Access to the wireless network: the TCP gateway concept

The EMEWS must operate in emergency situations when public networks can be damaged or overloaded. So the system cannot rely on public terrestrial services such as GSM. Satellites services are evolving fast but are still expensive compared to land radio communications. Two land radio technologies have been selected: amateur packet radio and spread-spectrum TDMA.

A control and acquisition application traditionally opens a serial port and sends/receives some data in order to communicate with a remote station. In the EMEWS there is only one radio network shared by all instruments. Since each instrument has its own control and acquisition application, conflicts will occur if each of them try to open the same serial port. This is why EMEWS introduces the concept of TCP gateways. A TCP gateway maps each remote station to a TCP port of the gateway computer. An application which needs to communicate with a remote station open a socket to the IP address of the gateway and the port matching the remote station. It then uses this socket to exchange data. It is up to the TCP gateway to convert the socket protocol in order to match the radio protocol. This concept allows several applications to share a radio network, it also hides the complexity of the radio technology to the control and acquisition application. So it will be possible to upgrade the radio technology without changing the acquisition applications. This and other advantages are summarized below:

  • applications don’t have to be rewritten when changing the network technology.
  • applications don’t have to be located on the machine or LAN of the transceiver, they can be located anywhere on the Internet
  • access to the instrument from anywhere on the Internet is possible
  • different applications can share the same wireless network and same base transceiver
  • simple access mechanism (socket) and available in all programming language

The control and acquisition applications sends the data directly to the EMEWS database (using ActiveX Data Object or JDBC) or, in case of seismic data, format it as Earthworm packets. If this is not possible, especially for commercial application which cannot be modified, a converter must be provided that will automatically insert new data into the EMEWS database.