Ethernet Frames

Ethernet was developed by the Xerox Corporation's Palo Alto Research Center (PARC) in the 1970s.  Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in 1980.  Shortly thereafter, DEC, Intel, and Xerox jointly developed and released and Ethernet specification (Version 2.0) that is substantially compatible with IEEE 802.3.  Today, the term Ethernet is often used to refer to all CSMA/CD LANs that generally conform to Ethernet specification, including 802.3.

Ethernet Frames

• Used at the Data Link layer to encapsulate packets handed down from the Network layer for transmission on a medium.
• Ethernet_II frames have a type field in their frame.
• 802.3 frames have a length field in their frame.
• Data size can be from 46 to 1500 bytes.
• FCS - Frame Check Sequence - used to store the CRC (Cyclic Redundancy Check) for the frame.
• 802.3 frame can't contain information about the upper layer protocols (Network Layer), so it is combined with the 802.2 (LLC) frame to provide this function.

The Four Types of Ethernet Frames:

• Ethernet II
• IEEE 802.3
• IEEE 802.2
• SNAP

Ethernet provides services corresponding to Layers 1 and 2 of the OSI model.  In Ethernet frames, the 2-byte field following the source address is a type field.  This field specifies the upper-layer protocol to receive the data after Ethernet processing is complete.  Ethernet is a broadcast LAN that uses CSMA/CD.

IEEE 802.3 specifies the Physical layer (Layer 1) and the channel access portion of the Data Link layer (Layer 2), but doesn't define a logical link control protocol.  In IEEE 802.3 frames, the 2-byte field following the source address is a length field, which indicates the number of bytes of data that follows this field and precede the frame check sequence (FCS) field.  Following this is the data field, which will contain data for the frame.  In the case of IEEE 802.3, the upper-layer protocol must be defined within the data portion of the frame.  IEEE 802.3 is also a broadcast LAN that uses CSMA/CD.

IEEE 802.2 is often referred to as the Logical Link Control (LLC).  It is extremely popular in LAN environments, where it interoperates with protocols such as IEEE 802.3, IEEE 802.4, and IEEE 802.5.  Upper-layer processes use IEEE 802.2 services through service access points (SAPs).  The IEEE 802.2 header begins with a destination service access point (DSAP) field, which identifies the receiving upper-layer process.  Following the DSAP address is the source service access point (SSAP) address, which identifies the sending upper-layer process.

The SNAP (Subnetwork Architecture Protocol) frame has its own protocol field to identify the upper-layer protocol.  This is a way to allow an Ethernet II frame to be used in an 802.3 frame.  SNAP frame's DSAP and SSAP are always set to AA with the command field set to 3.  SNAP was created because not all protocols worked well with the 802.3 frame which has no ether-type field.  802.2 frame is an 802.3 frame with the LLC info in the data field of the header (has DSAP and SSAP).  To allow the proprietary protocols created by application developers to be used in the LLC frame, the IEEE defined the SNAP format.  SNAP is mostly seen with proprietary protocols such as Appletalk and the Cisco CDP.

MAC Addressing

• 48-bit address.
• Manufacturer's identification (OUI- Organizationally Unique Identifier) is the first 24-bits and is assigned by the IEEE.
• Manufacturer assigns a unique value to the second 24-bit section

• The MAC address uniquely identifies the device from any other device in the world.
• The MAC address is a 48 bit address represented by 12 hexadecimal digits.
• The first 6 digits contain the manufacturer's Unique identifier (OUI) and the last 6 digits are the unique serial number assigned by the manufacturer.
• The MAC Address is usually burned onto a NIC (Network Interface Card) in its ROM (Read Only Memory).

3 Types of Media Access

1. Contention (Ethernet)
2. Token Passing (Token Ring, FDDI)
3. Polling (IBM Mainframes, 100VGAnyLAN)

Ethernet

• Uses a logical bus topology - signal runs from one end of the segment to the other.
• Baseband technology - when a station transmits, it uses the entire bandwidth.
• Uses CSMA/CD.
• Best effort delivery.

Each of the 802.3 (Ethernet) standards defines an AUI

• 10BaseT - uses AUI - 1 bit at a time
• 100BaseT - uses MII - 4 bits at a time
• 1000BaseT - uses GMII - 8 bits at a time

AUI - Attachment Unit Interface

MII - Media Independent Interface

GMII - Gigabit Media Independent Interface