Does 21CN require new fibre to be blown?

Hi guys,

I have been thinking of this question for ages and it has me stumped:

When BT upgrade an exchange to 21CN, do they use a separate pair of fibres than the old ATM based backhaul for 20CN/Blow new fibres so the 20CN service is not disrupted?

What exactly happens to the physical backhaul part of it? From what I've heard, 21CN uses Ethernet based backhaul.

Any ideas?

Cheers

Hi Chris,

Multiplexing transport protocols, such as SDH (Synchronous Digital Hierarchy) and WDM (Wave Division Multiplexing) allow for multiple services (such as ATM and IP) to be carried over the same optical circuit without, to use your phrase, "disrupting" each other.

It's often cheaper to upgrade the multiplexer, to one with a higher line rate/number of optical channels, than to lay long distances of new fibre.

Hi, thanks for your reply.

When I used the phrase 'disrupting', I meant having to unplug the old backhaul, taking all of the equipment offline and then plugging it back in (I should have been more clearer), effecting all users for a short time (The 20CN DSLAMs will still be in use when it's upgraded to 21CN and after, I believe).

Thanks again for your reply

Edited by chris6273 (Tue 04-Oct-11 08:40:24)

People on 20CN should see zero downtime.

the key word there is "should" as i know from previous experience with bt this was not the case

21CN MSANs are installed alongside 20CN DSLAMs but if you move to a 21CN servcie there will be a short break in service

Sorry for not understanding your question exactly.

An MSAN (and its backhaul) be will brought into service before any subs (end users) are connected to it.
Any DSLAM, as you say, won't be taken out of service until it's last sub has gone, whether that's all at once in an overnight cut-over or in dribs and drabs over a period of time.

Backhaul is (a lot) more than just a fibre patch lead.
Even if they are physically compatible, you couldn't simply disconnect the SDH STM fibre interface of a DSLAM and re-patch the same lead into an MSAN and expect it to work without a significant amount of network re-configuration.

As you probably already know, there's a (hopefully) short period of down time, when an end user's copper pair is re-jumpered over from a DSLAM to a C-MSAN.
As it's more cost effective to migrate end users in bulk, overnight, many users could experience service interruption on the same night for that reason.

Is that any better?

Chris

Each exchange has one or more fibre cables serving it. These could be from 8 fibre pairs to 192 fibre pairs in each. So it is unlikely that BT would need to pull in a new cable.( You couldn't blow a 24pair cable!).

21C uses Ethernet for it's transmission technology rather than the ATM that 20C BB uses so the BB equipment is all new.

However BT could run both over the same WDM transmission system if they wanted to, as each Wavelength can carry an STM64 ( 64 STM1 pipes) or a 10G ethernet path and you can get from 1 to 160 Wavelengths over the same fibre. The more you carry the more the equipment costs!

There should be no disruption when the 21C kit is provided and connected, only when a customer is physically transferred to the new kit. However most faults are caused by people! so there is a likelyhood that faults may occur when someone is working on the new kit especially if it is physically close to the old.

Thanks guys,

The replies I have received are more or less what I was looking for .

Obviously there would be downtime when being re-jumpered but I was talking about when a multiplexer is upgraded for 21CN - is the whole exchange taken offline affecting all customers for a short amount of time while all the new upgraded kit is plugged in?

Cheers

The existing multiplexer is left untouched, a brand new one is installed.

So the whole exchange is NOT taken offline for a short time.