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5 Image Transmission over Gigabit Ethernet Setting Inter Packet Delay The cameras provide the features GevSCPSPacketSize and GevSCPD. GevSCPD represents the Inter Packet Delay and is expressed in microseconds. Its value can range from 0 to 1000µs and should be set according to number of cameras connected to a certain network interface card and GevSCPSPacketSize. The GevSCPSPacketSize feature represents the size of packets and is expressed in bytes, the default camera packet size is at 1500 bytes, but can be larger if the network hardware supports Jumbo Frames. Assuming that the GevSCPSPacketSize is 1500 bytes (effective Ethernet packet size including inter-frame gap, preamble, header and CRC on the wire is 1538 bytes), maximum of 81274 packets are sent every second via the Ethernet interface. The time required to transfer one packet is 12,3µs. The GevSCPD should be a bit longer than the time required to transfer one packet, in order to ensure that packets from second camera will fit in the vacant time slot. On the other hand, if the camera is producing 60000 packets per second (50 frames per second, 1200 packets per frame), total transfer time must not exceed 16,67µs if frame rate is to be preserved. Example Three cameras are connected to one PC, and are sending 1500 byte packets each. GevSCPD should be such that packets from all three cameras are serialized to the PC’s Network Interface Card. Setting inter packet delay to 25µs (12,3µs + 12,3µs ≈ 25µs) will ensure that packets from other two cameras will fit in the gap between two consecutive packets. Camera 1 Packet 3 Delay Packet 2 Arriving packet flow Camera 2 Delay Packet 2 Delay Packet 1 Packet 1 Packet 1 Camera 3 Delay Packet 2 Delay Figure 105: Packet flow example with three cameras and inter packet delay 133 ©SMARTEK Vision Giganetix User Manual Version 2.1.1
