Data Link Layer

The link layer, which is the method used to move packets from the network layer on two different hosts, is not really part of the Internet protocol suite, because IP can run over a variety of different link layers. The processes of transmitting packets on a given link layer and receiving packets from a given link layer can be controlled both in the software device driver for the network card, as well as on firmware or specialist chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium.

For Internet access over a dial-up modem, IP packets are usually transmitted using PPP. For broadband Internet access such as ADSL or cable modems, PPPoE is often used. On a local wired network, Ethernet is usually used, and on local wireless networks, IEEE 802.11 is usually used. For wide-area networks, either PPP over T-carrier or E-carrier lines, Frame relay, ATM, or packet over SONET/SDH (POS) are often used.

The link layer can also be the layer where packets are intercepted to be sent over a virtual private network. When this is done, the link layer data is considered the application data and proceeds back down the IP stack for actual transmission. On the receiving end, the data goes up the IP stack twice (once for routing and the second time for the VPN).

The link layer can also be considered to include the physical layer, which is made up of the actual physical network components (hubs, repeaters, fiber optic cable, coaxial cable, network cards, Host Bus Adapter cards and the associated network connectors: RJ-45, BNC, etc), and the low level specifications for the signals (voltage levels, frequencies, etc).