## 4 Description of the Proposed Transport System
### Low-Latency Packet Forwarding (LLPF) Scheme  
IEEE P802.1 Qbu [22] introduces a concept of frame preemption (FP) to transport the different priority traffics in time-sensitive Ethernet-bridged networks. In a frame preemption operation, the transmission of low-priority (preemptable) frames is suspended to allow the transmission of one or multiple high-priority (express) frames. After that, the transmission of low-priority frames from the same point is resumed. A low-priority frame that was in the process of transmission has not been completely transmitted, but is rather preempted and buffered until no high-priority frame is left in the queue. The preemptive queuing system enables the cut-through transmission of time-critical packets and minimizes the queuing delays. With this approach, the high-priority frames are transmitted through the switching nodes at low queuing delays without waiting in the queues for the complete transmission of low-priority frames. However, in fronthaul bridged networks, all the eCPRI streams have an equal priority, and simply employing the concepts of frame preemption is unfeasible for traffic streams of equal priority. Therefore, a mechanism that not only allocates the low-load routes to eCPRI traffic streams but also employs the sophisticated priority schemes such as frame preemption to minimize the queuing delays of equal priority eCPRI streams would be more feasible, as discussed below. Under high traffic load conditions, a few RRH traffic streams are expected to experience the high queuing delays, despite the load balancing and efficiently utilizing the bandwidths [18], especially those that have long routes toward the BBU pool in term of delays and hop counts. As a result, the E2E latencies of such streams become intolerable. Thus, these high-latency experiencing RRHs streams (HERSs) can be given priority over the low-latency experiencing RRHs streams (LERSs) that have a margin of bearing slight queuing delays at intermediate nodes due to their physical or logical locality of being closer to the BBU pool. By considering the packets of HERSs as express frames and packets of LERSs as preemptable frames, the HERSs can be routed at low queuing delays through the nodes that the LERSs are connected to, and eventually, E2E latencies can be minimized. The global matrix of the LAPC algorithm provides the details about the high-latency experiencing RRHs streams over the most optimal paths. However, to manage the impact of preemption and retain the E2E latencies of the LERSs within limits, a frame of LERS should be delayed only a predefined number of times. The simulation result shows that a frame of the LERS can be delayed at maximum four to seven times under the considered scenarios, and after this threshold, a preempted frame of the LERS would be transmitted first instead of an express frame. In the case where a preempted frame is not transmitted according to a predetermined threshold, the E2E latencies of HERS might be low, but it will increase the E2E latencies of the LERS. The preempted frames are reassembled at the next nodes before processing so that these appear as valid Ethernet frames to the physical layer [22]. In the case, when more than one of the HERSs arrive at the bridges of a route, the HERSs that are connected with the most distant bridges, which is in terms of delays and hop counts from the BBU pool, would be given priority over the one with less distant connected bridges. As specified in the standard [22], packet tags are allocated to the preempted frames of the different streams that will be used at the receiver to reassemble the preempted frames. An example of the frames' arrival on the ingress ports, the preemption of frames, and transmissions of frames on the output port of the bridge are depicted in Figure 3. It can be observed that the HER streams of two different priorities (i.e., the priority of stream-3 is more than the priority of stream-2) and a LER stream of low priority (i.e.