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Question re: collisions & half-dup

Depends on the specific implementation. For the more common varieties that are likely to be used in half-duplex mode separate pairs are used for transmit and receive. Regardless, this is not a function of duplex--half- or full- duplex the same pairs are used, what changes is the signals.

10baseT and 100baseTX for example always use separate pairs for transmit and receive, while 1000baseT uses all four pairs bidirectionally.

Yes, it is possible to have collisions any time you are running half-duplex. There is little point to running half-duplex to a switch port but if you do that then if the workstation and the switch port attempt to transmit at the same time a collision will occur.

The manner in which collisions are detected and handles twisted pair is somewhate different from the manner on coax, which might be what is confusing you.

It is always possible, regardless of duplex, to have discarded packets. Ethernet does not guarantee delivery. This does not mean that the connection is unreliable, it means that guaranteeing of delivery has to be handled at a higher level. If you need guaranteed delivery at layer 2 then Ethernet is not the right protocol to be using.

As far as "reputable articles" go this is covered by the Ethernet specification which can be downloaded from . There is a charge for any release less than 6 months old but the questions you raise have been in the spec for a decade or more.

Reply to
J. Clarke
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I want to know the answer to several questions regarding half-duplex and collisions.

1) If you have a workstation talking half-duplex on a UTP cable to a switch (not a hub), does the workstation use a separate conductor for transmitting and receiving while in half duplex? If so, is it ever possible to have collisions while running in such a way (from what I've read it seems that under half-dup on UTP to a switch you use a separate xmit and rcv but can still have collisions).

2) If you hook half-dup from the workstation up to full-dup on the switch, then do you get discarded packets on the workstation-end, resulting in an unreliable connection? I've read that this is so.

Finally, if anyone can answer these questions can you also send me reputable articles (if possible) that back up your conclusion?

Thanks in advance,

Tom

Reply to
Tom Edelbrok

transmitting

Yes, there are different pairs for transmitting and receiving if you use

10BASE-T or 100BASE-TX.

If both stations start transmitting a frame at the same time (or within a very short time interval from each other), the stations will detect that they're receiving something from an other station while they're transmitting. In half duplex mode, such an event is called a collision, regardless of the underlying physical medium.

If you bring the link up in that way, the workstation will declare collisions every now and then, and the bridge will not react properly to them.

Go to

formatting link
and look for IEEE Std 802.3.

Reply to
Michael

Hi,

Here is a model that has worked well for me for a number of years. It allows predictions to be made about the errors generated when duplex mis-matches occur.

I don't know exactly how accurate it is but I think that is is pretty close.

The key thing is that in a HD transceiver (lets call it) the TX and RX cables are *connected* inside the transceiver. This means that for example when the HD side is transmitting, if the FD side transmits too the HD side detects a collision. In this case the HD side might record a late collision (it is not necessarily late) and the FD side will record a FCS error, and/or align error.

FCS is computed by a method known as CRC and the terms are often used interchangeably.

###########################################

The Tx cable and Rx 'cable' are each connected to a seperate twisted pair outside the transciever.

The little boxes are intended to represent the boundary between the external 'big bad world' signalling methods 'e.g Manchester encoding' and the reliable internal signalling 'e.g TTL logic'.

The collision detection comparator compares each Tx and RX bit and signals a collision if the they are different.

This is 10BaseT Ethernet, there will be small, but at this level insignificant, differences between 10 and 100 (and 1000). Co-ax cable ethernets use a different collision detection mechanism.

Connect XX to XX and YY to YY for FD-HD missmatch.

Half Duplex -- Tx-->>--------------------| |-------------------------->XX || -- /\ || || || || || || \/ ||Tx signal copied to Rx Collision Collision ||Rx signal copied to Tx Detected

Reply to
anybody43

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