Hubbry Logo
Optical Carrier transmission ratesOptical Carrier transmission ratesMain
Open search
Optical Carrier transmission rates
Community hub
Optical Carrier transmission rates
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Optical Carrier transmission rates
Optical Carrier transmission rates
Optical Carrier transmission rates
View on Wikipedia
from Wikipedia

Optical Carrier transmission rates are a standardized set of specifications of transmission bandwidth for digital signals that can be carried on Synchronous Optical Networking (SONET) fiber optic networks.[1] Transmission rates are defined by rate of the bitstream of the digital signal and are designated by hyphenation of the acronym OC and an integer value of the multiple of the basic unit of rate, e.g., OC-48. The base unit is 51.84 Mbit/s.[2] Thus, the speed of optical-carrier-classified lines labeled as OC-n is n × 51.84 Mbit/s.

Optical Carrier specifications

[edit]

Optical Carrier classifications are based on the abbreviation OC followed by a number specifying a multiple of 51.84 Mbit/s: n × 51.84 Mbit/s => OC-n. For example, an OC-3 transmission medium has 3 times the transmission capacity of OC-1.

OC-1

[edit]

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.

OC-3

[edit]

OC-3 is a network line with transmission data rate 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).

OC-3c / STM-1

[edit]

OC-3c (c stands for "concatenated") concatenates three STS-1 (OC-1) frames into a single OC-3 look alike stream. The three STS-1 (OC-1) streams interleave with each other so that the first column is from the first stream, the second column is from the second stream, and the third 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-4

[edit]

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 were commonly used by ISPs as wide area network (WAN) connections, or connecting xDSL customers to a larger internal network [3]

This connection speed was popular with mid-sized (below Tier 2) internet customers, such as web hosting companies or smaller ISPs buying service from larger ones.

OC-24

[edit]

OC-24 is a network line with transmission speeds of up to 1244.16 Mbit/s (payload: 1202.208 Mbit/s (1.202208 Gbit/s); overhead: 41.472 Mbit/s). Implementations of OC-24 in commercial deployments are rare.

OC-48 / STM-16 / 2.5G SONET

[edit]

OC-48 is a network line with transmission speeds of up to 2488.32 Mbit/s (payload: 2405.376 Mbit/s (2.405376 Gbit/s); overhead: 82.944 Mbit/s).

With relatively low interface prices, with being faster than OC-3 and OC-12 connections, and even surpassing gigabit Ethernet, OC-48 connections are used[when?] 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 a transmission speed for tributaries from OC-192 nodes in order to optimize card slot utilization where lower speed deployments are used. Slower cards that drop to 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 terminal would not allow for full use of the available bandwidth due to the number of cards that would be required.

OC-192 / STM-64 / 10G SONET

[edit]

OC-192 is a network line with transmission speeds of up to 9953.28 Mbit/s (payload: 9510.912 Mbit/s (9.510912 Gbit/s); overhead: 442.368 Mbit/s).

A standardized variant of 10 Gigabit Ethernet, called WAN PHY, is designed to inter-operate with OC-192 transport equipment while the common version of 10 Gigabit Ethernet 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.

OC-768 / STM-256 / 40G SONET

[edit]

OC-768 is a network line with transmission speeds of up to 39,813.12 Mbit/s (payload: 38,486.016 Mbit/s (38.486016 Gbit/s); overhead: 1,327.104 Mbit/s (1.327104 Gbit/s)).[4][5]

On October 23, 2008, AT&T announced the completion of upgrades to OC-768 on 80,000 fiber-optic wavelength miles of their IP/MPLS backbone network.[6] OC-768 SONET interfaces have been available with short-reach optical interfaces from Cisco since 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. In November 2008, an OC-768 connection was successfully brought up on the TAT-14/SeaGirt transatlantic cable,[7] the longest hop being 7,500 km.

OC-3072 / STM-1024 / 160G SONET

[edit]

OC-3072 is a network interface with transmission speeds of 159,252 Mbit/s (payload 153,944,064 Mbit/s). [8]

OC-12288 / STM-4096 / 640G SONET

[edit]

OC-12288 is a network interface with transmission speeds of 637,009.92 Mbit/s.[citation needed]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Optical Carrier transmission rates

View on Grokipedia
from Grokipedia
Optical Carrier (OC) transmission rates are a hierarchy of standardized bit rates defined for optical signals in Synchronous Optical Networking (SONET), a protocol suite for high-speed synchronous transmission of digital traffic over optical fiber in telecommunications networks. These rates, denoted as OC-n where n represents the level, start with the base rate of OC-1 at 51.84 Mbps and scale as integer multiples of this value, providing a modular framework for aggregating and transporting multiple lower-speed signals with built-in redundancy and network management capabilities. Developed to enable reliable, high-capacity backbone transport, OC rates support applications from local area networks to long-haul intercontinental links, though they have largely been supplanted by denser wavelength technologies in modern deployments. SONET, including its OC transmission rates, originated in the early as a response to the limitations of the (PDH), which suffered from issues in diverse carrier signals. The standard was formulated by the Exchange Carriers Standards Association (ECSA) for the (ANSI) and first published in 1988, with key specifications outlined in the ANSI T1.105 series of documents. Internationally, aligns with the Synchronous Digital Hierarchy (SDH) standard developed by the (), where OC-3 corresponds to (155.52 Mbps), OC-12 to , and so on, facilitating global . This ensures precise timing across the network, minimizing bit errors and enabling efficient add-drop at intermediate nodes. The OC hierarchy encompasses a range of levels to accommodate varying bandwidth needs, with the most commonly deployed rates summarized below:
OC LevelLine Rate (Mbps)Common Applications
OC-151.84Basic SONET entry point, equivalent to T3/DS3
OC-3155.52 networks, video transport
OC-12622.08Regional backbones, early peering
OC-482,488.32High-capacity inter-city links
OC-1929,953.28Long-haul core networks
OC-76839,813.12Ultra-high-speed research and legacy upgrades
These rates include overhead for framing, error correction, and management, with payload capacities slightly lower (e.g., OC-1 payload is 50.112 Mbps). While OC-9, OC-18, OC-24, OC-36, and OC-96 exist as defined multiples, they are less common and often considered "orphaned" in practice due to limited equipment support. Overall, OC transmission rates laid the foundation for modern optical networking by introducing standardized, scalable transport that supported the explosive growth of data traffic in the late 20th and early 21st centuries.

