Example for a Data flow through MAC layer.
The logical channels coming from higher layer – RLC will be terminating in the MAC. It is the functionality of MAC to perform logical to transport channel mapping. Many number of logical channels can be mapped into one transport channel.
Here I consider the logical channels DCCH/DTCH that are coming from RLC. A data packet arriving from DCCH/DTCH logical channel triggers the Transport channel type selection. In this example, the FACH transport channel is selected.
Next, the multiplexing unit adds a C/T field – this indicatesthe logical channel instance where the data originates. For common transport channels, such as FACH, this field is always needed. For dedicated transport channels (DCH) it is needed only if several logical channel instances are configured to use the same transport channel. The C/Tfield is 4 bits, thereby allowing the mapping of upto 15 logical channels to a transport channel.(Value 1111 is reserved for future use).
The priority tag (not part of the MAC PDU) for FACH and DSCH is set in MAC-d and used by MAC-c/sh when scheduling data onto transport channels. Priority for FACH can be set per UE; for DSCH it can be set per PDU. A flow control function in Iur interface is needed to limit buffering between MAC-d and MAC-c/sh (which can be located in different RNCs). After receiving the data from MAC-d, the MAC-c/sh entity first adds the UE identification type (2 bits), the actual UE identification (C-RNTI 16 bits, or U-RNTI 32 bits), and the Target Channel Type Field (TCTF, in this example 2 bits) which is needed to separate the logical channel type using the transport channel .Now the MAC PDU is ready and the task for the scheduling/priority handling function is to decide the exact time when the PDU is passed to Layer 1 via the FACH transport channel (with an indication of the transport format to be used).
Here I consider the logical channels DCCH/DTCH that are coming from RLC. A data packet arriving from DCCH/DTCH logical channel triggers the Transport channel type selection. In this example, the FACH transport channel is selected.
Next, the multiplexing unit adds a C/T field – this indicatesthe logical channel instance where the data originates. For common transport channels, such as FACH, this field is always needed. For dedicated transport channels (DCH) it is needed only if several logical channel instances are configured to use the same transport channel. The C/Tfield is 4 bits, thereby allowing the mapping of upto 15 logical channels to a transport channel.(Value 1111 is reserved for future use).
The priority tag (not part of the MAC PDU) for FACH and DSCH is set in MAC-d and used by MAC-c/sh when scheduling data onto transport channels. Priority for FACH can be set per UE; for DSCH it can be set per PDU. A flow control function in Iur interface is needed to limit buffering between MAC-d and MAC-c/sh (which can be located in different RNCs). After receiving the data from MAC-d, the MAC-c/sh entity first adds the UE identification type (2 bits), the actual UE identification (C-RNTI 16 bits, or U-RNTI 32 bits), and the Target Channel Type Field (TCTF, in this example 2 bits) which is needed to separate the logical channel type using the transport channel .Now the MAC PDU is ready and the task for the scheduling/priority handling function is to decide the exact time when the PDU is passed to Layer 1 via the FACH transport channel (with an indication of the transport format to be used).
No comments:
Post a Comment