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MTA Questions

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Anonymous (not verified)
MTA Questions


I've read through the forums and can't exactly find the answers to my questions. Sorry if the answers to my questions are readily available in a different post, and if so please direct me there. To begin, a little back story I guess is necessary. To make it short, my work has recently introduced me to the world of DOCSIS. I have a very basic grasp of the underlying technology involved in HFC networking and the provisioning of cable modems. I have been presented with many MTAs (Arris TM502) which are 2 port versions and are also provisioned with a configuration for data. For example, the modem has a config file that gives it 2Mb / 384Kb QoS. Applying everything i've ever known of phone lines, I'm assuming each line uses 64kbit of data per phone line.

1) Is this QoS going to be shared with the phone as well as the data or is there another QoS setup for the phone only?

I'm assuming that if both lines were in use, that's 128Kb/s bandwidth being used, taking away from the 384Kb/s upstream. Using SNMP I am able to view all the signal levels and also the modem configuration, but this only shows the one configuration.

The next question kind of ties into this as well. I have been informed by a co-worker that an MTA is actually 2 cable modems in one. I'm having a hard time believing this considering 1 coax interface on the modem. Applying everything I know of VOIP and cable modem data provisioning, the way I believe an MTA works is that it actually has an interface that connects to a SIP server(depending on the configuration of course but in this case I know it's using SIP).

2) Am I wrong in saying that an Arris TM502 is not actually 2 cable modems in one, but more of a SIP device with an HFC interface?

The way it appears to me is the phone portion of the MTA interfaces with the SIP server which is all transmitted back over the same interface as the data. I think of it as one of those modem / router combos, but with phone being used to provide a dial tone. The arguement that was presented was that when having transmission issues with fax machines connected to an MTA, and no packet loss was present at the time of testing, this was because the wrong modem was being polled for this information. I attribute fax issues, where no packet loss is shown by pinging the internal IP address of the MTA, to a wiring / fax machine on either end/bad ports on the MTA. A co-worker argues that if I was able to poll the correct modem, I would show the problems as my ping tests are only testing the data portion of the Arris TM502 MTA.

I appologize for the long post, but I appreciate any answers to these questions. Also, if you could provide me with any kind of official documentation, this would be greatly appreciated. Thank you for your time.

1. Depends on what version

1. Depends on what version of Docsis you use,

in Docsis 1.0 modems there is one upload speed and one downstream speed.

docsis 1.1+ modems use, usually multiple, Service Flows to determine what speed to use generally most modem manf can support 32 flows (16 upstream and 16 Downstream) but it varies from manufacturer.

The Service Flows control bandwidth usage but service Classifiers determine which service flow traffic flows across, Service classifiers can classify traffic by a lot of different criteria such as Source port/ip dest Port/ip etc..

So one way MSOs prioritize VOIP traffic is to create separate Service Flow for VOIP traffic and create classifiers for VOIP traffic. The way I do it is create a classifier that matches the SIP proxy server and gives it its own 256k of bandwidth up and down and a higher priority.

2. Your analogy about an EMTA (embedded MTA) is mostly correct. In voip terms it is basically an embedded ATA (or Two) in the cable modem.

If your going to use Arris EMTAs I would recommend calling up Arris and getting their PacketAce Software, it creates Docsis and MTA config files and includes multiple example Configs that can help you grasp how it all works together.


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