The data link layer is split into logical sublayers, the upper sublayer, known as LLC, that communicate with the network layer and the lower sublayer, known as MAC Layer, that communicates with the physical layer below. This article presents a brief function both sublayers of the data link layer. The Figure below illustrates the structure of LLC and MAC Sublayers۔
LLC Sublayer Overview and Function
The upper sublayer of the data link layer is known as Logical Link Control (LLC). It communicates with upper layers of the OSI model. It gets the network protocol data, which is usually an IPv4 packet. LLC also adds control information to help deliver the packet to the destination. The LLC communicate with the upper layers of the application and transition the packet to the lower layers for delivery.
LLC is implemented in software, and its implementation is independent of the hardware. The driver software of computer NIC and be considered as LLC. The driver is a software program and instruction that interacts directly with the computer NIC and other hardware. Driver software also enables the communication between the network device, computer, and operating system as well as with other network computers and network devices.
As I mentioned earlier that LLC communicates with the uppers i.e network layer. When the data link layer received a frame from the physical layer, the LLC is looking at the Layer 3 Protocol type and handing over the datagram to the correct Layer 3 protocol. It is called the de-multiplexing process. When Layer 3 sends a packet, LLC takes it from different Layer 3 protocols like IP, IPX, ARP etc., and forward it over to the MAC layer after adding the Layer 3 protocol type in the LLC header portion of the frame multiplexing. This process is called the multiplexing process.
LLC optionally present reliable frame forwarding. By the help of LLC the sending node numbering each frame during transmission called sequence number, at the other hand, the receiving node acknowledged each received frame using the acknowledgement number. If the frame lost during transmission, the sending node retransmits lost frames. The LLC also provides flow control optionally by allowing the receivers to control the sender’s speed through control frames known as Receive Delay and Receive Not Delay. LLC provides the following types of services.
Unacknowledged Connectionless Service
Connectionless service is also known as the best effort service e.g. IP datagram service, without prior connection establishment between source and destination and without acknowledgement. In the connectionless service, the data is sent to the peer directly without any connection establishment. The connectionless service is also unreliable because reliability, flow control and error control are provided at the transport layer. It is provided flow control optionally.
Connectionless Acknowledged Service
In this type of service, the data is directly sent between Layer2 peers without any prior logical link establishment. But the difference with previous is that this service mark each frame with its sequence numbers and the peer acknowledges each frame received using an Acknowledgment number field. This service mode is used where data reliability is needed. The data sender can track lost or damaged frames and retransmit these frames to accomplish reliability. The wireless links use connectionless acknowledged service, where the quality of the link is not good.
Before sending data, a logical connection is established between peers in this type of service. Before start data transfer, the service exchanges the control frames known as Supervisory Frames to establish a logical connection. Actual data transfer starts after the connection establishment phase is over. There are two types are used in connection-oriented service, one is without acknowledgement and another is with acknowledgement.
Connection-oriented service Without Acknowledgment
In this type of service a logical link is established before actual data transfer start but without any concept of frames being numbered and acknowledged through Sequence number and acknowledgement number fields. The example of this service is HDLC, PPP and LAPB.
Connection-oriented service with Acknowledgment
In this type of service a logical link is established before actual data transfer start, but with the concept of frames being numbered and acknowledged through Sequence number and acknowledgement number fields. Reliability and flow control services are also provided by the LLC through the use of sequence number, acknowledgement number and retransmission of lost frames. Sliding window mechanism provides Flow control. Most of the internet use this type of services because the Internet uses TCP,
The function of MAC sublayer
MAC is the lower sublayer of the data link layer. It is listed in the IEEE 802.3 standards. The figure above lists common IEEE Ethernet standards. It also shows how the data link layer divided into the LLC and MAC sublayers. The LLC communicates with the network layer while the MAC sublayer allows various network access technologies. For instance, the MAC sublayer communicates with Ethernet LAN technology to send and receive frames over copper or fibre-optic cable. The MAC sublayer also communicates with wireless technologies such as Wi-Fi and Bluetooth to send and receive frames wirelessly.
As we know that the Media Access Control (MAC) layer is between the Logical Link Control (LLC) sublayer and physical layers, The MAC sublayer has the following key tasks:
- Data encapsulation
- Media access control
The data encapsulation process includes frame assembly from the transmission node and disassembly on the receiving node. To form the frame, the MAC layer adds a header and trailer to the network layer PDU. Data encapsulation provides three key functions:
- Data Framing-The framing process provides a sequence of one or more characters to recognize a group of bits that make up a frame. These sequences bits provide synchronization between the transmitting and receiving nodes.
- Addressing -The data link layer is the lowest in the OSI model concerned with addressing. Data Link Layer receive Layer 3 PDU. The encapsulation process in the data link layer provides data link layer address as well as labelling information with a particular destination location. Every device on a network has a unique number, generally called a physical address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine get to it properly.
- Error detection –The data link each frame contains a trailer that used to detect any errors in transmissions.
Media Access Control
The Media Access Control sub-layer second responsibility is to control access to the media. This sub-layer is also responsible for the placement of frames on the media and the removal of frames from the media. The MAC sub-layer communicates directly with the physical layer.