optical carrier n

Optical Carrier

Optical Carrier levels describe a range of digital signals that can be carried on SONET fiber optic network. The number in the Optical Carrier level is directly proportional to the data rate of the bitstream carried by the digital signal.

The general rule for calculating the speed of Optical Carrier lines is when a specification is given as OC-n, that the speed will equal n × 51.8 Mbit/s.

Optical Carrier specifications (in use)


OC-1 is a SONET line with transmission speeds of up to 51.84 Mbit/s (payload: 50.112 Mbit/s; overhead: 1.728 Mbit/s) using optical fiber. This base rate is multiplied for use by other OC-n standards. For example, an OC-3 connection is 3 times the rate of OC-1.

OC-3 / STM-1x

OC-3 is a network line with transmission speeds of up to 155.52 Mbit/s (payload: 148.608 Mbit/s; overhead: 6.912 Mbit/s, including path overhead) using fiber optics. Depending on the system OC-3 is also known as STS-3 (electrical level) and STM-1 (SDH).

When OC-3 is not multiplexed by carrying the data from a single source, the letter c (standing for concatenated) is appended: OC-3c.


OC-3c (c is for concatenates) concatenates three STS-1(OC-1) frames into a single OC-3 look alike stream. The three STS-1 (OC-1) streams interleaved with each other such that the first column is from the first stream, the second column is from the second stream, and the three is from the third stream. Concatenated STS(OC) frames carry only one column of path overhead because they cannot be divided into finer granularity signals. Hence, OC-3c can transmit more payload to accommodate a CEPT-4 139.264 Mbit/s signal. The payload rate is 149.76 Mbit/s and overhead is 5.76 Mbit/s.

OC-12 / STM-4x

OC-12 is a network line with transmission speeds of up to 622.08 Mbit/s (payload: 601.344 Mbit/s; overhead: 20.736 Mbit/s).

OC-12 lines are commonly used by ISPs as WAN connections. While a large ISP would not use an OC-12 as a backbone (main link), it would for smaller, regional or local connections. This connection speed is also often used by mid-sized (below Tier 2) internet customers, such as web hosting companies or smaller ISPs buying service from larger ones.


OC-24 is a network line with transmission speeds of up to 1244.16 Mbit/s (payload: 1202.208 Mbit/s; overhead: 41.472 Mbit/s). Implementations of OC-24 in commercial deployments are rare.

OC-48 / STM-16x / 2.5G Sonet

OC-48 is a network line with transmission speeds of up to 2488.32 Mbit/s (payload: 2405.376 Mbit/s; overhead: 82.944 Mbit/s).

With usually cheap interface prices and being faster than OC-3, OC-12 connections, and even surpassing gigabit Ethernet, OC-48 connections are used as the backbones of many regional ISPs. Interconnections between large ISPs for purposes of peering or transit are quite common. As of 2005, the only connections in widespread use that surpass OC-48 speeds are OC-192 and 10 gigabit Ethernet.

OC-48 is also used as transmission speed for tributaries from OC-192 nodes in order to optimize card slot utilization where lower speed deployments are used. Dropping at OC-12, OC-3 or STS-1 speeds are more commonly found on OC-48 terminals, where use of these cards on an OC-192 would not allow for full use of the available bandwidth.


OC-96 is a network line with transmission speeds of up to 4976.64 Mbit/s (payload: 4810.752 Mbit/s; overhead: 165.888 Mbit/s). Implementations of OC-96 in commercial deployments are rare, if ever used at all.

OC-192 / STM-64x / 10G Sonet

OC-192 is a network line with transmission speeds of up to 9953.28 Mbit/s (payload: 9621.504 Mbit/s; overhead: 331.776 Mbit/s). A standardized variant of 10 gigabit Ethernet (10GbE), called WAN-PHY, is designed to inter-operate with OC-192 transport equipment while the common version of 10GbE is called LAN-PHY (which is not compatible with OC-192 transport equipment in its native form). The naming is somewhat misleading, because both variants can be used on a wide area network.

As of 2005, OC-192 connections are most common for use on backbones of large ISPs.

OC-768 / STM-256x

OC-768 is a network line with transmission speeds of up to 39,813.12 Mbit/s (payload: 38,486.016 Mbit/s; overhead: 1327.104 Mbit/s).

As of December 2007, AT&T has deployed 50,000 wavelength-miles of OC-768 in its Internet/MPLS backbone network OC-768 SONET interfaces have been available with short-reach optical interfaces from Cisco since as early as 2006. Infinera made a field trial demonstration data transmission on a live production network involving the service transmission of a 40 Gbit/s OC-768/STM-256 service over a 1,969 km terrestrial network spanning Europe and the U.S.

Optical Carrier specifications (unused)

  • Note: All of the following OC lines are theoretical. None of these are currently in use.


Will be able to provide transmission speeds of around 19.90656 Gbit/s.


Will be able to provide transmission speeds of around 79.62624 Gbit/s. It is unknown if such standards will be implemented in the near future. As of 2007, the majority of work beyond 40 Gbit/s is focusing on 100 gigabit Ethernet, in the IEEE's Higher Speed Study Group.


Will be able to provide transmission speeds of around 159.25248 Gbit/s


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