Hello Friends! Welcome back to the world of 4G. In our previous video we discussed how the
data were segregated using Logical channel and How they were mapped on to the corresponding Transport
Channels. Once the data is processed by MAC layers ,they
are known as MAC PDUs or Transport block. These PDUs are transmitted over the air interface
through Physical Channels. It’s the job of MAC layer to decide the
amount of data from each logical channel to be included in the MAC PDUs. The PDU size is decided on the basis of Uplink
resource grant Message. In our previous video, We mentioned that,
MAC layer or Team Y also prioritizes the goods. As the length of MAC PDU is fixed, if we keep
on filling the MAC PDU in the order of their priority, there would be a possibility that
the data in lower priority logical channels would not get chance to be transmitted. This particular scenario is known as “Channel
Starvation “. To avoid this, one Prioritized Bit Rate is added for each logical channel. So let us understand how PBR Helps in preventing
channel starvation. Consider packets are provided to MAC layer
through 4 logical channels (A,B,C,D). where channel A, has highest priority and
channel D, has lowest priority. Let’s say logical channel A has 20 bits
to send, B has 15 bits, C has 10 bits and D has 3 bits. But the length of the MAC PDU is of 20 bits. If we go on filling the MAC PDU on the basis
of priority, then only channel A which is of 20 bits will occupy the entire MAC PDU. The channels of lower priority will have to
wait for the next MAC PDU. So to avoid this, a PBR is configured for
each channel. Let’s assume for Channel A, it is 5bits, for
B 4 bits, for C 3 bits and for D 2 bits. So in the first round each logical channel
is served in decreasing order of priority, but the max amount of data that can be put
in MAC PDU is limited by their PBR. that means in first round only 5 bits of channel
A is filled in the PDU, then 4 bits of channel B then 3 bits channels C and finally 2 bits
of channel D. So the total space filled in MAC PDU after
the first round is 14 bits. Once the logical channels are served by their
PBR, we can see that there is still 6 bits space left in the MAC PDU, So the remaining
space shall be filled in the order of priority but without PBR. In this case after first round the channel
A has 15 bits. So,the remaining 6 bits space in the MAC PDU
shall be filled with data from the channel A which is of highest priority. But let’s say that the PDU size is 45 bits,
then all the remaining 15bits of Channel A will be included in the PDU without considering
the PBR. Then 11 bits from Channel B will be added. The remaining 5bits will be filled by Channel
C. This process is repeated till all the data
has been transmitted. This is how we can avoid problem caused by
channel starvation. So friends, We have seen how data are logically
prioritized and assembled into MAC PDU’s. In our next video we will discuss the function
of MAC control and the brain behind all these operation which is MAC scheduler module. We will also discuss the functioning of Team
Z or the Physical Channels which actually transports the datum. You can comment your doubts or suggestions
and subscribe to our channel to stay updated. Happy Learning


  1. thanks for the informative video.very thoughtful. but the Bgm is little louder (sudden sounds). please adjust that.

  2. Thanks a million for videos..its the best video I have come across… could you please upload video related to MAC scheduling and radio bearers..

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