Chapter 2 Structural Overview of ISP Networks

An Internet Service Provider (ISP) is a telecommunications company that offers its customers access to the Internet. This chapter specifically covers the design of a large Tier 1 ISP that provides services to both residential and enterprise customers. Our primary focus is on a large IP backbone network in the continental USA, though similarities arise in smaller networks operated by telecommunication providers in other parts of the world. This chapter is principally motivated by the observation that in large carrier networks, the IP backbone is not a self-contained entity; it co-exists with numerous access and transport networks operated by the same or other service providers. In fact, how the IP backbone interacts with its neighboring networks and the transport layers is fundamental to understanding its structure, operation, and planning. This chapter is a hands-on description of the practical structure and implementation of IP backbone networks. Our goal is complicated by the complexity of the different network layers, each of which has its own nomenclature and concepts. Therefore, one of our first tasks is to define the nomenclature we will use, classifying the network into layers and segments. Once this partitioning is accomplished, we identify where the IP backbone fits and describe its key surrounding layers and networks. This chapter is motivated by three aspects of the design of large IP networks. The first aspect is that the design of an IP backbone is strongly influenced by the details of the underlying network layers. We will illustrate how the evolution