a few megabits per second. Another problem in the uplink direction is congestion when many users share the same channel. A cable modem may jump to another channel when severe congestion occurs. Uplink congestion can also be solved by assigning time slots at the HE. In this case the cable modem termination at the HE divides uplink channels into TDM slots and assigns those slots to end points that want to send data. As we saw earlier, the cable TV network provides existing media for other services such as data and voice. It is an attractive alternative for high- speed data access and many cable TV operators offer it with better terms than telecommunications operators can provide DSL access. The major difference between the cable modem and DSL offerings is that users of a cable TV net- work share the data capacity and performance depends on the activity of other users. Another major concern is security because every user of a cable modem system may receive data directed to other users in the same fiber feed. 6.4.5 Wireless Access DSL and cable TV access rely on existing cable networks and they are very cost-effective solutions for the operators that own the access network. They are not willing to lease their cable network on reasonable terms to their com- petitors although there is a lot of political pressure to open access network competition. Wireless technology for fixed access provides cost-effective broadband access alternative for new service providers with much lower ini- tial investments. Some operators use WLAN technologies with directional antennas to provide fixed wireless broadband access. In some countries special frequency Introduction to Telecommunications Network Engineering bands are allocated for this application. A basic wireless access system consists of a LAN at the customer premises and a radio relay system connecting the LAN via radio waves to a service provider's router that is connected to Internet. 6.4.6 Fiber Cable Access Access via fiber optic cable is superior in terms of quality and ban