Please could you tell me what the difference between PBX and key system is?
Thanks in advance
That is a really good question and I don't have a strong background in traditional telephony. To me the lines are increasingly blurred, but traditionally the functionality was quite different.
With a PBX each user is assigned an extension number and dialtone (at least initially) is pulled from the PBX itself. When a user dials an access code like 9, that is part of the call routing rules on the pbx to hand the call to the PSTN. Typically there would be no appearance of a trunk line. Incoming calls would be routed to a user through a dial plan, auto or actual attendant. The user likely would have no idea of the status of the pstn trunk ports. This is basically a scaled down switch that is somewhat similar to that of the phone company.
With a Key system, a user hooks to an outside line for an incoming or outgoing call. Each phone typically would have all of the trunk ports that the user should utilize. If an incoming call comes in the phone may ring and the trunk port will flash on the phone. Multiple users may share this group of ports. When a user needs to place an external call, it is likely that he or she picks up one of the trunk ports and draws dial tone from the pstn. Internal calls would use the intercom side of the Key system. The Key system is more of a way to make use of multiple trunks than a true switch but these lines are constantly grayer.
If you think back to the analog key systems of about 20 years ago, you will remember a traditional desk phone with a row of buttons on the bottom. This was an analog key system. There may be a couple of lines for phone to phone communication, a green and a red line for example. There also may be a button dedicated to a paging system that is tied into the system. The remaining buttons may be trunk ports. So how does this work? A call comes in on one of the trunk ports. Since it is the responsibility of the reception to answer these calls, she picks it up when those blink and or ring. He or she gets some information. Then she pages Mr. Manager to pick up line one. If the manager is of the nature that he don't want to be bothered by just any one, he may want conversation with the receptionist first. In this case, the receptionist might page and say, "Please pick up the green line." The receptionist and the manager may have a conversation over the green line as to how to handle the call.
Now with digital key system, there is the combination of this functionality with the ability to call phone to phone (intercom). Additionally, most PBX have key system functionality and most Key Systems have PBX functionality. However, most are better at one or the other. I would say Cisco's products function better under the PBX model than the Key System Model. This is crucial when understanding what is being replaced and how it will function.
People simply don't like change in their phone system and it is extremely important to lab up the call flow and demonstrate and guy the customer buy in even before providing the final bill of materials. Some scenarios need phones with many line appearances. These are typically harder to manage, but in instances when everyone needs to see the status of everyone else there is a need for a number of line appearances equal to the number of user cubed plus one or more (users X users + 1 or more). If you find yourself in this situation you really need to have some in-depth discussions. However, users used even a very small key system are accustomed to havang 15 or more lights indicating trunk or handset status. Compare this to the typical Cisco 7941 (2 lines), and the 7961 (6 lines). These are extremely important concepts and discussions that should be agreed on early in the planning process of any system. Hope this helps.
Thanks a lot for your post, Paul.
In the beginning i thought there wouldn't be much difference. Now it seems a bit difficult for me cause i have no analog telephony background either and i got acquainted with IPT only few weeks ago. Though I got some vague notion of this i still need to read some docs on it. I hope after having read some more info on this subject your explanation(for which i am really grateful to you) will not sound too difficult for me.