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New Member

Why we are wasting two pairs in Cat5e cable?

Hi,

I'm learning about the UTP cables and the two  standards T568A and T568B. I learnt that only two pairs (Pins: 1,2,3,6)  are used for transmit and receive.

Then why we need other two pairs (Pins:4,5,7,8) ?

Any idea?

Thanks in advance.

Regards,

Chandu

Regards, Chandu
14 REPLIES
Hall of Fame Super Gold

Why we are wasting two pairs in Cat5e cable?

Future proofing.  You'll need two pins for GigabitEthernet and another two pins for PoE.

New Member

Why we are wasting two pairs in Cat5e cable?

Thanks leo.

But i didn't get what you meant by Gigabit Ethernet because as far i learnt: Ethernet,FaEth and GigEth can be run over cat5e so abviously they will use the same 4 pins, right?

Can you please elobarate a bit more?  Please don't mind if my question is silly.

Thanks in advance.

Regards,
Chandu

Regards, Chandu
Hall of Fame Super Gold

Why we are wasting two pairs in Cat5e cable?

You can run Ethernet, FastEthernet and or GigabitEthernet with the same PHYSICAL CABLE. 

For Ethernet and FastEthernet, you need two pairs.  For GigabitEthernet, you need a pair of wires.  So for GigabitEthernet, you need a TOTAL OF six wires. 

New Member

Why we are wasting two pairs in Cat5e cable?

HI Leo,

Do you have any doc or link about this? I wanna learn,

Regards,
Chandu

Regards, Chandu
Cisco Employee

Why we are wasting two pairs in Cat5e cable?

Leo,

For Ethernet and FastEthernet, you need two pairs.  For GigabitEthernet,  you need a pair of wires.  So for GigabitEthernet, you need a TOTAL OF  six wires.  

Are you sure about this? I am absolutely positive that for a working GigabitEthernet, you need to have all four pairs, i.e. a total of eight wires, connected properly, as all eight wires will be used simultaneously to carry data in both directions. This also follows from the per-pair signalling rate of 125 MHz, which coupled with 2 bits per modulation change (PAM-5) and the use of all four pair simultaneously gives you 125x2x4=1000 Mbps.

A nice knowledge base about Gigabit Ethernet can be found here:

https://www.iol.unh.edu/services/testing/ge/knowledgebase/

Best regards,

Peter

New Member

Why we are wasting two pairs in Cat5e cable?

Your correct, Gigabit uses all 4 pairs!

The other pairs were used for POE etc.

Martin

Hall of Fame Super Gold

Why we are wasting two pairs in Cat5e cable?

Are you sure about this? I am absolutely positive that for a working GigabitEthernet, you need to have all four pairs,

Hi Peter,

I'm talking about the wire mapping from the direction of the way Catalyst appliances "works" the cable.  There are significant differences between the way IEEE standards and the way GigabitEthernet switches implement this rule. 

My apologies for the confusion everyone.  I should've made this clear early on.

Cisco Employee

Why we are wasting two pairs in Cat5e cable?

Hi Leo,

No problem... but I must admit I still do not understand what exactly you have in mind.

I'm talking about the wire mapping from the direction of the way  Catalyst appliances "works" the cable.  There are significant  differences between the way IEEE standards and the way GigabitEthernet  switches implement this rule. 

Wire mapping? I am sorry, I am lost. Can you elaborate more on this please?

Best regards,

Peter

Hall of Fame Super Gold

Why we are wasting two pairs in Cat5e cable?

Wire mapping? I am sorry, I am lost. Can you elaborate more on this please?

Ok, GigabitEthernet, in the eyes of IEEE uses all four pairs.  Eight wires. 

But this is not the case with the way switches interpret them.  Using the same "logic" to TDR, Pair A is signaling.  Pair B is Layer 2.  Pair C is PoE and Pair D is GigabitEthernet. 

I must be using the terms wrong.

This is why I always gets confused when someone starts talking about IEEE standard and the way I "see" it via TDR.

Cisco Employee

Re: Why we are wasting two pairs in Cat5e cable?

Leo,

Hmm... Do you mean the test cable-diagnostic tdr? This is an output I've captured right now and I do not see any hint to the terminology you've used:

TDR test last run on: February 23 12:07:23

Interface Speed Local pair Pair length        Remote pair Pair status

--------- ----- ---------- ------------------ ----------- --------------------

Gi0/36    1000M Pair A     11   +/- 4  meters Pair A      Normal             

                Pair B     11   +/- 4  meters Pair B      Normal             

                Pair C     12   +/- 4  meters Pair C      Normal             

                Pair D     12   +/- 4  meters Pair D      Normal             

Best regards,

Peter

Hall of Fame Super Gold

Why we are wasting two pairs in Cat5e cable?

I do not see any hint to the terminology you've used

Yes.  It's the TDR.

The best way I can explain is this:  GigabitEthernet standard requires four pairs active.

If you get a cable and connect this to a switch and disable Pair "C" (which is the PoE pair) then the interface is still able to "run" at GigabitEthernet.

New Member

Why we are wasting two pairs in Cat5e cable?

Lots of information. Thanks.

So the overall confirmatin is Gigabit Ethernet uses 3pairs for data and 4th pair for PoE.

Correct me if i'm wrong.

Thanks in advance.

Regards,
Chandu

Regards, Chandu
Cisco Employee

Why we are wasting two pairs in Cat5e cable?

Chandu,

So the overall confirmatin is Gigabit Ethernet uses 3pairs for data and 4th pair for PoE.

Correct me if i'm wrong.

Well, I don't believe this until I test it It should be in a day or two. The PoE can be done over any pair that also carries data (this technology is ancient - old microphones have already used it and called it a phantom power).

Best regards,

Peter

New Member

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