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CRC is an acronym for
cyclical (or cyclic) redundancy check. This procedure is used in checking for
errors in data transmission. CRC error checking uses a math calculation to generate a number
based on the data transmitted. The sending device performs the calculation before transmission
and sends its result to the receiving device. The receiving device repeats the same calculation
after transmission. If both devices obtain the same result, it is assumed that the transmission was
error−free. The procedure is known as a redundancy check because each transmission includes
not only data but extra (redundant) error−checking values. Communications protocols such as
Xmodem and Kermit† use cyclical redundancy checking. CRCs do not correct errors.
Errors occur in telecommunications systems primarily from disturbances (noise) that are relatively
long in time duration. Thus, a noise burst of 0.01 second is not uncommon, and if it occurs
during a 1,200 bit per second data transmission, we would expect (at worst case) to receive 12
erroneous data bits known as an error burst of 12 bits long. A cyclic redundancy check code
known as CRC−12 has a 99.955% chance of detecting error bursts of 12 bits in length
1 - دستور show interface serial رو روی اکسس سرورت وارد کن ببین مقدار CRC چقدر هست .
2 -بعد از آن دستور clear counter را برای پاک کردن مقدار عددی CRC وارد کن تا صفر بشه .
چند ساعت بعد دوباره دستور 1 رو بزن ببین تغییراتی داشتی یا نه . اگه داشتی که خطت نویز داره و هر چه تغییرات بیشتر باشه میزان نویز خط بیشتر هست .
تذکر 1 - بعد از دستور اول باید 0:15 یا 1:15 یا 2:15 و ... رو بلافاصله وارد کنید این عدد بستگی به سریالی داره که کابل مودم به اون وصل هست .
تذکر 2 - دستور دوم فقط مقدار عددی CRC را کم می کند و ربطی به رفع شدن کلی این پیغام خطا نداره .
این موارد رو چک کنید تا بعداً بیشتر راهنمایی کنم .