What is Data Link Layer?

Data Link Layer is used to move packets from the network layer on two different hosts. The process of transmitting packets on a given link layer and receiving packets from a given layer can be controlled both in the software device driver for the network card as well as on firmware or specialist chip sets. These will perform data link functions such as adding a packet header to prepare it for transmission, and then actually transmit the frame over a physical medium. For Internet access over a dial up model, IP packets are usually transmitted using PPP (Point to Point Protocol). For broadband Internet access such ADSL or cable modem PPPoE is used. On a local wired network, Ethernet is usually used and on wireless network IEEE 802.11 is usually used. For wide area network either PPP over T-Carrier or E-Carrier lines, Frames relay, ATM or packet over SONET/SDH (POS) is 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 (Virtual Private Network).

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