Overview and Background

Definition and Purpose

Optical Carrier (OC) transmission rates refer to a standardized hierarchy of optical signal levels defined within the (SONET) framework, which specifies line rates for high-speed data transmission over networks. Developed by the (ANSI), SONET establishes OC-n levels where "n" denotes the multiplexing multiplier applied to the base rate, enabling consistent optical signaling for infrastructure. These rates serve as the optical counterparts to the electrical Synchronous Transport Signals (STS-n), converting electrical data streams into light pulses for fiber-optic propagation. The primary purpose of OC transmission rates is to facilitate the reliable of lower-speed digital signals, such as DS1 (1.544 Mbps) and DS3 (44.736 Mbps), into higher-capacity optical streams suitable for long-haul backbones. By aggregating these tributary signals into a unified optical carrier, OC levels support efficient bandwidth utilization across wide-area networks, allowing carriers to transport voice, data, and video services over vast distances with minimal latency. This structure ensures among diverse network equipment from multiple vendors, promoting scalable deployment in regional and national fiber-optic systems. Key benefits of OC transmission rates include synchronous timing, which aligns all network clocks to a common reference, eliminating the need for buffering and reducing signal jitter in multiplexed environments. Additionally, the embedded overhead bytes in SONET frames enable robust error detection and performance monitoring, such as parity checks and path status reporting, to maintain signal integrity over fiber links. Scalability is achieved through integer multiples of the base OC-1 rate, expressed as the line rate equation: OC-n = n × 51.84 Mbps, where n is the level multiplier, allowing seamless upgrades from basic to ultra-high-capacity transmissions without redesigning the underlying hierarchy.

Historical Development and Standardization

The development of Optical Carrier (OC) transmission rates emerged in the 1980s amid the U.S. telecommunications industry's need for standardized fiber-optic interfaces following the 1984 divestiture, which left regional Bell operating companies (RBOCs) requiring interoperable connections to multiple long-distance carriers using disparate proprietary optical (TDM) systems. Bellcore (now Telcordia Technologies), established to support the RBOCs, proposed the Synchronous Optical Network (SONET) framework in 1984-1985 as an to address these challenges, defining OC rates as the optical counterparts to SONET's electrical Synchronous Transport Signal (STS) levels, starting with OC-1 at 51.84 Mbps. Standardization efforts accelerated with ANSI initiating work in 1985 through its T1X1 committee, culminating in the approval of ANSI T1.105 in 1988, which formally specified SONET interfaces, including OC rates and formats for optical transmission. Concurrently, the ITU-T (then CCITT) began parallel efforts in 1986 to create a global equivalent, known as Synchronous Digital Hierarchy (SDH), with initial recommendations (G.707, G.708, G.709) issued in 1988 and finalized in 1990, establishing STM (Synchronous Transport Module) rates as SDH's optical equivalents (e.g., STM-1 aligning with OC-3). This led to a divergence: ANSI's SONET/OC became the North American standard, tailored to the 1.5/6/45 Mbps DS hierarchy, while ITU-T's SDH/STM gained international adoption, accommodating Europe's 2 Mbps E1 rates, though the two were designed for compatibility to enable global interconnectivity by the early 1990s. The transition from proprietary systems to /OC marked a shift toward vendor-neutral standards, reducing costs and fostering multi-vendor environments; Bellcore's role was pivotal in coordinating this evolution, ensuring OC rates supported efficient of lower-speed signals. Early deployments began with field trials in 1989, followed by successful multi-vendor mid-fiber meets in 1990, primarily for interoffice trunks in RBOC networks to enhance reliability and capacity. By the mid-1990s, OC rates were integral to backbone infrastructure, with widespread rollout peaking in the 2000s to support surging , where OC-48 (2.5 Gbps) and higher levels formed the core of North American data networks.

