SDH / SONET Explained in Functional Models: Modeling the Optical Transport Network

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Bit errors, of the transfer process. Functional modeling 23 Two basic transport entities can be distinguished based on the capability to monitor the integrity of the transferred information. Connections can be distinguished further by the topological component to which they belong: Therefore, from a transport network functional viewpoint the adaptation function falls between the layer networks. All the reference points belonging to a single layer network can be visualized as lying on a single plane.

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In packet-oriented data transmission, such as Ethernet , a packet frame usually consists of a header and a payload. The header is transmitted first, followed by the payload and possibly a trailer , such as a CRC. In synchronous optical networking, this is modified slightly. The header is termed the overhead , and instead of being transmitted before the payload, is interleaved with it during transmission.

Synchronous optical networking

Part of the overhead is transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted. For STS-1, the frame is transmitted as three octets of overhead, followed by 87 octets of payload. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload. The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC designation refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: The first nine columns of each frame make up the section overhead and administrative unit pointers, and the last columns make up the information payload. Thus, an OC-3 circuit can carry Carried within the information payload, which has its own frame structure of nine rows and columns, are administrative units identified by pointers.

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Also within the administrative unit are one or more virtual containers VCs. VCs contain path overhead and VC payload. The first column is for path overhead; it is followed by the payload container, which can itself carry other containers. Administrative units can have any phase alignment within the STM frame, and this alignment is indicated by the pointer in row four.

The overheads contain information from the transmission system itself, which is used for a wide range of management functions, such as monitoring transmission quality, detecting failures, managing alarms, data communication channels, service channels, etc. The transport overhead is used for signaling and measuring transmission error rates , and is composed as follows:. Data transmitted from end to end is referred to as path data.

It is composed of two components:. For STS-1, the payload is referred to as the synchronous payload envelope SPE , which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of bytes. Note that wherever the line or path is terminated, the section is terminated also.

SONET regenerators terminate the section, but not the paths or line. Higher-speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. The highest rate commonly deployed is the OC or STM circuit, which operates at rate of just under DWDM circuits are the basis for all modern submarine communications cable systems and other long-haul circuits.

Another type of high-speed data networking circuit is 10 Gigabit Ethernet 10GbE. User throughput must not deduct path overhead from the payload bandwidth, but path-overhead bandwidth is variable based on the types of cross-connects built across the optical system. The physical layer refers to the first layer in the OSI networking model. The photonic layer is the lowest SONET layer and it is responsible for transmitting the bits to the physical medium.

The section layer is responsible for generating the proper STS-N frames which are to be transmitted across the physical medium.

It deals with issues such as proper framing, error monitoring, section maintenance, and orderwire. The line layer ensures reliable transport of the payload and overhead generated by the path layer. It provides synchronization and multiplexing for multiple paths. It modifies overhead bits relating to quality control.

It takes data to be transmitted and transforms them into signals required by the line layer, and adds or modifies the path overhead bits for performance monitoring and protection switching. To handle all of the possible management channels and signals, most modern network elements contain a router for the network commands and underlying data protocols.

Nevertheless, as network architectures have remained relatively constant, even newer equipment including multi-service provisioning platforms can be examined in light of the architectures they will support. Thus, there is value in viewing new, as well as traditional, equipment in terms of the older categories. Traditional regenerators terminate the section overhead, but not the line or path. Regenerators extend long-haul routes in a way similar to most regenerators, by converting an optical signal that has already traveled a long distance into electrical format and then retransmitting a regenerated high-power signal.

Since the late s, regenerators have been largely replaced by optical amplifiers. Also, some of the functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems. STS multiplexer and demultiplexer provide the interface between an electrical tributary network and the optical network.

Description

Add-drop multiplexers ADMs are the most common type of network elements. Traditional ADMs were designed to support one of the network architectures, though new generation systems can often support several architectures, sometimes simultaneously. ADMs traditionally have a high-speed side where the full line rate signal is supported , and a low-speed side , which can consist of electrical as well as optical interfaces.

The low-speed side takes in low-speed signals, which are multiplexed by the network element and sent out from the high-speed side, or vice versa. Advanced DCSs can support numerous subtending rings simultaneously. These architectures allow for efficient bandwidth usage as well as protection i.

Switching is based on the line state, and may be unidirectional with each direction switching independently , or bidirectional where the network elements at each end negotiate so that both directions are generally carried on the same pair of fibers. In unidirectional path-switched rings UPSRs , two redundant path-level copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines which copy has the highest quality, and uses that copy, thus coping if one copy deteriorates due to a broken fiber or other failure.

UPSRs tend to sit nearer to the edge of a network, and as such are sometimes called collector rings. Any other nodes on the ring could only act as pass-through nodes. Bidirectional line-switched ring BLSR comes in two varieties: BLSRs switch at the line layer. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring on the protection fibers. BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability to support "extra traffic" that can be pre-empted when a protection switching event occurs.

SDH/SONET explained in functional models: modeling the by Huub van Helvoort

In four-fiber ring, either single node failures, or multiple line failures can be supported, since a failure or maintenance action on one line causes the protection fiber connecting two nodes to be used rather than looping it around the ring. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities. Because a BLSR does not send redundant copies from ingress to egress, the total bandwidth that a BLSR can support is not limited to the line rate N of the OC- N ring, and can actually be larger than N depending upon the traffic pattern on the ring.

The worst case is when all traffic on the ring egresses from a single node, i.

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