Wired communication refers to the transmission of data over a cable. Some examples include telephone networks, cable television or broadband internet

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2 Wired communication refers to the transmission of data over a cable. Some examples include telephone networks, cable television or broadband internet access. The most common used wired media are pairs of copper wires (that are usually twisted and can be shielded or unshielded), coaxial cables and fiberoptic cables, but you can also find less common devices such as waveguides, used for high-power and high frequency applications.

3 Copper pairs have been used for telephone communications for more than 100 years. They are still used as the last mile access technology for most homes and small business, as they are part of a huge infrastructure that it is very expensive to replace. The design is simple and robust: one or several copper pairs connect a home with a street node, where they are connected to a multi-pair cable, usually with 50 to 500 copper pairs. This cable is connected in another node to a bigger cable that goes to a local exchange office of the public switched telephone network. Some of these cables have 2400 copper pairs. These networks were developed for narrow band voice communications and were first used for data transmission. They did this by employing low bandwidth modems that adapted digital signal to the voice channel, but in the last 20 years a family of digital standards called DSL (for Digital Subscriber Line) have kept growing the bandwidth that can be obtained from a copper wire, using very clever modulation techniques that squeeze the transmission capacity of wires, far beyond of what was considered possible some years ago. These technologies can send data in parallel with traditional voice calls, as they use a different part of the frequency spectrum (that is why you have to use a filter for regular telephones if you have a DSL Internet access).

4 The most common DSL technology is ADSL (for Asymmetric Digital Subscriber Line), that offers different speeds to upload and download data (as the common user will normally be more interested in high download speed than in upload speed), the fastest versions of ADSL are called VDSL. There are also symmetric DSL technologies as SDSL or HDSL. The bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction of the customer ( called downstream), depending on DSL technology, distance from node, line conditions, and service-level implementation. Bit rates of 1 Gbit/s have been reached in trials, but if you want to install it at your home, the usual offer you will find will be between 10 and 100 Mbits/sg, depending on the area where you live and how far your home is from the node (and in some rural cases you will only have access to even lower bandwidths). To end with telephone networks, we will mention Integrated Services for Digital Networks. ISDN was created in the late eighties as the future technology for the digitalisation of telephone networks opening them to digital services, but its low bandwidth (basic access 2 64 kb/sg channels to the end user) compared with DSL, has relegated it as a standard used to connect PBXs to the switched telephone network with extended services.

5 A coaxial cable is a type of cable with a central conductor surrounded by a tubular insulating layer that is covered by a cylindrical conducting shield. Over the shield it has a protecting plastic layer usually called plastic jacket. It is called coaxial because all layers share the same axis. The central conductor and the shield are typically made of copper or aluminium. Its configuration protects the signal from external electromagnetic interference and guarantees a good bandwidth. That is why they are used as transmission lines for radio frequency signals. You can find them in feed-lines connecting radio transmitters and receivers with their antennas, in computer network connections, in analog and digital baseband video distribution, in digital audio (S/PDIF), and in the distribution of analog and digital radio-frequency cable television signals. Most cable-tv network operators have migrated their network to digital and are using them to offer broadband Internet access to their subscribers using specifically designed cable-modems.

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