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Ethernet Testing Parameters


Testing Ethernet Services

The Ethernet connections must be tested to ensure
that they are operating correctly and also they are
performing to the required levels.This is done by testing
 the bandwidth, the delay and the loss of frames in the
 connection. In Ethernet terms these are called
Throughput, Latency and Frame Loss.

Throughput

Data throughput is simply the maximum amount of data,
that can be transported from source to destination.
However the definition and measuring of throughput is
complicated by the need to define an acceptable level of
quality. For example, if 10% errored or lost frames were
deemed to be acceptable then the throughput would be
measured at 10% error rate. Here we have
generally accepted definition that throughput should be
measured with zero errors or lost frames.

In any given Ethernet system the absolute maximum
throughput will be equal to the data rate, e.g. 10 Mbit/s
100 Mbit/s or 1000 Mbit/s. In practice these figures
cannot be achieved because of the effect of frame size.
The smaller size frames have a lower effective
throughput than the larger sizes because of the addition
of the pre-amble and the interpacket gap bytes, which do
not count as data.

Latency
Latency is the total time taken for a frame to travel from
source to destination. This total time is the sum of both
the processing delays in the network elements and the
propagation delay along the transmission medium.
In order to measure latency a test frame containing a
time stamp is transmitted through the network. The time
stamp is then checked when the frame is received. In
order for this to happen the test frame needs to return to
the original test set by means of a loopback (round-trip
delay).

Frame Loss
Frame loss is simply the number of frames that were
transmitted successfully from the source but were never
received at the destination. It is usually referred to as
frame loss rate and is expressed as a percentage of the
total frames transmitted. For example if 1000 frames
were transmitted but only 900 were received the frame
loss rate would be: (1000 – 900) / 1000 x 100% = 10%
Frames can be lost, or dropped, for a number of reasons
including errors, over-subscription and excessive delay.

Errors - most layer 2 devices will drop a frame with an
incorrect FCS. This means that a single bit error in
transmission will result in the entire frame being
dropped. For this reason BER, the most fundamental
measure of a SONET/SDH service, has no meaning in
Ethernet since the ratio of good to errored bits cannot be
ascertained.

Oversubscription - the most common reason for frame
loss is oversubscription of the available bandwidth. For
example, if two 1000 Mbit/s Ethernet services are
mapped into a single 622 Mbit/s SONET/SDH pipe (a
common scenario) then the bandwidth limit is quickly
reached as the two gigabit Ethernet services are loaded.
When the limit is reached, frames may be dropped.

Excessive Delay - The nature of Ethernet networks
means that it is possible for frames to be delayed for
considerable periods of time. This is important when
testing as the tester is “waiting” for all of the transmitted
frames to be received and counted. At some point the
tester has to decide that a transmitted frame will not be
received and count the frame as lost. The most common
time period used to make this decision is the RFC
specification of two seconds. Thus any frame received
more then two seconds after it is transmitted would be
counted as lost.






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