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SONET/SDH Transmission Hierarchy

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 Mbps—SONET 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.

Table 10.2 SONET/SDH signal hierarchy

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:

  Optical Carrier (OC) is the definition of the SONET optical signal. The defined OC levels begin at OC-1 (51.84 Mbps) and culminate in OC-255 (13.2192 Gbps).
  Synchronous Transport Signal (STS) is the electrical equivalent of the SONET optical signal; it is known as Synchronous Transport Module (STM) in SDH. The signal begins as electrical and is converted to optical for transmission over the SONET fiber facilities. Each STS-1 frame is transmitted each 125µs, yielding raw bandwidth of 51.84 Mbps. The STS frame includes five elements, Synchronous Payload Envelope, Section Overhead, Line Overhead Path Overhead, and Payload.
  Synchronous Payload Envelope (SPE) is the envelope that carries the user payload data. It is analogous to the payload envelope of a X.25 packet. The SPE consists of 783 octets (87 columns and 9 rows of data octets).
  Transport Overhead (TOH) consists of Section Overhead and Line Overhead.
  Section Overhead (SOH) of 9 octets is dedicated to the transport of status, messages, and alarm indications for the maintenance of SONET links.
  Line Overhead (LOH) of eighteen (18) bytes controls the reliable transport of payload data between network elements.
  Path Overhead (POH), contained within the SPE, comprises 9 octets for the relay of OAMandP information in support of end-to-end network management.
  Payload is the actual data content of the SONET frame and rides within the SPE. Total usable payload at the OC-1 level consists of up to 49.54 Mbps, into which a T3 frame fits quite nicely. The balance of the 51.84 Mbps is consumed by Transport Overhead and Path Overhead.
  Multiplexing is on the basis of direct time division multiplexing. Either full SONET speeds or lesser asynchronous and synchronous data streams can be multiplexed into the STS-N payload, which is then converted into an OC-N payload. In other words, an appropriate combination of FDS-1, DS-1, DS-2 and DS-3 signals can be directly multiplexed into an electrical STS-1 payload, which is then converted into an optical OC-1 payload. The multiplexing process involves byte-interleaving, much as described for traditional TDMs.

SONET/SDH Topology

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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