Data Link Layer

Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multi-access media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sub-layering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages. The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines, and coaxial cables), includes a complete data link layer which provides both error correction and flow control by means of a selective repeat sliding window protocol. Both WAN and LAN service arrange bits from the physical layer into logical sequences called frames. Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions. For example, every fifth bit of the FDDI bit stream is not used by the layer.

Services provided by Data Link Layer

Data Link Layer is basically second layer of seven-layer Open System Interconnection(OSI) reference model of computer networking and lies just above Physical Layer.

This layer usually provides and gives data reliability and provides various tools to establish, maintain, and also release data link connections between network nodes. It is responsible for receiving and getting data bits usually from Physical Layer and also then converting these bits into groups, known as data link frames so that it can be transmitted further. It is also responsible to handle errors that might arise due to transmission of bits.

Service Provided to Network Layer :
The important and essential function of Data Link Layer is to provide an interface to Network Layer. Network Layer is third layer of seven-layer OSI reference model and is present just above Data Link Layer.

The main aim of Data Link Layer is to transmit data frames they have received to destination machine so that these data frames can be handed over to network layer of destination machine. At the network layer, these data frames are basically addressed and routed.

1. Actual Communication :
In this communication, physical medium is present through which Data Link Layer simply transmits data frames. The actual path is Network Layer -> Data link layer -> Physical Layer on sending machine, then to physical media and after that to Physical Layer -> Data link layer -> Network Layer on receiving machine.

2. Virtual Communication :
In this communication, no physical medium is present for Data Link Layer to transmit data. It can be only be visualized and imagined that two Data Link Layers are communicating with each other with the help of or using data link protocol.

Types of Services provided by Data Link Layer :

The Data link layer generally provides or offers three types of services as given below :

1. Unacknowledged Connectionless Service
2. Acknowledged Connectionless Service
3. Acknowledged Connection-Oriented Service 

1. Unacknowledged Connectionless Service :
   Unacknowledged connectionless service simply provides datagram styles delivery without any error, issue, or flow control. In this service, source machine generally transmits independent frames to destination machine without having destination machine to acknowledge these frames.
   
   

   This service is called as connectionless service because there is no connection established among sending or source machine and destination or receiving machine before data transfer or release after data transfer.

   In Data Link Layer, if anyhow frame is lost due to noise, there will be no attempt made just to detect or determine loss or recovery from it. This simply means that there will be no error or flow control. An example can be Ethernet.

2. Acknowledged Connectionless Service :
   This service simply provides acknowledged connectionless service i.e. packet delivery is simply acknowledged, with help of stop and wait for protocol.

   In this service, each frame that is transmitted by Data Link Layer is simply acknowledged individually and then sender usually knows whether or not these transmitted data frames received safely. There is no logical connection established and each frame that is transmitted is acknowledged individually.

   This mode simply provides means by which user of data link can just send or transfer data and request return of data at the same time. It also uses particular time period that if it has passed frame without getting acknowledgment, then it will resend data frame on time period.

   This service is more reliable than unacknowledged connectionless service. This service is generally useful over several unreliable channels, like wireless systems, Wi-Fi services, etc.

3. Acknowledged Connection-Oriented Service :
   In this type of service, connection is established first among sender and receiver or source and destination before data is transferred.

   Then data is transferred or transmitted along with this established connection. In this service, each of frames that are transmitted is provided individual numbers first, so as to confirm and guarantee that each of frames is received only once that too in an appropriate order and sequence.