Digital Private Network Signaling System (DPNSS)/Digital Access Signaling System 2 (DASS 2) Extensions to the IUA Protocol
Extracted elements (24)
A new message class (13) was introduced rather than reusing existing IUA message classes because DPNSS/DASS 2 boundary primitives must be distinguishable from IUA transport messages and from other IUA extensions such as V5 and GR-303.
Separate SCTP Payload Protocol IDs for DUA (10) and IUA (1) are recommended as the primary option even when both protocols share an association, to allow network entities to identify the payload type without parsing the DUA/IUA message header.
The DLCI field format departs from IUA to conform with the DPNSS specification (ND1301:2001/03 / BTNR 188), which defines a different channel-numbering and virtual-channel model than ISDN. DASS 2 uses only 32 DLCs (no virtual channels), constraining the channel number range to 0–31.
As an option, the IUA SCTP Payload Protocol ID (value 1) MAY also be used for DUA to allow backhauling both ISDN and DPNSS over the same SCTP association. However, using separate IDs (10 for DUA, 1 for IUA) is the recommended primary option even in mixed scenarios.
The IUA Message Header MUST be used with DPTM messages, but the DLCI field in the DLCI parameter is formatted differently from IUA to conform with ND1301:2001/03 (BTNR 188).
The SCTP Payload Protocol ID value of 10 SHOULD be used for DUA when only DPNSS is backhauled over the SCTP association (i.e., simultaneous ISDN and DPNSS backhauling is not required).
'Channel Number out of range' (0x1c) is sent when a received message contains a channel number greater than 63 for DPNSS or 31 for DASS 2. 'Channel Number not configured' (0x1d) is sent when the channel number is valid in range but not provisioned.
DPNSS layer 2 has no unit data primitive, so Unit Data Messages (Request and Indication) are invalid in DUA. Data Request (message type 1) and Data Indication (message type 2) are used instead.
DUA defines three new Management message types for DLC status exchange: type 5 (DLC Status Request), type 6 (DLC Status Confirm), and type 7 (DLC Status Indication). These messages carry the common message header followed by the IUA message header.
DUA inherits IUA error codes but invalidates codes 0x0a (Unassigned TEI), 0x0b (Unrecognized SAPI), and 0x0c (Invalid TEI/SAPI combination) as they are ISDN-specific. Two new error codes are added: 0x1c (Channel Number out of range) and 0x1d (Channel Number not configured).
DUA introduces a new Message Class 13 for DPNSS/DASS2 Boundary Primitives Transport Messages (DPTM) to distinguish them from IUA and other IUA extension messages (V5, GR-303). Valid message classes for DUA also include 0 (Management), 3 (ASPSM), and 4 (ASPTM).
For DASS 2, the DLC status value 00 (Out Of Service) is invalid because DASS 2 DLCs do not have this state. The Idle state is a transient local state and has no allocated status bit value.
Only Notify and Error Management messages are valid for DUA; TEI Status messages are not used. The ERR message is sent when an invalid value or unrecognized message is found in an incoming message.
The V-bit in the DLCI field determines scope: value 0 means the action applies to all DLCs in the carrier (channel number ignored); value 1 means the action applies to the single DLC specified by the channel number. The V-bit is used only by Establish and Release messages; Data messages ignore it.
IANA assigned SCTP Payload Protocol Identifier value 10 for DUA. This value SHOULD be used when only DPNSS/DASS 2 is backhauled over an SCTP association.
The security considerations from IUA (RFC 3057, Section 6) and from the SIGTRAN Protocols security document (RFC 3788) apply in full to DUA. No additional DUA-specific security requirements are stated.
DLC reset for a single DLC (Ind=1): SG sends SABMR to PBX; on receipt of UA, SG sends Est Cfm to MGC and DLC enters Reset Completed (RC) state. On failure (NT1/NT2 expiry with no UA), SG sends Rel Ind with RELEASE_OTHER and DLC enters Reset Attempted (RA) state.
Information transfer on a DLC: MGC sends Data Req to SG; SG sends UI(C) to PBX; PBX responds with UI(R); SG forwards Data Ind to MGC.
Link takedown for a single DLC (Ind=1, RELEASE_MGMT): for DPNSS the DLC is marked Out Of Service; for DASS 2 it is marked Reset Attempted. SG returns Rel Cfm (Ind=1) to MGC.
Link takedown for all DLCs (Ind=0, RELEASE_MGMT): for DPNSS all DLCs are marked Out Of Service; for DASS 2 DLCs are marked Reset Attempted. SG returns Rel Cfm (Ind=0) to MGC.
Reset of all DLCs (Ind=0): SG sends SABMR to each DLC; after all complete or time out, SG sends Est Cfm (Ind=0) to MGC without updating DLC statuses. MGC then issues a Status Request to obtain current DLC states via a Status Confirm. Remaining out-of-service DLCs may be retried individually (Ind=1) or collectively (Ind=0).
The DLC Status parameter (Tag 0x12) for a T1 interface covers up to 46 DLCs with D23 marked Not Applicable (timeslot 23 reserved for signaling). For DASS 2 over T1, only 23 DLCs exist (no virtuals). The message length is 12 for a DASS 2 DLC Status message.
The DLC Status parameter (Tag 0x12) for an E1 interface is a 128-bit field encoding the status of up to 60 DLCs using 2 bits per DLC, with D0 and D16 marked Not Applicable (timeslots reserved for framing and signaling). Four status values are defined: 00=Out Of Service, 01=Reset Attempted, 10=Reset Completed, 11=Information Transfer.
The DUA DLCI field is a 16-bit field containing: 7 reserved bits (set to 0, ignored by receiver), 1 V-bit indicating single-DLC or all-DLC scope, a Channel Number field, and a trailing 1-bit. Valid channel numbers are 0–63 for DPNSS and 0–31 for DASS 2.