this is a perfectly fine back- haul network. We recommend continuing the process to reduce the cost of backhaul. For each base station, we look for a cheaper hop. We consider each transport link the base station list and evaluate the cost impact of switching the downstream link from its current link to this one. Base station b currently sends its packets downstream to node C on its way to the MSC. Node C could be another base station, it could be a POP, or it could be the MSC itself. We are considering changing that path from C to d. In our thought experiment, the total packet traffic from b and any base stations upstream from b is going to be sent along the link to d, so we add the cost of that link. We also add the cost of increasing the flow downstream from d by the packets from b and up- stream. The cost saved is the cost of the link from b to C and any cost saved by remov- ing all the b packets from C downstream to the POP. We do this for all the transport Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Backhaul Planning Backhaul Planning links on our list from b to anywhere and then move on to the next base station to look for improvement. As before, we continue this iterative investigation until no improvement is found. When we reach this local optimum, we have a good backhaul network using the trans- port backbone. The backbone network for backhaul is complete when this optimization is finished. 35.4 Data Direct from the Base Station Another possibility is to have packet traffic for subscriber data services go directly from the base station to the public packet data network (PPDN). This makes sense when there is a large packet data market in the subscriber community served by the base station. Having data services going directly from the base station reduces the need for backhaul transport from that base station to the MSC. As we des