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Jam signal and collision detection

 
 
jameshanley39@yahoo.co.uk
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      04-20-2005
i figure that a jam signal would cause the receiving NIC to detect a
bad CRC and reject the frame.

but i'm wondering about the purpose of the jam signal in relation to
the transmitting NICs.
Given 2 transmitting NICS A and B - does A's jam signal trigger the
backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
collision detection?

one site says that a jam signal is required because although the
collision will propagate, the collision loses energy(current/voltage?)
and may not reach the transmitting NIC that is further from the
collision. So the jam signal ensures that the transmitting NIC that is
further away knows that a collision has occurred. and i imagine that
implies that A's jam signal received by B will trigger backoff in B

 
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Brad
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      04-20-2005
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
backoff in B's NIC.

If B detects the collsion himself then no he sends his own jam and goes
into backoff. If B doesn't detect the collision then the jam sent by A
will corrupt any frames and they will be discarded by B because of CRC
error. B will not go into backoff unless he detects a collsion himself.

>Or is B's backoff only triggered by its(B's) own collision detection?


Yes, backoff is only initiated when a NIC detcts a collsion.

 
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polleke7
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      04-20-2005
If i'm right, a jam is used in half-duplex environments to notify the
transmiter to holddown the output queue and retransmit the current packet a
few miliseconds later on.

It is mostly found when a dual-speed hub is used with a 10Mbit host on one
side and a 100Mbit on the other side that "pushes" its data to fast to
handle.

================================================== =====
NEW on the Internet: cisco irc chat groep
irc://irc.intersecting.net/cisco.certification
================================================== =====


<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> i figure that a jam signal would cause the receiving NIC to detect a
> bad CRC and reject the frame.
>
> but i'm wondering about the purpose of the jam signal in relation to
> the transmitting NICs.
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
> backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> collision detection?
>
> one site says that a jam signal is required because although the
> collision will propagate, the collision loses energy(current/voltage?)
> and may not reach the transmitting NIC that is further from the
> collision. So the jam signal ensures that the transmitting NIC that is
> further away knows that a collision has occurred. and i imagine that
> implies that A's jam signal received by B will trigger backoff in B
>



 
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Brad
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      04-20-2005
>If i'm right, a jam is used in half-duplex environments.

Yes, in a full duplex environment collision-detection is turned off.
Without collisions there are no jam signals.

>It is mostly found when a dual-speed hub is used with a 10Mbit host on

one side
>and a 100Mbit on the other side that "pushes" its data to fast to

handle.

Jam signals were around long before 100M ethernet was developed.

 
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Dana
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      04-20-2005

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> i figure that a jam signal would cause the receiving NIC to detect a
> bad CRC and reject the frame.


Kind of. The jam signal is heard by all the receivers on that lan.

>
> but i'm wondering about the purpose of the jam signal in relation to
> the transmitting NICs.
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
> backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> collision detection?


Once a receiver detects a jam signal, its backoff is triggered.

>
> one site says that a jam signal is required because although the
> collision will propagate, the collision loses energy(current/voltage?)
> and may not reach the transmitting NIC that is further from the
> collision. So the jam signal ensures that the transmitting NIC that is
> further away knows that a collision has occurred. and i imagine that
> implies that A's jam signal received by B will trigger backoff in B


And A's backoff will be triggered as well.

>



 
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Dana
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      04-20-2005

"polleke7" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> If i'm right, a jam is used in half-duplex environments to notify the
> transmiter to holddown the output queue and retransmit the current packet

a
> few miliseconds later on.
>
> It is mostly found when a dual-speed hub is used with a 10Mbit host on one
> side and a 100Mbit on the other side that "pushes" its data to fast to
> handle.


Actually in Ethernet, collisions are expected to happen on half duplex lans
>
> ================================================== =====
> NEW on the Internet: cisco irc chat groep
> irc://irc.intersecting.net/cisco.certification
> ================================================== =====
>
>
> <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) oups.com...
> > i figure that a jam signal would cause the receiving NIC to detect a
> > bad CRC and reject the frame.
> >
> > but i'm wondering about the purpose of the jam signal in relation to
> > the transmitting NICs.
> > Given 2 transmitting NICS A and B - does A's jam signal trigger the
> > backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> > collision detection?
> >
> > one site says that a jam signal is required because although the
> > collision will propagate, the collision loses energy(current/voltage?)
> > and may not reach the transmitting NIC that is further from the
> > collision. So the jam signal ensures that the transmitting NIC that is
> > further away knows that a collision has occurred. and i imagine that
> > implies that A's jam signal received by B will trigger backoff in B
> >

>
>



 
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Dana
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      04-20-2005

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> i figure that a jam signal would cause the receiving NIC to detect a
> bad CRC and reject the frame.
>
> but i'm wondering about the purpose of the jam signal in relation to
> the transmitting NICs.
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
> backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> collision detection?


