Baumer EXG03 Manuel de l'utilisateur - Page 31

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Baumer EXG03 Manuel de l'utilisateur

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.
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