# Extending the Performance of Hybrid NoCs beyond the Limitations of Network Heterogeneity
## 4 Evaluation
### Impact of the Proposed Router on WiNoCs  
For an extensive validation of the performance benefits of the proposed router in emerging WiNoCs, the M5 simulator [57] is employed to acquire memory access traces from a full system running PARSEC v2.1 benchmarks [58], which is used to stimulate our version of the Noxim simulator. For correlation purposes, we have used the same setup as [47]. Hence, in the setup, 64 two-wide superscalar out-of-order cores with private 32 KB L1 instruction and data caches, as well as a shared 16 MB L2 cache are employed. Following the methodology presented in Netrace [59], the memory traces are post-processed to encode the dependencies between transactions. Consequently, the communication dependencies are enforced during the simulation. Memory accesses are interleaved at 4-KB page granularity among four on-chip memory controllers. A summary of the benchmarks is presented in Table 5. Thus, we apply a wide range of benchmarks with varied granularity and parallelism to study the effects of the proposed bypassing technique on the state-of-the-art wireless communication fabrics on WiNoCs. For each trace, we simulate at least 100 million cycles of the PARSEC-defined region of interest (ROI), where we schedule two threads per core. Five evenly-distributed nodes in the WiNoC are equipped with transceivers. All other nodes have receivers. For WiNoCs with bypass techniques, the receiving nodes are enhanced with SlideAcross routers. Similarly, for WiNoCs with SmallWorld, the receiving nodes are enhanced with the seven-port small world routers, which have long links with repeaters that connect directly to wireless nodes. Thus, packets can exploit both the bypass links and adaptive routing (Buff NVH) within the wireline layer to access the wireless and destination nodes. To model the effect of different bit-error-rates (BER) of the wireline and wireless layer on the network performance in terms of packet latency, we employ the packet error ratio (which dictates the probability of packet retransmission) [47]:  
\[p_{p}=1-(1-p_{e})^{|P|} \tag{1}\]  
where \(|P|\) is the packet length in bits and \(p_{e}\) is the bit error probability, which is the expectation value of the BER for the communication fabric. Thus, Equation (1) is modelled and imported into the NoC simulator to assign the probability of retransmission of different communication fabrics at different packet injection rates. The alternating bit protocol is used for transmitting and receiving data and the credit flit (ACK/NACK). While wormhole flow control is used for the wireline layer, FDMAmedia access control is adopted to give more than one node the right to transmit over the shared wireless medium at a data rate of 256 Gbps in one clock cycle over 128 carrier frequencies. Fixed BERs of \(10^{-7}\), \(10^{-14}\) and \(10^{-13}\) are used for mm-wave, wire and SW, respectively (Table 6). Figure 13: Average packet latency of various inhomogeneous 3D NoCs. Figures 14 and 15 show the normalized packet delays of various WiNoCs. Particularly, it can be deduced from Figure 14 that SlideAcross increases the performance of mm-wave significantly, compared to SmallWorld and conventional mm-wave WiNoCs. Besides having a larger crossbar with a seven-port router and longer input buffer waiting time, SmallWorld routing involves intermediate buffering, which increases the router pipeline and hence contention in the network. Consequently, packets in SlideAcross experience shorter delays in the reduced pipelined routers, which allow bypassing of the input buffers and crossbar. Moreover, the performance improvement is more profound when these routers are employed in SW-enabled WiNoCs (Figure 15). This is expected as SW WiNoCs are less congested compared to mm-wave WiNoCs, as there are less retransmissions along the wireless channel. Consequently, the performance benefits of the bypass links in SlideAcross are more pronounced in SW than mm-wave.