ANSI and ITU Conversion

This section provides you with conversion information between ANSI (ISUP) and ITU (ISUP).

ANSI (ISUP) to ITU (ISUP)

The ANSI (ISUP) to ITU (ISUP) conversion is the most complex conversion, because these variants are mostly incompatible.

For example:

• The point code format is 24-bit in ANSI while its is 14-bit in ITU
  
  
• ANSI does not support the ITU Fast Answer Management with the Connect (CON) message while ITU supports this message type
  
  
• Circuit Group Management range is 24 octets while this range in ITU is 32 octets.

These two conversion requirements force the ANSI to ITU converter to keep real-time state information. Parameter conversion is also extensive.

For Signaling Connection Control Part (SCCP), the Called and Calling Party Address parameters are converted according to the new point code (PC) size and the PC and Subsystem Number (SSN) indicator location in the Address Indicator (see Party Address Conversion).

SIO Priority Assignment

The ANSI variant has priority bit defined based on message type. Possible values are 0, 1, 2, and 3.

Priority 0: Blocking Ack. Signal (BLA), Blocking Signal (BLO), Confusion (CFN), Circuit Group Blocking (CGB), Circuit Group Blocking Ack. (CGBA), Circuit Group Unblocking (CGU), Circuit Group Unblocking Ack. (CGUA), Circuit Group Query (CQM), Circuit Group Query Response (CQR), Circuit Group Reset (GRA), Circuit Group Reset (GRS), Initial Address (IAM), Reset Confirm (RSC), Unblocking Ack. (UBA), and Unblocking (UBL)

Priority 1: Address Complete (ACM), Continuity Check (CCR), Continuity (COT), Call Progress (CPG), Facility (FAC), Forward Transfer (FOT), Information (INF), Information Request (INR), Loopback Ack. (LPA), Release (REL), Resume (RES), Suspend (SUS), and Unequipped CIC (UCIC)

Priority 2: Answer (ANM) and Release Complete (RLC)

Priority 3: None

Fast Answer Feature

The Fast Answer call setup is a feature present in both ANSI and ITU networks, but the message sequence responsible is different. In ANSI, a Fast Answer condition is satisfied when a call is setup without an ACM (in other words, the IAM is responded immediately by an ANM). In ITU, the ACM is again excluded but a CON message is sent instead of the ANM. Because the CON message is not supported in ANSI, and an ANM without a prior ACM is not supported in ITU, a call state-based conversion is required for ANSI to ITU conversion. A simpler conversion is defined for ITU to ANSI conversion.

Circuit Group Message Management

The circuit range maximum in ANSI is 24 bits while ITU uses 32 bits. Messages received with an out-of-bound range is discarded.

If an ITU Circuit Group (CG) query message circuit range exceeds the ANSI maximum value, the message must be broken into two messages which together manage to cover the given range while staying in the ANSI available range. The corresponding acknowledgement messages must be reassembled to obtain the proper single acknowledgement message for the original ITU request message. Therefore, the ITU to ANSI conversion breaks down the CG queries while the ANSI to ITU conversion direction manages the acknowledgment reassembly.

In ITU to ANSI, the converter splits a CG message if its Range and Status parameter’s range value exceeds 23; else, the message is passed through. The first resulting message has the first 16 bits while the other message has the remaining bits (maximum of 16 bits). The messages’ CIC value is modified accordingly (original CIC + 16) to properly identify the new split range. Only the CG request messages CGB, CGU, CQM and GRS require verification. The acknowledgement messages CGBA, CGUA, CQR, and GRA are passed through, because no acknowledgement from an ANSI message is divided. On the odd case of an unexpected acknowledgement message, the ANSI switch processes with its own validation.

Because the acknowledgements may be received on a different peer due to load sharing, the pending unification of the acknowledgements notification must be broadcast. Since both resulting ANSI CG messages have the same Signaling Link Selection (SLS), their corresponding acknowledgements are received by the same peer but not necessarily the one which received the original request. Therefore, for increased efficiency, the peer which received the second acknowledgement message is responsible for the unification.

The CG message pending notification with a DPC-OPC-CIC-MT key and original range value is broadcast to all peers. Upon receipt of a notification, the timer of configurable length set to the CG Ack Timer attribute in the Converter Configuration object (see Table General Configuration Screen Attribute Listing and Description) is started to wait for the two acknowledgement messages in the ANSI to ITU conversion direction.

In ANSI to ITU, the converter reassembles the split CG acknowledgement messages CGBA, CGUA, CQR, and GRA. The CGB, CGU, CQM, and GRS are passed through.

