Customers often ask "What is the reliability of Frame Relay versus analog phone lines?". In our experience, the Frame Relay service has been more reliable. The following presents some evidence and some technical information that supports this improved reliability. We also note points of failure where analog circuits and Frame Relay have equivalent failure modes and rates of occurrence.
With analog circuits, the phone company must terminate the lines at the customer sites with the receive levels set to approximately -16 dBm, presuming a transmitting signal of 0 dBm. This attenuation is set in a Data Station Termination Unit. These are small boxes that typically have DIP switches used to set the level at the customer site. Depending on the length of the cable from the central office to the customer, the signal might be just right, have to amplified or have to be attenuated (reduced). Signal levels that are too high or too low at installation time have occasionally been a problem with analog circuits. More often, the Data Station Termination Unit at the customer site has failed. This requires an on-site visit from the phone company, a trip they would rather not make, so the on-site visit is often delayed as long as possible while other failure possibilities are explored.
Another problem with analog circuits is accumulated noise. Each time an analog circuit is amplified, the noise is amplified along with the information. An analog repeater can not differentiate from data. This is similar to dubbing an audio tape. Each generation of an audio tape will have more noise that the previous generation. Analog amplifiers are located on the customer premise in the form of the Data Station Termination Unit and in the central office equipment, including the analog ports on carrier equipment and in analog bridges used for multipoint lines. Noise sources can be difficult to track down. At any point in the circuit path where an analog amplifier exists, additional noise can enter the system.
Digital circuits do not require levels to be set like analog circuits. And there is no Data Station Termination Unit required for a 56 or 64 Kbps digital circuit, for example. Usually, the digital link to the customer site will either work or not work... no in-between conditions. Digital Dataphone System (DDS) circuits do not usually exhibit degraded behavior.
When digital circuits are regenerated at various points along the way from end to end, each regeneration is 100% new. There is no possibility for noise to be added through the regeneration process. If all the data is received correctly as 1s and 0s, it will be reproduced correctly. It is like making a copy of a CD or computer disk, where there is no degradation from one generation to the next.
To be fair in a discussion of analog circuits, they have become much more reliable in recent years. This is because carrier equipment has become nearly 100% digital. It is common today for an analog circuit to be analog only from the customer premise to the nearest central office. A circuit can go from coast to coast of the United States, perhaps 3000 miles, and be digital for all but a few thousand feet of cable from the central office to the customer. If a customer has the phone service coming to the premise over a T1 link, the analog portion of the link could be as short as a few hundred feet or less. When the analog portion of the link is that short, the reliability of analog circuits approaches the reliability of digital circuits.
Frame Relay circuits add another dimension of troubleshooting assistance to digital circuits. On a Frame Relay network, the customer equipment is required to send a Keep Alive data packet to the Frame Relay network (phone company's Frame Relay switch) every 10 seconds. If this Keep Alive does not show up as required, the Frame Relay network equipment sets an alarm. If a customer calls when a circuit fails, the Frame Relay network vendor can go to a console to look at the circuit. Often this console has a visual display that shows the customer equipment in various colors. A customer node that is up might show green, a customer node from which there are no Keep Alive packets may show up in red. The Frame Relay vendor instantly knows there is a problem and at which end of the circuit.
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