Introduction

 

The IEEE C37.94™-2002 standard (reaffirmed in 2008) defined a multi-vendor optical transmission interface to be used by power utility companies to replace existing electrical supervisory control and data acquisition links. Low level electrical links were considered unreliable for use in the power industry as they are prone to interference and damages due to intra-substation electromagnetic interference (EMI), electric shock, signal ground loops, ground potential rise and safety, among other electrical issues. Optical links and fiber optics don’t have such problems.

 

C37.94 was originally defined as an 850 nm MMF interface with ST (BFOC/2.5) connectors, but 1310 nm MMF and SMF variants with other connector types could be found today. It is capable of transmitting a single data stream at Nx64 kbit/s, with N being 1 to 12 (64 to 768 kbit/s). It uses a simple framing structure and a line rate of 2048 kbit/s. Every bit in the data channel is encoded as a two-bit symbol consisting of the original data bit and its complement (01 or 10) assuring enough transitions to help with clock recovery.

 

The frame alignment signal is carried in the header, as a 16-bit pattern that alternates with every frame:

 

• Framing pattern 1: 1001 1011 0000 1111

• Framing pattern 2: 11y1 1111 0000 1111 (y = yellow alarm indication bit. 1 indicates alarmed condition)

 

The framing pattern is followed by a 48-bit p -p q -q r -r s -s 1 0 1 0 1 0 1 0…1 0 overhead pattern, where the pqrs bits represent the N value or data rate multiplier. The alternate trailing 10101010 pattern in the following five time slots is used to seed the clock recovery circuitry and serve as a preamble to client data.

 

Loss of Signal (LOS) and Loss of Frame Alignment (LOF)

 

Although the standard recognizes the ITU-T G.706 LOF definition as the event that starts after the detection of three consecutive incorrect frame alignment signals, IEEE considers loss of framing a link Loss of Signal (LOS) condition. In C37.94, LOS is declared when the optical signal is too low or the frame alignment is lost.

 

Per IEEE, the LOS condition is declared upon receiving two or more errors in eight consecutive framing patterns, and cleared upon receiving eight consecutive correct framing patterns.

 

VeEX test sets repot Frame (induced) LOS and Signal (induced) LOS separately to provide extra information to users for better problem isolation and troubleshooting purposes.

 

Remote Defect Indication (RDI) and Yellow Alarm

 

Although the IEEE C37.94 refers to RDI path defects (as defined by ITU-T G.775), their official name for this far-end alarm status is Yellow alarm. RDI has become a more common term, but some users may use RDI and others Yellow, to refer to the alarm sent by the far end to indicate that there is a problem with the received signal. The yellow alarm indicator is carried by the third bit (y) in the second frame alignment signal.

 

Alarm Indication Signal (AIS)

 

When a network element declares loss of signal at its receiver, it sends a Yellow (RDI) alarm back to the originating equipment to notify the problem and an AIS alarm to all other equipment located upstream (in the direction of the original transmission). The AIS consist in replacing data channels’ line bits (time slots 8 to 31) with an All 1s pattern.