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The Broadband Doctor

What is a Bridge Tap?

What is a Bridge Tap?

Bridge Taps on NBN Connections

NBN Fibre to the Node (FTTN), Fibre to the Curb (FTTC) and legacy ADSL technologies use the existing cabling in the street and your premises that was previously installed for landline telephones, to transmit data to and receive data from your modem.

Unfortunately, past wiring practises for landline telephony are not always optimal for data transmission.

A bridge tap (or bridged tap, or star wiring) is an unterminated or unused branch of a telecommunications circuit. When a junction is installed in the telephone circuit to provide an additional outlet somewhere else, a bridge tap is created.

A common scenario is when the original telephone line comes into a house and terminates on a socket in the kitchen. Then some years later someone requires a telephone in the bedroom so a technician may come out and cut the cable in the roof or under the floor and connect another cable to the bedroom using 3-way connectors or simply twisting the wires together. This works fine for telephony but not for data.

Data services require a straight circuit between the DSLAM (or node) and the modem, without any other branches going off elsewhere. Even if there is nothing connected at the unused outlets, this wiring configuration will result in lower achievable speeds and sync dropouts.

The same principle applies to multiple sockets wired in daisy chain (or bus) configuration. If the line from the node goes to the kitchen outlet, then from the kitchen outlet to the living room outlet, and then from the lounge room outlet to the bedroom outlet, the best position for the modem is the bedroom outlet as it is at the end of the circuit and the circuit is straight, without branches. If the modem is at the kitchen or the living room, the cable leaving the socket acts in the same as a bridge tap and will also result in lower achievable speeds and sync dropouts.

When questioned about internet dropouts and slow speeds, a common response from internet service providers is that they have detected a bridge tap in your wiring.

NBN and your internet service provider can remotely detect the presence of a bridge tap by viewing a Hlog graph of your connection. If the graph slopes off evenly, there is no bridge tap. If it is up and down, there is a bridge tap.

The above tests were conducted at a premises that was approximately 780m from the node. The client was on a 50/20 plan, but only achieving 26/13 at the socket. The test at the wallbox outside achieved 54/22. The bridge tap on the internal wiring halved the achievable speed, and was causing sync dropouts as well.

Remediating the premises wiring by locating and removing any bridge taps (or rewiring your premises with a new cable) will result in achieving the maximum speed your line is capable of at the socket and reducing or eliminating dropouts. It also prevents your internet service provider from blaming connection problems on your internal wiring and forces them to address any issues properly with NBN.