DOCSIS Question

A technical question for you DOCSIS experts out there:

On the downstream, DOCSIS specifies MPEG formating with the reasoning (provided by a few articles) that this allows these data packets to be multiplexed within the digital CATV traffic, which is MPEG encoded. My question is, if DOCSIS traffic is sent over a seperate, dedicated 6MHz channel (not interleaved with digital CATV channels) why is there a need to MPEG encode?

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First of all, DOCSIS data is not MPEG encoded. Encoding is the process of taking a digital media stream and compressing it to a lower data rate. Encoding is part of MPEG2, but you are talking about the MPEG2 Transport Stream (TS). The TS is a mechanism for packetizing a data or media stream and sending it across a link. MPEG2 TS also includes hooks for maintaining clock synchronization, which is critical for video data.

As to your question, here are a few reasons of the top of my head:

o) By using the MPEG TS, DOCSIS data and video can share the same 6Mhz channel. This allows for a finer granularity of bandwidth allocation by the cable company. For example, a MPEG2 TS formatted 6MHz channel could carry

6-12 standard definition video channels and 15-20Mbps of data.

o) Cable infrastructure equipment is MPEG TS based. DOCSIS can co-exist with video equipment very easily and some equipment can even be shared. The pre-DOCSIS proprietary cable modems required completely separate equipment, which drove operating costs up by not allowing equipment or knowledge sharing.

o) Future cable set-top boxes (STB) will uses DOCSIS for the control channel (google NGNA). Again, the common use of MPEG TS allows for re-use of hardware within the STB, driving down costs in this cost sensitive device.

There are probably a bunch more reasons, but the bottom line is the use of MPEG TS allows cable operators to operate a more homogenous network at a lower cost than running separate networks for data, video and voice.


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