Fundamentals of SONET

Frame Structure and Overhead

The Synchronous Transport Signal level 1 (STS-1) forms the foundational unit for Optical Carrier (OC) transmission rates in Synchronous Optical Networking (SONET). It consists of 810 bytes organized into a matrix of 90 columns by 9 rows, with each frame transmitted at a fixed interval of 125 microseconds. This configuration establishes the base transmission rate of 51.84 Mbps for STS-1, derived from the frame's byte count, bit encoding, and repetition frequency. The bit rate is precisely calculated using the formula: Bit rate=810 bytes/frame×8 bits/byte125×10−6 s/frame=51.84 Mbps\text{Bit rate} = \frac{810 \, \text{bytes/frame} \times 8 \, \text{bits/byte}}{125 \times 10^{-6} \, \text{s/frame}} = 51.84 \, \text{Mbps} The frame divides into transport overhead and synchronous payload envelope (SPE), with the transport overhead spanning the first three columns (27 bytes total). This overhead splits into section overhead (rows 1–3, 9 bytes) for functions between network elements and line overhead (rows 4–9, 18 bytes) for and supervision between multiplexers. Section overhead includes A1 and A2 bytes for framing alignment, ensuring by providing a fixed across all frames, and B1 for BIP-8 error monitoring, which computes even parity over the previous frame to detect bit errors. Additionally, D1–D3 bytes allocate a 192 kbps channel (DCC) for messaging, while J0 serves as a section trace identifier to verify transmission paths and Z0 reserves space for future enhancements. Line overhead encompasses H1 and H2 pointer bytes, which define the starting position of the SPE and enable dynamic adjustments for payload alignment amid clock discrepancies, and K1 and bytes for automatic protection switching (APS), which coordinate ring or path protection by signaling switch requests and channel bridging. The SPE occupies the remaining 87 columns (783 bytes), integrating path overhead (9 bytes in the first column) with the user of up to 774 bytes. This supports either direct mapping of higher-rate signals like DS3 or virtual tributary (VT) grouping for lower-speed tributaries, such as VT1.5 for DS1 signals, organized into seven VT groups within the payload area. Pointer adjustments, indicated by H1 and H2, allow the SPE to float freely within the frame or across frames, using positive stuffing (inserting an extra byte) or negative stuffing (utilizing H3 for data) to compensate for timing variations, with adjustments spaced across at least three consecutive frames for stability. Error detection relies on BIP-8 mechanisms embedded in overhead bytes: B1 for section-level parity over the entire prior frame (excluding section overhead), B2 for line-level parity (excluding transport and section overhead), and B3 within path overhead for end-to-end monitoring. For fault recovery, K1 and K2 bytes enable APS protocols, where K1 encodes protection switch requests (e.g., signal failure or degradation) and K2 specifies the bridging channel, facilitating rapid traffic rerouting in linear or ring topologies.

Multiplexing and Hierarchy

In SONET, synchronous combines multiple lower-rate signals into higher-rate aggregates through byte-by-byte interleaving, where the transport overhead of signals is frame-aligned before interleaving their payloads. This process forms an STS-n signal from n individual streams, maintaining synchronization across the to support efficient scaling without significant buffering delays. The hierarchy establishes at 51.84 Mbps as the base level, with higher aggregates such as , STS-12, and beyond created by integer multiples of via the interleaving method specified in ANSI T1.105. These electrical STS-n signals are mapped to corresponding optical OC-n carriers for transmission, with the overall rate scaling linearly as n×51.84n \times 51.84 Mbps and overhead bytes distributed proportionally to preserve frame integrity and management capabilities. Pointer mechanisms, implemented via transport overhead bytes like H1 and H2, enable frequency justification by indicating the offset to the synchronous payload envelope (SPE) and supporting positive or negative to align plesiochronous tributaries. This allows dynamic adjustments for timing variations without disrupting data flow, as outlined in ANSI T1.105. In concatenated modes, such as OC-3c, multiple SPEs are merged into a single contiguous payload envelope, bypassing individual pointers to deliver unbroken high-capacity channels suitable for services like . The inherent synchronous architecture of , bolstered by these pointers, controls and wander by compensating for low-frequency phase drifts and high-frequency variations through minimal elastic store buffering, thereby ensuring reliable end-to-end timing across network elements.

OC Rate Levels

OC-1

OC-1 represents the foundational transmission rate in the Synchronous Optical Network () hierarchy, operating at a line rate of 51.84 Mbps, with a capacity of 50.112 Mbps and an overhead of 1.728 Mbps. This rate serves as the base unit for all higher-level optical carriers in , enabling the synchronous transport of digital signals over . As the optical counterpart to the Synchronous Transport Signal level 1 () electrical signal, OC-1 transmits data using a single on , facilitating direct conversion from electrical to optical domains without additional at this base level. For payload mapping, OC-1 supports the encapsulation of one DS3 signal at 44.736 Mbps in a direct synchronous mapping or up to 28 DS1 signals at 1.544 Mbps each through Virtual Tributary (VT) structures, such as VT1.5 for individual DS1s, allowing flexible grooming of lower-speed tributaries. In typical applications, OC-1 was employed for short-haul links in early deployments and laboratory testing to validate network and , though it is rarely deployed as a standalone rate in modern networks due to the prevalence of higher aggregated speeds. Optically, OC-1 operates over single-mode fiber at wavelengths of 1310 nm for shorter reaches or 1550 nm for extended distances, with transmitter power levels and receiver sensitivities defined in accordance with Telcordia GR-253-CORE specifications to ensure interoperability and reliability.

