Introduction VLBI_UDP is an application being developed to transfer VLBI data using the UDP protocol. Initially developed by Richard Hughes- Jones for continuous streaming of data, features are being added to allow streaming of VLBI data from the data acquisition devices PCEVN and Mark5A, as well as the ability to selectively lose UDP packets. The architecture of VLBI_UDP can be seen in Fig. 1. VLBI_UDP Simon Casey*, Richard-Hughes Jones #, Ralph Spencer*, Matthew Strong* * Jodrell Bank Observatory, The University of Manchester; # HEP Group, The University of Manchester Results Fig. 3 shows a non-stop 24 hour run between Jodrell Bank and Manchester. Each point represents the average received wire rate over a 30 second sample, and demonstrates the long term stability of the application. Fig 1. VLBI_UDP architecture Architecture At the sender, the input thread either reads data from a file or generates random data and places these in to the ring buffer. The output thread removes packet size chunks of data from the ring buffer and encapsulates the data part of a UDP container. The header of this container is then filled with a sequence number which increments by 1 for each packet sent. This packet is given to the IP stack which places it on the network. At the receiver, the receive thread places incoming packets directly into the next position in the ring buffer. The sequence number is read from the header, and this reveals whether the packet is at the correct position in the buffer. If the sequence number increment is more or less than 1, then the packet is moved forwards or backwards in the buffer, as indicated in Fig 2. Fig 2. Simulation of packets being placed into ring buffer The output thread removes blocks from the ring buffer and either discards or writes them out to a file, as specified by the user. A further mode of operation has been added whereby data are read from a file and filtered through a packet-dropping function which selectively drops packets according to the selected algorithm. The data are then written out to a file without being sent over the network. This is explained in more detail in the corresponding paper ‘Investigating the effects of missing data upon VLBI correlation using the VLBI_UDP application’. Fig Hour flow Fig. 4 shows the GÉANT2 core network, and the telescopes with high-speed connections used as the test bed. In December 2006, a 3 station e-VLBI experiment was emulated by simultaneously transmitting data from 3 locations into PCs at JIVE. The results of this can be seen in the 3 plots of Fig. 5. Fig 4. GÉANT2 network & telescope connections Packet 3 missing, move packet 4 forwards Packet 3 out of order, move packet 3 backwards Fig 5. Simultaneous UDP flows into JIVE The absence of packet loss, clearly show the superior performance of the UKLight lightpath when compared with the packet switched production network. This work was performed in collaboration with the EXPReS project, EC FP6 contract number