Delay and throughput

Two questions decide how fast a network feels: how long does one packet take to arrive (delay), and how many bits per second get through (throughput)?

Four delays at every hop

Transmission and propagation are often confused. Transmission is about the link's rate (bits per second): a 1500-byte packet on a 10 Mbps link takes 1.2 ms to put on the wire, whatever the distance. Propagation is about distance: signals travel at about 2 × 10⁸ m/s in cable, so 2000 km takes 10 ms, whatever the packet size.

packet_bits = 1500 * 8          # a 1500-byte packet
rate = 10_000_000               # 10 Mbps
distance = 2_000_000            # 2000 km, in metres
speed = 2e8                     # metres per second in cable

transmission = packet_bits / rate
propagation = distance / speed
print("transmission:", round(transmission * 1000, 3), "ms")
print("propagation:", round(propagation * 1000, 3), "ms")
print("total (no queueing):", round((transmission + propagation) * 1000, 3), "ms")

Throughput and the bottleneck

A path is only as fast as its slowest link, the bottleneck. A file crossing a 100 Mbps link, then a 4 Mbps home line, then a 50 Mbps link arrives at 4 Mbps.

links = [100, 4, 50]            # Mbps, in order along the path
bottleneck = min(links)
file_bits = 32 * 1_000_000 * 8  # a 32 MB file
seconds = file_bits / (bottleneck * 1_000_000)
print("bottleneck:", bottleneck, "Mbps")
print("time for 32 MB:", seconds, "seconds")

in_flight = 10_000_000 * 0.010  # 10 Mbps x 10 ms
print("bits in the pipe:", int(in_flight))

Note the units: networks count bits, files count bytes (8 bits), and network "mega" means 10⁶, not 2²⁰.

The bandwidth-delay product

Rate × propagation delay is the number of bits "in the pipe" at once. On a 10 Mbps link with 10 ms of propagation delay, 100,000 bits are on the wire at any moment. A sender that waits for a reply after every packet leaves most of that pipe empty: this is why the Reliable delivery lesson sends many packets at once.

Mark lesson complete