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The Anatomy of a Cable Modem

Originally published  August, 1996
by Carlo Kopp
1996, 2005 Carlo Kopp

A typical cable modem scheme will use an assymetrical arrangement, with high bandwidth downstream and much lower bandwidth upstream. While this asymmetry may be restrictive for typical computing applications, it is well matched to the typical domestic end user who needs most of his bandwidth from the web or news servers to his domestic platform.

The downstream channel will occupy the 6 MHz bandwidth of a television channel, allowing the provider to slot cable modem channels into unused television slots in his cable bandwidth. The favoured carrier modulation scheme for downstream traffic is Quadrature Amplitude Modulation or QAM. In a QAM scheme two carriers are transmitted, locked in frequency and phase shifted by 90 degrees. Information is then encoded in both the amplitude and the phase of the resulting signal (this clever piece of analogue witchcraft is the foundation of colour TV transmission, used both in NTSC and PAL-B, the French choosing essentially FM for their SECAM system). The advantage of QAM schemes is that providing the signal to noise ratio is suitable, you can pack multiple bits into into a single time slot. This means that one can carry a much higher bit rate than by using simplistic schemes, in a given bandwidth.

The most popular arrangement in current cable modems appears to be QAM64 (6 bits/time slot), which provides the user with about 36 Mbps across a 6 MHz channel. This is cca 3.5 times the speed of an Ethernet. The inclusion of forward error correction codes and MAC level encapsulation will however cost some fraction of this bandwidth.

The upstream channel requires robust handling of noise and interference. A typical arrangement is to slot the upstream traffic into the 5 to 40 MHz low band, basically sneaking in under the TV and downstream traffic. Upstream traffic typically uses a Quadrature Phase Shift Keying or QPSK scheme, which offers better noise immunity to QAM but achieves lesser bandwidth utilisation. A typical upstream channel will provide cca 2 Mbps, although higher speeds have been reported.

A cable modem is substantially more complex than our familiar voiceband modems. To receive the VHF or UHF radio band downstream carrier the modem must have a superhet receiver not unlike the tuner in an FM radio, or TV. A QAM coherent demodulator will be required to extract the raw bit stream from the carrier.

Once the raw bit stream is extracted, it must be fed into a MAC (Medium Access Control) protocol engine which will identify which packets are intended for the modem in question and which are to be discarded. A packet intended for the modem must then be buffered, checked for errors and then dumped typically to an Ethernet controller for transmission to the user's machine. A 10 Base T or 100 Base T Ethernet interface appears to be the preferred choice at this time (although some vendors have opted for internal (eg PCI) bus cards to simplify development at the cost of cross platform portability).

A packet being sent from the user's machine is received by the modem's Ethernet interface, stripped of its Ethernet MAC layer fields, encapsulated with the MAC level protocol fields for upstream transmission, and fed into a modulator. A master oscillator, most likely using an crystal oscillator and synthesizer chip will produce the RF carrier, which is QPSK modulated by the modulator. The modulated carrier is then fed into a power amplifier which drives the cable.

At the distribution center, the reverse of this occurs, and the packets are then fed into or taken from an IP router via which the network is accessed.

A real cable modem must also have network management support, which would typically mean that its master CPU must run an SNMP agent, and may also have to provide routing between the address space of the local network and the outside world. Moreover, encryption may also be used for some message types, and a user interface must exist so a supervisor can configure the device IP address mappings, channels and authentication to the network.

A cable modem is thus a complex little animal which combines a split RF transceiver, many router and supervisory functions, and MAC level interfaces to the user's Ethernet and the cable network. As it must operate in an arguably messy transmission environment, it must be robust at all levels of the design, and it must be cost competitive with top of the line voiceband modems in order to provide for a commercially viable service.

In a typical service environment, an Ethernet like strategy would see many users sharing a single 6 MHz channel. Providing that the provider (no pun intended) is sensible about loading levels on his service, and provides adequate router throughput, a cable modem network should deliver performance comparable or faster than a lightly loaded 10 base X Ethernet network. If providers try to do what many commercial computer sites do, and overload the routers and channels with users, a cable network will suffer similar problems to saturated Ethernets. The latter is the more likely outcome, even so it will provide a significant improvement over established voiceband links.




$Revision: 1.1 $
Last Updated: Sun Apr 24 11:22:45 GMT 2005
Artwork and text 2005 Carlo Kopp


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