Structural Overview of ISP Networks

Abstract

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 a hands-on description of the practical structure and implementation of IP backbone networks and 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. 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, such as the evolution of the Dense Wavelength-Division Multiplexing (DWDM) layer. The second aspect presents the use of Multi-Protocol Label Switching (MPLS) in large ISP networks. The separation of routing and forwarding provided by MPLS allows carriers to support Virtual Private Networks (VPNs) and traffic engineering on their backbones much more simply than with traditional IP forwarding. The third aspect is how network outages manifest in multiple network layers and how the network layers are designed to respond to such disruptions, usually through a set of processes called network restoration. This is of prime importance because a major objective of large ISPs is to provide a known level of quality of service to its customers through service level agreements. We address how the implementation of the various components of the IP protocol suite fits in the context of a Tier 1 ISP network, and how it can be used to deliver application services, including real-time delivery of video. We describe how a robust IPTV backbone may be constructed, building on various components for robust IP backbone design described in this chapter.