CRC-16 code can detect nearly 99.999% errors. In computer communications Ethernet packets are dropped when an error is detected. This saves resources (time and power) as a receiving computer does not need to process these corrupt packets. Since CRC is not perfect, other error detection mechanisms are necessary to detect those errors which go undetected at this stage. It may appear that one in 100 000 undetected errors is sufficiently low. However when one considers that a typ- ical Ethernet link can carry more than 200 million packets per hour, then this undetected error performance is quite inadequate. Present Ethernet standards employ a CRC-32 code, which according to estimates in one paper only lets between one in 16 million to one in 10 billion errors go undetected [5]. Again any packet arriving with error is discarded by the Ethernet receiver and is not passed to the higher layer applications. While the CRC performance is not perfect, it helps improve the system efficiency significantly. Dest. Source Checksum address address 8 bits 8 bits 4000 bits 16 bits Figure 6.7 The first proposed Ethernet packet structure. Source: Ref. [4] Broadband Telecommunications Technologies and Management Case Study Questions What is the technological trade-off in selection of CRC code? Why does Ethernet protocol use a CRC? Why are CRC codes suitable considering the Ethernet data payload size? What was the motivation behind selection of the CRC-32 code? What is the impact of the data field size on overall probability of error reception? Bit Error Rate and Block Error Rate As demonstrated in Case Study 6.1, in digital communications data are usually sent in blocks of a predetermined size. As shown, a block of data must be correctly received before it can be passed on to higher layers to complete the telecommunications process. Block Error Rate (BLER) - also known as Frame Error Rate (FER) - is a measure of the rate of erroneously received blocks (e.g. percentage of blocks received in error). Clearly BLER is related to BER discuss