1.5.7. Gateway and Client

As a minimum, a gateway consists of the standard communication stack and an additional gateway component. This makes the gateway into a network node. In principle, each node can be extended to a gateway by integrating this component. A runtime system can also take on the role of a gateway. Via a special gateway protocol this component provides external clients with access to nodes within the network, in which case communication is via the gateway instead of the client. The gateway has no other special role within the network. A network can therefore have any number of gateways, and the gateway can be positioned anywhere in the network. Two positions are particularly suitable in order to avoid unnecessary bandwidth problems:

  • As top-level node: In general this is where the network with the highest bandwidth is located. In addition, access to all branches involves the same level of complexity.

  • Near the target system: This solution is particularly suitable if the gateway is mainly intended for a particular subnet that may be connected to the rest of the network via low (available) bandwidth.

If more clients can communicate with a target system than the number of channels the system can make available on Layer 4, the gateway can multiplex the requests for different clients on one channel. To this end the gateway opens a single channel for all clients and sends requests to the clients in turn, waits for the reply, sends the next request, etc. While this technique slows down communication of the individual clients to some extent, for sporadic requests it uses significantly less resources in the target system.

Common clients are tools for programming (e.g. CODESYS itself), visualization, remote diagnostics, etc. The main feature of a client is that it is generally only connected to the network on a temporary basis. In addition, a client is always the active communication partner, i.e. a node will usually not establish communication with, but only respond to requests from a client.

The communication between client and gateway is based on communication drivers. In contrast to communication between block drivers this communication is connection-oriented. In addition there is no (direct) limitation of the maximum packet sizes. However, since the main part of the communication is forwarding of Layer 7 packets, the packet size is effectively limited by the negotiated size of the communication buffer plus administrative data.

Gateway requests include the following components:

  • Network scan: From the scan the gateway generates a request for the name service and supplies all the answers of the available nodes in the network as response. Network scan (broadcast) is not supported by the CmpBlkDrvTcp.

  • Address services: Initiation of address assignment, etc.

  • Layer 4 services: Setup and removal of a channel, sending of packets via this channel. By default the content and structure of these packets (e.g. monitoring request) is not known to the gateway. It merely forwards these packets to the associated receiver without interpreting them.

  • Diagnostics and administration of the gateway itself.

  • …

These services can be extended as required.

A basic client implementation is available for clients (GWClient), that covers communication with the gateway, integration of the communication drivers, etc. Specific clients should use this library in order to avoid getting bogged down in details of the communication with the gateway.