User’s Guide | Protocols and Packet Assembly Features |
In most telemetry applications, the data is fed from the host in a stream with little or no delay between characters. When there is a delay at the end of a stream, this setting can have the modem close and forward the packet.
3.The maximum packet buffer length set in +WS74 (default 1000) is reached. You may wish to set this value smaller if you are encountering too many packet errors and retransmissions. Although larger packets reduce overhead, if large packets are being retransmitted, throughput may improve by using shorter packets. Setting it below 5 will preclude use of the TIES to exit data mode. The time dependent escape sequence is still available. See below for details.
The actual size of TCP packets transmitted may be less than the size of the PAD buffer. TCP includes an exchange during the session opening handshake in which the largest expected packet size is declared. If this size is less than the buffer length, the DART 300 will automatically fragment the buffer into multiple packets to meet connection restrictions.
4.The time dependent escape sequence (guard time, +++, guard time) is received. The escape characters are discarded and the packet is closed and sent. The modem then goes to command state
CAUTION
If you disable the primary forwarding character (+WS63=0) and the timer
(+WS70=0), the only practical way to forward packets will be to fill the packet buffer. This is very strongly discouraged.
NOTE
Using DTR (with the &D2 setting) or the Time Independent Escape Sequence (TIES) will discard any partially assembled packet. A partial packet will also be discarded if the PAD Session Timeout (S30) expires with data still in the packet buffer.
If DTR is used with &D1, the partial packet remains in the buffer (not forwarded). If the session is then closed, the packet buffer is discarded. If the local host goes back on- line without closing the session, the partial packet is still in the buffer; subsequent data will be appended.
6.6.3.Escaping Data State
When using a PAD service mode, the transition from data state to command state can be achieved in the following ways:
•TDES – Time Dependent Escape Sequence. This is the traditional “+++” style escape using a guard time before and after the escape characters to prevent accidental escape during a binary transfer.
•TIES – Time Independent Escape Sequence. This does not require special timing by the host.
•DTR – Data Terminal Ready control line. An
•PAD Session Timeout – There is a possibility that the modem will lose signal and/or registration causing PAD sessions to timeout on inactivity.
•TCP closing handshake – Connections can be closed by the remote terminal. If this happens, the NO CARRIER message is presented. The modem will close the session and revert to command state.
Each is described in detail below.
6.6.3.1.Time Dependent Escape Sequence (TDES)
Where the local host device is capable of controlling the timing of data being fed to the modem, the more traditional time dependent escape sequence can be used. This sequence requires that transmit data to the modem be idle for a minimum guard time (set in register S12) followed by an
2110212 Rev 1.0 | Page 47 |
