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【文章內(nèi)容簡介】 at random points in a two dimensional grid. Links (represented by edges between nodes) are added to the network by considering all possible pairs of nodes and then deciding whether a link should exist according to a probability function involving how far apart the two nodes are and how many links are expected to be in the whole network. The original intention of this approach was to generate networks for paring Minimum Steiner Tree algorithms. It has several serious drawbacks when used for generating typical internets. First, the networks don39。t resemble the handdrawn topological maps of real networks. There is no sense of a backbone or hierarchy, and the existence of links clear across the network is unrealistic.Second, it does not guarantee a connected network. Each generated network must be checked for connectivity, then discarded or modified if the check fails. Either option involves extra work. Third, as the number of nodes in the network grows, the number of links grows in a similar fashion. This is unlike real networks, where new links are added for redundancy, not just because more nodes joined the network. Various modifications to Waxman39。s method have been proposed by the authors and others. Some of these attempts to restrict the longest links in the network, while others reduce the number of edges from any particular node. Still other modifications introduce a simple hierarchy to the network. None of them produce convincingly realistic networks. 2 A Better MethodOver the past few years a better method has been devised independently by the authors for generating graphs that reflect the hierarchical domain structure and locality that is present in the Internet [1, 4]. Three levels of hierarchy are modeled, corresponding to transit domains, stub domains, and LANs attached to stub nodes. The method constructs the graph piecewise, where the pieces correspond to domains at the different levels in the hierarchy. The connectivity within a domain (intranet work connectivity) is dealt with separately from that between domains (internetwork connectivity). ParametersTwo sets of parameters control the coarse properties of the networks generated. These parameters are chosen to provide reasonably simple control over the important structural characteristics of the graph. The parameters chosen also have obvious effects on the networks that are produced. The first set of parameters governs the relative sizes of the three levels in the hierarchy: T, the total number of transit domains, and NT, the average number of nodes per transit domain. Note that= 1 and NT=1. S, the average number of stub domains per transit domain, and NS, the average number of nodes per stub domain. Note that S= 1 and NS=1. L, the average number of LANs per stub node, and NL, the average number of hosts per LAN. LANs are modeled as star topologies with a router node at the center of the star and the host nodes each connected to the center router. As pared to using a plete graph connecting all hosts in the LAN, this significantly reduces the number of edges in the graph and reflects the lack of physical redundancy in most LANs. Note that L= 0 and NL=1. The total number of routing nodes, NR, and the total number of hosts, NH are given by: NR =TNT(1 + SNs) NH =TNTSNSLNL Note that the parameter values are taken as the basis for distributions used to obtain the actual value for each run of the algorithm. Extra information can be associated with each parameter to describe the distribution of the parameter. For instance, an upper and lower bound on the number of nodes in a stub domain and the function for distributing the value between the bounds could be described. The second set of parameters governs the connectivity within a domain (intranetwork connectivity) and the connectivity between domains at the same or higher and lower levels (internetwork connectivity).ET, the average number of edges from a transit node to other transit nodes in the same must be large enough so that the graph corresponding to each transit domain can be connected. (ET= 2.) ES, the average number of edges from a stub node to other stub nodes in the same domain. ES must be large enough so that the graph corresponding to each stub domain can be connected. (ES=2.) ETT, the average number of edges from a transit domain to another transit domain. ETT must be large enough so that the transit domains can be connected to one another. . (ETT =2.) EST , the average number of edges from a stub domain to a transit domain. Every stub domain must be connected to at least one transit domain, thus EST =1。 multihomed stub domains will have more than one edge to a transit domain. ELS, the average number of edges from a LAN to a stub node. Every LAN must be connected to at least one stub node, thus ELS= 1. LANs may be connected to more than one stub node in the assigned stub domain. Generation ProcessKnowledgework level: Advisors to both top and middle management who are often experts in a particular area.Tactical level: Middle managers who deal with planning, organizing and the control of the organization.Strategic level: Strategic managers who make decisions that guide the manner in which business is done.Each successively lower level has different OIS requirements and a different, and less extensive, view of the organization. Obviously, the higher the level, the more interrelated the business functions bee until, at the very top, they are viewed as one homogeneous