Packet Data Over Cellular Networks: The CDPD Approach Svet Naydenov. CS 556. Overview. What is CDPD? CDPD Overview The Physical Layer Medium Access Control Forward Link Structure Reverse Link Structure The Medium Access Procedure Logical Link Control. What is CDPD?.
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- mobile terminal (PC, PDA, etc)
- CDPD radio modem attached to the mobile terminal and manages the radio link and protocols
- generic IS: simple router without knowledge of CDPD and mobility issues
- MD-IS: mobile data IS is a specialized IS that provides switching, accounting, registration, authentication, encryption, and mobility management functions
- passes data between M-ESs and MD-IS and does not perform networking functions
- creates and manages the air interface the M-ESs and the CDPD backbone.
- encapsulates LPDUs into frame structures to establish the LPDU boundaries
- encodes LPDUs to provide error protection
- detects and corrects bit errors within received frames
- busy/idle flag is a 5-bit sequence transmitted on the forward channel once every 60 bits, providing binary info whether the reverse channel is busy/idle.
- decode status flag is a 5-bit sequence indicating whether the MDBS has decoded the preceding block.
On successful decoding (no decoding errers) the flag is 00000, on unsuccessful decoding 11111.
- point-to-point data links are used to transport info between single M-ES and it’s serving MD-IS.
- broadcast data links are used for point-to-multipoint or multipoint-to-multipoint communications on a CDPD channel stream.
- unacknowledged mode is used for information transfer on a broadcast channel, which makes transmission unreliable (no error or flow control mechanisms are used).
- acknowledged mode is used for information transfer on a point-to-point data link channel. This channel also supports unacknowledged mode for information transfer.