April 14, 2024
Synchronous Optical Network

The Evolution and Legacy of Synchronous Optical Networking (SONET) From Backbone to Obsolescence

Synchronous Optical Networking (SONET) is a standard-based telecommunication protocol that allows transparent transmission and switching of multiple digital hierarchy signals. SONET was initially developed in the 1980s and deployed extensively in North America and other parts of the world during the 1990s. While its adoption has declined in recent years due to maturation of Internet protocols and emergence of technologies like Ethernet, SONET continues to serve as the backbone for numerous telco and enterprise networks worldwide.

History and Development

The need for a synchronous optical transmission standard emerged in the 1980s as telcos looked to upgrade their backbone networks to support higher transmission rates. At the time, most trunk lines used asynchronous time-division multiplexing (TDM) which suffered from poor quality and lack of standardization. In response, Bellcore (now Telcordia Technologies) spearheaded the development of SONET which was formally approved as an ANSI standard in 1989.

Some key design goals of SONET included synchronous transmission of multiple services, self-healing capabilities, support for integration of new technologies, and interoperability between equipment from different vendors. This led to development of SONET’s highly structured frame format along with error detection and correction mechanisms. Initial implementations supported rates up to OC-3 (155.52 Mbps) which aligned with prevalent DS3 interface. Higher rates up to OC-192 (10 Gbps) were standardized over time.

Frame Structure

The basic building block of SONET is the Synchronous Transport Signal (STS-1) frame with a payload capacity of 51.84 Mbps and transmission rate of 52 Mbps after addition of overhead. Higher rates are integer multiples of the STS-1 known as STS-N frames. For example, OC-3 operates at 3 x STS-1.

The 90 bytes STS-1 frame is precisely structured with 9 rows and 90 columns. It transports client data in a synchronous payload envelope (SPE) while carrying overhead in the synchronous transport signal (STS) container. Overhead bytes carry vital functions for operations, administration, maintenance and provisioning (OAM&P) like frame alignment, error monitoring and ring switching. SONET frames maintain byte and frame synchronism end-to-end.

Self-Healing Rings

SONET networks are typically built in rings or mesh topologies with dual fiber rings providing redundancy. SONET supports self-healing capabilities using ring switching and protection protocols. Key aspects include physical layer protection using line switched rings (LSR) and path protection using path switched rings (PSR).

In LSR, each fiber segment has dedicated transmit and receive paths. During a fiber cut, a protection switch activates to reroute traffic over the alternate ring segment. PSR provides end-to-end path protection for higher layer payloads within 50ms of a failure. Together, these protection schemes enable quick restoration of failed circuits without needing operator intervention – a key value proposition of SONET.

Multiservice Transport

One of SONET’s strong suits is its ability to efficiently transport multiple client signals simultaneously over the same physical infrastructure. Using multiplexing and mapping techniques, SONET networks consolidate lower rate interfaces like T1s, E1s, DS3s onto higher STS structures in transparent fashion. Virtual Tributaries (VT) allow sub-STS payloads to be rapidly provisioned.

Client interfaces Synchronous Optical Network are mapped into the SPE as fixed size frames (VT1.5) or variable size packets (VT2). This way, bandwidth is readily scalable from first generation DS0 services all the way up to 40G and beyond. SONET’s flexible structures enabled telcos to cost-effectively upgrade plant capacity as demand grewrapidly in the Internet era of the 1990s and beyond. Its integration with WAN protocols like IP/MPLS created powerful combination.

Ethernet, DWDM and the demise of SONET

While SONET served networking very well for two decades, two technologies finally began displacing it around 2000. First, Gigabit Ethernet emerged as a lower cost LAN solution. Second, dense wavelength division multiplexing (DWDM) enabled scaling fiber capacity without relying on SONET. Ethernet also gained popularity due to synergies with IP while DWDM systems directly supported optical networking.

Together, these technologies decoupled transmission from switching/multiplexing layers. SONET’s highly integrated approach became redundant now that these could be separated. By 2015, most new deployments began using Ethernet and DWDM only with SONET circuits migrating to optical transport gradually. SONET did achieve its goal of enabling Internet growth but fell victim to its own success as the technologies it helped spur ended up making its architecture superfluous over time.

Legacy Deployments

While no longer growing, SONET remains entrenched worldwide in older networks that have not been upgraded. According to some estimates, over $20 billion worth of SONET equipment is still in operation globally. Especially in access networks closer to the customer edge, SONET equipment installed during the 1990s-2000s continues functioning without replacement. Telcos are now managing these legacy infrastructures focusing on reliability over new feature additions.

Vendors also offer modern SONET platforms targeting lifecycle extension through modular upgrades. Advanced operations tools monitor health and performance enhancing longevity. As 5G wireless and other bandwidth-intensive applications emerge, SONET assets may see new importance before finally retiring. Their rock-solid dependability makes migration to packet-optical networks a gradual multi-year process for many operators even today.

1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it