A NIC (network interface card) is designed to communicate over a computer network. It allows users to connect to each other either by using cables or wirelessly if the NIC is a wireless NIC (WiFi/WNIC). Every entity on a network, a PC, printer, router, etc., that needs to communicate with other devices must have a NIC if it is to communicate over the network. On older computers, the NIC is probably an expansion card, usually PCI or PCI express. High performance cards can cost less that $30. NIC functionality is now often integrated into the motherboard chipset or implemented with a dedicated Ethernet chip on the motherboard.

A similar situation is true for laptop computers. At one time, a PCMCIA network card would be used in a laptop computer for the NIC just as the PCI card was used in desktop computer, but now, NIC functionality is often integrated with the motherboard.

Ethernet is the dominant standard for cable connections for wired computer networks. An Ethernet connector looks similar to a telephone connector, only larger. This connector is called “RJ45”. Ethernet cables are either a shielded or unshielded cable of four twisted pairs of 24 AWG connectors, specified at 100 ohm impedance. Maximum cable length for CATX cables is 100 meters.

Early versions of Ethernet cables were CAT3 or CAT4 (CAT being short for category). These versions were not long lived. CAT5 and CAT5e are currently the most commonly used cables (bandwidth of 100 MHz, 100 Mbps), with CAT6 (bandwidth of 250 MHz, 1 Gbps) available and the configuration of the near-future. A CAT7 cable (bandwidth of 600 MHz) specification is in development, and should be available in a few years.

Each Ethernet NIC has a unique serial number called a “media access code” (MAC address) that is used to identify the NIC and associated computer on the network. No two NIC will have the same address, because the NIC manufacturers must purchase blocks of addresses from the Institute of Electrical and Electronics Engineers (IEEE).

NIC cards are capable of different speeds. Speeds of up to one gigabit per second (Gbps) are now available. Two NIC can communicate if they differ in speed ratings, but they will communicate at the rate of the slower NIC.

On a very simple network, NIC can be used to link personal computers (PC). If the computers are connected directly to one another, the network is a “peer-to-peer” (P2P, also called an “ad hoc”) network. If computers are connected directly to one another, a “cross-over” Ethernet cable is needed (also called a “Null-Modem cable”). This cable is not “straight-through” like standard Ethernet, but crosses the send and receive connectors, so that send line from computer A connects to the receive line of computer B.

For networks of a few computers, a “hub” can be used, with all of the computers connected to the hub. Any message sent from any PC will be seen by all of the computers, but only the computer with the correct MAC address will receive the message. P2P networks are useful for many purposes. File and printer sharing are the most common applications.

Networks with more than four active computers at a time will benefit from using a “network switch” instead of a hub. A network switch will direct the message to the appropriate destination, instead of each message packet being broadcast across the network. Computer A can send a message to computer B, while simultaneously, computer C sends a message to computer D. This increases the efficiency of the network. This is a simple form of a server network. An example of a client-server design is a computer server where the clients initiate a download or upload of files and the server reacts. The server would also often be responsible for interacting with printers and other servers on other networks, including satellite networks (SATCOM) or the Internet.

“Latency” is the delay caused by a network to communicate data. Latency causes not only slow service, but can result in data loss as well. Latency is usually tested by sending a message packet that is immediately returned to the sending computer. The round-trip time is defined as the latency.

Networking performance can be optimized for either latency or throughput. Networks can use a technique known as “interrupt moderation” to increase throughput and lower CPU utilization by queuing message packets and issuing fewer interrupts to the CPU. A network optimized this way would favor large transfers, reducing transfer overhead. CPU and network throughput benefit, but network latency would increase. The alternative is to fragment each message and pipeline the fragments through the network. Multiple paths from source to destination can allow overlapping. Latency can also be addressed with techniques such as prefetching (anticipating the need for data requests) and/or using multiple execution threads (multithreading).

There are two indicator LEDs on a typical NIC. A single lit green LED indicates the computer is connected to the network. This is called the “link” light. The second LED is amber in color. A flashing amber LED indicates message packet collisions are occurring. Occasional collisions are normal on a busy network, but a frequently lit amber LED is an indication of problems. A quickly flashing link LED (green) is a network activity indicator, meaning that communication is occurring. If the green link light is off, and the amber LED is blinking, then the NIC is in “power save” mode.

A wireless network interface controller (WNIC) is a NIC for connecting to a wireless network (WiFi). The standard for WiFi is IEEE 802.11. A WNIC uses an antenna to communicate with an Rf signal, most often at 2.4 GHz. Wireless connectivity development in recent years has closely paralleled wired connectivity. Like its wired cousin, a wireless card was once integrated with a PCI or PCMCIA add-on card. Now, integrated WNIC’s are becoming common, especially for laptop computers.