Test data travels through a System on Chip (SOC) from the chip pins to the Core-Under-Test (CUT) and vice versa via a Test Access Mechanism (TAM). Conventionally, a TAM is implemented using dedicated communication infrastructure. However, also existing functional interconnect, such as a bus or Network on Chip (NOC), can be reused as TAM; this will reduce the overall design effort and associated silicon area. For a given core, its test set, and maximal bandwidth that the functional interconnect can offer between test equipment and core-under-test, our approach instantiates a test wrapper for the coreunder-test such that the test length is minimized. Unfortunately, it is unavoidable that along with the test data also unused (idle) bits are transported. This paper presents a holistic TAM bandwidth under-utilization analysis when functional interconnect is considered for test data transportation. We classify the idle bits into four types that refer to the root-cause of bandwidth under-utilization and pinpoint design improvement opportunities. Experimental results show an average bandwidth utilization of 80%, while the remaining 20% is consumed by the idle bits.