Technical: More About Connectivity

Improving Latency

One thing to notice is that the 38.4 Kbps serial connection between the computer and the modem, which many people don't think of as being the bottleneck, turns out to be responsible for 52 ms of the delay. In fact, it's the single biggest contributor - almost twice as much as the actual communication over the telephone line. What can we do about this? If you can connect the modems at both ends at 115.2 Kbps instead of 38.4 Kbps, the serial port delay can be reduced to 9 ms at each end. Better still, if you can use an internal modem on a card instead of one connected through a serial port, the delay can be eliminated entirely, leaving a round-trip delay of only 156 ms.

Having eliminated the serial port delay, the next biggest contributor to delay is the fixed 50 ms overhead built into the modem itself. Why is there a fixed 50 ms overhead? The reason is that modern modems offer lots of "features" - namely, compression and automatic error correction. To get effective compression and error correction, modems must work on blocks of data, which means characters are corralled in a buffer until the modem has received a block big enough to work on efficiently. While the characters accumulate in the modem's buffer, they're not being sent over the phone line. Imagine you're sending a small amount of data, 100 bytes. That's not enough for the modem to work on effectively, so it would like a bigger block. After you have sent the 100 bytes to the modem, it waits to see if more characters arrive. After some time - about 50 ms - it decides no more characters are coming, so it compresses and ships what it has. That 50 ms the modem spends hoping for more data is unrecoverable, wasted time.

Modems were originally designed with remote terminal access in mind. They were meant to take characters - typed by a user on one end and transmitted by a mainframe on the other - and group them into little blocks to send. The only indication that a user had finished typing (or that the mainframe had finished responding) was a pause in the data stream. No one told the modem when no more characters would be coming for a while, so it had to guess.

This is no longer the case. Most people use modems to connect to the Internet, not old mainframes, and Internet traffic is made up of discrete packets, not a continuous stream of characters.

There's a simple fix for this problem. We could make modems aware that they are sending Internet packets. When a modem sees the PPP (Point to Point Protocol) End-Of-Packet character (0x7E), it could realize that the packet is complete and immediately begin compressing and sending the block of data it has, without pausing for 50 ms. This simple fix would eliminate the 50 ms fixed overhead, and should allow us to achieve a 56 ms round-trip delay over a modem PPP connection - almost five times better than what typical modems achieve today.

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