ALTERNATING BIT PROTOCOL
\ˈɔːltənˌe͡ɪtɪŋ bˈɪt pɹˈə͡ʊtəkˌɒl], \ˈɔːltənˌeɪtɪŋ bˈɪt pɹˈəʊtəkˌɒl], \ˈɔː_l_t_ə_n_ˌeɪ_t_ɪ_ŋ b_ˈɪ_t p_ɹ_ˈəʊ_t_ə_k_ˌɒ_l]\
Definitions of ALTERNATING BIT PROTOCOL
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(ABP) A simple data link layer protocol thatretransmits lost or corrupted messages.Messages are sent from transmitter A to receiver B. Assumethat the channel from A to B is initialised and that there areno messages in transit. Each message contains a data part, achecksum, and a one-bit sequence number, i.e. a value thatis 0 or 1.When A sends a message, it sends it continuously, with thesame sequence number, until it receives an acknowledgment(ACK) from B that contains the same sequence number. Whenthat happens, A complements (flips) the sequence number andstarts transmitting the next message.When B receives a message from A, it checks the checksum. Ifthe message is not corrupted B sends back an ACK with the samesequence number. If it is the first message with thatsequence number then it is sent for processing. Subsequentmessages with the same sequence bit are simply acknowledged.If the message is corrupted B sends back an negative/erroracknowledgment (NAK). This is optional, as A will continuetransmitting until it receives the correct ACK.A treats corrupted ACK messages, and NAK messages in the sameway. The simplest behaviour is to ignore them all andcontinue transmitting.
By Denis Howe
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