7. What is inside a Cable Modem?

Cable Modems are different, but the basic architecture is more or less the same as shown above. The major components are outlined below, along with an indication of some companies that are know to deliver products to the open market. Many other companies are working in the field, but may not be so well known to me - or may only produce components for their own use.

Tuner

The tuner connects directly to the CATV outlet. Normally a tuner with build-in diplexer is used, to provide both upstream and downstream signals through the same tuner. The tuner must be of sufficiently good quality to be able to receive the digitally modulated QAM signals. Companies like ALPS, Sharp, Temic and Panasonic are strong suppliers here.

A new concept of a silicon tuner is in the works. This is basically a tuner on a chip, and is expected to cut the cost down quite a bit compared to a more conventional tuner module.

Demodulator

In the receive direction, the IF signal feeds a demodulator. The demodulator normally consists of A/D converter, QAM-64/256 demodulator, MPEG frame synchronization, Reed Solomon error correction.

The clear leader here is Broadcom, with a single chip demodulator. Other companies are Stanford Telecom wit a combined demodulator and burst modulator, but also companies like SGS Thomson, VLSI Technologies, LSI Logic and Fujitsu play a role here. The demodulator component is required both in a cable modem and in the more mature product, the digital (receive-only) set-top box, so many companies have developed products for this part of the game.

Burst modulator

In the transmit direction, a burst modulator feeds the tuner. The burst modulator does Reed Solomon encoding of each burst, modulation of the QPSK/QAM-16 on the selected frequency and D/A conversion. The output signal is feed though a driver with variable output level, so the signal level can be adjusted to compensate for the unknown cable loss.

The burst modulator is unique to the cable modem (and some two-way set-top boxes), so less component are available here. Broadcom leads the pack, with Stanford Telecom, Analog Devices, SGS Thomson and others playing catch-up

Combined demodulator and burst modulator chips are also available as the integration race drives more and more functions into a single chip..

MAC

A Media Access Control mechanism sits between the receive and transmit paths. This can be implemented in hardware or split between hardware and software. The MAC is pretty complex compared to an ethernet MAC, and in reality no MAC's are able to handle all of the MAC layer function without some microprocessor "help".

For DOCSIS cable modems, Broadcom and Libit (now Texas Instruments) are known to have MAC ASIC's available as a standard products Connexant is also in the market with a MAC that rely more on software to handle the various functions, supposedly giving more flexibility. Other companies are known to be working on various MAC chips for both DOCSIS and DVB/DAVIC, with different partitions of what goes in software and hardware. Some cable modem manufacturers even develop their own MAC apparently in an attempt to be more competitive or to differentiate their products.

Interface

The data that pass through the MAC goes into the computer interface of the Cable Modem, be it Ethernet, USB, PCI bus or whatever.

CPU

The microprocessor is not explicitly shown on the diagram, but for external cable modems a CPU is required. Some work is being done on host based processing cable modems, that uses the processor in the host (PC or Mac) to do all (or almost all) processing. Much like how analog telephony modems (WinModem) rely on the PC processor to do the processing.

For external cable modems with Ethernet interface, the Motorola embedded PowerPC series of microprocessors are popular, but other RISC based architectures are also used.

Single devices combining MAC, demodulator, burst modulator, processor, ethernet/PCI/USB interfaces and more are emerging, in effect integration the guts of a cable modem in a single chip. There will still be some additional parts for memory, tuner, analog stuff, power supply etc. so we are still no-where near the true single-chip cable modem - even though that is what the marketing guys tout.

<< Back to Business - Proceed to Downstream >>

Table of contents

Good solid introduction to making reliable measurements in the lab and in the field. (Hewlett-Packard Professional) Prentice Hall (February 1995). Hardcover - 337 pages. at Amazon.com |  Details and customer reviews