crossover and straight thru

hi, im trying to figure out crossover and straight thru. I know the similar devices like switch to switch and router to router requires a cross over. I know that like switch to a router requires a straight thru. i came across a article on the internet, it said that a Nic to a router is crossover cable? I might be wrong here but isn't a nic to a router a straigt t hru since they are different devices Thanks for the help.

Reply to
CREAM
Loading thread data ...

I don't know the official terms but there are two kinds of devices. PC, routers, printers, fax machines are all one kind. Switches are the other kind.

Originally for UTP Ethernet it was hubs one way and everything else the other way which was simple enough. PC to hub was straight through, hub to hub or PC-to-PC needsd a crossover.

Then we got routers and bridges which were not hubs and so were in the everything else category.

Then switches replaced hubs.

Now of course some switches are really routers as well as being switches:-)

A router is normally the same as a PC and it needs a crossover to connect to a PC.

Of course many cisco and other switches now auto detect and work with either cable (auto-mdix) and as far as I recall GBE (gigabit Ethernet) works with either too - always.

Reply to
bod43

"Hub" has always been a confusing term; the first UTP-ish ethernet things were repeaters.

Switches replaced repeaters, but they were a direct replacement - no wiring changes needed (usually).

Don't confuse the children! If it's capable of being a switch (hub) then it's a switch for this purpose. If it's only a router then it's only a router.

This is true. You can usually connect two reasonably modern PCs together with a straight through cable and they'll work - but see below.

GigE (1000base-T) is specced to work with a straight through cable - always! That's because it uses all four pairs of wires in both directions simultaneously. Using a crossover cable (of which there are at least two sorts, but let's not go there) really ought to confuse it, but the standards guys were canny enough to see that coming and allow crossover cables. AFAICT you should never need one for GigE (anyone care to differ?) and the fact that many PCs have 10/100/1000 ports in them these days means that a straight through cable will work for them. Needless to say you shouldn't need a crossover to connect two routers or two switches with 1000base-T interfaces.

HTH

Sam

Reply to
Sam Wilson

Ah yes, I *never* use the term "hub" for the reasons you mentioned. Except this once it seems:-((

Thanks for fixing it. Dunces corner for me.

It seems to me that a cable with all 4 pairs crossed will be indistinguishable *by principle* from a straight through and so the standards people had no additional work to do.

I am guessing. I don't actually know how it works:-)

If the "end host" and "concentrator" ports are identical then the above will be the case. The port can't "know" what it is talking to or whether a cross cable is in the line or not.

No additional work from the designers.

If the ends are not identical and say each signal their "preferred" type to the other then of course the case of a crossover cable will need to be considered by the end devices.

Most crossovers I encountered have been full 4 pair crossovers and I am pretty sure I have used them on GBE ports.

I will post this to comp.dcom.lans.ethernet (probably the first on-topic post for months) too and perhaps Mr Seifert will comment on whether either end ever becomes aware that there is a crossover there or not? It's not important at all now since in my experience it all magically just works so maybe he will consider it not worth the time?

Reply to
bod43

My contribution to terminology based on vague and error-prone recollections of "the old days:"

repeater - two ports - what is seen on one port is always repeated (at the physical layer) on the other port

hub - a repeater with more than two ports. what is seen on one port is always repeated on the remaining ports.

bridge - two ports - what is sean on one port is always forwarded at the data link layer on the other port

learning bridge - a bridge that learns which MACs are on which side and makes forwarding decisions based on that

switch - a bridge with more than two ports.

learning switch - a switch that learns which MACs are on which port and makes forwarding decisions based on that.

some might replace "learning" with "smart"

I might yield on learning/smart vs not on terminology and go with "dumb bridge" as the one that does not learn and "bridge" as one that does - same for switch.

rick jones

Reply to
Rick Jones

In comp.dcom.lans.ethernet bod43 wrote: (snip, someone wrote)

For most 10baseT and 100baseTX ports, transmit and receive are on separate pairs, and must be straight or crossed as appropriate.

When I do make crossover cables (not very often), I cross the 100baseTX pairs (1,2) with (3,6), and also pairs (4,5) with (7,8). I have heard that others don't cross the latter.

