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Often called layer 2 protocols, data link protocols exist in the protocol layer just above the physical layer relative to the OSI protocol model. Data link protocols provide communication between two devices. Because there are many different ways to connect devices, there are many different data link protocols. The defining factors are
The PPP (Point-to-Point Protocol) that people use to connect to the Internet via a dial-up modem is an example of a data link protocol. Because the link between two systems is point to point, the bits are always delivered from sender to receiver in order. Also, unlike shared-media LANs in which multiple stations attempt to use the network, there is no contention.
Data link protocols may provide any of the following services:
Reliable services were essential in the days of dumb terminals that did not have the capabilities to perform error or frame checking. Today, end systems have their own processing power, so reliability services are not essential in the network itself. Instead, reliable services are executed on end systems. TCP is a reliable transport layer protocols that can replace network- level reliability services.
Common Data Link Protocols
The most common data link level protocols are listed here with a short description. Note that most of these data link protocol are used for WAN and Modem Connections. LLC is a LAN data link protocol.
LAN Data Link Controls
The IEEE (Institute of Electrical and Electronics Engineers) has defined a number of LAN technologies in the data link layer, including Ethernet, Fast Ethernet, Gigabit Ethernet, and token ring. You can refer to the related entries page for more information.
The actual data link layer is split into two sublayers, called the MAC (Medium Access Control) sublayer and the LLC (Logical Link Control) sublayer, as shown in Figure D-11. The lower MAC layer defines the media access method, which can be CSMA/CD (carrier sense multiple access/collision detection), token ring, or other IEEE physical interface. The LLC sublayer provides a way for the network layer to communicate with one of these protocols.
In computer networking, a frame is a data packet on the Layer 2 of the OSI model. A frame is "the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet." Examples are Ethernet frames (maximum 1500 byte plus overhead), PPP frames and V.42 modem frames.
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with preamble and start frame delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any headers for other protocols (e.g. Internet Protocol) carried in the frame. The frame ends with a 32-bit cyclic redundancy check which is used to detect any corruption of data in transit.