Ericsson ZAT 19.2 Operation Manual page 219

Flowcontrol and buffering SRE ke
8.1. Flowcontrol
Buffering is performed when the modem cannot transfer data over
the line as fast as the DTE transfers data to the modem or when the
line speed is higher than the DTE speed. Since this clearly causes a
problem with buffer overflow there has to be a mechanism which
controls the dataflow, i.e. flowcontrol. The following figure illustrates
the different buffers which may need flowcontrol.
Local Remote
Terminal Modem Modem
8.1.1 Terms
DTE_TX
This buffer contains data from the DTE which is to be sent on
the V24 interface to the modem.
DCE_TX
Data from the DTE to be transmitted to the remote modem/DTE.
DCE_RX
Data from remote modem/DTE to be transmitted to the DTE.
DTE_RX
This buffer contains data which the DTE has received from the
modem but not processed internally.
When data is placed in a buffer faster than it is removed, this
buffer will of course overflow. This means loss of data and must
be prevented. The following overflows are possible :
DTE_TX
Internal DTE problem.
DTE_RX
When the DTE is receiving data from the modem faster than it
can process them it must stop the modem from sending more
data. This is called DTE to DCE flowcontrol and utilises
hardware (RTS) and/or software (XON/XOFF) signalling.
DCE_TX
When the DTE is transmitting data to the modem faster than the
modem transmits data to the line, the modem must stop the
DTE from sending more data. This is called DCE to OTE
flowcontrol and utilises either hardware (CTS) and/or software
(XON/XOFF) signalling.