Invention of CRC-based Framing
StrataCom produced the first (pre-standard) ATM commercial product, the IPX. The IPX used 24 byte cells instead of ATM's 53 byte cells, and the field definitions were slightly different, but the basic idea of using short, fixed length cells was identical. StrataCom's first product had T1 (1.544 Mbit/s) based links which included a 5 bit header CRC, similar to ATM's 8 bit header CRC.
T1 is a time-division multiplexing (TDM) protocol with 24 byte payloads carried in a 193 bit frame. The first bit of each frame carries one bit out of a special pattern. A receiver finds this special pattern by sequentially looking for the bit position in the receive data where a bit from this pattern shows up every 193rd byte. It was convenient for StrataCom to make the length of one cell equal to the length of one T1 frame because a useful T1 framer Integrated Circuit from Rockwell was on the market. This device found the 193 bit long TDM frame and put out the 24 bytes in a form that could be used effectively.
When it came time to produce a European product, the benefit of using 24 byte frames became a liability. The European T-carrier (E1) format has a 32 byte frame of which 30 bytes could carry data. The development team's first proposal used the HDLC protocol to encapsulate a sequence of 24 byte cells into a byte stream collected from the 30 byte E1 payloads. This was highly inefficient because HDLC has a heavy and data-dependent overhead. The project team subsequently realized they could base the framing on the CRC. A circuit was designed which examined the incoming byte stream emerging from the E1 framer device and found a byte position for which the header CRC value was consistently correct. This team also went on to create a more error tolerant form of the technique.
A related technique was patented in 1984. That technique uses the CRC to find the start of 50 bit frames composed of a 36 bit data payload, a 13 bit CRC, and a single 1 bit start-of-frame indicator.
Read more about this topic: CRC-based Framing
Famous quotes containing the words invention of, invention and/or framing:
“We have much studied and much perfected, of late, the great civilized invention of the division of labour; only we give it a false name. It is not, truly speaking, the labour that is divided; but the men.”
—John Ruskin (1819–1900)
“Justice is a moral virtue, merely because it has that tendency to the good of mankind, and indeed is nothing but an artificial invention to that purpose. The same may be said of allegiance, of the laws of nations, of modesty, and of good manners. All these are mere human contrivances for the interest of society.”
—David Hume (1711–1776)
“In framing a government which is to be administered by men over men ... you must first enable the government to control the governed; and in the next place oblige it to control itself.”
—James Madison (1751–1836)