## 4 Description of the Proposed Transport System
### E2E Latency-Aware Path Computation (LAPC) Procedure  
In the considered fronthaul networks, a software-defined central controller (SDCC) [33] before the transmissions get a global view of the topology by collecting an information metric using the IS-IS (intermediate system to intermediate system) protocol, which was extended by IEEE 802.1 Qca [34] for the time-sensitive networking. The IS-IS protocol provided the details about the connection of nodes, link capacities, costs of the links, and distances between the nodes of the topology. Typically, the numbers of RRHs connected to the nodes and accordingly configured eCPRI line rates of the RRHs are well determined in a fronthaul topology [26]. We assumed that all the nodes to which the RRHs are linked also connected with the SDCC, and a path computation element (PCE) is employed at the SDCC. The placement of the BBU pool is also predetermined in the topology, which is preferably connected to a node with which the maximum numbers of nodes are linked as shown in Figure 2. The PCE generates the initial paths for all the eCPRI-based RRHs streams in accordance with the k-th shortest paths algorithm [35] and maintains the paths in a global matrix. The SDCC selects the shortest paths for the individual traffic streams using the SPB algorithm based on the link costs, loads, and delays constraints. Then, for each stream, the SDCC calculates the E2E latency using equations (1) to (4) and updates the latencies in the global matrix along with the pre-selected shortest paths. Alternative paths  
Figure 2: Proposed Ethernet-based fronthaul bridged networks. are allocated to the traffic streams that could not satisfy the E2E latency requirements of the eCPRI, such as yield more than 250 \(\upmu\)s. For this purpose, the first traffic load from the over-utilized adjacent links (ALs) of the destination node (DN) are offloaded, and then from the other links of previous ALs and so on. For simplicity, ALs are referred to as the first-level links (FLLs), the links that are adjacent to the FLLs are referred to as the second-level links (SLLs), next after the SLLs are referred to as the third-level links (TLLs), and so on, up to N-level links (NLLs), as illustrated in Figure 2. In this mechanism, the traffic loads from the FLLs to NLLs are systematically offloaded and uniformly distributed on the links. When a traffic load from a high-load link is shifted to a low-load link on each iteration, the according routes of the high-latency experiencing streams that were passing through the over-utilized high-load links are updated in the matrix. These routes were selected by searching from the k-th shortest paths by ensuring that it was not previously selected; then, E2E latencies on the new routes for the streams are recomputed. In the case when offloading the load from the high-load links results in E2E latencies that are greater than the threshold, under such scenarios, those paths are selected for the RRH traffic streams that yield the minimum E2E latencies for the maximum number of traffic streams. The above procedure is repeated until the E2E latencies of all the RRH streams reach the lowest level, and the further shifting of traffic loads between the links does not minimize the latencies. The global matrix contains the routes for each RRH traffic stream along with the E2E latencies computed with the proposed scheme. The output such as the global matrix of the low-latency paths computation algorithm will be used as the input of the low-latency packets transmission scheme, which is proposed in the next section. The proposed low-latency packet forwarding scheme cut down the queuing delays of those RRHs traffic streams, which could not comply with the stringent E2E latency requirements of the eCPRI, despite the low-latency paths computation procedure. The procedure of the proposed low-latency path computation scheme is described in Algorithm 1.