The data link layer, also known as layer 2, is responsible for node-to-node delivery of data on the same network. The data link protocol establishes the format of data on the network and handles the transfer of data between network nodes. Some key responsibilities of the data link layer include:
Framing
The data link layer divides the input data stream into manageable data frames. This framing of data packets allows the successful block-by-block delivery of data even when packet sizes differ. Some common data link layer protocols like Ethernet use physical addressing to identify the source and destination of each data frame.
Addressing
The data link protocol uses MAC (media access control) addressing to identify the source and destination of each data frame. MAC addresses uniquely identify each network node and allow data frames to be correctly delivered to the intended recipient.
Error Detection and Correction
Data can become corrupted as it travels across the physical medium between network nodes. The data link protocol employs methods like CRC checksums to detect errors in each frame. If errors are detected, measures like retransmission of frames can be used to correct them.
Flow and Error Control
The data link protocol regulates the flow of data frames between nodes. Techniques like buffering help prevent node overrun. Error control mechanisms like ARQ (automatic repeat request) are used for error recovery when data frames are corrupted or lost.
Multiplexing
Nodes can have multiple simultaneous upper layer connections. The data link protocol interleaves frames from different sources over a single physical link through multiplexing techniques like time-division multiplexing.
How Does Ethernet Handle Framing?
Ethernet is a widely used data link layer protocol. It handles framing as follows:
- Data from the network layer is divided into payloads of up to 1500 bytes.
- A header of 14 bytes is added to each frame. It contains the source and destination MAC addresses.
- A 4 byte frame check sequence (CRC) trailer is added for error detection.
- Padding bytes are added if the payload is less than 1500 bytes to meet the minimum frame size.
This framing process allows reliable link-level delivery of blocks of data between Ethernet nodes.
How Does Ethernet Handle Addressing?
Ethernet uses 48-bit MAC addresses burned into the network interface hardware to identify devices. Addressing works as follows:
- The source device inserts its MAC address into the source address field of the frame.
- The desired destination address is added in the destination field.
- Network nodes examine the destination address to determine if they should accept the frame.
This addressing mechanism ensures each frame is correctly delivered to the target device based on its unique MAC address.
How Does Ethernet Detect and Correct Errors?
Ethernet uses a cyclic redundancy check (CRC) to detect errors in each frame:
- The transmitting node calculates a CRC value based on the frame contents.
- This CRC is included as a trailer to the frame.
- The receiving node independently calculates the CRC of the received frame.
- If the calculated CRC matches the received CRC, the frame is accepted. Else it is discarded.
Damaged frames detected via CRC mismatches may be retransmitted to correct errors.
How Does Ethernet Control Flow and Errors?
Ethernet uses methods like the following for flow and error control:
- Buffering – Nodes have buffer storage to temporarily hold frames during congestion.
- Backpressure – Sending nodes are notified to pause transmission when buffers are full.
- CSMA/CD – Carrier sense multiple access with collision detection avoids frame collisions.
- Retransmission – Damaged frames are retransmitted to recover from errors.
These mechanisms prevent congestion and ensure reliable delivery of frames.
How Does Ethernet Allow Multiple Logical Connections?
Ethernet multiplexes frames from different upper layer connections through the use of unique MAC addresses. Steps include:
- Each connection gets assigned a different source and destination MAC address pair.
- Frames from simultaneous connections are interleaved together.
- The receiving node filters frames based on the destination MAC address.
- Frames are then forwarded to the appropriate connection based on the MAC address.
This multiplexing allows multiple simultaneous logical connections over a shared physical Ethernet link.
Advantages of Ethernet
Some key advantages of using Ethernet at the data link layer include:
- High speed – Ethernet is fast with high bandwidth utilization.
- Low cost – Ethernet hardware is inexpensive compared to alternatives.
- Simplicity – Ethernet is easy to deploy and configure.
- Standardized – Ethernet uses standard IEEE 802.3 framing.
- Compatible – Works over common network media like copper and fiber.
Disadvantages of Ethernet
Some potential disadvantages of Ethernet include:
- Limited scalability – Large networks may exceed collision domains.
- Security issues – Ethernet lacks native encryption and authentication.
- No quality of service – Ethernet does not prioritize time-sensitive traffic.
- Prone to errors – Error detection but not correction natively.
Conclusion
The data link layer handles the node-to-node transfer of data frames over the physical network medium. Protocols like Ethernet provide addressing, framing, error detection, and media access control mechanisms. These allow reliable exchange of data between devices on local area networks. An understanding of data link protocols like Ethernet is key to effectively administering and troubleshooting LANs.