OC-12 (622 Mbps) is the next step up from OC-3 (155 Mbps) in the SONET optical carrier hierarchy. You’ll find this service readily available in Metropolitan Area Networks (MAN) and as a dedicated Internet access connection for mid-size Internet service providers. Large organizations also have OC12 bandwidth connections, especially those with thousands of employees on-site or a need to support high bandwidth applications like medical image transmission or video production and distribution. Not too many years ago, OC12 was considered a good choice for a carrier fiber optic core network. Now, most regional and national carriers have moved up to OC-48 at 2.5 Gbps or OC-192 at 10 Gbps. Long haul and undersea fiber lines are running at OC-768 (40 Gbps) and moving quickly to support 100 Gbps.
OC12 is most often deployed on a protected SONET ring topology, with two counter-rotating fiber rings. If something in one ring fails, either equipment or a cable break, the other ring will pick up 100% of the traffic within 50 mSec.
Note that an OC12 service is probably not the only traffic on a particular fiber strand. At 622 Mbps, OC12 comes nowhere near using available bandwidth of the fiber. SONET is designed as a Time Division Multiplexed (TDM) system that can easily add or drop common OC service levels such as OC3 and OC12. In addition to the electrically multiplexed services on the fiber, each fiber may also be optically multiplexed into a dozen or more wavelengths using Dense Wavelength Division Multiplexing (DWDM) equipment.
Carrier Ethernet over fiber is based on packet switching rather than time division multiplexing. Without the strict synchronized channels of TDM, Ethernet is far more scalable. You can order 600 Mbps Ethernet as a replacement for OC12 SONET. You can also order 500 Mbps if that is all you reasonably need. Even more importantly, you can start increasing bandwidth to 700 Mbps, 800 Mbps, 900 Mbps and 1 Gbps as business requirements dictate.
With SONET, the service provider will install a managed router as Customer Premises Equipment (CPE) that includes an interface module specifically designed for the particular OC service level you have installed. The modules for OC3, OC12, and OC48 will plug into the same port on the router, but they are not interchangeable. You need to match the particular interface to the OC service level. That becomes important when you want to implement a service change. The provider will need to roll a truck to either swap out interface modules or completely replace the CPE router.
With Carrier Ethernet, your service provider will install a managed router that can support a particular port speed. For Ethernet over Copper, this is likely 100 Mbps. For fiber, you are likely looking at 1 Gbps, although you can often get equipment rated to 10 Gbps if you expect to be upgrading to that speed level in the foreseeable future. With a 1 Gbps port, you can order whatever service level you need between, say, 100 Mbps and 1 Gbps, in 100 Mbps or smaller increments. No equipment changes are needed as you change service levels within that range. You simply call your provider and tell them to increase bandwidth and you’ll see it happen within a few days, sometimes within a few hours.
Beyond scalability, Ethernet has a cost advantage over SONET in most situations. Sometimes the cost difference is dramatic, even half the cost, for Ethernet versus SONET of the same bandwidth. The one limitation that you’ll run into is that SONET has been around a lot longer and is more available than Ethernet over Fiber. Even so, competitive carriers have been aggressively building out their IP networks and may have fiber service closer than you think.
Do you require fractional Gigabit bandwidth services? If so, it is worth your while to compare 600 Mbps Ethernet to OC12 SONET for cost and availability. The same advice is true for other service levels from T1 copper bandwidth on up to 10 Gbps and higher.
