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In addition to data frames which contain user data, L2 also transmits and receives control frames. The control frames can be distinguished from data frames by three bits in the first symbol. There are two types of control frames:
* One type ("AFC- Acknowledgement and L2 Flow Control", 3 symbols) serves to acknowledge successfully received data frames.
* The other type ("NAC", 2 symbols) notifies the corresponding transmitter that an incorrect frame has been received.
Note that these L2 types of control frames are sent autonomously by L2.
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