Grids are built by user communities to offer an infrastructure helping the members to solve their specific problems. 
Hence, the geographical topology of the Grid depends on the distribution of the community members. Though there might be
 a strong relation between the entities building a virtual organization, a Grid still consists of resources owned by 
different, typically independent organizations. Heterogeneity of resources and policies is a fundamental result of this.
 Grid services and applications therefore sometimes experience a quite different resource behavior than expected. 
Similarly, a distributed infrastructure with ambitious service demands stresses the capabilities of the interconnecting 
network more than other environments. Grid applications therefore often identify existing bottlenecks, either caused by 
conceptual or implementation specific problems, or missing service capabilities. Some of these issues are listed in this
 document..