There are two key measurable parameters for any communications link carrying packet data, these are the utilisation and the throughput.
The utilisation is defined as the total number of bits transferred at the physical layer to communicate a certain amount of data (at a higher layer) divided by the time taken to communicate the data. It is normally expressed as a percentage of the physical layer data rate (line speed of the physical layer clock).
The utilisation includes the bits in all types of frames (supervisory, unnumbered, and information) and counts frames irrespective of whether they are corrupted or correctly received. It is therefore a measure of the amount of the link capacity which is used by the communication process.
The throughput of a given protocol layer is defined as the number of bits transferred per second from the given layer to the upper layer as a result of a conversation between two users of the given layer. This is always less than the utilisation due to the overhead of protocol headers added at each layer of a protocol stack.
The throughput considers only data which are forwarded to the protocol layer above (in the case of the link layer, those bytes which are forwarded to the network layer). It is a measure of the performance of the service provided by a particular layer.