Ethernet Tutorial
By Jason John Schwarz
LANs
Networks are collections of separate computers that communicate with each
other over a common cable or radio medium. Local area networks (LANs)
tend to be confined to one geographical location, while wide area networks
(WANs) tend to span many physical locations.
Ethernet
Ethernet is the most popular LAN technology currently in use. Other
popular LAN types include Token Ring, Fast Ethernet, Fiber Distributed
Data Interface (FDDI), Localtalk, Ethertalk, and Arcnet. Ethernet is
popular due to it's low cost, multitude of wiring types, and market
acceptance.
The Ethernet standard is defined by the Institute for Electrical and
Electronic Engineers (IEEE). The IEEE Standard 802.3 defines the
rules for configuring Ethernet as well as the protocol that allow
the computers to communicate.
Protocols
Network protocols are standards that allow computers to
communicate with each other. A protocol defines how
the computers should identify each other on the network,
the form that the data should take in transit, and how
the information should be reconstructed once it reaches
its final destination. Protocols also define how to handle
damaged transmissions. IPX, TCP/IP, DECnet, AppleTalk, LAT,
SMB, DLC, and NetBEUI are examples of network protocols.
Although each protocol is different, they all use the
physical cabling in the same manner,
which allows them to peacefully coexist. This concept is
known as "protocol independence", which means that the physical
network and the protocols are not directly connected.
Media and Topologies
One of the most important parts of designing and installing a
network is deciding on which cabling medium and wiring
topology to use. There are four major types of media in use today:
Thickwire,
thin coax,
unshielded twisted pair (UTP),
and fiber optic.
Ethernet media are used in two basic topologies called "bus" and
"star". The topology defines how a node (which is any device
such as a computer, printer, or hub) is connected to the network.
A bus topology consists of nodes connected together by a single
long cable. Each node "taps" into the bus and directly communicates
with all other nodes on the bus. The major advantage of this topology
is the easy expansion, by adding extra "taps", and the lack a hub.
The major disadvantage is that any break in the cable will cause
all nodes on the cable to loose connection to the network.
A star topology links exactly two nodes together on the network. A
hub is used to collection point where many of the
connections come together. The major advantage is any single break
only disables one host. The major disadvantage is the added cost of
a hub.
Thickwire Ethernet
Thickwire, or 10BASE5 ethernet, was generally used to create large
"backbones". A network backbone joins many smaller network segments
into one large LAN. Thickwire made an excellent backbone because
it can support many nodes in a bus topology and the segment can
be quite long. It can be run from workgroup to workgroup where
smaller networks can then be attached to the backbone. A thickwire
segment can be up to 500 meters long and have as many as 100 nodes
attached. New nodes are connected to the cable by drilling into
the media with a device known as a "vampire tap". Nodes must be
spaced exactly 2.5 meters apart to prevent signals from interfering
with one another.
Thickwire is being replaced by thin coax and fiber optic cabling in
most cases. The expense of the cable, coupled along with the expense
of the vampire taps, has begun to eliminate this form of cabling.
Thin Coax Ethernet
Thin coax, or 10BASE2 ethernet, offer the advantages of
thicknet's bus topology, with reduced cost and easier installation.
Thin coaxial cable is thinner and more flexible than thickwire,
but it can support only 30 nodes per segment, and each node must
be at least 1.5 meters apart.
A thin coax cable has BNC type connectors on both ends. You then
connect the segments of cable together with a "T" connector, and
connect the third connection of the "T" to the node. Each end
of the long segment must be terminated with a 50 ohm resistor and
grounded.
Twisted Pair Ethernet
Unshielded twisted pair (UTP), or 10BASE-T ethernet, cable is a 4 pair cable which
is very similar to telephone cable in both appearance and end
connector appearance. It comes in a variety of grades, with
level 1 being the lowest quality and level 5 being the best.
Level 1 and 2 cabling should only be used for voice and low speed
transmissions (less than 5 Mbps). Level 3 may be used for data
speeds up to 16Mbps, while level 4 can handle speeds up to 20Mbps.
The finest cable avaliable, level 5, can handle speeds up to 100Mbps.
A 10BASE-T ethernet network uses a star topology, with each node
being connected directly to a hub. The major limitation to this
cable is a maximum calbe length of 100 meters, and that each node
must have its own connection to the hub.
Fiber Optic Ethernet
Fiber-optic, or 10BASE-FL ethernet, is similar to twisted pair.
Fiber-optic cable can handle 100Mbps transmission speeds, but is
not affected by electrical emissions or electro-magnetic
interference . Lighting strikes, which can
be transmitted by other cabling types, is not transmitted by
Fiber-optic cable. The major advantage of fiber-optic cable is
the 2 kilometer maximum length. The disadvantage being the higher
cost of cable and equipment.
Fast Ethernet
With the addition of large data steams such as real time video
and audio, networks have begun to require high transmission speeds.
The new ethernet standard established to handle this requirement
is called Fast Ethernet, or 100BASE-T. It is defined by IEEE standard
802.3u, which raises the maximum speed from 10 megabits per second
to 100 megabits per second. There are three types of Fast Ethernet
currently avaliable:
100BASE-TX for use with level 5 UTP cable
100BASE-FX for use with fiber-optic cable
100BASE-T4 which has an extra two wires for use with level 3 UTP cable
Currently the 100BASE-TX standard has become the most popular due to its
close compatilibity with the 10BASE-T ethernet standard.
Hubs
A hub is a central point where multiple cables come together. A hub
usually allows 8, 16, or 64 node connections to communicate. If any
single connection disconnects or is having problems the hub can
partition (remove from the network) it and allow all other nodes to
continue to communicate.
Tranceivers
Transceivers, also known as Media Attachment Units (MAUs), are
used to connect nodes to the various ethernet media. Generally
the transceiver allows the attachment of 10BASE-T or 10BASE-2
cable on one side, and the connection via a 15 pin D-shell connector,
known as an Application User Interface (AUI), on the other.
The user would connect the AUI connection to the computer and the
10BASE-T or 10BASE-2 connection to the network media.
Repeaters
Repeaters are used to connect two or more ethernet segments of any
media type. They can be used to extend a segment beyond its maximum
length or maximum number of nodes by restoring signal quality and
timing. Repeaters can also be used to connect segments consisting of different media types
together into one larger segment.
It must be noted that a repeater counts as a node on every segment to
which it is attached.
Bridges
The function of a bridge is to connect separate ethernets together.
Bridges map the ethernet addresses of the nodes residing on each
network segment and then allow only the necessary traffic to pass
through the bridge. A bridge can also filter out certain traffic
and prevent it from passing through.
When a packet is received by the bridge, the bridge determines the
destination and source segments. If the segments are the same, the
packet is dropped("filtered"); if the segments are different, the
packet is forwarded to the proper segment. Additionally, bridges
prevent all bad or misaligned packets from spreading by not forwarding
them.
Bridges are called "store-and-forward" devices because they look at
the whole ethernet packet before making their filtering or forwarding
decisions.
Switches
An ethernet switch is a bridge which can connect more
than two segments together. The idea behind a switch
is that it removes all unneeded traffic from each segment
by only forwarding the traffic needed on that segment,
which provides better performance on the network.
Routers
Routers work in a manner similar to switches and
bridges in that they filter out network traffic. Rather
than doing so by packet addresses they filter by specific
protocol.
An IP router can divide a network into various subnets so
that only traffic destined for particular IP addresses can
pass between segments. The price paid for this type of
intelligent forwarding and filtering is usually calculated
in speed of the network, because this protocol filtering
usually takes more time than packet filtering.
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