Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00 4.10

When using TCP/IP protocol in the WFW3.11 or Windows 95 or Windows NT, for CITECT LAN communication, the response time on the client could be more than ten times slower than when NETBEUI protocol is used. If possible you should use the NETBEUI protocol to work around this problem. In many cases you will want to use the TCP/IP protocol to allow the protocol to be routed. This problem is also much worse Windows 95 and Windows NT than Windows for Workgroups.

This problem may show up in Citect with the Out of Buffers in lan.write.pool error message. This will occur as Citect cannot transmit the data fast enough across the network and Citects write buffers may overflow.

Microsoft has made extensions to the TCP/IP protocol to support piggybacking and windowing of packets. These features are designed to make the most efficient use of the network bandwidth, however they may add slight delays to the network transfer. These extensions increase the performance when using file transfers however they degrade the performance of any application requiring real time communication. As Citect is a real time applications these extension will degrade the performance of Citect. If you have any other real time applications these extensions will also be degrading their performance as well.

Currently there is no known way to disable these extensions in TCP/IP. The only way to work around this problem is to increase the number of transmit buffers on the server (as detailed below) or use a different protocol. This same problem exists with NetBEUI see also Q1721 and IPX/SPX see Q1711. However you can disable the effect on Windows for Workgroups. We are currently researching this problem and so far we cannot find a way to disable this effect on Windows 95 and Windows NT for any network protocol, we are working with Microsoft to find a solution to this problem

You may work around the real time limitation of the network protocol by increasing the number of transmit buffers Citect will use. You will only need to increase the transmit buffers on Citect which are sending a large number of network packets. For example increase on the I/O Server or Alarm/Trend/Report servers. You should not need to increase transmit buffers on a Citect client. You adjust the transmit buffers with the parameter [LAN]SesSendBuf. See online help for full documentation on this parameter. You must be careful when adjusting this parameter as you can cause the network protocol to run out of transmits buffers if you set this number too high. If the protocol runs out of transmit buffers you will suffer a major degrade of network performance and you will get hardware errors. The default for this parameter is 2 and you should try to slowly increasing this parameter. Values of 8 to 12 with TCP/IP will give good performance without causing the protocol to run out of transmit buffers. Warning Windows for Workgroups and Windows 95 has a maximum limit of 42 pending NCB See Q1861 for details. When you hit this limit the network performance will greatly degrade. Windows NT does not have this limitation and you should set the SesSendBuf=32, which is the maximum allowable.

We are in the processing of creating a work around for this problem and it is expected this will be available in the next release of Citect (possibly version 3.40 and 4.20). If you require a solution to this problem before the release of these versions contact Citect Support.