Mobile Broadband Backup for Fibre Broadband Service

Introduction

After making do with 10 Mb/s of Internet bandwidth delivered using ADSL2 over a traditional copper telephone line for several years, the opportunity to swap to Fibre-to-the-Premises (FTTP) instead – with the prospect of Gigabit speeds if required – was very appealing. The Chellaston telephone exchange was one of the priorities for switching to full-fibre in the Derby area and the fibre network made it as far as the telegraph pole by the gate to the lane early in 2022. Getting it the last 425m, which required new underground ducting, took a little while longer and the FTTP service was commissioned on 30 August 2022. It works very well – except when it doesn’t; there were two major outages in the first 9 months of operation.

The problem is that the fibre ‘cable’ runs overhead through trees along the lane, and when the trees blow around in the wind the fibre gets stretched and can break. Typically the fibre break is inside its outer protective sheathing so it’s not immediately obvious where the problem is and it can take days or weeks to get the fault located and fixed.

The obvious mitigation is to have a secondary Internet connection and to switch to that when the primary connection fails. There’s reasonable 4G mobile reception in the area so that’s the obvious alternative and as a very temporary measure it’s possible to ‘tether’ specific WiFi clients to a mobile phone – but that leaves any wired Ethernet clients offline, and some of them complain (in particular the alarm system takes objection to the fact that its secondary connection path has gone away). A better option is to connect a mobile broadband Internet link into the same router as is used for FTTP.

Some domestic broadband routers make provision for this and some ISPs (notably BT and Vodafone) offer integrated fall-back to a mobile broadband service – albeit with several constraints that make them unsuitable for my requirements.

In broad terms, the required solution needs to consist of three sub-components:

  • A hardware device which makes the mobile broadband Internet connection available in an easily-consumable fashion – ideally presented as an Ethernet interface
  • A suitable mobile broadband subscription
    • This sounds easy but the complication is having a little bit of data available all the time then a lot of data available (as cheaply as possible) when the FTTP goes down
  • An automated means of detecting when the FTTP connection has gone away and switching to the mobile broadband – and then switching back again
    • This also needs some way to notify that it has switched (and is starting to consume mobile broadband data) – so that the FTTP fault can be reported

Hardware Interface

The fundamental requirement is for some sort of hardware interface device that:

  • Has a socket for a SIM card (or potentially provision for an e-SIM)
  • Has an aerial for communicating with the mobile phone network – either built-in or (preferably) external, attached via some sort of standard RF connector
  • Connects to a computer network via USB or (preferably) Ethernet

Previously I’ve used a simple USB ‘dongle’ that takes a SIM card and connects directly to a broadband router or other computer. While these can work OK, I’ve had problems getting good enough reception from their internal aerial (especially since they need to be close to the host computer – ideally attached directly) and finding the right drivers for their interface chips can be problematic.

With the likelihood of having to make regular use of this fall-back, I opted for a more ‘professional’ solution using the Teltonika TRB140. This provides very good functionality and ticks a lot of boxes:

  • The mobile signal aerial connects via an SMA connector so the supplied aerial can be replaced with an alternative model – or moved further from the device
  • The network connection is via an RJ45 10/100/1000 Ethernet interface
  • While it does not support standard Power-over-Ethernet, it can use passive PoE
  • There’s an outdoor enclosure available with an integrated aerial (see here) which enables the whole unit to be mounted outdoors, with just a single Ethernet cable connecting back to the network and passive PoE source

The main drawbacks are that:

  • It only supports 3G (which is being phased out) and 4G, not also 5G
  • It uses a SISO (Single-In, Single-Out) antenna, rather than MIMO (Multi-In, Multi-Out) which can run with multiple parallel 4G data streams for higher throughput

CC BY-SA 4.0 Mobile Broadband Backup for Fibre Broadband Service by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.