of the second frame. After a frame has been sent,
all stations on a 10-Mbps Ethernet are required to wait a
minimum of 96 bit-times (9.6 microseconds) before any station
may legally transmit the next frame. On faster versions of
Ethernet the spacing remains the same, 96 bit-times, but the
time required for that interval grows correspondingly shorter.
This interval is referred to as the spacing gap. The gap is
intended to allow slow stations time to process the previous
frame and prepare for the next frame. A repeater is expected to
regenerate the full 64 bits of timing information, which is the
preamble and SFD, at the start of any frame. This is despite
the potential loss of some of the beginning preamble bits
because of slow synchronization. Because of this forced
reintroduction of timing bits, some minor reduction of the
interframe gap is not only possible but expected. Some Ethernet
chipsets are sensitive to a shortening of the interframe
spacing, and will begin failing to see frames as the gap is
reduced. With the increase in processing power at the desktop,
it would be very easy for a personal computer to saturate an
Ethernet segment with traffic and to begin transmitting again
before the interframe spacing delay time is satisfied. After a
collision occurs and all stations allow the cable to become
idle (each waits the full interframe spacing), then the
stations that collided must wait an additional and potentially
progressively longer period of time before attempting to
retransmit the collided frame. The waiting period is
intentionally designed to be random so that two stations do not
delay for the same amount of time before retransmitting, which
would result in more collisions. This is accomplished in part
by expanding the interval from which the random retransmission
time is selected on each retransmission attempt. The waiting
period is measured in increments of the parameter slot time. If
the MAC layer is unable to send the frame after sixteen
attempts, it gives up and generates an error to the network
layer. Such an occurrence is fairly rare and would happen only
under extremely heavy network loads, or when a physical problem
exists on the network. Web Links Backoff
http://www.usyd.edu.au/is/comms/ networkcourse/USydNet_mod3_
ethernet.html#tocBackoff
Content 6.2
Ethernet Operation 6.2.5 Error handling The
most common error condition on an Ethernet is the collision.
Collisions are the mechanism for resolving contention for
network access. A few collisions provide a smooth, simple, low
overhead way for network nodes to arbitrate contention for the
network resource. When network contention becomes too great,
collisions can become a significant impediment to useful
network operation. Collisions result in network bandwidth loss
that is equal to the initial transmission and the collision jam
signal. This is consumption delay and affects all network nodes
possibly causing significant reduction in network throughput.
The considerable majority of collisions occur very early in the
frame, often before the SFD. Collisions occurring before the
SFD are usually not reported to the higher layers, as if the
collision did not occur. As soon as a collision is detected,
the sending stations transmit a 32-bit “jam” signal that will
enforce the collision. This is done so that any data being
transmitted is thoroughly corrupted and all stations have a
chance to detect the collision. In Figure two stations listen
to ensure that the cable is idle, then transmit. Station 1 was
able to transmit a significant percentage of the frame before
the signal even reached the last cable segment. Station 2 had
not received the first bit of the transmission prior to
beginning its own transmission and was only able to send
several bits before the NIC sensed the collision. Station 2
immediately truncated the current transmission, substituted the
32-bit jam signal and ceased all transmissions. During the
collision and jam event that Station 2 was experiencing, the
collision fragments were working their way back through the
repeated collision domain toward Station 1. Station 2
completed transmission of the 32-bit jam signal and became
silent before the collision propagated back to Station 1 which
was still unaware of the collision and continued to transmit.
When the collision fragments finally reached Station 1, it also
truncated the current transmission and substituted a 32-bit jam
signal in place of the remainder of the frame it was
transmitting. Upon sending the 32-bit jam signal Station 1
ceased all transmissions. A jam signal may be composed of any
binary data so long as it does not form a proper checksum for
the portion of the frame already transmitted. The most commonly
observed data pattern for a jam signal is simply a repeating
one, zero, one, zero pattern, the same as Preamble. When viewed
by a protocol analyzer this pattern appears as either a
repeating hexadecimal 5 or A sequence. The corrupted, partially
transmitted messages are often referred to as collision
fragments or runts. Normal collisions are less than 64 octets
in length and therefore fail both the minimum length test and
the FCS checksum test. Web Links Error Handling
http://www.usyd.edu.au/is/comms/networkcourse/ USydNet_mod3_
ethernet.html#tocDuplex
Content 6.2
Ethernet Operation 6.2.6 Types of
collisions Collisions typically take place when two or more
Ethernet stations transmit simultaneously within a collision
domain. A single collision is a collision that was detected
while trying to transmit a frame, but on the next attempt the
frame was transmitted successfully. Multiple collisions
indicate that the same frame collided repeatedly before being
successfully transmitted. The results of collisions, collision
fragments, are partial or corrupted frames that are less than
64 octets and have an invalid FCS. Three types of collisions
are: To
create a local collision on coax cable (10BASE2 and 10BASE5),
the signal travels down the cable until it encounters a signal
from the other station. The waveforms then overlap, canceling
some parts of the signal out and reinforcing or doubling other
parts. The doubling of the signal pushes the voltage level of
the signal beyond the allowed maximum. This over-voltage
condition is then sensed by all of the stations on the local
cable segment as a collision. In the beginning the waveform in
Figure represents normal Manchester encoded data. A few cycles
into the sample the amplitude of the wave doubles. That is the
beginning of the collision, where the two waveforms are
overlapping. Just prior to the end of the sample the amplitude
returns to normal. This happens when the first station to
detect the collision quits transmitting, and the jam signal
from the second colliding station is still observed. On UTP
cable, such as 10BASE-T, 100BASE-TX and 1000BASE-T, a collision
is detected on the local segment only when a station detects a
signal on the RX pair at the same time it is sending on the TX
pair. Since the two signals are on different pairs there is no
characteristic change in the signal. Collisions are only
recognized on UTP when the station is operating in half duplex.
The only functional difference between half and full duplex
operation in this regard is whether or not the transmit and
receive pairs are permitted to be used simultaneously. If the
station is not engaged in transmitting it cannot detect a local
collision. Conversely, a cable fault such as excessive
crosstalk can cause a station to perceive its own transmission
as a local collision. The characteristics of a remote collision
are a frame that is less than the minimum length, has an
invalid FCS checksum, but does not exhibit the local collision
symptom of over-voltage or simultaneous RX/TX activity. This
sort of collision usually results from collisions occurring on
the far side of a repeated connection. A repeater will not
forward an over-voltage state, and cannot cause a station to
have both the TX and RX pairs active at the same time. The