CRC-based framing

CRC-based framing

The concept of CRC-based framing was developed by StrataCom, Inc. in order to improve the efficiency of a pre-standard Asynchronous Transfer Mode (ATM) link protocol. This technology was ultimately used in the principal link protocols of ATM itself and was one of the most significant developments of StrataCom. An advanced version of CRC-based framing was used in the ITU-T SG15 G.7041 Generic Framing Procedure (GFP), which itself is used in several packet link protocols.

Overview of CRC-based framing

The method of CRC-Based framing re-uses the header cyclic redundancy check (CRC), which is present in ATM and other similar protocols, to provide framing on the link with no additional overhead. In ATM, this field is known as the Header Error Control/Check (HEC) field. The header CRC/HEC is needed for another purpose within an ATM system, to improve the robustness in cell delivery. Using this same CRC/HEC field for the second purpose of link framing provided a significant improvement in link efficiency over what other methods of framing, because no additional bits were required for this second purpose.

A receiver utilizing CRC-based framing bit-shifts along the received bit stream until it finds a bit position where the header CRC is correct for a number of times. The receiver then declares that it has found the frame. A hysteresis function is applied to keep the receiver in lock in the presence of a moderate error rate.

In links where there is already a byte lock mechanism present such as within an E-carrier or SDH frame, the receiver need only byte-shift (rather than of bit-shifting) along the receive data stream to find lock.

Length/HEC-Based Framing

An advanced, variable frame size version of CRC-Based framing is used in ITU-T SG15 G.7041 GFP links where it is known as Length/HEC-based framing. An offset to the next valid header is present in a fixed position relative to the CRC/HEC. The receiver looks for a position in the receive data stream following the rules that the header CRC/HEC is correct and the byte offset correctly points to the next valid header CRC/HEC.

Invention of CRC-based framing

The story behind the invention of CRC-based framing is as follows: 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 [ [ Previous Stratacom patent using DS-1's framing] ] 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. This factor may have ruined the economics of product for the European market. In the design review, everyone was lamenting what a bad solution HDLC was for the problem. Brian Holden, who was one of the reviewers present, then uttered the sentence "Why don't you just frame on the CRC?". Everyone in the room went "Oh!" as the lightbulb flashed on. Participating in this design review and uttering this one high-value sentence was Brian's only involvement in this E1 project and the consequent development of CRC-based framing.

The project team went on to base the framing of the E1 project on the CRC [ [ Original Stratacom CRC-based framing patent] ] . 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. The developers Rick Enns and Paul O'Hare properly got the patent for the development as Brian Holden did just utter the one sentence.

This team also went on to create a more error tolerant form of the technique. [ [ More error tolerant Stratacom CRC-based framing patent] ]


External links

* [ Official ITU-T website]
* [ List of all ITU-T Recommendations]

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