OC-3

OC-3, or Optical Carrier level 3, represents a fundamental multiplexed rate in the , serving as an entry-level optical signal for transporting multiple lower-rate tributaries over fiber optic links. It operates at a line rate of 155.52 Mbps, achieved by multiplexing three base-rate signals each at 51.84 Mbps. Of this total capacity, the envelope provides 150.336 Mbps for user data, while 5.184 Mbps is allocated to overhead for transport, path, and section monitoring functions. The structure of OC-3 involves byte-interleaving three independent frames to form a single signal, enabling flexible mapping of lower-speed services without concatenation. This non-concatenated configuration supports virtual tributary (VT) mapping, where T1 signals (1.544 Mbps) are accommodated via VT1.5 structures and E1 signals (2.048 Mbps) via VT2 structures, allowing up to 84 T1s or 63 E1s across the three s. Additionally, it permits direct mapping of DS3 signals (44.736 Mbps) into the synchronous payload envelope (SPE) of each , facilitating efficient transport of higher-capacity digital streams. In practical deployments, OC-3 has been extensively applied in ring architectures, leveraging SONET's self-healing capabilities to ensure network resilience against fiber cuts or equipment failures. It played a key role in early provisioning, delivering scalable bandwidth for emerging data services in the 1990s and early 2000s. Furthermore, OC-3's 155 Mbps rate aligns directly with (ATM) user network interfaces, enabling seamless integration for voice, video, and packet transport in hybrid environments. While many optical interfaces implement the concatenated OC-3c variant for undivided payloads, the standard OC-3 emphasizes the non-concatenated form to support channelized operations across the three STS-1s. The signal is engineered for high reliability, targeting a (BER) below 10−910^{-9} under normal operating conditions, with (FEC) available as an optional enhancement for improved margin in longer spans.

OC-3c / STM-1

OC-3c represents a concatenated form of the Optical Carrier level 3 in the (SONET) standard, operating at a line rate of 155.520 Mbps with a synchronous payload envelope (SPE) dedicated entirely to data transport at 149.760 Mbps, without subdivision into virtual tributaries (VTs) that would otherwise support lower-rate multiplexing for voice or other services. This concatenation treats the three underlying signals as a single contiguous , enabling efficient carriage of high-speed data streams such as Packet over SONET (POS) or 100 Mbps , which maps directly into the SPE with minimal overhead waste after accounting for framing and error correction. In contrast to non-concatenated OC-3, which allocates portions of the payload to VTs for T1 or DS3 channels, OC-3c prioritizes undivided bandwidth for applications. The equivalent in Synchronous Digital Hierarchy (SDH), designated , aligns closely with OC-3c under the G.707 standard, utilizing an Administrative Unit type 4 (AU-4) with a pointer to justify the Virtual Container 4 (VC-4) payload, which also achieves 149.760 Mbps for asynchronous or synchronous mapping of client signals. This structure facilitates dynamic alignment between the VC-4 and the frame, accommodating rate variations in mapped traffic like 100 Mbps Ethernet via generic framing procedure (GFP). While functionally similar, subtle differences exist in overhead bytes; for instance, the A1 framing bytes in SDH follow an all-ones pattern in certain positions to distinguish regenerator section overhead, differing from SONET's byte-interleaved approach that emphasizes compatibility with existing hierarchies. STM-1 gained widespread global adoption as the foundational rate in the framework, particularly outside where SDH supplanted regional variants, serving as the baseline for international backbone and access networks throughout the and . In regions like and , equipment dominated deployments for its standardized multiplexing and management, enabling scalable transport over fiber optic links. To bridge and SDH domains in cross-border connections, interworking gateways were employed to transcode overhead and pointers, ensuring seamless integration of OC-3c and signals in multinational infrastructures during this period.

OC-12 / STM-4

The OC-12, also known as Synchronous Transport Signal level 12 (STS-12) in , operates at a line rate of 622.08 Mbps, which is derived as 12 times the base OC-1 rate of 51.84 Mbps. This rate supports mid-capacity backhaul in networks by 12 interleaved STS-1 signals or equivalently 4 OC-3 signals into a single optical carrier. The corresponding SDH equivalent is , defined in Recommendation G.707, which maintains the same line rate of 622.08 Mbps and follows a similar multiplexing structure using 4 frames. In its standard multiplexed form, the OC-12 provides a capacity of 601.344 Mbps, with 20.736 Mbps allocated to overhead for , section, line, and path management. It also supports OC-12c concatenated mode, where the 12 payloads are combined into a single contiguous Synchronous Envelope (SPE) to enable efficient of larger, undivided data streams, yielding a of 599.04 Mbps and overhead of 23.04 Mbps due to optimized path overhead allocation (one Path Overhead column per three STS-1 groups). For STM-4, the concatenated counterpart is AU-4-4c, which similarly delivers approximately 600 Mbps of capacity within the STM-4 frame for high-bitrate applications requiring minimal fragmentation. OC-12/STM-4 finds primary applications in regional networks for aggregating and transporting mid-range traffic volumes, including broadcast video signals where its capacity supports multiple streams. Common mappings include up to 10 DS3 signals (each at 44.736 Mbps) or traffic encapsulated via protocols like Generic Framing Procedure (GFP), leveraging the concatenated mode for seamless integration without excessive overhead. Optically, OC-12/STM-4 typically employs 1310 nm transceivers, achieving reaches of up to 120 km without optical amplification on low-loss single-mode , making it suitable for metro and regional spans.

