5.3.5 Data flows through Layer 2
25.3013GPPRadio interface protocol architectureTS
Data flows through layer 2 are characterised by the applied data transfer modes on RLC (acknowledged, unacknowledged and transparent transmission) in combination with the data transfer type on MAC, i.e. whether or not a MAC header is required. The case where no MAC header is required is referred to as "transparent" MAC transmission. Acknowledged and unacknowledged RLC transmissions both require a RLC header. In unacknowledged transmission, only one type of unacknowledged data PDU is exchanged between peer RLC entities. In acknowledged transmission, both (acknowledged) data PDUs and control PDUs are exchanged between peer RLC entities.
The resulting different data flow cases are illustrated in Figures 6 – 9. On the level of detail presented here, differences between acknowledged and unacknowledged RLC transmission are not visible. Acknowledged and unacknowledged RLC transmission is shown as one case, referred to as non-transparent RLC.
NOTE: The term "transparent transmission" is used here to characterise the case where a protocol, MAC or RLC, does not require any protocol control information (e.g. header). In transparent transmission mode, however, some protocol functions may still be applied. In this case an entity of the respective protocol must be present even when the protocol is transparent. For the RLC protocol the segmentation/reassembly function may be applied. This can be performed without segmentation header when a given higher layer PDU fits into a fixed number of RLC PDUs to be transferred in a given transmission time interval. In this case segmentation/reassembly follows predefined rules known to sending and receiving RLC entities. For instance in the user plane, the segmentation/reassembly function is needed for the case of real-time services using high and possibly variable bit rates. For such services higher layer PDUs shall be segmented into reasonably sized RLC PDUs of fixed length allowing efficient FCS error detection on the physical layer. The higher layer PDU can be reassembled by simply concatenating all RLC PDUs included in a transport block set as implied by the used transport format.
Figure 6 and Figure 7 illustrate the data flows for transparent RLC with transparent and non-transparent MAC transmission, respectively.
Figure 8 and Figures 9, 9a, 9a1, 9b, 9c illustrate the data flows for non-transparent RLC with transparent and non-transparent MAC transmission, respectively.
Figure 10 illustrates the data flow for transparent RLC with transparent and non-transparent MAC transmission.
In Figure 9a, with MAC-hs configuration, MAC-d operation is non-transparent, when multiple logical channels are multiplexed on the same MAC-d flow, otherwise it is transparent.
A number of MAC PDUs shown in the figures shall comprise a transport block set. Note, however, that in all cases a transport block set must not necessarily match with a RLC SDU. The span of a transport block set can be smaller or larger than an RLC SDU.
Each mapping between a logical channel and a transport channel as defined in Figure 4 and Figure 5 in combination with the respective RLC transmission mode implies a certain data flow that is specified on a general level in the following.
Figure 6: Data flow for transparent RLC and MAC
Figure 7: Data flow for transparent RLC and non-transparent MAC
Figure 8: Data flow for non-transparent RLC and transparent MAC
Figure 9: Data flow for non-transparent RLC and MAC
Figure 9a: Data flow for non-transparent RLC and MAC mapped to HS-DSCH (MAC-hs configured)
Figure 9a1: Data flow for non-transparent RLC and MAC mapped to HS-DSCH (MAC-ehs configured)
Figure 9b: Data flow for non-transparent RLC and MAC mapped to E-DCH (MAC-e/es configured)
Figure 9c: Data flow for non-transparent RLC and MAC mapped to E-DCH (MAC-i/is configured)
Figure 10: CCCH data for transparent RLC and non-transparent MAC mapped to E-DCH (MAC-i/is configured, FDD and 1.28 Mcps TDD only)
5.3.5.1 Data flow for BCCH mapped to BCH
All RRC PDUs transmitted on BCCH have a fixed length and fit into one RLC PDU (and, equivalently, MAC PDU, as defined by the transport format). No RLC header is needed, i.e. the transparent data transfer mode of RLC is applied.
No MAC header is needed since only one BCCH logical channel is mapped onto a BCH. Figure 6 is applicable.
5.3.5.2 Data flow for BCCH mapped to FACH
No RLC header is needed, i.e. the transparent data transfer mode of RLC is applied. A MAC header is required for identification of the logical channel carried by the FACH. The data flow shown in Figure 7 is applicable.
5.3.5.3 Data flow for PCCH mapped to PCH
No RLC or MAC header is needed, i.e. the data flow in Figure 6 is applicable.
