Digital communications
Digital communications

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Digital communications

4.4.2 Passive optical networks and Ethernet in the first mile

If FTTH uses a dedicated fibre link between each house and the telephone exchange then each house carries the cost of two sets of terminal equipment: one in the house and one in the exchange.

A significant saving can be made with the passive optical network (PON) configuration (Figure 30). Here, as with FTTCab, a single fibre runs from the exchange to a hub, but now the splitting is done in the fibre (with a splitter like that described in Section 3.1) and the drop to each house is over fibre. With this configuration there is an optical terminal in each house, but the terminal at the exchange is shared between several houses. It is possible, furthermore, to have transceivers - combined transmitters and receivers – on the fibre and use one fibre for both directions of transmission.

Figure 30
Figure 30 A passive optical network

In the downstream direction – from the exchange to the home – multiplexed signals for all users are broadcast to all users, and the equipment in the home has to extract its ‘own’ signal. In the upstream direction there needs to be some mechanism for ensuring that the signals from each user do not interfere with each other, and the equipment at the exchange has to be able to extract and identify signals from each user. These are significant problems; trials of PONs have been run and the concept found to be technically feasible, but as with other FTTH experiments the economics are still in question.

One specific use of the PON concept is the Ethernet passive optical network (EPON) in the Ethernet in the first mile (EFM) standard being developed by the 802.3ah EFM Task Force of the IEEE 802 LAN/MAN standards committee.

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