Introduction to the continuous ARQ protocol

The protocols in the network are diverse. Today we will introduce the continuous ARQ protocol. Maybe many of my friends have never heard of this agreement. Compared with the tcp ip protocol, this protocol is indeed a bit unpopular. But it doesn't matter. Let's take a look at it.

Go-back-n (GBN) and selective repeat (ARQ). The two Protocols are the combination of Sliding Window Technology and request retransmission technology, since the window size is large enough, the frame can flow continuously on the line, so it is also called the continuous ARQ protocol. The continuous re-sending request ARQ solution is that the sender can send a series of information frames continuously, that is, the next frame can be sent without waiting for the previous frame to be confirmed, which greatly improves the efficiency, a large buffer storage space (referred to as repost) needs to be set on the sender to store several frames of information to be confirmed. When the sender receives a confirmation frame for a message frame, this information frame can be deleted from the re-publication. Therefore, the link transmission efficiency of the continuous re-sending request ARQ solution is greatly improved, but a larger buffer storage space is required. In this protocol, after the sending site sends a data frame, it does not stop and wait for the response frame, but can send several data frames consecutively. If the receiving end receives the response frame again, the data frame can be sent again. As the waiting time is reduced, the throughput of the entire communication is increased ｡

The main functions of the continuous ARQ protocol can be summarized as follows:

1. reliable delivery on unreliable links ｡

This can be illustrated in the following figure ｡

Unreliable link for reliable delivery

Because the erroneous frames are quietly discarded by the CRC validator, therefore, the frames received by the receiving end are all frames without transmission errors. The data link layer protocol also ensures that the sequence of received frames is consistent with that sent by the sending end, because frames with incorrect serial numbers are not accepted, the continuous ARQ protocol on the data link layer achieves reliable delivery on the unreliable link layer, that is, what the sender sends, what is received by the receiving end. All retransmission is transparent to the above ｡

In short, reliable delivery is: no error, no loss, no repetition, in order ｡

2. Supports traffic control ｡

The concept of traffic control at the link layer is that when the receiving end cannot receive traffic (for example, the cache space is insufficient or the CPU is too busy ), you can reduce the transmission rate of data frames by not sending confirmation frames ｡

For the continuous ARQ protocol, special attention should be paid to the handling of error frames and frames not arriving in order ｡

Handle error frames:

The CRC tester automatically discards it, and there are no other actions in the Protocol. However, to avoid wasting time, you can specify how many times a frame has been retransmitted for an error, in this case, do not re-transmit the data, but report the upper-layer software, indicating that the communication line is unavailable ｡

Processing of frames that do not arrive in sequence (this frame has no transmission error ):

If the receiving end does not have a data frame to send, the receiving end discards the frame and repeatedly sends the last validation frame that has been sent to the sending end ｡

If the receiving end has a data frame to be sent, the receiving end can forward the confirmation information to the other party in the following data frame ｡

The receiving end can also send the last validation frame that has been sent to the sending end at intervals ｡