5.3.5.4 Data flow for CCCH mapped to FACH/RACH
For CCCH, transparent transmission mode on RLC is employed on the uplink (when mapped to RACH). Unacknowledged transmission mode on RLC is employed on the downlink (when mapped to FACH). A MAC header is used for logical channel identification (BCCH, CCCH, CTCH, SHCCH, DCCH, DTCH). If the transparent RLC transfer mode is applied, the data flow Figure 7 is applicable. If the unacknowledged RLC transfer mode is applied, the data flow Figure 9 is applicable.
5.3.5.5 Data flow for SHCCH mapped to USCH
For SHCCH mapped on USCH, transparent transmission mode on RLC is employed. A MAC header may be used for logical channel identification (SHCCH, DCCH, DTCH). When no MAC header is used, SHCCH must be the only channel mapped to USCH/DSCH. Depending on whether the MAC header is needed or not, either the data flow Figure 6 or Figure 7 is applicable.
5.3.5.6 Data flow for SHCCH mapped to FACH/RACH
For SHCCH, transparent transmission mode on RLC is employed on the uplink (when mapped to RACH). Unacknowledged transmission mode on RLC is employed on the downlink (when mapped to FACH). A MAC header may be used for logical channel identification (BCCH, CCCH, CTCH, SHCCH, DCCH, DTCH). When no MAC header is used, SHCCH must be the only channel mapped to RACH/FACH. If the transparent RLC transfer mode is applied, depending on whether the MAC header is needed or not, either the data flow Figure 6 or Figure 7 is applicable. If the unacknowledged RLC transfer mode is applied, depending on whether the MAC header is needed or not, either the data flow Figure 8 or Figure 9 is applicable.
5.3.5.7 Data flow for DCCH mapped to FACH/RACH
For DCCH, both unacknowledged and acknowledged transmission mode on RLC is employed. A MAC header is mandatory for FACH/RACH carrying DCCH. The data flow shown in Figure 9 is applicable.
5.3.5.8 Data flow for DCCH mapped to DSCH
For DCCH, both unacknowledged and acknowledged transmission mode on RLC is employed. A MAC header is optional, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.9 Data flow for DCCH mapped to USCH
For DCCH, both unacknowledged and acknowledged transmission mode on RLC is employed. A MAC header is needed if DCCH and DTCH logical channels are multiplexed in MAC before mapping to a USCH, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.10 Void
5.3.5.11 Data flow for DTCH (non-transparent RLC) mapped to FACH/RACH
Mapping to FACH/RACH implies a DTCH with acknowledged or unacknowledged transmission on RLC. A MAC header is mandatory for FACH/RACH when carrying DTCH. The data flow shown in Figure 9 is applicable.
5.3.5.12 Data flow for DTCH (non-transparent RLC) mapped to DSCH
Mapping to DSCH implies a DTCH with acknowledged or unacknowledged transmission on RLC. A MAC header is optional, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.13 Data flow for DTCH (non-transparent RLC) mapped to USCH
Mapping to USCH implies a DTCH with acknowledged or unacknowledged transmission on RLC. A MAC header is needed if DCCH and DTCH logical channels are multiplexed in MAC before mapping to a USCH, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.14 Data flow for DTCH (transparent RLC) mapped to DCH
Continuous DTCH data stream is segmented into transport blocks on RLC and mapped on a DCH transport channel on MAC. The transport block size is naturally implied by the data rate. Both RLC and MAC sublayers are transparent, i.e. no protocol control information is added, when no multiplexing of DTCH on MAC is applied. The data flow shown in Figure 6 is applicable. If multiplexing on MAC is performed, a MAC header is needed, and Figure 7 applies.
5.3.5.15 Data flow for DTCH (non-transparent RLC) mapped to DCH
In this case acknowledged or unacknowledged transmission on RLC is applied. A MAC header is needed only if multiple DTCH logical channels are multiplexed in MAC before mapping to a DCH, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.16 Void
5.3.5.17 Data flow for DCCH mapped to DCH
In this case non-transparent or transparent transmission mode on RLC is applied. A MAC header is needed only if DCCH and DTCH logical channels are multiplexed in MAC before mapping to a DCH, i.e. either the data flow in Figure 8 or Figure 9 is applicable.
5.3.5.18 Data flow for CTCH mapped to FACH
For CTCH, unacknowledged transmission mode on RLC is employed. A MAC header is used for logical channel identification (BCCH, CCCH, CTCH, SHCCH, DCCH, DTCH). The data flow shown in Figure 9 is applicable.
