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.

Local access and transport areas

What determines if the telecom service is local or long distance? As part of the agreement between AT&T and the Justice Department at the time of AT&T’s divestiture, this question was one of the most important to be answered. Why? The negotiators on the LEC side of the table and on the AT&T side of the table haggled over who got to carry which calls. Ultimately, the question was: “Who gets to earn the revenue from these calls?” A huge amount of money was at stake. In the end, it was decided to divide the country into 777 LATAs.

LATA boundaries were determined by population and, in most cases, LATAs are the size of two to five counties. In some cases, the LATA is almost the same geographically as the area code, but they are not the same. Most LATAs are contained within a state, but some cross state lines. The Chicago LATA, for example, includes Gary, Indiana. A call from Gary to Chicago is an intralata call, even though it is an interstate call.

In 1984, as part of the agreement between the Justice Department and AT&T, local telephone companies would provide all telephone lines (access) and carry all telephone calls (transport) within the LATA. Calls between LATAs, or interexchange calls, were to be carried by long-distance carriers. Figure (x) shows the two LATAs in Washington state. A call from Spokane to Seattle to Tacoma is an intralata call and should be carried by the local carrier. A call from Seattle to Spokane, however, is an interlata call and is to be carried by the long-distance carrier.


Figure (x): Local access and transport areas in Washington state.
Although the breakup of AT&T was designed to benefit consumers because competition would bring prices down, the situation initially proved to be very confusing for consumers. It was unclear to customers which company was responsible for which service. This was very frustrating, especially for customers whose lines needed to be repaired. Bell and AT&T often blamed each other. Meanwhile, the customer lost valuable phone calls while the carriers were busy pointing fingers instead of repairing lines. Today, local carriers and long-distance carriers still bicker about who is responsible when a customer experiences technical difficulties with the phone lines.