Jam signal and collision detection

Discussion in 'Cisco' started by jameshanley39, Apr 20, 2005.

  1. 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
    jameshanley39, Apr 20, 2005
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  2. jameshanley39

    Brad Guest

    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.
    Yes, backoff is only initiated when a NIC detcts a collsion.
    Brad, Apr 20, 2005
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  3. jameshanley39

    polleke7 Guest

    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

    NEW on the Internet: cisco irc chat groep
    polleke7, Apr 20, 2005
  4. jameshanley39

    Brad Guest

    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.

    Jam signals were around long before 100M ethernet was developed.
    Brad, Apr 20, 2005
  5. jameshanley39

    Dana Guest

    Kind of. The jam signal is heard by all the receivers on that lan.
    Once a receiver detects a jam signal, its backoff is triggered.
    And A's backoff will be triggered as well.
    Dana, Apr 20, 2005
  6. jameshanley39

    Dana Guest

    Actually in Ethernet, collisions are expected to happen on half duplex lans
    Dana, Apr 20, 2005
  7. jameshanley39

    Dana Guest

    The station that was transmitting goes into backoff.
    Dana, Apr 20, 2005
  8. jameshanley39

    stephen Guest

    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).
    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).
    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
    stephen, Apr 20, 2005
  9. jameshanley39

    thrill5 Guest

    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

    thrill5, Apr 21, 2005
  10. Well, no. "transceiver" is a device that can receive and
    transmit, but not necessarily both at the same time.
    Walter Roberson, Apr 21, 2005
  11. jameshanley39

    Brad Guest

    Good definition Walter.

    Scott, when ethernet is in half-duplex mode then it can only transmit
    OR receive not both at the same time.
    Brad, Apr 21, 2005
  12. but then howcome it says * in the 802.3 spec

    the spec seems to say that the jam signal elongates the duration of the
    collision for it to be detected by transmitting stations!

    yet you and many others, seem to me to say that the collsiion is
    detected by the transmitters, without the jam. The jam is so the
    receivers reject the frame.

    either way, a prob I noticed with all this, is that if transmitters
    immediately transmit a jam signal after detecting a collision, then
    wouldn't jam signals collide?

    infact, according to one site, the jam signal 'unambiguously destroys
    the colliding transmissions'. So the stations better detect the
    collsion before that happens!

    btw, do the standard CCNA Cisco books from ciscopress iron out these

    * (quoted from 802.3 spec - available free from IEEE)
    ""Transmit Media Access Management enforces the collision by
    transmitting a bit sequence called jam. ....
    --This ensures that the duration of the collision is sufï¬cient to be
    noticed by the other transmitting station(s) involved in the
    After the jam is sent, Transmit Media Access Management terminates the
    transmission and schedules another transmission attempt after a
    randomly selected time interval."
    jameshanley39, Apr 21, 2005
  13. jameshanley39

    Dana Guest

    Kind of. What happens is if you have two stations spaced out to the max
    distance, first A transmitts, because of propagation delay it takes a
    certain amount of time to travel down the pipe. Now say station B who is at
    the max distance from A transmits a few microseconds after A., A collision
    will occur closer to B than A, since both A and B were transmitting, they
    both start to transmit the Jam signal. Now the signal has to travel from B
    back to A, After B sends out it's jam signal, after a few microseconds where
    A notices the collision, it as well will transmit the jam signal.
    B finishes its 48 bit jam signal, but A still hears this jam signal from B
    because of the propagation delay. When the B jam signal at A finally ends, A
    is still in the process of transmitting it's own jam signal.

    All receivers here the jam signals from A and B, but only A and B go into
    backoff before attempting to retransmit.

    This is why frame length is important. If frame length was shorter, more
    collisions would occur, as the stations would be transmitting before a
    signal propagates through the medium between the two stations separated the
    By the transmitting stations, not the actually transmiter in the NIC.
    Remember this is a transceiver, it can transmit and receive, simultaneously
    in full duplex only.

