This read-only octet string (up to 15 characters) reports the Source Access Point Identifier actually being received right now on an OTU2e (enhanced OTU2, roughly 11.1 Gb/s) trunk carrying an oversized client signal such as 10GbE LAN, as opposed to what was configured as expected. An administrator compares this live received value against the configured expected SAPI to diagnose a TIM alarm or confirm a newly turned-up circuit is receiving trace from the correct origin. For example, if OTU2e shows a received SAPI reading 'SITE-B-TX' when 'SITE-A-TX' was expected, that mismatch is the direct evidence used to track down a fiber patched to the wrong far-end site.