The present document defines a set of Circuit Bearer Services to be provided to PLMN subscribers by a PLMN itself and in connection with other networks. This TS should also be used as a reference for defining the corresponding required mobile network capabilities.

Bearer Services are described by attributes, which are intended to be independent. These attributes are described and defined in TS 22.001. They are grouped into four categories:

1. Information transfer attributes, which characterize the network capabilities for transferring information from a user access point in a PLMN to a user access point in another network. (Refer to TS 22.001 ).
2. Access attributes, which describe the means for accessing network functions or facilities as seen at the access point in the PLMN (see TS 22.001).
3. Interworking attributes, which describe properties of the terminating network and its access point. The terminating network may include another PLMN or the originating PLMN.
4. General attributes, which deal with the service in general.

Figure 1 shows the relation between the groups of attributes and their fields of applicability.

The following Table lists the individual attributes in each of the four groups. The Bearer Service definitions in this specification are based on the "Minimal Set" of attributes.

Attributes that are not part of the minimal set provide further technical detail and are required to fully define the use of each Bearer Service. See specifications TS 27.001, TS 27.002, TS 27.003, TS 27.005 for information about the Signalling Access Protocols, Information Access Protocols and related access attributes. Supplementary services are defined in TS 22.004. Intercommunication is required with services in the PSTN, ISDN and other PLMNs. The capabilities that describe the Interworking Attributes are described in specifications TS 29.002 to TS 29.007 and [15].

All Bearer Service categories provide information transfer between the reference points and allow the use of sub-rate information streams which are rate-adapted. The Bearer Services can be grouped into the following categories:

• Unrestricted Digital Information (UDI);
  Provides the transfer of unrestricted digital information.
• 3,1 kHz (External to the PLMN);
  Used to select a "3,1 kHz audio" interworking function at the MSC. This service category is used when interworking with the ISDN or PSTN "3,1 kHz audio" service and includes the capability to select a modem at the interworking function. "External to the PLMN" indicates that the "3,1 kHz audio" service is only used outside of the PLMN, in the ISDN/PSTN. The connection within the PLMN, user access point to the interworking function, is an unrestricted digital connection.

This clause provides a list of the existing Bearer Services and indicates the values for each attribute in the minimal set. The following attributes have the same value for all Bearer Services. Their values are as follows: All asynchronous NT Bearer Services may support data compression to enhance user data throughput. NT Bearer Services 20 may support V.120 interworking, enabling data terminals connected to an UE to interwork with V.120 [12] terminal adapters on the ISDN as shown in the Figure 2 below.

Table 2 contains the list of the Bearer Services and the values for the remaining attributes in the minimal set.