PDF of this tutorialThere are three significant categories of combined cable and fiber systems used for Internet access. According to Figure 7, they are as follows:
Figure 7. Fiber and Coax
Cable TV Hybrid Fiber/Coax (HFC) System (Top)
Traditional systems have only downstream broadcast capability. These traditional cable–TV systems broadcast downstream in the 50- to 550/750-MHz band with 6-MHz channels.
Cable modems are used to allow Internet and data transmission in the downstream direction of the HFC system. Internet data speeds up to the 30–Mbps range can be realized in a nominal 6-MHz video channel. The upstream signal is provided by an existing telephone channel using VBD or ISDN.
Bidirectional HFC system (Middle)
These newer systems have transmission capability in both directions. Such bidirectional cable–TV systems typically broadcast downstream in the 50- to 750-MHz bandwidth of the coax within the 6-MHz nominal video channels. The upstream bandwidth is shared among all the homes passed by the coax cable and is nominally limited to the 5- to 40-MHz frequency band.
Downstream Internet data speeds up to the 30–Mbps range in 6-MHz channels can be realized. Upstream data is contention based and operates at claimed speeds of up to 10 Mbps. In practical multiuser environments, however, actual throughput speeds will be significantly less.
Cable modems can either be overlaid onto the HFC system or be an integrated part of the HFC system.
Switched Digital Broadband (SDB) Systems (Bottom)
SDB is classified as a baseband digital system with nominal 50–Mbps point-to-point downstream rates that can be apportioned as desired between digital video and data. For data, a 1.5–Mbps nominal, contention-based, upstream data bandwidth is available. Though the system is contention based, there is always a minimum guaranteed upstream data rate available—typically in the order of 16 kbps.
The three architectures described all have provisions for both analog and digital video broadcast capability.
Both the bidirectional HFC and SDB systems are broadband systems that are applicable to telephony, video, Internet/data, and PCS wireline access. Note that the architectures have a number of similar characteristics and components. The bidirectional HFC system provides fiber distribution to the fiber node. At the fiber node, signals are collected and distributed to multiple-coax feeds that cover a given residential area. Fiber nodes are designed to serve from 500 to 2000 homes.
SDB systems push fiber closer to the end-user. In typical systems, feeder fiber can be optically split. Optical network units (ONUs) terminate the fiber and provide individual coax (and twisted-pair) drops to subscribers. A typical ONU can serve from 4 to 60 homes. Thus, SDB provides fiber closer to the customer.
In many ways, HFC, SDB, and PON (discussed shortly) can be viewed as a continuum of technology where fiber moves ever closer to the customer premises.
