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.
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.
Optical Transport Network (OTN) technology represents both a technical leap forward in optical networking over SONET/SDH and a business opportunity for carriers and service providers alike.
| OTN | SONET/SDH |
|---|---|
| Asynchronous mapping of payloads | Synchronous mapping of payloads |
| Timing distribution not required | Requires right timing distribution across networks |
| Designed to operate on multiple wavelengths | Designed to operate on multiple wavelengths |
| Scales to 100Gb/s (and beyond) | Scales to a maximum of 40 Gb/s |
| Performs single-stage multiplexing | Performs multi-stage multiplexing |
| Uses a fixed frame size and increases frame rate to match client rates | Uses a fixed frame rate for a given line rate and increases frame size as client size increases |
| FEC sized for error correction to correct 16 blocks per frame | Not applicable (no standardized FEC) |
