PHYSICAL LAYER DRY CONTACT TCP/IP SWITCHING
By: Don McClatchie
Transmission Control Protocol and Internet Protocol TCP/IP is one of the most common methods used to transmit and receive data from one computer or switch to another. It consists of seven layers, the Application layer (FTP, SNMP or HTTP), Presentation layer (ASCII, MIDI or PNG encryption), Session (Syn and Ack), Transport layer (UDP, TCP Port Numbers), Network layer (Routers, IP), Data Link layer ( MAC, switches), and a seventh one, the Physical layer (8P8C connectors and cable). It is the physical layer that is the connection point for all points outside of the computer so for security purposes this is the final control gate for any data that will enter or exit your computer.
A network plug called an 8P8C type connector is the most common way an internet connection is made to the computer and it is through this connector that the physical layer completes the connection to the other switches and computers on the internet. There are many reasons why you might want to disconnect this data cable from your computer from time to time. For instance, during a lightning storm high voltages can damage your computer if it stays connected, or if you suspected malware was operating on your computer, or you wanted to restrict data access during off hours when building security is lower, or you wanted to prevent data exchange during a building alarm condition or at the end of the day. I am sure you could think of other situations that would require a data cable interruption.
You can program a computer to disable the modem or physical layer inside the computer, but that approach can be hacked and disabled easily and disabling the modem still leaves it physically connected to the computer, what we call a wet connection. A wet connection is one that allows current flow into the computer as far as the modem electronics and can be used to destroy the modem in the case of lightning strikes or malicious intent. Any data wire connected to the inside of your computer can be a problem when full isolation of your computer is desired.
If your data communication is vital and cannot be allowed to fail it is a good idea to have a back up computer running that can be switched to in case of failure or corruption of the primary computer. This should be done with an external physical layer A/B switch that uses dry contact switching. Dry Contact Switching is a system that uses C form relays for all wire connections that have an Air Gap between the un-used data channel. An air gap relay will prevent all but the most direct lightning strikes from damaging your computer modem and it cannot be hacked through the data paths.
You can also use physical layer A/B switches for Zone Switching in IP security camera applications. An example would be at a vacation home, your customer might want in door cameras in all the rooms so they can call up and see the interior of the residence for security checks, but when they come to visit they would want them turned off for privacy during their stay. In this scenario you can have more cameras than you have NVR inputs by A/B switching the NVR camera input ports from one camera to another camera with an external data relay. Or take the hospital that needs a camera in a room during examination but then must turn off the camera when the room is being used for other patient needs. Yes, you can just arrange to cut the power to the camera, but it may be easier and more secure to cut the camera feed.
So, if you find a need to switch IP data using a dry contact method look at the IPX-24. It is an A/B switch controlled by any 24 VDC power source. When the power is applied the unit switches to the B channel and when the power is removed, it reverts back to the A channel. All the ports are transparent to the data rates, protocols and formats. This unit gives you simple and direct control over the physical layer of any data network up to 10 Mbps.
If you need dry contact air gap A/B switching of video using BNC connectors like TVI, CVI, or any of the analog HD and standard video formats you can use the VS-24 for 24 volt use or VS-12 for 12 volt use.
For stereo audio dry contact air gap A/B switching you can use the SAR-2 Stereo Audio Relay. It is designed to switch balanced or unbalanced stereo audio channels. The standard control voltage is 24 VDC, also available in 12 VDC control.
All of these unusual products and more can be viewed at www.fmsystems-inc.com
or call 800-235-6960 for more information and pricing. E-Mail me at: fmsystemsinc@sbcglobal.net
