Migrating a private 4 wire circuit to a VPN over ADSL

The company I work for is traditionally a radio communications company. We do so much more nowadays but radio communications is core to the business. As such we look after a lot of taxi companies. Most of the taxi companies we look after had several private 4 wire circuits between their premises and our aerial mast site. A private 4 wire circuit is similar to a telephone line except that all 4 wires are used and they are a dedicated line between two sites. As such they're maintained by either BT or Virgin Media and are expensive.

Each taxi company has a minimum of 2 radio channels (1 has 4). One channel is dedicated for voice and the other(s) for data. Diagram below shows the basic setup for one channel whether it be voice or data:

As already mentioned a dedicated 4 wire private circuit is expensive and most of our customers had at least 2! We came up with a more cost effective and modern solution that also included a backup! So how would we achieve this? ADSL and Wifi! The 4 wire circuit was replaced with a dual WAN router which would have a VPN tunnel configured per WAN port to another router at the aerial mast site. A single VPN connection could of course service more than one radio channel therefore saving the customer £1000s! To get the voice channel across the VPN we used identical VOIP units at either end. They convert line level audio to IP (the main advantage of this is there's no loss and you can literally stream anywhere in the world!) and stream to an identical unit which converts it back to line level audio.

As for the data channels, the operators enter jobs into a PC which then transmits it to an IP-to-serial unit. This then converts the serial data to audio using a modem and then connects to the radio base station. All this equipment is at the aerial mast site so the dispatcher PCs connect to the IP-to-serial unit over the VPN.

As specified earlier the primary VPN connection routes via ADSL. As ADSL fails now and again we had to come up with a backup plan. So we utilised another companies wireless network (whom we have an excellent working relationship with) and put up a wifi connection either by direct line of site to the aerial mast or via other nearby wifi devices which will then route to the aerial mast!

Below is a diagram of the improved setup.

Initially we used Draytek Vigor modems and routers which worked well to begin with but then we had no end of problems which were all VPN related! I'll go through all that in another post. We now use Mikrotik routers which are simply excellent and do exactly what they're programmed to do! The customer has saved £1000s a we have can monitor all IP equipment effectively and know of a fault before the customer does which is how it should be!