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.
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Happy Learning