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So far in this chapter, we have dealt with layer one devices and concepts. Starting with the
network interface card, the discussion moves to layer two, the data
link layer, of the OSI model.
In terms of appearance, a network interface card (NIC card or NIC) is a printed circuit board that fits into the expansion slot of a bus on a computer’s motherboard or peripheral device. It is also called a network adapter. On laptop/notebook computers NICs are usually the size of a PCMCIA card. Its function is to adapt the host device to the network medium. 
 
NICs are considered Layer 2 devices because each individual NIC throughout the world carries a unique code, called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network. You will learn more about the MAC address later. As the name implies, the NIC controls the host's access to the medium.
In some cases the type of connector on the NIC does not match the type of media that you need to connect to. A good example is your Cisco 2500 router. On the router you will see
AUI (Attachment Unit Interface) connectors and you need to connect the router to a UTP Cat5
Ethernet cable. To do this a transceiver (transmitter/receiver) is used. A transceiver converts one type of signal or connector to another (e.g. to connect a
15-pin AUI interface to an RJ-45 jack, or to convert electrical signals to optical signals). It is considered a Layer 1 device, because it only looks at bits, and not at any address information or higher level protocols.
NICs have no standardized symbol. It is implied that whenever you see networking devices attached to network media, there is some sort of NIC or NIC-like device present even though it is generally not shown. Wherever you see a dot on a topology, there is either a NIC
or an interface (port), which acts like at least part of a NIC.
To find out more about commercially available NICs, visit:
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