Com-Servers replace analog leased lines

Many serial devices used in alarm systems, telecontrol systems, time and attendance systems, etc. are connected to higher-level systems via leased lines and the corresponding leased-line modems. With the phase-out of analog dedicated lines, users must rethink their approach and find suitable alternatives. Using the Internet through the paired deployment of Com-Servers is, of course, one of the first ideas that comes to mind, and it offers several advantages:

• Easy integration into the network, which is usually already in place
• Spatial flexibility of the serial devices
• Independence from the Internet service provider

Box-to-Box mode

In Box-to-Box mode, the Com-Servers establish a permanent TCP connection with each other, through which the data flow from the serial devices is tunneled. All serial data received by the communication servers is transmitted to the respective remote end over the network as quickly as possible. In Box-to-Box mode, one COM server acts as the master (TCP client) and the other as the slave (TCP server). The perimeter router on the Slave-Com-Server side must therefore be able to forward incoming connections on the WAN side to the desired IP address on the intranet via NAT (application example NAT). When line breaks or faults, voltage drop-outs etc. occur, the automatic recovery functions of Box-to-Box mode ensure that the connection is quickly reestablished.

Requirements for Box-to-Box mode:

• WAN-side fixed IP addresses on both DSL routers
• At least one router must support NAT
  

TCP Client mode

If no static IP addresses are available, TCP client mode can be used as an alternative to Box-to-Box mode. In this case, one of the COM servers is configured as a TCP client so that, upon receiving serial characters, it establishes a connection to the partner COM server specified by host name. As with Box-to-Box mode, a bidirectional, transparent TCP tunnel is then available for transporting the serial data. TCP client mode is particularly well-suited for serial master/slave protocols in which polling is initiated by a master. On this page, the COM server configured as a TCP client is installed. A prerequisite for using TCP client mode is that the perimeter router on the serial slave side must be able to change its WAN IP address to a dynamic name server service such as DynDNS.

Prerequisite for TCP Client mode:

• The perimeter router(s) must have a DynDNS client
• At least one router must support NAT

Box-to-Box mode in VPNs

An alternative to replacing analog leased lines is to connect the various locations using VPNs (Virtual Private Network). What the various implementation options for this type of linking have in common is that two networks are connected over the Internet using encryption. From the point of view of the clients a VPN tunnel can be considered like a simple infrastructure component, so that no special factors need to be considered when establishing Box-to-Box or TCP Client mode. The configuration always takes place just like within a conventional Intranet. Only the TCP port numbers used by the Com-Servers may need to be enabled in the firewalls by the system administrator.