| Octets per second (octet/s) | Zettabits per second (Zbit/s) |
|---|---|
| 1 Octet per second | 8 × 10-21 Zbit/s |
| 2 Octets per second | 1.6 × 10-20 Zbit/s |
| 3 Octets per second | 2.4 × 10-20 Zbit/s |
| 4 Octets per second | 3.2 × 10-20 Zbit/s |
| 5 Octets per second | 4 × 10-20 Zbit/s |
| 10 Octets per second | 8 × 10-20 Zbit/s |
| 20 Octets per second | 1.6 × 10-19 Zbit/s |
| 25 Octets per second | 2 × 10-19 Zbit/s |
| 50 Octets per second | 4 × 10-19 Zbit/s |
| 100 Octets per second | 8 × 10-19 Zbit/s |
| Reference | Octets per second (octet/s) | Zettabits per second (Zbit/s) |
|---|---|---|
| A dial-up modem | 7000 octet/s | 5.6 × 10-17 Zbit/s |
| Typical home broadband | 12500000 octet/s | 1 × 10-13 Zbit/s |
| Gigabit Ethernet | 125000000 octet/s | 1 × 10-12 Zbit/s |
| Streaming a 4K film | 3125000 octet/s | 2.5 × 10-14 Zbit/s |
The octet per second is a unit of data transfer rate equal to eight bits per second. Its symbol is octet/s. It is the unit in which software reports transfer speeds, as against the bits per second in which hardware and network services advertise them, and the factor of eight between the two conventions is the source of endless confusion.
The division of labour is consistent once it is understood. Anything describing a physical link — an Ethernet port, a fibre connection, a radio channel, a broadband package — is quoted in bits per second, because bits are what the signalling actually carries. Anything describing a file moving from one place to another is quoted in octets per second, because files are measured in octets.
A download manager that reports 12 megaoctets per second on a connection sold as 100 megabits per second is not disagreeing with the advertisement; it is stating the same rate in the other convention, and the arithmetic between them is a division by eight. Recognising this immediately removes the most common cause of complaint about internet speed.
Individual octets per second appear in the same narrow places as individual bits: deep-space telemetry, low-power sensor links, and the slowest legacy serial connections. A rate of a hundred octets per second would move a page of text in about twenty seconds, which was a normal experience in the 1970s and is unimaginable now.
The unit also underlies the way disc and interface throughput is described. A drive that sustains 500 megaoctets per second is moving four gigabits per second across its interface, and matching the two figures is a routine part of system design: an interface rated in gigabits must be compared with a drive rated in octets, and the factor of eight decides whether one starves the other.
Storage tools reinforce the convention. Every file copy utility, backup program and command-line transfer tool reports in octets per second or its multiples, and every network measurement tool reports in bits per second. A person reading both at once has to keep the conversion in mind, which is exactly what makes the distinction worth stating explicitly.
One octet per second equals 8 bits per second, 0.008 kilobits per second, or 0.001 kilooctets per second.
The zettabit per second is a unit of data transfer rate equal to a thousand exabits per second. Its symbol is Zbit/s. No system on Earth moves data at this rate, and none is planned; the unit exists because the metric system defines every prefix for every unit, whether or not the combination has yet been needed.
To see how far off it is, take the whole internet. Global traffic at present runs at roughly one exabit per second on average, so the entire planet's communications would have to grow a thousandfold to reach one zettabit per second. At the growth rates of the last two decades that would take somewhere between twenty and thirty years, which is precisely the sort of extrapolation that has been wrong in both directions before.
A zettabit per second is 125 exaoctets per second. Since global data storage manufacturing runs at a few hundred exaoctets a year, a link at this rate would transfer the world's entire annual production of new storage capacity in a couple of seconds. Nothing could be stored at the far end; the data would have to be processed and discarded as it arrived.
That last point is not as fanciful as it sounds. Several existing systems already discard almost everything they receive: particle detectors, radio telescope arrays and network monitoring systems all process far more than they keep, because keeping it is impossible and unnecessary. A zettabit-per-second link would be an extreme case of an architecture that already exists.
The physical obstacles are less absolute than they might appear. The theoretical capacity of a single optical fibre is far above what is used today, and the practical limits come from amplifier noise, non-linear effects and the electronics at each end rather than from the glass itself. Aggregating enough fibres would reach a zettabit per second; the difficulty is that nobody has a reason to.
For a converter, the unit matters because forecasts and capacity models are written in whatever unit keeps the numbers legible. A projection that reaches into the 2050s may reasonably state totals in zettabits per second, and a reader needs to be able to convert that into something familiar.
One zettabit per second equals 1,000 exabits per second, 125 exaoctets per second, or about 0.8470 zebibits per second.