Ethernet networks are the backbone of modern communication, both in commercial IT and industrial environments. A crucial aspect of Ethernet networking is the frame size, which directly affects data transmission efficiency. This article explores standard Ethernet frame sizes, the impact of VLAN tagging, the role of industrial protocols such as Parallel Redundancy Protocol (PRP), and how certain switches accommodate larger frame sizes through features like jumbo frames.
Standard Ethernet Frame Size
The standard Ethernet frame size, as defined by IEEE 802.3, consists of the following components:
- Preamble (8 bytes) – Synchronization sequence.
- Destination MAC Address (6 bytes)
- Source MAC Address (6 bytes)
- EtherType or Length (2 bytes)
- Payload (46 to 1500 bytes) – The data being transmitted.
- Frame Check Sequence (FCS) (4 bytes) – Error detection.
This results in a total frame size of 64 to 1518 bytes. However, various extensions and protocols can increase this size.
Impact of VLAN Tagging on Frame Size
VLAN (Virtual Local Area Network) tagging is a method used to logically segment network traffic. The IEEE 802.1Q standard introduces an additional 4-byte VLAN tag inserted between the source MAC address and the EtherType field. This increases the total Ethernet frame size to 1522 bytes.
In networks that employ Q-in-Q (Double VLAN tagging), two VLAN tags are added, increasing the frame size further to 1526 bytes. This is commonly used in carrier networks for traffic segregation.
Industrial Protocols and Increased Frame Size
Industrial networks often utilize specialized redundancy and high-availability protocols, which can further increase Ethernet frame size. One such example is Parallel Redundancy Protocol (PRP), defined in IEC 62439-3. PRP enables seamless network failover by duplicating frames across two independent network paths. Each PRP frame includes a Redundancy Control Trailer (RCT) of 6 bytes, increasing the frame size to 1528 bytes when combined with a VLAN tag.
Additional industrial protocols, such as High-availability Seamless Redundancy (HSR), also introduce extra headers, contributing to increased frame sizes beyond standard limits.
Jumbo Frames and Customizable MTU
As Ethernet networks handle larger payloads, some switches offer jumbo frame support, allowing users to specify the Maximum Transmission Unit (MTU) size manually. A typical jumbo frame can accommodate up to 9000 bytes of payload, significantly reducing transmission overhead in data-intensive applications such as industrial automation, video streaming, and storage networks.
Conclusion
Understanding Ethernet frame sizes is critical for optimizing network performance, particularly in industrial settings where VLANs, redundancy protocols, and large data packets are commonly used. By leveraging switches that support jumbo frames, network engineers can better manage increased frame sizes and ensure smooth communication within complex network infrastructures.
