GMPLS

Following the success of MPLS in the provision, configuration and management of virtual circuits for IP networks, some thought was given as to whether MPLS might be used to handle other sorts of virtual circuits, not as a transport mechanism, but as a signaling and control plane for:

§  Layer-2 virtual circuits for Frame Relay,

§  TDM virtual paths for SONET and SDH,

§  Wavelengths in an optical transport network (OTN),

§  Fiber segments linked via spatial physical port switches.

Thus was born Generalized MPLS (GMPLS), which applies the MPLS control plane (with extensions) to these other layers—the focus is typically on SONET/ SDH and optical (wavelength) networks. Cross-connects and Add-Drop Multiplexers in these networks need to exchange GMPLS protocol messages. This is not necessarily strange—all these devices today run element managers or SNMP agents that communicate via a management IP layer. In the TDM world, MPLS label allocation/de-allocation is identified with time-slot allocation; in the optical world it is equivalent to wavelength allocation.

In fact, GMPLS has had a mixed reception in the world’s carriers. Optical and transmission engineers don’t necessarily believe that the IP guys know best when it comes to controlling their networks. There is a history of virtual path management in SONET/SDH networks and optical channel management for OTNs being organized through the network management systems. With increasing element intelligence and more powerful management tools, it is widely felt that the management plane is adequate, and that replicating its functions in a new signaling plane is not required. Of course, opinions differ.

IP/MPLS Transport and Routing Layer

This is the classic Internet model. In an all-IP world, hosts, or end systems (computers, servers, or anything that can run an IP stack) communicate over any convenient wired or wireless access transmission link (wet string) to edge routers. These edge routers look at packet headers and then forward them on the correct “next hop,” to the next router in the chain, or to the final destination host. Routers started as ordinary computers running routing software (this still works!) in the earliest days of the Internet, and then became special purpose machines with a custom architecture. Initially focused on enterprise applications, a new generation of ultralarge and ultrareliable machines came into service in the Internet boom of 1999–2001. The current state of the art is the Terabit router, the Tera prefix (10^12) indicating aggregate router throughput of thousands of billions of bits per second.

Only routers at the edge of modern Service Provider networks actually see IP packets. The Provider Edge routers encapsulate a packet into MPLS by attaching a label to the front of the packet that indicates its final destination (unlike IP addresses, labels are only locally unique and may be altered at each intermediate label-switching router, thereby supporting scalability). Interior or core routers forward the labeled packets—based on their label information—along label-switched paths. The threading of label-switched paths through network routers is under the explicit control of the operator, and this control is used for a number of purposes:

§  Load-balancing between alternative routes,

§  The creation of virtual private networks (VPNs) for enterprises,

§  Segregation of traffic between different quality of service classes,

§  Network survivability via failover to backup label-switched paths.

There used to be many concerns about the robustness and service quality of IP networks in general, and the Internet in particular. But as the Internet has become more central to business, significant care and attention, as well as capital resources have been invested by telecom carriers. The Internet is no longer a byword for flakiness and delay. Many carriers privately believe that the Internet is currently “too good,” and as the inexorable rise of Internet traffic fills up the currently rather empty pipes, expect to see a harder-nosed commercial attitude emerging.

Many carriers will focus their NGNs on connectivity services based directly on IP such as Internet access and MPLS-based VPNs. Services such as leased lines, frame relay, and ATM will either be discontinued, or will be supported only on legacy platforms that will eventually be phased out—this may take a while for leased lines services based on SDH, for example. However, some carriers want to phase out and decommission legacy networks early, to get the OPEX advantages of a simpler network infrastructure, but still leave these legacy services in place to avoid disruption to customers.

Surprisingly, there is a way to do this. It involves emulating leased line, Frame Relay, and ATM services over the new MPLS network, using service adaption at Multi-Service Access Nodes (MSANs) or Provider Edge routers at the edge of the network. There is obviously a considerable cost in MSAN or edge router device complexity, service management overhead and in dealing with the immaturity of the MPLS standards for doing this (using MPLS pseudo-wires). The advantage seems to be in decoupling platform evolution to the NGN from portfolio evolution to “new wave” products and services.