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Re: Path Status (G1) latching

The G1 Path Status is sent every 125usec by the receiver (far end) of a
SONET signal to tell the sender (near end) of that signal how it
looks when it is received at the far end, ie it is a real time
(upstream) feedback signal. Thus, that Rx register at the near end is
reloaded every 125usec when the G1 byte is received from the far end.
The near end Rx chip that contains this register interfaces to a
processor that must read the register. The software has to be designed
hae a real time loop that runs to completion every 125usec.
The REI-P field contains an estimate of the number of bit errors
contained in the Synchronous Payload Envelope to
which it refers. The sender/near-end accumulates the byte-size XOR of
all the bytes of the SPE, puts that result-byte in B3 of the next
SPE/frame and ships it (downstream). Receiving/far end calculates the
XOR across the SPE and XORs it with B3, counts up the number of 1s in
the result and that is the REI-P--it is a reasonable estimator of the
number of bit errors in that frame. The far-end returns the G1 byte in
which the RDI and ERDI field identifies all other monitored Path layer
error conditions. REI-P refers only to bit errors (or Coding Violations
as they are called in the lingo of SONET).

Thus, the register to which you refer is changing all the time in
response to the real-time conditions of the outbound/downstream link

Naturally, this process is ongoing at both ends such that signal
integrity in both directions is monitored and fed back to the senders.
And reported to the NMS of course.

In SONET lingo, downstream refers to the direction of signal flow,
upstream to the reverse direction.

Prosun Chatterjee wrote:

> Hi all,
> This regards the G1 path status maintained
> in register 2.12.7:0 (section of
> the draft).
> This states ".... become set to the value
> of the received octet and remain set until
> it is read via the management interface."
> A typical SONET equippment will set this
> to 02h (= "0000" REI & "001" RDI (no error
> ERDI indication) & '0').
> Since a non-zero value, once latched, will
> not be removed till read, the non-zero
> normal operation condition is what will
> get latched practically all the time.
> All error conditions will be lost to this
> status byte practically for all time.
> Given this, can someone tell what purpose
> this status byte hopes to achieve?
> regards,
> Prosun


Dr. Gary A. Nelson
Zynrgy Group Inc
20708 Deerpath Road
Barrington, IL 60010-3787
+1.847.304.1929 fax