Two years ago, ADSL testing consisted almost entirely of physical-layer tests on simulated local loops. New ADSL chip set and full transceiver designs still require rigorous physical-layer testing, but in production, manufacturers may perform a few parametric tests. Manufacturers more often test for bit-error rate and other functional attributes. Chip sets have loopback features that let you test the devices' transmitter and receiver circuits. The loopback tests can take place within the chips or over a simulated local loop to a remote transceiver.
Manufacturers often start testing by using the chip set's local-loopback feature.
Figure 1 shows a typical setup for remote loopback testing. Loopback tests let manufacturers know that the chip set's transmitter and receiver function properly. Because ADSL is asymmetric, upstream and downstream directions use different data rates. The upstream's slower speed limits testing to about 600 kbps. Because they often perform functional tests only, manufacturers typically test at the maximum upstream rate only and use the same type of device at both ends of a loop.