A primary goal of SONET is survivability: a self-healing network that recognizes fiber cuts and reroutes traffic before service is interrupted or degraded. All carriers that deploy SONET follow the same set of standards. However, it's the network architecture and equipment over which SONET is deployed that makes a difference in survivability and restoral times. SONET deployments announced by some carriers are linear in design; Netstar's backbone networks are ring-based.
 

SONET Architecture

Linear SONET:

A linear SONET architecture is configured point-to-point. Traffic moving from point A to point B has only one route to follow. If there is backup cable in place, it usually follows the same route. In this design, a fiber cut or other service disruption requires rerouting traffic along other routes in the network, if capacity is available. Service stops while restoration is implemented. 

Ring-based SONET:

NetStar chose a bi-directional, line-switched ring (BLSR) architecture that sends traffic only in the required direction during normal conditions. When a fiber cut is detected, the traffic is routed around the failure in the opposite direction. This happens in about 60-120 milliseconds -- less than a blink of an eye -- so rapidly that it's virtually transparent to the user. NetStar's approach allows sharing of the protection capacity and increases traffic capacity.
 

SONET Ring - Normal Operation During normal operation, all traffic on a BLSR SONET ring rides on the working channel.

SONET Ring - Break Fiber cut or network outage between POPs: Should a fiber cut or other network outage occur, traffic is rerouted back around the ring in milliseconds -- so service continues uninterrupted. This millisecond restoral capability means you can count on the reliability of a BLSR SONET network for your mission-critical communications.

• Supports high-bandwidth applications such as multimedia 
• Operates in multi-vendor environments 
• Increases transmission network capacity 
• Enhances network reliability and survivability 

Enables flexible bandwidth allocation to meet changing application and communication needs

Transmission Speeds and Capacity 

Transmission Speeds

The SONET rate hierarchy sets a standard for transmitting and multiplexing signals at much higher speeds than those currently used in asynchronous networks. SONET standardizes line rates, frame formats and operations, administration, maintenance and provisioning (OAM&P) protocols. Transmission rates range from 51.84 Mbps to 2.4 Gbps and higher, increasing in standard increments of 51.84 Mbps. The SDH basic bit rate is 155.52 Mbps. SONET equipment converts electrical signals to optical signals, and interfaces with asynchronous transmission equipment to convert digital signals.

Capacity

SONET doubles the capacity across the network, from 1.7 billion bits of information through asynchronous transmission to 2.5 billion bits of information per second through synchronous transmission. And the expanded capacity requires no additional fibers. This increased capacity capability is becoming increasingly important as new and evolving applications continue to put increased demands on the network.
 

For More Information 

If you would like to learn more about SONET, get the SONET 101 tutorial.  If you are wanting in depth information about advanced SONET networking, take a look at the Intro to SONET tutorial. And finally, if you want to get into the really heavy stuff and take a peak at some highly advanced technology utilized in SONET transmission, download the SilkRoad Refractive Synchronization Communicatiions whitepaper from Silkroad.