Regardless of the topology implemented or the specific medium used, the cable plant cannot simply attach to the back of a computer. Some kind of interface is required between the computer system and the medium. This is the role of the LAN adapter.
The process of generating a message for transmission to another computer is simply an output operation, and receiving a message is simply an input procedure. A computer is well-suited to performing input/output (I/O) operations—all it needs is the appropriate hardware. If the I/O is to a disk drive, a drive controller is needed; if the I/O is to a video monitor, a video controller is needed; if the I/O is to a LAN, a LAN adapter is needed.
A LAN adapter has two interfaces. One interface connects to the host computer. Associated with this interface is a parallel connector that physically attaches to the host, and the circuitry to support exchanges between the host and the LAN adapter. The other interface connects the adapter to the LAN. Associated with this interface is the hardware to perform parallel-to-serial conversion, a serial connector, a line driver to boost the signal strength, and the circuitry necessary to support transmissions to and from the LAN.
Also found on the LAN adapter is one or more integrated circuits (IC) responsible for controlling the operation of the adapter. One of the responsibilities of the LAN adapter in a shared-bandwidth LAN is to gain access to the medium. The Media Access Control (MAC) protocol that provides this capability is implemented within the adapter.
Finally, the LAN adapter must implement some amount of memory to be used as buffers. When a transmission is received from the LAN, it must be buffered until the LAN adapter can verify its correctness. When a transmission is received from the host, it must be buffered until the LAN adapter can gain access to the LAN. If the host system submits several transmissions before the LAN adapter can secure access to the network, each must be buffered and then processed on a first in, first out (FIFO) basis.
The operation of a LAN adapter is straightforward. If the host computer needs to transmit, it builds a frame that includes the address of the LAN adapter in the transmitting system, the address of the LAN adapter in the intended recipient, and the information to be transmitted. There might also be other control information. The host computer passes this frame to the LAN adapter where it is buffered. The LAN adapter then secures access to the LAN. When access is secured, the LAN adapter begins transmission, calculating a frame check sequence (FCS) as it does so. The FCS is appended after the final bit of the frame.
If a LAN adapter detects a transmission in progress on the LAN, it reads the transmission until it has read the entire destination address. If the LAN adapter recognizes its own address as the destination, it copies the entire frame from the LAN, calculating the FCS as it does so. When the entire frame is copied, it checks its calculated FCS against the one appended to the frame by the transmitter. If they match, the frame is passed to the host computer; otherwise it is discarded.
Types of LAN Adapters
The visual depicts three of the more common forms of LAN adapters. The network interface card (NIC) is a specialized expansion card, a printed circuit board that attaches to the input/output (I/O) bus inside microcomputers, both desktops and servers. Along one edge of the NIC is the edge connector, which is a plug that inserts into a socket, called an expansion slot on the I/O bus. Via this connection, the NIC has access to the parallel signals flowing down the I/O bus within the computer. Because different computers implement different types of I/O buses, the NIC must implement the appropriate interface for the I/O bus of the computer in which it will be installed.
When the host computer wishes to send a message to another computer, it simply performs an I/O operation, passing the information to be transmitted to the NIC along the I/O bus. The NIC stores the information to be transmitted in a set of buffers, or random access memory (RAM), on the NIC itself.
Along another edge of the NIC (i.e., one that can be seen outside of the computer) is a connector that attaches to the LAN medium. NICs have a socket for a modular eight pin connector (commonly referred to as an RJ 45 connector) for twisted pair. NICs can have other types of connectors as well (i.e., fiber connectors).
The PC Card slot is a socket found on most laptops and palmtops. The slot is designed to hold one or more PC Cards, a credit card size device originally designed as a modular memory expansion card. The Personal Computer Memory Card International Association (PCMCIA) developed the standards that describe the PC Card and the PC Card slot. It did not take the industry long to note that other devices could be implemented as PC Cards and exploit the PC Card slot. As a result, there are PC Card modems, fax cards, LAN adapters, and various combinations of these.
By far, however, the market has moved towards computers being network-ready. This trend reflects the reality that most computers sold today are destined for attachment to a LAN at home or the office. This means that, alongside the keyboard and video ports, there is now a LAN port on the back of many computers that is integrated into the system board. This is true for laptops, IP telephones, peripherals, desktop systems, and servers.