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 .
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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
UDP Mode Description
User Datagram Protocol is a basic transport protocol that provides a best-effort, connectionless delivery
service with minimum overhead. The protocol does not guarantee delivery of packets. There is no
checking or retransmission of the data packets. It does provide minimum overhead as it only adds the
source and destination port numbers to the header. Since the protocol is unreliable, the application must
provide checking, acknowledgments and retransmissions if the data is critical. In many applications, the
data is not critical because updates are sent periodically and occasional losses can be tolerated.
UDP sessions may be originated by the local application host or by the network. The DTR signal must be
asserted or S211=1 (pretend DTR is always asserted) for the unit to enter into a data communications
session. The session is terminated by dropping DTR or resetting the unit.
UDP Modes of Operation
Command:
ATS82 = <mode>
Where
<mode> = Timer value in seconds
0 = Regular mode
1 = Not used (reserved)
2 = Half-Open mode
Originating a UDP Session
A session is originated by the application host issuing the ATD (Dial) command:
ATDPnnn.nnn.nnn.nnn/ppppp
ATDP specifies a UDP session. nnn.nnn.nnn.nnn is the destination IP address and ppppp is the
destination application port number. If no port is specified then a port number of 0 is assumed. It should
be noted that UDP and TCP make extensive use of port numbers, please be aware that the modem will
use the port number as a criteria for accepting and passing data to the host application.
CONNECT [terse 1] result code will be issued and all ensuing data sent from the host application will be
assembled into a UDP packet and transmitted to the destination application.
ERROR [terse 4] result code will be generated if the modem is not currently registered on a CDPD
network.
PAD (Packet Assembly and Disassembly) function will transmit data when the inter-character idle time-out
occurs or when the PAD buffer is full. (256 or 512 bytes). The idle time-out is specified in register S50 in
1/10 seconds.
Receiving Data - Once the session is started, any packets sent to the modem's IP address with the port
number specified in register S110 will be processed and the data portion will be transmitted over the serial
link to the host application. Any packets received with a different port number specified will be discarded.
Termination - The session is terminated by dropping DTR or resetting the unit.
