![]() A frame with a bad CRC is dropped and must be retransmitted by the sender. A receiving station determines if a bit has been changed or dropped by calculating the CRC again and comparing the result to the CRC in the frame. The sending station places the value of the CRC in the frame. In packet-switched networks, a sending station calculates a CRC based on the bits in a frame. With packet-switched networks, it makes more sense to measure frame (packet) errors because a whole frame is considered bad if a single bit is changed or dropped. ![]() As discussed in the "Checking the Status of Major Routers, Switches, and Firewalls" section later in this chapter, you can also use Cisco show commands to gain an understanding of errors on a serial interface, which is a more common practice on modern networks than using a BERT. You can use a BER tester (also called a BERT) on serial lines to test the number of damaged bits compared to total bits. Chapter 2 talked about specifying network accuracy as a bit error rate (BER).
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