Synchronous Optical Network (SONET) is a standard for carrying digital traffic over fiber in fixed-rate channels, developed in North America and widely used by telephone companies from the late 1980s onward. It defines a hierarchy of speeds, known as optical carrier (OC) levels, such as OC-3 and OC-48, and is known for fast, automatic protection switching on fiber rings. SONET carried much of the voice and data traffic of its era, including T1 lines multiplexed into larger circuits. Today it is a legacy technology: buyers mostly meet it when carriers retire it or when replacing old circuits.
At a glance
- SONET is a time-division multiplexing (TDM) standard that carries traffic in fixed-rate channels over fiber.
- Speeds come in OC levels: OC-3 at about 155 Mbps, OC-12 at about 622 Mbps, OC-48 at about 2.5 Gbps, OC-192 at about 10 Gbps.
- Ring designs can switch traffic to a protection path very quickly, often within about 50 milliseconds, when a fiber is cut.
- SDH is the closely related international standard used outside North America.
- Many carriers have stopped selling new SONET services and are retiring older equipment.
What problem it solves
Before SONET, carriers used a mix of proprietary and incompatible optical systems, making it hard to connect equipment from different vendors or hand traffic between networks. SONET standardized how digital signals are framed, timed and multiplexed on fiber, so equipment could interwork and lower-speed circuits could be combined into larger ones predictably.
It also brought strong resilience. SONET rings and protection schemes could detect a fiber cut and move traffic to a backup path fast enough that phone calls generally stayed up. For decades that made SONET the backbone of carrier networks and the basis of many enterprise private lines.
How it works
Synchronous framing. SONET equipment shares precise timing across the network. Traffic is carried in frames that repeat at a fixed rate, with each lower-speed signal occupying set time slots. That fixed structure made it easy to add or drop a single circuit at a node without unpacking everything else.
Rate hierarchy. The base rate, STS-1 or OC-1, is about 51.84 Mbps. Higher rates are multiples of it, and lower-speed signals such as T1s and T3s are mapped into the frames.
Rings and protection. Many SONET networks are built as fiber rings, with traffic able to travel either direction. If a fiber or node fails, protection switching reroutes traffic around the ring. Whether a given circuit is protected, and how, depends on how the carrier provisioned it.
Fixed capacity. Because bandwidth is allocated in fixed slots, unused capacity in a channel cannot be shared with other traffic. This made SONET less efficient for bursty data than packet-based Ethernet and IP.
Successors. Carriers have largely moved to Ethernet, IP and DWDM systems, often using the Optical Transport Network (OTN) standard to wrap and manage high-speed signals on wavelengths.
If you are replacing legacy optical circuits, our Wavelength solution page covers one common migration path.
When it matters for buyers
- Carrier retirement notices. A notice that SONET-based services are ending sets a migration deadline; start early, because replacements may need new equipment or construction.
- Cost reviews. Legacy private lines are often priced well above modern Ethernet services of similar or higher speed.
- Voice and TDM dependencies. Circuits carrying T1s for phone systems or older equipment may need conversion or a broader voice migration.
- Resilience requirements. If you relied on SONET ring protection, confirm what protection the replacement offers; it may differ.
- Inherited networks. Old OC-level circuits often surface in acquisitions or audits of telecom spend.
Questions to ask vendors
- Which of our circuits run over SONET, and what are your plans and dates for retiring them?
- What replacement services do you recommend, at what speeds and prices?
- Is the replacement circuit protected, and how quickly does it recover from a fiber cut compared with our current service?
- Will we need new equipment at our sites, and who provides it?
- Can any TDM traffic, such as T1s for voice, be carried on the new service, or does it need to be converted?
- Is there a migration window during which both services run in parallel?
How it differs from OTN
SONET and the Optical Transport Network (OTN) both define how carriers package and manage traffic on fiber. SONET was built for a world of voice circuits, with fixed rates up to around 10 Gbps in common use and fixed time slots. OTN is the newer standard, designed to carry and manage much higher-speed signals, such as 100 Gbps and beyond, on DWDM wavelengths. It can wrap Ethernet, IP and even SONET signals and adds stronger error correction and monitoring. In many carrier networks, OTN and packet systems have taken over the role SONET used to play.