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Cabling ?

Posted on 2004-10-29
Medium Priority
Last Modified: 2013-11-09
Ok, can someone verify this for me?

1. 10base T and 100 base T are both wired the same. 10baseT runs at 10mbps  and 100baseT typically runs at 100mbps

2. Cat3 and Cat5 are terminated the same. The physical difference is that Cat5 has more twists in it?

3. Half duplex uses 1 pair (thats what my Cisco book says?). One wire transmits, one receives.  Must do one or the other. Cannot do both simultaneously.   Half duplex 10baseT operates at 10mbps.  Half duplex 100baseT operates at 100mbps (still uses 1 pair).

4. Full duplex uses 2 pair. 2 wires transmit, 2 receive at the same time.  Full duplex 10baseT can run up to 20mbps.  Full duplex 100baseT can run up to 200mbps.

I thought half duplex used 2 wires to transmit, 2 wires to receive (not simultaneously). Buy my Sybex Cisco book is saying otherwise ?
Question by:dissolved
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LVL 69

Assisted Solution

Callandor earned 1000 total points
ID: 12447590
1. true
2. true
3. How do you transmit on one wire?  I think half duplex means signals travel in one direction at any given time, so two wires are being used, but at any given moment, one side is the transmitter and the other is the receiver.  They can flip positions, but can't be both at the same time.
4. Two pairs are used, but the transmission limit is still 10mbps for 10baseT and 100mbps for 100baseT.  The limit is determined by the number of twists in the wire, to reject noise.  Gigabit transmission requires cat6, which has more twists than cat5.

Accepted Solution

stockhes earned 1000 total points
ID: 12447624
Hi Dissolved

1 OK

2 Cat5 is better shielded as well

3. Half duplex uses 1 pair AT a time, 1 pair transmits and another pair receives but NOT simultaneously.
Half duplex is necessary when connecting more host on the same LAN eg via a HUB.

4. FULL duplex 100baseT can run up to 200mbps. theoretically yes 100 up and 100 down simultaneously
needs "starconnected" net each host need to be connected to a switch

I thought half duplex used 2 wires to transmit, 2 wires to receive (not simultaneously).

If using RJ45 and ethernet manchestercoding(normal lan) you are

Buy my Sybex Cisco book is saying otherwise ?

Its possible like in the old coax( same as antenna cable) days where everything was half duplex
but its not normal these days


Author Comment

ID: 12447666
Thanks guys. So half duplex does indeed use 4 physical wires ?  1 pair transmits, 1 pair receives. But not at the same time? Is that correct?
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LVL 69

Expert Comment

ID: 12447681
I stand corrected on 4 - each pair of wires is limited to 10 or 100 mbps, so two pairs can deliver 200 mbps in full duplex.  There is a standard for gigabit that uses 4 pairs of cat5, pushing 250 mbps on each pair to deliver 1gbps total.
LVL 69

Expert Comment

ID: 12447736
Half duplex can use 2 pairs or 1 pair.  A reason for going with half duplex, though, is to slash cabling costs, or because there is one talker and many listeners.  http://news.managingautomation.com/fullstory/5828
LVL 11

Expert Comment

ID: 12448309
I dug up this, it might just be a rerun of whats been said. If so I'm sorry.

    My understanding was that when Cat5 cable is used with network adapters, a reference signal was sent through one wire, and data through the other, and therefore noise would affect both equally so the receiving device could reverse any noise in the data.

    But the audio device you suggest using the cable with wouldn't know or care about this. How do twisted pair cables stay noise-free?


You're almost describing a balanced connection, but not quite. In balanced interconnects, you have two signal conductors, and one ground. The two signal conductors each carry the same signal, but they have opposite polarity - when the waveform's rising on one of them, it's falling on the other. So one conductor carries a signal that's exactly the same as whatever you want to feed down the cable, and the other one carries that same signal, but inverted.

The idea here is that noise will affect each conductor the same, so that when you un-reverse the inverted conductor's signal and combine it with the un-inverted conductor's signal at the other end of the line, the noise will cancel itself out.

In any case, that's not how Ethernet network cables work. Standard 10/100BaseT RJ45-connector cables use only four of the eight terminals on the connector, and they use them for two simple two-conductor loops, one for transmit and one for receive. No fancy balanced stuff is going on.

Twisting two conductors around each other, as opposed to running them parallel to each other, turns them from one big (if narrow) loop into lots and lots of little loops, each successive one being reversed compared to the one before it. The multiple reversed loops reduce common mode noise (potential differences between the conductors and ground), because the polarity of noise created by external interference is reversed in each loop, and thus it tends to cancel itself out. Twisted pair cable certainly isn't perfect, but it's a lot less interference-prone than the same cable without the twists.


If you want to read of the start of Ethernet and a full explanation of it all, you can read this:

It's great for explaining all that about full/half duplex and how it all began.


Expert Comment

ID: 12449051

Another way(instead of twisting) of avoiding the capacitance being created, when signalling on parallel wires, is using HDB3 coding, which basically inverts every second 1 being send, referencing a ground.  first 1 is positive ,second 1 is negative and so on.



Assisted Solution

stockhes earned 1000 total points
ID: 12449081
here is a link explaining the manchester coding used in a normal ethernet


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