The following sections provide information about attenuation and interoperability for SONET/SDH PICs.
SONET/SDH transceivers in the different reach classes—short reach (SR), intermediate reach (IR), and long reach (LR)—generate different output power levels and tolerate different input power levels. Transceivers that have a longer reach can transmit enough power to saturate the receivers on PICs that have a shorter reach. Specifically, LR transceivers can saturate IR PICs, and both IR and LR transceivers can saturate SR PICs. Transceivers in the same reach class can also potentially saturate one another.
To prevent saturation, you might need to attenuate power at the PIC receiver, particularly if you know that it has a shorter reach than the transceiver that is sending the signal. Determine the amount of attenuation needed by measuring the power level at each receiver. Attenuate the power to bring it within the allowable range; for short lengths of fiber, with fiber and connector loss close to zero, an attenuator of 5 to 10 dB should be sufficient.
For example, the quad-wide SONET/SDH OC48 PIC used in an M20 or M40 router is an intermediate-reach (IR) PIC. The single-wide SONET/SDH OC48 PIC used in an M40e or M160 router is a short-reach (SR) PIC. The IR transceiver might transmit more power than the SR PIC can receive without experiencing saturation, so you might need to attenuate power at the SR receiver. The IR receiver should not require attenuation, because the SR transmit levels do not exceed the IR transceiver's reception levels.
For information about the maximum and minimum input and output levels for various transceiver types, see Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, document number GR-253-CORE, published by Telcordia Technologies (formerly Bellcore).
The SONET/SDH OC192 transceiver is an SR-2 transceiver, with a transmit wavelength of 1550 nm. Some OC192 transceivers from other vendors are SR-1 transceivers which have a transmit wavelength of 1310 nm. The OC192 transceiver can receive at both 1310 nm and 1550 nm.
To ensure interoperability with other vendors' SR-1 and SR-2 transceivers, follow these guidelines:
The receiver of a SONET/SDH OC192 PIC has very high sensitivity and low minimum and maximum allowable power. A direct fiber connection from transmitter to receiver, whether end-to-end or loopback, can easily overload the receiver. To prevent the receiver from overloading, you need to attenuate the receiving end.
When connecting two OC192 transceivers, follow these guidelines:
When connecting two OC192 transceivers in the same room, you would use a short length of fiber, and the connector and fiber cable loss would be close to zero. In this situation, a 15-dB attenuator inserted before the receiver should be sufficient to bring the receiver within the allowable range.