When you are testing WAN links, applying different patterns would be good idea to isolate an issue. Here is output of each testing pattern.
0x0000 – Sending binary all 0’s (zeroes) to see a failure if the line is mis-coded
0xFFFF – Sending binary all 1’s (ones) to see a failure if the line is mis-coded. All 1’s also tests for bad equipment. The repetative 1’s signal stresses out equipment. This is a good indicator of equipment which is starting to go bad, due to the fact that it cannot keep the heavy ‘1’ signal.
0x5555 – Sending binary alternating 1’s and 0’s to see a failure on bad repeater
0x8080 – Sending patterns 3-in-24 to see a failure on bad repeater, this is ‘1 in 8’
0x1000 – To see a failure on bad MUX/DACS