Network Basics: Switch Duplex/Simplex Communication
Network switches can operate in either duplex or simplex mode. The distinction makes a fundamental difference in the throughput and reliability of these devices on your network.
The terms duplex and full duplex are synonyms, as are simplex and half duplex. A simplex device is not capable of sending and receiving data at the same time. Think of the old CB radios or most of the FRS radios on the market. These devices can transmit your voice, or they can listen to the other people on the channel, but they are not able to do both at the same time, ever.
The same is true for half duplex or simplex devices. These devices are only capable of sending or receiving data at any given time, so they are constantly changing from sending to receiving. This prevents them from sending data as fast as they possibly can, because after sending data, they need to wait for responses from the target systems.
The other option, full duplex, or duplex, enables network devices to send and receive data at the same time and to send it continuously, because they can receive responses to the sent packets as they are returned. When dealing with inter-switch links, or connections between switches, full duplex is the way the links need to be configured.
Using half duplex links between switches will have a grave impact on your data throughput. For example, if your network is composed of two switches with your clients’ computers on one switch, your servers on the other, and the ports linking the two switches are set to half duplex, you have a recipe for disaster.
Similar to a repaving project where traffic in both directions shares one lane on the road, as traffic queues up to pass over the one-way-at-a-time link between the switches, the clients' devices on one switch will constantly be delayed trying to "talk" to the servers on the opposite switch.
Simply changing switches to full duplex on an inter-switch link can dramatically change the performance of the network.