N O V A T E L W I R E L E S S T E C H N O L O G I E S L T D .
9
A T C o m m a n d S p e c i f i c a t i o n - E x p e d i t e a n d M e r l i n
W i r e l e s s I P M o d e m s
Data Mode Description
The modem contains an integrated TCP/IP protocol stack. It is accessed via AT commands that put the
modem into either a TCP or UDP data communications mode. If the application host contains a TCP/IP
stack, PPP or SLIP can be used to transfer IP packets between the application host and the modem.
The Data Terminal Ready (DTR) line is used by the modem as a qualifier for its operation in a data mode,
TCP, UDP, SLIP or PPP. The DTR line is to be asserted before entering a data mode to indicate to the
modem that the host computer is available for operation. The DTR line is de-asserted to terminate a data
session. The modem will not sustain a data mode session without DTR being active. The use of DTR
can be overridden by setting register S211 to 1: the equivalent of asserting DTR permanently.
The DSR Signal is driven by the modem to indicate the existence of a data session. When the local host
asserts DTR, the modem will respond with the DSR signal to indicate that the modem has entered data
mode. DSR is de-asserted to indicate that the modem has exited data mode.
The NRM provides Host and Network originated access to the connection-oriented service using TCP/IP.
Host origination is accomplished via the DIAL (ATDT) command. Upon successful completion of the DIAL
command (i.e. CONNECT result code), the NRM switches to TCP-Data mode. DTR must be asserted
prior to issuing the ATDT, ATDP, AT\ASLIP or AT\APPP. If this is not followed the modem will not engage
the session in a manner where it can be sustained. The NRM signals a network origination to the Host
using the RING result code. Failure to have DTR asserted while in “Listen” mode, network origination, will
result in a dropped session. The PAD functions are active during TCP-Data mode only, however, the
data buffering functions apply in all modes.
For Listen mode applications, the modem must see DTR asserted when the TCP Session request is
received. If DTR is not asserted the session request will be rejected. When the DTR signal is asserted,
the modem will issue a RING followed by a CONNECT to indicate the beginning of an incoming data
session. Here RING indicates the source of the session as being a remote server while the CONNECT
indicates the transition from AT Command mode to Data mode. . In this mode, the NRM will “Listen” for
TCPP packets with port numbers which match that specified with the NRM’s IP address (re: S110). Any
TCP packets received in AT-CDPD mode, or packets received in TCP-Data mode without a matching port
number, are rejected by the NRM with no indication to the Host. DTR can be used to terminate the
session or the remote server can initiate the termination.
A session may be terminated by the Host de-asserting the DTR line. The remote host may also terminate
a session. Connection status is maintained on the DSR line. Once the session has been terminated, the
NRM returns to AT-CDPD mode. Note that session termination is the only method to return to AT-CDPD
mode
.
For UDP/IP, a connection-oriented service is “fabricated” within the NRM. The DIAL command (ATDP) is
used to place the NRM into UDP-Data mode. In this mode, the NRM will “Listen” for UDP packets with
port numbers which match that specified with the NRM’s IP address (re: S110). Any UDP packets
received in AT-CDPD mode, or packets received in UDP-Data mode without a matching port number, are
rejected by the NRM with no indication to the Host. In UDP-Data mode, the port number for transmitted
UDP packets is specified with the destination address associated with the DIAL command. Only the Host
Origination and Host Termination scenarios described above apply to UDP operation. The DTR and DSR
lines, and the PAD functions, operate as in TCP-Data mode.
3
The Hayes compatible escape sequence is not supported.
