Overview Forward Error Correction (FEC) is a technology which allows the receiver of a message to detect and correct most single bit errors, and many multi-bit errors ( error bursts) without referring to the transmitter of the message. While commonly implemented in hardware digital logic, FEC can also be implemented in either firmware or software (such as a device driver) as described here. Effect of Error Rate upon Throughput In PC based communications, using simple protocol technologies such as SLIP / PPP / HDLC / SDLC / and most custom protocols, the data error rate has an effect upon data throughput. Simply put, the higher the error rate, the lower the throughput. This effect is vastly amplified by the connection's propagation delays.

Simply put again, the larger the propagation delay, the reduction in data throughput can be HUGE. In a typical case where 22mS of propagation latency exists due to geosynchronous satellite connection, a simple single or multi bit error can stall a link for periods far greater than the 44mS of retransmit propagation burden. This is often due to the additional burden imposed by detection of dropped packets being performed in a layer which is separated from the physical link layer, such as scenario where TCP is a remote layer from the serial port device.

Serial I/O and Data Communication Basic Concepts in Serial I/O • Interfacing requirements: – Identify the device through a port number. • Memory-mapped. Corrections - Salon. Salon strives to publish accurate information at all times. Minor errors of spelling, punctuation and the like will be corrected on our website.

Stall times arising from single bit errors on SLIP (Serial Link IP), for example, of 1 to 5 seconds are not unusual. Serial Port Forward Error Correct ('SP FEC') Technology This all done in a fashion whereby the serial port appears as a standard serial port to Operating Systems, Routers, and Application software. Family Photos Of Serial Actress Gayathri Arun on this page. Most common single bit and multi bit error burst no longer stall the link, resulting an improvement in throughput. Intangibly also, by eliminating data stalls when browsing content, can transform end user frustration into end user satisfaction.

Data Flow Diagram Sales and Product Information: White papers which describe this technology are available. Click for more information, or for sales and licensing of this technology. Constellation Data Systems, Inc. 10296 Springfield Pike, Suite #100 - Cincinnati, Ohio 45215 USA Voice: 513.984.4491, Fax: 513.984.4697 © 2005-2013 Constellation Data Systems, Inc.

Contents • • • • • • • • • • • • Introduction [ ] There are 3 main types of handling errors: • acknowledge or retry (ACK-NAK). • 'Forward Error Correction' (FEC) • Pretend It Never Happened ACK-NAK [ ] Each packet is checked by the receiver to make sure it is 'good'.

If it *is* good, the receiver (eventually) tells the sender that it came through OK -- it acknowledges (ACK) the packet. All versions of ACK-NAK absolutely require. How does the receiver know it's good? [ ] The sender calculates a checksum or CRC for the entire packet (except for the footer), then appends it to the end of the packet (in the footer/trailer).

The typical CRC is 32 bits, often a. Aside: Note that the checksum or CRC are forms of hashing, ie, irreversibly shrinking data. Checksums and CRCs are weaker algorithms than 'cryptographically strong' message authentication code algorithms such as MD5 or SHA variants. Cryptographically strong algorithms can detect errors better than checksums or CRCs, but they take more time to calculate. Whenever the receiver receives a packet, the receiver calculates exactly the same checksum or CRC, then compares it to the one in the footer/trailer. If they match, the entire packet is (almost certainly) good, so the receiver sends an ACK.

When there's even the slightest question that the packet has any sort of error (which could be *either* in the actual data *or* in the header *or* in the checksum bits -- there's no way for the receiver to tell), the receiver discards it completely and (in most cases) pretends it never saw it. If it's not good, the sender sends it again. How does the sender know it wasn't good? [ ] It never got the ACK. (So either the packet was corrupted, *or* the ACK was corrupted -- there's no way for the sender to know).

'Stop-and-wait ARQ' [ ] The simplest version of ACK-NAK is 'Stop-and-wait ARQ'. The sender sends a packet, then waits a little for an ACK. As soon as it gets the ACK, it immediately sends the next packet. If the sender doesn't hear the ACK in time, it starts over from the beginning, sending the same packet again, until it does get an ACK. The receiver waits for a packet. If the packet passes all the error-detection tests perfectly, the receiver transmits an ACK (acknowledgment) to the sender. Subtleties: If the receiver receives the packet perfectly, but the ACK message is delayed too long, then the transmitter sends another copy of the message (a 'communication echo').