OC-24

OC-24 is a transmission rate within the Synchronous Optical Network () hierarchy, providing a line rate of 1.24416 Gbps through the byte-interleaving of 24 signals, each operating at the of 51.84 Mbps. This structure allows for the synchronous of lower-rate signals while maintaining the rigid 125 μs frame timing characteristic of . The effective payload capacity of OC-24 stands at 1.202688 Gbps, supporting data transport, while 41.472 Mbps is allocated to overhead for functions such as framing, error monitoring, and . This overhead ratio ensures robust over fiber optic links but limits the usable bandwidth compared to the total line rate. In contrast to more prevalent rates, OC-24 lacks a direct equivalent in the Synchronous Digital Hierarchy (SDH) standard, though it loosely maps to two concatenated signals, each at 622.08 Mbps. This mapping arises from the bit-rate alignment but does not conform to standard SDH multiplexing hierarchies, which favor levels like , , and STM-16. Developed as a extension primarily by North American carriers for handling specific bundles of DS3 signals—accommodating up to 24 DS3 channels at 44.736 Mbps each—OC-24 served niche applications in private networks during the and early . Its deployment enabled efficient aggregation of high-volume DS3 traffic in regional backbones, yet limited vendor restricted broader adoption. As a non-ANSI-standardized rate outside the core SONET levels, OC-24 faced challenges in multi-vendor environments and was largely phased out by the early 2000s in favor of widely supported options like OC-48. Optical transmission specifications for OC-24 mirror those of OC-12—such as NRZ encoding and similar modulation—but are scaled to accommodate the doubled , requiring enhanced receiver sensitivity for longer spans.

OC-48 / STM-16

OC-48, also known as STS-48 in its electrical form, operates at a line rate of 2.488 Gbps, achieved by multiplexing 48 times the base STS-1 rate of 51.84 Mbps. The synchronous payload envelope (SPE) capacity is 2.405 Gbps, with 82.944 Mbps allocated to transport overhead; path overhead adds 27.648 Mbps in multiplexed mode, yielding a user payload of approximately 2.378 Gbps. These rates are defined in the American National Standards Institute (ANSI) T1.105 standard. OC-48 serves as a high-capacity optical interface in Synchronous Optical Network (SONET) systems, enabling efficient transport across fiber optic links. The structure of OC-48 supports byte-interleaved multiplexing of 48 individual signals or equivalently 4 OC-12 signals, allowing flexible accommodation of lower-rate tributaries through pointer-based justification as outlined in multiplexing hierarchies. For applications requiring undivided high-bandwidth channels, the concatenated variant OC-48c reduces path overhead to one column per three STS-1 (16 columns total), delivering an effective user payload of 2.396 Gbps with total overhead of 92.16 Mbps, higher than the multiplexed user payload. In the Synchronous Digital Hierarchy (SDH), the equivalent rate is STM-16, standardized by the Telecommunication Standardization Sector () in recommendation G.707, which multiplexes 16 Administrative Units level 4 (AU-4) to achieve the same 2.488 Gbps line rate. OC-48 and STM-16 are designed for seamless interworking, with mappings that preserve integrity across /SDH boundaries, facilitating global network . OC-48/STM-16 found widespread application in long-haul trunk lines during the early 2000s, serving as a backbone for aggregating and transporting high volumes of voice and data traffic in core networks. It supported early implementations of Ethernet over (EoS), enabling the carriage of emerging 10 Gbps Ethernet services via concatenated s or virtual concatenation techniques. Deployment peaked around 2000, with OC-48 units comprising the majority of new installations amid the boom. Optically, OC-48/STM-16 interfaces are compatible with dense wavelength-division multiplexing (DWDM) systems, allowing multiple channels to share a single fiber pair for enhanced capacity. Standard long-reach transceivers support unrepeated spans up to 80 km at 1550 nm wavelengths, while (FEC) enhances performance for extended distances, often exceeding 300 km in amplified DWDM configurations.

OC-192 / STM-64

OC-192, equivalent to STM-64 in the , operates at a line rate of 9.953 Gbps, achieved by 192 times the base rate of 51.84 Mbps. The synchronous envelope (SPE) capacity is 9.622 Gbps for standard multiplexed configurations, with transport overhead of 332 Mbps and path overhead of 111 Mbps, yielding a user of approximately 9.511 Gbps. In concatenated mode, designated OC-192c, path overhead is reduced to one column per three (64 columns total), enabling a user of 9.585 Gbps with total overhead of approximately 368 Mbps—higher than the multiplexed user . This level can also be formed by combining four OC-48 signals, leveraging the for scalability from lower rates. In the SDH domain, STM-64 corresponds to 64 AU-4 administrative units, providing a frame structure optimized for international compatibility and supporting mappings for 10 Gigabit Packet over SONET (10G POS) interfaces. Early implementations also facilitated mapping of 10 Gigabit Ethernet (10GE) traffic into the OC-192/STM-64 envelope, enabling efficient transport of LAN speeds over wide-area optical networks before the widespread adoption of native Ethernet optics. OC-192/STM-64 found primary applications in transcontinental backbone links during the internet expansion of the 2000s, where it handled surging data demands for and enterprise connectivity across vast distances. These deployments relied on Erbium-Doped Fiber Amplifier (EDFA) technology for signal boosting in long-haul scenarios, operating predominantly at the 1550 nm wavelength to minimize attenuation in silica fibers. Key challenges in OC-192/STM-64 systems included stringent control to maintain over high-speed paths, often requiring advanced and designs in line cards. Optical specifications typically supported spans up to 600 km before requiring electrical regeneration to restore the signal, balancing dispersion, , and nonlinear effects in dense environments.

