In SS7 network sccp is the transport layer . It is the user of MTP3 layer and service provider to the TCAP layer. MTP3 layer is the network layer which does the routing based on ss7 point code. SCCP does the routing on SSN and global title (GT). Like any other protocol addressing , SCCP have source and destination address in header. Source address is named as calling party address and destination named as Called Party address. Here we will mostly discuss about the SCCP address format and SCCP GT translation. In real network mostly routing is done based on the global title. SS7 node which woks as intermediate route is Signaling Transfer Point (STP).
Why SCCP address is important? As we have already said that MTP3 does the routing based on point codes , so why GT is required? In real telecom network , global title is either IMSI or MSISDN. Both parameters identifies the home network or the sim card provider. When a mobile phone attaches to the VLR/MSC in roaming network, imsi digits woks as SCCP called address. VLR derives called party GT from imsi. Following is an example conversion.
IMSI = MCC + MNC + MSIN ----> Called GT = CC + NDC + MSIN
With the CC and NDC , a message can reach to the home network. Where subscriber profile is present.
SCCP Address Format :
Being a standard protocol, SCCP address have a standard protocol format. The ITU Q.713 describes the address format. This section describes the various components of sccp address.
Address Indicator (AI) :
This is the first byte in SCCP address. SCCP address indicator represents the parameters present in the address and how the routing will be done. Address Indicator have following sub parameters.
Routing Indicator (RI), a one bit parameter. If 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 have some optional parameters , depends on 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.
|0||NO GT is present|
|1||global title includes nature of address indicator only|
|2||global title includes translation type only|
|3||global title includes translation type, numbering plan and encoding scheme|
|4||global title includes translation type, numbering plan, encoding scheme
and nature of address indicator
SSN Indication, if bit is set then ssn is present in address else not. Mostly SSN is mandatory to add while sending ss7 message from one application to another. E.g If VLR wants to send a update location message while attaching to the network, to the HLR , then VLR sets the SSN (6) of HLR in sccp called address. Then only update location can reach to the HLR, else receiver can not know , to which application message has to deliver. We will discuss where the ssn can be optional , in GT translation and routing section.
Point Code (PC) Indication, bit represent if the SS7 point code is present or not. If point code is present and routing is set on route on GT. Then routing does not use the point code.
SS7 Point Code, this is the address of a ss7 node. If a point code is an international point code , then from anywhere in 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. For other variant it is different , e.g ANSI 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 SS7 stack. You can say that , it is similar to the port in TCP protocol. And point code is similar to the IP address.
Translation Type: It is a one byte parameter. It is used to select translation node. E.g , if same GT can be translated on any of the target node. Then which node the message should deliver , translation type decides this.
Numbering Plan, it is a four bits value. Each value represent the number type.
Nature of Address Indicator (NAI), represent the type of number this is . E.g a number can be a national number or international number. SS7 uses , NAI to decode the number , it is in national format , then GT digits don't have country code.
Encoding Scheme, how the digits has been encode . E,g can be BCD .
Digits (1 to n), the actual digits of global title.
SCCP Address translation (GT translation):
In network, mostly message is routed based on GT. Each node does a SCCP address or GT translation. After translation next outgoing link is selected. When the message reaches to the end node ( to which called sccp address belongs). GT is translated to the local SSN and message reaches to the application. In following we will cover a simple example of GT translation.
Suppose a subscriber dials a O2 mobile number (+448781234567) from an Indian Airtel number. Called number have country code (44 as UK), NDC (878 ) code of mobile operator and subscriber number (1234567). Airtel MSC routes this message out from India just looking at country code. The STP (Signaling Transfer Point) which connects India and UK receives the message and then routes to the UK. There may be other intermediate STPs.
When messages reaches to 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 vary and a STP may have different capabilities for messaging routing. In following we are giving an example of simple sccp address translation.
For each translation there is a rule. Each rule have GT selector and GT translator. There can be multiple rules for translation configured on STP. When SCCP receives a message , if RI is route on GT. Called Party Address in message compared against all the configured rules. If any rule matches then Called party address is modified as per translation in that rule. If no matching of rule, message is dropped. GT translator also have next point code, where message should send. It can be a local point code or a remote point code.