o alternative routing can save facilities and reduce blocking. The Poisson distribution depends on time-independent call arrivals, that the likelihood of a call being placed is independent of other calls. This makes sense for millions of people independently using their telephones, but a link is going to be blocked for some period of time during which there will be a burst of overflow traffic. Since the extra 10 percent is bursty rather than smooth, the likelihood of multiple links being blocked is actually higher than if they block independently. The likelihood of a tandem link being blocked is less than the sum of their individual blocking probabilities because the sum double counts the case where both links are full. (At least this one works in our favor; the truth is better than our assumptions.) With more sophisticated calculations taking these effects into account, we still realize an enormous benefit from alternative routing. As a telephone network gets bigger, there are more alternative routes, and the facilities can be used more effectively. In real telephone networks, the actual calculation of blocking rates with alternative pathways is a huge calculation. Most of the time networks are growing and changing fast enough that the engineer's job is to stay ahead of the new demand for service rather than trying to figure out exact blocking rates. It is still important to estimate the present and future point-to-point demand throughout the network, to enumerate primary and alternative routes, to develop internal routing tables for the network, and to provision enough facilities to meet the demand. However, with the rapid growth that telephone markets have seen for the last century, and with few signs of that growth ending anytime soon, overprovisioning is more often a matter of buying facilities too soon rather than buying too much transport. 23.5 IP Services (DSL) With the rapid growth of digital subscriber line (DSL) technology, one would have to wonder about the traffic engineering issues. DSL generates a 