## 1 Introduction
### Contributions  
In this paper, we formulate and study the LDCP problem, which consists in selecting a subset of PP nodes in which the DU and CU radio processing entities have to be placed, assuming QoS (latency) guarantees for FH, MH, and BH flows carried over a common packet-switched transport network. According to our best knowledge, this problem has not been studied in the literature so far. In the majority of prior works, the radio processing functions placed at different network sites were connected using dedicated optical links [20; 21; 22; 23]. For this reason, these works did not account for latency guarantees for traffic flows in a packet-switched network. The authors of [9] studied a packet-switched network with FH and BH flows; however, queuing latencies were not taken into account. In [24; 25], dynamic latencies were included into analysis; however, only one type of traffic flow, namely MH flow, was assumed. Eventually, in [26; 27], we formulated the flow allocation problem that concerned the assignment of DU nodes to a set of RUs and routing of FH and MH flows in the NGFI network under latency constraints. Still the problem addressed in [26; 27] was simplified since BH flows were not included into analysis, and the capacities of processing nodes were assumed to be unbounded.  
Given the above, our main contribution concerns modeling and solving the LDCP problem in the NGFI network in which latency-sensitive fronthaul, midhaul, and backhaul flows are jointly carried in a convergent packet-based network. Our particular contributions are as follows:  
1. development of an MILP optimization model for latency-aware DU/CU placement;
2. in the MILP model, consideration of three different traffic flows (FH, MH, BH) realized jointly in the NGFI network;
3. in the MILP model, consideration of limited PP processing capacities in the NGFI network;
4. reporting and discussion of results of numerical experiments assessing performance of the MILP optimization model proposed and evaluating NGFI network performance in different scenarios.  
We would like to stress that the LDCP-MILP model proposed can be applied practically for solving an essential optimization problem in the NGFI network. This problem concerns the allocation of processing resources for realization of radio processing function in a virtualized and centralized radio access network, which is connected using a packet-switched network. In particular, the LDCP optimization problem appears when planning the placement of distributed and centralized units in the NGFI network. Note that the NGFI network is one of the most promising C-RAN solutions, and it is very likely to be deployed in 5G communication networks. In this work, we show the results of such optimization assuming different network topologies and configurations.  
The remainder of the article is structured as follows. In Section 2, the network scenario, traffic model, and latency model considered in this work are presented. In Section 3, the LDCP problem is described. Moreover, an MILP formulation of the problem is proposed. In Section 4, numerical experiments are performed. Finally, we present concluding remarks in Section 5.