What is shared line appearance?

I've heard about it, but never got to learn what SLA is... I came across this term while I was setting up my SPA942 with an Asterisk server...
rodstevensAsked:
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Ron MalmsteadInformation Services ManagerCommented:
Shared line appearance, is having the same extension onmore than one phone.

For example... I have 4 reps.  They have 4 line appearances, all the same extensions. 101,102,103,104
When a call comes in, rep 1 picks up 101, and the other reps can see that the call was answered.
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feptiasChief DudeCommented:
I would say that what xuserx2000 described is a Busy Lamp Field, not a shared line appearance. There is a distinction, historically, between lines and extensions - the line is a trunk connection (e.g. between the CO and the PBX). So on xuserx2000's phones the lamps for extensions 102 and 103 would come on when there was an internal call between them. A Shared Line Appearance, the lamp should only come on to show when a call is active between the PSTN and your system. It is a throwback to the old key telephone systems which had several analogue trunk lines so each handset would have a key/lamp for each line. When they wanted to make a call, they would press the button to seize one of the lines - this would give dial tone and then they would dial the number. The key/lamps would also be used to answer an incoming call - for example, if two incoming calls arrived at the same time you would be able to choose whether to answer the one on line 1 or line 2 by pressing the button for whichever line.

Asterisk SLA was an attempt to mimic the key phone systems, but (a) it has too many weaknesses to be worth bothering and (b) the whole concept breaks down when the trunks are SIP rather than analogue because the channel numbers are virtual rather than physical.

The SPA942 can be integrated with some service providers/hardware to use a kind of shared line appearance - I've not seen it in action, but possibly Broadsoft, PBXnSIP and Sylantro are names that seem to come up in this respect. Unfortunately, the SPA942 is not too great when used with Asterisk in terms of it not having any keys you can program to act as BLF keys. This makes it unsuitable for use with Asterisk SLA (at least it did when I tested it with Asterisk v1.4).
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Ron MalmsteadInformation Services ManagerCommented:
http://www.asterisk.org/node/48342
That explains Asterisk SLA pretty well and provides an example.

There is absolutely a distinction between lines (trunk) and extensions which can be mapped to/from trunks...I guess I just used the wrong terminology and over simplified it.....
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feptiasChief DudeCommented:
If you have a few hours to spare, you could read the article on my web site which goes into a little more detail:
http://www.smartvox.co.uk/astfaq_asterisk_sla.htm
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