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The MD-IS acts as the liason between the cellular portion and the fixed, or land-line, portion of the network. It accepts packets sent from the MDBS and converts them to IP format for transmission over the fixed network. Similarly, it accepts packets from the fixed network and encrypts and further processes them for transmission over the airlink via SNDCP (Sub-network Dependent Connection Protocol). The MDIS also handles registration (connection establishment) for the MES's in its service
area. MDIS's can be divided into 2 categories: home and serving. A home MDIS is an MES's primary MDIS, the one it normally registers to. A serving MDIS is an MDIS in another coverage area which an MES registers to if I is within that area. Home MDIS's are responsible for forwarding packets to the appropriate serving MDIS's of all of its MES's.
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