Ethernet data link layer

The Data Link Layer (DLL) is responsible for forming the frames, addressing and basic error detection mechanism. Ethernet is used in a connection-less manner to simplify the transfer at DLL as much as possible. Further mechanisms ensuring the transfer robustness and reliability are usually left to upper-level protocols in the hierarchy.

Connectionless operation (used by Ethernet)

• best-effort service
• no guarantees are made
• each frame is handled independently of all others
• no error recovery mechanisms are invoked
• error detection prevents corrupted frames from being delivered to the higher-layer client
• efficient for channels with low error rate
• higher-layer mechanisms to recover from errors are needed

Connection-oriented operation (used by other protocols from the 802 family, not Ethernet)

• error and flow control
• the communicating partners must maintain state information
• connection establishment mechanism necessary
• additional overhead and complexity
• resource management is needed

Ethernet Frame Structure and Addressing

The Ethernet frame is structured as depicted in Figure 1 below:

• Preamble
  • 7 bytes of 0x55, SFD – 1 byte of 0xD5, i.e. 62 bits of ones and zeroes followed by 2 ones. This is important for the synchronization of the receiver
• Destination (DA) and source address (SA) – see Figure 2 below
  • 48 bit address of the receiver/sender
  • The first bit of the destination address is called I/G bit (individual/group) … I/G=0 for unicast, I/G=1 for multicast. Multicast destination addresses are destined for a group of recipients.
  • Source address is always unicast. The first bit of the source address is always 0 for Ethernet.
  • The addresses can be locally or globally administered, i.e. the network administrator can assign locally administered addresses and is responsible for their uniqueness within the network.
  • Organizationally unique identifier (OUI) is assigned by IEEE to organisations who produce network interfaces. It is their responsibility to ensure that the MAC addresses are unique within the assigned OUI address space. If an organisation runs out of addresses within the OUI address space it can ask for another OUI.
• Frame Check Sequence (FCS)
  • checksum from DA through the end of the data field, 32-bit cyclic redundancy check (CRC)
• Type (EtherType)
  • identifies the above-running client protocol. It allows Ethernet multiplexing upward among higher-layer protocols, such as IP, Profinet, etc. In such a way, the attached process can be determined to deal with the data delivered with the Ethernet frame.
  • The Type field values are maintained by the IEEE and are in the range of 0x0600 – 0xFFFF. Originally, the assignments were made by Xerox Corporation.

The ordering is big endian for the octet order (leftmost octet is sent first) and little endian for bit order (rightmost bit, i.e. LSB, is sent first).

The following list shows some Organization Unique Identifiers (OUIs) you can encounter in industrial communication systems based on Ethernet. In all these OUIs, the I/G bit, i.e. the multicast bit, is 0.

• Siemens AG … 40-EC-F8, 08-00-06, 00-23-41
• WAGO … 68-23-4B, 00-30-DE, 00-07-06
• Hirschmann … EC-E5-55, 00-D0-26, 00-80-63, 00-1A-7C
• Harting … D4-7B-75, 2C-80-65, 00-11-FC, 00-0A-ED
• Profibus & Profinet International (PI) … 00-0E-CF

The following list shows OUIs for multicast addresses, where the multicast bit is 1.

• FF-FF-FF-FF-FF-FF … broadcast to everybody
• 01-0E-CF-00-00-00 … multicast to PROFINET devices. Notice the difference to OUI of PI as shown in the list above. The multicast bit in the first byte of the address is the lowest significant bit, which is sent as first because of the little-endian ordering. This ordering of bits is also used in the frame structure in Figure 2 above.
• 01-80-C2-00-00-0E … multicast for LLDP