h-definition TV delivery process, including the delivery of these services to portable and mobile devices. We revisit terrestrial broadcasting both in terms of physical layer evolution and network topology in Chapter 15. 2.11 Copper Access - ADSL and VDSL Evolution As stated earlier, optical transmission systems have a target to achieve less than one error bit for every trillion bits sent and received (1 in 1012). Copper access has a target to achieve one error bit for every 10 billion bits sent or received (1 in 1010), wireless systems have a target to achieve one error bit for every 1000 bits sent or received (1 in 103. However, it is not just the error rate but the distribution of the error rate that matters. If errors occur in bursts then coding techniques will be needed to mitigate the effect this will have on throughput efficiency, a topic we cover in more detail in the next chapter. Error bursts are bursts that are inherently more likely in unguided media and less likely in guided media though error coding is typically still used in guided media to minimise the impact of error rates. Most voice conversations and data exchanges or web or internet access sessions travel over copper cables at some point in their end-to-end journey including of course mobile to land line voice or data exchanges. In the early years of telecommunications copper cable was just a cable made of copper. In 1880 Oliver Heaviside came up with the idea of coaxial cables. Coaxial cables have an inner conductor surrounded by a flexible tubular insulating layer and an outer shield. The derivation of coaxial refers to the geometric relationship between the inner conductor and outer shield. The advantage of coaxial cabling is that the electromagnetic field carrying the signal is guided through the space between the inner and outer conductors. This means that the cable is resilient to external electromagnetic interference and can be laid next to metal objects, for example gutters and metal conduit. The cable acts as an efficient waveguide at