Hello, everyone.
Recently we've connected the second downstream to our network. New cable modems that have just been connected to one of the upstreams assosiated with the second downstream, go online way too slowly. It looks like they find the frequency of the first downsream first and try to connect. But since the upstream they are connected to is not assosiated with the first downstream, they just keep struggling over and over. After some time they start scanning again, find the frequency of the second downstream and get online quite fast.
I guess I'm not the only one who has varios downstreams visible to every cable modem in the network. Is there a solution?
Thanks in advance.
What's too slow, time wise? what does your hfc/head end combining look like? what cmts do you use?
Yes, most mso's have at least two downstreams, but they are bonded with all the upstreams of a given cmts blade or card. if you have two different cmts's, is it possible to feed nodes 1-20 with the first and 21-40 with the second for example?
narrow cast the downstreams so they only go to those nodes? do the modems have the current firmware?
~Carl
>What's too slow, time wise?
15 minutes to 4 hours.
>what does your hfc/head end combining look like?
All upstreams are physically separated from each other (it is done to make the upstreams' snr better). All downstreams are "visible" to every cable modem in the network.
>what cmts do you use?
uBR7246vxr with one 2ds-8us card.
>do the modems have the current firmware?
yes. we are using Arris and Scientific Atlanta. both have their firmware upgraded to the current version.
So, as far as I understand, the solutions I have is either to interconnect the upstreams so that every cable modem could join any of them, or filter out the downstream signal for the cable modems (upstreams) that are not supposed to see it.
Can I ask you some more questions?
With only one MC28 do you have a downstream bandwidth problem? If your not using more than 35 Mbs of Downstream bandwidth you could turn your MC28 into a MC18 by turning off one downstream, and mapping the extra upstreams to the other downstream.
Do you have separate forwards i.e. is one downstream feeding one laser and the other feeding another? if so change your combining network so the the downstream is injected right before the laser. This way Laser 1 will see its downstream and laser two will see its downstream. You can even use the same Frequency for both.
Are you using both downstreams for the same physical foward carrier? If so look into doing Downstream/Upstream load balancing. That way whichever DS it finds it will be able to lock on.
>With only one MC28 do you have a downstream bandwidth problem?
Yes. We've contracted more bandwidth recently (70mb/s). So I can't use just one downstream anymore.
>Do you have separate forwards i.e. is one downstream feeding one laser and the other feeding another? if so change your combining network so the the downstream is injected right before the laser. This way Laser 1 will see its downstream and laser two will see its downstream. You can even use the same Frequency for both.
Sorry, I'm not an RF tecnician and have no idea what exactly happens with the signal after it leaves the CMTS :-( My coworker is now investigating if we can filter out the DS0 frequency for the upstreams assosiated with DS1, which I guess is close to what you are suggesting.
>Are you using both downstreams for the same physical foward carrier? If so look into doing Downstream/Upstream load balancing. That way whichever DS it finds it will be able to lock on.
Is it actually possible? We have all upstreams physically separated from one another. Is it possible to configure some sort of load balancing in a network with such design?
Lets start with an easier question? then.....
How many customers are you serving?
How many forward lasers do you have feeding your cable plant?
How many nodes are you serving?
If you have one forward laser with the two downstreams, which I suspect you do, you have a couple of options.
1. split the forward with a second laser and use the same docsis frequency for both. This requires a combining network change.
2. use downstream and upstream loadbalancing Some docs found here more available online
Disadvantages for each
1. Forward lasers are not cheap, and depending on how your combining network is setup, may be easy or hard to setup.
2. I have not set up Downstream Load balancing, but I have set up upstream load balancing and it is a little tough to get your head around the ideas behind it. Ideally you would want to connect both downstreams together and the upstrams in pairs. The setup is a quite complex and only works with docsis 1.1+ modems for downstream switching, I believe. Complexity is the main drawback of this approach.
Advantages for each
1. Physically separate the downstreams at a Cable plant level, allows you to only assign one freq/channel for docsis and reuse it on each forward laser. We employ this approach and use ch 58/429Mhz for docsis, we have around 3-4 forward lasers per MC28U and we reuse the frrequency/channel for our own network. As far as the modems are concerned there is only 1 downstream carrier and they quicly lock online.
2. With dynamic Loadbalancing you can balance out your customers by network usage/ modem count or statically. This allows you to take advantage of all your bandwidth, and better respond to demand.
Load balancing is possible with upstreams physically separated, however would require combining some of them.
i.e. for example if you currently have the following nodes connected to CMTS as follows
US0 US1 US2 US3 US4 US5 US6 US7
1 2 3 4 5 6 7 8
on the upstream it would look like after Load balancing it would look like
US0-US4 US1-US5 US2-US6 US3-US7
1 & 2 3 & 4 5 & 6 7 & 8
The upstreams are still physically separated but not as much.
DS0 would be associated with US0,1,2,3
DS1 would be associated with US4,5,6,7
So if modem a in node 1 came up on DS0 it would lock on US0
if it came up on DS1 it would lock on US4
With Dynamic load balancing the CMTS can tell new modems to use a different frequency if current downstream is full. (requires Docsis 1.1)
If you answer my other two questions would be happy to offer a reccomendation.
Your absolutly correct in your analysis of the modem being locked to the other downstream. Whats even worse is that they are transmitting back upstream based on the UCD's that they see in the downstream signal that they are listening to. They are starting out at a default transmit level and ramping up to their full output capability trying to get a response back from the CMTS module.
When this happens this is a bad interfering signal to you correctly registered modems not to mention potentially clipping your upstream lasers if you have an HFC network.
You need to move away from this.