Well, for gigabit, which both sends and receives on all four pairs, it isn't so hard for it to figure out which pair connects to which, and extract the bits accordingly.

Also, many 100baseTX ports have this ability, presumably with extra hardware.

-- glen

Reply to
glen herrmannsfeldt

No, these two cases *are* distinguishable because the signals on the four pairs have different semantics (meanings). The 1000 Mb/s data stream is sent as four streams carrying 250 Mb/s each, and then recombined at the far end. It is important that the signal be recombined in the proper order, so you cannot "mix up" the wire pairings indiscriminately.

Also, Auto-Negotiation (which is mandatory in 1000BASE-T) is performed on only two pairs--the same two pairs that are used for 10/100BASE-T. This is necessary because it is entirely possible that a 1000 Mb/s device at one end is Auto-Negotiating with a 10/100 Mb/s device at the other end, with the result of 100 Mb/s operation. So, again, the pairing is important.

Of course, it is easy to figure out whether you have a straight- through cable (Auto-Negotiation signals from the other end will show up at pins 1/2 on your input) or a crossover cable (Auto-Negotiation signals from the other end will show up at pins 3/6 on your input). Once determined, you can organize the data signals for proper presentation to the four pairs : 1/2, 3/6, 4/5, 7/8 so that they can be decoded and recombined at the other end.

*Some* additional work from the designers, but not a whole lot. Even before Gigabit Ethernet, many Ethernet chip manufacturers had incorporated "auto-crossover-detect" in their 10/100 Mb/s products, because it made life easier for the installers and network administrators--no manual setting of MDI/MDI-X switches on devices, and it doesn't matter which flavor of cable you use.

There has indeed been a dearth of on-topic postings to this newsgroup; I only check it every week or so now. This is not a statement about interest in Ethernet, but a statement about the general demise of Usenet.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Reply to
Rich Seifert

Thanks Rich illuminating as always.

Of course usenet has obviously declined and sadly no clearly defined replacement is forthcoming. Mostly in my view this is due to the more recent development of tools that make it trivial for any Joe to knock up their own version of any forum. There have been many cases of people making usenet posts requesting that users divert their attention to competing forums.

I have no idea where to find the equivalent of comp.dcom.lans.ethernet or comp.dcom.sys.cisco as they were only a few years ago. When I first encountered them (along with the now obsolete lans.fddi and lans.token ring newsgroups) they were filled with learning opportunities. I have as yet discovered no alternatives and I miss them sorely.

Another key factor is the capacity of a very few obstreporous individuals to flood a newsgroup with venemous posts such that it becomes unusable. Oh oh! 100baseTx vs the other one (T4?). What were the names, Vernon S vs HP man as I recall? I think HP lost. Also switched full-duplex token-less ring-less token-ring vs switched FD Ethernet. Those were the days:)

Usenet was an experiment and it turns out that unmoderated discussion forums have effectively been proven not to work - is my view. Any functional internet discussion forum needs as far as I can see a driven and dedicated individual to manage a team of moderators such that the chief moderator is able to enforce his vision of the purpose of the group. Otherwise chaos inevitably seems to reign.

If anyone has any cisco discussions that they might care to recommend I am all ears:)

Thanks again everyone.

Reply to
bod43

The "replacement" for Usenet is web-based fora. (Fora is *really* the plural of forums.) Of course, the value of such fora is purely a function of the knowledge of the participants, and their willingness to share. In this regard, they are no different from Usenet.

It was Fast Ethernet vs. 100VG-AnyLan (IEEE 802.12), with Dan of HP (the proponent of VG) and Vernon going at it on a daily basis. Fortunately, I was able to maintain a friendship and good working relationship with both of them.

On this we can disagree. Usenet was not an experiment, but a forum for interactive communications that thrived for more than 20 years. Things change, technology changes, and Usenet is no longer the force it was. So be it. But that doesn't mean it wasn't successful. To say that would be to say that companies like DEC, that thrived for decades before falling by the wayside weren't "successful".

So a dictatorship is preferable to a democracy, because it is more ordered and consistent in its vision and purpose? Chaos can be good.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Reply to
Rich Seifert

Anything is possible, but that never happens, at least not for any significant length of time. Google Groups (your news server) is probably the worst thing UseNet has ever experienced noise wise.