    Yes, and this can be done by either looking at the current, as if only one
    station is transmitting, the current level would be X, (I think I remember
    this as being in the neighborhood of 16 to 20 miliamps.) now if another
    station transmits, this current being seen by a receiver now increases to
    X+.(Which would probably be over 22 miliamps).
    Another way is for the transmitter to compare what it received to what it
    transmitted, during the transmission phase. If the CRC's match, probably no
    Well they will reject the frame as it would be garbled, the jam signal is
    notification that the collision happened.
    At some point on the transmission media yes they would.
    To see how this works look at two stations separated the max distance of
    2800 meters. Lets have A transmit first, now B transmits before the signal
    from A reaches B, somewhere closer to B the signals collide, since the
    collision occured closer to B, B detects the collision first, and sends it's
    jam signal. After B sends it's jam signal, A also dected the collision and
    it also sends it's Jam signal. After B sends its jam signal and stops
    transmitting, the entire jam signal has still not made it to A, so while A
    is transmitting it's jam signal, B's jam signal is still on the line, After
    B's jam signal is off the media, A is still transmitting it's 38 bit jam
    This is where frame length and max distance between stations come into play.
    When station A or B transmitts, there is a certain amount of time before
    either B or A receives this transmission.
    CCNA does not go into this much detail, so the answer is no. Some older CCNA
    books may have this. As look at the CCNA book now compared to one that came
    out in the mid 90's, quite a bit of material has been dropped off the test.
    Dana, Apr 21, 2005
  14. There is lots of stuff on this in comp.dcom.lans.ethernet (newsgroup
    recommended by charles - the author of the 'definitive ethenet guide').

    A big authority on this is richard seifart on that newsgroup mentinoed

    The transmitting NIC has to stop transmitting - it doesn't want the
    receiving station to read the frame. But if it stops transmitting, then
    the frame may be so so small (< 96 bits) that - on 500m coax, the
    collisino frame will degrade and be unreadable if the other
    transmtiting station is at the other end. (as seifart says regarding
    the jam "it was designed in the coax days.") So, the jam signal is
    32-bits long. Station B that detected the collision first, will
    complete its preamble and SFD(if it hasn't already), then will transmit
    the jam signal as a continuation, so it transmtis a frame with a bad
    CRC that is long enough not to degrade. (this is a different kettle of
    fish to the much larger minimum frame size of 512 bits). That collision
    frame from B will reach station A, (it will reach A while A is still
    transmitting, because the 512 bit minimum frame size ensures that). A
    will then stop transmitting and emit the jam signal which will append
    onto its transmission.
    Suppose this reaches B !!! If B is still sending its jam, i.e. its
    transmitting, then it won't pay an attention to any more collisions
    while its sending its jam (to quote an earlier post fro 1998 "a station
    sends the fixed length jam once and ignores what is happening while it
    is transmitting it. ") .
    Suppose it reaches B and B isn't transmitting - so B like any other
    receiving frame isn't detecting collisions, and will read the bad CRC
    and reject the frame.

    why a min frame size ? so that a transmitting station will always hear
    collision while its transmitting. Why is that important? because then
    it can resend the frame. If however, the transmitting frame were to
    complete tarnsmitting and then etect a collisionl it won't be storng
    the farme anymore, it doesn't archive old frames. It clears its memory
    once its transmitted - it assumes that tarnsmission is successful. once
    it has transmtitd it isn't listening for a collision.

    The 96 bit thing is to last out twice the cable length while still
    emitting. its purpose is just to be long enough to reach the other end
    - it can stop being emitted long before .

    as mentioned. jam signals don't trigger backoff. collisions do, and jam
    signals are bits within a collision anyway. the jam signal just ensures
    that the collision is long enough (96 bits) to reach the other end of a
    500m coax cable without degrading. They also give/maintain a bad CRC.
    (i say 'maintain' because i'm sure that a collided frame would have a
    bad CRC anyway). (i say 'give' because if the collisino is detected
    during the preamble and SFD then the premable and SFD is finished, and
    the jam signal would give a bad CRC). since calculation of the CRC
    begins immediately after the preamble and SFD.

    i'm dizzy now. i hope that's more-a-less correct!!
    do check other posts - tere are many on this topic in that newsgorup
    mentinoe earlier, specially check richard seifart's posts.
    q_q_anonymous, Apr 28, 2005
  15. jameshanley39

    stephen Guest

    its not about signal degrading in this co-ax - its about sending a long
    enough signal that it still looks like a "jam" locally, and on other
    segments after passing through up to 4 repeaters.

    That collision
    No (or not in the general case) - the signal has to propagate a lot further.
    at 10 Mbps the collision domain spans multiple repeaters, and several
    segments in parallel. or on a point to point fibre without repeaters - up to
    4 Km.

    The minimum length bit string is to make sure signals propagate end to end
    across the entire collision domain.
    stephen, Apr 28, 2005
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