13.6 Network Interface Card

The network interface card (NIC) is a circuit board designed for insertion into the expansion slot of a specific type of computer, such as an IBM PC or compatible computer, a Micro Channel IBM PS/2 computer, or an Apple Macintosh. This card contains a series of chips or an integrated chip set that implements the network access protocol, as well as RAM memory, and other chips designed to perform specific functions. Both the hardware design and card functionality can have a considerable bearing on the ability of the adapter card to transfer data onto and receive data from the network.

13.6.1 Performance Considerations

The key to the ability of a NIC to transfer data onto and from a network is the method by which data transfer occurs between the adapter card and the computer in which it is inserted. Data transfer between the adapter card and the computer can be implemented in several ways, including direct memory access (DMA), I/O mapping, and shared memory.

13.6.1.1 DMA Interface

A direct memory access (DMA) interface is a data transfer technique in which the transfer of data between the adapter card and the computer's memory can occur simultaneously with other operations. To accomplish this, the DMA transfer is initiated by a board processor storing a starting and ending address and then initiating a DMA transfer operation, after which the processor can perform other operations during the time that data is transferred from the adapter card's memory to the workstation's memory. If the adapter card does not include an onboard processor, circuitry can be used to implement a DMA transfer. To accomplish this, an on-board buffer area on the adapter card receives data from the network. As the buffer fills, circuitry on the adapter recognizes this condition as a signal to generate a DMA transfer.

Although a DMA transfer permits the transfer of data from the NIC to the computer's memory to occur with other operations, it is generally slow in comparison to other methods of data transfer. The reason for this is due to the time required to set up and initiate a DMA transfer. A second factor that limits the capability of a DMA transfer is the fact that it requires contiguous memory in the host computer for its transfer operations. Because frames are often assembled from several different areas in memory, each DMA transfer may be limited to the amount of data that can be transferred.

13.6.1.2 I/O Mapping

I/O mapping is a data transfer mechanism based on the shared use of an I/O port between the adapter card and the host computer. This method of data transfer is faster than the use of a DMA transfer because it occurs at the I/O channel speed and eliminates the DMA transfer setup time.

13.6.1.3 Shared Memory

In a shared memory method of data transfer, a portion of the computer's memory is set up so that it can be used by both the adapter card and the computer. This results in a frame avoiding an actual transfer between the adapter and the computer, thus enhancing the data transfer between the computer and the network.

Most modern high-performance network adapter cards use some type of shared memory scheme to expedite the transfer of data from the computer to the network, and vice versa.

Typically, shared memory can be set up in increments of 16 Kbytes, from 16 to 64 Kbytes. Because the use of a LAN program and an application program will typically use a majority of conventional memory, many computers are limited to a setup in which only 16 Kbytes are used as shared memory. While this amount of memory will not adversely affect the operation of NICs connected to a 4-Mbps Token Ring or a 10-Mbps Ethernet network, the opposite can occur if you are connected to a 16-Mbps Token Ring network. This is because the 16-Mbps Token Ring network can have an information frame whose length can exceed 16 Kbytes.

In comparing the method used by the NIC (network interface card) for transferring data, performance can significantly differ based on the technique used. Although you may be tempted to simply select a vendor's adapter card based on their stated data transfer rate, there is one additional item you should consider prior to doing so. That additional item is the transfer rate of the hard disk of the computer in which you intend to install the NIC.

The transfer rate of the hard disk will vary based on the type of disk and disk controller used, as well as the bus interface of your computer. For example, antique PC XT computers have a disk I/O transfer capability under 1 Mbps. Thus, installing a shared memory network interface card with a 2-Mbps transfer capability would probably be extravagant because any sustained transfer of data to or from the computer's hard disk would be limited to 1 Mbps regardless of the type of NIC used. Thus, you should select a NIC in conjunction with the performance characteristics of the computer in which it is to be installed.