People are often concerned about measuring the maximum data throughput in bits per second of a communications link or network access.
In fact, there are many overheads accounted for in throughput in addition to transmission overheads, including latency, TCP Receive Window size and system limitations, which means the calculated goodput does not reflect the maximum achievable throughput.
However, the convention adopted by Windows systems is to define 1 kilobyte is as 1,024 (or 210) bytes, which is equal to 1 kibibyte.
In actuality, a 64 kilobyte file is 64 × 1,024 × 8 bits in size and the 64 k circuit will transmit bits at a rate of 64 × 1,000 bit/s, so the amount of time taken to transmit a 64 kilobyte file over the 64 k circuit will be at least (64 × 1,024 × 8)/(64 × 1,000) seconds, which works out to be 8.192 seconds.
This can be done invisibly to the user, so a highly compressible file may be transmitted considerably faster than expected.
Assuming your data cannot be compressed, the 8.192 seconds to transmit a 64 kilobyte file over a 64 kilobit/s communications link is a theoretical minimum time which will not be achieved in practice.
If it is set up to have no parity and one stop bit, this means the byte transmission rate is 23.04 kbyte/s.
In this type of interface, a clock signal is added on a separate wire, and the bits are transmitted in synchrony with the clock — the interface no longer has to look for the start and stop bits of each individual character — however, it is necessary to have a mechanism to ensure the sending and receiving clocks are kept in synchrony, so data is divided up into frames of multiple characters separated by known delimiters.
If each frame carried but a single byte, the data throughput efficiency would be extremely low.
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.
The way the frame is electrically defined on a connection between two systems is different from the typically wide-area networking technology that uses HDLC or PPP implemented, but these details are not important for throughput calculations.
Ethernet is a shared medium, so that it is not guaranteed that only the two systems that are transferring a file between themselves will have exclusive access to the connection.
If several systems are attempting to communicate simultaneously, the throughput between any pair can be substantially lower than the nominal bandwidth available.
[9] Asynchronous Transfer Mode (ATM) uses a radically different method of carrying data.
Rather than using variable length frames or packets, data is carried in fixed size cells.
[10] Multiprotocol Label Switching (MPLS) adds a standard tag or header known as a 'label' to existing packets of data.
For example, a data packet could have two MPLS labels attached via 'label-stacking', then be placed as payload inside an HDLC frame.
This generates more overhead that has to be taken into account that a single MPLS label attached to a packet which is then sent 'natively', with no underlying protocol to a receiving system.
[11] Few systems transfer files and data by simply copying the contents of the file into the 'Data' field of HDLC or PPP frames — another protocol layer is used to format the data inside the 'Data' field of the HDLC or PPP frame.