SIGTRAN in Telecom: Architecture, Protocol Stack, & Use Cases

Do you think phone calls, text messages, and mobile data sessions just happen? They don’t, because before a call connects, an SMS reaches your phone or a data session begins, it’s the signaling that works behind the scenes. Traditionally, telecom networks relied on SS7 running over TDM links for signaling, which worked well for many years. But as operators moved to IP networks, they needed something that could help them carry SS7 signaling over modern IP networks. That’s where SIGTRAN in telecom came in and solved the problem.

It enables operators to keep trusted signaling protocols while adopting modern infrastructure, which is why it is still used across mobile networks for voice calls, SMS, roaming, and various other telecom services.

This post will break down what SIGTRAN is, how it works, where it’s used, and why it remains a critical part of modern telecom networks.

What is SIGTRAN in Telecom?

SIGTRAN might look like a highly complex technical term, but when you learn about it, it’s actually pretty simple. SIGTRAN stands for Signaling Transport. Rather than being a single protocol, it is actually a group of protocols developed by the Internet Engineering Task Force (IETF). The main purpose behind creating SIGTRAN was to create a way to carry existing SS7 signaling over IP networks

Why the Telecom Industry Needed SIGTRAN?

For decades, SS7 signaling ran over dedicated TDM circuits. Those networks were reliable, but came with certain limitations like expensive expansion, time-consuming management, etc. At that time, telecom networks were also transitioning to Internet technology (IP). Operators wanted a single network for voice, data, and signaling, and keeping a separate TDM network just for signaling no longer made sense. 

SIGTRAN provided a new transport layer to the SS7 signaling and allowed operators to move to IP networks while keeping the same signaling services running.

Here are the beenfits it brought:

• Lower network and maintenance expenses
• Better use of existing IP infrastructure
• Easier network expansion
• Higher bandwidth for signaling traffic
• Built-in redundancy and failover
• Faster deployment of new telecom services

And, this is what made SIGTRAN an important component of modern telecom networks. 

Understanding Telecom Signaling and SS7 

While we have learned about the SIGTRAN, it is useful to understand what telecom signaling actually does. Signaling is the control system of a telecom signaling network that tells the network what action to take. If signaling is not there, calls wouldn’t connect, messages wouldn’t be delivered, and mobile devices wouldn’t be able to move between networks.

Let’s learn in detail.

What is Telecom Signaling? 

Telecom signaling is the process by which control messages are exchanged between network elements. Don’t think of these messages carrying your voice, text, or data; these messages only tell the network what to do.

In simple words, voice traffic carries the conversation that happens between two people. Signaling traffic sets up, manages, and ends that conversation. Similarly, when you send a text message. The network uses signaling to find who the recipient is and to route the message.

So if we have to define what jobs signaling does, then here’s the thing:

• Setting up and terminating calls
• Delivering text/ SMS messages
• Authenticating users/ subscribers
• Supporting roaming between networks
• Updating user/ subscriber location
• Routing network requests

What is SS7? 

SS7 stands for Signaling System No. 7. It is the signaling protocol that runs over dedicated TDM links. Telecom networks have been used for several years, when IP technology was not available.

Even today, SS7 performs many core telecom functions, including call routing, SMS delivery, roaming support, and access to subscriber information.

Though telecom networks today have moved to IP, SS7 hasn’t disappeared. Many operators still use SS7 signaling over IP using SIGTRAN for millions of calls and messages every single day. And that fact alone makes SIGTRAN so important today

How SIGTRAN Works: Step-by-Step Working Process 

By following a signaling message from beganning till end can help us understand how SIGTRAN works: 

Step 1: An SS7 message is created 

When a call is initiated by a mobile user or an SMS is sent, the network creates an SS7 signaling message. This message does not contain the voice or SMS text, but the instructions.

Step 2: The Signaling Gateway receives the message 

The SS7 message reaches a Signaling Gateway, which prepares the message for transport over IP.

