A D V E R T I S E M E N T
Ethernet uses star topology for the
physical wiring layout. A diagram of a typical ethernet network layout
is shown below.
On a network, a hub is basically a repeater which is used to re-time
and amplify the network signals. In this diagram, please examine the
hubs closely. On the left are 4 ports close to each other with an x
above or below them. This means that these ports are crossover ports.
This crossover is similar to the arrangement that was used for serial
cables between two computers. Each serial port has a transmitter and
receiver. Unless there was a null modem connection between two serial
ports, or the cable was wired to cross transmit to receive and vice
versa, the connection would not work. This is because the transmit port
would be sending to the transmit port on the other side.
Therefore note that you cannot connect two computers together with a
straight network jumper cable between their network cards. You must use
a special crossover cable that you can buy at most computer stores and
some office supply stores for around 10 dollars. Otherwise, you must use
a hub as shown here.
The hub on the upper left is full, but it has an uplink port on the
right which lets it connect to another hub. The uplink does not have a
crossover connection and is designed to fit into a crossover connection
on the next hub. This way you can keep linking hubs to put computers on
a network. Because each hub introduces some delay onto the network
signals, there is a limit to the number of hubs you can sequentially
link. Also the computers that are connected to the two hubs are on the
same network and can talk to each other. All network traffic including
all broadcasts is passed through the hubs.
In the diagram, machine G has two network cards, eth0 and eth1. The cards
eth1 and eth0 are on two different networks or subnetworks. Unless machine G is
programmed as a router or bridge, traffic will not pass between the two
networks. This means that machines X and Z cannot talk to machines A through F
and vice versa. Machine X can talk to Z and G, and machines A though F can talk
to each other and they can talk to machine G. All machines can talk to machine
G. Therefore the machines are dependent on machine G to talk between the two
networks or subnets.
Each network card, called a network interface card (NIC) has a built
in hardware address programmed by its manufacturer. This is a 48 bit address and
should be unique for each card. This address is called a media access control
(MAC) address. The media, in our specific case will be the ethernet.
Therefore when you refer to ethernet, you are referring to the type of network
card, the cabling, the hubs, and the data packets being sent. You are talking
about the hardware that makes it work, along with the data that is physically
sent on the wires.
There are three types of networks that are commonly heard about. They are
ethernet, token-ring, and ARCnet. Each one is described briefly here, although
this document is mainly about ethernet.
The network interface cards share a common cable. This cable structure does
not need to form a structure, but must be essentially common to all cards on the
network. Before a card transmits, it listens for a break in traffic. The cards
have collision detection, and if the card detects a collision while trying to
transmit, it will retry after some random time interval.
Token ring networks form a complete electrical loop, or ring. Around the ring
are computers, called stations. The cards, using their built in serial numbers,
negotiate to determine what card will be the master interface card. This card
will create what is called a token, that will allow other cards to send data.
Essentially, when a card with data to send, receives a token, it sends its data
to the next station up the ring to be relayed. The master interface will then
create a new token and the process begins again.
ARCnet networks designate a master card. The master card keeps a table of
active cards, polling each one sequentially with transmit permission.