Level 3 Packet Level - Texas Instruments 990 User Manual

LEVEL 3 PACKET LEVEL
I
1
-----,
L
MESSAGE WITH PACKET HEADER
I
SEND:
DETERMINES
WHERE MESSAGE
DESTINATION IS.
ATTACHES HEADER.
RECEIVE: GETS SEND 10.
I
REMOVES HEADER
ROUTES TO
PROPER BUFFER.
I
I
I 1 - '
1 - - 1
---.......1
I
1
NETWORK
I
I
L
---1
INFORMATION
TABLES
(NIT)
- - - - - I
I
' - - - I
L_~
I
MESSAGE
I
' - - - - - . BUFFER
I
L ___
--.J
2280548
Introduction
LEVEL 3/LEVEL 2
QUEUE
-
FIFO
QUEUE
Figure 1-7 Level 3 Packet-Level Block Diagram
Message IDs are stored in the network information tables (NITs),
which are further discussed under level 4 (paragraph 1.4.2.4).
For the packet and higher levels, the NIT functions as a look-up
table to determine message routing. For a transmission, the
packet level software extracts data from the NIT for inclusion in
the packet header. The combined message and header is then
forwarded to the HOLe level via a first-in first-out (FIFO)
queue.
Window sizes at the packet level were discussed in the preceding
paragraph. At this level, the window size is largely a function
of the degree of sophistication of the operating system; for
example, a TX5 secondary has a window size of 7 while a PM550 or
a TM990 has a window size of only 1. Any attempt to transmit a
message when the corresponding window is full results in an error
si tuation from which the system attempts to recover.
A packet that contains data or a message destined for another
task is termed a data packet, and the associated packet header
contains relevant address and control information, including a
sequence count. The sequence count is provided as a means of
countering the loss or duplication of messages between stations.
For certain control functions, control and address information is
transmitted without any accompanying message and independently of
a task. These control packets are described in the following
par agr aphs •
2270526-9701 1-19