The distribution network is the part of a cable television system that connects the head-end of the system (video and media sources) to the customer's equipment. Traditionally, the local connection has been composed of a coaxial cable that allows for the one-way transmission of a maximum of one hundred and twenty 6 MHz RF television signals or approximately one hundred 8 MHz RF channels.

Core Network

The core network is the central network portion of a communication system. The core network primarily provides interconnection and transfer between edge networks. Core networks in CATV systems are commonly setup as fiber rings and spurs. A fiber ring is an optical network of network topology with a connection that provides a complete 

Figure 1.16 shows a typical cable distribution system that uses a combination of fiber optic cable for the core distribution and coaxial cable for the local connection. This diagram shows that the multiple RF television channels at the head-end of the cable television system are shifted in frequency to allow distribution through high-speed fiber cable. The fiber cable is connected in a loop around the cable television service area so that if a break in the cable occurs, the signal will automatically be available from the other part of the loop. The loop is connected (tapped) at regular points by a fiber hub that can distribute the optical signals on fiber spurs. The fiber spurs end into fiber nodes that convert the optical signals into RF television signals that are distributed on the local coaxial cable network. 

Access Network

Access network is a portion of a communication network that allows individual subscribers or devices to connect to the core network. For CATV systems, the end user device (e.g. set top box) communicates to the system using RF channels. There are several types of RF channels that may be used in CATV systems and these include analog channels, digital channels, out of band control channels and data channels.

Analog RF channels transfer television programming in analog form (e.g. NTSC or PAL). These signals may be available for anyone to use or they may be scrambled so the receiver must decode (descramble) the analog program. 

Digital channels transfer television programming in digital form (e.g. MPEG). The programs transferred by digital channels are usually compressed so each digital RF channel carries multiple television programs. Digital channels may be available for anyone to use or they may be encrypted so the receiver must decode (decrypt) the digital program.

Cable systems send (and optionally receive) control information to the devices such as channel identification information and programming guides. The control messages may be sent on the television channel that is displaying the program (In band) or on separate control channels (out of band). When the data is sent in band, the control data shares the bandwidth with the television programming. When the data is sent on out of band channels, it is sent on other RF channels. 

The channel information is typically repeated in carousel form so that receivers can capture and store the information when it becomes available. If the receiver cannot obtain the entire data block of information, 

Data RF channels are designed to efficiently transfer user data (such as Internet data) between users and the cable system. Data RF channels can use a very efficient form of modulation (such as QAM) from the system to the end user allowing the cable system to provide high speed data from the system to the receiver (up to 30 Mbps to 40 Mbps) per RF channel. Because signals from multiple users are combined when they are sending data to the cable system, this increases the amount of noise level so a more robust (less efficient) modulation form is used (such as QPSK), which can provide medium speed data from the users to the system (up to 2 to 5 Mbps) per RF channel. Data channels are defined in the DOCSIS specification available from CableLabs.

Because data channels can transfer information much faster than the older out of band (OOB) channels, when data channels are available in a system, the data channel can also send the system information so the out of band channels do not need to be used. However, systems that upgrade to data channels may continue to use the OOB channels as the existing customer equipment may not have data channel capability.

User devices (such as STBs) may only have the capability to receive one type of RF channel at a time (single tuner) or it may be able to simultaneously receive multiple channels at the same time (multiple tuners). For cable receivers that only have one tuner, the user may be interrupted when data (e.g. program guides) is gathered. For cable receivers that have multiple tuners, the control information can be received and sent without interrupting the viewer's program display.

Figure 1.17 shows how the access portion of a CATV uses RF channels to communicate with a set top box. This example shows a CATV network that has a mix of analog TV, digital TV, control channels and data communication channels. System information may be repeatedly sent using in band, out of band or on data channels. This example shows that the CATV system may communicate with older set top boxes (STBs) that only have one tuner or it may be communicating with set top boxes that have multiple tuners. The STBs with multiple tuners can simultaneously receive programs and system information that allows the viewer to continue to watch their programs without interruption.

Premises Distribution

A premises distribution network is the equipment and software that is used to transfer data and other media in a customer's facility, home or personal area. A PDN is used to connect terminals (computers) to other networks and to wide area network connections. Some of the common