SONET/SDH Interfaces Overview


Synchronous Digital Hierarchy (SDH) is a CCITT standard for a hierarchy of optical transmission rates. Synchronous Optical Network (SONET) is a USA standard that is largely equivalent to SDH. Both are widely used methods for very high speed transmission of voice and data signals across the numerous world-wide fiber-optic networks.

SDH and SONET use light-emitting diodes or lasers to transmit a binary stream of light-on and light-off sequences at a constant rate. At the far end optical sensors convert the pulses of light back to electrical representations of the binary information.

In wavelength-division multiplexing (WDM), light at several different wavelengths (colors to a human eye) is transmitted on the same fiber segment, greatly increasing the throughput of each fiber cable.

In dense wavelength-division multiplexing (DWDM), many optical data streams at different wavelengths are combined into one fiber.

The basic building block of the SONET/SDH hierarchy in the optical domain is an OC1; in the electrical domain, it is an STS-1. An OC1 operates at 51.840 Mbps. OC3 operates at 155.520 Mbps.

A SONET/SDH stream can consist of discrete lower-rate traffic flows that have been combined using time-division multiplexing (TDM) techniques. This method is useful, but a portion of the total bandwidth is consumed by the TDM overhead. When a SONET/SDH stream consists of only a single, very high speed payload, it is referred to as operating in concatenated mode. A SONET/SDH interface operating in this mode has a “c” added to the rate descriptor. For example, a concatenated OC48 interface is referred to as OC48c.

SONET and SDH traffic streams exhibit very few differences in behavior that are significant to Juniper Networks SONET/SDH interfaces; in general, this chapter uses SONET/SDH to indicate behavior that is identical for the two standards. However, there is one important difference that requires you to configure the interface specifically for SONET or SDH mode. That difference is in the setting of two bits (the ss-bits) in the pointer. SONET equipment ignores these bits, but SDH equipment uses them to distinguish a VC-4 payload from other types. When configured in SDH mode, Juniper Networks SONET/SDH PICs set the ss-bits to s1s0 2 (binary 10). For more information, see Junos OS Administration Library.


To extend the life of the laser, when a SONET/SDH PIC is not being actively used with any valid links, take the PIC offline until you are ready to establish a link to another device. To do this, issue the request chassis pic offline fpc-slot slot-number pic-slot slot-number operational mode command:

user@host> request chassis pic offline fpc-slot slot-number pic-slot slot-number

After you have connected the PIC to another device, bring the PIC back online by issuing the request chassis pic online fpc-slot slot-number pic-slot slot-number operational mode command.

user@host> request chassis pic online fpc-slot slot-number pic-slot slot-number

For information about taking a PIC offline or online, see request chassis pic.