Step 3: SCTP carries the message 

The Gateway uses SCTP, i.e., Stream Control Transmission Protocol, to send the message. SCTP supports multiple paths, so if one path fails, traffic can switch to another without interrupting the connection.

Step 4: The message travels across the IP network 

The signaling message then travels through the IP, which is the biggest change that SIGTRAN brings. Operators can use their IP infrastructure while keeping the same SS7 signaling

Step 5: The destination receives the message 

The message reaches another signaling gateway or a telecom network element such as an STP, HLR, or MSC. The destination processes the message and completes the requested action. That could be setting up a call, delivering an SMS, or checking subscriber information.

This signaling process remains the same for SMS, mobile calls, roaming, subscriber authentication, and other telecom services

SIGTRAN Protocol Stack Explained

As we mentioned before, the SIGTRAN protocol stack isn’t one protocol, but a collection of protocols that work together like a team. Each protocol handles one part of the journey. Like one prepares the message, another transports it, and another delivers it to the right application. Together, they carry the SS& signaling safely over an IP network.

SIGTRAN Architecture 

A SIGTRAN network typically consists of four components

• Signaling Gateway (SG) 

The SG sits between the traditional SS7 network and the modern IP network. It receives the SS7 messages from TDM links and moves them forward to IP. The SG also does the reverse when the message comes back.

• Application Server (AS) 

The Application Server is responsible for handling signaling applications. It processes requests such as call setup, SMS, routing, roaming, and subscriber queries.

• SCTP 

SCTP It is the transport protocol used by SIGTRAN. SCTP is responsible for carrying signaling messages across the IP network. Contrary to TCP, SCTP supports multiple network paths and better fault recovery. That’s why it’s widely used in telecom.

• IP Network

It is the path that carries signaling traffic through the standard IP infrastructure.

Protocol Stack Layers 

Layer	What it Does
Adaptation Layer	Converts SS7 signaling into a format that works over IP.
SCTP	Reliably transports signaling messages between network nodes.
IP	Carries the packets across the network.

Common Adaptation Protocols of SIGTRAN

SIGTRAN includes different protocols that adapt to different types of SS7 signaling.

Protocol	Common Use
M2UA	Connecting SS7 links through a Signaling Gateway
M2PA	Replacing traditional SS7 links with IP links
M3UA	Connecting application servers such as MSCs, HLRs, and SMSCs
SUA	Database queries, roaming, and subscriber services
IUA	ISDN signaling between gateways and controllers

Amongst all these, M3UA is one of the most commonly used protocols in modern telecom networks.

SIGTRAN vs SS7 

Now let’s compare SIGTRAN and SS7 directly to understand how they are related but not the same thing.

Feature	SS7	SIGTRAN
Transport	TDM links	IP networks using SCTP
Network Type	Circuit-switched network	IP-based network
Scalability	Limited. Adding capacity often needs new hardware.	Easy to scale by expanding the IP network.
Cost	Higher infrastructure and maintenance costs	Lower costs by using existing IP infrastructure
Reliability	Very reliable with dedicated links	Very reliable with SCTP redundancy and failover
Performance	Stable but limited by TDM capacity	Faster and better suited for growing traffic
Deployment	Best for legacy telecom networks	Best for modern and next-generation networks

Applications of SIGTRAN in Modern Telecom Networks 

From phone calls to sending text messages, SIGTRAN is used widely across many telecom services that millions of people rely on every day. Let’s see the most common ones

Mobile Network Signaling 

Signaling messages are constantly exchanged between different mobile networks. So when a mobile user’s phone connects to the network, makes a call, or moves between cell towers, it is the signaling that makes it happen. If signaling is not there, the user’s phone should not know where to connect or how to reach another user

SMSC Signaling 

Suppose you sent an SMS message. It doesn’t go directly to the recipient. At first, the message passes through an SMSC, which is a Short Message Service Center. SIGTRAN carries the signaling that makes the process possible. This means that it helps with several important functions, including

• Routing SMS messages to the correct network
• Supporting store-and-forward messaging when the recipient’s phone is unavailable
• Sending delivery reports back to the sender
• Finding the recipient’s current network before delivery

The content is carried by the text message; SIGTRAN carries the instructions that inform the network where the message should be routed.

