Data Link Layer: Concepts, Protocols, and Standards
Introduction
The data link layer is the second layer of the Open Systems Interconnection (OSI) model. It is responsible for the exchange of data between nodes on a network. The data link layer provides two main functions:
- Media access control: The data link layer controls access to the physical medium. This includes determining which node can transmit data at any given time and resolving collisions that occur when multiple nodes attempt to transmit simultaneously.
- Error detection and correction: The data link layer detects and corrects errors that occur during data transmission. This is done by adding a checksum to each frame of data. The checksum is used to verify the integrity of the data when it is received by the destination node.
Network Devices
The data link layer is implemented in network devices such as switches, routers, and network interface cards (NICs). These devices are responsible for connecting nodes to the network and for managing the flow of data between nodes.
Media
The data link layer uses a variety of media to transmit data, including:
- Copper cable: Copper cable is a type of electrical cable that is used to transmit data over short distances.
- Fiber optic cable: Fiber optic cable is a type of optical cable that is used to transmit data over long distances.
- Wireless: Wireless media uses radio waves to transmit data over the air.
Network
A network is a group of nodes that are connected to each other. The data link layer is responsible for the exchange of data between nodes on a network.
Logical Link Control
Logical Link Control (LLC) is a sublayer of the data link layer that is responsible for providing a logical connection between nodes on a network. LLC provides services such as flow control, error control, and addressing.
Media Access Control
Media Access Control (MAC) is a sublayer of the data link layer that is responsible for controlling access to the physical medium. MAC provides services such as collision detection, collision avoidance, and media arbitration.
Coding
Coding is a method used to convert a stream of bits of data into a predefined code. The codes are groups of bits used to provide a predictable pattern that can be recognized by both the sender and the receiver. The use of predictable patterns helps to distinguish the data bits from the control bits and provides better error detection in the media.
Signaling
The physical layer must generate wireless, optical, or electrical signals representing”” and”” in the media. The method of representation of bits is called a signaling method. The physical layer standards must define what kind of signal represents a”” and a””. This can be as simple as a change in the level of an electrical signal, an optical drive, or a more complex signaling method.
Digital Bandwidth
Digital bandwidth measures the amount of information that can flow from one place to another over a period of time.
Shielded Cable and Twisted Pairs
The types of shielded cable or twisted pairs of wire are designed to minimize signal degradation due to electronic noise.
Optical Fiber
- Singlemode optical fiber: Singlemode optical fiber carries a single ray of light, usually emitted from a laser. This type of optical fiber can transmit pulses at very long distances, since the laser light is unidirectional and travels through the center of the fiber.
- Multimode optical fiber: Multimode optical fiber often uses LED emitters that do not generate a single wave of coherent light. In contrast, an LED light enters the multimode fiber at different angles. The fiber lines can generate large pulses unclear when received at the receiving end because light entering the fiber at different angles require different periods of time to travel through the fiber. This effect, known as modal dispersion, limits the length of the segments of multimode fiber.
Wireless Media
Wireless media transport electromagnetic signals through radio frequencies and microwave frequencies that represent the binary digits of data communications. As a means of network, the wireless system is not limited to drivers or gutters, as in the case of the means of fiber or copper.
IEEE Standards
The first LAN (Local Area Network) in the world was the original version of Ethernet. Robert Metcalfe and his colleagues at Xerox’s designed more than thirty years. The first Ethernet standard was published by a consortium of Digital Equipment Corporation, Intel and Xerox (DIX). Metcalfe wanted Ethernet was a shared standard from which all could benefit, so that was launched as an open standard. The first products developed from the estEthernet walk were sold in the early 1980s. In 1985, the Standards Committee for Local and Metropolitan Networks Institute of Electrical and Electronics Engineers (IEEE) published standards for LANs. These standards begin with the number 802. The Ethernet standard is 802.3. The IEEE wanted to ensure that their standards are compatible with the OSI model of the International Organization for Standardization (ISO). To ensure compatibility, IEEE 802.3 standards should cover the needs of Layer 1 and the lower portions of the OSI model Layer 2. As a result, some minor modifications to the original Ethernet standard were made in 802.3.
The LLC sublayer protocol takes data from the network, usually an IPv4 packet, and adds control information to help deliver the packet to the destination node. The Layer 2 provides communication with the upper layers through the LLC. The LLC is implemented in the software and its implementation depends on the hardware. In a computer, the LLC may be regarded as the controller of the network interface card (NIC). The driver of the NIC (network interface card) is a program that interacts directly with hardware on the NIC to pass data between the media and the binder of the Media Access Control (MAC).
Data Encapsulation Data encapsulation provides three main functions: Addressing frame Delimitation Error Detection The process of encapsulation of data includes the assembly of the frame before the transmission and analysis of the plot at the time of receipt of a frame. When a web form, the MAC layer adds a header and trailer to the PDU for Layer 3. The use of frames facilitates the transmission of bits as they are placed in the media and the grouping of bits in the receiver node. The process framework provides important anchors that are used to identify a group of bits that make up a frame. This process provides synchronization between the transmitter and receiver nodes. The encapsulation process also enables the routing of data link layer. Each added to the Ethernet header frame contains the physical address (MAC address) that allows the fabric is sent to a destination node. An additional function of the encapsulation of data is the error detection. Each Ethernet frame containing a trailer with a cyclic redundancy check (CRC) of the contents of the plot. Once it receives a frame, the receiving node creates a CRC for comparison with the plot. If these two CRC calculations match, it can be assumed that the frame was received without errors.
Media Access Control MAC sublayer controls the placement of frames in the media and the removal of frames from the media. As its name implies, is responsible for managing the media access control. This includes the start of frame transmission and recovery from transmission failure due to collisions.