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Headend Combining

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astrit
Headend Combining

Hi all,

I would like to ask somebody who has a long experience working in cable ISP. I just wanted to know some general numbers like:

- How many fiber nodes per CMTS upstream port you have connected?
- How many ports (home passed) per nodes you have connected? , and
- What kind of technology for upstream combining and splitting you are using; do you use any Return Path Multiplexer (RPM)?

I appreciate your help on these questions.

Best Regards,
Astrit

DocsisAdmin
Headend Combining

A couple years back I used to run 2-3 nodes per upstream port. Each node added increases noise (reduces SNR), so adding more was harmful. To remain competitive with speeds I had to increase bandwidth and go down to 1 node per upstream port. That trend continued so today I'm running 4 upstream ports for each node -- two 6.4mhz wide ATDMA (docsis 2.0) ports and two 3.2mhz wide TMDA. All ports are 16QAM. This gives me about 48Mbps upstream available for each node.

Most nodes have 300-500 data modems (1200-1500 homes passed). We used to run a multiplexer system but ended up installing additional fiber instead. The multiplexer was often the source of problems.

omadon
Headend Combining

What is SNR needed for QAM16 TDMA & ATDMA on upstream port.

DocsisAdmin
Headend Combining

16 QAM isn't too difficult with post-equalization / noise cancellation turned on. TMDA ports will run well above 21db and ATDMA will run well above 26dbmv. These figures are just from memory and experience; there are probably official specifications published from the cmts vendors. Motorola has some ATDMA/Docsis 2.0 papers at:
http://broadband.motorola.com/ips/pdf/spectrum_mgmt.pdf
http://broadband.motorola.com/ips/pdf/migrating_to_docsis_2.pdf
http://broadband.motorola.com/ips/pdf/Logical_Channels.pdf