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 II
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
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
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.
802.2 SNAP
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
Function of a MAC address
- 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).
MAC Address Examples
| MAC Address |
Manufacturer Code |
Serial Number |
| FF34.2344.13FD |
FF34.23 |
44.13FD |
| 44CC.7800.34FF |
44CC.78 |
00.34FF |
| 00A0.CC60.1388 |
00A0.CC |
60.1388 |
3 Types of Media Access
- Contention (Ethernet)
- Token Passing (Token Ring, FDDI)
- 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
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