# Internal Backpressure for Onboard Crospoint-Buffered Switches with Port Multiplexing
## 3 Design of Internal BP Architecture
### BP Mechanism  
However, in the practical operation of the aforementioned model, if multiple input ports simultaneously send data to the same output port, and the data bus rate does not match the output port rate, data will accumulate at a specific node in the CBM, leading to traffic congestion. The core principle of the BP mechanism lies in utilizing real-time status feedback from downstream queues to regulate upstream traffic transmission [39]. It sends a clear status signal upstream through the downstream queue (for example, a 0 signal indicates that the system is in normal operation and able to receive data. A 1 signal is used as an overload warning, indicating that the current processing capacity has reached the upper limit), which is used as the basis for regulating the traffic [40]. Specifically, a buffer queue added at the output port can quickly reach saturation when multiple data streams send data to one of the queues at the same time. Once the queue capacity reaches the upper limit, in order to avoid data overflow, a BP signal is sent to the input end to improve the problem caused by traffic overload in the buffer queue. After the data in the queue is gradually processed and emptied, the output port sends a signal to the input port, indicating that the queue is empty and data can be sent again. At this point, the BP state will be removed. When the queue is full again, the same BP process is triggered to ensure that the transmission of data is always kept under control. This dynamic feedback mechanism can effectively avoid traffic congestion. As illustrated in Figure 4, input port IP(0,0) and IP(\(N/k\)\(-\)1,0) are simultaneously transmitting data towards output port OP(0,0). However, the data processing rate at the OP end is considerably slower, amounting to only \(1/k\) of the bus rate, which can lead to the congestion of traffic at the Crosspoint Queue (CQ). This congestion blocks the scheduling of traffic from other input ports, such that data originating from IP(0,\(k\)\(-\)1) is prevented from being routed to OP(0,\(k\)\(-\)1) via CQ(0,0). To avert such a case, the OP(0,0) dispatches a BP signal to the input end IP(0,0). Upon the reception of this BP signal, IP(0,0) ceases the transmission of new data to OP(0,0), effectively easing the traffic burden on the OP(0,0). At this point, OP(0,0) gains sufficient buffer capacity to address the accumulated data packets at CQ(0,0), ensuring the orderly scheduling of data from IP(0,\(k\)\(-\)1). Once the queue length of OP(0,0) descends below the designated minimum threshold, the BP state is canceled, thereby restoring the normal traffic of data transmission. This paper proposes the following BP mechanisms: \begin{table}
\begin{tabular}{c c} \hline
**Symbolic** & **Description** \\ \hline SIM(\(n\)) & The \(n\)th input module \\ OAM(\(m\)) & The \(m\)th output module \\ IP(\(n\),\(x\)) & SIM(n) \(x\)th input port of the \(n\)th input module \\ OP(\(m\),\(x\)) & OAM(m) \(x\)th output port of the \(m\)th output module \\ CQ(\(n\),\(m\)) & The Crosspoint Queue connecting SIM(n) and OAM(m) \\ IB(\(n\)) & The \(n\)th input bus in the crosspoint \\ OB(\(m\)) & The \(m\)th output bus in the crosspoint \\ \hline \end{tabular}
\end{table} 
Table 1: Meaning of symbols. * _BP coarse_: When the system detects that a particular output queue has reached its full capacity, this mechanism comprehensively blocks all traffic destined for the internal bus associated with that overloaded queue. While this approach rapidly and effectively reduces traffic pressure, its drawbacks are concurrently evident. * _BP fine_: Upon reaching full capacity, a specific output queue triggers the BP mechanism solely for the VOQs directed towards it. This prevents the VOQs from forwarding data to the CQ, ensuring the queue does not overload until its state improves. * _BP load detect_: The system identifies overloaded CQ queues based on the congestion status of intermediate nodes.