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.
