The Media Access Control
mechanism is normally implemented in hardware or in a combination of hardware and
software. The primary purpose of the MAC is to share the media in a reasonable way. Both
the CMTS and the Cable Modem implements protocols to do
Ranging to compensate for different cable
losses. It is essential that the upstream bursts from all Cable Modems are received in the
Head-End at the same level. If two Cable Modems transmit at the same time, but one is much
weaker than the other one, the CMTS will only hear the strong signal and assume everything
is okay. If the two signals are same strength, the signal will garble and the CMTS will
know a collision occurred.
Ranging to compensate for the different cable
delays. The size of a CATV network calls for fairly large delays in the millisecond range.
Assigns frequencies etc. to the Cable Modems.
The Cable Modem first listens to the downstream to collect information about where and how
to answer. The it signs on to the system using the assigned upstream frequency etc.
Allocate the time-slots for the upstream.
It is impossible to give more detailed
information about the MAC, without going into the specific standards. This is one of the
areas that are most closely tied to the specific standard. |
