 functionality provides error correction by re-transmissions in acknowledged mode at Layer 2. The HARQ functionality ensures delivery between peer entities at Layer 1. The HARQ within the MAC layer is characterized by an N-process stop-and-wait protocol and re-transmission of transport blocks upon failure of earlier transmissions. A total of eight HARQ processes are supported [29]. The 3GPP LTE-Advanced system extends the capabilities of 3GPP LTE Rel-8 with support of carrier aggregation, where two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz and for spectrum aggregation. A user terminal may simul- taneously receive or transmit one or multiple component carriers depending on its capabilities. From the UE perspective, the Layer 2 aspects of HARQ are similar to those of Rel-8. There is one transport block (in absence of spatial multiplexing, up to two transport blocks in case of spatial multiplexing) and one independent HARQ entity per scheduled component carrier. Each transport block is mapped to Table 3-1 Summary of the Differences between MAC and RRC Control [33] MAC Control RRC Control Control Entity Signaling Type Physical control channel MAC control PDU RRC message Signaling Reliability 10-2 (No HARQ ~10-3 (HARQ 10-6 (ARQ re-transmission) re-transmissions) re-transmissions) Control Latency Very short Short Longer Extensibility Limited Security No integrity protection No integrity protection Integrity protected No ciphering No ciphering Ciphering CHAPTER 3 IEEE 802. 16m Reference Model and Protocol Structure a single component carrier on which all HARQ re-transmissions may take place. A UE may be scheduled over multiple component carriers simultaneously, but at most one random access procedure will be ongoing at any time. Whenever a UE is configured with only one component carrier, the 3GPP LTE Rel-9 DRX is the baseline. In other cases, the same DRX operation will be applied to all configured component carriers. Therefore, the Layer 2 structure of th