the data link layer the layer 2 of osi model is provides the physical transmission of the data and handles error notifications, network topology, and flow control. This means that the Data link layer will ensures that messages are delivered to the proper device on a LAN using hardware addresses and will translate message from the network layer into bits for then physical layer to transmission across the network.
two domains determine data transport reliabilty:
- Broadcast domain : a group of nodes that can receive each other’s broadcast messages and are segmented by routers.
- Collision Domain : a group of nodes that share the same media and are segmented by switches. A collision occurs if two nodes attempt a simultaneous transmission. carrier Sense Multiple Access Collision Detection (CSMA/CD) is an access method that sends a jam signal to notify the devices that there has been a collision. The devicesthen halt transmission for a random back-off time.
note : *Routers segment broadcast domains, whereas switches segment collision domains*
The data link layer is the only layer of the OSI model that has sublayers. The two sublayers in question define the IEEE Ethernet 802.3 frame, which in turn provides physical addressing and flow
control. Also, routed protocol information (IP, IPX, AppleTalk, and so on) is provided to the upper layers.
The IEEE Ethernet 802.3 sublayers are Media Access Control (MAC) and Logical Link Control (LLC), and are described in the following sections.
Media Access Control (MAC)
The MAC address is the hard-coded address on the network interface controller (NIC) of the Physical layer node attached to the network. Although the source address will always be a unicast or single destination address, the destination address can be a unicast, multicast (a determined subset of nodes), or broadcast (all nodes in a broadcast domain) address.
Each MAC address must be unique and follow this format:
- It must consist of 48 bits.
- It must be displayed by 12 hexadecimal digits (0-9, A-F).
- The first 6 hexadecimal digits in the address are a vendor code or organizationally unique identifier (OUI) assigned by the NIC manufacturer.
This is an example of a MAC address: 00:00:07:A9:B2:EB
note : *Know the structure of a MAC address and that the broadcast address value is FFFF FFFF FFFF*.
Logical Link Control (LLC)
The LLC sublayer complements the MAC sublayer in the ethernet model; the LLC is responsible for framing, error, and flow control. LLC provides a service access point (SAP) identifier in the frame. The SAP field of the frame consists of one byte that identifies an upper layer protocol (for example, 06 = IP, whereas E0 = IPX). The LLC inserts a destination SAP (DSAP) and a Source SAP (SSAP) in the frame. image on bottom provides an example of an ethernet frame:
Two devices are used at the Data Link layer:
- Bridges : Bridges connect two segments in a single network or two networks together.They simply forward data between those segments/networks without performing an analysis or redirection of the data.
- Switches : at Layer 2, switches are multi-port bridges that utilize Application Specific Integrated Circuit (ASIC) to forward frames. Each port of the switch has a dedicated bandwidth.
Although both devices create a separate collision domain for each connected device, all the devices connected to either are a part of the same broadcast domain. Remember that broadcast domains are segmented at the Network layer by routers.
Switches and bridges identify MAC addresses by scanning for the source MAC address of each frame received.
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