SCCP Address in SS7 network explained.

In the SS7 network, SCCP is the transport layer. It is the user of the MTP3 layer and service provider to the TCAP layer. The MTP3 layer is the network layer that does the routing based on the ss7 point code. SCCP does the routing on SSN and global title (GT). SCCP has a source and destination address in the header like any other protocol addressing. The source address is named the calling party address, and the destination is called the Called Party address. Here we will mainly discuss the SCCP address format and SCCP GT translation. In a real network, routing is primarily done based on the global title. SS7 node, which works as the intermediate route, is the Signaling Transfer Point (STP).

Why is SCCP address essential? As we have already said that MTP3 does the routing based on point codes, so why is GT required? The global title in the real telecom network is either IMSI or MSISDN. Both parameters identify the home network or the sim card provider. When a mobile phone attaches to the VLR/MSC in the roaming network, IMSI digits work as SCCP called party address. VLR derives called party GT from IMSI. Following is an example of conversion.

IMSI = MCC + MNC + MSIN —-> Called GT = CC + NDC + MSIN

With the CC and NDC, a message can reach the home network. Where the subscriber profile is present.

SCCP Address Format :

Being a standard protocol, the SCCP address has a standard protocol format. The ITU Q.713 describes the address format. This section describes the various components of the SCCP address.

sccp address format
SCCP address format
Address Indicator (AI) :

This is the first byte in the SCCP address. The SCCP address indicator represents the parameters present in the address and how the routing will be done. Address Indicator has the following sub-parameters.

Routing Indicator (RI), a one-bit parameter. If the routing indicator bit value is zero, SCCP routes the message by doing GT translation; else, routes on SSN.

GT Indicator– According to ITU-T specifications, GT has some optional parameters, depending on the gt format type. GT indicator mentions what type of GT is present in the address. The possible values are from zero to four. Following is the table for possible GT indicator values.

GT Indicator Meaning 
0NO GT is present
1the global title includes the nature of the address indicator only
2the global title includes translation type only
3the global title has a translation type, numbering plan, and encoding scheme
4the global title includes translation type, numbering plan, encoding scheme, and nature of address indicator

SSN Indication, if the bit is set, then SSN is present in the address else not. SSN must be added while sending ss7 messages from one application to another. E.g., If VLR wants to send an update location message to start the camping with the network, then VLR sets the SSN (6) of HLR in SCCP called party address. Then only an update location message can reach the HLR process. The receiver ss7 stack can not determine which user application an incoming message has to deliver. We will discuss cases where the SSN can be optional in the GT translation and routing section.

Point Code (PC) Indication, bit represents whether the SS7 point code is present. If point code is present and routing is set on the route on GT. Then routing does not use the point code.

SS7 Point Code, this is the address of an ss7 node. If a point code is an international point code, then from anywhere in the world, a message can be sent to the node by doing point code-based routing. For ITU-T, it is of 14 bits value in 3-8-3 format. It may differ for other variants, e.g., ANSI, and its 24 bits 8-8-8. The format is Network-Cluster-Member.

SSN or Sub System Number, a one-byte value, identifies the application over the SS7 stack. You can say that it is similar to the port in TCP protocol. And point code is identical to the IP address.

                  Translation Type:  It is a one-byte parameter. It is used to select a translation node, E.g., if the same GT can be translated on any target node. Then which node the message should deliver to, and what is the translation type decided?

              The Numbering Plan is a four bits value. Each value represents the number type.

             The nature of the Address Indicator (NAI) represents the type of number this is. E.g., a number can be a national number or an international number. SS7 uses NAI to decode the number, and it is in national format, then GT digits don’t have country code.

             Encoding Scheme,  how the digits have been encoded. E,g can be BCD.

            Digits (1 to n) are the actual digits of the global title.

SCCP Address translation (GT translation):

Most of the time, messages are routed based on GT in the network. Each node in the path does an SCCP address or GT translation. After translation, the following outgoing link is selected. When the message reaches the end node ( to which called SCCP address belongs). GT is translated to the local SSN, and the message comes from the application. In the following, we will cover a simple example of a GT translation.

Suppose a subscriber dials an O2 mobile number (+448781234567)  from an Indian Airtel number. The called number has a country code (44 as in the UK), NDC (878 ) code of the mobile operator, and subscriber number (1234567). Airtel MSC routes this message out from India just by looking at the country code. The STP (Signaling Transfer Point), which connects India and UK, receives the message and then routes it to the UK. There may be other intermediate STPs.

When messages reach the UK, STP looks for NDC, and now it routes the message to the core network of O2. For call, the receiving core network element is GMSC.

GT Translation Configuration:

The configuration can be varied, and an STP may have different capabilities for messaging routing. In the following, we give an example of a simple SCCP address translation.

For each translation, there is a rule. Each rule has a GT selector and a GT translator. There can be multiple rules for translation configured on STP when SCCP receives a message if RI is set to route on GT and called Party Address in the message compared against all the configured rules. If any rule matches, then the Called party address is modified as per the translation in that rule. If there is no matching rule, the message is dropped. GT translator also has the next point code, where the message should send. It can be a local point code or a remote point code.