Description

This article is from the Amiga Networking FAQ, by Richard Norman with numerous contributions by others.

02 Hardware

    
At the heart of a network is a shared cable often called a Backbone. In the simple
case this is a PARnet cable to connect two Amigas via  the parallel port. Both
machines share the cable.  A more complex  example is an   ethernet cable    which
without special equipment can  be 1000ft in length or more with a hundred or so
computers attached  all interconnecting at once. This is known as a LAN or Local
Area Network.  A cheaper but far more limited LAN Backbone is Localtalk which
Apple unleashed upon the world.

bridges, routers, and gateways   Oh my!

To overcome the distance and node limitation of ethernet wiring you  need at least
a bridge which basically acts as a repeater. A bridge can  also do a limited
amount of filtering so that traffic between the LAN  segments is more efficient.

There are also distance limitations with bridges, so a more complex  piece of
equipment is needed called a router. A router provides many  more tools for
controlling the flow of information between segments,  and can even provide some
level of security. Special security  configurations of routers are know as
firewalls. For really long  distances leased lines or satellite links are used
between the routers thus forming a Wide Area Network or WAN. These links are
usually  provided by common carriers or some   WAN providers  .

This all works great as long as the two machines are the same brand,  but since
there is more than one vendor there is more than one  "language" called a protocol
for communicating. A gateway must be  used to translate between the protocols. As
an alternative to a  gateway, some routers are able to handle multiple protocols
at the same time.  Gateways are also used most heavily for converting  between
electronic mail formats or to go between two different physical  media such as
ethernet and Localtalk or ethernet and SLIP. See the Envoy specific question
section for an example of an ethernet to SLIP gateway.

The gateway provides access to other parts of a network that would  not otherwise
be directly accessible. A router is dedicated to keeping track of routes through
gateways and other routers to various domains.  On large networks your default
gateway will often point to a router.

The distinction between gateways, routers, and bridges is not absolute since many
of the functions of each can be included in a single  product. In fact some
companies call their product a brouter because  it performs both as a bridge and a
router.

Computers use numbers, but  humans use names. Therefore, another device that is
used on large networks is called a  _NAMESERVER_. A nameserver maintains a
database of machine names and their corresponding numeric addresses. The
nameserver allows the computer to look up  the numeric address when you use a
name. In addition to the nameserver you can maintain a HOSTS file locally which is
used first by your computer when trying to translate a name into a number.

The above terminology is slanted towards the TCP/IP protocol  because that is (for
better or worse) what you will most likely  encounter.

 ethernet cable 
------------------------------------

While looking through the What's New page of Mosaic, I stumbled  across the
Ethernet Web Page. It references an ethernet FAQ from the comp.dcom.lans.ethernet
news group. So if you don't find your answer  in this crude introduction then try
their FAQ.

the URL for the web page is:

 http://wwwhost.ots.utexas.edu/ethernet/ethernet-home.html

the gopher URL for the FAQ is:

 gopher://mojo.ots.utexas.edu/11/netinfo/ethernet/ethernet-faq

An ethernet cable comes in several flavors. The maximum length of  your LAN
segment is determined by which flavor you choose or  which flavors you intermix.
There is twisted pair, thin coax and thick  coax ethernet cables. Each of these
are rated at 10Mbit per second.

Note that this is the TOTAL capacity (bandwidth) of the cable NOT the speed
between any two nodes. The speed between nodes is  determined by how many nodes
are trying to communicate at any  given time. Even with only two nodes
communicating you will NOT get the entire bandwidth of the cable. The maximum is
usually only around three Mega bits per second. 

The thick coax was the first used. It ran as a backbone through a building with
taps and drop cables for each node (computer). 

Thin coax came into favor because of lower cost and ease of  installation. Thus
thin coax is often called cheaper net. It has a shorter  overall maximum length
than thick. It is routed in a daisy chain style  using tee connectors at each
node. There are adapters to go between  thick and thin, but your overall length
can be reduced to that of thin. There are devices called hubs which may not reduce
the limit and can  provide conversion between one cable type to another.

Twisted pair is the current rage because it can be used for other things as well,
such as voice. You can wire an entire building with twisted pair and decide at the
wiring closet what service will be provided.  Ethernet over twisted pair is called
10BaseT and is most often configured as a star with an ethernet concentrator at
the center usually in a wiring closet. The concentrator allows for the longer
length required for a star configuration. Using a concentrator provides the LAN
administrator a  lot of other benefits and options such as diagnostic tools and
functions  for monitoring the health of the LAN segment.   

Several grades of twisted pair wire exist. The best class can also carry FDDI
which is a fairly new high speed token-ring style network architecture. FDDI can
handle speeds of 100Mbit per second. FDDI is usually carried over fiber optic
cable for long distances. There are also Gigabit per second network architectures
for short distance applications such as a cluster of compute servers. As far as I
know there are NO FDDI or faster adapters for the Amiga at this time.

So the same wire can carry either FDDI or Ethernet so how do they differ? Good
question. It is obviously NOT just the wire.  Ethernet and FDDI are also
specifications for how the electrical signals will be transmitted and interpreted
over the wire. The Ethernet spec was originally developed by Xerox and DEC, and is
now defined by the  IEEE standards committee. IEEE 802.3 is one of the main
ethernet standards in use.  

One reason FDDI came into being is that ethernet performance  degrades rapidly as
you approach the capacity of the bandwidth. This  means you are limited in the
number of nodes that a LAN segment can support.  Depending on the activity level
of the nodes you may  be  able to support as many as 100 nodes on a LAN segment.
Beyond  that and you should consider subdividing into multiple LAN segments with
bridges and routers.

A new wrinkle is FAST ethernet which also runs at 100 Mbs but unlike FDDI it can
coexist on the same wire as regular ethernet. They even make hybrid cards that can
do both 10 and 100 Mbs depending on what the host they are talking to can support.
The catch to FAST ethernet is that it has an even shorter distance limitation than
regular ethernet. Also, FAST ethernet only runs over category 5 twisted pair or
thick coax not the thin. Unfortunately there are no FAST ethernet cards for the
Amiga yet.
    

 

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• prev: 01 What is a network?
• Index
• next: 03 Protocols

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