Why Selt, Delt & Melt do not work
Learn About > Why Selt, Delt & Melt do not work
Many CSPs have had the misfortune of having been persuaded by their DSLAM vendors to invest in integrated "SELT, DELT and MELT" as a universal solution to customer service measurement.
SELT = Single Ended Line Test
DELT = Double Ended Line Test
MELT = Metallic Line Test
The DSLAM can generate masses of statistics on DSL performance, it can also pull off statisticss from the customer's DSL modem: -at least as long as the customer has not decided to use an incompatible modem! Loads of "useful" information can be derived from this data. This is all very impressive but unfortunately it is only of very limited use in practice in solving real customer service problems, because SELT and DELT only works while there is a DSL connection established.
The customer however only really gets hot and bothered in the converse situation, when his or her DSL connection does not work at all. This will definitely guarantee a call to the CSPs helpline. Under these circumstances SELT and DELT will tell you that it is not possible to establish a DSL connection. It cannot tell you why! The fault may be due to a faulty line card port on the DSLAM, a faulty DSL modem at the customer's end or neither of these, it could be a real line fault!
SELT and DELT enable the CSPs call centre operative to confirm that there is a fault but it cannot identify the source of the fault.
Some DSLAM vendors claim that they can test the copper line using their built in "MELT" functionality. Unfortunately this is another half truth. Yes the DSLAM can carry out AC tests on the line but these tests are of little value as they cannot identify common line faults such as Leg to Ground leakage. All of the really useful line tests require DC coupling of the test circuitry to the line. This is however not possible on the line card because the DSLAM port is AC coupled to the line via a line transformer. - One of our major customers carried out an exercise to validate DSLAM "MELT". They took ten years worth of real line fault data and checked how many of these "real" faults could have been detected using the DSLAM MELT test functionality. The answer was a single digit percentage!
Faced with a "don't know" answer from the integrated SELT DELT and MELT, the unfortunate operative at the CSPs call centre is left with few options. He can only ask the customer to cold start his PC and Modem and check the status of the modem indicator LEDs. These LEDs again only verify that there is no service but cannot identify the cause of the fault. – "Yes we can verify that you have no service Sir" does not do very much for the customer who was complaining about of this fact.
Unable to sort out the problem, the call centre operative escalates the fault to the CSPs "technical helpdesk" who lacking a proper testhead and test access switch will not be able to do much either. The first thing they will do is to further annoy the customer by asking him again to cold start his PC and Modem. – "technical professionals" generally do not trust the actions of the "unskilled" call centre operatives. If this fails they will try to eliminate the line. In the case of an "unbundled" copper pair this will mean asking the incumbent operator to do a proper line test. If the line tests OK, then the CSP will usually be charged for the test. If it tests OK it still leaves the big question open – has the DSLAM line card port failed or is there a problem with the customer's DSL modem. The only way that this can be checked is to send a technician to the DSLAM to manually break the line and to carry out tests with a hand held tester or plug a known good working modem onto the line at the customer's premises. In either case it means expensive truck rolls to resolve the problem.