12. What is MAC?
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.
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