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newbie question on for the PBX world.

Hello to you all... I am posting this question here because EE does not have a dedicated section for telephony questions.  My question is in regard to the function of standard PBX systems.  I have worked for companies where every cubical/office has a phone connected to the companies central PBX system.  This office also has its own "phone number" where people (From outside the office) can call directly to.  I have since switched jobs and work for an organization that uses older key systems ..etc with a receptionist (not automated) forwarding calls.  I would like a  Medium - High level explanation on how PBX's are able to have direct lines to people's desk with their own phone numbers.  

Say you had 500 employees and 500 phones, if you wanted to have a setup for each employee to have their own outside phone number would you have to then have 500 lines which somehow forwarded  by the PBX? This is not practical so I am assuming no.  What is a good ratio to keep between outside lines available and actual phones?

thanks all for helping with my learning process!
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akant74
Asked:
akant74
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1 Solution
 
Joel_SiskoCommented:
The term you are searching for is DID, or Direct inward Dial,

DID is a service that allows for a company to buy a block of numbers 555-1200 thru 555-1300 lets say, then they supply 10 circuits, these are essentiaally voice paths, that when any one of those numbers are dialed the call is sent over them. The PBX recieves the digits and routes the call that has bed desiganted for that number.

This is the High level explanantion, I will include a more detailed in the following post.

Kindest regards

Joel _Sisko
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Joel_SiskoCommented:
Also here are some resources: (Netwons dictionay is the bible for the PBX industry)

Some books to study:

Newtons Telecom Dictionary, by Harry Newton

Guide to T-1 Networking: How to Buy, Install & Use T-1 From Desktop to Ds-3
by William A. Flangan


Here is a list of links to keep you busy this weekend:

http://www.protocols.com/pbook/VoIP.htm
http://www.protocols.com/pbook/VoIPFamily.htm    basic overview of VoIP

http://www.cisco.com/warp/public/788/signalling/net_signal_control.html      basic telecom link

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a0080174804.shtml    debug sample

http://www.cisco.com/application/pdf/en/us/guest/products/ps5037/c2001/ccmigration_09186a008011bd39.pdf   all of IOS voice commands

MGCP rfc's below

http://www.faqs.org/rfcs/rfc2705.html
http://www.faqs.org/rfcs/rfc3525.html
http://www.faqs.org/rfcs/rfc3660.html
http://www.faqs.org/rfcs/rfc3661.html 

SIP RFC's

http://www.ietf.org/rfc/rfc3261.txt
http://www.ietf.org/rfc/rfc3311.txt
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akant74Author Commented:
What type of connection is normal supplied from the teleco for DID service.  I assume you cannot have them simply install regular POTS line jacks at your location and you have to have a T1 or something similar setup?  

Am I correct in understanding your comment as: If you purchase a DID block of numbers from the teleco for 555-1200 - 555-1300 you would logically have 100 numbers for your organization... but with only 10 circuits you can only have 10 Active, External conversations going on at any given time. ??
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Joel_SiskoCommented:
In regards to DID service it was typically supplied via specialzed analog circuits that you ordered specific for the service. Today you can still get the service via analog, but most carriers push towards using digital circuits (T1/PRI). Typically the rule of thumb is 10 DID numbers per DID circuit, depends on call volume type of business.

So the scenerio is this:

The PBX is programmed to handle DID service, phones are assigned typically the last 3 or 4 digits of the DID number. Depending upon your PBX, the phone could be programmed to have multiple keys that have the same DID number assigned to them. Rather than thinking of it as an individaul number, think of it as voice paths.

So you would program,lets say the first 3 keys on the digital phone for the DID number 555-1200, then you have the carrier only send you the last 4 digits of the inbound calls.

So you call from the outside for 555-1200, the carrier routes the call to the first DID circuit, it sends a siganl to PBX a call is coming, the PBX responds, then the carrier sends the digits 1200. The PBX routes the call to the extenstion that has beed programmed for 1200. The phone rings you pick it up. Now as you are speaking with the first caller, a second person dials 555-1200, the carrier routes the call to the second DID circuit, same handshake, then PBX routes the call to the phone, it then rings the second button, which you could then put caller 1 on hold, pick up caller 2 and talk. Then caller 3 dial 555-1200, same events happen. The fourth caller dials, but since the phone was only programmed for 3 voice paths, the PBX determines the phone is busy and forwards based upon programmed rules (typically goes to Voice mail).

This is the medium level explanantion. Let me know if you need adtional help on this.

