When one looks at the core transport technology, it has been SONET for a few decades now. Given how quickly technologies change, it is baffling to know that this technology has sustained itself for so long. However, the very design and logic in SONET makes it highly flexible and versatile which has allowed it to adapt to the changing customer needs. None showcases this better than the OC12 link in the SONET hierarchy.
The basic building block in SONET is called the STS1 which is a block of 51.84 Mbps. This is typically created and processed at the electronic level within the SONET equipment circuitry. There is no photonic equivalent for STS1.
The fundamental photonic layer in SONET is the Optical Carrier level 3 or the OC3. This consists of three STS1 frames. Being synchronous, the scalability in SONET is proportional. Thus the OC3 link has transmission speeds that are 3 times the bandwidth of STS1 or 155.52 Mbps. Such scaling really allows SONET to remain the standard core transport technology despite changing customer needs.
The next logical level in the SONET structure is the OC12 or the optical carrier level 12. Following the synchronous concept from above, an optical carrier level 12 link has four times the transmission rates of optical carrier level 3 or 622.08 Mbps. During the initial period of SONET, this was the ideal link speed for core networks and optical carrier level 12 was in fact used for this purpose. An important reason for this was that the first SONET networks were designed to support voice traffic only which required lower bandwidths.
With time, the number of users and average bandwidth utilization per user has increased exponentially. As a result, optical carrier level 12 is not more ideally suited for backbone infrastructure. It has been taken up by higher levels like OC 192 with bandwidths of nearly 10 Gpbs.
While the utility as a backbone network transport link may have diminished, there are other avenues where OC12 links are becoming indispensible. For the service providers, this link forms the perfect transmission medium to connect the remote locations to their main networks. Similarly for CLECs this link is the perfect way to tap into the ILEC high speeds networks to cater to their customers.