MAC Layer
it would be almost impossible to explain everything about MAC but i have tried to simplify MAC by diagram.
lets start to read diagram first and see what MAC does? i have taken diagram from 3GPP TS:36.321 and modified according to me to simplify MAC
MAC Layer
- Mapping between logical channels and transport channels;
Query_1: what kind of channel used for mapping in UE/eNodeB?
UE --> Logical channel (CCCH, DCCH and DTCH) <--> Transport Channel (RACH and UL-SCH)
CCCH, DCCH, DTCH are all mapped to UL-SCH.
There is a special channel called 'RACH', but this does not have any corresponding logical channel.
eNodeB --> Logical channel (PCCH, BCCH, MCCH, MTCH, CCCH, DCCH and DTCH) <--> Transport Channel (PCH, BCH, MCH and DL-SCH)
PCCH (Logical Channel) is mapped only to PCH (Transport channel).
Some BCCH is mapped to BCH and Some BCCH is mapped to DL-SCH. Do you know which one is which ? BCCH for MIB is mapped to BCCH and BCCH for SIB is mapped to DL-SCH.
DL-CCCH, DCCH, DTCH are all mapped to DL-SCH.
Query_2: What channel modified or what is not modified by MAC?
from Diagram:
i) All the logical channes (PCCH, BCCH, CCCH, DCCH, DTCH) goes through MAC layer. (too simple reading ? -:)
ii) It seems that PCCH does not get manipulated by MAC in any special way. It just looks as if it is by passing MAC process. It does not use HARQ procedure meaning 'no retransmission' mechanism being used.
iii) CCCH, DCCH, DTCH all go through the same procedure (Prioritization, Mux/DeMux, HARQ).
iv) Some BCCH goes through HARQ, but some BCCH does not go through HARQ. Do you know which BCCH go through HARQ and which does not ? BCCH for MIB does not go through HARQ, but BCCH for SIB go through HARQ. (But this HARQ is a little different from normal HARQ that we know of.. It does not expecting any ACK/NACK response from the reciever.. but perform 'retransmission' based on a predefined rule).
v) Random Access process orignated within MAC layer, it does not have the correspoding logical channel. (This may apply to Msg1, Msg2 of RACH procedure.. Msg3,4,5 involves RRC layer intervention as well).
Query_3: what kind of information carry channel?
Logical Channels : Type of information
Transport Channels : How the information is transferred
Try to correlate each messages in the call flow with the channel mapping. And the try to correlate this mapping with the diagrams described above. Ask a couple of questions to yourself, e.g which of these message use HARQ ? Which message is carried by a 'Common' channel and which messages are carried by a 'Dedicated' channel.
- Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels;
- de-Multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;
Query_4: How data multiplex/de-mux happen?
- Scheduling information reporting;
Query_5: How scheduling report?
- error correction through HARQ;
Query_6: How error correction happen through HARQ?
- Priority handling between UEs by means of dynamic scheduling;
- Priority handling between logical channels of one UE;
- Logical Channel prioritization
Query_7: how logical channel prioritize
- Transport format selection.
Query_8: how mac select transport format?
Mac Procedure
- Random Access
Query_9: How RACH happen?
- DL-SCH Data Transfer
Query_10: How DL-SCH data transfer?
- UL-SCH Data Transfer
Query_11: How UL-SCH transfer?
MAC PDU:
MAC Control Element
RNTI
lets start to read diagram first and see what MAC does? i have taken diagram from 3GPP TS:36.321 and modified according to me to simplify MAC
MAC Layer
- Mapping between logical channels and transport channels;
Query_1: what kind of channel used for mapping in UE/eNodeB?
UE --> Logical channel (CCCH, DCCH and DTCH) <--> Transport Channel (RACH and UL-SCH)
CCCH, DCCH, DTCH are all mapped to UL-SCH.
There is a special channel called 'RACH', but this does not have any corresponding logical channel.
eNodeB --> Logical channel (PCCH, BCCH, MCCH, MTCH, CCCH, DCCH and DTCH) <--> Transport Channel (PCH, BCH, MCH and DL-SCH)
PCCH (Logical Channel) is mapped only to PCH (Transport channel).
Some BCCH is mapped to BCH and Some BCCH is mapped to DL-SCH. Do you know which one is which ? BCCH for MIB is mapped to BCCH and BCCH for SIB is mapped to DL-SCH.
DL-CCCH, DCCH, DTCH are all mapped to DL-SCH.
Query_2: What channel modified or what is not modified by MAC?
from Diagram:
i) All the logical channes (PCCH, BCCH, CCCH, DCCH, DTCH) goes through MAC layer. (too simple reading ? -:)
ii) It seems that PCCH does not get manipulated by MAC in any special way. It just looks as if it is by passing MAC process. It does not use HARQ procedure meaning 'no retransmission' mechanism being used.
iii) CCCH, DCCH, DTCH all go through the same procedure (Prioritization, Mux/DeMux, HARQ).
iv) Some BCCH goes through HARQ, but some BCCH does not go through HARQ. Do you know which BCCH go through HARQ and which does not ? BCCH for MIB does not go through HARQ, but BCCH for SIB go through HARQ. (But this HARQ is a little different from normal HARQ that we know of.. It does not expecting any ACK/NACK response from the reciever.. but perform 'retransmission' based on a predefined rule).
v) Random Access process orignated within MAC layer, it does not have the correspoding logical channel. (This may apply to Msg1, Msg2 of RACH procedure.. Msg3,4,5 involves RRC layer intervention as well).
Query_3: what kind of information carry channel?
Logical Channels : Type of information
Transport Channels : How the information is transferred
Try to correlate each messages in the call flow with the channel mapping. And the try to correlate this mapping with the diagrams described above. Ask a couple of questions to yourself, e.g which of these message use HARQ ? Which message is carried by a 'Common' channel and which messages are carried by a 'Dedicated' channel.
- Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels;
- de-Multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;
Query_4: How data multiplex/de-mux happen?
- Scheduling information reporting;
Query_5: How scheduling report?
- error correction through HARQ;
Query_6: How error correction happen through HARQ?
- Priority handling between UEs by means of dynamic scheduling;
- Priority handling between logical channels of one UE;
- Logical Channel prioritization
Query_7: how logical channel prioritize
- Transport format selection.
Query_8: how mac select transport format?
Data is sent from the physical layer to the MAC layer in the form of transport channels. The MAC layer is responsible for mapping the transport channels onto logical channels and transmitting the data up to the RLC layer, and thereverse - receiving data on logical channels from the RLC layer and mapping that data onto transport channels for the physical layer. On those channels, MAC layer is also responsible for segmenting long messages from higher layers into blocks, and reassembling blocks from the physical layer into messages for the higher layers.
MAC layer is also responsible for selecting the transport formats used on the transport channels, and distinguishing between different UEs using the common channels.
Every TTI (1ms) a set of Transport Blocks is transferred from the MAC to the Physical layer. The data will be coded and split into blocks to be sent out every 10ms radio frame.
Data from MAC could be sent in a single large Transport Block or in a set of smaller Transport Blocks. The latter approach allows the number of blocks (and hence the data rate) to be varied more simply in each TTI.
- Random Access
Query_9: How RACH happen?
- DL-SCH Data Transfer
Query_10: How DL-SCH data transfer?
- UL-SCH Data Transfer
Query_11: How UL-SCH transfer?
MAC PDU:
MAC Control Element
RNTI
1 comments:
I think We won't use HARQ retransmission for SIB..
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