OC3 - Optical Carrier Level 3
OC3 - The Fiber Optic Transport And Access Solution
Optical Carrier 3 or OC3 as it is commonly referred to as is the basic network link in SONET. Traditionally, it has been the ideal backbone link that carries the bulk calls between central offices. Today, it is fast becoming a great access link in buildings with many customers demanding higher bandwidth connections. This is a great building block in the flexible and scalable network solution called SONET.
SONET today works so well, as it was built to overcome the problems with an equally popular and prevalent solution: the Plesiochronous Digital Hierarchy or PDH. The basic block for PDH networks was the 64kbps voice line or the DS0. 24 DS0s are aggregated to make a DS1 and 28 DS1s form a DS3. This was the first digital hierarchy solution that supported the growth of telephone networks initially.
PDH was the perfect first step towards traffic aggregation in digitized networks. However, the approach it took towards the aggregate lines had some inherent drawbacks. The main problem was synchronization between network elements. To attain synchronization, PDH introduced overheads which could align the digital pulses for synchronization.
The overheads though created a new problem with regards to adding and dropping lines. As the overheads skewed the frame structure, whenever one needed to add or delete a DS0 in a DS1 frame, the entire frame had to be dismantled and built again. This was a very inefficient approach to aggregation.
SONET addressed this problem through a synchronized and standardized frame structure. The SONET frame has all the necessary overheads included into the synchronous frame. As a result, adding or dropping individual frames from the master SONET frame did not require rebuilding. This really allowed SONET networks to become highly scalable.
Within SONET the fundamental building block from a link perspective is the OC3 link. Each OC3 consists of 3 STS-1 which are the basic building blocks for the entire SONET hierarchy. An STS-1 capacity is 51.84 Mbps which is large enough to hold a DS3 frame. Thus Optical Carrier level 3 can support 3 DS3 links.
The real measure of Optical Carrier level 3 capacity can be gauge when one looks at the number of live voice lines it can carry. With 3 DS3s, this number comes to as high as 2000 live voice calls. This is why OC3 has been one of the best backbone links since the inception. However today with increasing bandwidth it is actually assuming a new role of access link too. Large commercial buildings with multiple bandwidth hungry enterprises mean that Optical Carrier level 3 is the ideal link to connect them to the core.