Wavelength Service in Optical Transport Networks

Optical WDM (Wavelength Division Multiplexing) networks are networks that deploy WDM optical fiber links, where each fiber connection with multi-wavelength channels. An All Optical Network (AON) is an optical WDM network, end-to-end optical paths by all optical nodes, the optical signal in the optical region remain without allowing conversion into electrical signals. AONs are generally optical circuit-switched networks, in which circuits of the intermediate nodes in the switchedGranularity of a wavelength channel. Therefore, a circuit-switched AON is also known as wavelength routing network, where optical circuits are equivalent wavelength channels.

A wavelength routing network consists of optical cross-connects (OXC) and optical add / drop multiplexer (OADM) are interconnected by WDM fibers. Transmission of data over the optical network via optical circuit-switching connections, known as lightpaths. An OXC is an N x N optical switch with N input fibers and NOutput fibers with each fiber leads W wavelengths. The OXC optical switch any incoming wavelength of its input fibers to the outgoing wave from his starting fibers. An OADM may terminate the signals on a range of wavelengths, and add new signals at these wavelengths. The remaining wavelengths pass through the OADM and transparent.

In order for a user (router A) to transfer data to a target user (router B), a circuit-switching connection is through the use of besta wavelength at every stage along the connection path. This unidirectional optical path is a light path and the node between each hop either an OXC or an OADM. Has a separate optical path, will build the basis of the different fibers to transfer in the opposite direction. To satisfy the wavelength continuity condition, the same wavelength is used at every step along the path of light. When a light path blocked, because the required wavelength is available, a converter in an OXC cantransform the optical signal transmitted from one wavelength to another wavelength.

Since the wavelength bandwidth is often much greater than that required by a single client, the transport used hot to allow the bandwidth of a lightpath to be used by many customers. The range in which a lightpath subrate divided units, customers can take one or more units subrate application traffic at lower interest rates. For example, information on an optical network are transmittedSONET (Synchronous Optical Network) Framing at rates of OC-48 (2.488 Gbps). A light path of OXC1 to OXC3 by OXC2 equipped with wavelength w, the unit for this lightpath subrate OC-3 (155 Mbps). A user on OXC1 can transfer a whole number of OC-3 subrate units up to a total capacity of 16 to the data to another user on OXC3. A network operator can use traffic-groomed lightpaths to subrate transportation services to the users by providing a virtualNetwork with the optical network.

Information on a lightpath is usually transferred with SONET framing. In the future, the information transmitted over optical network, the new ITU-T G.709 standard, is known as digital wrapper. In the ITU-T, an optical network is known as the Optical Transport Network (OTN). Below are some of the features of the G.709 standard:

1) The standard allows the transmission of different types of traffic: IP packets, and Gigabit Ethernet frameswith Generic Framing Procedure (GFP), ATM cells, and SONET / SDH synchronous data.
2) It supports three granularities bit rate: 2.488 Gbit / s, Gbit / s and 9.95 Gbit / s 39.81.
3) It provides functions to connect to an end-to-end basis across multiple carriers, as well as from a single carrier.
4) identify uses G.709 Forward Error Correction (FEC) to correct links to some physical impairments due to errors in transmission.

Lightpath can be either static or dynamic. StaticLightpaths are using network management procedures and can stay for a long time. Virtual Private Networks (VPN) can be set with static lightpaths. Dynamic lightpaths are set up in real time with signaling protocols such as IETF GMPLS (Generalized Multi-Protocol Label Switching) and proposed UNI (User Network Interface) by Optical Internetworking Forum (OIF). GMPLS is an extension of MPLS, and was designed to provide MPLS label switching techniques to timeDivision multiplexing (TDM) networks and wavelength routing networks, in addition to packet switching networks. The OIF UNI specifies methods for signaling clients to automatically create, delete, and query a connection via wavelength routing network. The UNI signaling is through the expansion of sales label out protocols, LDP and RSVP-TE.

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