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SONET defines the Synchronous Transport Signal Level-N (STS-N) as the electrical signal. When converted to an optical signal for transport over a standard fiber optic medium, the term Optical Carrier-N (OC-N) is applied. As shown in Table 10.2, the basic building block of the digital hierarchy is OC-1 at 51.84 MbpsSONET begins at broadband levels. Notably, the various STS levels are considerate of the existing digital signal hierarchy, thereby achieving backward compatibility with legacy systems. For example, a T3 frame maps comfortably into a STS-1 signal which becomes an OC-1 frame. Similarly, multiple T1 frames can be aggregated to form and map into an STS-1 signal, which then becomes an OC-1 frame.
Optical Carrier (OC) Level | SONET STS Level | SDH STM Level | Signal Level | Equivalent DS3 (45 Mbps) Channels | Equivalent DS0 (64 Kbps) Channels |
---|---|---|---|---|---|
OC-1 | STS-1 | 51.84 Mbps | 1 | 672 | |
OC-2 | STS-2 | 103.68 Mbps | 2 | 1,344 | |
OC-3* | STS-3 | STM-1 | 155.52 Mbps | 3 | 2,016 |
OC-4 | STS-4 | STM-3 | 207.36 Mbps | 4 | 2,688 |
OC-9 | STS-9 | STM-3 | 466.56 Mbps | 9 | 6,048 |
OC-12 | STS-12 | STM-4 | 622.08 Mbps | 12 | 8,064 |
OC-18 | STS-18 | STM-6 | 933.12 Mbps | 18 | 12,096 |
OC-24 | STS-24 | STM-8 | 1.24416 Gbps | 24 | 16,128 |
OC-36 | STS-36 | STM-12 | 1.86624 Gbps | 36 | 24,192 |
OC-48 | STS-48 | STM-16 | 2.48832 Gbps | 48 | 32,256 |
OC-96 | STS-96 | STM-32 | 4.976 Gbps | 96 | 64,512 |
OC-256** | STS-256 | 13.2192 Gbps | 256 | 171,360 | |
Note: ANSI also has specified OC-192, which operates at a line rate of 9.953278 Gbps, providing the equivalent of 5,376 DS1s and 192 DS3s. The ITU-T has not formalized that standard on an international basis. |
*OC-3 was defined by the CCITT as the basic transport rate for B-ISDN [10-17].
**OC-256 is the theoretical maximum transmission rate of SONET; it has not been defined fully.
At OC-1, for instance, a T-1 bit stream of 45 Mbps is presented as a STS-1 signal of 51.84 Mbps, with an actual maximum payload rate of 49.54 Mbps [10-18]. When converted from an electrical signal to a fiber optic photonic signal, the bit stream is known as OC-1. The OC-1 comprises 810-byte frames transmitted at a rate of 8,000 frames per second. SONET speeds range from OC-1 (51.84 Mbps) to OC-96 (4.976 Gbps), as currently defined in full. The maximum theoretical speed is that of OC-255 (13.2192 Gbps). Fractional speeds are achievable at virtually any level, with subrate transmissions below OC-1 being multiplexed to form an OC-1 channel.
SONET terms of significance include the following:
SONET networks are highly redundant, with dual fibers providing backup. Although they can be laid out in a linear, star, ring or hybrid fashion, the optimum topology is a dual, counter-rotating ring. In a SONET ring, one fiber transmits in one direction, with the other transmitting in the other direction. Through this layout, it is highly unlikely that any device on the network can be isolated through a catastrophic failure, such as a cable-seeking backhoe. There are two primary implementations of the SONET physical topology, path-switched, and line-switched rings.
Path-Switched Rings
These employ two fibers. As illustrated in Figure 10.3, all traffic moves in both directions, thereby providing protection from network failure. A path-switched approach also improves error performance, as the receiving stations examine both data streams, selecting the better signal.
Figure 10.3 Path-switched SONET implementation.
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