Cable Modems
Modem Architecture
Cable Modems are different, but the basic architecture is more or less the same as shown below:
Tuner
The tuner connects to the cable outlet and separates the TV data from the
Internet data.
In addition, it is used to separate the downstream data arriving from the CMTS
from upstream data arriving from other modems connected to the CMTS. Upstream
data is transmitted in lower frequencies (5-42 MHz), and downstream data is
transmitted in the higher frequencies (42-850 MHz).
Demodulator
The signal going out of the tuner goes into the demodulator. Demodulator is used in the downstream path and performs the following tasks:
* A/D converter - the modulator converts the analog wave received from the tuner into its digital representation.
* Error Correction - the modulator checks the received signal in order to detect errors in the data. In case an error is detected, the modem may request retransmission of the malformed data.
Modulator
The modulator role is to convert the digital representation received from the
computer into analog waves for transmission to the CMTS. Before converting the
data from digital to analog, the modem inserts additional information that will
be used to error correction in the receive side.
The modulator is also called a burst-modulator, because of the burst nature of
most data traffic in the upstream path (from the subscriber to the CMTS).
MAC
As mentioned above, all subscribers of a given CMTS share the bandwidth between them and the CMTS. The primary purpose of the Medium Access Control (MAC) is to share the media logically and efficiently. Both the CMTS and the modems connected to it implement the MAC protocols in order to achieve the following goals:
Allocating Frequencies and Time Slots for Upstream
Each modem collects information from the downstream data flowing in the
CMTS local network, and then decides on the frequency and timeslot to
transmit its upstream data. This is done in order to prevent collisions between
upstream flows of different modems connected to the CMTS.
Ranging
It is essential that signals transmitted from all modems of a certain
CMTS will be received by the CMTS with the same voltage level.
If two modems are located in different distances from the CMTS, the signal
received by the CMTS from the closer modem may be stronger than the signal from
the far modem. The CMTS may, mistakenly, not notice the signal from the
far modem and continue as if everything is ok (instead of detecting a collision
and request retransmission).
The MAC protocol ensures that every modem transmits signals in proper strength.
Cable Modems
Modem Architecture