A method of modelling the composition of cable segments (C1, C2, C3) in a network, said network comprising a plurality of circuits (L1, L2) each connected to a respective customer premises (106, 108), each circuit comprising at least one cable segment and at least one cable segment is shared between a plurality of circuits, and wherein cable composition comprises length, gauge and material of the cable, said method comprising: a) determining a signal loss measurement for each circuit; b) setting a circuit loss probability distribution for each circuit in dependence on the signal loss measurement for the respective circuit; c) setting a signal loss distribution for each cable segment to a respective initial signal loss distribution; for a selected cable segment: d) estimating an observed signal loss of the selected cable segment by subtracting, from the circuit loss probability distribution of a circuit comprising the selected cable segment, the signal loss distributions of any other cable segments in said circuit; e) repeating step d) for other circuits comprising the selected cable segment to estimate further observed signal losses; f) determining an updated signal loss distribution for the selected cable segment in dependence on the observed signal loss estimates from steps d) and e), and setting the signal loss distribution for the selected cable segment to the updated signal loss distribution; g) repeating step d) to f) for other selected cable segments; h) mapping the signal loss distribution for each selected cable segment onto respective cable composition probabilities by comparing the observed loss distribution to each of a plurality of cable composition distributions and determining the probability of any given cable composition distribution contributing to the observed loss distribution; and i) using the cable composition probabilities to determine the speed of a broadband service that can be received over each circuit; characterised in that: each respective initial signal loss distribution is set to a proportion of the circuit loss probability distribution based on a weighted average of the circuit loss probability distribution. a) determining a signal loss measurement for each circuit; b) setting a circuit loss probability distribution for each circuit in dependence on the signal loss measurement for the respective circuit; c) setting a signal loss distribution for each cable segment to a respective initial signal loss distribution; d) estimating an observed signal loss of the selected cable segment by subtracting, from the circuit loss probability distribution of a circuit comprising the selected cable segment, the signal loss distributions of any other cable segments in said circuit; e) repeating step d) for other circuits comprising the selected cable segment to estimate further observed signal losses; f) determining an updated signal loss distribution for the selected cable segment in dependence on the observed signal loss estimates from steps d) and e), and setting the signal loss distribution for the selected cable segment to the updated signal loss distribution; g) repeating step d) to f) for other selected cable segments; h) mapping the signal loss distribution for each selected cable segment onto respective cable composition probabilities by comparing the observed loss distribution to each of a plurality of cable composition distributions and determining the probability of any given cable composition distribution contributing to the observed loss distribution; and i) using the cable composition probabilities to determine the speed of a broadband service that can be received over each circuit; characterised in that: each respective initial signal loss distribution is set to a proportion of the circuit loss probability distribution based on a weighted average of the circuit loss probability distribution.