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کامپیوتر و شبکه::
زمان handover
Specifically, the scheme has four modules: (1) a train uses its on-board sensors to obtain its speed, location, and RSSI (Received Signal Strength Indicator) of train-trackside WiFi channel; (2) the train reports these real-time measurements to the railway control center with the legacy GSM-R networks; (3) according to the measurements, the control center or its WiFi AP adaptively customizes the protected codestream bitrate and AP-train handover time; (4) the train renders the received codestream which passes the authenticity verification process.
Therefore, similar to GSM- R (Global System for Mobile Communications-Railway) handover [15], the WiFi handover time is shortened significantly such that the AP-train connectivity remains stable at handover time.
In particular, location-aware handover schemes are attractive as they are able to reduce handover time by exchang- ing communication context among neighboring APs [26]- [28].
and easy to be manipulated by anyone with a WiFi transceiv- er; (3) Multimedia are broadcast in plaintext in some real subways in order to avoid the complicated cryptography key management and compatibility problem existing between the video decoders from different suppliers; (4) broadcast system designers focus on reducing handover time and packet loss rate [39], worry about protection methods which may add overhead and propagate the packet errors; and believe that the subway is physically secure as the operator controls the subway infrastructure.
The handover process consists of three phases: scanning AP, authentication of moving station and association between new AP and the moving object, where the scanning AP may cost 80% handover time, or up to 400ms [54].
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