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Author Topic: ADSL/ATM Error recovery and retransmission  (Read 5514 times)

kitz

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Re: ADSL/ATM Error recovery and retransmission
« Reply #15 on: November 14, 2015, 09:26:19 PM »

Sorry, just noticed that ejs has said basically similar to me

Quote
I don't think there is any retransmission without G.INP, otherwise they wouldn't have needed to do G.INP separately

The quote ejs found from technet, and specifically the part he embolded,  confirms what I said in my post above.
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WWWombat

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Re: ADSL/ATM Error recovery and retransmission
« Reply #16 on: November 14, 2015, 10:08:49 PM »

While you are considering the ATM angle, I can point you separately in the direction of what mechanisms exist in DSL systems in general, courtesy of the Broadband Forum...

Google for TR-197, "DQS: DSL Quality Management Techniques and Nomenclature".

Note that this document covers the full spectrum of techniques, but not all are employed by BT. And techniques have evolved separately in 20CN, 21CN and FTTC.

You would think that, if ATM could offer any quality facilities (such as a retry mechanism) to DSL, it would be mentioned in that document. Even if only as part of the " External" section.

Instead, the only mention of ATM comes within the DSL retransmission section (which is the G.INP mechanism mentioned earlier), saying that retransmission DTU units must be sized to hold an integer number of ATM cells.

Having read that made me think ... We do remember that, while ADSL is designed to accommodate ATM cells, it isn't actually ATM, dont we? Even if ATM included a retry mechanism (I don't think it does either - it is a low latency, guaranteed latency, design), it wouldn't apply because the link between the DSLAM and ADSL modem is not ATM.

That link is governed by the ITU ADSL specifications, which don't have retries. If it did, G.INP would have to accommodate it, and it would be mentioned in TR-197.
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aesmith

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Re: ADSL/ATM Error recovery and retransmission
« Reply #17 on: November 15, 2015, 02:15:46 PM »

Thanks for all the comments.  What I'm finding is that proving a negative is difficult, there's no trace of an ATM retransmission mechanism mentioned in any of the ITU documents, rfcs or SINs that I've looked at, other then G.INP (ITU G.998.4).   I'm finding lots of stuff that I'm not certain about, for example Wombat has just said that DSL is not ATM - but does it not carry ATM?  That's got me wondering, I always assumed that the modem recreated the ATM PVC from the DSL, then rebuilt PPP and in turn the Layer 3 packet from the ATM.  However I'm not sure where I got that from, maybe just made the assumption because Cisco calls the DSL interface ATM (and the terminology itself of course, PPP over ATM).

I find it perfectly logical to deduce that if ATM already had a retry mechanism, then either G.INP would not be required, or the definition would explain how it differs and offers an improvement over the existing ATM mechanism.



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ejs

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Re: ADSL/ATM Error recovery and retransmission
« Reply #18 on: November 15, 2015, 09:11:30 PM »

I did eventually track down an ATM cell retransmission mechanism, but I don't think it's used with ADSL, if it were then it would surely be mentioned somewhere in some ADSL related document.

I'll send it in a PM to aesmith rather than post it publicly, just in case someone at Plusnet reads this and then points to the document as proof it exists.
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aesmith

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Re: ADSL/ATM Error recovery and retransmission
« Reply #19 on: November 16, 2015, 10:00:41 AM »

Got your PM thanks.
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WWWombat

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Re: ADSL/ATM Error recovery and retransmission
« Reply #20 on: November 16, 2015, 11:36:07 PM »

Thanks for all the comments.  What I'm finding is that proving a negative is difficult, there's no trace of an ATM retransmission mechanism mentioned in any of the ITU documents, rfcs or SINs that I've looked at, other then G.INP (ITU G.998.4).

There's probably a reason why!

Quote
I'm finding lots of stuff that I'm not certain about, for example Wombat has just said that DSL is not ATM - but does it not carry ATM?
I think the best way to think of this is to separate ATM into two - one being the networks, protocols, routing, rules, specs; the other being the packets of data: cells of 53 bytes, made of 5 bytes of header and 48 bytes of payload.

At one time, BT's core network will have been based on ATM. The whole enchilada, including networks, protocols, routing, rules ... all to carry those 53 byte cells.
http://www2.bt.com/static/i/media/pdf/atm_vpn_dec05.pdf

(In the days of poor QoS in the IP world, ATM was the telecomms solution for a converged network - small cells allowed low latency voice to coexist with high volume data; "ATM" as an overall concept is all about the routing & transmission of these small cells to keep voice working well).

But right on the access edge, the network hits the copper lines and ADSL. Here, the protocols and rules are all pure ADSL, not ATM. At this point, there is little need for the "big"  network concepts, and every need for techniques to cope with long, dodgy copper lines. However, the data being carried, right down at the bottom level, remained in the form of little 53 byte packets, aka ATM cells.

The 5 bytes of header become a terrible overhead in there, given that most lines carry a single virtual circuit.
http://aa.net.uk/kb-broadband-how-atm.html

So we get to see that ADSL is carrying ATM cells, but it isn't actually performing any real function that we'd think of as a core ATM network function.

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That's got me wondering, I always assumed that the modem recreated the ATM PVC from the DSL, then rebuilt PPP and in turn the Layer 3 packet from the ATM.  However I'm not sure where I got that from, maybe just made the assumption because Cisco calls the DSL interface ATM (and the terminology itself of course, PPP over ATM).

Yup, it is indeed that way - at least when ATM mode is used in ADSL. There are also modes for SDH and PTM (which is more used in NGA). More information in the area here...
http://blog.farnz.org.uk/2010/02/on-pppoa-pppoe-atm-and-adsl.html
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aesmith

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Re: ADSL/ATM Error recovery and retransmission
« Reply #21 on: November 19, 2015, 02:42:18 PM »

Thanks.  I've sort of let ATM pass me by since the early days when it was supposed to be "the way forward".  You probably know the time period I mean, when Fore was going to be the next Cisco, IBM backing ATM to the desk rather than Fast Ethernet, etc etc.
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