Ascend Archive
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: (ASCEND) max6k 100Mb FD?

On 10 Jun 98 at 16:21, Danny Williams quoted someone as saying:

>   We've discussed this at length previously, but here again is      
>    
> the obvious. How does a transmitter know when to shutup because a
> wire is busy? 

The wire is never busy on Switched FDX. Both end stations can happily
transmit and recieve simultaneously, with no collisions. That's what 
full-duplex means. That's the point of full-duplex. That's why Cisco 
call it "Collision-Free" in the Catalyst manual.

I run  "collapsed" 100BaseT FDX backbones between switches, and into 
my core routers, in my machine room. Works like a dream and network 
performance is second to none.

> FDX lacks the feedback to tell it. Frames will be
> lost, "the network is slow" phone calls happen.

As far as I know, CSMA/CD rules are implemented in 100Base-T FDX 
anyway, so there is a feedback system of some sort - but in a 
swictched FDX network, there will be no need for it. Quite why anyone 
would wish to try running a shared 100Base-T FDX network is beyond 
me.

In the case of things being dropped on busy links, the layer 3 and 
above protocols can handle things like retransmission. However, if 
you've got capacity/collision problems on 100Base-T FDX, then you 
need to look at some sort of stronger solution still (FDDI, LANE over 
ATM, or Gigabit), or at your network design in general.

Also, some switches can dynamically alter switching modes (from 
cut-through to store-and-forward, etc) and also exercise some form of 
flow management.

>   Switches/routers in series between transmitter and receiver       
>   
> can buffer some frames, but only some and you don't know how many
> (because some are in use from other traffic).
>   FDX does not double throughput, in can markedly reduce       
> throughput (because of lost frames). Unless you can demonstrate no
> problems and a strong need then turn it off.

Is this supposed to be a troll? If so, I've fallen for it hook 
line and sinker! :-)

Mike
-- 
Mike Hughes - Network Services         mike@dircon.net
Tel: 0181 297 0300                     http://www.dircon.net/
Fax: 0181 463 9820
++ Ascend Users Mailing List ++
To unsubscribe:	send unsubscribe to ascend-users-request@bungi.com
To get FAQ'd:	<http://www.nealis.net/ascend/faq>
References: