An optical circuit's rate is only the beginning of its specification. To understand an inherited SONET or SDH service, identify what it carries, how it obtains timing, which failures it can survive, and where responsibility changes hands.
Separate the service from its carrier
SONET means Synchronous Optical Network; SDH means Synchronous Digital Hierarchy. Both organize transmission into repeating frames with payload capacity and operational overhead. SONET terminology is associated with North American standardization; SDH provides the related international hierarchy. ITU-T G.707/Y.1322 defines the SDH network-node interface.
- Client serviceA voice circuit, packet link, Ethernet service, or other supported payload.
- Mapping and multiplexingThe service is placed into the required payload structure and combined with other traffic.
- SONET/SDH framingTiming, overhead, monitoring, and configured protection support transport between the endpoints.
- Optical pathFiber spans and optical equipment carry the signal; a wavelength system may also be involved.
For example, RFC 2615 specifies PPP over SONET/SDH. A router's packet link and the underlying optical transport are distinct things to inspect. ATM and Ethernet mappings use their own arrangements; record the actual mapping instead of inferring it from an OC or STM label.
Overhead helps locate faults. SONET distinguishes section, line, and path; SDH uses regenerator section, multiplex section, and path. Cisco's framing comparison explains why matching line rates still leaves framing and overhead details to verify.
Read line rate separately from payload
| SONET optical signal | SDH signal | Line rate, Mbit/s |
|---|---|---|
| OC-1 | No corresponding STM-1 level | 51.840 |
| OC-3 | STM-1 | 155.520 |
| OC-12 | STM-4 | 622.080 |
| OC-48 | STM-16 | 2488.320 |
| OC-192 | STM-64 | 9953.280 |
These are decimal line rates, including transport overhead. The correspondence does not guarantee interchangeable equipment or identical payload configuration. Electrical STS terminology, optical OC terminology, concatenation, and channelization describe additional aspects of a SONET service.
The Cisco SONET primer (PDF) documents the hierarchy and frame structure. It is historical equipment documentation, useful here for established transport concepts rather than present-day product selection.
Interpret a circuit record before ordering a replacement
Protection mechanisms differ. The G.841 protection-architecture recommendation is a starting point for SDH terminology. Ask which failure triggers switching, whether the alternate route is physically independent, and what happens during maintenance or multiple failures. A quoted switching time applies to defined conditions; application recovery includes more than transport switching.
During an incident, preserve timestamps, alarms, error counts, path trace, clock source, and protection state at both ends. Identify whether the alarm is local, remote, transport-level, or associated with the client service. Compare the records before attributing every error to the nearest fiber span.
Replace the required function
| Option | Role to examine | Evidence still required |
|---|---|---|
| Ethernet or routed packet service | Connectivity for packet-native endpoints | Handoff, traffic profile, latency, loss, MTU, security, and failover behavior. |
| Circuit emulation | Preserve a supported legacy circuit over a packet network | Timing recovery, jitter-buffer behavior, end-to-end delay, and endpoint compatibility. |
| OTN | Digital transport, client mapping, and monitoring | Exact supported client signal, mapping, protection, and operational handoff. |
| DWDM | Carry multiple wavelengths on fiber | The client service and digital transport still need an appropriate implementation. |
For the adaptation and transport roles, consult RFC 4553's structure-agnostic TDM circuit emulation and ITU-T G.709's OTN interfaces. These are architectural references, not assurances that any particular service implements the required features.
- Inventory every carried service, endpoint, dependency, and contract or operational requirement.
- Agree on measurable acceptance criteria and the conditions under which recovery will be tested.
- Validate the replacement in a representative setup, including timing-sensitive traffic where present.
- Plan cutover, monitoring, escalation contacts, and rollback while the original path remains recoverable.
- Retire the old circuit only after the service owner accepts the observed behavior.
The useful outcome is continuity of the required service, with understood changes in its operation. Bandwidth is one field in that record.