 Therefore, not only is analysis of group-server queues  regarded as a new interesting research direction, but it also exists many theoretic challenging , basic difficulties and open problems in the area of queueing networks.
<meaning-changed>  Therefore, not only is analysis of group-server queues  regarded as a new interesting research direction, but it also exists many theoretic challenging , basic difficulties and open problems in the area of queueing networks.
 Therefore, not only is analysis of group-server queues is regarded as a new interesting research direction, but it also exists many theoretic challenging , basic difficulties and open problems in the area of queueing networks.