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Author Topic: ATM VPI VCI ADSL (an old chestnut, apol)  (Read 739 times)

Weaver

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ATM VPI VCI ADSL (an old chestnut, apol)
« on: May 02, 2018, 07:57:43 AM »

I noticed that the factory default ATM VPI VCI settings on my DLink modems (ADSL) are something like VPI = 0, VCI =33 (or maybe 34, I forget). I doubt that VCI-33 works.

Kitz points out that it should be 0 / 38 for BT lines. Is it different for some LLU services?

I don't understand how these fields work with virtual circuit-based muxing and RFC2684 (RFC1483)? I realise that I've never understood it properly. Do these fields get set uniquely for each vc that is set up to carry each type of ‘higher level’ protocol eg IPv4 that is in AAL5 payload?

If that theory is correct, and the values change anyway per carried protocol, then what do the standard per-ISP runes quoted - such as 0 / 38 - mean?

Any reading matter / links ?

I looked at the wikipedia article on ATM for guidance on these matters, absolutely as clear as mud.
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CarlT

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Re: ATM VPI VCI ADSL (an old chestnut, apol)
« Reply #1 on: May 02, 2018, 08:36:10 AM »

Different depending on telco carrying it. VP + VC = PVC. PVC needs to be correct to enter ATM switch and inform it what to do with it. In the case of ADSL the DSLAM is on ATM switch duty and lines are mapped to VPs statically. The PVC they arrive with is configuration on the BT side, it could have anything listed there as the ATM headers are rewritten anyway for transport from DSLAM.

Think about the layering. IP is inside PPP, PPP is inside ATM. Everything on the circuit is inside PPP so it all goes inside that PVC.

PVC 0/33 is probably default for another telco somewhere.
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Weaver

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Re: ATM VPI VCI ADSL (an old chestnut, apol)
« Reply #2 on: May 02, 2018, 08:58:51 AM »

Good point about the layering. I forgot about PPP.

In fact in my own case it would all be PPP in PPPoE in ethernet in rfc2684 in aal5 as I am PPPoEoA. So just as you say, only one PVC in fact.

Thanks for clearing that up.
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CarlT

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Re: ATM VPI VCI ADSL (an old chestnut, apol)
« Reply #3 on: May 02, 2018, 11:24:06 AM »

You're very welcome. After working in networking for as long as I have you'd hope I have my uses  :D
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ejs

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Re: ATM VPI VCI ADSL (an old chestnut, apol)
« Reply #4 on: May 02, 2018, 07:32:47 PM »

I think that with multicast TV over ADSL2+, the general Internet traffic goes on VP/VC 0/38, and the multicast TV traffic goes on 0/35.
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burakkucat

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Re: ATM VPI VCI ADSL (an old chestnut, apol)
« Reply #5 on: May 02, 2018, 11:51:58 PM »

I believe that KCOM use VP=1 & VC=50 just to be different.  :-\
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