If no pending unification is found for a given acknowledgment message, the message is passed through; else, the information is added to the pending acknowledgement response message. When both acknowledgement messages are received, the lifespan timer and pending unification are cleared and the new unified ITU acknowledgement message is forwarded to the ITU network. If the timer expires, the collected information is discarded.

USI TMR Conversion (ANSI to ITU)

The Transmission Medium Requirement (TMR) parameter in the IAM message is mandatory in ITU and must be added based on the ANSI IAM mandatory User Service Information parameter. The TMR is of fixed size one octet. The TMR is not used in the ANSI network. See User Service Information for parameter definition.

The ANSI USI is also converted to the ITU variant. If the DPC associated ITU SS7 Variant Route Mapping ITU USI Discard is enabled, the USI is not converter to ITU but simply discarded following TMR generation.

The USI conversion is as follows:

• If the second octet is extended, remove all extensions.
  
  
• If there is an octet 2.1 (multi-rate), remove octet.
  
  
• If there are no layer protocols, conversion is complete.
  
  
• For layer 1, if protocol is set to 0x02 (G.711 u-law), reset to 0x03 (G.711 A-law).

Fast Answer Conversion

A call state flag is kept for each DPC-OPC-CIC combination key. This implementation does not require any call state information distribution as the message sequence under study is always route to the same converter peer except on link failure.

The fast answer flag is managed given the call message sequence and the configurable lifespan Fast Answer Timer attribute defined in the Converter Configuration object. On receipt of an ACM, the flag is set and the timer started. On receipt of ANM, if the flag is set, the ANM is passed through; else, the ANM is converted into a CON message and the flag and timer are reset. On receipt of a REL or a RLC or on timer expiry, the flag and timer are reset.

For efficiency, a fast answer record is created for each OPC-DPC combination possible for the given Route Mapping records in the ANSI and ITU SS7 variants. Each FA record has a table of CIC values (12-bit maximum) which are empty for unused CICs. A lifespan timer pointer is added to the relevant CIC position to enable the fast answer flag.

ANSI to ITU Parameter Definitions

This section provides you with the ANSI to ITU parameter definitions.

Backward Call Indicators

ITU (ISUP) to ANSI (ISUP)

This section provides you with ITU (ISUP) to ANSI (ISUP) information.

USI TMR Conversion (ITU to ANSI)

The Transmission Medium Requirement parameter in is not used in ANSI network but is mandatory in ITU IAM. The TMR will be discarded from the IAM. In its place a User Service Information parameter is mandatory. See User Service Information for parameter definition.

If the USI is not present in the ITU IAM, add the default USI (hex 80 90 a2) to the ANSI IAM.

If the USI is present, the following conversion will apply:

• If the information transfer rate is set to 0x18 (multi-rate) make sure the 3rd octet is present for the rate multiplier.
  
  
• If there is no layer protocol, conversion is completed.
  
  
• For layer 1, if the protocol is set to 0x03 (G.711 A-law), reset to 0x02  (G.711 u-law).

The TMR Prime parameter is converted to a USI Prime in an SS7-IP deployment if fall back USI management is used. Additional information is  added if CPAP is ever reused.

Subsequent Message Management

The SAM message is used to send additional digits following an IAM in ITU. The IAM message needs to be buffered until the lifespan timer expires or the terminating signal (ST) is received from one or more SAM messages. The ITU SAM Overlap Timer attribute is configurable in the Route Mapping record based on the originating switch with default 30 seconds. On timer expiry, the IAM is sent as is.

On receipt of a SAM, check if a pending IAM exists. If the SAM is unexpected, discard the MSU. Append the SAM additional digits to the IAM Called Party Number parameter. If the ST signal is received, the IAM is complete and is sent out while the SAM itself is discarded. The IAM can also be deemed complete if a certain number of digits is received or a maximum number of SAM messages are received. The ITU Max CLD ANSI IAM and ITU Max Number of SAM attributes are configurable in the Route Mapping record based on the originating switch with default 32 and 2 respectively.

Fast Answer Conversion

On receipt of an ITU CON, convert to an ANSI ANM. A straight conversion is possible due to the compatibility between the two messages.

Send Twice Management

Some circuit group request messages need to be sent twice for old switches. If the corresponding ANSI Route Mapping record ANSI Send Message Twice attribute is enabled, send the resulting ANSI CBG, CGU and GRS twice.

The following table provides you with the ITU (ISUP) to ANSI (ISUP) message conversion details.

The following table provides you with the ITU (ISUP) to ANSI (ISUP) parameter conversion details.