However, because of the integration of voice, data and video (convergence) and the emergence of fiber connectivity, companies are now beginning to understand the value of good preventive maintenance and upkeep of the once neglected hardware.
Today, modern PBX's are housed in environmentally-controlled rooms. They are usually installed on elevated floors to help with air-flow and with easily accessed cable plant.
There are three major PBX issues that you should be aware of:
- A PBX is usually DC powered, supported by backup batteries.
- The Platform's architecture should be totally redundant.
- A PBX's central function is high speed switching. Hence the term - Switch.
If you see another cabinet beside the PBX - it is probably a Voice Messaging System. Some call it Voice Mail - some call it Phone Mail.
PBX's and Voice Messaging systems communicate via analog ports. I will go into this in greater depth later.
Back to the PBX - let's use a 2000 port Platform as an example:
Physically, this type of system will probably be three separate cabinets.
Within each cabinet will be three or four shelves.
Each shelf will probably contain 24 slots
Each slot will contain a board, blade or card.
Each card will support 8 or 16 ports.
Each port will support a single phone. (Hard phone line.)
There are usually two shelves that contain the telephony control cards along with the hard-drives and CPUs for the PBX.
The processor clock speed within the PBX typically runs at 386 Mhz. Newer platforms run 486 processors.
Each of the two telephony control shelves is a duplicate of the other.
Total redunancy. In a failure or switchover scenario, this redunancy ensures that calls will not be dropped or cut off.
Another component of the PBX architecture is the universal back-plane within the cabinets. Each cabinet shelf has a separate back-plane. This is where the Platform is linked via amphenol* connections to the Main Distribution Frame. *Most are hard-wired.
Cards/Boards/Blades
Cards or blades within a PBX can support either analog, digital, 4-wire E&M or T1 connections.
OK - enough of this stuff.
Tell me how I get a phone (Ext 2275) connected from the PBX to the desktop.
Here we go:
- Shelf within PBX = 02
- Slot within PBX = 01
- Free Card/Blade port = 14 out of 16
- Phone circuit address = 02-01-14
From the back-plane a feed-cable runs to a circuit break-out panel on the Main Distribution Frame. If you read the labeling - you should easily find the circuit address.
If the line is going to an area within the same building - then the circuit is jumpered with one pair of 24 (AWG) gauge cable to an Intermediate Distribution Frame. Let's say Pair 202 runs on a hundred pair intra-building feed-cable to the second floor.
On the second floor (usually in the janitor's closet again.) - find Pair 202 on the Intermediate Distribution Frame. We want to go to Station 275 which runs to Room 275. Jumper a cross-connect from Pair 202 on the Intermediate Distribution Frame to Patch Panel Station Cable 275.
Bingo. Go to Room 275 - plug in the phone. Dial Ext 2275. You should get a busy signal.
Seems a little difficult, but it's not really. What if I told you that some sites do not have any kind of numbering plan or intermediate frames. How would you put in a phone to the desktop in such a case?
Stay tuned for tips and tricks later.
Now that we have dial-tone - how is that phone going to function?
Are we going to give the user long distance access, transfer capabilities, intercom, call-back, etc.?
How is it done? The quick answer to that lies within the PBX software.
The features of each individual phone are controlled by the Class of Service. Each function can be enabled or disabled by the person who controls thePlatform. He or she is usually known in the profession as a System Administrator.
As a technician, you will soon discover that no-one on the premises fully understands Class of Service functionality.
Should you care? Yes, you should because if you set up a working phone and are able to place calls on it - you may be under the illusion everything is simply wonderful and leave the premises.
Forty miles down the freeway - you are called and told that the phone you installed does not work. You return only to find that the phone works fine but the user could not place a long distance call. Therefore, the phone doesn't work, right?
You are left looking like an idiot even though you had no control of the Class of Service which controls whether a person can place a long distance call.
Therefore, be sure to ask the right questions once you have installed a phone. It will stand you in good stead.
Plea with your boss to take a PBX Administration Class. That way - you're covered, no matter what.
You might meet with some resistance at first, because training costs on PBX Platforms are not cheap. Employers may spend up to $3,000 to have you trained on a Platform only to find that you are then lured away by another company in need of your newly acquired system administration certification.
There is an ethical question here that only you will be able to answer. Your company spends $10,000 on your training and certification and another company whispers in your ear to offer you a substantial raise to move over to them. What do you do?
Your call. Just remember that you owe your present employer a lot of thanks for investing in you. Is that faith in you worth anything? In my opinion - a lot.
Never sell your boss short.
This concludes the Private Branch Exchange (PBX) page. I hope you are still with me and that things are beginning to fall into place for you. |