Wednesday, September 14, 2011

IP PBX vs Legacy PBX

A customer asked for the differences between IP PBX and legacy PBX.
IP PBX is usually a powerful linux/unix server with an Asterisk-based PBX software.  Examples are Elastix and Trixbox.  A PRI T1/E1 or FXO card is required to connect the telco side.
Vendors such as Digium, Snom and Grandstream also manufacture all-in-one appliances that simplifies the installation and configuration. 
Features and flexibility
Being a software, IP PBX  is more feature-rich and customizable than a traditional PBX.   For instance, we can customize IVR, call queues, ring groups, followme, voice recording and many other features in the IP PBX.
The IP PBX also provides a web-based utility for IT staff to conveniently perform daily tasks.  
Transmit audio over internet to save money
Voice is transmitted over a packet network (Internet, LAN) rather than phone lines.  Though it sounds trivial, it has important implications:  it can help to save money. 
The main office can communicate with remote branch offices over internet and enjoy free calling among the extensions.  No IDD charge is incurred. 
Note that we need to use IP phones with IP PBX.   These phones are connected to your company LAN, rather than telephone lines.  There are many brands of IP phones and they are inter-changeable.  There is basically no vendor lock-in.
The IP nature also enables remote extensions whereas we can have extensions outside office, such as home, trip or even our mobiles* as long as internet access is available.  It is an extra convenience that legacy PBX cannot offer. 
*a sip client app is required
Voice quality
You will find voice quality the same.   The company LAN is a 10/100M ethernet, which is a pretty sufficient bandwidth for voip conversation.  A typical two-way call merely consumes around 150kbps.
Nevertheless, IT administrator should pay more attention to conditions that could saturate the LAN, e.g packets sent from infected PC or lengthy download/upload.
Separating voice and data into two subnets is suggested.
Ongoing support
In the long run, customer will see savings in maintenance costs as the IP PBX configuration is self-serviced. 
When your business grow, you need to add more extensions.   It can be accomplished easily in IP PBX without adding ports as legacy PBX requires.  You will find IP PBX more scalable.
Buying considerations : Replacement or parallel run
Customer is generally more interested in IP PBX when starting a new office or the legacy PBX is almost fully depreciated.  Some customers also chooses to install the IP PBX in front of the legacy PBX such that they can start using the new features while keeping the old investment.  In this scenario, the IP PBX has a 2-port T1 card, one port connecting to telco and the other connecting to the legacy PBX via a cross cable.


  1. An IP PBX is a complete telephony system that provides telephone calls over IP data networks. All conversations are sent as data packets over the network. It is much easier to install & configure than a proprietary phone system. Thanks a lot.

  2. I actually prefer the quality on my SIP phone. I used to laugh at the claims of "HD" voice, but I am convinced. It is pleasant.