问题
In a serial communication link, what is the prefered framing/sync method?
- framing with SOF and escaping sequences, like in HDLC?
- relying on using a header with length info and CRC?
It's an embedded system using DMA transfers of data from UART to memory. I think the framing method with SOF is most attractive, but maybe the other one is good enough?
Does anyone has pros and cons for these two methods?
回答1:
Following based on UART serial experience, not research.
I have found fewer communication issues when the following are included - or in other words, do both SOF/EOF and (length - maybe)/checkcode. Frame:
- SOFrame
- (Length maybe)
- Data (address, to, from, type, sequence #, opcode, bytes, etc.)
- CheckCode
- EOFrame
Invariably, the received "fames" include:
- good ones - no issues.
- Corrupt due to sender not sending a complete message (it hung, powered down, or rump power-on transmission) (receiver should timeout stale incomplete messages.)
- Corrupt due to noise or transmission interference. (byte framing errors, parity, incorrect data)
- Corrupt due to receiver starting up in the middle of a sent message or missing a few bytes due to input buffer over-run.
- Shared bus collisions.
- Break - is this legit in your system?
Whatever framing you use, insure it is robust to address these messages types, promptly validate #1 and rapidly identifying 2-5 and becoming ready for the next frame.
SOF has the huge advantage of it it easy to get started again, if the receiver is lost due to a previous crap frame, etc.
Length is good, but IMHO the least useful. It can limit through-put, if the length needs to be at the beginning of a message. Some low latency operations just do not know the length before they are ready to begin transmitting.
CRC Recommend more than 2-byte. A short check-code does not improve things enough for me. I'd rather have no check code than a 1-byte one. If errors occur from time-to-time only be caught by the check-code, I want something better than a 2-byte's 99.999% of the time, I like a 4-byte's 99.99999997%
EOF so useful!
BTW: If you protocol is ASCII (instead of binary), recommend to not use cr or lf as EOFrame. Maybe only use them out-of-frame where they are not part of a message.
BTW2: If your receiver can auto-detect the baud, it saves on a lot of configuration issues.
BTW3: A sender could consider sending a "nothing" byte (before the SOF) to insure proper SOF syncing.
来源：https://stackoverflow.com/questions/17073302/what-type-of-framing-to-use-in-serial-communication