eclared, and a random access procedure is triggered. The scheduling request procedure of 5G differs from that of LTE by allowing multiple SR configurations. Each SR configuration consists of a set of PUCCH resources tied to a numerology, BWP, and cell. Whether a particular logical channel can use a particu- lar SR configuration is also explicitly configured by RRC. With multiple SR configura- tions, it then becomes possible to reserve more SR opportunities for URLLC services and identify the logical channel(s) requesting resources directly from the SR used to request them. 7.6.6 Logical Channel Prioritization and Multiplexing The mapping of RLC PDUs from the logical channels onto transport blocks to be trans- mitted by the physical layer is governed in the MAC sublayer by a process called logical channel prioritization, or LCP for short. LCP in 5G builds upon the basic principles introduced in LTE. Two loops are used to determine how many bits can be transmitted for each logical channel: the first loop relies on a token bucket algorithm to serve the logical channels in order of priority up to their configured prioritized bit rate (PBR), and the second loop serves logical channels in strict order of priority only. The first loop 7.6 MAC Layer is used to guarantee that logical channels of low priority will not starve as long as they have a PBR configured. After LCP, the MAC sublayer indicates to each RLC entity how many bits are available for transmission for the corresponding logical channel. But unlike in LTE, the MAC sublayer is responsible for the multiplexing of RLC PDUs from each logical channel (see Section 7.5.1), and an RLC entity produces as many PDUs and PDU segments as allowed by the number of bits indicated by MAC (see Section 7.5.2). When a logical channel is empty, others will be transmitted, and if none have data available for transmission, padding will be used to fill the transport block. Another main difference in 5G lies in the addition of LCP restrictions. With LCP restrictions, it becomes possi