  New generation radio interferometers encode signals from thousands of antenna
feeds across large bandwidth. Channelizing and correlating this data requires
networking capabilities that can handle unprecedented data rates with
reasonable cost. The Canadian Hydrogen Intensity Mapping Experiment (CHIME)
correlator processes 8-bits from N=2048 digitizer inputs across 400~MHz of
bandwidth. Measured in $N^2~\times $ bandwidth, it is the largest radio
correlator that has been built. Its digital back-end must exchange and
reorganize the 6.6~terabit/s produced by its 128 digitizing and channelizing
nodes, and feed it to the 256-node spatial correlator in a way that each node
obtains data from all digitizer inputs but across a small fraction of the
bandwidth (i.e. `corner-turn'). In order to maximize performance and
reliability of the corner-turn system while minimizing cost, a custom
networking solution has been implemented. The system makes use of Field
Programmable Gate Array (FPGA) transceivers to implement direct, passive,
full-mesh, high speed serial connections between sixteen circuit boards in a
crate, to exchange data between crates, and to offload the data to a cluster of
256 graphics processing unit (GPU) nodes using standard 10~Gbit/s Ethernet
links. The GPU nodes complete the corner-turn by combining data from all crates
and then computing visibilities. Eye diagrams and frame error counters confirm
error-free operation of the corner-turn network in both the currently operating
CHIME Pathfinder telescope (a prototype for the full CHIME telescope) and a
representative fraction of the full CHIME hardware providing an end-to-end
system validation.
  An analysis of an equivalent corner-turn system built with Ethernet switches
instead of custom passive data links is provided.
