User Manual

Protocol support: Service Data Units (SDUs)

Integrator’s Guide – RIM 801D OEM Radio Modem

ServiceDataUnits(SDUs)

The DataTAC network transfers user data in packets called SDUs, short for

Service Data Units.

DataTAC Logical Link Identifier (LLI)

The DataTAC Logical Link Identifier (LLI) is a 32-bit number which

uniquely identifies each radio on a DataTAC network. This number is

assigned by the DataTAC network operator, and is usually printed on the

radio modem.

An LLI is conceptually analogous to a telephone number. In addition to its

own LLI, a radio modem can belong to up to 16 Group LLIs, which allows

it to receive a copy of all data sent to the Group.

SDU Types

DataTAC defines many different SDU types. If you have looked into the

DataTAC Open Protocol Specification, then you already know that there are

dozens of different SDU types. However, most of these SDU types are for

private use between the radio modem and the DataTAC network.

There is only one SDU type that is of interest to a programmer. User data

SDUs contain user data which is destined for the DataTAC network.

SDU headers

All user data SDUs start with a variable length user header.

VHO Length User header data

The first two bytes define the user header length and are referred to as the

UHO length. This is a 16-bit binary value in big-cadian format (MSB, LSB).