Also the sequence of events are the same for the most part in regards to analog DID or Digital DID.

Kindest regards,

Joel Sisko

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Joel_SiskoCommented:
In regards to the block of numbers, you buy the numbers and then you but the circuits, so though you have a 100 numbers, you can only haqve 10 conversations if you only have 10 circuits. But you are not limited to this, you could have 50 circuits with a 100 numbers. Or 2 circuits for a 100 numbers.

The goal of DID service is to provide automatic call routing without the high cost of individual leased lines and physical equipment.

Also in the early days the circuits were only for inbound, but that has changed in many areas and the same circuits can be used for outbound dialing also, this is typically on a digital circuit, and referred as a DIOD (Direct Inward/Outward Dial)
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akant74Author Commented:
I need to clarify here for one moment PRI is an arconym for ISDN wich has an A and B channel combited to 128k A T1/DS1 has 24 channels which all could be "partioned" for voice traffice like in our example above.  So if we had a T1 for this service it could have 24 voice paths because it has 24 channels?  Likewise an ISDN/PRI could have 2 voice paths because it has 2 channels?  or am I way off?

I have an idea what to expect if we orded DID service using a digital line... but if they use a specialized analog circuit to deliver service what can I expect the phone company to install for me.  Would they put in a large block of some type that I would have to wire to my FXO interfaces?  So if I had 10 voice paths delivered to me on this special analog circuit I could expect a punch down block with 10 connection points that would then need to be hardwird to my PBX?

Thanks :D

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Joel_SiskoCommented:
Analog DID service is connected to specialzed PBX cards for that purpose, if you connect a phone directly to it it can cause many types of problems. The analog DID circuit is a two wire circuit that is normally wired to the PBX making sure to reverse the polarity when doing so. From a wiring perspective, the analog DID and POTS circuits use the same wiring.

In regards to T1/PRI: This is something 90% of Data and Telecom people get mixed up, the book on Guide to T1 Networking is good to have.

T1 is really not a service at all, but over the years the term has ben misused but excepted as the actual service. T1 is the actual medium that digital services are delivered on. The simple definintion of the T1 is a 4 wire circuit that has a data thruput of 1.544 MB per second. The circuit uses repeaters/signal booster every 6,000 feet. The services that can run on the T1 are PRI and DS1.

DS1 (Digital signal 1) is 24 DS0 (DS0 is a 64K channel), DS1 is that what is normally reffrered to as the T1 service in regards to voice,
PRI is also a service that is comprised of 23 B channels and 1 Data channel for signaling. So DS1 deleivers 24 channels of voice, where as PRI delivers 23. There are variations of this, but this you can find out from the forementioned books.

The best way to look at T1 and the services, the T1 is the highway, DS1 aqnd PRI are the type of cars you are driving.
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akant74Author Commented:
Ok I was way off then on my definitions.  BTW, I have ordered the T1 networking book that you recommended ;)  So in regards to ISDN, PRI has no dealings with this?  I thought that using a ISDN line could be used in the same was as a T1 online for smaller installs.  You are right about the confusion with the T1 term.  There are even some cable internet companies selling T1 service and use this term only represent the speed the speed that you would be receiving. Hmmm ;)

As for DID service if an analog circuit was the only delivery method in an area for the service (we have some remote sites in the sticks) how would that then be connected to an Asterick type of installation?  From what I understand Digitum or whatever does not make cards for this service and would require the use of the T1/E1 cards... so therefor would not be a possible solution.
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Joel_SiskoCommented:
ISDN has two flavors: BRI and PRI

BRI is 2B+1D
PRI is 23B+1D

In regards to the analog DID issue, you could use a Cisco router that supports analog DID, then have the router be a SIP gateway for the Asterisk pbx.
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Joel_SiskoCommented:
akant74,

Also Newtons Telecom Dictionary has all the defeinitnions you would ever want. Almost a thousand pages strong.

Hope all this has helped!

Kindest regards,

Joel_Sisko

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Joel_SiskoCommented:
akant74,

Hope all is well, do you need any more info?

Kindest regards,

Joel _Sisko
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rwotherspoonCommented:
Joel_Sisko has earned this one .... well done indeed!   ....
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Joel_SiskoCommented:
Thanks rwotherspoon!
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akant74Author Commented:
Joel,  I was going back over this question you answerd for me last year and am still damn impressed that you provided this much detail.  It is hard to find people in this world who will take the time to teach you something.  I really do appreciate this!

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