OC3 is the abbreviated term for Optical Carrier level 3, used to specify the speed level of fiber optic networks over SONET. Speed is measured through SONET (Synchronous Optical Network) standards.

OC3 works as a reliable fiber optic backbone for large networks with large voice/data/video traffic needs.

OC-3 is most often made up of what is called a "SONET ring" to maintain it's reliability (redundancy) during it's high speed transmissions.

A common method of back up used with SONET ring is called a "bi-directional ring topology." This bi-directional ring is a physical topology which deploys two sets of fiber strands.

The first strand for the connection is for sending and receiving. The second strand will reroute traffic from the original transmission in another direction should the first strand break or malfunction.

Bi-directional ring topology helps ensure constant reliability within an OC-3 SONET based network.

Primary function is to take , synchronize , and transport data and voice at the speed of 155 Mbps reliably.

OC3 is capable of 2,000 simultaneous voice transmissions with each transmission carrying variable data types.

Common sources of this data include ATM & Frame switches, T1 and T3 multiplexers, and large IP networks.

STS-3 is the abbreviated term standing for Synchronous Transport Signal level 3. STS and SONET OC carry the same electrical equivalent.

STS-3 is an optical carrier frame consisting of the following characteristics:

SPE (Synchronous Payload Envelope), SOH (Section Overhead), POH (Path Overhead), and LOH (Line Overhead).

SPE is the electronic envelope that carries the data over the transmission line otherwise known as the Payload.

SOH maintains all the links within OC3 while relaying line status to the end user. The information within SOH includes the status of transported data, the notification of messages, as well as alarm levels.

POH is responsible for the functionality of the data envelope within OC-3 while in transit. Such functionality includes vital information for the management of end-to-end networks. POH travels within the data envelope until the data is demultiplexed through multiplexing equipment.

LOH controls the reliability transit of the data over the line.

LOH and SOH combined on a SONET OC-3 network create what is referred to as TOH (Transport Overhead). TOH is one of the elements responsible for signaling and control over OC3 network lines.

1. Photonic Layer - This layer converts transmitted electrical signals that usually travel via copper into optical signals then transfers them back into electrical signals at the end of the transmission.

2. Section Layer - This layer monitors the condition and progress of signal in route to and from signal amplifiers and applied hardware.

3. Line Layer - This layer monitors and provides administration of multiplexers as well as provides synchronization of multiple signals. Through this synchronization, all the signals are put into one stream of traffic for high speed transfer.

4. Path Layer - Assembles and disassembles transmitted data and voice into frames.