NETWORK INTERFACE CARD


The role of the network adapter card in a network

Network Adapter/Interface Card (NIC) act as the physical interface or connection between the computer and the network cable. The cards are installed in an expansion slot in each computer and server on the network. The role of NIC is to: 

  • Prepare data from the computer for the network cable
  • Send data to another computer
  • Control the flow of data between the computer and the cabling system.
Preparing the Data

Before data can be sent over the network, the NIC must convert it from a form the computer can understand to another form which can travel over the network cable. Data moves through a computer along the paths called busses. 

Older busses, such as those used in the original IBM personal computer, were known as 8-bit busses because they could move data 8 bits at a time. Nowadays, many computers use 32-bit busses. When data travels on a computer?s bus, it is said to be traveling in parallel. Think of a 32-bit bus being a 32-lane highway with 32 cars moving side by side (moving in parallel), each carrying one bit of data. 

On the network cable, data must travel in a single bit stream. When data travels on a network cable, it is said to be traveling as a serial transmission because one bit follows another. In other words, the cable is a one-highway. The data on these highways always travel in one direction. The NIC takes data traveling in parallel as a group and restructures it so that it will flow through the 1-bit wide serial path of the network cable. This is accomplished through the translation of the computer?s digital signals into electrical and optical signals that can travel on the network?s cables. The component responsible for this is the transceiver.

The NIC also particpates in several other functions in taking data from the computer and getting it ready for the network cable:

1. The computer and the NIC must communicate in order to move data from the computer to the NIC. On NICs that can utilize Direct Memory Access (DMA), the computer assigns some of its memory space to the NIC.

2. The NIC signals the computer requesting the computer?s data.

3. The computer?s bus moves the data from the computer?s memory to NIC.

Sending and Controlling Data

Before the sending NIC actually sends data over the network, it carries on electronic dialog with the receiving NIC so that both NICs agree on the following:

a. Maximum size of the groups of data to be sent

b. The amount of data to be sent before confirmation

c. The time intervals between sending data chunks

d. The amount of time to wait before confirmation is sent

e. How much data each NIC can hold before it overflows

f. The speed of the data transmission

If a newer, faster NIC needs to communicate with an older, slower model, both NICs need to find a common transmission speed each can accommodate. When all of the communication details have been determined, the two NICs begin sending and receiving the data.

Configuration Options and Setting for NIC

NICs often have configurable options that must be set for the NIC to function properly. The examples include:

  1. a. Interrupt (IRQ)
  2. b. Base I/O port address
  3. c. Base memory address
  4. d. Transceiver
1. Interrupt (IRQ)
Interrupt request lines are hardware lines over which devices such as input/ouput ports, the keyboard, disk drives, and the NICs can send interrupts or requests for service to the computer?s processor. Interrupt request lines are built up in to the computer?s internal hardware and are assigned different levels of priority so that the processor can determine the relative importance of incoming service requests. When the NIC sends a requests to the computer, it uses an interrupt-an electronic signal sent to the CPU. Each device in the computer must use a different interrupt request line or interrupt (IRQ). In most cases, IRQ3 or IRQ 5 can be used for the NIC. IRQ5 is the recommended setting if it is available, and it is the default for most systems. We can use several tools such as Microsoft Diagnostic (MSD) to determine which IRQs are already being used.
2. Base I/O Port
The base input/output/ (I/O) port specifies a channel through which information flows between the computer?s hardware (such as the NIC) and its CPU. The ports appears to the CPU as an address. Each hardware device in a system must have a different base I/O port number. The port numbers (in hexadecimal format) below are examples:

- Port: 270 to 27F è Device: LPT3

- Port: 2F0 to 2FF è Device: COM2

3. Base Memory Address
The base memory address identifies a location in a computer?s memory (RAM). This location is used by the NIC as a buffer area to store the incoming and outgoing data frames. This setting is sometimes called the RAM start address.
4. Transceiver
The NIC may have other settings that need to be defined during configuration. For example, some cards come with an external and an on-board transceiver. In this case, you would have to determine which tarnsceiver you want to use and then make the appropriate choice on NIC.

NIC Performance

Because of the effect it has on data transmission, the NIC has a great affect on the performance of the entire network. If the NIC is slow, data will not pass to and from the network quickly. On a bus network, where no one can use the network until the cable is clear, a slow card can increase wait times for all users.

After identifying the physical requirements of the card-the type of connector it needs and the type of network in which it will be used- it will be necessary to consider several other factors which affect the capabilities of the NIC.

Although all NICs conform to certain minimum standards and specifications, some cards feature enhancements which greatly improve server, client, and network performance.

We can speed up the movement of data thorugh the card with the following:

a. Direct Memory Access (DMA)
With this method, the computer moves data directly from the NIC?s buffer to the computer?s memory, without using the computer?s processor.
b. Shared adapter memory
In this method, the NIC contains RAM which it shares with the computer. The computer identifies this RAM as if it is actually installed in the computer.
c. Shared system memory
In this system, the NIC?s processor selects a section of the computer?s memory and uses it to process data.
d. Bus mastering
With bus mastering, the NIC takes temporary control of the computer?s bus, by-passes the computer?s CPU, and moves data directly to the computer?s system memory. This speeds up computer operations by freeing the computer?s processor to concentrate on other task. These types of NICs are expensive, but they can improve network performance by 20 to 70 percent. Both EISA and Micro Channel Architecture NIC s offer bus mastering.
e. RAM buffering
Current network traffic travels at a speed which is often too fast for most NICs. RAM chips on the NIC form a buffer. When the NIC receives more data than it can process immediately, the RAM buffer holds some of the data until the NIC can process it. This can speed up the NIC?s performance and keeps the NIC from becoming a bottleneck.

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