Wavelength service in optical transport networks
Optical WDM (wavelength division multiplexing) networks are networks that implement WDM fiber optic links where each fiber link carries multiple wavelength channels. An all-optical network (AON) is an optical WDM network that provides end-to-end optical paths by using all optical nodes that allow the optical signal to remain in the optical domain without conversion to an electrical signal. AONs are typically optical circuit switched networks where circuits are switched by intermediate nodes at the granularity of one wavelength channel. Therefore, a circuit switched AON is also called a wavelength routing network where the optical circuits are equivalent to the wavelength channels.
A wavelength routing network consists of optical cross-connects (OXCs) and optical add/drop multiplexers (OADMs) interconnected by WDM fibers. Data transmission over this optical network is done using optical circuit switched connections, known as light paths. An OXC is an N x N optical switch with N input fibers and N output fibers, each carrying 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 signals at various wavelengths and insert new signals at these wavelengths. The remaining wavelengths pass through the OADM transparently.
For a user (router A) to transmit data to a destination user (router B), a circuit-switched connection is established using one wavelength at each hop along the connection path. This unidirectional optical path is called a light path, and the node between each hop is an OXC or an OADM. A separate light path must be established using different fibers to set up transmission in the opposite direction. To satisfy the wavelength continuity constraint, the same wavelength is used at each hop along the light path. If a light path is blocked because the required wavelength is not available, 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 greater than that required by a single client, traffic shading is used to allow many clients to share the bandwidth of a light path. The bandwidth of a light path is divided into subrate units; customers can request one or more sub-rate units to carry traffic flows at lower rates. For example, information is transmitted over an optical network using SONET (Synchronous Optical Network) frames with a transmission speed of OC-48 (2.488 Gbps). A light path is established from OXC1 to OXC3 via OXC2 using wavelength w, the available subrate unit in this light path 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-ready light paths to provide subrate transport services to users by adding a virtual network to the optical network.
Information about a light path is usually transmitted using SONET frames. In the future, information transmitted over the optical network will use the new ITU-T G.709 standard, known as digital envelope. In ITU-T, an optical network is called an optical transport network (OTN). The following 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 frames using the Generic Framing Procedure (GFP), ATM cells and SONET/SDH synchronous data.
2) Supports three bit rate granularities: 2.488 Gbps, 9.95 Gbps, and 39.81 Gbps.
3) Provides capabilities to monitor an end-to-end connection across multiple carriers as well as a single carrier.
4) G.709 uses forward error correction (FEC) to detect and correct bit errors caused by physical deficiencies in transmission links.
Lightpath can be static or dynamic. Static lightpaths are established by network management procedures and can remain active for a long time. Virtual Private Networks (VPNs) can be configured using static light routes. Dynamic light paths 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 the Optical Network Interconnection 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, as well as packet switched networks. The OIF UNI specifies signaling procedures for clients to automatically create, drop, and query a connection over a wavelength routing network. UNI signaling is implemented by extending the label distribution protocols, LDP and RSVP-TE.
