## Network hierarchy

#### The PSTN provides a good example of transmission hierarchy within a network, where the connections carrying little traffic are served with low capacity links (for example, the twisted pair cable connection to the average home which can support up to 56 kbps with a modem, or 2 x 64 kbps with a special ISDN (Integrated Services Digital Network) line), through to the interconnections between the major cities which often comprise very high speed optical fibre links carrying in excess of 500 Mbps and supporting over 7000 voice calls each. |