The Japanese variants (JNTT and JTTC) differ from the ANSI and ITU specifications and from each other. This topic describes those differences.
The JNTT and JTTC variants:
Use 16 bit point codes.
May use a signaling route test message, acknowledgement, and negative acknowledgement to determine destination availability (SRT/SRA/USN).
Use the high order two bits of the length indicator byte for message priority.
Do not support the SIPO message. If a processor outage condition occurs, the link is brought down.
The JTCC variant:
Does not use timer T5, the second changeback timer.
Uses congestion messages.
Allows only 40 outstanding unacknowledged packets rather than the full standard window of 127 packets. As a result, JTTC is likely to reach congestion faster than other configurations.
Ignores 2 of 3 bad BSNs or FIBs.
Discards packets with a bad FSN and generates a negative acknowledgement.
Always uses emergency alignment.
Does not support the user part unavailable message (UPU).
Allows multiple destinations for transferring and routing set test messages and their cluster variants (TFP, TFR, TFA, TCP, TCR, TCA, RSP, RSR, RCP, RCR).