Echelon OpenLDV - Manual (Page 69)

OpenLDV Programmer’s Guide 61
Layer 2 Buffer Structure
The following sections provide an overview of the Layer 2 buffer structure shown
in Figure 6. Bit transmission order within a byte is “most significant first”,
meaning that the most significant bit is transmitted first. Byte transmission
order is also “most significant first”, meaning that the most significant byte of a
field is transmitted first.
A Layer 2 network interface uses the Layer 2 buffer structure for most messages.
However, local network management messages use the Layer 5 buffer structure,
regardless of which layer the network interface uses for network messages.
For a more complete description of the Layer 2 buffer structure, see the
ISO/IEC 14908-1 Interconnection of information technology equipment –
Control Network Protocol - Part 1: Protocol Stack.
Application Layer Header
The application layer header contains the network interface command (and
queue) and a byte that indicates the length of the rest of the message. The most
significant nibble of the network interface command contains the command code
(for example, niCOMM for network messages), and the least significant nibble
contains the queue code, if any. These nibbles combine to form the
command/queue byte, which is the network interface command.
An OpenLDV application sends these commands using the ldv_write() function,
and receives them using the ldv_read() function. See Network Interface
Commands on page 76 for a description of the network interface commands.
Layer 2 Header
The Layer 2 header is a single byte that includes the following fields:
• A 1-bit field to specify the priority of the data packet. 0 = Normal; 1 =
Priority.
• A 1-bit field to specify the channel to use, primary or alternate. This field
allows transceivers that have the ability to transmit on two different
channels and receive on either one, without the need to instruct the
transceiver to explicitly receive on a specific channel. The transport layer
sets this bit.
• A 6-bit unsigned field (≥ 0) to specify the channel backlog increment to be
generated as a result of delivering this packet. The backlog represents
the number of messages that the packet shall cause to be generated upon
reception. This value is used by the Smart Transceiver or Neuron Chip
MAC algorithm.
NPDU
The Network Protocol Data Unit (NPDU) encapsulates the physical packet data.
The NPDU includes the following fields:
• Protocol version (2 bits)
• Physical packet type (2 bits)
• Address format (2 bits)