Texas Instruments 990 User Manual page 244

Communications Structures
*
A more-data bit, M. This bit must
always be set to zero.
*
peS), the transmit (send) packet
level counter
Bytes 3 and 4 are the session control word. This is fixed
in value at >0007 for all data packets.
Bytes 5 and 6 are dest ina tion
decimal and
genera tion.
IDs coded
assigned
in binary coded
during network
Bytes 7 and 8 are source IDs coded in binary coded decimal
and assigned during network generation.
Bytes 9, etc. are user data. The maximum size is usually
256 or 260 bytes.
0.4 DATA PACKET FLOW
The orderly flow of data is enhanced by defining a window size
for each permanent virtual circuit (PVC). The window size
specifies the maximum number of sequentially numbered data
packets that a station is authorized to transmit and have
outstanding at any given time without receiving a receive ready
packet as an acknowledgement between packet transmissions. With
modulo 8 arithmetic a window size of 1 through 7 can be specified
but the size must be the same at the primary and secondary
station for a given PVC. A window size of 1 specifies that the
sending station must receive an acknowledgement before sending
the next data packet. A window size of 7 specifies that the
sending station may not transmit more than 7 data packets or have
more than 7 outstanding before receiving an acknowledgement. The
window size is determined by the memory available for buffering
input and output data packets, including the memory required to
manage these buffers.
If the recelvlng station window is exceeded, a procedural error
has occurred and a reset request packet should be sent as a
reply.
Examples of transmitted data packets follow.
0-10 2270526-9701