# PCCNoC: Packet Connected Circuit as Network on Chip for High Throughput and Low Latency SoCs
## 5 Implementation and Evaluation
### Latency and Throughput  
First, a set of experiment is designed to compare the latency and throughput between PCCNoC and conventional PS NoCs. Noxim [40] simulator is used to model and simulate the traditional packet switching NOC. Noxim is a well-known cycle accurate NoC simulator to analyze performance of a NoC. In order to justify the performance of the proposed work, routers and network interfaces for PCCNOC have been implemented in Noxim. The  
Figure 11: Pipelines for different switching method for a \(3\times 3\) mesh NoC using the adaptive X-Y routing method. The data transmission task is from node 0 to node 8, as shown in (**c**). The subfigures are: (**a**) PS cut through data flow; (**b**) Conventional CS data flow; (**d**) Virtual channel(VC) PS data flow; (**e**) VC PS data flow with conflict at location 2; (**f**) Wormhole(WH) PS data flow; (**g**) WH PS data flow with conflict at location 2; (**h**) Packet connected circuits (PCC) data flow; (**i**) PCC data flow with conflict at location 2. Because the datapath is short, the grant signal and unlink signal are purely combination logic. PCCNoC is compared with a standard VC PS NoC model on Noxim. The detailed NoC configurations are presented in Table 2.  
Figure 12 shows the comparison of simulation results with different mesh size under different injection rates (IRT). Average latency refers to the average clock delay of messages arriving at the destination, and average network throughput refers to the the average packet number received by destination node at every clock. From Figure 12, it can be seen that the average packet delay of our scheme for mesh networks with different sizes has been significantly decreased thanks to the extremely low delay of circuit switching after packet switching, which reduces the overall average packet delay. It can be observed from the figure that the average network throughput has been significantly improved, as the established circuit can achieve full bandwidth transmission. In addition, it can be seen that performance saturation point of conventional PS NoC occurs when the injection rate is 0.08. By contrast, the average delay and network throughput of PCCNoC have good linear characteristics with variation of injection rate, and the saturation point is much higher. For injection rate 0.12, the throughput of PCCNoC improves 32%, 61%, 85% respectively for 4 \(\times\) 4, 6 \(\times\) 6 and 8 \(\times\) 8 PS Noc and the latency decreases 92%, 95%, 95% respectively. Transmission length is set as another experimental variable. Because the data port bandwidth is firm, we adjust the packet number for every transmission to achieve the transmission length variation effect. The size of every packet is 4 bytes and additional 7 bits are used for PCCNoC. The detailed NoC configurations are the same with Table 2 except that the packet number changes for every experiment and the injection rate keeps 0.01. \begin{table}
\begin{tabular}{c c} \hline NoC model & Mesh: 4 \(\times\) 4, 6 \(\times\) 6, 8 \(\times\) 8 \\ \hline Flit width & 34 bits (2-bit flit type + 32-bit(4 Byte) message data) \\ \hline Packet length & 12 flits (1 head flit + 14 body flits + 1 tail flit), total 48 byte message data. \\ \hline Operation time & 10,000 cycles \\ \hline Traffic model & Uniform random \\ \hline Route algorithm & adaptive-XY \\ \hline Synthesis time & 100,000 clock cycles \\ \hline \end{tabular}
\end{table} 
Table 2: Detailed information for each part of the router interface. Figure 12: Comparison for (**a**) average latency and (**b**) average throughput for different NoC with different IRT. Figure 13 shows the comparison of simulation results with different mesh sizes while the packet number varies. It can be seen that the average latency of the conventional packet switched NOC network increases with packet number increasing, and the network throughput is saturated after a slight improvement.