Here is an example of the log message with the binary data emitted in a raw format (nonprintable or non-ASCII bytes were replaced by "?"):

2016-01-29 12:35:31,797 | TRACE | entLoopGroup-9-1 | AsyncSshHandlerWriter | 249 - org.opendaylight.netconf.netty-util - 1.0.0.SNAPSHOT | Writing request on channel: [id: 0xf21aeb32, local:E:f21aeb32 => local:netconf], message:
#93
$EXI????????
rpc-reply?????message-id?m-38data

The stuff in the log message appears to be Netconf RPC reply in EXI format. As EXI is a binary format, it should be either hexlified or emitted in the following format (which allows the ASCII data embedded in it to be readable without special processing):

2016-01-29 12:35:31,797 | TRACE | entLoopGroup-9-1 | AsyncSshHandlerWriter | 249 - org.opendaylight.netconf.netty-util - 1.0.0.SNAPSHOT | Writing request on channel: [id: 0xf21aeb32, local:E:f21aeb32 => local:netconf], message: 0A "#93" 0A "$EXI" EF BF BD 00 40 0E 5C 00 00 "'urn:ietf:params:xml:ns:netconf:base:1.0" 0A "rpc-reply" 02 04 00 01 01 01 0B "message-id" 06 "m-38" 01 03 04 05 "data" 00 00 0A "##" 0A

i.e. runs of ASCII characters from chr(32) to chr(126) (with possible exclusion of '"') at least 2 characters long emitted in "", everything else hexlified).