The station that was transmitting goes into backoff.

>
> one site says that a jam signal is required because although the
> collision will propagate, the collision loses energy(current/voltage?)
> and may not reach the transmitting NIC that is further from the
> collision. So the jam signal ensures that the transmitting NIC that is
> further away knows that a collision has occurred. and i imagine that
> implies that A's jam signal received by B will trigger backoff in B
>



 
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stephen
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      04-20-2005
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> i figure that a jam signal would cause the receiving NIC to detect a
> bad CRC and reject the frame.
>
> but i'm wondering about the purpose of the jam signal in relation to
> the transmitting NICs.
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
> backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> collision detection?


its sort of a moot point - once the jam signal is on the wire, any device
sending at that point should backoff and retry (and there is no limit - 3 or
more devices could be involved in a collision).
>
> one site says that a jam signal is required because although the
> collision will propagate, the collision loses energy(current/voltage?)


No - the distance limits on a segment are chosen to make sure signals get
transferred reliably - jam is just another kind of signal (although badly
formed, so might decay a bit more quickly).

> and may not reach the transmitting NIC that is further from the
> collision.


this is the correct bit - but the reason is more about timing skew.

imagine a max length fibre segment @ 10 Mbps, no repeaters (4 Km for 1/2
duplex) - signals take a finite time to move across that glass.

if a collision happens at 1 end of the signal, the jam is needed to extend
the time for the event - as the jam is propagating back down the cable
towards the far end. The jam is long enough that both ends "see" a
collision, event though the overlap in packets is different due to
propagation delay

So the jam signal ensures that the transmitting NIC that is
> further away knows that a collision has occurred. and i imagine that
> implies that A's jam signal received by B will trigger backoff in B
>

Agreed.

--
Regards

Stephen Hope - return address needs fewer xxs


 
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thrill5
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      04-21-2005
The jam is sent to ensure that all devices on an Ethernet segment know that
a collision has occurred and that errors detected by other devices receiving
the signal do not mistake this as noise. When a collision occurs both
transmitters will detect the collision. Receivers will only detect the
collision by receiving the jam signal. Transmitters detect the collision by
comparing what is being sent, by what is being received (this is why it is
called a "transceiver" because it transmits and receives at the same time).
If the received signal differs from the transmit, a collision has occurred,
and it sends a jam to notify the receivers to drop the frame and mark it as
a collision. The collision is what triggers the backoff timer on the
transmitter, not the jam signal. Both transmitters will always detect the
collision.

Scott

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>i figure that a jam signal would cause the receiving NIC to detect a
> bad CRC and reject the frame.
>
> but i'm wondering about the purpose of the jam signal in relation to
> the transmitting NICs.
> Given 2 transmitting NICS A and B - does A's jam signal trigger the
> backoff in B's NIC. Or is B's backoff only triggered by its(B's) own
> collision detection?
>
> one site says that a jam signal is required because although the
> collision will propagate, the collision loses energy(current/voltage?)
> and may not reach the transmitting NIC that is further from the
> collision. So the jam signal ensures that the transmitting NIC that is
> further away knows that a collision has occurred. and i imagine that
> implies that A's jam signal received by B will trigger backoff in B
>



 
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Walter Roberson
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      04-21-2005
In article <(E-Mail Removed)>,
thrill5 <(E-Mail Removed)> wrote:
>Receivers will only detect the
>collision by receiving the jam signal. Transmitters detect the collision by
>comparing what is being sent, by what is being received (this is why it is
>called a "transceiver" because it transmits and receives at the same time).


Well, no. "transceiver" is a device that can receive and
transmit, but not necessarily both at the same time.
http://searchnetworking.techtarget.c...213670,00.html


--
Are we *there* yet??
 
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