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5.5 Packet Resend

Due to the fact, that the GigE Vision

datagram protocol - for data transfer, a mechanism for saving the "lost" data needs to be

employed.

Here, a resend request is initiated if one or more packets are damaged during transfer

and - due to an incorrect checksum - rejected afterwards.

On this topic one must distinguish between three cases:

5.5.1 Normal Case

In the case of unproblematic data transfer, all packets are transferred in their correct order

from the camera to the PC. The probability of this happening is more then 99%�

5.5.2 Fault 1: Lost Packet within Data Stream

If one or more packets are lost within the data stream, this is detected by the fact, that

packet number n is not followed by packet number (n+1). In this case the application

sends a resend request (A). Following this request, the camera sends the next packet and

then resends (B) the lost packet.

In our example packet no. 3 is lost. This fault is detected on packet no. 4, and the re-

send request triggered. Then the camera sends packet no. 5, followed by resending

packet no. 3.

5.5.3 Fault 2: Lost Packet at the End of the Data Stream

In case of a fault at the end of the data stream, the application will wait for incoming pack-

ets for a predefined time. When this time has elapsed, the resend request is triggered and

the "lost" packets will be resent.

standard stipulates using a UDP - a stateless user

◄ Figure 43

Data stream without

damaged or lost pack-

ets.

◄ Figure 44

Resending lost packets

within the data stream.