Binary Messages

By Wolfgang Laun
Date: Friday, 25 July 2003 13:20
Duration: 40 minutes

The leitmotiv of the talk is pack and unpack for binary messages. It is
shown how protocols that typically are made up from a variety of binary
message frames can be implemented while keeping the effort required for
defining a message type down to a minimum.

Part I is about a short exercise in pack and unpack, dealing with the
(de)composition of GPS-like messages. Binary data including reals (such as
satellite position and speed) are squeezed into a 250-bit CRC-protected
message. This has ben used for cross-checking a set of algorithms used in
the Navigation Land Earth Station of the European Geostationary Navigation
Overlay System.

Part II deals with an application where pack is used to implement the core of
the preparation of binary messages used for testing a subsystem of ELEKTRA,
ALCATEL Austria's electronic railway interlocking system. A base class Frame
is presented which permits the dynamic definition of a class hierarchy,
extending the base class or any of its subclasses with additional fields or
default values for individual fields, or both.

Part III shows how pack and unpack are used in the context of a package
implementing the Telepath Short Message Peer to Peer Protocol used for
communication between applications and a Short Message Service Center. The
required set of messages is derived from a common abstract base class
providing a method for the simple definition of a message type. This results
in a list of field names (which can then be used as accessors to field
values of a message object) and the template for pack and unpack used in the
conversion between the binary message and the unpacked object.

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