MSU Conversions

In this section:

Message- and parameter-based conversions are defined on the originating and destination SS7 variants. These conversions may also be specific to a switch. This section outlines the special message and parameter processing required to convert the incoming Level 4 Converter traffic for supported SS7 variant combinations.

Routing Label Conversion

An SS7 Routing Label is composed of the Signaling Indicator Octet (SIO), the Destination Point Code (DPC), the Originating Point Code (OPC), and the Signaling Link Selection (SLS). The conversion of these fields is consistent for all Level 4 Converter SS7 messages.

SIO Conversion

In ANSI and Japan, the SIO has priority assignments on a per message type basis whereas ITU-related variants use the default value of 0 for all messages. Message priority is assigned on a per message type basis. For an example, refer to SIO Priority Assignment

Point Code and CIC Conversion

Point code conversion is based on network architecture and system deployment. This conversion is based on mirror point codes to address possible point code incompatibilities between SS7 variants. The SS7 Variant Route Mapping objects (refer to Creating and Configuring Route Mappings) are responsible for the associated mapping between Home point codes and their corresponding mirror point codes in the other network.

The carrier identification code (CIC) value is passed through without modification. If mirror point codes are difficult to acquire (such as in ITU networks), the Level 4 Converter can be deployed in conjunction with the Point Code Emulator (PCE). Contact your Ribbon Sales Representative for more details on the Ribbon feature.

SLS Conversion

To support load sharing, the SLS values follow a bit rotation algorithm unless otherwise specified by the SS7 specification. This procedure is managed at the MTP3.

Message Conversion

Message conversion is based on the incoming variant Message Rules (refer to Creating and Configuring Message Rules). By default, messages are passed through with only routing label conversion. If a different action is required, a Message Rule must be defined for the given message ID.

Parameter Conversion

Parameter conversion is based on the incoming variant Message and Parameter Rules (refer to Creating and Configuring Message Rules and Creating and Configuring Parameter Rules). By default, parameters are passed though as they are. If a different action is required, a Parameter Rule must be defined for the given parameter ID with an associated Message Rule for the given message ID.

SCCP Conversion

This section provides you with information about SCCP conversion.

Payload Constraints

Typically, the SCCP transports TCAP traffic. With the complexity of the GSM data, TCAP messages can exceed the maximum SS7 MSU size and require partitioning using extended unitdata (XUDT) messages. SCCP handles the reassembly at the receiving node.

The Level 4 Converter connects to MTP3 not SCCP, therefore, the SCCP MSUs received at the Level 4 Converter may contain partial payloads. For this reason, the Level 4 Converter is not equipped to manipulate the SCCP Data parameter. The converter performs routing label mapping similar to ISUP traffic with the addition of the Party Address parameters conversion.

Party Address Conversion

The SCCP connectionless traffic messages (UDT, UDTS, XUDT, and XUDTS) have mandatory variable length Called and Calling Party Address parameters. Both these parameters have the same format which includes (if present) the following:

Address Indicator
PC
Subsystem Number (SSN)
Global Title (GT)

If present, the Party Address PC requires conversion similar to the Routing Label OPC and DPC. The SSN is not modified due to similar subsystem configuration in the SS7 network for the same SCCP payload. If GT data is present, the signal information is untouched but certain subfields such as the Nature of Address might be removed depending on the Address Indicator (AI) GT bits.

Special consideration is required for ANSI-ITU conversion or ANSI-IT-based such as China.

National/International Indicator

The 8^th bit in the Address Indicator is defined as the national/international routing indicator in ANSI. Setting the bit to 1 defines national routing while 0 defines international routing. For ITU-based variants such as ITU/ETSI, China, Japan, Pakistan, New Zealand and so on, the 8^th bit is reserved and must be set to 0.

For outgoing ITU-based messages, the 8^th bit is forced to 0. For outgoing ANSI messages not originating from an ANSI network, the 8^th bit is forced to 1. ANSI to/from ANSI keeps the 8^th bit as is.

PC and SSN Indicators

For the Address Indicator, the PC and SSN bits are reversed in ITU-based versus ANSI networks. In ITU, the SSN is the 2^nd bit while the PC is the 1^st bit. Reverse the order for ANSI AI. Following the AI byte, the order of the PC and SSN values are also reversed. In ITU, the PC is first then followed by the SSN.

Global Title Indicator

For the GT data, you must first convert the GT AI bits which are the 6^th, 5^th, 4^th, and 3^rd in the AI octet. A value of 0 is defined as no GT data. In ITU, the values 1 through 4 are defined while in ANSI only 1 and 2 are defined. ITU GT AI of 3 is a direct map to ANSI GT AI of 1. The ANSI and ITU GT AI of 2 are identical. Unfortunately, the ITU GT AI or 1 and 4 are not supported in ANSI as they include the Nature of Address subfield.

The ITU GT AI of 1 is discarded as no equivalent ANSI GT can be devised.

For ITU GT AI of 4, it is mapped to ANSI GT AI of 1 (same as ITU GT AI of 3) but the Nature of Address octet which follows the 2 octets Translation Type and the Numbering Plan/Encoding Scheme is excluded. Therefore, the 3^rd octet of the ITU GT is removed.

Variant-specific Conversion

Certain variants such as Japan NTT and TTC have additional conversion requirements beyond the standard PA conversion (see Party Address Conversion).

Japan NTT

If the Called Party Address is set to route on GT, add an SSN of zero if not already defined.

SCMG GTP and GTC messages are defined for Global Title status management as national. These messages are not defined in ANSI/ITU SCCP and, therefore, are discarded from Japan NTT to the other variant.

SCMG messages (SSP, SST, SSA, and so on) are not supported in Japan NTT. Therefore, all SCMG messages to Japan NTT from the other variant are discarded.

Japan TTC