- Ethernet Standards -What is Ethernet?
Ethernet has become the standard technology used in LAN networking. Over
time, the Ethernet standard has evolved to satisfy bandwidth requirements,
resulting in various IEEE “categories” of Ethernet:
• 802.3 - Ethernet (10 Mbps)
• 802.3u - Fast Ethernet (100 Mbps)
• 802.3z or 802.3ab - Gigabit Ethernet (1000 Mbps)
Various subsets of these Ethernet categories exist, operating at various speeds,
distances, and cable types:
Half-Duplex vs. Full-Duplex
Ethernet devices can operate either at half-duplex, or full-duplex. At half
duplex, devices can either transmit or receive data, but not simultaneously.
Full-duplex allows devices to both transmit and receive at the same time.
Devices connected to a hub can only operate at half-duplex, whereas devices
connected to a switch can operate at full-duplex.
Half-duplex Ethernet uses Carrier Sense Multiple Access with Collision
Detect (CSMA/CD) to control media access. Devices monitor the physical
link, and will only transmit a frame if the link is idle. If two devices send a
packet simultaneously, a collision will occur. When a collision is detected, both
NICs will wait a random amount of time before resending their respective
packets. Full-duplex Ethernet does not use CSMA/CD.
Port speed and duplex can be either manually configured or auto-negotiated
with a hub or switch. However, a duplex mismatch will occur if one side is
configured manually, and the other configured for auto-negotiation.
Ethernet (10 Mbps)
The first incarnation of Ethernet operated at 10 Mbps, over thinnet
(10base2), thicknet (10base5), or twisted pair (10baseT) mediums.
Ethernet’s specifications were outlined in the IEEE 802.3 standard.
Even though the term “Ethernet” is widely used to describe any form of
Ethernet technology, technically the term refers to the 10 Mbps category.
The most common implementation of Ethernet is over Category 5 twistedpair
cable, with a maximum distance of 100 meters.
Full Duplex Ethernet allows devices to both send and receive
simultaneously, doubling the bandwidth to 20 Mbps per port. Only devices
connected to a switch can operate at Full Duplex.
Fast Ethernet, or IEEE 802.3u, operates at 100 Mbps, utilizing Category 5
twisted-pair (100base-TX) or fiber cabling (100base-FX).
Full Duplex Fast Ethernet allows devices connected to a switch to both send
and receive simultaneously, doubling the bandwidth to 200 Mbps per port.
Many switches (and hubs) support both Ethernet and Fast Ethernet, and are
commonly referred to as 10/100 switches. These switches will autonegotiate
both port speed and duplex.
As mentioned earlier, it is also possible to statically configure this
information. Both the device and switch must be configured for autonegotiation
(or both configured with the same static settings), otherwise a
duplex mismatch error will occur.
Gigabit Ethernet operates at 1000 Mbps, and can be utilized over Category
5e twisted-pair (1000baseT) or fiber cabling (1000baseSX or 1000baseLX).
Gigabit Ethernet over copper is defined in the IEEE 802.3ab standard.
Full Duplex Gigabit Ethernet allows devices connected to a switch to both
send and receive simultaneously, doubling the bandwidth to 2000 Mbps.
Newer switches can support Ethernet, Fast Ethernet, and Gigabit Ethernet
simultaneously, and are often referred to as 10/100/1000 switches. Again,
switches and devices can auto-negotiate both speed and duplex.
10 Gigabit Ethernet has also been developed, defined in the IEEE 802.3ae
standard, and currently can operate only over fiber cabling.
Twisted-pair cable usually contains 2 or 4 pairs of wire, which are twisted
around each other to reduce crosstalk. Crosstalk is a form of
electromagnetic interference (EMI) or “noise” that reduces the strength and
quality of a signal. It is caused when the signal from one wire “bleeds” or
interferes with another wire’s signal.
Twisted-pair cabling can be either shielded or unshielded. Shielded twistedpair
is more resistant to from external EMI. Florescent light ballasts,
microwaves, and radio transmitters can all create EMI.
There are various categories of twisted-pair cable, identified by the number
of “twists per inch.”
• Category 3 (three twists per inch)
• Category 5 (five twists per inch)
• Category 5e (five twists per inch, pairs are twisted around each
Category 5 (and 5e) twisted-pair cabling usually contains four pairs of wire
(eight wires total), and each wire is assigned a color:
• White Orange
• White Green
• White Blue
• White Brown
Types of Twisted-Pair Cables
Various types of twisted-pair cables can be used. A straight-through cable
is used in the following circumstances:
• From a host to a hub (or switch)
• From a router to a hub (or switch)
The pins (wires) on each end of a straight-through cable must be identical.