Unit Uni
Wavelength Service in Optical Transport Networks
Optical WDM (Wavelength Division Multiplexing) networks are networks that deploy optical WDM fiber links where each fiber link carries multiple wavelength channels. An All Optical Network (AON) is an optical WDM network which provides end-to-end optical paths by using all optical nodes that allow optical signal to stay in optical domain without conversion to electrical signal. AONs are usually optical circuit-switched networks where circuits are switched by intermediate nodes in the granularity of a wavelength channel. Hence a circuit-switched AON is also called a wavelength routing network where optical circuits are equivalent to wavelength channels.
A wavelength routing network consists of optical cross-connects (OXC) and optical add/drop multiplexers (OADM) interconnected by WDM fibers. Transmission of data over this optical network is done using optical circuit-switching connections, known as lightpaths. An OXC is an N x N optical switch with N input fibers and N output fibers with each fiber carries W wavelengths. The OXC can optically switch all of the incoming wavelengths of its input fibers to the outgoing wavelengths of its output fibers. An OADM can terminate the signals on a number of wavelengths and inserts new signals into these wavelengths. The remaining wavelengths pass through the OADM transparently.
In order for a user (router A) to transmit data to a destination user (router B), a circuit-switching connection is established by using a wavelength on each hop along the connection path. This unidirectional optical path is called a lightpath and the node between each hop is either an OXC or an OADM. A separate lightpath has to be established using different fibers to set up transmission in the opposite direction. To satisfy the wavelength continuity constraint, the same wavelength is used on every hop along the lightpath. If a lightpath is blocked because the required wavelength is unavailable, a converter in an OXC can transform the optical signal transmitted from one wavelength to another wavelength.
Since the bandwidth of a wavelength is often much larger than that requires by a single client, traffic glooming is used to allow the bandwidth of a lightpath to be shared by many clients. The bandwidth of a lightpath is divided into subrate units; clients can request one or more subrate units to carry traffic streams at lower rates. For example, information is transmitted over an optical network using SONET (Synchronous Optical Network) framing with a transmission rate of OC-48 (2.488 Gbps). A lightpath is established from OXC1 to OXC3 through OXC2 using wavelength w, the subrate unit available on this lightpath is OC-3 (155 Mbps). A user on OXC1 can request any integer number of OC-3 subrate units up to a total of 16 to transmit data to another user on OXC3. A network operator can use traffic-groomed lightpaths to provide subrate transport services to the users by adding a virtual network to the optical network.
Information on a lightpath is typically transmitted using SONET framing. In the future, the information transmitted over optical network will use the new ITU-T G.709 standard, known as digital wrapper. In ITU-T, an optical network is referred to as the optical transport network (OTN). The following are some of the features of G.709 standard:
1) The standard permits transmission of different types of traffic: IP packets and gigabit Ethernet frames using Generic Framing Procedure (GFP), ATM cells and SONET/SDH synchronous data.
2) It supports three bit rate granularities: 2.488 Gbps, 9.95 Gbps and 39.81 Gbps.
3) It provides capabilities to monitor a connection on an end-to-end basis over several carriers, as well as over a single carrier.
4) G.709 uses Forward Error Correction (FEC) to detect and correct bit errors caused by physical impairments in the transmission links.
Lightpath can either be static or dynamic. Static lightpaths are established using network management procedures and may remain up for a long time. Virtual Private Networks (VPN) can be set up using static lightpaths. Dynamic lightpaths are established in real time using signaling protocols, such as IETF’s GMPLS (Generalized Multi-Protocol Label Switching) and UNI (User Network Interface) proposed by Optical Internetworking Forum (OIF). GMPLS is an extension of MPLS and was designed to apply MPLS label switching techniques to Time Division Multiplexing (TDM) networks and wavelength routing networks, in addition to packet switching networks. The OIF UNI specifies signaling procedures for clients to automatically create, delete and query a connection over wavelength routing network. The UNI signaling is implemented by extending the label distribution protocols, LDP and RSVP-TE.
About the Author
Paul Ngai is an engineering consultant specialized in Telecommunications Network and Systems. He is also the principal of Network Systems Technologies LLC (http://www.nstecs.net), a telecommunications consulting firm provides planning, analysis, design, testing and operation support services.
G-Unit In Connecticut – Sacred Heart University, Fairfield CT
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