Alternating bit protocol

networking (ABP) A simple data link layer protocol that

retransmits lost or corrupted messages.

Messages are sent from transmitter A to receiver B. Assume

that the channel from A to B is initialised and that there are

no messages in transit. Each message contains a data part, a

checksum, and a one-bit sequence number, i.e. a value that

is 0 or 1.

When A sends a message, it sends it continuously, with the

same sequence number, until it receives an acknowledgment

(ACK) from B that contains the same sequence number. When

that happens, A complements (flips) the sequence number and

starts transmitting the next message.

When B receives a message from A, it checks the checksum. If

the message is not corrupted B sends back an ACK with the same

sequence number. If it is the first message with that

sequence number then it is sent for processing. Subsequent

messages with the same sequence bit are simply acknowledged.

If the message is corrupted B sends back an negative/error

acknowledgment (NAK). This is optional, as A will continue

transmitting until it receives the correct ACK.

A treats corrupted ACK messages, and NAK messages in the same

way. The simplest behaviour is to ignore them all and

continue transmitting.

(2000-10-28)