This table identifies the paths established on a GSN. The path may be established between a GGSN and SGSN or GGSN and charging gateway or GGSN and MAP converting GSN. The path between GGSN and charging gateway is established when the charging gateway is configured on the GGSN. The path between GGSN and MAP converting GSN is established when the MAP converting GSN is configured on the GGSN. The path between a GGSN and SGSN is established in the following series of steps: 1. When Mobile System (MS) wants service, it sends packets to a SGSN with specific APN. 2. SGSN uses the DNS to resolve this APN to the IP address of the GGSN which is the designated as the gateway to this APN. 3. SGSN establishes a path to the GGSN using GTP protocol. 4. The SGSN and GGSN peer maintains path by sending echo request message to each other. If one side fails in echo reply for certain times, the other side will send a trap to NMS. A SGSN can have multiple GGSN as peers, while a GGSN can have multiple SGSN peers, depending on routing path. Moreover, a GSN can have more than one path to a peer, each of which will have a unique port.