# PCCNoC: Packet Connected Circuit as Network on Chip for High Throughput and Low Latency SoCs
## 2 Related Work  
There are many packet-switched NoCs and Circuit-switched NoCs in the literature. PS NoC has been intensively studied by researchers, and there are several kinds of PS NoC [7; 8; 9; 10; 11; 12; 13; 14; 15], mainly including virtual channel (VC) and wormhole (WH) switching. In the virtual channel technique, a packet is decomposed into flits, which are then routed consecutively through the network. The adjacent router needs to store the whole packet when network congested in VC as described in Figure 2a. The memory size takes up a large area of the system on chip. The wormhole switching technique reduces the usage of the buffer, as it uses backpressure to stall the route path when the network congestion happens as shown in Figure 2b. However, segmented storage of flits is more likely to cause link blocking and deadlock [16]. Figure 2: The flit status when the is network congested for (**a**) virtual channel PS NoCs and (**b**) wormhole PS NoCs. Generally speaking, wormhole switching NoCs with virtual channels is well accepted and utilized [10]. [PERSON]'s book [5] has covered almost every aspect of such a PS NoC, while [PERSON]'s book [17] has mentioned the design methodology of such NoCs. VCS [7] (virtual circuit switch) uses a flow-control policy that exclusively reserves a virtual path for routing packets in an adaptive mode for each communication flow in order to guarantee that delivery of all packets belongs to the same communication flow. It allows sharing of the reserved path among packets of different communication flows. The method needs additional memory unit for storing all unordered packets, and needs extra channels to for path reservation. MRBS [9] manages all input buffers in a router as a register file style instead of dedicating a single buffer per input port, which efficiently utilizes the buffer space, especially for multicast traffic patterns. MRCN [8] uses destination router partitioning and traffic-aware adaptive branching technique to reduce packet routing hops and disperse channel traffic. It can ensure deadlock freedom by utilizing extended routing and router labeling rules. DancerFly [15] is an order-aware network-on-chip router which resolves out-of-order packet delivery issue in two steps. First, it performs in-buffer reordering by reordering packets queuing in the input buffer. Second, packets from different input ports are re-ordered before going through the router. Ref. [18] proposes a solution that guarantees in-order packet delivery while packets are routed through multiple paths in the network, and [19] proposes a solution that supports in-order packet delivery under adaptive routing by reserving an alternate virtual path during the VC allocation process. Yet, such approaches are relatively expensive to implement in terms of silicon footprint and have high power consumption due to the extensive use of buffers. There are studies in the literature studying adaptive NoCs [19; 20] to improve application parallelism abilities and fast path NoCs [21; 22; 23], which improves latency and throughput by redesigning the PS routers. Adapt-NoC [20] introduces a reinforcement learning (RL)-based control policy. It can dynamically allocate several disjoint regions of the NoC, called subNoCs, which can provide efficient communication support for concurrent application execution. Adapting NoC can handle a variety of traffic patterns for running multiple applications at the same time. It is capable of adapting to a given topology such as a mesh, cmesh, torus, or tree, thereby tailoring the topology [19]. The above-mentioned studies represent a hot research domain. Nevertheless, there are drawbacks in terms of uncertain transmission times for real-time applications. Multi-hop bypassing has been researched intensively to design a near-optimal interconnection network [21]. FastPass [22] promotes a packet to traverse the network bufferlessly, and provides multiple pre-defined non-overlapping lanes.