And how, pray tell, does one actually "measure" T1 vs. T2 networks?
That's easy. You define a set of criteria by which you can measure the networks on some scale, and then set two thresholds. Networks which exceed the higher threshold are Tier 1, those which only exceed the lower threshold are Tier 2. I have seen people do this by counting the number of ASes that a network connects to. And I have seen this done with nodes by summing up the bandwidth of all circuits connected to a node. Even though the network is a dynamic partial mesh, researchers can learn a lot about the behavior by imposing various types measurement hierachy on the network. Thus, Tier 1 and Tier 2 are not inherent characteristics of the Internet; rather they are characteristics of a particular view of the network at a particular point in time. There are probably people who are trying to measure a hierarchy of latency or a hierarchy of jitter. The more views, the merrier. --Michael Dillon