dynamically between these in the event of link failure or congestion. The routing tables are generated centrally based on the known topology of the network, the numbering plan, and analysis of traffic data. These are then downloaded to each exchange in the telephone operator's network. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls between telephone numbers on the same network can be routed based on their area code and prefix using these routing tables. Some calls however cannot be routed on the basis of prefix alone, for example non-geographic numbers, such as toll-free or freephone calling. In these cases the Intelligent Network is used to route the call instead of using the pre-computed routing tables. In determining routing plans, special attention is paid for example to ensure that two routes do not mutually overflow to each other, otherwise congestion will cause a destination to be completely blocked. According to Braess's paradox, the addition of a new, shorter, and lower cost route can lead to an increase in overall congestion. Hybrid routing Hybrid routing uses numbering plans and routing tables to permit the colocation, in the same area code, of switches using a deterministic routing scheme with switches using a non-deterministic routing scheme, such as flood search routing. Routing tables are constructed with no duplicate numbers, so that direct distance dialing service can be provided to all network subscribers. This may require the use of ten-digit telephone numbers. Trunk reservation When congestion causes many calls to use indirect alternative routes that pass through more exchanges than a direct route, the overall capacity of the network is reduced since each of these calls occupies several inter-exchange trunks. Trunk reservation reduces this effect by reserving a fraction of the capacity of each trunk for directly routed calls. Dynamic alternative routing Dynamic alternative routing (DAR) is a method of decentralized dynamic routing of telephone calls that uses only a limited amount of local information, which consists of trunk reservation thresholds, or simply knowing the collection of outgoing trunks from the exchange. An 1989 US patent by British Telecommunications describes dynamic alternative routing as:A call between two nodes interconnected by a direct link is first offered to the direct route, and if that is blocked it is offered to a currently nominated two-link alternative route between the two nodes. If that route is busy, the call is lost,