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3. Layer 3 signaling (over SRB1/2)
Next transmission requirement comes from UE dedicated EPC and E-UTRAN signaling required for - Initial Attach, Tracking Area Update, Service Request/Paging, Bearer setup/management, Handover etc. [23.401:5.3.2/3/4, 5.4, 5.5]. In nutshell, this is Layer 3 (NAS/RRC) signaling over SRB1/2 .
With reference to channel mapping diagram (fig 1.6.3 and 1.6.4), this transmission goes over SCH requiring DL and UL allocations. Note that SRB 0/1/2 maps to separate MAC LCIDs (Logical Channel IDs) [36.321:6.2.1].
4. Data path (DRB0-7)
Layer 3 bearer setup procedures (refer articles on UE Initial Attach and Dedicated Bearer Setup) configures Layers below for User data transmission. As seen in Data path example, allocation mechanism is needed for user data transmission and associated layer 2/1 signaling .
We can now list down these requirements for which we would need allocation mechanism:
1. system information messages (except MIB and SIB1)
2. RAR and DL RRC Connection Setup message
3. Layer 3 (NAS/RRC) signaling over SRB1/2
4. user data transmission and associated layer 2/1 signaling
Above requirements basically lead to [chunk of data] that MAC & PHY has the responsibility to put on air. The amount of data depend on type of data as per requirements listed above; also the requirements have differing timing constraints. This is one side of job, the other side of the job is to adapt transmission to the signal conditions and accordingly run the allocation mechanism.
Signal conditions vary, so the [chunk of transport data] may require one allocation unit (RB to be specific) or n allocation units. If we base our allocation mechanism on one-unit-at-a-time, we will have good adaptability/flexibility, but more signaling overload - every unit will have to have allocation signaling overhead ! On the other hand, if we allocate more than one unit at a time, we gain in overhead, but at the expense of granularity and adaptability. So the goal is “Allocation mechanism should have minimal overhead and at the same time, maintain granularity so as to give the sense of adaptive/flexible and efficient air interface ”.
Till now we used the word "Allocation" loosely to indicate allocation of radio resources. Let us now look at it in little more detail before we see allocation mechanism. Allocation mean specifying 1) quantity 2) location 3) modulation & coding 4) user (UE) & (transport) channel of the radio resources. Of course, both DL/UL allocations happen through DL.
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