A MS in Dual Transfer Mode (DTM) has resources for an RR (Radio Resource) connection and is simultaneously allocated resources for one or more TBFs (Temporary Block Flow in GPRS network), provided that the BSS co-ordinates its allocation of radio resources. DTM is optional
both for the MS and the network. A DTM mobile is a class A mobile.
Introduction
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 would result in MS that are required to operate in two different frequencies either in the same time slot or different time slots.
Also, operators have expressed their needs to offer services that allow the simultaneous existence of a CS connection and a PS session in GSM/GPRS coverage area due to unavailability of UMTS in all areas.
The DTM solution overcomes the restrictions mentioned above and makes possible to have simultaneous CS and PS active connections. This is achieved by sending PS data (signaling and user data) on the time slot used by the CS connection or other time slot(s) that in both directions are contiguous with the time slot used by the CS connection.
Also, operators have expressed their needs to offer services that allow the simultaneous existence of a CS connection and a PS session in GSM/GPRS coverage area due to unavailability of UMTS in all areas.
The DTM solution overcomes the restrictions mentioned above and makes possible to have simultaneous CS and PS active connections. This is achieved by sending PS data (signaling and user data) on the time slot used by the CS connection or other time slot(s) that in both directions are contiguous with the time slot used by the CS connection.
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PS data established over DTM |
GPRS signalling over DCCH
In DTM, the MS has resources allocated for an RR connection and for one or more TBFs. So the current GPRS resources are used for the transfer of GPRS signalling instead of the main DCCH (Dedicated Control Channel).
The use of the main DCCH is only allowed when the MS is in dedicated mode.
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GPRS signaling over DCCH |
MS-SGSN tunnelling
If GPRS signaling needs to be sent during a voice call (i.e. in dedicated mode), then LLC (Logical Link Connection) frames are sent on the main DCCH with Layer 2 SAPI 0. This uses a new Protocol Discriminator in Signaling Layer 3 message structure for LLC: GTTP (GPRS Transparent Transport Protocol).
The GPRS information from upper layers (i.e. GMM or SM) is always sent inside an LLC frame. This LLC frame can now be passed down:
The GPRS information from upper layers (i.e. GMM or SM) is always sent inside an LLC frame. This LLC frame can now be passed down:
- to RLC (Radio Link Control) and transmitted on a TBF; or
- to RR, if the MS is in dedicated mode, and transmitted on the main DCCH.
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LLC PDU over DCCH |
The LLC PDU is inserted in a new Layer 3 message, with a new Protocol Discriminator (GTTP) so that the BSC identifies the tunneling mechanism without the need to analyze the Message Type. This Layer 3 message is sent to the BSC on the main DCCH, with the existing Layer 2 mechanisms. The BTS re-assemblies the Layer 3 message and sends it to the BSC.
Single slot operation
If the MS in DTM has one TimeSlot allocated for the CS services, it is possible to reuse the same TS for the transmission of GPRS signaling and user data. In such case the CS information and RLC/MAC blocks are multiplexed in the same TS of the TDMA frame.
The configuration of logical channels used in that case is called ‘TCH/H+ PDCH/H’, which means that on one physical channel network configures one CS half rate connection and PS connection uses the remaining half of the physical channel.
This configuration is very useful when it is impossible for the network to allocate a TBF in some circumstances on separate TS (e.g. congested cell, multi slot capabilities not supported in the serving cell).
On the 'TCH/H' part, the support of AMR as the speech codec is mandatory.
The PDCH/H is a resource dedicated (allocated exclusively) to the MS in both directions and can not be shared with other GPRS MS. For instance, if an uplink TBF is established, the network may send a control message in any of the downlink blocks. No downlink data, however, are sent without a previous downlink assignment.
In the uplink, the MS transmits in any of the blocks of the PDCH/H, irrespective of the USF in the previous blocks.
The PDCH/H is a resource dedicated (allocated exclusively) to the MS in both directions and can not be shared with other GPRS MS. For instance, if an uplink TBF is established, the network may send a control message in any of the downlink blocks. No downlink data, however, are sent without a previous downlink assignment.
In the uplink, the MS transmits in any of the blocks of the PDCH/H, irrespective of the USF in the previous blocks.
Multi slot operation
In multi slot operation, the GPRS data is sent on a PDCH. The number of time slots comprising the PDCH is decided by the network after taking into account the multi slot capabilities supported by the MS. The PDCH/F may be shared with other GPRS MS.
This article explains the basic GPRS signaling in DTM. More would be coming up to explain the signaling procedures during MO and MT call. Furthermore, I will discuss the need of DTM in today's scenario where UMTS and HSPA are taking lead.
