What is the Role of the Data Link layer in the OSI model?
Data Link Layer and the OSI model:
The data link layer is the second layer in the OSI model. A data link layer uses the services of the underlying physical layer and provides services to the network layer. When a network layer has to deliver a user data packet to the destination (a remote IP address). The network layer passes the packet to the data link layer for the next hope.
The next hope may be the final destination or an intermediate routing node. The data link layer is responsible for the reliable delivery of IP packets to the next hope. For reliable delivery, the data link layer does error control, and sequence delivery of frames.
Addressing of data link layer:
The communication between the two data link layers uses source and destination data-link addresses. A data link-layer address is the address of the physical address of the card or port, which connects to the network channel. E.g In a LAN, the address of an ethernet card or MAC address is the address of the data link layer. ARP is the protocol that maps an IP address to a MAC address.
What type of services data link provides?.
The data link layer provides three types of services to the internet layer. Each service type is designed to meet a specific application requirement. The error correction and reliable delivery is the optimization at the data link layer. Because the upper transport layer anyhow, does the reliable delivery of the messages from source to destination.
- Connectionless unacknowledged service:
- Acknowledge connectionless service:
- Connection-Oriented service with acknowledge:
What are the Functions of the data link Layer?
A-Frame is a group of bits. Each frame starts with a bit pattern and ends with another bit pattern, named as start and end patterns, respectively. The framing is over a point-to-point physical channel. Each frame in the data link layer has checksum bits for error control.
Error Control :
The frame may get corrupted because of the noise in the physical channel. The data link layer ensures reliable delivery of the frames from source to destination. For that, it adds the checksum bits to the frames. On the receiver, the checksum bits are again calculated from the received frame. If checksum bits are different from received, the fame is marked a corrupted, and the sender will retransmit the same frame.
Flow control is the process of restricting the fast sender when the receiver is slow. The flow control enables reliable and fast communication when the sender and receiver have differences in the processing capability. The most straightforward flow control protocol is the stop and wait. Where the sender sends a network packet and waits for an acknowledgment.
Shared channel access control:
Access control is the process of getting access to the channel when there are multiple users of the channel. The process is called medium access control. If there will be no access control, then there are chances of collision if more than one user tries to access the shared channel or medium. There are various protocols for access the underlying channel or medium.
What are the Protocols in the data links layer?
The OSI model provides the guidelines for the data link layer. Actual protocols are developed for specific use for the communication network. Following is the list of protocols from the reference.
- SDLC (Synchronous Data Link Protocol)
- HDLC (High-level Data Link Control)
- SLIP (Serial Line Interface Protocol)
- PPP (Point-to-Point Protocol)
- LAP (Link Access Procedure)
- Frame Relay
- LLC (Logical Link Control)