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The Data Link Layer provides the functional and procedural means to transfer data between network entities .
Posted by sagitraz
searchnetworking.techtarget.com/sDefinition/0,,sid7_gci211899,00.html - 62k -
Posted by crouse
The data link layer is responsible for taking the Layer 3 packet (regardless of which protocol created it—IP, IPX, and so on) and preparing a frame for the packet to be transmitted on the media. There are different layer 2 frame types, they are:
• Ethernet
• Frame Relay
• Point-to-Point Protocol (PPP)
• High-Level Data Link Control protocol (HDLC)
• Cisco Discovery Protocol (CDP)
The type of frame created depends on the type of network service in use; if it is an Ethernet interface, obviously it will be creating Ethernet frames. A router serial port can create several different frame types, including PPP, HDLC, and Frame Relay.
The data link layer uses flat addressing—not hierarchical as in Layer 3. In Ethernet, the addresses in question are MAC addresses. MAC stands for Media Access Control. A MAC address is a number assigned by the manufacturer of a NIC, burned in at the factory. For this reason, it is sometimes called a hardware or physical address, again as opposed to the logical addressing at Layer 3. A valid MAC address will consist of 12 hexadecimal characters. The first six characters are called the OUI (Organizationally Unique Identifier), and identify the company that made the card. The last six characters are the card serial number. Following are some valid MAC addresses as examples:
00-0F-1F-AE-EE-F0
00-00-0C-01-AA-CD
A MAC address must be unique within a broadcast domain. This is because one of the functions of Ethernet is that a host will broadcast an ARP request to find out the MAC address of a particular IP; if there are two identical MACs in that broadcast domain, there will be serious confusion.
In other Layer 2 network types, the addresses are not MACs but serve an equivalent purpose. Frame Relay, for example uses DLCIs (Data Link Connection Identifiers). A dial-up link using regular analog phone or digital ISDN will use the phone number as the Layer 2 address of the IP you are trying to reach. Remember that you must always resolve an IP address down to some type of Layer 2 address, and there will always be a mechanism to do so.
Layer 2 devices include switches and bridges. These devices read MAC addresses in frames and forward them to the appropriate link.
Posted by griffinLincoln
Data link layer do error detection & flow control on physical link
& for more info:
http://www.tech-faq.com/data-link-layer.shtml
Posted by Hash007
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