The easiest way to think of a Layer 1 (L1) switch, also known as a physical layer switch, is as an electronic patch panel. Completely transparent connections between ports is performed based on software commands sent to the L1 switch over its control interface. In testing environments, this allows the tests to be as accurate as if there were a patch cord between the devices.

Using Layer 2 (L2) switches for connection purposes in a test environment can cause a number of problems. In L2 switches, the output bit stream is different from the input bit stream in that MAC control frames, such as pause frames, are discarded by the MAC layer. Also the differing clock timing forces the PHY on the L2 switch to add/delete idle characters to compensate, making it impossible to compare input data streams to output data streams when testing using an L2 switch.

L1 switches on the other hand, are fully transparent to the traffic going through them. Once a connection is made between two ports the attached devices are essentially directly connected. L1 switches have very low latency and do not store or manipulate a single bit in the data stream. The internal hardware architecture of the L1 switch allows the L1 switch to duplicate any incoming data to any number of output ports at full wire speed without dropping a single bit. This allows testing of multiple devices from a single test set or output. Software simulation of cable breaks (port flapping) can also be simulated using software-defined duration times and repetitions of the simulated cable break.

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