ISO/IEC 15055:1997 PDF

Overview

ISO/IEC 15055:1997 - "Information technology - Telecommunications and information exchange between systems - Private Integrated Services Network - Transit counter additional network feature (ANF‑TC)" - defines the Transit Counter feature for Private Integrated Services Networks (PISN). The standard provides stage 1 (feature principles) and stage 2 (functional model and information flows) specifications to limit the number of network exchanges a call or signalling connection may traverse during call establishment, preventing indefinite looping.

Key topics and requirements

• Feature purpose: ANF‑TC limits the number of network exchanges transited by a call request during establishment (used to protect the network from loops).
• Counter range: The transit counter initial value is normally zero; the network‑specific maximum is in the range 1 to 31 (allowed range for the counter is 0..max).
• Operation: The counter is incremented at each network exchange. If the counter exceeds the PISN limit before the call reaches its destination or leaves the PISN, the call is aborted. Other network actions are out of scope.
• Functional model (stage 2):
  • FE1 - TC Initialize: Detects invocation and sets the initial transit counter.
  • FE2 - TC Execute: Intermediate FE2 compares the counter to the limit and increments it; Final FE2 terminates ANF‑TC.
  • Collocation rules: FE1 collocated with Originating CC, Incoming Gateway CC, or Transit CC; intermediate FE2 with Transit CC; final FE2 with Terminating CC or Outgoing Gateway CC.
• Information flows: Defines the ra‑TC unconfirmed information flow containing the mandatory service element Transits count (Integer) and its use with r2‑Setup requests.
• Conformance: To claim conformance, a stage 3 standard must specify signalling protocols and equipment behaviour adequate to support the stage 1 and stage 2 aspects relevant to the interface or equipment.

Applications and users

Practical uses of ISO/IEC 15055:1997 include:

• Preventing routing loops and runaway signalling in PISNs and private ISDN-like networks.
• Enforcing hop limits for calls that traverse multiple exchanges or gateways.
• Ensuring predictable network behaviour for call transfers, call forwarding and other supplementary services that can reroute calls.

Who uses this standard:

• Telecom standards engineers and protocol designers implementing stage 3 signalling.
• Network architects and PISN operators designing routing and loop‑protection policies.
• Equipment vendors (PINX/PISN exchange manufacturers) implementing FE collocation and ra‑TC signalling.
• Test and interoperability teams validating ANF‑TC behaviour.

Related standards

Relevant references cited by ISO/IEC 15055:1997:

• ISO/IEC 11574 (basic services)
• ISO/IEC 11579‑1 (PINX reference configuration)
• ISO/IEC 13863, 13865, 13866, 13872 (related ANF/SS specifications)
• CCITT/ITU‑T Recommendations I.130, I.210, Z.100

Keywords: ISO/IEC 15055:1997, Transit Counter, ANF‑TC, PISN, PINX, functional model, information flows, transit counter limit, ISDN supplementary services, signalling, loop protection.