## 2 State of the Art--Ethernet Based Fronthaul Networks  
Due to prior investments and backward compatibility issues, the existing standard bodies are focusing on the Ethernet-based fronthaul implementations. Currently, the IEEE 1914.3 [20] working group is investigating the techniques to encapsulate the CPRI samples within the Ethernet frames. This standard is also working on defining the techniques to improve the statistical multiplexing gains in the CPRI over Ethernet networks. The IEEE 802.1 CM [21] is striving to standardize the Ethernet-based fronthaul systems for transporting the eCPRI streams in bridged networks. This standard is in the process of defining the default configurations, procedures, and profiles to carry the time-sensitive eCPRI streams in the MFH at low latencies. However, no mechanism has been defined explicitly in the standards to compensate the constraints of queuing delays at bridges and simultaneously transport the multiple eCPRI streams in Ethernet-based fronthaul bridged networks by satisfying the latency requirements. Recently, due to the cost-benefits of Ethernet networks over the eCPRI-based optical switching networks, several academicals research proposals have been submitted to carry the time-sensitive fronthaul traffic in Ethernet networks. Most of the initial studies addressed the delay, jitter, and throughput challenges of the fronthaul networks by implementing the priority schemes [22] and packets scheduling [23] protocols, which were initially proposed for time-sensitive networking (TSN). The study [24] implemented the frames priority mechanism and traffic scheduling algorithm for the Ethernet-based MFH networks. This study shows that traffic priority can be effective in minimizingthe delays in the Ethernet networks, but cannot fulfill the CPRI demands. The author claims that the queuing delays could be mitigated at switches with the traffic scheduling algorithm for a few CPRI streams. However, the parameters and configurations under which the results were obtained were not detailed in the paper. A performance analysis of passive optical networks (PON)-based fronthaul systems for long-range transmissions has been reported in [25]. This study shows that PON systems could not fully satisfy the eCPRI requirements without several hardware upgrades. This study also proposed to employ an expensive erbium-doped fiber amplifier (EDFA) at selected locations of the networks for achieving the QoS parameters within the eCPRI requirements. It is suggested to use an EDFA before the BBU pool for the considered fronthaul scenario to comply with the eCPRI demands. The studies [26] and [27] discussed the traffic scheduling and buffering techniques for Ethernet switching systems to improve the multiplexing gains in the fronthaul networks while retaining the delays and jitter within the CPRI limits. These studies showed that the CPRI traffic flows can be transported through at maximum one or two Ethernet switching nodes without violating the CPRI requirements. However, these schemes become ineffective for retaining the E2E latencies within the limits in more realistic fronthaul bridged networks, which comprises several intermediate bridging nodes between the RRHs and the BBU pool. The constraint-based and QoS aware routing schemes would be alternative solutions to manage the latencies in multi-hop Ethernet networks. Since then, these routing schemes have been well studied in the past few years to tackle the network constraints such as the cost, capacity, throughputs, and delays. [PERSON]'s shortest path algorithm that allocates the weights to different links, based on the available bandwidths and distances, has been studied in [28] for TSN networks, which could be an efficient solution for the delay constraint routing problems. Mutual constraint-based routing procedures have been discussed in [29] to fairly utilize the limited resources in the Ethernet bridged networks.