5.3.5.19 Data flow for DCCH mapped to HS-DSCH
For DCCH, both unacknowledged and acknowledged transmission mode on RLC is employed. With MAC-hs configuration, a MAC-d header is needed only if DCCH and DTCH logical channels are multiplexed on the same MAC-d flow, i.e. the data flow in Figure 9a is applicable. With MAC-ehs configuration, a MAC-d header is not needed, i.e. the data flow in Figure 9a1 is applicable.
5.3.5.20 Data flow for DTCH (non-transparent RLC) mapped to HS-DSCH
Mapping to HS-DSCH implies a DTCH with acknowledged or unacknowledged transmission on RLC. With MAC-hs configuration, a MAC-d header is needed only if DCCH and DTCH logical channels are multiplexed on the same MAC-d flow, i.e. the data flow in Figure 9a is applicable. With MAC-ehs configuration, a MAC-d header is not needed, i.e. the data flow in Figure 9a1 is applicable.
5.3.5.21 Data flow for DCCH mapped to E-DCH
For DCCH, both unacknowledged and acknowledged transmission mode on RLC is employed. A MAC header is mandatory when the DCCH is mapped to the E-DCH, i.e. the data flow in figure 9b and 9c is applicable.
5.3.5.22 Data flow for DTCH (non-transparent RLC) mapped to E-DCH
Mapping to E-DCH implies a DTCH with acknowledged or unacknowledged transmission on RLC. A MAC header is mandatory when the DTCH is mapped to the E-DCH, i.e. the data flow in figure 9b and 9c is applicable.
5.3.5.23 Data flow for MCCH (non-transparent RLC) mapped to FACH
For MCCH mapped to FACH, unacknowledged transmission mode on RLC is employed. In case of MAC multiplexing the MAC header is needed for logical channel service multiplexing. The data flow in either Figure 8 or Figure 9 is applicable.
5.3.5.24 Data flow for MSCH (non-transparent RLC) mapped to FACH
For MSCH mapped to FACH, unacknowledged transmission mode on RLC is employed. In case of MAC multiplexing the MAC header is needed for logical channel service multiplexing. The data flow in either Figure 8 or Figure 9 is applicable.
5.3.5.25 Data flow for MTCH (non-transparent RLC) mapped to FACH
For MTCH mapped to FACH, unacknowledged transmission mode on RLC is employed. In case of MAC multiplexing the MAC header is needed for logical channel service multiplexing. The data flow in either Figure 8 or Figure 9 is applicable.
5.3.5.26 Data flow for CCCH mapped to HS-DSCH/RACH (FDD only)
In FDD, for CCCH, transparent transmission mode on RLC is employed on the uplink (when mapped to RACH). Unacknowledged transmission mode on RLC is employed on the downlink (when mapped to HS-DSCH). A MAC header is used for logical channel identification (CCCH). If the transparent RLC transfer mode is applied, the data flow Figure 7 is applicable. If the unacknowledged RLC transfer mode is applied, the data flow Figure 9a is applicable.
5.3.5.27 Data flow for PCCH mapped to HS-DSCH
No RLC header is needed. A MAC header is mandatory when the PCCH is mapped to the HS-DSCH. For FDD, an H-RNTI specific for PCCH transmission is used on HS-SCCH to identify the logical channel carried by the HS-DSCH. For 1.28 Mcps TDD, the HS-SCCH less operation is used. The logical channel ID in MAC header is set to a reserved value.
5.3.5.28 Data flow for BCCH mapped to HS-DSCH (FDD and 1.28 Mcps TDD only)
No RLC header is needed, i.e. the transparent data transfer mode of RLC is applied. A MAC header is mandatory when the BCCH is mapped to the HS-DSCH. An H-RNTI specific for BCCH transmission is used on HS-SCCH to identify the logical channel carried by the HS-DSCH. The logical channel ID in MAC header is set to a reserved value.
5.3.5.29 Data flow for CCCH mapped to HS-DSCH/E-DCH (FDD and 1.28 Mcps TDD only)
For CCCH transmission, transparent transmission mode on RLC is employed on the uplink (when mapped to E-DCH). Unacknowledged transmission mode on RLC is employed on the downlink (when mapped to HS-DSCH). A MAC header is used for logical channel identification (CCCH). If the transparent RLC transfer mode is applied, the data flow Figure 10 is applicable. If the unacknowledged RLC transfer mode is applied, the data flow Figure 9a is applicable.