What is a Subsystem Number (SSN)? Understand this ‘secret key’ to telecom networks in detail

The world of telecommunications is vast. When you call or send a message on your phone, thousands of tiny processes take place behind the scenes in just a hundredth of a second. Network protocols are essential for ensuring these processes run smoothly. Deep within this network lies a term—Subsystem Number (SSN).

If you’re a telecommunications engineer or studying in this field, you’ve likely heard of SS7 (Signaling System No. 7). SS7 is an essential part of the SS7 network. In today’s special article, we’ll explore the A-Z of Subsystem Numbers.

What is a Subsystem Number?

In simple terms, a Subsystem Number (SSN) is a unique identification number used to identify a specific software application or ‘service’ within a telecommunications network.

Let’s understand this with an example. Imagine a large building (this is your mobile network). There are many rooms in that building, and each room houses a different department (e.g., accounts, sales, reception). Now, if a courier arrives from outside, how will it know which department to go to? To do this, each department is assigned a ‘room number.’

In a telecom network (such as an MSC or HLR), the Subsystem Number is the same as the ‘room number.’ It tells the network which service an incoming signaling message (such as an SMS, location update, or prepaid balance check) should be routed to.

In technical terms, the SSN is used in the SCCP (Signaling Connection Control Part) layer to indicate which ‘user application’ the message is to be delivered to.

Why Subsystem Numbers Exist 

You may think that when we have a Point Code and an IP address, then what is the need for an SSN?

In fact, a single network node (such as a server) can perform multiple functions simultaneously. For example, the same node can serve as both an HLR (Home Location Register) and an EIR (Equipment Identity Register).

• Accurate Delivery: Without SSN, the network will know which machine (Node) to send the data to, but it will not know which ‘software’ inside that machine will process that data.
• Multi-tasking: Allows multiple telecom services to run on a single machine.
• Standardization: SSN numbers are distributed based on a global standard so that telecom equipment around the world can understand the same language.
• Efficient Routing: It helps to direct network traffic in the right direction so that data packets do not go astray.

Common SSN Assignments 

The International Telecommunication Union (ITU-T) has designated specific numbers for certain services. These numbers are generally the same worldwide. Here are some prominent examples:

If a packet has an SSN of 6, the network immediately understands that the message is to be sent to the HLR. Similarly, an SSN of 8 means that the MSC is required for call handling.

How Routing with SSN Actually Works

Let us understand this through a step-by-step process:

1. Creation of the message: Imagine you switch on your phone. Your phone sends a “Location Update” message to the network.
2. Addressing: This message contains two main things – ‘Point Code’ (machine address) and ‘SSN’ (service address).
3. Function of MTP layer: The lower layer of the signaling network (MTP – Message Transfer Part) looks at the point code and delivers the message to the correct machine (Node).
4. Function of SCCP layer: When the message reaches that machine, the SCCP layer there looks at the ‘SSN’.
5. Delivery: If the SSN is ‘7’, the machine hands over the message to the ‘VLR’ software module.

In this way, the SSN ensures that the message knocks on the right ‘door’ inside the machine.

The Standards Behind Subsystem Numbers

SSNs are not created arbitrarily. They are governed by strict international rules and standards:

• ITU-T Recommendations: The format of SCCP and SSN is mainly described in detail in the Q.713 standard.
• GSM/3GPP Standards: 3GPP has defined specific SSNs for mobile networks (e.g., for MAP).
• National vs International: Some SSNs are fixed internationally, while others can be used within a country (national network) as needed. Typically, the range from 0 to 255 is used.

SSN in the Age of SIGTRAN (SS7 over IP)

Nowadays, the old SS7 network has been replaced by IP-based networks (SIGTRAN). But does this mean SSN is dead?

No way!

In SIGTRAN, we send SS7 messages over IP (Internet Protocol). The SUA (SCCP User Adaptation) layer is used here. This layer encapsulates the SSN (Subsystem Number) information. Even though the data is traveling over the Internet Protocol instead of a dedicated data cable, the SSN is still just as crucial for identifying the recipient application as it was 20 years ago. Without the SSN, the SIGTRAN network wouldn’t know which application to deliver the message to.

Typical Troubleshooting Questions

If you are working in the telecom sector and face any network issues, here are some common SSN-related questions and their solutions:

• Message is being dropped (Subsystem Prohibited): This means that the SSN (service) you are sending the message to is currently ‘down’ or unavailable.
• Unknown SSN Error: If you have configured the wrong SSN, the receiving node will reject that packet.
• Where to check?
  • Wireshark Logs: Wireshark is a tool where you can capture packets to see which SSN is going where in the SCCP layer.
  • Routing Tables: Check the routing table of your switch or gateway to see if a route has been created for the corresponding SSN.
  • STP (Signaling Transfer Point) Logs: This is where the message turns.

Best Practices When Working with Subsystem Numbers

If you’re a network administrator, keep the following in mind:

1. Comply with the standard: Always use the standard SSN as prescribed by ITU-T. Do not change the number at will.
2. Documentation: Keep a list of all SSNs in use within your network, especially if you are using some ‘National Reserved’ numbers.
3. Monitoring: Always keep track of which Subsystem (SSN) is going down and how often. This improves network performance.
4. Redundancy: Ensure that the same SSN (e.g., HLR) is configured on the backup server as well, so that the service does not stop if one server fails.

Key Takeaways

What did we learn from this article? Let’s recap:

• Identification: The SSN is the ‘address’ of a specific service or application within the telecom network.
• Place: It is found in the SCCP layer of the protocol stack.
• Importance: This ensures that calls, messages, and data reach the correct software module.
• Continuity: From legacy SS7 networks to modern SIGTRAN (IP) networks, the role of the SSN remains essential today.
• Management: It is very important for a telecom engineer to have proper knowledge of SSN to troubleshoot network issues.

Conclusion:

The Subsystem Number (SSN) may sound like a small, technical term, but it’s part of the backbone of global telecommunications. Without it, our mobile phones would be mere boxes, as the network wouldn’t know which request to send where. Hopefully, this article helped you understand the complexities of the SSN in simple terms.

If you have any other questions related to this topic, please ask in the comment box below!

Frequently Asked Questions