LTE MAC Layer Interview Question and Answer 1-5

Que 1. What is Full Form of Mac Layer?, Out of 3 Layers [L1,L2,L3], where it lies?

Ans. Mac Layer stands for (Medium Access Control Layer), Out of 3 Layer it present in L2 Layer of lte.

Physical layer called L1 Layer, Mac, RLC, and PDCP called L2 Layer, RRC and NAS called L3 Layer in LTE.

Que 2. What is the Main function of MAC Layer in LTE?Ans The following functions are supported by MAC sub-layer?

Ans here are the some main functions of mac layer are defined.

Ø mapping between logical channels and transport channels;

Ø multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels;

Ø de-multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;

Ø scheduling information reporting;

Ø error correction through HARQ;

Ø priority handling between UEs by means of dynamic scheduling;

Ø priority handling between logical channels of one UE;

Ø Logical Channel prioritization;

Ø Transport format selection.

Que 3. How do you know how many RAPIDs are there and length of the RAR?

Ans When RAR message received and we try to decode them we check E=1 means there is 1 more sub-header is present, E=1 show only next sub-header presence, Until E=0 means this is last sub-header present in received RAR message, Each E [E=1,E=0] Carry’s each UE preamble Relevant information, So we can easily find out how many RAPID’s are present in RAR message and it’s Length.

Que 4. How UE Mac know that the received message is RAR?

Ans There is Two options

1.  UE’s Physical layer inform UE’s Mac layer that this message is received in PDSCH against RA-RANTI,

2.  A parameter include in received message header “Message Type”.

Que 5. How many times UE1 tries to re-transmit Preamble if its preamble information is not present in received RAR Msg?

Ans If [UE does not receive a matching Random Access response (RAR) in ra-ResponseWindowSize (12ms), (hence UE considers RACH attempt as failed) and PREAMBLE_TRANSMISSION_COUNTER (n) is less than PREAMBLE_TRANS_MAX (n20)]

    Then [UE retransmits the Preamble given in the PDCCH Order ].