7 Network interworking

09.043GPPInterworking between the Public Land Mobile Network (PLMN) and the Circuit Switched Public Data Network (CSPDN)TS

7.1 Checking bearer capabilitys

The table shows bearer capability elements and the values. Just the element user rate is optionally.

Table 3: Shows parameters which have to be considered by the IWF.

Parameter name

In band

Out of band

SETUP or CALL CONF

ITC, Info transfer capability

UDI

UR, User rate

2400,4800,9600

SA, Sync/Async

Sync

CE, Connection

T

RCR, Radio Channel Requirement

HR,FR,dualFR,dualHR

7.1.1 Network interworking mobile originated

Bearer capability compatibility checking of the mobile originated call is carried out by the IWF to determine the appropriate data transmission service class in the CSPDN. If it is not possible for the IWF to provide a bearer service match, then the IWF shall fail the call and indicate the reason to the user.

As well as compatibility checking, the network shall check the subscribers subscription parameters, see GSM 02.01, to ensure that the calling mobile does subscribe to the requested service.

7.1.1.1 Home IWF access

Home IWF access would be available to a mobile subscriber if the home PLMN has a CSPDN IWF. Figure 09.04/1 illustrates both the case when the mobile subscriber is present in the home PLMN and when present in a visited PLMN that does not support a CSPDN IWF.

NOTE 1: Home IWF Access via home PLMN for a mobile in VPLMN may not be possible (HPLMN as transit network).

NOTE 2: IWF for ISDN must be provided in VPLMN. This connection presumes E.164 addressing capability and two step dialing.

Figure 09.04/1: Home IWF access

7.1.1.2 Visited IWF access

A visited IWF access would be available to a mobile subscriber if the visited PLMN has a CSPDN IWF. This is shown in figure 09.04/2.

country A : country B

———————:———————-

:

+—–+ :

| DTE | :

+—–| :

| DCE | :

+—–+ :

| :

+——-┼——+ : +————-+

| CSPDN :……┼————–┼.. CSPDN |

+————–+ : +-┼———–+

: |

: +-┼——–+

: | : IWF |

: +-┼——–+

: |

: +-┼——–+

: | : PLMN B |

: +-┼——–+

: +-┴–+

: | MS |

: +—-+

Figure 09.04/2 Visited IWF access

7.1.2 Network interworking mobile terminated

Bearer capability compatibility checking of the CSPDN originated call is carried out by the IWF to determine the appropriate bearer service selection in the PLMN. If it is not possible for the IWF to provide a bearer service match, then the IWF shall fail the call and indicate the reason for the user (if possible).

As well as compatibility checking, the network shall check the subscribers subscription parameters, see GSM 02.01, to ensure that the called mobile does subscribe to the requested service.

* If an interrogating node in ISDN is not supported

** IWF for ISDN/PLMN interworking must be provided in VPLMN.

Figure 09.04/3 Terminating call

7.2 Transparent service support

7.2.1 MSC to IWF rate adaptation scheme

This link consists of a 64 kbit/s channel with the information, both user data and in band parameter information rate adapted in conformance to GSM 08.20.

7.2.2 Rate adaptation in the IWF

This process is a reverse of that provided in the Terminal Adaptation Function of the MS. Specification GSM 04.21 refers to the rate adaptation mechanism to be provided.

+———+ +————–+ TO CSPDN

+——| RA1 +——-| X.51 MAPPING +——

MSC +—-┴—+ +———+ +————–+

—| RA2 |

+——–+

7.2.3 Mapping of signalling MS/IWF to CSPDN signalling

In GSM SS no7 is used for signalling. Towards CSPDN channel associated signalling defined in CCITT X.71 is used. Therefore the interworking function has to map between those two signalling systems.

Signalling +———————-+

————| mapping |

| <———– | Signalling and traffic

Traffic | mapping +———–

————| ———–> |

+———————-+

7.2.4 Establishment of end-to-end terminal synchronisation

Prior to exposing the traffic channel of a PLMN connection to transmission of user data, the controlling entities of the connection have to assure of the availability of the traffic channel. This is done by a so called synchronisation process

– starting on the indication of "physical connection established" resulting from the PLMN-inherent outband signalling procedure. This indication is given on sending message CONNECT in case of MOC, CONNECT ACKNOWLEDGEMENT in case of MTC.

– ending by indicating the successful execution of this process to the controlling entity, which then takes care of the further use of the inband information (data,status).

Network interworking within an IWF is concerned with the terminating side (to the MS) and the transit side (to the fixed network) of a connection. Both sides have to be treated individually related to the synchronisation process.

With respect to the terminating side the procedure is as follows:

– sending of synchronisation pattern 1/OFF (all D bits "1", all S bits "1", all E bits "1") to the MS using the RA1/RA2 rate adaption function

– searching for detection of the synchronisation pattern 1/OFF from the MS within valid V.110 frames. This implies that the E1, E2 and E3 bits for each V.110 frame shall be checked for legal user rate in order to distinguish TCh synchronisation pattern from BSS Idle bit pattern.

When the 1/OFF from the MS has been recognised as a steady state, the IWF continues sending the synchronisation pattern 1/OFF to the MS unless a timer T expires. From this time the information on the receiving lines from the MS and from the fixed network are directly mapped to the respective sending lines.

During the synchronisation process described above, i.e. while the synchronisation pattern is being sent by the IWF, the IWF will not send X.51-frame structure to the CSPDN transit network. Once timer "T" expires the synchronisation pattern will continue to be transmitted from the IWF to the MS, however, the IWF will start mapping the information received from the MS into X.51 frames to the CSPDN transit network. The IWF will start looking for the CSPDN frame alignment to be received from the CSPDN. On recognising frame alignment the IWF will cease sending its synchroniation pattern to the MS and connect the CSPDN through to the MS.