PC Hardware in a Nutshell, 3rd Edition

Choosing UART type

Which UART you need to use for maximum throughput depends on three factors:

DCE-to-DCE rate

DCE-to-DCE rate specifies the actual link speed between the two communications devices. For example, V.90 “56K” modems actually talk to each other at a maximum of 53 Kb/s (rather than 56 Kb/s, due to FCC regulations limiting signal amplitude on a phone line). ISDN devices communicate with each other at 64 Kb/s when using one B channel, or 128 Kb/s when using two, and so forth.

Data compressibility

Data compressibility is the degree to which source data can be compressed by the communications device before being placed on the DCE-to-DCE link. Some data (e.g., text, web pages, and databases) contains a great deal of embedded slack space, and can be compressed as much as 4:1 by the communications hardware. Other data (e.g., images, executables, etc.) is much less compressible.

DTE-to-DCE rate

DTE-to-DCE rate specifies the actual link speed between the local serial port and the local communications device. Setting this speed faster than the DCE-to-DCE rate allows the serial port to provide compressible data to the communications device quickly enough that the DCE-to-DCE link never runs out of data to transfer. The compression algorithms used by most communications devices allow 4:1 compression in theory, although 1.5:1 or 2:1 is more common in practice. This means that the DTE-to-DCE data rate must be at least one and one-half or two times the DCE-to-DCE rate (and, ideally, four ...