Understanding FTTC data path to internet

[BlueComp]

I have an interesting issue where I have three sites on an industrial estate with FTTC.  1 site is on Zen, 1 is BT and 1 is Gamma Telecoms.  Both the Zen and BT lines are suffering from 10+% packet loss.  The Gamma line is fine.

In troubleshooting I've found that support and Openreach engineers appear to have sketchy or diverging understanding about what actually happens to data at / beyond the DSLAM - some refer to 'frames at the exchange', others to BRAS, some claim that BRAS are no longer used, etc.  I've looked at a lot of the ADSL stuff on Kitz but think it's outdated for FTTC.

1. Where does the network change from switched to routed?  My first hop OUT is 81.134.160.1 and my last hop IN is 213.120.163.197.  Are these at the cabinet, the exchange or deeper towards core network?
2. Why does 81.134.160.1 'BT-ADSL LNS' only feature on the outbound route?
3. Am I correct that the layer 2  / PPP path goes: copper to cabinet > fibre to exchange ?L2TP Access Concentrator?  > fibre to  ???   L2TP Network Server  ? 
4. This is an incidental question, but Gamma are described as having their own access network - do they have their own cabinets / DSLAMs?  Or use a different architecture?

Thanks for helping my understanding 🙂 

[BlueComp]

**This is just for fun / speculation as the issue is under investigation by Enhanced Support / OpenReach / BT Wholesale**

Two sites on Zen and BT, FTTC, IPs 88.97.13.173 & 81.133.250.29, sites are about 150 meters apart.  Both have suffered 5-20% packet loss for around 6 weeks.  Copper status and VDSL statistics are great, great speeds, no drops, few errors etc.  Many many Pair Quality Tests have been conducted 😉

Logging shows a very noticeable improvement in packet loss in the wee small hours (daily):

Lift and shift has been carried out for want of a better idea.

Running traceroute and pathping, the loss appears to occur on the last hop  to the premises - ie from outside you can ping all the hops except the fault premises without loss

Engineers have also ruled out CPE, hot VP, electrical interference (REIN, SHINE).

I've now got a fiver on 'broken BRAS', but I know diddly squat about VDSL delivery, so would love to hear other wild guesses.  Have at it!

[Keith_Beddoe · Accepted solution]

@BlueComp 

You would be better posting on the BT Business forum, as this is the residential one.

https://business.forums.bt.com/

Thanks