The definition of GPRS class A mode of operation in Release 97 assumes a total independence between the CS and PS domains. Thus the direct implementation of the existent standards for class A would result in mobile stations that are required to operate in two different frequencies either in the same timeslot, in timeslots n and n + 3 or their adjacent ones. This complicates enormously the internal architecture of the ME, resulting in a very high development cost, which both operators and manufacturers would prefer to avoid.
Nevertheless, operators have expressed their need for this type of mobiles, since they want to offer services that demand the simultaneous existence of a CS connection and a PS session. This is particularly important during the coexistence of GSM/GPRS with UMTS, as these capabilities will exist in UMTS. However, UMTS coverage may not be available in some areas where there is GSM/GPRS coverage (e.g. deep inside buildings or when roaming to a 2G network). As coverage is a vital service, in order for an operator to be able to sell "UMTS class A services" it is necessary to be able to imitate class A services in areas of only GSM coverage. On the other hand, the provision of class A services with GERAN technology is also essential for operators without UMTS coverage.
A constant aim throughout this document is to reuse the existing functionality when possible, in order to minimise the impact on current implementations. In general, the changes proposed have little impact on the core network elements (i.e. MSC and SGSN) and 3G TS 24.008
The solution outlined in this document overcomes the restrictions mentioned above and makes possible to have simultaneous CS and PS active connections. This is achieved by sending PS data (signalling and user data)
on the timeslot use by the CS connection
on timeslot(s) not used by the CS connection
The possible timeslot configurations are based on two restrictions in Release 99:
the number of timeslots allocated to the CS connection is limited to one;
the timeslots allocated in each direction are contiguous.
More flexible proposals are left for further study. In addition, for the definition of DTM multislot classes, the restrictions in 3G TS 45.002
for multislot capabilities shall apply.
shows an example of a multislot configuration (2 uplink, 3 downlink).
class A mobile station in dual transfer mode.
In a similar manner to UMTS, the A interface is modified so that the BSC knows the IMSI associated with each SCCP connection to the MSC. This means that the BSC is able to ensure that 'packet paging' messages can be delivered to mobile stations which have a connection to the MSC. The same functionality can be reused to deliver MSC originated pages to mobiles in packet transfer mode while the network is in mode of operation II (i.e. no Gs interface).
Mobility management is basically the same as is specified in TS 23.060
for class A mobiles, but using the same techniques as UMTS for control of "in connection" cell, routeing area and location area updates (e.g. System Information 6 message is extended to contain the Routing Area Code).
If GPRS signalling needs to be sent during a standalone voice call, then it is proposed that these LLC frames can be sent on the main DCCH (FACCH or SDCCH) with layer 2 SAPI 0. This uses a new Protocol Discriminator in TS 24.007
for LLC: GTTP (GPRS Transparent Transport Protocol). The use of the main DCCH for GPRS signalling is subject to certain restrictions to reduce the harm to the speech quality.
Inter-BSC handover is planned to be controlled by A interface signalling. The Old BSS to New BSS information element is used to indicate to the target BSC that the mobile station is in DTM.
DTM Handover procedure is realized by utilizing in parallel the handover procedures that are defined in TS 43.129
for the PS domain and in TS 23.009
for the CS domain.
For paging, the behaviour of the mobile station is as in class B mode of operation: the PCH takes priority to PPCH, and both to CBCH.
The implementation described in this document also applies the restriction that the mobile station shall not be required to operate in two different frequencies in the same moment in time. However, GSM CS and GSM GPRS services will be still supported simultaneously. Thus, the feature here described is a subset of the GPRS class A capabilities.
The mentioned subset will be referred as DTM.
The specification of an unrestricted class A mode of operation that requires the mobile station to operate in different frequencies simultaneously shall not be forbidden.