Solomon encoding of each downstream burst, performs QPSK or QAM16 modulation on the designated upstream frequency, and D/A conversion. The Burst Modulators output signal is fed through a variable output amplifier, so the signal level can be adjusted to compensate for cable loss(Ostergard, 1998). Both the upstream and downstream traffic travels through the cable modems Media Access Control mechanism. The MAC mechanisms functions are fairly complex. The MAC mechanisms main purpose is to implement MAC protocols under the direction of the CMTS. MAC protocols are used to time-share the cable media among the various cable modems in a cable data network. The MAC processes can be implemented in hardware, or a combination of software and hardware. Both the CMTS and the MAC mechanism implement MAC protocols to perform ranging procedures to compensate for cable media delays and line losses. The CMTS also interfaces with the MAC mechanism in each cable modem to...
