## 3 Cause of Network Scan Failures and Delay  
When a network scan is performed, the cause of scan failure or no response is unknown, and the security of the network and terminals cannot be guaranteed. It is important to know why there is no response from the network scan to determine the necessary countermeasure. There are cases in which terminals being scanned may not be reachable, and this can be a result of the terminals not being powered on or not being connected to the network. On the other hand, it is possible that the failure to acquire a response from the scanned terminals is a result of wireless network error. Therefore, to ensure that the network and terminals are constantly secure, a technique to determine the cause of scan failure or no response is necessary. First, we identify various network conditions that can impact the outcome of a network scan. The network conditions can be categorized into three scenarios:Normal scenario--a condition in which network scan can be carried out smoothly without any negative impact on the network QoS, and scan response is acquired. 2. Low SNR (low signal-to-noise ratio) scenario--a condition in which the network is degraded because the propagation loss of a certain terminal is large as a result of the terminal moving at a high speed or positioned behind a building. In this situation, some terminals may not respond to the scan, or the round-trip time (RTT) is large in those terminals. 3. Congestion scenario--a condition in which the network is congested as a result of regular communication, and the scan response is not acquired from most terminals or the RTT is large due to retransmission of scan packet. From the network conditions that may cause the network scan to fail/delay stated above, it is possible to categorize the causes of network scan failure/delay into four categories, as explained in [4]. These causes can be summarized as follows:  
1. Port blocking--scan responses are obtained for some (but not all) ports in a terminal. 2. No network connection--scan responses are not obtained for all ports in a terminal. 3. Insufficient communication quality (SNR insufficient)--a few terminals have large latency compared with normal situation. 4. Network congestion (NW congestion)--many terminals have large latency compared with normal situation. The first two causes of scan failure/delay (e.g., port blocking and no network connection) are a result of the situation on the network layers (i.e., errors not caused by wired or wireless channel). In such a situation, it is expected that there will be a response from the open port on the terminal for the cause of scan failure/delay to be considered as \"port blocking\", while for \"no network connection\", there is no response at all in the ports of the relevant terminal. From these differences, it is possible to distinguish between \"port blocking\" and \"no network connection\" by checking whether there is a port with a response from a terminal or not. If there is no response on all the ports for the network scanning to the terminal, it is judged that the terminal is not connected to the network. On the other hand, if there are responses from some ports, the terminal(s) can be judged as \"port blocking\" (the port that responds is judged as an open port). It is assumed that the terminal is secure when the port does not return a response; however, the real port status is unknown (i.e., open/close). Therefore, a rescan may be necessary. On the other hand, there may be a momentary interruption or similar, but since these states are expected to be maintained for several hours, a rescan is performed in different appropriate time zones (such as when the terminal power is on). Furthermore, the last two (e.g., SNR insufficient and NW congestion) are the failure/delay caused by the wireless channel error (i.e., the situation on the PHY/MAC layer). In the insufficient communication quality, the wireless channel condition becomes worse by moving behind buildings or moving at a high speed and is expected to recover in a short time.