PDF of this tutorialAcceptable test results for optical broadband interfaces differ depending on which optical interface classification the component or network element is designed for. Interface classifications specify a number of things, including whether the component or network element is intended for use in short-haul or long-haul applications, for low or high data rates, and what wavelength will be used. These requirements determine the different expected test results.
The SONET Interface Standard
The SONET standard defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme that simplifies multiplexing.
The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as synchronous transport signal level 1 (STS–1), which operates at 51.84 Mbps. Higher-level signals are integer multiples of STS–1, creating the family of STS–N signals in Table 1. An STS–N signal is composed of N byte-interleaved STS–1 signals. This table also includes the optical counterpart for each STS–N signal, designated optical carrier level N (OC–N).
| SONET Signal | Bit Rate (Mbps) | Capacity |
| STS–1, OC–1 | 51.840 | 28 DS–1s or 1 DS–3 |
| STS–3, OC–3s | 155.520 | 84 DS–1s or 3 DS–3s |
| STS–12, OC–12 | 622.080 | 336 DS–1s or 12 DS–3s |
| STS–48, OC–48 | 2488.320 | 1344 DS–1s or 48 DS–3s |
| STS–48, OC–48 | 2488.320 | 1344 DS–1s or 48 DS–3s |
Table 1. SONET Hierarchy
The SONET interface standard for optical signals allows for three adopted and approved classifications based upon distance (see Table 2). Short reach (SR) is intended for very short interconnect distances, typically less than 2 km. Intermediate reach (IR) is intended for distances up to approximately 15 km. Long reach (LR) (without an optical amplifier) distances are 40 km for 1310 nm wavelength and 60 km for 1550 nm wavelength. At this time the very long reach (VR) classification is being studied. This classification offers the promise of interconnects spanning up to 160 km at 1550 nm.
| Application | |||||||||
| Short Reach | Intermediate Reach | Long Reach | Very Long Reach | ||||||
| Distance (km) | less than or equal to 2 | 15 | 40 | 60 | 120 | 160 | |||
| Wavelength (nm) | 1310 | 1310 | 1550 | 1310 | 1550 | 1310 | 1550 | ||
| OC–1 | SR | IR–1 | IR–2 | LR–1 | LR–2 | LR–3 | VR–1 | VR–2 | VR–3 |
| OC–3 | SR | LR–1 | LR–2 | LR–3 | VR–1 | VR–2 | VR–3 | ||
| OC–12 | SR | IR–1 | IR–2 | LR–1 | LR–2 | LR–3 | VR–1 | VR–2 | VR–3 |
| OC–48 | SR | IR–1 | IR–2 | LR–1 | LR–2 | LR–3 | VR–1 | VR–2 | VR–3 |
| OC–192 | — | IR–1 | IR–2 | LR–1 | LR–2 | LR–3 | VR–1 | VR–2 | VR–3 |
Table 2. SONET Interface Classifications
The SDH Interface Standard
The SDH standard is a worldwide standard designed to address the interworking between worldwide and ANSI transmission hierarchies. (SONET, therefore, can be considered a subset of SDH.)
The SDH interface standard (ITU–G.957) allows for three classifications based upon distance. Intraoffice (I) is intended for very short interconnect distances, typically less than 2 km. Short haul (S) is intended for distances up to approximately 15 km. Long haul (L) distances are 40 km for 1310 nm wavelength and 60 km for 1550 nm wavelength. At this time the very long haul (V) and ultra long haul (U) classifications are being studied. These offer the promise of interconnects spanning up to 160 km (see Table 3).
| Application | ||||||||
| Intra-Office | Short Haul | Long Haul | Very Long | Ultra Long | ||||
| Distance (km) | less than or equal to 2 | 15 | 40 | 60 | 120 | 160 | ||
| Wavelength (nm) | 1310 | 1310 | 1550 | 1310 | 1550 | 1550 | ||
| STM–1 | I-1 | S–1.1 | S–1.2 | L–1.1 | L–1.2 | L–1.3 | — | — |
| STM–4 | I-4 | S–4.1 | S–4.2 | L–4.1 | L–4.22 | L–4.3 | — | — |
| STM–16 | I–16 | S–16.1 | S–16.2 | L–16.1 | L–16.2 | L–16.3 | V–16.3 | U–16.3 |
| STM–64 | I–64 | S–64.1 | S–64.2 | L–64.1 | L–64.2 | L–64.3 | V–64.3 | — |
Table 3. SDH Interface Classifications
The bit rate and capacity of SDH signals are summarized in Table 4.
| SDH Signal | Bit Rate (Mbps) | Capacity |
| STM–0 | 51.840 | 28 E-1s |
| STM–1 | 155.520 | 84 E-1s or 1 E-4 |
| STM–4 | 622.080 | 252 E-1s or 4 E-4s |
| STM–16 | 2488.320 | 1008 E-1s or 16 E-4s |
| STM–16 | 9953.280 | 4032 E-1s or 64 E-4s |
Table 4. SDH Hierarchy
SONET and SDH converge at the 52 Mbps level, sometimes referred to as synchronous transport module (STM)–0. The base level for SDH is STM–1, which is equivalent to SONET’s STS–3 (3 x 51.84 Mbps = 155.5 Mbps). By changing the SONET standard from bit interleaving to byte interleaving, it became possible for SDH to accommodate both transmission hierarchies.
SDH allows nonsynchronous 2 Mbps tributaries to be multiplexed to the STM–1 level in a single step. SDH recommendations define methods of subdividing the payload area of an STM–1 frame in various ways so that it can carry combinations of synchronous and asynchronous tributaries. Using this method, synchronous transmission systems can accommodate signals generated by equipment operating from various levels of the nonsynchronous hierarchy.
