, which forward the downstream packets to all subscribers within their networks. A CO is connected to a subscriber's premises optical network terminal Broadband Telecommunications Technologies and Management (ONT) either directly by an optical fibre through a splitter, or via a DSL/coaxial cable. The upstream signals are communicated in a TDM fashion through scheduling by the ONT, which allocates a time window to each ONU. An ONU acts as a DSLAM, buffering and aggregating the traffic from subscriber premises. Fibre-to-the-x As noted above, DSL technologies are effective only for distances of up to 4 km due to copper medium characteristics. One way to increase the range of DSL coverage is by incorporating optical fibre into the DSL system. In these configurations, a CO is connected to a remote DSLAM using optical fibre lines in a node known as a remote terminal (RT). A number of configurations exist as shown in Figure 9.23. The difference between all these configurations is essentially how close the fibre comes to the end-user premises and remote terminal. The two most common configurations are fibre to the node (FTTN), where the DSLAM is located at 'node' in a neighbourhood with a radius of typically 1 km. The other common configuration is fibre to the building (FTTB), where fibre is brought to an apartment building. A DSLAM is then placed at typically the telephone box at the basement of the building from where all individual apartments connect to the DSLAM with copper. Both FTTN and FTTB configura- tion facilitate the roll-out of VDSL, and provision of data rates in the order of several tens of megabits per second. Optical fibres Metallic cables >1000ft. (300 m) <1000ft. (300 m) Figure 9.23 Fibre-to-the-x configurations. FTTN, fibre to the node; FTTC, fibre to the cabinet; FTTB, fibre to the building; FTTH, fibre to the home [18]. Source: Rich (own work) [GFDL (http://www.gnu. org/copyleft/fdl.html) or CC-BY-SA-3.0 (https://commons.wikimedia.org/wiki/File%3AFTTX.svg)] Fixed Broadband Communications Systems Copp