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Optical Network Technical Background

Optical Network: Optical Network (ONW), whether Long-Distance or Junction, is simply defined as a collection of Optical Nodes (ON) interconnected by Optical Circuits (OC), so as to enable communication between them. Optical Nodes (ON) and Optical Circuits (OC) are defined as basic transmission entities. Optical networks are actually materialized in the physical world into active optical equipment. Optical circuits are materialized into passive fiber materials, such as Fiber Optic Cables, splice closures, etc.

Optical Nodes (ON) and Optical Circuits (OC) are defined as basic transmission entities. Optical networks are actually materialized in the physical world into active optical equipment. Optical circuits are materialized into passive fiber materials, such as Fiber Optic Cables, splice closures, etc.
Optical Network Technical Background
Optical Network details

Optical Node (ON)

Optical Node (ON) is a generic term used in this guideline to describe the nodes participating in the Optical Network (ONW). Optical Nodes (ON) are actually materialized as telecom equipment with optical transmission capabilities. Three types of Optical Nodes (ON) are distinguished in this guideline:

➥Optical Service Node (OSN).
Optical Expansion Node (OEN).
Optical Regeneration Node (ORN).

Optical Network (ONW) is defined by its Physical Network Topology (PNT). Several PNTs are described in the technical literature. However, in this guideline, only three PNTs are emphasized.

1. Point-to-Point (P2P): P2P is the more simple and plain topology that can be designed. It contains only two Optical Nodes (ON) interconnected with a single Optical Circuit (OC).

2. Protected Point-to-Point (PP2P): Two Optical Nodes (ON) interconnected with two different Optical Circuits (OC) is renamed as "Protected Point-to-Point" (PP2P) because one Optical Circuit behaves as a redundant alternative for the main Optical Circuit.
3. Ring: Ring topology contains 3 or more Optical Nodes (ON) interconnected forming a ring shape. Thus, the first and the last node coincide. The sequence in which the ONs are connected in the ring might be a design requirement.
(a) Point-to-Point (b) Protected Point-to-Point (c) Ring
(a) Point-to-Point (b) Protected Point-to-Point (c) Ring

Optical Service Node (OSN)

Optical Service Node (OSN) is a term used to describe a type of Optical Node (ON), wherein telco connectivity end-services must be provided. The location of OSNs is fixed
and predetermined, and thus, can’t be changed during the F-OSP design stage.

Optical Expansion Network (OEN)

Optical Expansion Node (OEN) is a term used to describe a type of Optical Node (ON) addressed to provide telco connectivity end-services in the future to an upcoming network expansion that will start from that location. An Optical Expansion Node is actually a virtual node whose existence is now anticipated. In practice, an OEN is an abstract network entity, usually on top of an existent underground civil network infrastructure, such as a manhole or vault.

Optical Regeneration Node (ORN)

Optical Regeneration Node (ORN) is a term used to describe a special type of Optical Node (ON) required for restoring the quality of the optical signal after being worsened by the effect of fiber transmission impairments. Therefore, the inclusion of ORN is driven solely by engineering needs.

The quality of optical signals is worsened progressively by fiber transmission impairments such as attenuation, dispersion and non-linearity. Therefore, optical signals have to be corrected or regenerated at every certain distance, wherever those fiber impairments degrade the signal quality below a specific threshold. Used in a broad meaning, this distance is called Regeneration Length (RL).

The need for ORNs is only determined after engineering the Optical Circuits (OC). The equipment is typically installed in ORNs: Optical Amplifiers, Dispersion Compensators, Optical-Electro-Optical Regenerators.

Junction Networks are unlikely to require plain ORN because those networks are usually formed with many OSNs separated by a short span. Instead, ORN are frequent in Long-Distance Networks because the distance between Optical Service Nodes (OSN) is longer, usually exceeding the Regeneration Length (RL).

The addition of ORNs increases the number of Optical Circuits (OC) of the original network.
With the inclusion of an ORN, a single OC is split now into two contiguous OCs at each side of the added ORN.

Optical Circuit (OC)

Optical Circuit (OC) is a generic term used to describe an all-optical end-to-end transmission line interconnecting two Optical Nodes (ON). Each OC is engineered as a single entity. Optical circuit consist of-

➥The IDs of the Optical Nodes (ON) at both ends.
➥The Transmission Technology
➥The Fiber-Type and Fiber-Count N.
➥The Regeneration Length (RL).

Optical Circuit technical definition
Optical Circuit

Optical Circuit Transmission Technology

The Transmission Technology used in Optical Circuits (OC) is dictated by the network
architecture and requirements of telecom operators. A number of different standardized technologies are available in both Long-Distance and Junction networks, such as WDM (coherent and not coherent), OTN, SDH, Ethernet (at various available bit rates), etc.

The Transmission Technology may condition or determine the Fiber-Type, the Fiber-Count and the Regeneration Length (RL) of the Optical Circuit (OC) to be designed.

Transmission Technology may be still unknown at early design stages, especially if Optical
Expansion Nodes (OEN) are included in the network. In order to be conservative in those
occasions, the affected Optical Circuits (OC) will be designed according to the most restrictive Transmission Technology.

Optical Circuit Fiber-Type and Fiber-Count

The Fiber-Type has to be selected appropriately according to the Transmission Technology* and the optical interfaces installed in the network terminals. For new network deployments, the Fiber-Type selected must be in the list of approved fiber types contained in the Materials Technical Specifications for Long-Distance or Junction Networks.

An Optical Circuit (OC) is transmitted over a generic number N of fiber-strands. That number is usually 2 (one fiber for each transmission direction), but some network interfaces allow to use only 1 fiber (both directions in the same fiber). However, as a broad definition used in this guideline, for each OC, the Fiber-Count can be defined by design as any number N. The number N may include some extra/spare fibers to provide future capacity, network upgrade, or to provision network resiliency capabilities.

Optical Circuit Regeneration Length

The Regeneration Length (RL) is the maximum optical transmission distance allowable for an Optical Circuit (OC) before the fiber transmission impairments degrade its signal quality below a specific threshold. The RL is expressed as the maximum permissible fiber-strand length and usually is in a range comprised between 80 and 120 Km. Optical signals must be processed to boost their power and improve their quality before exceeding the RL quality threshold.

The term Regeneration Length (RL) is used in a broad meaning in this topic, including not only optical-electrical-optical signal regeneration, which is formally its technical definition, but also to include any other type of pure optical signal improvement (such as optical amplification or dispersion compensation).

Each Optical Circuit (OC) has its own Regeneration Length (RL), and its value depends on the Fiber-Type, Transmission Technology*, number and bit rate of the optical channels, etc. Regeneration Length (RL) determines an upper limit on the distance between two contiguous Optical Nodes (ON). Whether those nodes are separated further, an Optical Regeneration Node (ORN) must be included in an intermediate point.

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