Ethernet was developed by 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, Digital Equipment Corporation,
Intel Corporation, and Xerox Corporation jointly developed and released an
Ethernet specification (Version 2.0) that is substantially compatible with
IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest
market share of any local-area network (LAN) protocol. Today, the term Ethernet
is often used to refer to all carrier sense multiple access
collision detect (CSMA/CD) LANs that generally conform to Ethernet specifications,
including IEEE 802.3.
When it was developed, Ethernet was designed to fill the middle ground
between long-distance, low-speed networks and specialized, computer-room networks
carrying data at high speeds for very limited distances. Ethernet is well
suited to applications where a local communication medium must carry sporadic,
occasionally heavy traffic at high peak data rates.
Ethernet and IEEE 802.3 specify similar technologies. Both are CSMA/CD
LANs. Stations on a CSMA/CD LAN
can access the network at any time. Before sending data, CSMA/CD stations “listen”
to the network to see if it is already in use. If it is, the station wishing
to transmit waits. If the network is not in use, the station transmits. A collision occurs when two stations
listen for network traffic, “hear” none, and transmit simultaneously.
In this case, both transmissions are damaged, and the stations must retransmit
at some later time. Backoff
algorithms determine when the colliding stations retransmit. CSMA/CD stations
can detect collisions, so they know when they must retransmit.
Both Ethernet and IEEE 802.3 LANs are broadcast networks. In other words, all stations
see all frames, regardless of whether they represent an intended destination.
Each station must examine received frames to determine whether the station
is a destination. If it is a destination, the frame is passed to a higher
protocol layer for appropriate processing.
Differences between Ethernet and IEEE 802.3 LANs are subtle. Ethernet provides
services corresponding to Layers 1 and 2 of the OSI reference model, whereas
IEEE 802.3 specifies the physical layer (Layer 1) and the channel-access portion
of the link layer (Layer 2), but does not define a logical link control protocol.
Both Ethernet and IEEE 802.3 are implemented in hardware. Typically, the physical
manifestation of these protocols is either an interface card in a host computer
or circuitry on a primary circuit board within a host computer.
IEEE 802.3 specifies several
different physical layers, whereas Ethernet defines only one. Each IEEE
802.3 physical layer protocol has a name that summarizes its characteristics.
The coded components of an IEEE 802.3 physical-layer name are shown in Figure
4-1.
Figure
4-1. IEEE 802.3 Physical Layer Name Components
A summary of Ethernet Version 2 and IEEE 802.3 characteristics appears
in Table 4-1.
Table 4-1. Ethernet Version 2 and IEEE 802.3 Physical
Characteristics
| Characteristic |
Ethernet Value |
IEEE 802.3 Values
|
| |
|
10Base5
|
10Base2
|
1Base5
|
10BaseT
|
10Broad36
|
|
Data rate(Mbps)
|
10
|
10
|
10
|
1
|
10
|
10
|
|
Signalingmethod
|
Baseband
|
Baseband
|
Baseband
|
Baseband
|
Baseband
|
Broadband
|
|
Maximum segment length (m)
|
500
|
500
|
185
|
250
|
100
|
1800
|
|
Media
|
50-ohm coax (thick)
|
50-ohm coax (thick)
|
50-ohm coax (thin)
|
Unshielded twisted-pairwire
|
Unshielded twisted-pairwire
|
75-ohm coax
|
|
Topology
|
Bus
|
Bus
|
Bus
|
Star
|
Star
|
Bus
|
Ethernet is most similar to IEEE
802.3 10Base5. Both of these protocols specify a bus topology network with
a connecting cable between the end stations and the actual network medium.
In the case of Ethernet, that cable is called a transceiver cable. The transceiver
cable connects to a transceiver device attached to the physical
network medium. The IEEE 802.3 configuration is much the same, except that
the connecting cable is referred to as an attachment unit interface (AUI), and the
transceiver is called a media attachment unit
(MAU). In both cases, the connecting cable attaches to an interface board
(or interface circuitry) within the end station.
Ethernet and
IEEE 802.3 frame formats are shown in Figure
4-2.
Figure
4-2. Ethernet and IEEE 802.3 Frame Formats
Both Ethernet and IEEE 802.3 frames begin with an alternating pattern of
ones and zeros called a preamble.
The preamble tells receiving stations that a frame is coming.
The byte before the destination address in both an Ethernet and an IEEE
802.3 frame is a start-of-frame (SOF) delimiter. This byte ends with two consecutive
one bits, which serve to synchronize the frame reception portions of all stations
on the LAN.
Immediately following the preamble in both Ethernet and IEEE 802.3 LANs
are the destination and source address fields. Both Ethernet and IEEE 802.3
addresses are 6 bytes long. Addresses are contained in hardware on the Ethernet and IEEE 802.3
interface cards. The first three bytes of the addresses are specified by the
IEEE on a vendor-dependent basis, and the last three bytes are specified by
the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single node) address,
whereas the destination address may be unicast, multicast (group), or broadcast
(all nodes).
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.
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 follow this field
and precede the frame check sequence (FCS) field.
Following the type/length field is the actual data contained in the
frame. After physical-layer and link-layer processing are complete, this data
will eventually be sent to an upper-layer protocol. In the case of Ethernet,
the upper-layer protocol is identified in the type field. In the case of IEEE
802.3, the upper-layer protocol must be defined within the data portion
of the frame, if at all. If data in the frame is insufficient to fill the
frame to its minimum 64-byte size, padding bytes are inserted to ensure at
least a 64-byte frame.
After the data field is a 4-byte FCS field containing a cyclic redundancy
check (CRC) value. The CRC is created by the sending device and recalculated by the receiving
device to check for damage that might have occurred to the frame in transit.
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