Technical: More About Connectivity

Understanding Transmission Delay

At each hop, overall transmission time has two components: per-byte transmission time and fixed overhead. Per-byte transmission time is easy to calculate, since it depends only on the raw transmission rate. The fixed overhead comes from sources like software overhead, hardware overhead, and speed of light delay.

For modems, the distance is typically short, so speed of light delay should be negligible. However, the data rate is low, so it takes a long time to send each byte. The per-byte transmission time should account for most of the time taken to send the packet. To send 100 bytes over a 28.8 Kbps modem should take:

100 bytes * 8 bits per byte / 28800 bits per second = 28 ms

That means the round-trip should be twice that, or 56 ms. In reality it's often more like 260 ms. What's going on? Two other factors contribute to the overall time.

First, modems are often connected via serial ports. Many modem users assume that if they connect their 28.8 Kbps modem to their serial port at 38.4 Kbps they won't limit their performance, because 38.4 is greater than 28.8. It's true that the serial port won't limit throughput, but it will add delay, and delay, once added, never goes away. So, sending 100 bytes down the serial port to the modem should take:

100 bytes * 10 bits per byte / 38400 bps = 26 ms

Second, modems try to group data into blocks. The modem will wait for about 50 ms to see if more data is coming that it could add to the block, before it starts to send the data it already has. Let's see what the total time is now:

26 ms (100 bytes down serial port to modem) 50 ms (modem's fixed waiting time) 28 ms (transmission time over telephone line at 28.8 Kbps) 26 ms (100 bytes up serial port at receiving end)

Thus, the total time is 130 ms each way, or 260 ms for the round- trip. To make things worse, imagine that the 100 bytes in question are used by an interactive game being played by two players. If both players are connected to their respective Internet service providers by modem, then the total player-to-player round-trip delay is 520 ms, which is hopeless for any tightly-coupled interactivity, and this is reflected in the state of today's networked computer games. Can we do anything to improve this?

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