| A visit to a Telstra Exchange | by DonQuay |
Because this excursion was as part of the Data Communications module for TAFE, we went to the exchange on 9.15am Tuesday 21st September 1999. On the way there, the facilitator (teacher) noticed a trench dug in the ground for some ETSA cables which were to be supplied through plastic pipes called conduits. There were ropes in conduits which are used to pull the cables through when they need to be placed. The trench which was dug seemed to be something which would be used in the Modbury Hospital.
The Modbury Exchange was built back in the 1960's, when the telephone cables were still very thick, and since then further work has been underway to switch up to the newer technology of fiber optic cables for increased bandwidth, and smaller cabling sizes.
Beneath the exchange is a large passageway, called the cable chamber, and there is a hatch which must be opened from inside the exchange in order to enter. This passageway requires a control unit to detect gas within the chamber, for any danger. The control unit has three lights on it indicating 'Warning Gas Present', 'Fan Running' & 'Fan Overload'. There are two switches which determine if the control unit is on, and if the operation of the unit is manual, or automatic. There is also a 'Fan Lock Out' lock which can be used as an override.
The passageway, has a small platform, and further down the corridor is a slope where any water which has entered the chamber will continue down, until it reaches a sump pump, where it may be pumped out. The first thing which is noticed about the cable chamber are the cables which are thick, and sticking out from the walls. The cables at the bottom are Copper wire twisted pair combinations of many small cables from the outlying areas. They have Lead Sheath on the outside. Later cables are similar, but consist of Nylon sheath over the lead. The fiber optic cables are much smaller than the copper cables and are covered by blue insulating material. At the exchange I visited, there were approximately 13 fibre optic cables for more than 30 large copper cables. There were holes which I presume cables came through before which were filled in. This is most likely because of the addition of the fiber optic cables to our phone system.
There can be problems observed with cables in the field, such that people may cut through cables, partly, or wholly in executions of other construction. It is then that a special facility for the copper cables is used. Air is blown through the cable to detect any tears in the cable. Every 3 months, or so, someone comes down into the chamber, and checks that the pressure in the cable is at its peak, or otherwise, it may be injured, and need replacement or mending. If however one of the wire pairs inside the cable has been cut, alarms will immediately be sounded in the exchange (possibly something less dramatic). Salts leaking into a wounded cable may form an electrolyte along with the two different types of metal constituting the cable, making a type of battery. This is undesireable, and may be detected by sending a noble gas through the cable, and have a employee check for this gas with a meter.
The cables were brought up to the roof, and split into their smaller cables, to be connected to the equipment upstairs. The cables have a ground which is connected by a large piece of copper metal connected to the wall, and into the ground in a mesh pattern. The ground should not have more than 0.5 ohms resistance. The Optic Fiber cables are not allowed to have too much bend in them, lest the glass inside them bend or shatter. Extra slack cable is wound in large circles, and the climb to the ceiling is gradual, and the cable is supported in many places. Each optic cable has between 36 and 68 optic fibers which are used in pairs to send the transmissions. Analogue signals through the copper network uses the frequencies 300Hz -> 3.4kHz, and one conversation will use about 64kbps of a digital signal. The copper wires can support 30 conversations for their bandwidth which is approximately 2Mbps. Optic fiber has a bandwidth of 565Mbps, which allows it about 265,000 conversations of 64kbps on a 60 fiber cable (30 fiber pairs).
There are two networks of optic fibers around Australia, and 2 optic fiber cables from Adelaide to Melbourne in case one is broken, so that it won't bring the network down. This cable is 2 1/2 meters deep, and is difficult to detect a break in it. They check the cable with a optical reflector check, and there is also a thin metal wire inside the cable which may be detected by a good metal detector.
Upstairs is the MDF (main distribution frame). A series of racks which contain wire to wire connections from the cables below. The early connections are soldered onto the boards, and a connector to the other side of the board is able to be disconnected if need be. Red and white wire pairs to these connectors signify normal customers, and green and white wire pairs signify special customers, such as ATMs and burgular alarms. A rack which was set up to read HPSD 03 had thicker wires on its connection. This was in fact a connection to another interchange, the Hampstead interchange. Newer connectors to racks are IDC, insulation displacement connectors which connect directly through the insulation, by the metal contactors cutting into the wire. There are fewer of these than there are soldered connections. A ladder which is connected to the roof contains all the tools needed for maintainance of all the connectors on the distribution frame, such as a soldering iron, IDC tools, and wire strippers & cutters. There are a few testing points on the grid which are indicated by plastic separators.
There is a small box (in comparison with the large grid) for all the optic fiber connections. The optic fiber is so small that it must be connected to this machine with a computer melting the fiber onto its input. A large part of the exchange used to be filled with a machine called CROSSBAR. This provided switching for the alternating current system. This had silver contacts, and required much maintenaince. The machines which are now there are the ones which convert analogue signal into digital, called System 12. These are made by a French company called Alcatel. Each board in the system contains 16 codecs for conversion. There is also an earlier digital system made by Ericcson, called AXE. All this digital equipment requires conversion from DC to AC, and from DC to DC, so that the digital circuits can operate on 5V, and the bell ring for the telephone lines is 90VAC 25Hz.
Telephones work on 50V (approximately 48V), which is produced by ETSA, but maintained by the local exchange through large batteries, and extra equipment. There are two series of batteries which are used to power the phone system in the event of a power failure. 24 big blue Exide Faure's provide at least 50V for the system, and they are constantly recharged. There is perhaps an hours worth of life in the battery, so after that, a diesel generator is switched in to provide the power for the system. The diesel generator is a Turbocharged 6 cylinder engine with a very large muffler to create less noise in its residential surroundings. It provides 325kWatt Amps, and is powered by a large store of diesel fuel underground and a small supply nearby. The starter motor on it requires 24V, and two lots of 4 series batteries connected in parallel.
It was calculated by all the equipment in the exchange that it required 665 Amps to work all the equipment in the exchange. Some more equipment that we located was some modems in a rack which were used for EFTPOS, and the like machines, some multiplexers to split the digital signals into their individual conversations, the cable TV connections, and some ISDN line devices.
This visit to the exchange showed me how everyone is connected to the large network of telecommunications in our system. Wires and fiber optics are connected to the outside world where they will be diverted to the customers, and their telecommunications systems. The phone system is large and complicated and has been evolving since someone invented the first wire. Telstra provides a working model of a very effective communications system.