Optical Carrier (OC-x) Levels

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Optical Carrier Levels, commonly abbreviated as OC-x, define a range of digital signaling speeds designed for use over Synchronous Optical Network (SONET) and Asynchronous Transfer Mode (ATM) networks. Originally architected to run over fiber-optic cabling, OC-x played a critical role in the telecommunication landscape, especially before the advent of more modern technologies.

Jump to:

  1. What are Optical Carrier Levels (OC-x)?
  2. OC-x Transmission Rates
  3. How It Was Used
  4. T-Carrier System: A Comparison
  5. Modern Alternatives
  6. Conclusion
  7. References
Optical Carrier Levels (OC-x)

1. What are Optical Carrier Levels (OC-x)?

OC-x stands for Optical Carrier Levels, is a set of signaling rates designed for transmission over Synchronous Optical Network (SONET) networks. Optical carrier (OC-x) levels also apply to Asynchronous Transfer Mode (ATM) networks. The term “optical carrier” indicates that SONET runs over fiber-optic cabling.

2. OC-x Transmission Rates

The following table shows some of the common OC-x transmission rates.

OC-151.85 Mbps
OC-3155.52 Mbps
OC-12622.08 Mbps
OC-241.244 Gbps
OC-482.488 Gbps
OC-1929.952 Gbps

OC-1 is roughly equivalent to T3 in the T-carrier system. It is a standard SONET Synchronous Transport Signal (STS) transmission rate of 51.85 Mbps. This data transmission rate is known as STS-1. Higher data transmission rates (OC-3, OC-12, and so on) are multiplexed STS-1 transmissions. In other words, OC-3 is three times faster than OC-1 (STS-1).

3. How It Was Used

Optical Carrier Levels served as the backbone of early high-speed data communications. Let’s delve deeper into their applications:

  1. Long-Distance Telecommunication: OC-x was fundamental in underpinning national and international telecommunication infrastructures. They were deployed in both public networks and large enterprise settings, proving especially useful for cross-border data exchange.
  2. ATM Networks: OC-x standards were a key enabler of Asynchronous Transfer Mode (ATM) networks, which demanded high-speed, low-latency data transmission. Financial institutions, internet service providers, and large corporations relied on ATM networks, underpinned by OC-x, for secure and fast data transfer.
  3. Data Centers: High-speed links between data centers were almost exclusively provided by OC-x during its peak years. These links were critical for real-time data replication and disaster recovery operations.

4. T-Carrier System: A Comparison

The T-Carrier system was another milestone in digital transmission but had some key differences and limitations compared to OC-x:

  • Transmission Medium: While T-Carrier predominantly utilized copper wiring, OC-x took advantage of the higher bandwidth and lower attenuation features of fiber optics.
  • Data Rates: The T-Carrier system had limitations in terms of data rates. For instance, a T1 line offers 1.544 Mbps, which is dwarfed by even the lowest OC-1 rate of 51.85 Mbps.
  • Flexibility: OC-x offered a broad range of data rates and was far more scalable. For example, you could easily scale from OC-1 to OC-192 as demand increased, providing greater flexibility for network engineers.
  • Equivalences: Generally speaking, a T3 line (45 Mbps) was considered equivalent to an OC-1, and a T4 line (274.176 Mbps) to an OC-3. However, these are rough equivalences and not exact matches.

5. Modern Alternatives

Although OC-x levels served their purpose well, several technologies have since taken the helm:

  1. Dense Wavelength Division Multiplexing (DWDM): This technology allows multiple data streams to be sent simultaneously over the same fiber, but at different wavelengths. This dramatically increases the total capacity of the fiber. (learn more)
  2. Ethernet Over Fiber: Many organizations are now moving to Ethernet over fiber for its ease of integration with existing Ethernet LANs. It offers a seamless transition from internal to external networks while providing similar, if not better, data rates.
  3. MPLS Networks: Multi-Protocol Label Switching (MPLS) offers a more flexible approach to routing, allowing for better optimization and management of traffic flows. This makes it ideal for modern, complex networks that require fine-grained traffic engineering.

With these advancements, OC-x and T-Carrier systems have mostly been relegated to legacy status but serve as a crucial part of telecommunications history.

6. Conclusion

While OC-x levels paved the way for high-speed, long-distance data transmission, newer technologies have since eclipsed them. However, understanding OC-x and its counterpart, the T-Carrier system, provides historical context and foundational knowledge for telecom professionals.

7. References