EMBEDDED APPLICATIONS FCT
TCP_STATUS_BYTE = 14, TCP_CONNECTED = 2),
or
2 Check the result code from a send or receive operation
and if the code indicates CONNECTING (1) then retry
the operation, with appropriate timeouts applied.
2.2.5
Testing Communications over TCP/IP
The TCP/IP stack supports ICMP operations, such as ping
responses these influence the operation of the low-level
stack. However, the application-writer cannot perform ping
tests from within a script. The only method for testing
whether a remote site can be accessed is to attempt either
a UDP or TCP operation to the host.
2.3
General
Restrictions
The TCP/IP interface supports a single socket meaning
that the embedded application can have only a single
active socket at any one time. Use of, for instance, a UDP
socket and a TCP socket in the same application requires
that the script closes one socket before attempting to open
or re-open another. For the majority of applications, the
module is only ever transferring data over a single socket,
so the restriction will not cause problems.
2.4
Resource
Restrictions
2.4.1
Packet Buffers
The TCP/IP system supports a maximum of six
simultaneous packets of size 1-1536 bytes, plus an
additional two packets of size 1537-3100 bytes. If, having
assigned all of these buffers, additional packets are
received, or queued for transmit, the packets will be
silently dropped. The packet buffers are shared with the
lower-level GPRS system, hence the limits apply to the
total data being buffered within GPRS and TCP/IP
software layers.
Due to the buffer considerations, it is strongly
recommended to use TCP rather than UDP to transfer
larger volumes of data, or any quantity of critical data. The
likelihood of packet loss within the radio device when
using UDP is quite high, even for infrequent ‘bursts’ of
data transfer.
LZT 123 8019 R1A
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