 is implemented externally) toward the operator network or a split-6 implementation to support some of the advanced centralized features such as joint scheduling. The ISC is a much more "closed" solution where high- and low-physical layer are typi- cally tightly coupled and running on colocated hardware. The ISC is also expected to be a small form factor (low power and cost), driving for a cost-optimized (ASIC-like) hardware solution with highly proprietary software. 34 Open Radio Access Network (O-RAN) Systems Architecture and Design Table 1-1 Categorization of nonideal backhaul (3GPP 36.932). Backhaul technology Latency (one-way) (ms) Throughput Priority Fiber Access 1 10-30 10M-10 Gbps Fiber Access 2 100-1000 Mbps Fiber Access 3 50M-10 Gbps DSL Access 15-60 10-100 Mbps Cable (DOCSIS) 25-35 10-100 Mbps Wireless Backhaul 10-100 Mbps typical, <1 Gbps max This goes against a lot of the "software-driven" principles of O-RAN system but shows the strength of the O-RAN standards: O-RAN defines interfaces rather than dictating an imple- mentation, allowing for tightly integrated solutions to exist in the market. The main benefit of the ISC solutions is the support for a nonideal backhaul to the opera- tor network. See Table 1-1 (copied from 3GPP 36.932). The table shows the two big challenges with nonideal backhaul: latency is too high to maintain HARQ roundtrip time (more about that later), and throughput is not high enough to support physical layer functional splits (e.g., 3GPP splits 7 or 8). An additional example is that of the industrial environment, which is a key target market for 3GPP systems. Industrial environments (the factory floor) often are prewired with an Ethernet/TSN network that is capacity-limited, say to 1 Gbps CAT-5 wiring. The near- 10 Gbps throughput numbers required for a CU/DU/RU split are simply not supported by such cabled backhaul. Asking the customer to outfit the factory floor with cabled networking to support a new wireless network can be too much to ask. Note that in either implementatio