RS-232C和电子邮件（7位字符集）

Question

The book "Designing Embedded Hardware" in the chapter "9.3. Old Faithful: RS-232C" mentions that emails are still sent in 7bit char set because of RS-232C:

It's also not unheard of to see RS-232C systems still using 7-bit data frames (another leftover from the '60s), rather than the more common 8-bit. In fact, this is one of the reasons why you'll still see email being sent on the Internet limited to a 7-bit character set, just in case the packets happen to be routed via a serial connection that supports only 7-bit transmissions.

How can I confirm the observation?

Answer 1

Check out the spec. The original rfc822 , for ARPA Internet Text Messages, explicitly states:

A message consists of header fields and, optionally, a body. The body is simply a sequence of lines containing ASCII characters.

Since ASCII is 7-bit, voila.

Note, however, that there are a whole bunch of additions to that original spec, all the MIME extensions, which allow message header extensions for non-ascii text.

Answer 2

The Quoted-printable MIME encoding is specifically designed to encode 8-bit data in 7-bit characters. This encoding is widely used to encode email.

Note also that the text you quoted says "in case the packets happen to be routed via a serial connection" which is misleading, especially if they're talking in a context of IP packets. IP packets assume an 8-bit data path, and cannot be sent directly over a 7-bit RS-232 link without additional encoding (and then it's not a 7-bit data path anymore, it's 8-bit).

Answer 3

The systems that were restricted to 7 bits were already old when email first became popular. The chances that you will find one today approach zero.

Since certain characters have special meaning to email programs (most notably the end-of-line character), it still makes sense to limit the character set.