What do they do?

Network interface cards (NICs) are installed in a computer to allow it to communicate with a network. They provide a transparent interface between the network and the computer. The computer gives the NIC a message for another networked device and the NIC formats that message for transport over the media. The reverse is also true, the NIC receives messages from the network and reformats them so that the computer can understand. NICs have their own CPU (central processing unit) and RAM (random access memory).

NICs come in many shapes and sizes. They can be installed internally or externally, although an internal installation is more likely. The picture illustrates an internally installed combo (more than one type of connection) card. This card is somewhat unusual in that it supports three connections:

• The upper connection is an RJ45 female for UTP media.
  
  
• The middle connection is a BNC connector for thin coaxial media.
  
  
• The lower connection is a DB-15 connector for a thick ethernet viper tap.

How do they do it?

The NIC has to be able to comply with the method of medium access control (eg. CSMA/CD) for the network. The MAC methodology is represented at the datalink sublayer of the network access layer of the TCP/IP protocol suite. Each NIC has it's own MAC address that is assigned to it by the manufacturer. It is up to the network administrator to assign an IP address to the device in which the NIC is installed.

In order for a NIC to operate effectively, it must be able to carry out its interface tasks with minimum disruption to the CPU of the computer in which it is installed. Four methods of NIC to computer data transfer are used:

• Bus mastering DMA (direct memory access):
• Data enters the NIC from the network.
• The NIC's own CPU stores data in the NIC's RAM.
• NIC's CPU sends data to computer's motherboard when the network transmission is complete.
• The computer's CPU is not interrupted - the NIC's CPU has ultimate responsibility for data transfer.
  
  
• DMA:
• Data enters the NIC from the network.
• The NIC's CPU interrupts the computer's CPU.
• The computer's CPU stops other tasks and transfers the network data into its RAM.
  
  
• Programmed I/O (input/output):
• Data enters the NIC from the network.
• The NIC's CPU loads the network data into a motherboard I/O address.
• The computer's CPU checks the I/O address for any network data.
• If there is any data, the computer's CPU transfers the data to its RAM.
  
  
• Shared memory:
• Data enters the NIC from the network.
• The NIC's CPU stores the data in the NIC's RAM.
• The NIC's CPU interrupts the computer's CPU.
• The computer's CPU stops other tasks and transfers the network data into its RAM.

What else can they do?

• Some manufacturers have developed motherboards with onboard NICs (not a seperate card, but a device intrinsic in the motherboard).
  
  
• Dual cards - some cards can support two speeds of data transfer, eg. 10/100 cards that can detect whether data is arriving at 10Mbps or 100Mbps.
  
  
• Multiport cards - suited for servers where spare expansion slots are at a premium. The NIC can have upto four ports on a single card.

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