HLR Signaling 

HLR stands for Home Location Register, which is a database that stores important subscriber information. When networks need to verify user identities or locate them, they query HLR. SIGTRAN transports these signaling messages between network elements. From looking up subscriber information to authenticating SIM cards, finding subscribers’ current location to routing calls and SMS to the correct network, SIGTRAN helps with all such HLR signaling tasks.

VoIP and IMS Networks 

With telecom networks shifting to IP, VoIP, and IMS have become the new standards. SIGTRAN allowed these newer technologies to communicate with legacy SS7 systems. It ensures reliable signaling between traditional telecom hardware and modern IP infrastructure, which is why operators can seamlessly connect VoIP with traditional phone networks. This is the reason why SIGTRAN became a crucial tool for coexisting with old and new systems during the ongoing evolution toward all-IP networks. 

Benefits and Challenges of SIGTRAN 

While SIGTRAN solved many of the limitations of TDM-based signaling, it also brought some challenges that operators need to manage.

Benefits of SIGTRAN 

IP-based signaling 

The biggest advantage of SIGTRAN is its ability to carry SS7 signaling over IP networks, which means operators no longer need separate TDM networks just for signaling. 

Lower operational costs 

Operators do not need to invest in dedicated TDM infrastructure, as they can run services using one IP network. This turns out to be less expensive as there is reduced hardware, maintenance, and network expansion costs.

High availability 

SIGTRAN makes use of SCTP, which routes signaling through redundant paths in case one path fails. This helps telecom networks ensure minimal downtime.

Scalability 

With an increase in the number of subscribers, operators can expand their IP networks more easily than traditional TDM networks, which makes handling of increassing siganling traffic a lot easier.

Better performance 

IP networks come with higher bandwidth and offer faster signaling transport, which helps networks respond instantly during call setup, SMS delivery, and other signaling operations.

Easy integration 

SIGTRAN fits naturally into the modern IP networks and enables telecom systems to easily connect to newer platforms with existing SS7-based services.

Challenges of SIGTRAN 

Legacy migration 

Operators that are still running the traditional SS7 equipment need to be careful while planning their transition from TDM to IP to avoid any service disruptions.

Security concerns 

Most of the legacy SS7 networks were private and had minimal security risks. But modern IP networks face a wide range of security threats, and therefore operators need to employ strong security measures to safeguard signaling traffic from attackers.

Configuration complexity 

Setting up a SIGTRAN network is a high-tech job as it involves several protocols, gateways, routing rules, and redundancy settings. Even a small configuration error can affect signaling across the network

However, the benefits offered by SIGTRAN simply outweigh the challenges.

How Modern SMS Platforms Use SIGTRAN 

When a text message is sent, the network needs to perform several functions, including finding the subscriber, choosing the right route, checking network availability, and confirming delivery. SIGTRAN helps with all of these.

Modern SMS platforms make use of SIGTRAN to exchange SS7 signaling over IP networks. Through this, these platforms are able to connect with core telecom elements such as SMSCs, HLRs, MSCs, and signaling gateways, without relying on traditional TDM links. This easy connectivity makes the platform faster, easier to scale, and better suited for today’s IP-based telecom networks.

REVE SMS platform is a carrier-grade solution built for telecom operators, SMS aggregators, and enterprises that handle large volumes of messaging traffic. It supports SIGTRAN connectivity and can communicate with SS7-based telecom networks over IP.

With SIGTRAN integration, REVE SMS can:

• Connect with SMSCs, HLRs, MSCs, and other core network elements
• Route SMS traffic efficiently across multiple operator networks
• Perform subscriber lookups for accurate message routing
• Support high-volume A2P messaging with reliable delivery
• Scale easily as messaging traffic grows
• Work alongside existing SS7 infrastructure without requiring a complete network overhaul

By combining SIGTRAN with a modern SMS platform, operators and aggregators get the best of both worlds. Get your free demo to learn more about our platform!

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