OC-768 / STM-256

OC-768, the (SONET) designation for the highest standardized rate level, operates at a line rate of 39.813 Gbps, equivalent to 768 times the base rate of 51.84 Mbps. This configuration multiplexes 768 signals or alternatively 4 × OC-192 signals into a single stream, providing a capacity of 38.486 Gbps when fully populated with individual payloads of 50.112 Mbps each, while the overhead totals 1.327 Gbps for framing, error correction, and management functions. In the concatenated variant, OC-768c treats the entire stream as a single 40 Gbps channel, optimizing for high-capacity applications by minimizing per-channel overhead and supporting undivided payloads up to approximately 38.5 Gbps. The SDH counterpart, STM-256, mirrors this line rate of 39.813 Gbps as defined in Recommendation G.707, structured around 256 Administrative Units level 4 (AU-4), each carrying a Virtual Container-4 (VC-4) . This yields a maximum of 38.342 Gbps when exclusively using VC-4 mappings (256 × 149.76 Mbps), with an overhead of 1.472 Gbps dedicated to section, line, and path monitoring. Experimental adaptations have mapped 40 (40GE) signals into STM-256c frames, enabling transparent transport of LAN-PHY or WAN-PHY 40GE over SDH infrastructure via generic framing procedure (GFP) and virtual concatenation. Despite its potential for ultra-high capacity in dense (DWDM) channels, OC-768/STM-256 has seen primarily laboratory demonstrations and field trials rather than broad commercial rollout, with notable tests in the mid-2000s achieving transmission over 3,000 km using advanced modulation. Early trials integrated it into carrier networks for 40 Gbps DWDM lambdas, but adoption remained limited due to the rapid shift toward (OTN) standards offering greater flexibility. Deployment faces significant technical challenges, including precise management of chromatic dispersion and polarization-mode dispersion at 40 Gbps symbol rates, which demand sophisticated compensation modules to maintain over long hauls. High optical power budgets are required to overcome and nonlinear effects in , often necessitating enhanced amplification similar to that used in OC-192 systems. Prototypes employing coherent detection techniques, combining phase-sensitive receivers with , have addressed these issues in experimental setups by enabling electronic dispersion compensation and improved receiver sensitivity.

Proposed Higher Rates

While the SONET/SDH standards formally define rates up to OC-768/STM-256, conceptual extensions to higher levels such as OC-3072/STM-1024 have been proposed to support aggregate capacities exceeding 100 Gbps, primarily through byte-interleaving multiples of the base STS-1 frame. The OC-3072 rate would achieve a line rate of 159.252 Gbps, derived from 3072 × 51.84 Mbps, with an approximate payload of 154 Gbps after accounting for overhead, positioning it as a theoretical framework for multiplexing 100G+ channels in legacy SONET architectures. Further escalation to OC-12288/STM-4096 has been outlined in preliminary ANSI discussions, targeting an ultra-high line rate of 640 Gbps (12288 × 51.84 Mbps), though detailed specifications remain undeveloped beyond basic scaling principles. These proposals draw influence from G.709 (OTN) frameworks, which introduce flexible multiplexing for terabit-scale transport, but they have not been standardized within core /SDH protocols like ANSI T1.105 or G.707. Implementation faces substantial technical hurdles, including nonlinear fiber effects such as and , which degrade at elevated power levels and beyond 100 Gbps. Additionally, at terabit scales exacerbates accumulation across multiplexed tributaries, complicating pointer adjustments and frame alignment in synchronous hierarchies. Research on these rates has thus been confined to simulations, with no commercial deployments realized. By 2025, these proposed SONET extensions are effectively obsolete for practical use, overshadowed by OTN equivalents like the ODU4 container, which delivers 100 Gbps payload with enhanced and management overhead tailored for modern dense systems.

Applications and Current Status

Deployment in Networks

Optical Carrier (OC) rates have been deployed in tiered architectures within networks, with lower rates such as OC-3 and OC-12 commonly used in access and networks (MANs) for efficient bandwidth allocation in local and regional segments. Higher rates like OC-48 and OC-192 serve core and long-haul backbone applications, enabling high-capacity over extended distances. These deployments often utilize ring topologies, including Unidirectional Path Switched Rings (UPSR) for access/metro redundancy and Bidirectional Line Switched Rings (BLSR) for core/long-haul self-healing capabilities, ensuring rapid protection switching in the event of cuts. In the United States, major carriers extensively deployed OC rates from the mid-1990s through the , integrating them into national backbones for voice, data, and video services. For instance, rolled out OC-12 services with functions for private line connectivity. Internationally, deployments leveraged Synchronous Digital Hierarchy (SDH) equivalents like (OC-3) and STM-16 (OC-48) in hybrid /SDH configurations, facilitating global interoperability through common optical LSIs and ring adaptations. OC rates integrate seamlessly with Dense Wavelength Division Multiplexing (DWDM) systems to multiply capacity, where signals are transported over multiple wavelengths, often using add-drop multiplexers (ADMs) at nodes to selectively insert or extract traffic without full optical-electrical-optical conversion. A notable example is the transatlantic cable, activated in 2001, which employed 16 OC-192/STM-64 channels at 10 Gbit/s each for high-speed intercontinental connectivity across 15,428 km. In cellular backhaul, OC-48 links have supported networks by aggregating TDM and Ethernet traffic in ring topologies, providing up to 2.5 times the bandwidth of alternatives. These deployments achieve carrier-grade performance, with network availability exceeding 99.999% through redundant paths and protection mechanisms. Operations, Administration, Maintenance, and Provisioning (OAM&P) functions are enabled via Data Communications Channel (DCC) bytes in the overhead, allowing centralized monitoring and fault isolation across the network.

