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User’s Guide UDP/TCP Service
2110212 Rev 1.0 Page 69
CAUTION
If the modem auto-answers a TCP session while the local host is unavailable to process
received packets (indicated by flow control), the packets are buffered at the modem. If
the local host remains unresponsive to the session, the session may close without the
host being advised.
When the local host does restore flow control the modem will have lost the session’s
data.
It is possible for the modem to auto-answer and process (buffer) multiple sessions
without attention from the local host. Each session’s data is lost if it closes without the
local host’s attention.
When the connection handshake is complete, and the modem has advised the local host device of
the change from command state to data state, the incoming packets will be presented to the host
(based on DTR). The modem is ready to accept host reply data for packet assembly and
transmission to the client.
The host can check the identity of the calling station by escaping data state and issuing the
+WVCALLERIP command. The response is the IP address of the current client in dotted-
decimal format.
Using an Open TCP Session
While a session is open, the modem handles packet assembly and disassembly for outgoing and
incoming data respectively. For details on packet assembly and forwarding see Section 6.6 above.
An intelligent or semi-intelligent host server will want to receive the packet data and respond to it.
A dumb host server will simply begin sending telemetry.
Receiving Data
Once the session is open, the modem will only accept packets from the connected partner. Any
connection requests from another client are declined (ignored).
RTS or DTR flow control can be used to suspend the flow of data from the modem to the local
host (DTE). If RTS or DTR is de-asserted, the modem will buffer incoming packets. When RTS
is re-asserted, the data held in the modem will be delivered to the local host. If DTR is being used
for flow control, the modem will not release the buffered data until a packet arrives while DTR is
asserted. In the event that the modem’s receive buffer fills, the modem will automatically assert
flow control with the CDPD network to have packets held there until it is ready to receive them;
although there is a limit to how long the network will hold packets, and the sender is waiting for
an acknowledgement from the modem. When TCP is being used, the modem will acknowledge
all received (and buffered) packets; but if network flow control is asserted, packets will go
unacknowledged until network flow control is de-asserted.
Escaping data state while keeping the modem in on-line condition (the session open), will cause
the modem to buffer incoming packets as with RTS flow control. If the modem is returned to data
state with the On-line command (O), the buffered data is then sent to the host. Should the local
host close the session (hang-up or reset), any buffered data is discarded.
Transmitting Data
All outgoing packets are directed to the current client IP address and port. The local host simply
transmits data to the modem PAD buffer. The PAD service will assemble and forward the packet
according to rules described in Section 6.6 above
.