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In the overlay model, the optical network uses the first control plane architecture described above (see also Figure 9.16). An IP client network is connected to the optical network via an edge IP router which has an optical interface to its ingress optical node,
i.e. the optical node to which it is directly attached. Before an edge IP router can transmit over the optical network, it has to request a connection from its ingress optical node. This is done by using a signaling protocol defined over a UNI. A connection over the optical network can be a lightpath (permanent or switched) or a subchannel. The edge router is not aware of the topology of the optical network; nor is it aware of its control and data planes. The control plane of the optical network can be based on GMPLS. However, UNI maintains a strict separation of the client networks and the optical network.
Finally, in the augmented model, the IP client networks and the optical network use separate control planes. However, routing information from one network is passed to the other. For instance, IP addresses from one IP client network can be carried by the optical network to another IP client network to allow reachability. Routing within the IP and optical networks is separate, but both networks use the same routing protocol. The interdomain IP routing protocol BGP can be adapted for exchanging information between IP and optical domains.
9.5 GENERALIZED MPLS (GMPLS)
The generalized MPLS (GMPLS) architecture is an extension of MPLS described in Chapter 6. MPLS was designed originally to introduce label-switching paths into the IP network, and as we saw in Chapter 6, it is also applicable to ATM, frame relay and Ethernet-based networks. The GMPLS architecture was designed with a view to applying label-switching techniques to time-division multiplexing (TDM) networks and wavelength routing networks in addition to packet-switching networks.
A TDM network is a network of SONET/SDH links interconnected by digital cross connect systems (DCS); see Section 2.5. A DCS terminates the SONET/SDH signal on each incoming link, converts it into the electrical domain, and then switches the contents of some of the virtual tributaries to different outgoing SONET/SDH frames. It also drops some virtual tributaries, and adds new ones to the outgoing frames. The outgoing frames are then transmitted out over the SONET/SDH output links of the switch. Aggregation of SONET/SDH payloads to a higher SONET/SDH level can also be done at the output links. A circuit-switching connection through such a SONET/SDH network can be set up by allocating one or more slots of a SONET/SDH frame along the links that make up the
GENERALIZED MPLS (GMPLS)
path (see Section 2.5). GMPLS can be used to configure the SONET/SDH DCSs, so as to set up a circuit-switching connection.
GMPLS can also be used to set up a lightpath in a wavelength routing optical network. In addition, it can be used to configure an OXC so that to switch the entire optical signal of an input fiber to an output fiber.
In GMPLS, IP routers, ATM switches, frame relay switches, Ethernet switches, DCSs and OXCs are all treated as a single IP network from the control point of view. There are no UNIs and NNIs, since GMPLS is a peer-to-peer protocol.
GMPLS is an architecture and its implementation requires a signaling protocol. Both RSVP-TE and CR-LDP have been extended to support GMPLS.
In the rest of this section, we describe the basic features of the GMPLS architecture and the extensions proposed to CR-LDP and RSVP-TE.
9.5.1 Basic Features of GMPLS
A GMPLS-capable LSR can support one or more of the following interfaces:
1. Packet-switch capable (PSC) interfaces: These are the different interfaces used to receive and transmit packets, such as IP packets, ATM cells, frame relay frames, and Ethernet frames. Forwarding of these packets is based on: an encapsulated label, the VPI/VCI field of the ATM cell header, or the DLCI field of the frame relay frame.
2. Time-division multiplex capable (TDM) interfaces: They forward data based on the data’s slot(s) within a frame. This interface is used in a SONET/SDH DCS.
3. Lambda switch capable (LSC) interfaces: They forward data from an incoming wavelength to an outgoing wavelength. This interface is used in OXCs.
4. Fiber-switch capable (FSC) interfaces: They forward data from one (or more) incoming fibers to one (or more) outgoing fibers. They are used in an OXC that can operate at the level of one (or more) fibers.
These four interfaces are hierarchically ordered (see Figure 9.17). At the top of the hierarchy is the FSC, followed by the LSC, then TDM, and finally PSC. This order of the interfaces is used by GMPLS to support hierarchical LSPs. (Recall from Section 6.2.4 that MPLS also supports hierarchical LSPs.) Consider an LSP that starts and ends at a packet-switching interface. This LSP can go through several types of networks, where it can be nested together with other LSPs into a higher-order LSP. The high-order LSP can start and end at a packet-switching interface, a time-division interface, a lambda switch interface, or a fiber-switch interface. In general, the nesting of LSPs into a high-order LSP is done following the hierarchy of the above four interfaces (see Figure 9.17).