It's interesting to see how things that we take for granted (ie. A
switch allows for multiple conversations while a hub can't) turn out to
be so complicated when you try to understand what's taking place "behind
the scenes". To sum up what has been uncovered thus far (and maybe put
an end to this thread)-
The issue in question was whether/how a 1 gig uplink can be filled by
100 meg attached stations on two switches.
I think that since the uplink is serial by nature (but full duplex) only
packets from one station can be sent and at the same point in time only
packets from one station can be received. The packets sent and received
may or may not be from the same station. Essentially, as Chuck said only
one packet is on the wire per direction at any given time. The switch
will buffer all packets and treat each session between any two stations
individually. Multiple data streams between different stations in the
same direction cannot exist at the same point in time on the wire.
(think of a twisted pair cable which has 4 pairs encased in the same
sheath /pipe). Data from multiple stations (represented by the
pairs)cannot travel over the uplink (represented by the sheath) at the
same time, resulting in 4 concurrent flows. Instead, one conversation
occurs per direction at any point in time.
Now about filling up that 1 gig link even though only two conversations
can exist on the wire at any given time-
I guess that the switch can switch between conversations fast enough and
send packets over the uplink at a much greater rate than what any
attached client can send. Put enough busy clients together sending
requests out the uplink and I can see how that 1gig uplink could become
saturated. The switch just increases the rate at which packets are sent
out of the Gb uplink.
Does this make sense?
Vijay Ramcharan
Vijay Ramcharan
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Chuck Larrieu
Sent: Wednesday, May 30, 2001 3:51 PM
To: [EMAIL PROTECTED]
Subject: RE: elementary? [7:6359]
I concur. I should have been a bit more clear in that I was addressing
the issue of a gig link between two switches.
For traffic that remains within a single switch, different things can be
done with the switch fabric, thus increasing the number of packets
handled.
But a single gig link between two switches, operating at full duplex,
can have only one packet per direct on the "wire" at one time.
Also, I still think that on any link between any end station and the
switch port, the transmitting end station still waits until it senses
nothing on the wire fore putting the next packet out that interface. The
end station, after all, does not know to what it is connected. Rules of
the game. Listen. If wire is empty, place packet onto wire, listen, if
wire is busy, wait. Perhaps some of the newer layer two drivers do
things a bit differently if they detect full duplex? I'm not so sure,
but then I'm just an old dog.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf
Of
Gareth Hinton
Sent: Wednesday, May 30, 2001 10:14 AM
To: [EMAIL PROTECTED]
Subject: Re: elementary? [7:6359]
I think everybody might be right here but arguing about different parts
of the process, or confusing the meaning of the previous post , so just
to add more confusion: Peter said that all every station could send as
much as they want, which I think he was referring to their own ethernet
segment/(switch port). On the Gig link, buffering will obviously have to
take place. Statistical multiplexing seems a good summary of what's
happening. I'm not sure exactly what you were saying in the last post
Alan, about the buffering. Full duplex operation will allow another
station to send to you while you are sending to it, so no buffering
required in that case.
As usual, open (prone) to correction,
Gaz
""W. Alan Robertson"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Chuck,
>
> It's pretty much an issue of semantics... Another station could send
> to you, but the frame would be buffered by the switch until the
> current frame had finished sending. It would be transmitted to you
> afterward.
>
> Also, to confirm Peter's statement that he's never seen a full-duplex
> hub... Such an animal does not/can not exist. This is one of the key
> differences between hubs and switches. A hub, by it's very nature,
> cannot provide full-duplex operation. It has no means of bufferring
> frames, nor of providing segmentation on a per node basis. A hub is
> layer 1 device, and the network is provides is a shared medium.
>
> Vijay, chances are that if it has a 1Gbps uplink, it is a switch, and
> depending on the number of connected 100Mbps stations, and your
> network traffic patterns, you very well might be able to saturate the
> uplink connection, because a switch allows for multiple simultaneous
> conversations. Under the right conditions, you could fill up
> virtually any pipe, but unless your traffic demands are really
> outlandish, you probably won't. If you do, you should examine the
> reasons, and revise the design of your network accordingly.
