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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [NUMBER ENTRIES]

E.164

Refer to the Number Entries page for information about the E.164 telephone number format recommendations

E-Model

The E-Model is a quantitative method for measuring the quality of a voice conversation (as with VoIP) created by the International Telephony Union (ITU). Measurements with the E-Model provide an alternative to MOS which measures quality subjectively. The E-Model does not compare the original signal with the received signal directly but, rather, calculates the sum of all measured impairments in the circuit to arrive at the "R" value ("impaiRment" value) for the conversation. These impairments include the codec used, echo, average packet delay, packet delay variation (jitter), and the percentage of dropped packets. See Converting MOS to R-Values for an explanation of how the R-Value and the MOS metric relate to each other.

Earth Curvature

In the context of RF, earth curvature refers to the fact that, as the distance between two antennae increases, the curvature of the earth can cause the earth itself to impinge upon the Fresnel Zone between the two antennae. Imagine a scenario where to antenna are so far apart that their line-of-sight just barely clears the horizon. Even though the antenna have visual line-of-sight, the link might not be viable because the horizon would cause the Fresnel Zone to be blocked. For a given distance between two antenna, there is a certain minimum height at which the antenna must be mounted in order to ensure that the earth's curvature doesn't block the Fresnel Zone.

From a height of 6-feet (the height of a person standing on the sea shore, for example) a departing boat would appear to reach the horizon at a point roughly 3.6 miles from the observer. As you multiply the height by 4 you double the distance to the horizon. Hence, if you were to mount an antenna on a tower that was 24-feet high (four times higher than the person standing on the sea shore) the greatest direct line of sight you could obtain would be roughly 7.2 miles. Refer to our discussion of antenna height for more information and a table of required tower heights for various situations.

EV-DO

"Evolution-Data Only" - The CDMA2000-1x Evolution-Data Only standard that is part of 3G cellular communication. EV-DO builds on CDMA2000-1x to boost data transfer rates from 56Kbps to between 300Kbps and 500Kbps with peak throughput of more than 2Mbps. The EV-DO standard is based on a technology originally called "HDR" (High Data Rate) or HRPD" (High Rate Packet Data) developed by the Qualcomm Corporation. EV-DO carries an international standard number of IS-856. It only deals with the transmission of data (as opposed to voice or voice-and-data) and requires a dedicated frequency band in which to operate. Where EV-DO supports data only, the next evolution (EV-DV - "Evolution Data and Voice") will integrate voice and data on the same frequency band.

Expert System

An expert system is a type of artificial intelligence program that uses pre-configured rules to analyze information about a specific class of problems.  The rules are designed to allow the program to mimic the way that a human expert would analyze the problem in question--hence, the name.  Because they must be programmed with all of the rules required to troubleshoot their problem domain, expert systems are not effective for general problem solving--the rule set would simply be too large.  Expert systems are most effective at solving problems in specific, focused domains, such as "diagnosing gastrointestinal problems based on their symptoms," or "troubleshooting a specific model of car."  In general, the more specific an expert system's domain, the more effective it can be.

Within the realm of protocol analysis, the term, "expert system," has commonly been applied to any protocol analyzer that automatically notifies the user of potential network problems.  In 1994, Network General's Sniffer was the first expert system protocol analyzer to gain widespread usage.  Since then, nearly all commercial protocol analyzers have come to include some level of expert analysis.  As a protocol analyst uses an expert analyzer, it's important to remember that the analyzer is only reporting what it believes to be anomalies on the network.  It's up to the protocol analyst to determine whether those anomalies are truly problems.