Building a resilient, high-capacity WAN has never been what I'd call simple. In the old days (you know, 10 years ago), it was typically a mishmash of frame-relay and point-to-point leased line circuits mixed with ATM in higher-end, converged applications. Today, those technologies and their successors are being displaced by a variety of IP-based Ethernet circuits. Although I think this shift to Ethernet-based WAN implementations is hugely liberating when it comes to WAN design, it has its drawbacks.
Chief among those drawbacks: It becomes a lot more difficult to know exactly what you're buying. When I buy a point-to-point T1, I know with some certainty that I'll have a 1.5Mbps channel from site A to site B. The data I push into the pipe from the router at site A will end up at the site B side in exactly the same order and typically with a very predictable and consistent latency.
Ethernet-based systems, while far more flexible and feature-rich, have a much higher degree of variability due in large part to the same flexibility they offer customers. As a result, you need to have a much better understanding of the technologies that modern Ethernet-based carriers use and what might differentiate the options you have available. Without that understanding, you might end up paying too much for a service that won't meet your needs in the long run.
The last mile comes first, and fiber is the best last mile
Perhaps the most crucial part of any WAN circuit is how the so-called last mile is delivered to you. In North America, there are several common ways used by carriers to deliver Ethernet-based services to your doorstep. The most important distinction is whether the last mile is based on copper (such as twisted-pair phone lines or the coaxial cable used in cable TV plants) or fiber optics.
With the correct equipment, fiber optics can push massive amounts of bandwidth (beyond 100Gbps over a single pair) -- far more than any long-distance electrical interface could hope for. However, despite many gains in the last few years, fiber to the premise is still uncommon in the United States. Unless you're in a well-developed urban area, you should consider yourself lucky if fiber is available to you without enormous upfront installation costs.
The gotchas of fiber last-mile connections
Even if you can get a fiber-based product to your premise, you may not have access to all that scalability. Carriers that construct fiber-optic networks to customer premises have two main choices when it comes to engineering their network: They can construct an active optical network, or they can create a passive one. That distinction isn't generally exposed to users, but it makes a big difference to the kinds of services you can receive.
Active Ethernet fiber networks involve a series of true point-to-point fiber-based Ethernet links tying various customer premises together, typically in rings for redundancy. In these networks, scalability is limited only by the equipment chosen to connect the customer sites; 1Gbps is common, but 10Gbps is possible in some locations where there's been enough market demand. Active Ethernet networks also have the most flexibility in offering a variety of services to different customers and in supporting larger distances between customers in the same area. But they are also significantly more expensive to build and maintain than their passive brethren because they require expensive customer-premise hardware.
PONs (passive optical networks) use a single fiber to serve a large number of customer premises, typically 16 to 32 at a time. These networks work by using separate light frequencies to handle downstream data and upstream data and by dividing that light among all the customer premises on the network. In basic terms, this means that each customer premise in a given area has to share the same downstream and upstream bandwidth -- much in the same way that copper-based cable TV networks do, but there's much more available bandwidth to be shared in PONs.
In fact, PONs are often used by "triple play" services (Verizon's FiOS is an example) that combine TV distribution with data services because the broadcast nature of PONs lends itself well to distributing high-bandwidth HD television signals. However, although PONs are cheaper to deploy, their bandwidth scalability is limited compared to active Ethernet deployments. PONs that don't implement encryption can also represent a security risk as every node on a PON segment ultimately can see the downstream traffic from every other node (though it would take a particularly savvy attacker to leverage this).