Editing Measuring network throughput (section)

===HDLC===

When using [[High-Level Data Link Control|HDLC]], rather than each byte having a start, optional parity, and one or two stop bits, the bytes are gathered together into a [[data frame|frame]]. The start and end of the frame are signalled by the 'flag', and error detection is carried out by the frame check sequence. If the frame has a maximum sized address of 32 bits, a maximum sized control part of 16 bits and a maximum sized frame check sequence of 16 bits, the overhead per frame could be as high as 64 bits. If each frame carried but a single byte, the data throughput efficiency would be extremely low. However, the bytes are normally gathered together, so that even with a maximal overhead of 64 bits, frames carrying more than 24 bytes are more efficient than asynchronous serial connections. As frames can vary in size because they can have different numbers of bytes being carried as data, this means the overhead of an HDLC connection is not fixed.<ref>Cisco System, Inc. (2001-2006). Cisco IOS IP Configuration Guide</ref>