Real Time Data Transfer for Very Long Baseline Interferometry Simon Casey, Richard Hughes-Jones, Stephen Kershaw, Ralph Spencer, Matt Strong The University of Manchester, UK

1
Real Time Data Transfer for Very Long Baseline
Interferometry Simon Casey, Richard
Hughes-Jones, Stephen Kershaw, Ralph Spencer,
Matt StrongThe University of Manchester, UK
Part of the VLBI network in Europe
Transporting VLBI Constant Bit Rate Data with
TCP/IP TCP is a byte-stream transport layer
protocol that guarantees reliable, in-order, and
non-duplicated delivery data from sender to
receiver. It uses acknowledgments (ACK) from the
receiver to slide a window (Cwnd) over the data
to regulate the transmission rate. If a packet
has been lost, TCP interprets this as congestion
on the network, and the standard (New Reno)
congestion avoidance algorithm decreases the
window by half and then slowly increases it by
one packet per round trip time. The packet
re-transmission and the decrease in throughput
delay the delivery of data to the receiving
application as shown in the centre plot below.
For e-VLBI this is undesirable and can lead to
loss of correlation. However, if incoming CBR
data can be stored in the TCP socket buffer
during the loss event and data can be sent on the
link faster than the CBR rate, then catch-up of
the data delivery times is possible.
Timely arrival of data
Effect of loss rate on message arrival time.TCP
buffer 1.8 MB (BDP) RTT 27 ms
With packet loss, TCP decreases the rate.TCP
buffer 0.9 MB (BDP) RTT 15.2 ms
Increase in throughput allows message catch-up.
TCP buffer 160 MB RTT 15.2 ms
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
encapsulates the VLBI data in a UDP/IP packet
together with an application header containing
sequence number which increments by 1 for each
packet sent. 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
not equal to 1, then the packet is moved forwards
or backwards in the buffer as appropriate. In
December 2006, a 3 station e-VLBI experiment was
emulated by simultaneously transmitting data from
3 locations over the GÉANT2 network into PCs at
JIVE. The achieved throughputs and packet loss
are shown in the plots to the right. The absence
of packet loss clearly shows the superior
performance of the UKLight lightpath when
compared with the packet switched production
network.
Moving VLBI data with UDP/IP
3 simultaneous flows into JIVE Throughput and
Packet Loss
Architecture of the VLBI-UDP Program
e-VLBI Science
RR001 The First Rapid Response Experiment
(Rushton Spencer)
Microquasar GRS1915105 (11 kpc) on 21 April
2006 at 5 Ghz using 6 EVN telescopes, during a
weak flare (11 mJy), just resolved in the jet
direction (PA140 deg). (Rushton et al.)

• The experiment was planned as follows
• Operate 6 EVN telescopes in real time with
  observations on 29th Jan 2007
• Correlate and analyse the results
• Select the sources for follow up observations
• Observe the selected sources on 1 Feb 2007
• The experiment worked we successfully observed
  and analysed 16 sources weak microquasars) read
  for the follow up run, but we found that none of
  the sources were suitably active at that time.

Microquasar Cygnus X-3 (10 kpc) (a) on 20 April
and (b) on 18 May 2006. The source as in a
semi-quiescent state in (a) and in a flaring
state in (b), The core of the source is probably
20 mas to the North of knot A. (Tudose et al.)
(b)
(a)
The ESLEA UK e-Science project is funded by
the EPSRC, PPARC and MRC Research Councils
This work was performed in collaboration with the
EXPReS project, EC FP6 contract number 026642