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[[File:Phone_systems_001.png|x300px]]
== Communications Lines ==
The first part is how many phone lines go into the system. There are many types for different volumes.
=== PSTN ===
Packet Switched Telephone Network. This is a standard phone line. It uses voice tones to deliver the call. One line, one number.
=== BRI ISDN ===
Basic Rate Interface Integrated Services Digital Network. This is a digital line. One line one number.
It is also possible to get Dual ISDN, which gives you two lines per connection. Two lines two numbers.
=== PRI ISDN ===
Prime Rate Interface Integrated Services Digital Network. This is a digital line. It contains 32 "channels" on a single connection. 2 are control channels and 30 are lines which can be used for conversations. When you have these channels used for voice, that allows 30 simultaneous calls in and out.
A PRI in Ireland will come with 100 virtual numbers.
PRIs are costly. There is a slightly cheaper option. The cable into the premises is the same but the phone supplier limits the usable channels to 16 instead of 30. This is called FRA or Fractional Rate Access.
The connection in technical terms is called an E1 (in Europe) or a T1 (in America)
=== Broadband ===
Phone calls can also be transmitted over an Internet connection. This technology is called VoIP. Voice over Internet Protocol. https://www.youtube.com/watch?v=2x3Ie6VZ_sg
A phone call can use tiny amounts of data. A good rule of thumb is 100kb up and down. Up means going from your premises to the internet. Down means coming into your premises. Not all broadband has equal up and down. It is important to check. So a 1Mb up and 1Mb down, should theoretically hand 10 simultaneous calls (10 x 100Kb).
To convert a phone call from the traditional PSTN network to a data connection requires an interface called a SIP trunk. Session Initiation Protocol. https://www.youtube.com/watch?v=n9cT1Lq9Lg8
== PBX ==
=== Introduction ===
So now we have some communication lines to bring in external conversations.
The traditional approach was to just handle phone calls. With the advent of data it has opened up the technology to also accept a number of other technologies such as email, livechat through websites and others. More on this below
When the call comes in, we need everyone in the business to be connected and ready to chat. A PBX or Public Branch Exchange is required. Previously these were called a Phone System. The new name is a Unified Communications Platform where it does more than just phone calls.
The name is an evolution from manual switching, through electronic switching to modern server computers.
{| class="wikitable"
|-
! scope="col" style="text-align:center;" | Manual
! scope="col" style="text-align:center;" | Electronic
! scope="col" style="text-align:center;" | Server
|-
| [[File:Phone_systems_002.png|x250px]]
| [[File:Phone_systems_003.jpg|x250px]]
| [[File:Phone_systems_004.png|x250px]]
|}
