RRC and NAS messages. New Radio Access Layer 2/3 Aspects and System Operation 217 A sufficiently low block error rate is not only required for URLLC services, but it also is important from a system-level perspective in terms of sustaining data rate performance. The TCP protocol (at transport layer) requires virtually error-free delivery of packets to the peer TCP layer. As an example, to obtain sustained data rates in the excess of 100 Mbps in TCP/ IP applications, a packet error rate of less than 10- is required, because the TCP protocol would consider packet errors due to network congestion and would trigger the congestion- avoidance mechanism to decrease the data rate, causing reduction of the overall data rate performance of the system. It must be noted that the infrequent transmission of RLC status reports compared to more frequent HARQ retransmission makes obtaining a reliability level 10-5 using RLC ARQ retransmissions more practical [17]. Therefore, complementing MAC sublayer HARQ protocol with RLC sublayer ARQ mechanism would achieve rela- tively lower latency and reasonable feedback overhead to satisfy the stringent requirements of URLLC applications. The RLC sublayer ARQ mechanism retransmits RLC SDUs or RLC SDU segments based on RLC status reports, where polling may be used to request RLC status reports. An RLC receiver can also trigger RLC status report after detecting a missing RLC SDU or RLC SDU segment [8]. The RLC-TM entity is configured to trans- mit/receive RLC PDUs through BCCH, DL/UL CCCH, and paging control channel logical channels. The RLC-UM entity is configured to transmit/receive RLC PDUs via downlink or uplink dedicated traffic channel (DTCH). The RLC-AM entity can be configured to trans- mit/receive RLC PDUs through downlink or uplink dedicated control channel (DCCH) or DL/UL DTCH logical channels. Functional models of RLC-AM and RLC-UM entities are illustrated in Fig. 2.11, where the functions that are only related to RLC-AM mode (ARQ functions) are marked with a dark color. The RLC P