User Manual

Message Routing and Migration
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Boomer II User Manual & Integrator’s Guide
Wavenet Technology 94 BM210012WT09
Note: To run your application on DataTAC 4000 or 6000 systems, you
must isolate the use of extended SCR or avoid it entirely.
Service Data Unit (SDU) Size
An application SDU consists of the complete message; both user data
and data header. See “Data Header Routing” below. The maximum size
of an application SDU is 2048 bytes for DataTAC 5000 and 6000
systems, and 2550 bytes for DataTAC 4000 systems. For this reason,
the recommended maximum SDU size for tri-system applications is
2048 bytes.
For transport over the air, the SDU is broken up into smaller physical
data units (PDUs). Most network operators price their service at cost
per PDU or cost per SDU. Gather data from your various operators to
develop an application design that favorably considers these cost
factors.
Note: As a general rule, it is less expensive to send fewer large packets
than many small packets. Try to take full advantage of the space
available in each packet.
Data Header Routing
To use the data header to route messages properly, first consider the
data header as a pointer to a destination.
In early DataTAC systems, all mobile units sent their messages to a
single host. Because all traffic went to one destination, there was no
need for a destination header. Later, a data header field was added to
inform the network where (to which host or peer) to direct a particular
SDU or message.
Although the data header field can range in size from 0 to 64 bytes, by
convention most applications are written using a 3-byte data header.
The DataTAC systems each use a different portion of these three bytes
as a pointer to a destination. This pointer is called a session ID. Setting
these three bytes (the entire data header field) to a common value
guarantees compatibility across all three systems.
Here are the specific differences in how the systems implement the
session ID:
On DataTAC 5000 systems, the first two bytes of the data header point
to a destination. (These two bytes are referred to as the session ID.)
On DataTAC 4000 and 6000 systems, the third byte only of the data
header points to a destination. (The third byte is also known as a host
slot on these systems.)
An example of these system differences is illustrated below. Each row
in the table depicts the application-visible portion of the DataTAC
system header for the identified system. In the data header column the
session ID bytes appear in bold typeface. The data header offset field
identifies the length in bytes of the data header field.