Build Your Defence: How to Choose the Right Firewall

Written by
Samuel Harrison

A firewall is a network security system designed to monitor traffic based on a set of predefined rules. The term “firewall” referring to technology was first used in the 1983 classic movie WarGames. The concept went from science fiction to science fact when the first paper on the topic was published only four years later, in 1987. In the telecoms industry, there are four types of firewalls in common use:

  • IT Security Firewalls – the most common type of firewall in the world, designed to stop attacks on computers, servers or websites.
  • Signalling Firewalls – these are designed to protect telecommunication networks specifically, stopping fraudsters from preying on vulnerabilities in the network to access subscribers’ SMS, voice calls and more.
  • Voice Firewalls – an expansion on the features of the signalling firewalls but focusing on protecting users against voice fraud.
  • SMS Firewalls – specialised firewalls designed to protect users and telecoms from fraud, keeping A2P traffic on monetised white routes instead of illegal grey routes.

History of SMS Firewalls

The first SMS firewalls emerged in the mid-2000s as a result of government legislation in many cases. Politicians were worried about SMS spam levels and asked networks to build firewalls to protect subscribers from this unwanted communication. The big change to SMS firewalls came around the time of a big change in the mobile industry, the introduction of the iPhone and the popularisation of smartphones from 2007 onwards. With the launch of the app store in 2008, suddenly apps and services used on mobile devices needed an easy way to verify identity via mobile, and the one-time-password (OTP) was born.

Originally A2P (application to person) data was delivered for the same meagre costs as P2P (person to person) messages, but once this traffic grew exponentially with the introduction of OTP and other automated messaging, telecom operators started to monetise this traffic differently. This was because A2P volume can be astronomically higher than P2P. Even the most avid texter is probably only sending a hundred or so messages across a span of 24 hours, but an application texting OTPs could be sending hundreds every second.

Networks started splitting A2P out from P2P traffic and to ensure that A2P was monetised correctly they put gates in the way using SMS firewall rules. Unfortunately, the fraudsters are always on the lookout for ways to deliver their traffic cheaper and have found new grey routes to get their traffic through for a lower cost. The modern SMS firewall is designed to protect users from spam messaging and protect the revenues of the telecoms that implement them.

Whenever an SMS is sent, it has to travel through a short message service centre (SMSC). These SMSCs use set protocols to transmit through networks. The most widely used is called SMTP. Messages are sent using SMTP and hop between SMSCs before they reach end-users. To ensure that A2P traffic is only transmitted through white routes, the telecoms industry implements firewalls at various points in this process.


Essential Elements of an Excellent Firewall

We spoke to our resident firewall expert Salman Nayyar, GMS’ Group Director for Products, Strategy and Innovation, to get his viewpoints on the essential elements needed for an excellent firewall. He told us that to ensure the firewall you choose is able to maximise the revenues of your network correctly, you need to make sure it has the following features:

  • Ability to read SMS content – your firewall must be able to view the content of the messages, not just the context, to be sure that the message is being sent through the appropriate channel.
  • Filtering & Segregation – your firewall must be able to filter and segregate what’s coming through to it, especially it should be able to filter international traffic from domestic traffic.
  • Future Proof – many firewalls are still legacy products, designed during the first generation of the technology to block spam messaging for users and built on and adapted to meet current needs. Something not designed to meet the current needs of the A2P industry will be far less capable than a more modern offering.
  • Scalable – your firewall needs to be able to grow with your business. There’s no point in getting a firewall installed that only caters to your current business needs. A2P messaging is growing as more and more services start using SMS for authentication. It’s best to make sure whatever you use will be able to handle larger traffic volumes and adapt to new frauds.
  • User Friendly – many of the first firewalls and even some more modern ones used text-based command-line interfaces to interact and control them with. In today’s world, that’s not up to scratch. Your firewall needs to have a graphical user interface (GUI) and be operable by somebody with limited technical knowledge. Everyone working on security for your network should know how it functions and be able to implement new rules, while ensuring they do not impact or negate existing ones.
  • Use Artificial Intelligence (AI) & Machine Learning (ML) – a firewall that relies on humans to do all the segregation, filtering, blocking, and other standard activities won’t be fast or good enough to do these activities in real-time or without disruption for users.
  • In-Depth Reporting – while AI and ML can be relied upon for much of what is needed from a firewall, creating and analysing in-depth reporting from your firewall is essential. Often a human can identify a pattern of fraud and help create a rule to prevent it quicker than an AI can.
  • SIM Box Detection – one of the most common forms of SMS fraud in the world at the moment is achieved using a SIM box, essentially a giant device filled with SIM cards that forwards on A2P messages masked as P2P. Your firewall needs to be able to detect and defeat these devices.

The Three Crucial Components Of Your SMS Security

You must think about three things when it comes to SMS firewalls- what, where, and who. What firewall are you going to use, where in the SMS chain it will be installed, and who will operate it. That final question of who will operate the firewall is the most important one of them all. It’s no use having an incredible firewall with every feature you could ever possibly want if nobody is updating the rules to combat innovations in the fraud space.

Salman Nayyar, GMS Group Director
Salman Nayyar
GMS Group Director – Products, Strategy and Innovation


A firewall is not unlike a racing car. It can be as fast as you want, but if you don’t have the right person behind the wheel, you’re not going to get anywhere.

GMS’ team of experts are skilled at network evaluation and are always vigilant and proactive when it comes to emerging threats and vulnerabilities. We have a global view of all the firewalls we operate for our clients, taking learnings from one network and applying them to others. Should we see a new type of fraud emerging in say, Pakistan, we write a rule to ensure that it is stopped there, and then we will apply it to the firewalls of all  clients before the fraud is even detected in other countries.  If you’d be interested in talking more about SMS firewalls and how we could operate them on behalf of your business, get in touch with one of our experts.

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