Specifications

AN93
Rev. 0.8 105
SMS Support
Short Message Service (SMS) is a service that allows
text messages to be sent and received from one
telephone to another via an SMS service center. The
Si2493/57/34/15/04 provides an interface that offers a
great deal of flexibility in handling multiple SMS
standards. This flexibility is possible because most of
the differences between standards is handled by the
host in the data itself. The Si24xx performs the
necessary modulation of the data and provides two
options for message packet structure (protocol 1 and
protocol 2 as defined in ETSI ES 201 912). The rest of
the data link layer and the transfer layer are defined by
the host system.
The Si24xx uses a V.23 half-duplex modulation to
transmit and receive the data over the PSTN. Two
packet structures are provided: protocol 1 and protocol
2. Protocol 2 differs from protocol 1 in that a packet is
preceded by 300 bits of channel seizure. ETSI ES 201
912 describes the other differences between protocols 1
and 2, but the host processor handles these when
structuring the data within the packet.
There are four commands that control the behavior of
the SMS feature.
To enable the SMS features on the Si24xx, the host
must send “AT+FCLASS = 256” to the modem prior to
handling an SMS call. The host can then dial or answer
an SMS call using the “ATDTxxxx;”, where xxxx is the
number to be dialed, or “ATDT;” commands,
respectively. Note the semi-colon at the end of the
command, which places the modem immediately into
command mode after dialing and responds with “OK”.
The host can then prepare the modem for transmitting
or receiving SMS data.
To receive protocol 1 or protocol 2 data, the host must
send “AT+FRM = 200”. This causes the modem to
return to data mode silently listening for data from the
remote SMS server. If the modem detects a valid
protocol 1 or protocol 2 packet, it responds with a
“CONNECT” message followed by the SMS message
(without channel seizure and mark). When the carrier
stops, the modem returns to command mode and
responds with “OK”.
To transmit protocol 1 or protocol 2 data, the host must
send “AT+FTM = 201” or “AT+FTM = 202”, respectively.
This causes the modem to return to data mode and wait
silently until data is received from the host processor for
transmission. Once data is received from the host, the
modem transmits the proper number of channel seizure
and mark bits followed by the data it received from the
host. After the modem has begun transmitting, it will
send marks when it does not have data to send and will
continue to do so until the host escapes to command
mode.
The content of the data message is entirely up to the
host including any checksum or CRC. ETSI ES 201 912
describes two standard data and transfer layers that are
commonly used. SMS typically relies on caller
identification information to determine if the call should
be answered using an SMS device or not. Please refer
to the section on caller ID for more information on how
to configure the modem for caller ID detection.
Table 77. Protocol 1
80 bits of mark (constant 1s) Message
Table 78. Protocol 2
300 bits of channel seizure
(alternating 1’s and 0’s)
80 bits of mark
(constant 1s)
Message
Table 79.
Command SMS Feature Behavior
AT+FCLASS = 256 Prepares the modem for handling
SMS calls.
ATDT; Goes off hook and returns to com-
mand mode. If a phone number is
provided, it is dialed prior to
returning to command mode.
AT+FRM = 200 Returns to data mode prepared to
receive an SMS message.
AT+FTM = 201 Returns to data mode prepared to
transmit an SMS protocol 1 mes-
sage.
AT+FTM = 202 Returns to data mode prepared to
transmit an SMS protocol 2 mes-
sage.