Lesson 6

Network Adapter Cards

NIC

A network adapter card is an expansion card which pushes into an expansion socket of the PC. The card has a socket into which a network cable can be plugged. These cards are also known as NICs - network interface cards.

(Some more modern PCs have a network interface right on the motherboard - so a NIC is not needed.)

Every NIC made has a numeric address programmed into its chips. These addresses are given to card makers by the IEEE. Every card made has a different address to all the others ever made.

What Does A NIC Do?

We think of sending data from one computer to another. In fact it is better to think of one Network Adapter sending data to another Network Adapter, as it these cards that actually talk to each other.

1. Sending Data

The NIC prepares, sends and controls the transmission of data.

a) Parallel to Serial Conversion

Data is moved around inside the PC on the data bus. This is a set of parallel copper tracks on the motherboard of the PC. For example, the binary number 10001010 01001101 11110111 01001001 can be moved to the network card in one operation, each wire of the bus carrying one bit.

The network cable can only send one bit at a time, so one job of the NIC is to take the incoming parallel data and covert it into a serial bit stream.

b) Talking to the Receiving Card to Agree The Rules.

Computer data is not sent as a continuous stream, rather it is broken up into units called packets. Each packet contains some data, the address of the sending card, and the address of the receiving card. Before data transmission can happen, the transmitting and receiving cards talk electronically to agree details of the packets:

1. How big will the packets be?

2. How many packets will be sent, before the receiver acknowledges receipt?

3. How much time must pass between sending each packet?

4. How long should the sender wait for the receiver to confirm that it has received the data, before re-transmitting?

5. How much data can the receiver hold before its memory gets full?

6. How fast will the data will be sent?

In this way modern, fast NICs can work with older, slower ones.

When the rules have been established, the sending card puts the addressed packets of data onto the cable so that they can be received by the other card.

The part of the card that actually puts the data signals onto the cable is called the transceiver.

We shall learn later how many computers can send data over a single cable without interfering with each other. For now understand that the details of this process are handle by the NIC - the host PC does not have to worry about such details.

2. Receiving Data

The receiving card must negotiate the rules for transmission with the sending card.

All the computers on the network share the same cable. The receiving card must check the address of each packet on the cable, read those for it, and ignore all others.

The Configurable Options of a NIC

When we put a new NIC into a computer there are settings that have to be made before the card can work properly. These settings are made using an installation program supplied on disk, along with the NIC.

1. Which Interrupt?

An interrupt is a way that devices in a PC, such as a mouse, or a network adapter, can interrupt a program to tell it that something happened. A PC as 16 IRQs, numbered 0 through 15. We must assign an unused IRQ to the network card. Normally IRQ5 or IRQ3 is used.

2. Base I/O Port

This is a place in the PC’s memory map that it used for transferring data between the adapter card and the rest of the PC. It is like a doorway. We must choose an address that its not being used for any other purpose.

3. The Card’s Base Memory Address

NICs need RAM to hold data as it is being sent or received. This is called a buffer. Some NICs have their own RAM chips right on the card. Others use part of the PCs memory. In this case, we must set the starting address of the memory that will be used, plus how much memory can be used. If we let the NIC use a lot of memory it may work faster, but there will be less memory for use by our programs.

4. Which Transceiver?

Some NICs have both an on-board transceiver and a special socket to allow the use of an external transceiver. With such a card we must tell the system which we will use.

What To Think About When Buying a NIC

1. The Card Must Fit The Expansion Slot

Different computers have different bus types, and so different expansion slots. Before buying a card you must find out if your computer has a ISA, EISA, MCA or PCI bus. You must then buy a card to fit.

2. The Sockets On Your Card Must Match The Cable You Plan To Use.

The card must have a BNC socket if you plan to use Thinnet coaxial cable, and AUI socket if you will use Thicknet co-ax or an RJ-45 socket for twisted pair cable.

Combi-cards have both a BNC socket and an RJ-45 on the same card.

3. Are Drivers Available?

Are driver programs supplied with the card, for the operating system you plan to use?

Enhancements To NICs to Improve Network Performance

1. Direct Memory Access.

Cards that can do DMA can move data into and out of any part of the PC’s memory map without help from the PCs CPU. This makes the computer work faster.

2. Bus Mastering

This is like DMA. The NIC takes control of the PC’s bus system and handles the movement of data to and from the card. This frees the main CPU to do other jobs. Bus mastering adapter cards can improve the network’s performance by up to 70%.

3. RAM Buffering

Sometimes the data comes off the network cable faster than the adapter card can process it. For this reason some cards have RAM chips to hold the data until the card can deal with it.

Back to top