OSI Layer 2 frames

Derek is studying to become a Red Hat Enterprise Linux Engineer and Data Scientist at Red Hat's headquarters in Raleigh, North Carolina. Please answer the following questions to help him to formula a study plan to master computer networking topics.

  1. In relation to OSI Layer 2, please give Derek a detailed explanation of frames: their purpose, what they do, and how they work.
  2. Give a detailed explanation of headers and trailers.
  3. Give a detailed description of MAC addresses.
  4. Give a detailed description of flow control.

Sure, Derek's focus on mastering computer networking topics, particularly within the context of OSI Layer 2, is a critical step toward becoming a skilled Red Hat Enterprise Linux Engineer and Data Scientist. Here's a breakdown of the key concepts Derek needs to understand:

1. Frames in OSI Layer 2


Frames are the basic data units used in Layer 2 of the OSI (Open Systems Interconnection) model, which is also known as the Data Link Layer. The primary purpose of frames is to encapsulate packets received from the Network Layer (Layer 3) into a format that is suitable for transmission over physical media. Frames add necessary headers and trailers to the packet, which include addressing and error-checking information, ensuring that the data can be correctly routed and reliably delivered to the destination.

How They Work

Frames work by encapsulating packets with a header and a trailer. The header typically contains the destination and source MAC (Media Access Control) addresses, which are used for local network addressing. It may also contain other control information, such as type fields to identify the payload type and control flags for flow and error control. The payload is the encapsulated data from the upper layer. The trailer often contains a Frame Check Sequence (FCS), used for error detection.

Upon receiving a frame, a device checks the destination MAC address against its own. If they match (or if it's a broadcast/multicast address the device listens to), the device processes the frame, verifies its integrity via the FCS, and then decapsulates it, sending the payload up to the Network Layer.

2. Headers and Trailers


Headers are added at the beginning of the data packet at each layer of the OSI model. In the context of Layer 2, the frame header contains:

  • Destination MAC Address: The physical address of the intended recipient.
  • Source MAC Address: The physical address of the sender.
  • Type/Length: Identifies the type of payload or its length.
  • Control Information: Used for various control purposes, including flow and error control.


The most common trailer used in Layer 2 frames is the Frame Check Sequence (FCS). The FCS is a type of checksum used for error detection. It allows the receiving device to determine if the frame was altered during transmission. If the calculated FCS of the received frame doesn't match the one contained in the frame, the frame is discarded, as this indicates an error occurred during transmission.

3. MAC Addresses

MAC addresses are unique identifiers assigned to network interfaces for communications on the physical network segment. They are used by Layer 2 protocols to ensure data is sent to the correct device on a local network. MAC addresses are 48 bits long, typically represented in hexadecimal format, and are embedded into network interface cards (NICs) by manufacturers. They play a crucial role in local area networking, especially in Ethernet networks.

4. Flow Control

Flow control is a mechanism used to manage the rate of data transmission between two devices to prevent a fast sender from overwhelming a slow receiver. In the context of Layer 2, flow control can be implemented using various techniques, such as:

  • Pause Frames: In Ethernet networking, devices can send pause frames to tell other devices to temporarily stop sending data.
  • Backpressure: Used in half-duplex networks to prevent data from being sent if the network is congested.
  • Window-based flow control: Involves controlling the size of the "window" of frames that can be sent before receiving an acknowledgment.

Flow control ensures a balanced data transfer rate, minimizing the risk of buffer overflow and data loss, and thus maintaining the integrity and efficiency of network communication.

Understanding these fundamental concepts will be instrumental for Derek as he prepares for a role that combines expertise in both network engineering and data science.

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