Technical > Photos of Broadband gear > Small CMTS installs

Small CMTS installs

4 Cisco 7246 chassis with wavecom upconverters

anoterh 7246 vxr with wavecom HD4000

more cabling

This is how ignorant I am on the cable side of things...

Each coax coming out of the unit handles how many actual client modems?

I am more familiar with the DSL 1:1 sync style more than anything cable. Is it the same in cable world or does each coax carry more than one subscriber signal?

Also what about fiber nodes? How do they and CMTS fit into the scope of things? I'm assuming it's the same as CO based DSLAMs and remote DSLAMS?

That all depends on who you ask I think right now Cisco is leaning towards 100-200 subscribers per upstream port (the red/orange cables in the pictures).
In the CATV world, it's like a tree where all the branches lead back to the CMTS ports for the return path (or flow from the CMTS in the downstream direction). Each Upstream port will generally have a node or two attached to it (A node feeds a neighborhood)
A fiber node is technically the point where the Coaxial network in the neighborhood is converted to fiber and runs back to the cable company. In the CMTS world, it's a description of the MAC domain that defines what nodes are tied to what up and downstream ports so the CMTS "knows" where modems are located.

(node diagram)
»www.cablefax.com/images/articles···9394.gif

(cable fiber node best practice)
»www.cisco.com/en/US/tech/tk86/tk···fd.shtml

These are small DOCSIS 2.0 installations. Larger installations utilizing the ARRIS C4 platforms and Cisco UBR10012 chassis (with edge QAMs) can include hundreds of upstream ports and serve tens of thousands of customers per chassis.
I have some pictures of installs like that around here somewhere...I'll try and find them

Here's an older document that outlines the combining network and goes over some of the underlying technology (it's a little outdated, but the principles are the same)

»www.cisco.com/en/US/tech/tk86/tk···69.shtml

I should also add that on a CMTS the upstream and downstream are separate ports. On the orange/black cmts cabling , the DS is black, and on the red/yellow, the DS is yellow.
You configure the UP/Downstream combining in software on the CMTS, as well as with RF splitters and combiners (and diplexers if you want to get specific) to combine multiple HFC nodes onto a single set of cables heading towards the CMTS.

reply to cablegeek01
Bang on!

Thanks for the info! This will help me wrap my head around it a little better. I come from the DSL/WISP/Telco side of things so cable is my weak point and it handles a little differently.

reply to cablegeek01
Great pics, thanks for sharing.

reply to cablegeek01
I have to say this is interesting stuff. I only finally bothered to look up what all those ports on the line cards in the Cisco are.

Originally I thought it was something like

Video "headend" -> (1 or 2 ports on right of linecard) -> left ports fan out to multiple nodes somewhere around the neighborhood

But it seems that the left hand ports are to handle upstream data from users, and the right hand ports are to handle downstream data to users?

Interesting scheme...

Each port has its own frequency Im guessing?

reply to cablegeek01
I've just always thought it was a hard process converting all that RF to fiber to RF etc. Or it would just be horribly lossy etc.

reply to TomS_

So the DS frequency output from those linecards is always the same, and you use a box to shift it to a new frequency to slot into your network?

reply to cablegeek01
On those particular cards (look like uBR-MC28X), and some of cisco's other linecards (uBR-MC16 series) yes. The cards output in the IF band, and you use an external up convertor to place the downstream on the frequency desired.

Cisco does however make a card in that form factor for that chassis (uBR-MC28U) that has a built in up convertor that you would specify the frequency to drive at in the CLI.

reply to cablegeek01
*Is bubbling with nerd giddyness*

Totally awesome info! For once I'm putting the puzzle pieces together and can see into some of the cable world!

Keep going! LOL

reply to voxframe
If you really want to geek out, here's a presentation that explains at the packet level how the modem and CMTS interact.

»shannon.cm.nctu.edu.tw/netexp/catv03.pdf

/If you couldn't tell already, I have a passion for DOCSIS :-D

reply to TomS_
As Killa200 said, on those particular cards, yes. And to go a bit further, the CMTS tells the CM(cable modem) what frequency to transmit back on by sending out packets in the downstream called UCDs or upstream channel descriptors. The CMTS then listens for modems on that frequency.

Cool stuff. Thanks for the info.

Sounds a bit like split system microwave radios. The modem cards in the IDU are tuned to fixed transmit/receive frequencies, but the ODU is an up/down converter, and shifts frequencies to or from the transmission frequency you configure/according to your licence.

Makes me want to get into this stuff.

reply to cablegeek01
Lemmme get this straight...

Between Blue and Green clouds is Coax (What we see in the pics)

From the Green cloud to the nodes is fiber...

From the Nodes is back out to Coax to the taps/etc.

What kind of method is both the TV/Data streamed with over the fiber so that it is easily converted to coax?

Maybe this is a limitation of my knowledge, but I'm still imagining 1 channel per freq etc. Is there really X number of transmitters in a node converting everything from the fiber to each individual freq?

This may also be a limitation that I'm thinking back to the analog channel days. Being digital for most everything I may be completely out of whack.

reply to cablegeek01
Looks good, but you guys use too many zip ties.....

reply to voxframe

reply to AMD Phreak