Simplex, Half Duplex, and Full Duplex

Simplex, half duplex, and full duplex are referred to as dialog modes, and they determine the direction in which data can flow through the network media.

Simplex allows for one-way communication of data through the network, with the full bandwidth of the cable being used for the transmitting signal. One-way communication is of little use on LANs, making it unusual at best for network implementations. Far more common is the half-duplex mode, which accommodates transmitting and receiving on the network but not at the same time. Many networks are configured for half-duplex communication.

The preferred dialog mode for network communication is the full-duplex mode. To use full duplex, both the network card and the hub or switch must support full duplexing. Devices configured for full duplexing are capable of transmitting and receiving simultaneously. This means that 100Mbps network cards are capable of transmitting at 200Mbps using full-duplex mode.

Media Interference

Depending on where network cabling (commonly referred to as media) is installed, interference can be a major consideration. Two types of media interference can adversely affect data transmissions over network media: electromagnetic interference (EMI) and crosstalk.

EMI is a problem when cables are installed near electrical devices, such as air conditioners or fluorescent light fixtures. If a network media is placed close enough to such a device, the signal within the cable might become corrupt. Network media vary in their resistance to the effects of EMI. Standard UTP cable is susceptible to EMI, whereas fiber cable with its light transmissions is resistant to EMI. When deciding on a particular media, consider where it will run and the impact EMI can have on the installation.

A second type of interference is crosstalk. Crosstalk refers to how the data signals on two separate media interfere with each other. The result is that the signal on both cables can become corrupt. As with EMI, media varies in its resistance to crosstalk, with fiber-optic cable being the most resistant.