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Rad-icon 1520 Detector
Teledyne DALSA
Flow Control
The GigE Vision standard defines an inter-packet delay that can be used to manage flow
control (i.e. the speed at which stream packet can be output to the network). This is useful
when connecting multiple detectors to the same port of the network card, or when the
network card/Ethernet switch (if used) is simply too slow to process those packets. A
careful selection of equipment will ensure that the network equipment is fast enough to
handle data transmitted to the wire-speed of 1 Gigabit per second. Therefore, inter-packet
delay is typically only used when multiple detectors are connected to the same port of the
network card, through an Ethernet switch.
It is important to consider that inter-packet delay inserts a minimum delay between
image packets to spread packet transmission over a longer period of time. This can
directly impact system latency as more time than could be necessary is put in between
those packets. The best approach for real-time imaging is to dedicate a different network
port to each detector. This way, the inter-packet delay can be eliminated in many cases.
Some network equipment also supports the optional IEEE802.3 PAUSE mechanism. This
is a low-level handshake to ensure the receiver of the packets is not overwhelmed by the
amount of data. It can propagate a pause signal back to the transmitter, asking to
momentarily stop the data transmission (with a possible impact on the overall system
latency). Again, by combining network equipment that can operate at wire-speed and
allocating a different network interface port for each detector in the system, we can
ensure these pause requests will not be used.
