Functionality - 3Com PathBuilder S200 Series User Manual

Functionality

Functionality

Introduction
Overview of TPA
2
This section describes Transparent Polled Async basic functionality.
A polled environment is present when a host computer communicates with remote
devices through multipoint modems or a digital sharing device. Figure 1 shows three
remote devices attached to the host's modem in a multipoint configuration. Through
proper addressing, the host can send a poll message to Remote Device. A and receive
a message in response to the poll. Using other addresses, the host can also send a poll
message to Remote Device B or C.
HOST
Modem
Figure 1. Transparent Polled Async Devices
In a typical PathBuilder S200 series switch asynchronous environment, data enters
the PAD and is transmitted across the network when certain data forwarding
requirements are met, for example:
• Data Forwarding Character is encountered
• Idle Timer expires
• Dynamic Data Forwarding is enabled
• Packet is full
Intercharacter delay, the delay between characters transmitted at the remote end, is
inconsequential. However, in a Transparent Polled Async environment, excessive
intercharacter delay may cause the remote user device to conclude incorrectly that
the end of the message has been reached. To ensure that data is transmitted properly
in this application, a PathBuilder S200 series switch PAD must not begin to transmit
a message to a user device until it has received the entire message from the network.
This is accomplished through proper use of the Data Forwarding Criteria and
Message Assembly parameters. Refer to the section "Configuration" for information
on these parameters.
Modem Modem
Remote
Remote
Device
Device
B
A
Modem
Remote
Device
C
Transparent Polled Async