When two devices are able to communicate information back and forth to each other, the system is said to be duplex. A system where information can be transmitted and received simultaneously by the same device is full-duplex. An example of a full-duplex system is a telephone call. Both parties can simultaneously talk and listen to each other. A system where transmission and reception cannot occur simultaneously on the same device is called half-duplex. An example of a half-duplex system is communication with walkie-talkies. One party can talk or listen to the other party, but they cannot do both.

When data is transmitted across a single channel, the system is said to be serial. In a serial system, bits in a packet are sequentially sent, one at a time. When data is transmitted across multiple channels, the system is said to be parallel. In a parallel system, multiple bits can be sent simultaneously.

Parallel systems can transmit data at a faster rate than serial systems. In a parallel system, data is being transmitted over several channels at the same time, whereas data is being transmitted within a single channel for serial systems. However, serial systems are less prone to corruption. In a parallel system, the bits being transmitted over each channel can be received at different times, subsequently corrupting the data. Since the serial system transmits bits one at a time, it is less likely that the data will be corrupted.

When two devices share a clock signal, the system is said to be synchronous. When they do not share a clock signal, the system is asynchronous. In order for the receiver to understand when data is being transmitted in an asynchronous system, control bits (such as start or stop bits) are used as flags. As such, in an asynchronous system, the bits transmitted in a packet are not entirely comprised of data.