Bits on the wire
Every message on a network starts as 1s and 0s. See how those bits become voltage on copper, light in fibre and radio waves in the air, then meet the two speeds every network engineer talks about: latency (how long one bit takes to arrive) and bandwidth (how many bits per second you can push out).
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01How many bits are in a 1,500-byte packet?
A byte is 8 bits, so 1,500 × 8 = 12,000 bits.
02About how long does a signal take to cross 1000 km of fibre, one way?
Light in glass moves at about two-thirds of its vacuum speed, roughly 5.0 µs/km. 1000 km × 5 µs = 5.0 ms.
03Your link is upgraded from 10 Mb/s to 1 Gb/s. What happens to the time the first bit needs to cross 1000 km?
Travel time (latency) depends on distance and the medium, not on the bit rate. The upgrade only makes the bits leave faster.
04At 10 Mb/s, how long does it take to push a 12,000-bit packet onto the wire?
Serialization time = bits ÷ rate = 12,000 ÷ 10,000,000 = 0.0012 s = 1.2 ms.
05Which of these carries bits as flashes of light?
Fibre sends light down a thin strand of glass. Copper uses voltage, and Wi-Fi and mobile networks use radio waves.
Glossary
- Bit
- The smallest piece of data: a single 1 or 0.
- Byte
- A group of 8 bits. The letter A is one byte: 01000001.
- Signal
- Something physical that changes to carry bits: a voltage, a light, or a radio wave.
- Fibre-optic cable
- A cable that carries bits as light inside thin strands of glass.
- Latency
- How long it takes data to get from one end to the other. Mostly set by distance.
- Bandwidth
- How many bits per second a link can carry, for example 1 Gb/s.
- Serialization delay
- The time it takes to push all the bits of a packet onto the wire: size ÷ rate.
- Packet
- A chunk of data sent across a network in one go, often up to 1500 bytes.
Notes and sources
- Speed in fibre is taken as 2/3 of c (199,862 km/s, 5.00 µs per km); real single-mode fibre is about c / 1.47, a little faster, and real cable routes are longer than the straight-line distance.
- The 1000 km link, the 1 Gb/s and 10 Mb/s rates and the 1,500-byte packet are typical example values (RFC 894 / IEEE 802.3 for the 1500-byte limit).
- In the figure one bead stands for one bit in the first steps and for about 1000 bits in the packet steps; timings are slowed down millions of times but keep their ratios.
- "A" = 01000001 in ASCII (RFC 20); Ethernet puts the least-significant bit of each byte on the wire first.
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