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Author Topic: TalkTalk MPF/LLU: Intermittent caller display and line reversal issues  (Read 581 times)

DonnyDave

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I recently moved my telephone line from BT to a service which uses TalkTalk MPF/LLU. Since then my caller display has been intermittent, working most of the time.

I have made many hundreds of test calls to try and find a pattern. This included disconnecting internal wiring, and using different telephones and caller display units not normally used on this line.

As this is a long posting I have added headings to aid readability.

Test method and fault found

With an analogue voltmeter connected I noticed that the line reversal that should occur to signal an incoming call does not always occur at the correct point in time. When it doesn't do as it should one or more of the caller display units fail. When the caller display units work, the line reversal occurs correctly.

If you're reading this and aren't familiar with the technical aspect of the line reversal here is an explanation: There is 50V DC across the pair (wires) when in idle state (handset on the cradle with no call coming in). When a call is made to the line the polarity reverses (known as a "line reversal"). Ringing current (which rings the bell/warbler/sounder on the phone) is not sent down the line until about a second after the call is made and the line reversal happens.

To emphasise: the caller gets ringing tone immediately but the phone line's ringing isn't in sync with that it is about one second behind it. The line reversal happens (or should happen) at the point the caller starts hearing the ringing tone, which is immediately after having dialled the last digit.

The problem is that the line reversal sometimes is delayed and does not happen until the ringing current starts. On those occasions that it is delayed in this manner, the meter's needle gives a little twitch and returns to the 50V mark at the point it should have.

BT's specification of CLI

BT SIN 227, which sets out the operation of the CLI service, and SIN 242, which sets out the operation of CLI service terminal equipment, show the line reversal to occur before the CLI data is sent. Thus, it would seem that malfunctioning line reversal would result in malfunctioning caller display.

What put me on to this

What put me on to the possibility of a line reversal issue with respect to my caller display fault was a thread in the TalkTalk Community forums: Caller ID works only sometimes (50/50)

The person had just moved from BT to TalkTalk LLU and caller display become intermittent. He had an answerphone/fax device which contained a relay that clicked twice before ringing started. Having moved to TalkTalk the timing of the clicks became irregular or non-existent. When the clicks didn't happen the unit did not show any CLI. It struck me that the trigger for the relay is the line reversal.

Steps taken to try and resolve the fault

TTB have made changes at the exchange (whatever they are), restarted the line card and carried out a lift-and-shift, all to no avail. The OP of the above thread apparently had his issue resolved following a lift-and-shift.

It has got to the point where a shrug of the shoulders has been given and a "there is nothing further we can do".

How common might this issue of poor/delayed line reversal on TalkTalk MPF be? Has anyone on here come across it?

Further issue with line reversal on my TalkTalk MPF line

A further issue I have found is that if a call is made to the line and the line reversal occurs correctly (when it should), and the call is terminated before ringing current gets a chance to start (i.e. terminated within about a second), the line stays permanently reversed. I tested this and left it in that state for a couple of hours. Only lifting the handset rights this. Perhaps this is a bug in the design of the line card and not a fault. This does not occur with my BT line and the line reversal is dropped for a call that is not long enough to ring the bell.
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burakkucat

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Re: TalkTalk MPF/LLU: Intermittent caller display and line reversal issues
« Reply #1 on: September 09, 2017, 10:58:29 PM »

Welcome to the Kitz forum.  :)

Having read through your post, I decided to experiment.

I am a TalkTalk customer but do not have "caller display". Even before I set up my experiment, your post got me thinking and I have to admit I am totally unaware of where (and how) a TalkTalk telephony services will interface with the national network. I'm perfectly clear with how the metallic pathway links the NTE5, in "The Cattery" (via the DP, PCP, MDF & HDF) and the TalkTalk MSAN in the local exchange (EABSE). My broadband connection to the Internet takes that metallic pathway and finally appears at the "far side" of the TalkTalk MSAN on an optical fibre and continues on their (TT) backhaul. As I understand it, my telephony circuit exits the "far side" of the TalkTalk MSAN but where, how and what happens next is a total blank.

The experiment.

I connected my Fluke 8021 to the pair and noted a steady +49.7 V DC. From a mobile phone I called my land-line telephone. After a brief pause whilst the circuit was set up, the polarity of the pair was reversed. The largest value noted was -60 V DC but, obviously there was a significant swing in sympathy with the "ringing" current. The voltage polarity remained negative. In significantly less than one second from the reversal of the line polarity, both the incoming call signal on the land-line telephone and the confidence tone, heard on the mobile phone, begun in absolute synchronism. At call clear-down, initiated on the mobile phone, there was a brief moment when the land-line telephone continued with its cacophony before ceasing (with a little squawk). The above experiment was performed a further two times . . . with absolutely identical results.

With my three out of three result, I am not sure how to interpret your anomalous results.  :-\
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DonnyDave

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Re: TalkTalk MPF/LLU: Intermittent caller display and line reversal issues
« Reply #2 on: September 10, 2017, 02:41:00 PM »

Hello burakkucat and thanks for the welcome.

I intend to upload a video of my test set-up, demonstrating correct operation and those instances where it doesn't work properly. I will post back with a link when it is done.
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burakkucat

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Re: TalkTalk MPF/LLU: Intermittent caller display and line reversal issues
« Reply #3 on: September 10, 2017, 06:23:45 PM »

I intend to upload a video of my test set-up, demonstrating correct operation and those instances where it doesn't work properly. I will post back with a link when it is done.

Thank you. That will be interesting to see.

Thinking about BT's SINs, they define how both BT's & other users' equipment should operate and respond when interfaced to the BT network.

In your case, described above, it appears that the TalkTalk equipment is not consistently operating in the SIN defined and expected manner when a signal is passed from the BT network to the TalkTalk network. If it does not cause a malfunction of or damage to the BT network I am uncertain what -- if anything -- can be done about it.  :-\
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