Multimedia ‘cable TV’
Cable TV now offers interactive services, video-on-demand, cheap tele- phone connection and internet access as well as the normal TV channels. For internet connection, a 2-way communication is necessary and this is obtained by allocating a relatively small part of the bandwidth with a bit rate of 500 kbps–1.5 Mbps to what is known as ‘upstream’ data communi- cation from the subscriber to the provider and a larger bandwidth with a bit rate of up to 35 Mbps to what is known as ‘downstream’ data commu-
nication from provider to subscriber. This is the same technique used in the telephone ADSL system described in the next chapter. Two modems are used: a cable modem at the subscriber’s end and cable modem termination system (CMTS) a the provider’s headend.
At the provider’s head-end, the CMTS provides many of the same func- tions provided by a broadband internet connection. The CMTS takes the traffic coming in from a group of subscribers on a single channel and routes it to an Internet Service Provider (ISP) for connection to the Internet (Figure 23.3). The cable providers will also have servers for accounting and logging, Dynamic Host Configuration Protocol (DHCP) for assigning and administering the IP addresses of all the cable system’s users, and control servers. In the narrower upstream, time division multiplexing, TDM is used. The bandwidth is divided into ‘bursts’ of time of few mil- liseconds, in which users can send commands, queries, etc. to the Internet.
At the subscriber’s end, the cable modem which includes a tuner, can be either internal or external to the computer or it can be part of a set-top cable box, requiring that only a keyboard and mouse be added for Internet access.
The two-way communication is also used to provide other services including video-on-demand as will be described in the next chapter.