Legacy Role and Transition to Modern Standards

Optical Carrier (OC) transmission rates, integral to Synchronous Optical Networking (SONET) standards, achieved widespread adoption in the 2000s as the primary technology for high-speed backbone transport in telecommunications networks. By 2025, however, OC rates have transitioned to legacy status, with accelerated decommissioning efforts across major carriers, particularly for OC-48 and OC-192 levels that still comprise the bulk of remaining deployments. The decline stems from 's design limitations for modern packet-oriented data traffic, which favors circuit-switched voice and (TDM), rendering it inflexible for bursty IP-based applications dominant today. Additionally, incurs high overhead—approximately 4% of bandwidth for and —contrasting with Ethernet's greater through statistical and lower protocol encumbrance. These factors, combined with escalating maintenance costs for aging equipment and vendor end-of-support announcements, have prompted carriers to phase out OC infrastructure. Transitions from OC rates focus on adopting the (OTN) framework defined by G.709, which supports scalable rates beyond 100 Gbps, such as the ODU4 container at approximately 100 Gbps payload capacity, enabling efficient mapping of legacy signals while adding and enhanced monitoring. Parallel shifts involve direct deployment of (DWDM) Ethernet at 100 Gbps and 400 Gbps wavelengths, bypassing entirely for cost-effective, high-capacity packet transport. Migration strategies emphasize gradual integration, such as overlaying circuits onto OTN layers to preserve existing TDM services during transition, minimizing disruptions through hybrid packet-optical architectures. Many operators are planning full decommissioning, aligning with vendor support lifecycles and regulatory pushes for spectrum efficiency, though timelines vary by region and . Despite the shift, OC rates endure in niche applications, including rural networks supporting legacy voice and TDM services where upgrade costs remain prohibitive, and select submarine cable systems relying on SONET/SDH for reliable long-haul protection. These remnants often coexist with IP-over-DWDM overlays, ensuring backward compatibility in mixed environments until complete retirement.