Works for me.

Like floods of spam from Google Groups...

Trying to moderate UseNet would be like pretending you know everything about everything there is to know. That is what RoadRunner tried to do to its own users before dropping UseNet.

Yes, some moderated Internet forums are great for commercial information specific to a product they sell. And that commercial content varies depending on how tied they are to the producer. But there is no better place in the world to find anti-BS information about anything. And there never will be better noncommercial information on any moderated forum.

UseNet has been made more difficult to get to, but that is not necessarily a fault. I suspect that the reduction in use corresponds to the worldwide depression as much as it has to do with anything else. Also, UseNet is not a friend of the one-way media, and someday maybe UseNet will be shut down as an enemy to the state. That possibility is the only way I can see for a real UseNet demise.

UseNet is the wisdom and folly of the world. Long live free speech.

Reply to
John Doe

In comp.dcom.lans.ethernet bod43 wrote in part:

Various web fora have been growing for 10+ years. Perhaps they lack certain advantages of USENET, but they have their own and crucially, have been attracting users.

Why should either be wrong? The cabel was one of the reasons for the decay of USENET. You cannot direct what people wish to talk about. You can only silence them, and only in some places.

Google? USENET has whithered as a result of many factors, including an autoaccelerating effect.

You call that unusable? That was exceptionally mild.

I have seen moderated fora fail worse. If nothing else, the delay stifles them. I am reasonably content to sift through 100 irrelevant/uninteresting posts to find one that is valuable. I worry that filtering [moderation] technologies are certain to generate false-positives, so I sift through 10 irrelevant/uninteresting posts to find _nothing_ of interest.

I do not know what you consider chaos. I do not expect harmony nor edited journal conformity (thank G-d!) I am quite willing to do _research_, including my own filtering.

I believe the Crisco website has some. They suffer all the usual 'blog disadvantages with an heaping dose of corporate editing.

-- Robert

Reply to
Robert Redelmeier

A fair number of questions that previously might have been asked here can now be answered through a Google search.

It seems to me that the reasone for so few posts in comp.dcom.lans.ethernet is that there really aren't that many questions to ask. Ethernet works! No more questions on the

3-4-5 rule.

Some other groups are doing well, others are choked with irrelevant posts. I used to read comp.lang.c, but it takes too long, and there really isn't much there. People post questions about programs presumably written in C (MS Office), that are unrelated to C. On the other hand, comp.lang.fortran is doing fairly well (and survives with posts that might be a little off topic). Comp.dsp also works fairly well.

I suppose so, but it has to have a real advantage to pull people away from something that works. Wikipedia may be one that does work, but there aren't so many of them.

(snip)

comp.compilers is moderated, and seems to work. The delay does make it hard sometimes to have a conversation, but mostly it works.

(snip)

Try comp.lang.c, at least last time I was there.

-- glen

Reply to
glen herrmannsfeldt

In comp.dcom.lans.ethernet glen herrmannsfeldt wrote in part:

Good points.

May I correct that? c.c _appears_ to work. You do not know what it could be, what gets dropped or those who get discouraged and do not post.

I bring that up because I have followed comp.lang.asm.x86 [moderated] and alt.lang.asm [unmoderated] for 15+ years. They cover basically the same topic, but have substantially different "feel". CLAX86 has had more trouble and gone (mostly approve) robomoderation. It is "cleaner" than ALA, but also less interesting, particularly with fewer novel ideas.

I did. ~40 posts/day , average thread depth ~20. Not enough spam nor flames :) Seems perfectly reasonable, but as a catchall (wide topic), it will necessarily have few threads of interest. So I skim. Light work, I've followed NGs with 100s of posts/day and shallow threads that are much more work to process. I don't expect my news preprocessed or otherwise edited. The editor cannot help but be wrong.

-- Robert

Reply to
Robert Redelmeier

In comp.dcom.lans.ethernet Robert Redelmeier wrote: (snip, I wrote)

Maybe it has improved over the years. It has been some years since I tried to follow it.

-- glen

Reply to
glen herrmannsfeldt

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.