Chapter 15 · General Datacom Concepts 147 detection is fairly simple, the theories explaining its efficacy involve complex mathematical proofs well beyond the scope of this book. In the CRC approach, the data to be checked is treated as a block of bits instead of a series of digital words to be summed. This data block is typically divided using a specially designed polynomial, and the result of the division is the checksum which is transmitted along with the data for verification by the receiver. Because of the properties of the specially designed poly- nomial, the remainder for any given block is one of a huge number of possible check values, and the probability that both pure and corrupted data would generate the same checksum is extremely low. CRC is the error detection approach used in Ethernet. F LOW C ONTROL In a data link, the transmitter might not be able to blindly send data forever, because the receiver might have limited memory capacity and could "overflow," causing data to be lost. For this reason, in some applications, the receiver needs to be able to signal the transmitter when to stop and start transmission--a process known as flow control. There are two types of flow control: hardware and software. In hardware flow control, addi- tional control lines are run between the receiver and transmitter; when the receiver's buffer starts filling up, it signals the transmitter to halt transmission. Once the receiver is ready to accept data again, it signals the transmitter to restart. In software flow control, specially desig-