Ethernet
Frequently Asked Questions:
Hardware
Basics
- What is a driver?
- What is SQE? What is it for?
- What is an SQE Test?
- What do they mean by means "IPG"?
- What is a runt?
- What causes a runt?
- What is jabber?
- What causes jabber?
- What is a collision?
- What causes a collision?
- How many collisions are too many?
- How do I reduce the number of collisions?
- What is a late collision?
- What is a jam?
- What is a broadcast storm?
- How do I recognize a broadcast storm?
- How can I prevent a broadcast storm?
- What is an Alignment Error?
- What is high traffic on an Ethernet?
- How can I test an Ethernet?
Q:
What is a driver?
A: Typically the software that allows an
Ethernet card in a computer to decode packets and send them
to the operating system and encode data from the operating
system for transmission by the Ethernet card through the network.
By handling the nitty-gritty hardware interface chores, it
provides a device-independent interface to the upper layer
protocols, thereby making them more universal and [allegedly]
easier to develop and use.
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Q: What is SQE? What
is it for?
A: SQE is the IEEE term for a collision.
(Signal Quality Error)
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Q: What is an SQE Test?
A: SQE Test (a.k.a. heartbeat) is a means
of detecting a transceiver’s inability to detect collisions.
Without SQE Test, it is not possible to determine if your
collision detector is operating properly. SQE Test is implemented
by generating a test signal on the collision pair from the
transceiver (or its equivalent) following every transmission
on the network. It does not generate any signal on the common
medium. The problem with SQE Test is that it is not part of
the Ethernet Version 1.0 specification. Therefore, Version
1.0 equipment may not function with transceiver that generates
the SQE Test signal. Additionally, IEEE 802.3 specifications
state that IEEE 802.3 compliant repeaters must not be attached
to transceivers that generate heartbeat. (This has to do with
a jam signal that prevents redundant collisions from occurring
on the network). Therefore, you must usually turn-off SQE
Test (heartbeat) between the transceiver and an 802.3 repeater.
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Q: What do they mean
by means "IPG"?
A: The InterPacket Gap (more properly referred
to as the InterFrame Gap or IFG) is an enforced quiet time
of 9.6 us between transmitted Ethernet frames.
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Q: What is a runt?
A: A packet that is below the minimum size
for a given protocol. With Ethernet, a runt is a frame shorter
than the minimum legal length of 60 bytes (at Data Link).
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Q: What causes a runt?
A: Runt packets are most likely the result
of a collision, a faulty device on the network, or software
gone awry.
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Q: What is jabber?
A: A blanket term for a device that is behaving
improperly in terms of electrical signaling on a network.
In Ethernet this is Very bad, because Ethernet uses electrical
signal levels to determine whether the network is available
for transmission. A jabbering device can cause the entire
network to halt because all other devices think it is busy.
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Q: What causes jabber?
A: Typically a bad network interface card
in a machine on the network. In bizarre circumstances outside
interference might cause it. These are very hard problems
to trace with layman tools.
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Q: What is a collision?
A: A condition where two devices detect
that the network is idle and end up trying to send packets
at exactly the same time. (Within 1 round-trip delay) Since
only one device can transmit at a time, both devices must
back off and attempt to retransmit again. The retransmission
algorithm requires each device to wait a random amount of
time, so the two are very likely to retry at different times,
and thus the second one will sense that the network is busy
and wait until the packet is finished. If the two devices
retry at the same time (or almost the same time) they will
collide again, and the process repeats until either the packet
finally makes it onto the network without collisions, or 16
consecutive collision occur and the packet is aborted.
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Q: What causes a collision?
A: See above. Ethernet is a CSMA/CD (Carrier
Sense Multiple Access/ Collision Detect) system. It is possible
to not sense carrier from a previous device and attempt to
transmit anyway, or to have two devices attempt to transmit
at the same time; in either case collision results. Ethernet
is particularly susceptible to performance loss from such
problems when people ignore the "rules" for wiring
Ethernet.
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Q: How many collisions
are too many?