>
> Alan
>
> ----- Original Message -----
> From: "Chuck Larrieu"
> To:
> Sent: Wednesday, May 30, 2001 11:14 AM
> Subject: RE: elementary? [7:6359]
>
>
> > Hhhmmmm....... Not so sure this is exactly right......
> >
> > With full duplex, you have effectively created two "directions" ---
> there
> > and back.
> >
> > I believe it is accurate to say that only one packet can be on the
> wire per
> > direction at one time.
> >
> > I can send to you at the same time you are sending to me. But
> Someone else
> > can not send to you at the time my packet is on the wire.
> >
> > Correct me if I'm wrong.
> >
> > Chuck
> >
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf
> Of
> > Peter I. Slow
> > Sent: Wednesday, May 30, 2001 7:40 AM
> > To: [EMAIL PROTECTED]
> > Subject: Re: elementary? [7:6359]
> >
> > Noooooooo.
> > nononononono.
> > CSMA/CD only gets used when you are not in full duplex. (/me ducks)
> ( i
> > have NEVER seen a full-dup. hub) meaning that if i am using a switch
> capable
> > of full duplex (as most are) ..conversations, every station can
> transmit as
> > much as they want. this is what differentiates between a hub and a
> switch.
> > (but not the only thing)
> > you are correct in that a 100 meg HUB with a gig uplink could never
> fully
> > utilize the link, but the case is completly different with a switch.
> >
> >
> >
> > ----- Original Message -----
> > From: "Vijay Ramcharan"
> > To:
> > Sent: Wednesday, May 30, 2001 9:54 AM
> > Subject: RE: elementary? [7:6359]
> >
> >
> > > Thanks everyone for their replies. As I now understand it, the
> 1Gb
> > > uplink just moves data faster than... say, a 100Mb uplink.
> Correct?
> > > Conversations between hosts on each switch still take place one at
> a
> > > time, thereby obeying Ethernet rules of one station transmitting
> at a
> > > time. Correct?
> > > Okay my next question. Is there any point at which this 1Gb uplink
> can
> > > become saturated, since it's only handling station to station
> sessions-
> > > one at a time.
> > > If a number of stations on each switch were doing large file
> transfers
> > > to each other via the uplink, would there be some point at which
> the
> > > uplink would be maxed out- in terms of bandwidth? Or is the only
> > > limiting factor, the workstations inability to pump data out fast
> enough
> > > to max out the uplink when they're only running 100Mb?
> > >
> > > I'm thinking that it's really not possible to max out a 1Gb uplink
> when
> > > stations are only running 100Mb. If this is correct then I lay
> this
> > > question to rest.
> > >
> > > Thanks.
> > >
> > > Vijay Ramcharan
> > >
> > >
> > > -----Original Message-----
> > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On
> Behalf Of
> > > Vijay Ramcharan
> > > Sent: Wednesday, May 30, 2001 12:06 AM
> > > To: [EMAIL PROTECTED]
> > > Subject: elementary? [7:6359]
> > >
> > >
> > > Forgive me if this sounds a little bit basic but this is what
> happens
> > > when you rush into things without understanding the fundamentals.
> > > Suppose a 24 port 100Mbit switch called A is uplinked to another
> 24 port
> > > 100Mb switch called B via a 1Gb connnection. Suppose hosts D
> through N
> > > are on switch A and hosts M through X are on Switch B. Would
> > > conversations between the hosts from Switch A to Switch B occur
> one at a
> > > time or are multiple conversations multiplexed over the 1Gb
> uplink?
> > >
> > > I'm just trying to find out if and how that 1Gb uplink is used up.
> > > Thanks in advance. I'd put TIA but I hate those little acronyms.
> No
> > > flames please.
> > >
> > > Vijay Ramcharan
> > > FAQ, list archives, and subscription info:
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