References

  1. https://www.tek.com/en/documents/primer/sonet-telecommunications-standard-primer
  2. http://www.photonics.byu.edu/FiberStandards.parts/DataRates.phtml
  3. https://www.cisco.com/c/en/us/support/docs/optical/synchronous-optical-network-sonet/13567-sonet-tech-tips.html
  4. https://www.carrierbid.com/sonet-optical-network-advantages/
  5. https://bytesolutions.com/data-rates-standard-ds-oc-services/
  6. https://www.gigapackets.com/articles/oc3bandwidth.php
  7. http://www.sonet.com/EDU/edu.htm
  8. https://www.ee.columbia.edu/~bbathula/courses/HPCN/chap04_part-1.pdf
  9. https://www.gevernova.com/grid-solutions/sites/default/files/2025-08/SONET_101.pdf
  10. https://techheap.packetizer.com/communications/sonet/sonet_101.pdf
  11. https://www.cisco.com/c/en/us/td/docs/routers/asr903/software/guide/cem/16-12-1/b-cem-eomer-xe-16-12-1-asr900/b-cem-eomer-xe-16-11-1-asr900_chapter_0111.html
  12. https://www.cs.fsu.edu/~hawkes/sonet.html
  13. https://blog.albedotelecom.com/sdh-and-sonet/
  14. https://www.gartner.com/en/information-technology/glossary/sonet-synchronous-optical-network
  15. https://www.scaler.com/topics/synchronous-optical-network/
  16. https://www.michael-henderson.us/Papers/SONET-SDH.pdf
  17. https://www.comsoc.org/node/19936
  18. https://catalogimages.wiley.com/images/db/pdf/078031168X.excerpt.pdf
  19. https://www.mdpi.com/2673-4001/3/4/35
  20. https://www.gl.com/Presentations/SONET-SDH-Technology-Overview-Presentation.pdf
  21. https://www.cisco.com/c/en/us/support/docs/optical/synchronous-optical-network-sonet/16147-concat-16147.html
  22. https://www.cse.wustl.edu/~jain/cis777-97/ftp/g_9snt.pdf
  23. https://www.fratec.net/faq/diccionario/sonet-sdh-technical-summary/
  24. https://www.cisco.com/c/en/us/td/docs/optical/15000r5_0/planning/guide/r50engpl/r50appb.html
  25. https://www.cisco.com/c/en/us/td/docs/optical/15000r5_0/planning/guide/r50engpl/r50snttr.html
  26. https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-253
  27. https://cleanroom.byu.edu/sonet-sdh-data-rates
  28. https://infocenter.nokia.com/public/7705SAR234R1A/topic/com.nokia.interface-config-guide/sonet-ai9kfynwxp.html
  29. https://cs.wmich.edu/~alfuqaha/Fall11/cs6570/lectures/saved/sonet.pdf
  30. https://www.gl.com/Presentations/SONET-SDH-Technology-OC3-Analysis-Presentation.pdf
  31. https://www.cisco.com/c/en/us/products/interfaces-modules/packet-oc-3-interface-processors/index.html
  32. https://www.centurylink.com/techpub/77412/77412_O_v1.pdf
  33. https://www.cisco.com/c/en/us/td/docs/routers/7300/line_cards/oc3_pos_line_card/oc_pos_lc/3436ovr.html
  34. https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.707-199603-S!!PDF-E&type=items
  35. https://www.exfo.com/contentassets/b3d7164d08a94b4094f3dfbb45baa907/exfo_anote125_next-generation-sonet-sdh_en.pdf
  36. https://www.cisco.com/c/en/us/support/docs/optical/synchronous-optical-network-sonet/16180-sonet-sdh.html
  37. https://lightyear.ai/tips/sonet-versus-sdh
  38. https://ieeexplore.ieee.org/document/512692
  39. https://www.itu.int/rec/T-REC-G.707/en
  40. https://grouper.ieee.org/groups/802/3/10G_study/public/sept99/nicholl_1_0999.pdf
  41. https://netinsight.net/wp-content/uploads/2019/07/OC12-STM4_module.pdf
  42. https://www.corp.att.com/cpetesting/wp-content/themes/cpetesting/pdf/tr54077.pdf
  43. https://www.optospan.com/120km-sfp-622mb-transceiver-sfp-622-k120t55/
  44. https://www.unitconverters.net/data-transfer/oc24-to-stm-4-signal.htm
  45. https://its.ntia.gov/publications/download/SP-93_FixedServicesSpectrum.pdf
  46. https://www.researchgate.net/publication/225964190_Grooming_Mechanisms_in_SONETSDH_and_Next-Generation_SONETSDH
  47. https://solveforce.com/ocx-optical-carrier/
  48. https://webstore.ansi.org/standards/atis/ansit11051995
  49. https://newsroom.cisco.com/c/r/newsroom/en/us/a/y2001/m03/cisco-captures-number-one-market-share-in-oc-48-optical-transport-segment.html
  50. https://www.investor.agilent.com/news-and-events/news/news-details/2001/Agilent-Technologies-Ethernet-Over-SONET-Mapper-is-First-to-Meet-New-Global-Standards-Breakthrough-Technology-Expected-to-Simplify-Corporate-Network-Maintenance-and-Reduce-Costs/default.aspx
  51. https://www.cisco.com/c/en/us/td/docs/optical/15000r8_5/dwdm/reference/guide/d85refr/d85txmxp.html
  52. https://www.cisco.com/c/en/us/td/docs/optical/15000r4_6/15454/sonet/reference/guide/r46refrn/r46eoptc.html
  53. https://zytrax.com/tech/data_rates.htm
  54. https://www.gigapackets.com/articles/oc192_ethernet.php
  55. https://www.cisco.com/c/en/us/td/docs/interfaces_modules/shared_port_adapters/configuration/ASR1000/asr1000-sip-spa-book/m_asr-spa-pos-ovw.html
  56. https://apps.dtic.mil/sti/tr/pdf/ADA305913.pdf
  57. https://opg.optica.org/oe/abstract.cfm?uri=oe-26-18-24190
  58. https://www.eetimes.com/understanding-jitter-issues-in-oc-48-oc-192-line-cards/
  59. https://documentation.nokia.com/html/365-372-300R9.1/365-372-300R9-1/op-specs-oc192-ptm.html
  60. https://www.imda.gov.sg/-/media/imda/files/industry-development/infrastructure/technology/itr3opticalphotonics.pdf
  61. https://www.cisco.com/c/en/us/products/collateral/routers/carrier-routing-system/product_data_sheet0900aecd80395bbe.pdf
  62. https://www.cisco.com/c/en/us/products/collateral/optical-networking/ons-15454-series-multiservice-provisioning-platforms/data_sheet_c78-643796.html
  63. https://opg.optica.org/abstract.cfm?uri=ofc-2006-PDP30
  64. https://www.photonics.ece.mcgill.ca/Plant/dvp_pdf/CP-2008-03.pdf
  65. https://sharetechnote.com/html/OpticalComm_OCx.html
  66. https://ieeexplore.ieee.org/book/8040003
  67. https://www.fujitsu.com/global/documents/about/resources/publications/fstj/archives/vol35-1/paper03.pdf
  68. https://www.cisco.com/c/dam/global/de_at/assets/docs/dwdm.pdf
  69. https://cyberwar.nl/d/20060300_Submarine-cables_CPNI-UK.pdf
  70. https://www.fujitsu.com/ca/en/Images/NGMobileBackhaulwp.pdf
  71. https://blog.telegeography.com/why-legacy-telecom-networks-are-giving-way-to-the-future
  72. https://www.exfo.com/contentassets/5c2bc3613e9c405990157c921c747dfc/exfo_anote152_sonet-sdh-ethernet-migration-testing_en.pdf
  73. https://ptgmedia.pearsoncmg.com/images/1587050706/samplechapter/1587050706content.pdf
  74. https://www.itu.int/rec/T-REC-G.709/en
  75. https://www.packetlight.com/resources/articles/what-is-otn-a-complete-guide-to-the-optical-transport-network-layer
  76. https://www.veexinc.com/files/DirectDownload/NIC/TechnicalNotes/WhitePaper_OTN.pdf
  77. https://ribboncommunications.com/solutions/service-provider-solutions/migrating-legacy-sdhsonet-ip
  78. https://www.nec.com/en/global/techrep/journal/g10/n01/pdf/100109.pdf
Add your contribution
Related Hubs
User Avatar
No comments yet.