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.
- What are Optical Carrier Levels (OC-x)?
- OC-x Transmission Rates
- How It Was Used
- T-Carrier System: A Comparison
- Modern Alternatives
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-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:
- 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.
- 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.
- 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:
- 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)
- 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.
- 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.
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.
- “SONET/SDH Demystified“, Steven Shepard
- “Introduction to Broadband Communication Systems“, Cajetan M. Akujuobi, Mohamed A. M. Mohamed
- “ATM Theory and Applications“, David E. McDysan, Darren L. Spohn
- RFC 4842 – Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) / Circuit Emulation over Packet (CEP)