Xylem SDI-12 Data Logger iRIS 220, iRIS 320, iRIS 350, iRIS 150, iRIS 300 - Manual (Page 74)

iRIS Datalogger User Guide V1.50
68 iQuest (NZ) Ltd - PO Box 15169, Hamilton, New Zealand Tel: +64 7 857-0810 Fax: +64 7 857-0811 Email: [email protected]
9.3.1 Aliased Gateway explained
The iRIS uses a gateway method called "Aliased Gateway Communication". In other words, it alters the
destination and source addresses of a received packet before it is redirected back out via the gateway.
The primary reason for this in a radio only situation is to completely separate radio communication paths
between stations. This is to avoid a packet that has been redirected through an intermediate station being
received and replied to by the far destination station at the same time. This might be due to unusual RF
propagation conditions where direct communications is not normally achievable between the originator and
destination stations but suddenly becomes possible.
By addressing the original packet from the originating node (usually HydroTel™) to a notional (non-existent)
number, the gateway iRIS can add an offset to the address (correcting to the real address) and pass it on to
the distant node. Any directly received packet will now be ignored by the distant iRIS as the address will not
match.
9.3.2
Gateway Example
An iRIS, set to address 14, is installed at a hilltop rainfall measuring site. This logger is within range of a
GSM network and it uses GPRS IP communications with the HydroTel™ base.
A second iRIS, which is set to address 67, is installed down in the valley at a water quality site. This site has
no wireless network coverage, so a pair of low-power radios is used to link the two sites. Each radio
connects to the RS232 port on its respective iRIS logger.
The hilltop site uses a special cable (see Figure 12 above) to enable non-dedicated RS232 telemetry mode
because it also has wireless connectivity. As the valley site has no modem, it is set to “RS232 only”
telemetry mode and it uses a standard RS232 cable to connect to the radio.
The HydroTel™ base computer (which has a communications address of 0) needs to access both iRIS
loggers. To achieve this, the hilltop iRIS is enabled for gateway communication and for convenience its offset
gateway is set to 1000. This means that the site will redirect any packet with a destination address between
1000 and 1099.
To enable alarm call-back from the iRIS at the valley site, its call-back address is also set to
1000 to cause the hilltop gateway logger to redirect messages to the HydroTel™ base.
The only change at the HydroTel™ base is to set the address for the valley site to 1067 in the
station configuration form, rather than its actual address of 67. The address gets changed as
it passes through the gateway iRIS logger as described below.
This is what happens to the data packets during a typical requestresponse data conversation.
 HydroTel™ sends a request packet to the valley site via GPRS (which has the same IP address as
the hilltop site in HydroTel™). It is addressed to destination address 1067 from sender’s address 0.
 The gateway logger matches the destination address (1067) to be in its gateway address range.
 The packet’s source and destination addresses are aliased by applying the gateway offset (1000).
 The gateway logger sends the aliased packet via the radio down to the remote iRIS. It is now
addressed to destination address 67 from sender’s address 1000.
 The remote iRIS processes the request and sends back its response via the radio. This packet is
addressed to HydroTel™ as destination address 1000 from the valley iRIS sender’s address of 67.
 The gateway logger matches the destination address (1000) to be in its gateway address range.
 The packet’s source and destination addresses are aliased by applying the gateway offset (1000).
 The gateway logger sends the aliased packet via the GPRS network to the HydroTel™ base. It is
now addressed to destination address 0 from sender’s address 1067.