Latency in Exadata DB's

I am struggling to understand the difference and relationship between "Average Synchronous Single-Block Read Latency" (hereafter ASSBRL) and "Cell Single Block Physical Read" (hereafter CSBPR). What I have gathered so far:

1. CSBPR is captured in v$eventmetric, while ASSBRL is captured in v$sysmetric.
2. CSBPR is captured in AWRs while ASSBRL is not.
3. CSBPR is always higher than ASSBRL, and largely proportional. i.e., when one goes up, so does the other.
4. It seems to me that ASSBRL is a component of CSBPR. Is this correct? Is ASSBRL the measure right at the disk while CSBPR is the full round trip from the db to disk and back?