ETSI - PRETS 300 099
Integrated Services Digital Network (ISDN) Specification of the Packet Handler Access Point Interface (PHI)
|Publication Date:||1 January 1991|
Implementation alternatives for ISDN packet mode services and applicability of this European Telecommunication Standard (ETS).
This specification define
The term ISDN packet mode services is defined in such a way as to include both case B (ISDN Packet Mode Bearer Services (PMBS)), and case A (PSPDN services), as defined in ETS 300 007 .
If a specification implementation alternative for the provision of PMBS and PSPDN is used in an ISDN or a Global System for Mobile communication (GSM) Public Land Mobile Network (PLMN), this ETS applies. The implementation alternatives and the exact location of the interface are described below.
In CCITT Recommendation I.324  local Connection Related Function (CRF) and Packet-Handling Function (PHF) are defined as being involved in the provision of PMBS. The local CRF includes the Exchange Termination (ET). Two basic implementation alternatives are mentioned in subclause 3.1.2 of CCITT Recommendation I.324  and in ETS 300 007 :
1) the PHF is integrated in the local CRF; and
2) the PHF is not part of the local CRF, Local CRF and PHFs are implemented with different physical equipment and in a multi-vendor environment. In addition, the Packet Handler (PH), although logically belonging to the ISDN, may be physically part of the PSPDN.
In the first case, the interface between ET and PHFs can be kept
internal and proprietary. In the second case, a standardized
interface between local CRF and PHFs is required. This ETS defines
If the second implementation alternative is chosen for the provision of packet mode services in an ISDN, this ETS applies for the implementation of the PHI. The existence of this ETS does, however, not preclude the choice of the first implementation alternative in an ISDN.
Considerations for the use of the PHI in private networks and interworking between private and public networks are outside the scope of this ETS. The PHI is a network-internal interface to be used in public networks.
ISDN packet mode services supported
The full scope of services defined in ETS 300 048  and ETS 300 049  (case B) and in ETS 300 007  (cases A and B) is supported by the PHI specification, see clause 5 for details. Subclause 5.4 contains a table giving an overview of the services, references to the relevant sections of the PHI specification and a conformance statement.
The PHI specification uses the term Packet Handler (PH) in both case A and case B services. For case A services, the PH assumes the role of the Access Unit (AU), see ETS 300 007 .
No other services are supported by the PHI specification
Local and remote access to the PHI
The PHI denotes the interface between the PH and the CRF it is directly connected to (called CRF-P, see subclause 4.3). At least in the initial phase of service offerings it can be expected that the number of local exchanges in an ISDN exceeds the number of packet handlers. The PHI will thus also have to support subscribers accessing it remotely, i.e. subscriber and PH are connected to different CRFs.
The following two scenarios are considered in this specification:
1) Local PHI Access:
An ISDN subscriber accessing packet mode services from the PH is directly connected to the CRF-P.
2) Remote PHI Access:
An ISDN subscriber accessing packet mode services from the PH is connected to the CRF-P via another local CRF (called CRF-S, see subclause 4.3) or series of CRFs (CRF-S and transit-CRFs).
In the second scenario the PHI specification defines only the interface between PH and CRF-P. The interface between CRF-S and CRF-P is thus not defined by the PHI specification.
NOTE 2: Requirements by the PHI specification on the common channel signalling system (signalling between CRF-S and CRF-P) are described in (informative) annex A.