If your enterprise extends across cities, countries, or continents, you can have three possible network topologies: hub and spoke, partial mesh, and in some cases, full mesh. With a hub and spoke topology, your branch office users have a single connection to the corporate headquarters. If they require access to the other branches, their branch router routes their traffic indirectly to the other branches by way of the headquarters router. In a fully meshed configuration, all locations have a single connection to every other location, enabling direct access to one another. A less extreme version of this is a partial mesh topology. As the name indicates, each branch maintains only a few connections to the other locations. Figure 4-6 illustrates the three branch office to central office access topologies.
Figure 4-6. Typical Branch Office Network Designs
You can use various site-to-site WAN technologies to connect branch office buildings to the headquarters location. More traditional WAN access includes circuit-switched connectivity, such as Public Switched Telephone Network (PSTN) dialup links. Circuit-switched technology provides dedicated connectivity between end points.
Slightly less traditional access technologies include various private packet-switched network types, such as ISDN, X.25, Frame Relay, and ATM. These technologies provide a logical dedicated link between offices but use a shared non-IP-based packet-switched core. They are therefore much less expensive than circuit-switched networks. Packet-switched networks enable network service providers to provision for customers who have fluctuating traffic levels, which is the case with the majority of organizations. That is, a customer's traffic may burst above the agreed bandwidth on occasion and not affect other subscribers in the shared network. Under these circumstances, the provider may not charge the customer with a violation in the agreed bandwidth contract.
You can also use IP-based private and public packet-switched networks to create tunnels to form a VPN between locations. MPLS VPNs and encrypted Internet VPNs are available to you, at a major cost savings over traditional site-to-site technologies. Additionally, the underlying Layer 1 and 2 technologies are often much less expensive for IP-based networks than their traditional counterparts, making them very attractive to today's emerging organizations.
You can easily insert QoS into traditional access technologies to provide delay-sensitive applications, such as video and voice, with guaranteed delivery across the network. Additionally, you can achieve QoS inherently within the MPLS protocol. However, because the Internet is uncontrolled by nature, traffic prioritization is impossible for most Internet VPNs. You will learn about QoS in detail in Chapter 6.
You will also learn how branch topologies benefit from edge content caching, distribution, and routing in Chapter 13, "Delivering Cached and Streaming Media," and Chapter 14, "Distributing and Routing Managed Content," respectively.