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Lakeland Web
P.O. Box 2004
Lakeland, Fl 33806
Phone: (863) 644 - 4292
www.lakelanweb.net
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Each network
has its own technology, topology and protocols.
These three aspects of the network govern how it
works. There are several popular network
technologies including Token Ring and Ethernet. The
two main protocols that are in use are TCP/IP and
IPX - the former is the protocol used by the
Internet and the latter is used internally by Novell
NetWare.
The
following sections go into some technical detail so
a summary section has been provided afterwards.
Token Ring
Computers in a Token Ring network are connected
together in a ring. A small packet called a token is
circulated along the ring. When a particular
computer receives the token it has the opportunity
to send a data packet.
When each computer in the ring receives the data
packet it compares its address with the data
packet's destination address. If they match the data
packet is copied before being sent to the next
computer in the ring. The packet continues around
the ring until it arrives back at the sending
computer. It must then pass the token on and the
process starts again.
Token Ring networks are more resilient than Ethernet
networks, but are also more expensive
Ethernet and CSMA/CD
Ethernet can support a variety of different cabling
standards, making it easy to design a network for
price and performance. It uses a system called CSMA/CD
which stands for carrier sense multiple access with
collision detection.
Carrier sense means that each computer
in the network is constantly "listening'' for
network traffic and will examine each piece of data
that is transmitted.
Multiple access means that any of the
computers connected to the network can choose to
send data at any time as long as the network appears
to be idle.
Because it takes a finite amount of time for
transmitted data to propagate through the physical
network, it is possible for two computers to both
think the network is not in use and begin
transmitting.
Collision detection means that the
computers are aware that such collisions can occur.
If two computers do transmit at the same time, they
will both detect the collision, cease transmitting
and wait a short, random amount of time before
attempting to transmit again.
Networks and Roads
This
type of behavior compares well to the idea of a
system of roads. the network can be thought of as a
main road, with all of the client computers being
side roads and the frames of data being cars
traveling along the roads. When cars reach a
give-way onto a main road they have to wait for a
break in traffic before joining on. This is exactly
what happens with transmitted data. In fact data on
networks is often referred to as network traffic.
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Ethernet sends data across a network in frames.
There are several different standards for Ethernet
frames which can all be used on the one physical
network. However for two computers to communicate
they must both be configured to use the same
standard.
Each frame basically contains a small amount of data
along with a destination address. When a frame is
transmitted successfully, every other computer on
the network will have listened and received it. Each
individual computer can then look at the frame's
address. The computer whose address matches the
destination address on the frame accepts the data
and the others ignore it.
(Note that every computer on the network receives
every frame that is transmitted so theoretically a
computer could examine the data in every piece of
network traffic. However there is normally too much
traffic on the network for this to be practical.)
Frames and Delivery Vans
A Frame can be thought of as a delivery van driving
on the ``road'' of the network - many different
types of packages can be carried by the one type of
van. The network must be set up to deal with one
type of frame, just like a company deciding to use a
certain type of delivery van.
Inside each frame is a packet of data which will
conform to a certain network protocol. A computer
can send some data using a certain protocol, but the
receiving computer must use the same protocol or
else it will not be able to understand the data.
Protocols and Delivery Vans
Network protocols can be thought of as types of
packaging being used by a company to deliver things
- any shape, size or type of packaging can fit into
the delivery van. The receiving computer must know
about the protocol used by the sending computer
because, after all, a customer would become very
confused if he was expecting a bill and received it
in a large, wooden box instead of an envelope.
Summary
Ethernet is a type of network technology. If a
network is using Ethernet then it will be using a
certain type of cabling and certain network
interface cards. Every computer on an Ethernet
network is constantly listening for data to be
transmitted. All of the computers on the network
receive data from a sender at once, but only the
receiving computer will accept the data - all of the
other computers will ignore it.
A computer waits for a gap in the network traffic
before it attempts to transmit data. Because it
takes a finite time to start transmitting it is
possible for two computers to transmit at once. If
this happens they will both detect a collision -
both computers will back off and wait for a random
amount of time before attempting to send the data
again. Data is transmitted across Ethernet
using frames. A frame is like a package with an
address on the outside and some data on the inside.
Only the physical network and the interface cards
deal with frames.
There are different network protocols which use
different types of data packets. Each packet is
enclosed inside a frame so it can be sent across the
network. However, both the sending computer and the
receiving computer must use the same protocol or
else they will not understand each other, even
though they are on the same network.
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