Tech Note 0030
Calculator for computing speeds and times
Enter a path speed and file size below to calculate its minimum transfer time.
The speed of any data path will be limited to its slowest component. This might be a WAN line, a local uplink, a hard-drive, a CPU, or a firewall. Tech Note 0009 lists many bottlenecks commonly found in network paths.
Units are important. One megabyte is about 8.4 megabits. When communicating about speeds and sizes, it is best to spell out units and avoid abbreviations. DEI software uses traditional base-two units for quantities of data: one kilobyte is 1024 bytes, one gigabyte is 1,073,741,824 bytes. Line speeds traditionally use base-ten units: one kilobit is 1000 bits, one gigabit is 1,000,000,000 bits. Use the calculator below to convert between bits and bytes.
Latency & Loss
Latency will have no significant effect on the speed of a long-lived MTP/IP data transfer. It only effects the length of very brief transfers, since it takes at least one round-trip to perform a transaction even if only 1 byte is moved. Packet loss is almost always a symptom of other traffic sharing the line and so its effects vary depending on the underlying causes. See Tech Note 0021 for details about how Loss, Latency, and Speed relate to each other.
Telecom providers usually report the speed of a line as the raw signaling rate. The actual maximum speed will be significantly lower due to various layers of overhead. For example, a line provisioned at 100 megabits per second will typically deliver at most 94 megabits per second of actual throughput. When estimating a path's speed, assume 6% to 10% overhead for most networks.
Aggregation, Multiplexing, and Bonding
Combining multiple slower lines is not the same as having a single line of a higher speed. For example, combining four lines of 1 gigabit per second each will not produce a path capable of moving data at 4 gigabits per second. Depending on the methods and equipment used, the maximum speed attainable for any one transfer will likely be at most that of a single line. In some cases, even the total speed of multiple parallel transfers may be limited to that of a single line if those transfers are between the same two machines.
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