A: This depends on your application and
protocol. In many cases, collision rates of 50% will not cause
a large decrease in perceived throughput. If your network
is slowing down and you notice the percentage of collisions
is on the high side, you may want try segmenting your network
with either a bridge or router to see if performance improves.
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Q: How do I reduce the
number of collisions?
A: Disconnect devices from the network.
Seriously, you need to cut- down on the number of devices
on the network segment to affect the collision rate. This
is usually accomplished by splitting the segment into two
pieces and putting a bridge or router in between them.
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Q: What is a late collision?
A: A late collision occurs when two devices
transmit at the same time, but due to cabling errors (most
commonly, excessive network segment length or repeaters between
devices) neither detects a collision. The reason this happens
is because the time to propagate the signal from one end of
the network to another is longer than the time to put the
entire packet on the network, so the two devices that cause
the late collision never see that the other’s sending until
after it puts the entire packet on the network. The transmitter
after the first "slot time" of 64 byte times detects
late collisions. They are only detected during transmissions
of packets longer than 64 bytes. Its detection is exactly
the same as for a normal collision; it just happens "too
late." Typical causes of late collisions are segment
cable lengths in excess of the maximum permitted for the cable
type, faulty connectors or improper cabling, excessive numbers
of repeaters between network devices, and defective Ethernet
transceivers or controllers. Another bad thing about late
collisions is that they occur for small packets also, but
cannot be detected by the transmitter. A network suffering
a measurable rate of late collisions (on large packets) is
also suffering lost small packets. The higher protocols do
not cope well with such losses. Well, they cope, but at much
reduced speed. A 1% packet loss is enough to reduce the speed
of NFS by 90% with the default retransmission timers. That’s
a 10X amplification of the problem. Finally, Ethernet controllers
do not retransmit packets lost to late collisions.
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Q: What is a jam?
A: When a workstation receives a collision,
and it is transmitting, it puts out a jam so all other stations
will see the collision also. When a repeater detects a collision
on one port, it puts out a jam on all other ports, causing
a collision to occur on those lines that are transmitting,
and causing any non-transmitting stations to wait to transmit.
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Q: What is a broadcast
storm?
A: An overloaded term that describes an
overloaded protocol. :-). Basically it describes a condition
where devices on the network are generating traffic that by
its nature causes the generation of even more traffic. The
inevitable result is a huge degradation of performance or
complete loss of the network as the devices continue to generate
more and more traffic. This can be related to the physical
transmission or to very high level protocols.
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Q: How do I recognize
a broadcast storm?
A: That depends on what level it is occurring.
Basically you have to be aware of the potential for it beforehand
and be looking for it, because in a true broadcast storm you
will probably be unable to access the network. This can change
dramatically for a higher level protocol. NFS contention can
result in a dramatic DROP in Ethernet traffic, yet no one
will have access to resources.
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Q: How can I prevent
a broadcast storm?
A: Avoid protocols that are prone to it.
Route when it is practical.
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Q: What is an Alignment
Error?
A: A received frame that does not contain
an integer number of octets and contains a frame check sequence
validation error. A frame in which the number of bits received
is not an integer multiple of 8 and has a FCS (Frame Check
Sequence) error. (802.3 standard, page 41)
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Q: What is high traffic
on an Ethernet?
A: High traffic is when things start slowing
down to the point they are no longer acceptable. There is
not set percentage point, in other words. Xerox used to use
a formula based on packet size over time, or something, but
the plethora of protocols available and how they react to
wire usage have significantly muddied the issue. I usually
start paying attention over 40-50%, or when things slow down.
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Q: How can I test an
Ethernet?
A: This depends on what level you want
to test. The most basic test (a.k.a., "the fire test")
is to connect pair of devices to the network and see if they
can communicate with each other. If you want to test the electrical
integrity of the wire (i.e., will it carry a signal properly),
a TDR or cable scanner that incorporates TDR and other functions,
would be the most comprehensive tool (though a great deal cab
be determined with a simple ohmmeter). If you need to test the
performance or troubleshoot protocol transmission problems,
you will need special and usually very expensive software, usually
coupled with custom hardware, to capture, optionally filter,
and analyze the network packets.
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