Multiplexer User's Guide
Confidential / Released
3.2 Multiplexer control and signaling lines
The following chapter covers all information you need to develop and set up a virtual driver.
Differences and restrictions in comparison to the unframed module are pointed out.
3.2.1 Flow control
Logical flow control
The internal logical flow control (FC-BIT in MSC message, see Chapter 4.3.9) represents the
existing flow control to the module. For example, if a data call is initiated and the customer
application transmits data to the module on this channel, the module will stop the data
transmission from time to time. This happens because the module operates with a bandwidth
of 9k6 on air, but the customer application uses a larger width. In this case the module sends
a MSC message with FC-BIT set. After all data stored in the internal buffer have been sent,
the module will send a second MSC message with FC-BIT reset. As already pointed out, the
logical flow control operates like RTS/CTS but with FC-BIT on every channel. The RTS/CTS
are not used for flow control because the traffic on the logical channels may cause a
temporary loss of bandwidth on another channel. This behavior has no impact on the
handshake V.24 lines.
RTS/CTS on the physical channels
Hardware flow control (AT\Q3) is recommended for use with multiplexer. For power saving it
is indispensable. The setting AT\Q3 needs to be made before switching on the multiplexer.
The customer application decodes and encodes the data. To prevent loss of data, the
application must be able process the information about internal flow control (MSC) regulated
by the module. Flow control information is transmitted within the data flow and contains
messages whether or not the channel is allowed to send. See chapter 4.3.9 for MSC.
As of Multiplexer Protocol version 2, the customer application must set RTS (in the direction
to the module) on channel 1. RTS shall not be switched off to prevent loss of data (control
data cannot be sent in this case). If flow control is needed, it is recommended to use logical
flow control on every channel.
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