802.3 vs Ethernet II.. Which is what?

You must have heard about Layer 2 Ethernet Frames used on Telecom Networks which contains most importantly MAC address of destination along with other fields.

However I have seen there is one confusion people generally have regarding the type of frame we generally use because of 2 types of Framing standard exists in Market today.

One is 802.3 Frame and other is Ethernet (specifically Ethernet II) Frame. We will discuss the difference between both of these in this blog and which version is most likely to be used by Telecom service providers for their networks.

In 1980-81, the first Ethernet standard was developed by Digital (DEC-Digital Equipment Corporation), Intel, and Xerox, and combined the first letter of all of their names. This standard is called DIX 1.0, sometimes referred to as Ethernet I. The standard was superseded in 1982, by DIX 2.0, the current Ethernet standard, also known as Ethernet II (or Ethernet Version 2).

In 1983, the IEEE (Institute of Electrical and Electronics Engineers) introduced the 802.3 Ethernet standard to standardize the protocol across all networking equipment, regardless of the manufacturer.

Now in Ethernet II, the frame begins with an 8-byte preamble that is used to alert and synchronize the Ethernet Network Interface Card (NIC) to the incoming data.
The next two fields are the Destination MAC Address and the Source MAC Address. Each field is 6 bytes.
Following the address fields, Ethernet Version 2 uses a 2-byte “type” (or EtherType) field that identifies the unique network protocol type embedded within the data field.

Following are the examples of EtherTypes:

0x0800 Internet Protocol version 4 (IPv4)
0x0806 Address Resolution Protocol (ARP)
0x8100 VLAN-tagged frame (IEEE 802.1Q) & Shortest Path Bridging IEEE 802.1aq
0x86DD Internet Protocol Version 6 (IPv6)
0x8847 MPLS unicast

The data field follows the EtherType field. This field is 46 to 1500 bytes.
Finally, the frame ends with a 4-byte Frame Check Sequence (FCS) that uses a 32-bit cyclical redundancy check (CRC) for error detection.

A typical Ethernet Version 2 frame looks like this:

DIX_Ethernet_Header

IEEE 802.3 Ethernet

This frame begins with a 1-byte preamble followed by a 7-byte start frame delimiter. Combining these fields produce field no different than the 8-byte preamble used by Ethernet Version 2.
The next two fields are the Destination MAC Address and the Source MAC Address. Each field is again 6 bytes.
Following the address fields, IEEE 802.3 Ethernet uses a 2-byte “length” field that includes the IEEE 802.2 Logical Link Control (LLC) bytes and the data bytes. The length field in IEEE 802.3 Ethernet frames is always less than hex ’05DC’. This corresponds to 1500 bytes which is the maximum frame size for Ethernet. NIC cards look at this field to determine which Ethernet standard is being used. If the field is less than ’05DC’, it represents a length field, and the frame is an IEEE 802.3 Ethernet frame. If the field is greater than ’05DC’, the frame is an EtherType field and is an Ethernet Version 2 frame.

IEEE 802.2 is not part of the definition of IEEE 802.3; however, it is always bound together with an IEEE 802.3 header.
The rest of the data unit follows the 802.2 LLC fields. This can be from 42 to 1496 bytes.
Finally, the frame ends with a 4-byte FCS field which is the same as that described in the Ethernet Version 2 standard.

A typical IEEE 802.3 Ethernet frame looks like:

eth_frame

So if you have read above, you can see major advantage with Ethernet II is less overhead and more space to send data through the frame. For Ethernet II, data field is 46 to 1500 bytes and for 802.3, data field is 42 to 1496 bytes.

I hope I was able to resolve some of your confusion and please let me know if you still have any 🙂

Regards

Mohit Mittal

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s