.. id Fiber and Coax(HFC) networks. Transmission over fiber-optic cable has two main advantages over coaxial cable: A wider range of frequencies can be sent over the fiber, increasing the bandwidth available for transmission; Signals can be transmitted greater distances without amplification. The main disadvantage of fiber is that the optical components required to send and receive data over it are expensive. Because lasers are still too expensive to deploy to each subscriber, network developers have adopted an intermediate Fiber to the Neighborhood (FTTN)approach.
Figure 3.3: Fiber to the Neighborhood (FTTN) architecture Various locations along the existing cable are selected as sites for neighborhood nodes. One or more fiber-optic cables are then run from the head end to each neighborhood node. At the head end, the signal is converted from electrical to optical form and transmitted via laser over the fiber. At the neighborhood node, the signal is received via laser, converted back from optical to electronic form, and transmitted to the subscriber over the neighborhood’s coaxial tree and branch network. FTTN has proved to be an appealing architecture for telephone companies as well as cable operators. Not only Continental Cablevision and Time Warner, but also Pacific Bell and Southern New England Telephone have announced plans to build FTTN networks.
Fiber to the neighborhood is one stage in a longer-range evolution of the cable plant. These longer-term changes are not necessary to provide Internet service today, but they might affect aspects of how Internet service is provided in the future. 3.2 ISDN Technology Unlike cable TV networks, which were built to provide only local redistribution of television programming, telephone networks provide switched, global connectivity: any telephone subscriber can call any other telephone subscriber anywhere else in the world. A call placed from a home travels first to the closest telephone company Central Office (CO) switch. The CO switch routes the call to the destination subscriber, who may be served by the same CO switch, another CO switch in the same local area, or a CO switch reached through a long- distance network.
Figure 4.1: The telephone network The portion of the telephone network that connects the subscriber to the closest CO switch is referred to as the local loop. Since all calls enter and exit the network via the local loop, the nature of the local connection directly affects the type of service a user gets from the global telephone network. With a separate pair of wires to serve each subscriber, the local telephone network follows a logical star architecture. Since a Central Office typically serves thousands of subscribers, it would be unwieldy to string wires individually to each home. Instead, the wire pairs are aggregated into groups, the largest of which are feeder cables.
At intervals along the feeder portion of the loop, junction boxes are placed. In a junction box, wire pairs from feeder cables are spliced to wire pairs in distribution cables that run into neighborhoods. At each subscriber location, a drop wire pair (or pairs, if the subscriber has more than one line) is spliced into the distribution cable. Since distribution cables are either buried or aerial, they are disruptive and expensive to change. Consequently, a distribution cable usually contains as many wire pairs as a neighborhood might ever need, in advance of actual demand.
Implementation of ISDN is hampered by the irregularity of the local loop plant. Referring back to Figure 4.3, it is apparent that loops are of different lengths, depending on the subscriber’s distance from the Central Office. ISDN cannot be provided over loops with loading coils or loops longer than 18,000 feet (5.5 km). 4.0 Internet Access This section will outline the contrasts of access via the cable plant with respect to access via the local telephon network. 4.1 Internet Access Via Cable The key question in providing residential Internet access is what kind of network technology to use to connect the customer to the Internet For residential Internet delivered over the cable plant, the answer is broadband LAN technology. This technology allows transmission of digital data over one or more of the 6 MHz channels of a CATV cable.
Since video and audio signals can also be transmitted over other channels of the same cable, broadband LAN technology can co-exist with currently existing services. Bandwidth The speed of a cable LAN is described by the bit rate of the modems used to send data over it. As this technology improves, cable LAN speeds may change, but at the time of this writing, cable modems range in speed from 500 Kbps to 10 Mbps, or roughly 17 to 340 times the bit rate of the familiar 28.8 Kbps telephone modem. This speed represents the peak rate at which a subscriber can send and receive data, during the periods of time when the medium is allocated to that subscriber. It does not imply that every subscriber can transfer data at that rate simultaneously. The effective average bandwidth seen by each subscriber depends on how busy the LAN is. Therefore, a cable LAN will appear to provide a variable bandwidth connection to the Internet Full-time connections Cable LAN bandwidth is allocated dynamically to a subscriber only when he has traffic to send.
When he is not transferring traffic, he does not consume transmission resources. Consequently, he can always be connected to the Internet Point of Presence without requiring an expensive dedication of transmission resources. 4.2 Internet Access Via Telephone Company In contrast to the shared-bus architecture of a cable LAN, the telephone network requires the residential Internet provider to maintain multiple connection ports in order to serve multiple customers simultaneously. Thus, the residential Internet provider faces problems of multiplexing and concentration of individual subscriber lines very similar to those faced in telephone Central Offices. The point-to-point telephone network gives the residential Internet provider an architecture to work with that is fundamentally different from the cable plant. Instead of multiplexing the use of LAN transmission bandwidth as it is needed, subscribers multiplex the use of dedicated connections to the Internet provider over much longer time intervals.
As with ordinary phone calls, subscribers are allocated fixed amounts of bandwidth for the duration of the connection. Each subscriber that succeeds in becoming active (i.e. getting connected to the residential Internet provider instead of getting a busy signal) is guaranteed a particular level of bandwidth until hanging up the call. Bandwidth Although the predictability of this connection-oriented approach is appealing, its major disadvantage is the limited level of bandwidth that can be economically dedicated to each customer. At most, an ISDN line can deliver 144 Kbps to a subscriber, roughly four times the bandwidth available with POTS. This rate is both the average and the peak data rate.
A subscriber needing to burst data quickly, for example to transfer a large file or engage in a video conference, may prefer a shared-bandwidth architecture, such as a cable LAN, that allows a higher peak data rate for each individual subscriber. A subscriber who needs a full-time connection requires a dedicated port on a terminal server. This is an expensive waste of resources when the subscriber is connected but not transferring data. 5.0 Cost Cable-based Internet access can provide the same average bandwidth and higher peak bandwidth more economically than ISDN. For example, 500 Kbps Internet access over cable can provide the same average bandwidth and four times the peak bandwidth of ISDN access for less than half the cost per subscriber. In the technology reference model of the case study, the 4 Mbps cable service is targeted at organizations. According to recent benchmarks, the 4 Mbps cable service can provide the same average bandwidth and thirty-two times the peak bandwidth of ISDN for only 20% more cost per subscriber.
When this reference model is altered to target 4 Mbps service to individuals instead of organizations, 4 Mbps cable access costs 40% less per subscriber than ISDN. The economy of the cable-based approach is most evident when comparing the per-subscriber cost per bit of peak bandwidth: $0.30 for Individual 4 Mbps, $0.60 for Organizational 4 Mbps, and $2 for the 500 Kbps cable servicesversus close to $16 for ISDN. However, the potential penetration of cable- based access is constrained in many cases (especially for the 500 Kbps service) by limited upstream channel bandwidth. While the penetration limits are quite sensitive to several of the input parameter assumptions, the cost per subscriber is surprisingly less so. Because the models break down the costs of each approach into their separate components, they also provide insight into the match between what follows naturally from the technology and how existing business entities are organized. For example, the models show that subscriber equipment is the most significant component of average cost. When subscribers are willing to pay for their own equipment, the access provider’s capital costs are low. This business model has been successfully adopted by Internex, but it is foreign to the cable industry.
As the concluding chapter discusses, the resulting closed market structure for cable subscriber equipment has not been as effective as the open market for ISDN equipment at fostering the development of needed technology. In addition, commercial development of both cable and ISDN Internet access has been hindered by monopoly control of the needed infrastructurewhether manifest as high ISDN tariffs or simple lack of interest from cable operators.