Ethernet
Ethernet, one of the pivotal technologies that made Local
Area Networks possible, was developed in the 1970s by
Digital, Intel and Xerox. This original design is
often referred to by the initials of its creators - DIX.
Ethernet works by connecting an almost unlimited number of
devices to the same cable. Usually, a host can just transmit
whenever the cable is not is use. In the relatively uncommon
case where two devices start transmitting at the same
time, a collision occurs. Both senders then
wait a random amount of time before transmitting again.
In any case, every device on the cable can receive every
packet, but discards all those not addressed to it.
This scheme, one of many that can regulate
access to a hardware medium, is referred to CSMA/CD,
an acronym for Carrier Sense, Multiple Access / Collision Detect.
There has been much debate over Ethernet performance.
The performance of any CSMA/CD network will depend
on several considerations, including
the method of determining silent times after a collision,
the length of the cabling,
the size of packets, and the amount of traffic.
The Ethernet standard defines how silent times are determined,
and the network engineer can seldom influence this feature anyhow.
The remaining factors are summarized in the table below,
though be aware that the standard
places constraints on several of these.
Performance Factor
Desirable Feature
Cable length
Short
Short cables reduce the chance of collisions, since
electrical signals take less time to propagate between hosts
Packet size
Large
Large packets reduce the chances of a collision,
since collisions can only occur during a fixed time window at
the beginning of a packet
Amount of traffic
Light (< 20% capacity)
More traffic means more collisions; for standard
10 Mbit/s Ethernet, try not to exceed 2 Mbit/s on any single segment
In the early 1980s, IEEE began developing standards for LAN
technologies, including Ethernet. Unfortunately, the new
standard, IEEE 802.3, is not compatible with the older
DIX specification. Due to Ethernet's already
large installed base, both the newer IEEE standard
and the persistent DIX pseudo-standard are in use.
Ethernet operates at 10Mbps per second, and its
packets are between 64 and roughly 1500 bytes in length.
A 6-byte address is used, which is divided into a
3-byte vendor ID and a 3-byte vendor-defined field.
Ethernet manufacturers are assigned a unique vendor ID,
and are then responsible for insuring that all of their
devices have unique addresses in the last 3 bytes.
An all-ones address is interpreted to mean broadcast,
and all hosts attached to the cable process such a packet.
A group of addresses are assigned for multicast use, and
most newer Ethernet cards permit software to select a
subset of the multicast addresses for reception.
The IEEE Ethernet Standard is not available online, because
the sale of standards documents are a major income source
for IEEE and other standards organizations.
Ethernet encapsulation standards describe how IP packets
should be converted into Ethernet packets. ARP (Address Resolution
Protocol) describes how IP addresses are translated to Ethernet
addresses.
