[lug] linux router project

[rm at mamma.varadinet.de]

On Wed, Aug 23, 2000 at 12:50:37PM -0600, PC Drew wrote:
> "To understand recursion, we must first understand recursion."
> 
> On Wed, 23 Aug 2000, Glenn Ashton wrote:
> 
> > Perhaps their objections are real.  10base2 can be very flakey.  Does
> > anyone else on the list know if the network people's objection to a media
> > converter is valid?  I'd be curious to know.
> 
> 
> Out of pure ignorance, here's a question: why not upgrade to Ethernet?  Granted,
> I don't know very much about token ring or any of these other topologies, but
> I do know ethernet.  And I do know that it works, it's fast, and it's good.
> 
> Do they not want to upgrade because it would cost too much money?
> 
> If anyone can elighten me as to why 10base2 is better than ethernet, I'd appreciate
> it.

10base2 IS ethernet ;-) 
The colloquial term ethernet stands for a whole family of both networking hardware
as well as network protocols (i.e. it stands for several related types of cabeling 
and the lowlevel protocols run over these cables). 

There are several types of cables used for ethernet:

 - the old 'thick wire' coaxial cable (10base5, wasn't it). Impressive red cables
   about a thumb thick, and a major pain to bend (i think the minimum bending
   radius was 50 cm). You atached a node to the cable by actually drilling a hole
   into the cable and sticking a connector into that hole (no joke!).
   
 - the slightly newer 'thin wire' coax cable, traditionally yellow. This is the
   10base2 cable. Like the thick wire this was a bus system (i.e. one long cable
   with nodes atached to it. Terminators on both ends of the cable). 

 - 10baseT cable (twisted pair category 5 cable). This is the cheap ethernet you
   probably know. On the cable-level this uses a star topology, but on higher levels
   it still is a bus (all nodes 'see' all packets on the bus).

 - 100baseT cable (very similar to 10baseT but much faster ...). Without going
   into too much detail, there are some important differences between the last 
   two, so it's not too easy to build a box that converts from one to the other
   (different binary encodings (i.e. the way the 'zeros' and 'ones' are represented
   'on the wire', different frames etc.).

 This is only the cable part: of course one can (and often does) run different
 network protocols over the same wire. AppleTalk as well as TokenRing commonly
 run on 10baseT (the hardware protocol is then called 'ethertalk'...). 

 What unifies the ethernet family then? It's the basic network concept behind
 the different protocols: You send data frames (packets) as if the wire where
 the radio ether (hence the name). Whenever one station wants to send it checks
 if the 'ether' is free, and if so starts sending. If not, the station waits 
 for a while (the exact algorythm for this waiting is actually pretty tricky)
 and tries to send again. As you can imagine, once the 'ether' starts getting 
 filled up, sending gets very hard (the number of unsucessfull sends increases
 dramatically--the net is saturated). For ethernet, once you reach 60% saturation
 the performance drops fast. This might result in certain nodes not being able
 to send packets at a decent speed. In a TokenRing network the bandwidth is
 split up evenly and there are no collisions. A 'token' is passed arround,
 and whoever has the token is allowed to send (yes, i know, this is grossly
 oversimplified).
    
 If you are interested in networking, there are of course a lot of good books
 arround. If you don't want to (or can't) spend a lot of money you might try
 the PDF book available online at:
   http://www.redbooks.ibm.com/
   (i just checked it, i think you now need to register ...)

 I would suggest 'IP Network Design Guide'. I think this book gives you a
 lot of valid information on networking (not only ethernet).

   Ralf