What is Bit Time?

Bit time is a computer networking term that measures how long one pulse or bit takes to travel from a transmitter to a receiver to produce a specific network data rate. It is sometimes confused with other terms such as bit rate or baud rate, which is the total number of bits per second (bps) transmitted, and slot time, which is the amount of time it takes for a pulse to travel through the longest length of a network medium. Bit time, however, only calculates the ejection of one bit, and instead of focusing on the network medium, it looks at how this bit transmits out of a network interface card (NIC) at a determined speed, such as 10 Mbits/s.

Many people have heard the term "bit" used in reference to computers, but they might not know exactly what one is or how it is used. A bit is a single binary number, either zero or one, that is used in network transmission to indicate the amount of voltage pulsing through a circuit. Thus, bit time is looking at one of these pulses and how quickly it responds to an instruction to leave the NIC. As soon as the logical link control layer 2 sublayer receives a command from the operating system, the measurement of bit time begins, calculating how long it takes for the bit to eject from the NIC. The basic formula for is as follows: Bit time = 1 / NIC speed.

Some common measurements of bit time are 10 nanoseconds for Fast Ethernet and 100 nanoseconds if the determined speed is 10 Mbits/s for the NIC. The bit time is 1 nanosecond for gigabit Ethernet. To put that another way, to transmit 1 Gbps of data, it takes just 1 nanosecond. Overall, then, the higher the data rate, the shorter the bit rate.

This measurement becomes a significant discussion in computer network analysis in regard to the issue of low latency networks. There is some question about whether a lower bit time combined with a higher transmission speed of the signal translates into lower latency. This issue seems to be up for debate.

Latency, along with throughput, is a basic measure of network performance. The latency measures how long it takes for a message to travel through a system. Therefore, low latency indicates that a short amount of time is required and that the network is efficient. Bit time, then, comes into play here as network managers continually work to improve on network performance and evaluate how different times affect latency.