SS7 is a time-tested signaling mechanism that connects and disconnects telephone calls, SMS, and other services on standard PSTN networks. But, with the rise of voice-over-IP (VoIP) technology, carriers are now looking for IP-friendly signaling schemes to implement in their network architectures like Sigtran.
It was developed by the Internet Engineering Task Force (IETF) to allow operators to transmit SS7 signaling traffic between a media gateway controller (MGC) and a signaling gateway (SG) or IP-enabled signaling control point (IP-SCP), thus permitting carriers to maintain their SS7 signaling schemes while using the IP network for transport. In this article, we’ll explain the key components that form the Sigtran protocol.
Sigtran Basics
In simple words, Sigtran is a set of networking protocols that includes stream control transport protocol (SCTP) and a suite of user adaptation (UA) layers (which change the visual appearance of SCTP into the lower layers of SS7). There are right now 4 SS7-related adaptation layers defined by Sigtran. These include:
- The services of MTP2 in a client-server situation, like SG to MGC, are provided by the message transfer part level 2 user adaptation (M2UA) layer. Its user will be MTP3.
- In a peer-to-peer situation, like SG-to-SG connections, the message transfer part level 2 peer-to-peer adaptation (M2PA) layer offers the services of message transfer part level 2 (MTP2). Its user would be MTP3.
- The services of MTP3 in a client-server situation, like SG to MGC, are given by the message transfer part level 3 user adaptation (M3UA) layer. Its users can be SCCP and/or ISUP.
- In a client-server architecture, like SG-to-IP SCP, the signaling connection control part (SCCP) is delivered by the SS7 SCCP user adaptation (SUA) layer. Its user can be TCAP, or another transaction-oriented application part.
The Four UA Layers
The Sigtran protocol comprises 4 UA layers: the M2UA, M2PA, M3UA, and SUA. All of them have a common purpose: to send protocol messages from the SS7 network’s termination point to the higher layer’s presence point over an IP network. Let’s discuss each layer in more detail:
1. The M2UA Layer
M2UA is used to transfer MTP2 user data between two devices. First, an SG (signaling) and second, an MGC (media gateway controller). It operates in a client-server way, where the MGC acts as the client and the SG acts as the server. This layer allows an MTP2 service to run on an MGC, which makes it feasible to extend SS7 (Signaling System No. 7) into the IP network. The MTP3 instance on the MGC relies on the MTP2 instance on the SG. Neither MTP2 nor MTP3 knows they are remotely connected. This process of sending signaling messages over IP, from one SS7 layer to another, is called backhauling.
The MTP3 user at the MGC is typically ISUP (ISDN User Part). This setup is valuable when there are limited SS7 links at one location (as low as one) but many different SG functions. Since SS7 links are spread out, it makes sense to host MTP3 in the MGC. The system’s SS7 address (pointcode) stays with MTP3. If each SG had its own MTP3 layer, many point codes would be required, which would make it complicated to manage a single gateway. Using M2UA, streamline this by centralizing MTP3 at the MGC.
2. The M2PA Layer
M2PA is similar to M2UA but works in a peer-to-peer way. Instead of linking MTP2 and MTP3 at two different locations, M2PA replaces an MTP2 link under MTP3. In M2UA, one end is MTP3, and the other is an SG interface. But in M2PA, MTP3 is at both ends. This layer allows two MTP3 layers in SGs to communicate directly. It also extends SS7 signaling over an IP network.
M2PA is mostly used for SG-to-SG connections. It helps connect separate SS7 networks that need to communicate. Each SG can connect to multiple other SGs without being aware of the higher layers they support. But SG must have MTP3 to carry out routing and managing MTP2/M2PA links. Since every SG has MTP3, it should also have its own SS7 point code. M2PA is not the same as SUA, as it is used for accessing an IP-SCP (IP-based Service Control Point). SUA has knowledge of higher layers like TCAP (Transaction Capabilities Application Part), while M2PA does not.
3. The M3UA Layer
M3UA functions similarly to M2UA. It follows a client-server model that provides an SS7 upper layer remote access to lower layers. However, M3UA runs between an SG and an MGC to offer an MTP3 service on the MGC. In M3UA, the MTP3 in the SG is unaware that its user is on another node. Likewise, the user layer at the MGC has no idea that it is being served by a remote MTP3. This process is called backhauling, where signaling messages pass from the SG to the MGC over IP.
This configuration is effective when there are many SS7 links. It is also useful when SS7 links are all connected at a single location. These situations are very common in North American networks, where SS7 links are isolated from voice circuits. In these networks, SS7 links are combined into a single medium, like a T1 line. Each SG in this case runs its own MTP3 instance and has a unique SS7 point code.
4. The SUA Layer
SUA is used to connect application parts, like TCAP, on an IP-based Service Control Point (IP-SCP) via an SG. It supports multiple IP-SCPs to connect through a single SG. These IP-SCPs lack their own MTP3 instances, so they do not require separate SS7 point codes. Instead, the MTP3 and pointcode stay inside the SG. Without SUA, SCCP has to be available in each IP-SCP, and the SS7 network must acknowledge each SCCP instance separately.
SUA is very flexible. It can support application parts working between two nodes totally within an IP network. This is very beneficial for modern networks that no longer depend on SS7. In this case, both nodes would use the same IP-based signaling system to make SS7 unnecessary.
In Conclusion
SIGTRAN has revolutionized the process of transmission of signaling data over IP networks for uninterrupted communication in telecom infrastructures. These protocols provide structured, reliable, and scalable SS7 signaling over IP to support the convergence of voice and data networks. As telecom continues to evolve, businesses need flexible SS7 solutions to stay ahead of the curve. ComCode Technologies specializes in SS7 software development services to offer cutting-edge solutions for seamless signaling over IP networks. So for high-performance SS7 software, contact us today.
