Data Link Layer Protocols: MAC, LLC, Token Ring, VLAN, STP Essentials

Understanding the Data Link Layer

The MAC sublayer operates at the bottom of the Data Link Layer (OSI Layer 2).

MAC Sublayer: Core Functions

The Ethernet MAC sublayer has two primary responsibilities:

• Data Encapsulation
• Medium Access Control

Some key functions of the MAC sublayer include:

• Controlling access to the physical medium.
• Adding the MAC address to frames.
• Frame delimitation (determining the beginning and end of a data frame within a string of bits received by the physical layer).
• Performing transmission error detection.
• Discarding duplicate or erroneous frames.

LLC Sublayer: Data Transfer and Control

The LLC (Logical Link Control) sublayer defines how data is transferred on the physical environment, providing services to higher layers. The LLC sublayer handles error control, flow control, framing, and MAC sublayer addressing.

The functions of this sublayer are:

• Grouping bits into frames.
• Handling transmission errors.
• Regulating the flow of frames (flow control).
• Managing the link layer.
• Translating protocols for heterogeneous networks.

Token Ring: Ring Topology Networking

Token Ring defines a Local Area Network (LAN) with a ring topology, using a token-passing method for medium access control. Its standard rates are 4 or 16 Mbps. It supports two types of basic tokens and frames for commands and data. The token is a special frame circulating around the ring.

Token Bus: Bus Topology Networking

Token Bus is a Local Area Network protocol similar to Token Ring, but designed for bus topologies instead of ring topologies. It is a medium access protocol where nodes are connected to a bus or channel. At any given time, a single token circulates among the network nodes, and only the node possessing the token is allowed to transmit. The main bus typically uses a coaxial cable.

Virtual Local Area Networks (VLANs)

A VLAN (Virtual Local Area Network) is a method of creating independent logical networks within a single physical network. Several VLANs can coexist on a single physical switch or a single physical network.

Spanning Tree Protocol (STP): Loop Prevention

The Spanning Tree Protocol (STP) is a network protocol operating at the second OSI layer (Data Link Layer). Its primary function is to manage the presence of loops in network topologies caused by redundant links. The protocol allows devices to automatically enable or disable redundant interconnection links to prevent broadcast storms and MAC table instability.