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Chapter 10
Broadband Network Infrastructure

Et loquor et scribo, magis est quod fulmine iungo. I speak and I write but more, it’s with light(ning) that I connect.

Giovanni Pascoli, describing his views of the telegraph, 1911.

translated by Daniel Minoli

Broadband Networking addresses the near and distant future of telecommunications—the need for speed! As we have discussed in previous chapters, the PSTN set the stage. It is still in place, is highly available, it works beautifully for voice and quite well for many nonvoice applications, and is highly reliable and cost-effective. However, it offers limited bandwidth, is assumed to be analog at the local loop level, introduces errors in transmission, is relatively slow in establishing connections, and is expensive for communications-intensive applications.

Conventional public data networks (PDNs) were significant improvements over PSTN. Although designed specifically for data communications, the PDNs still relied, at least in part, on the PSTN model for basic connectivity (local loops, data nodes collocated with voice switches, and common transport facilities). The traditional data model included both dedicated and switched connections, as well as a packet-switched alternative, X.25. Conventional PDNs provided improved performance in terms of speed, error control, throughput and network management.

The broadband network model is still emerging. Although some broadband network technologies are still immature, they are rapidly being deployed in backbone networks in developed countries. They also now are being extended directly to large end users. Broadband networks also rely on the voice model to some extent, often using the same, although upgraded, local loop; equipment collocated in PSTN end offices; and common, although upgraded, backbone transmission facilities. Some broadband technologies are intended specifically for data transmission, although others were designed to support a full range of voice, data, video and image traffic.

Broadband networks, generally speaking, share a number of characteristics. Bandwidth, by definition, is greater than DS3 ([ge]45 Mbps). Additionally, bandwidth is often provided on-demand in order to address dynamic bandwidth requirements of applications such as videoconferencing, multimedia conferencing, and bursty LAN-to-LAN traffic. Error performance is excellent, network management is robust, and resiliency is high due to substantial redundancy. While broadband networks are cost-effective for bandwidth-intensive applications, the absolute cost of acquisition and deployment is quite high. As a result, the necessary infrastructure is being deployed cautiously.

In previous chapters we explored the fundamentals of transmission systems (Chapter 3), the nature and specifics of PSTN (Chapter 6), and conventional digital and data networks (Chapter 8). Those discussions set the stage for an in-depth presentation of the latest in access and transport technologies. This chapter will deal with a number of developing local loop technologies. We will also examine SONET, the standard for fiber-optic backbone networks.

Local Loop Technologies

Regardless of the sophistication and elegance of the backbone network technology, it is still necessary to gain access to it. Despite the hyperbole about fiber optic cable or hybrid fiber/coax connections to the premise, little investment has been made in such access technologies. Some large user organizations in commercial office parks or high-rise buildings have direct fiber connections from LECs, IXCs or CAPs. A few privileged residential and small business customers have access in areas where various field trials are underway. The rest of us must still contend with the limitations of the twisted-pair local loop. However, significant advances have been made in the use of UTP. Additionally, fiber and wireless technologies have been developed to extend broadband capabilities to the premise. Specifically, we will explore the local loop access technologies of Asymmetric Digital Subscriber Line (ADSL), High-Bit Rate Digital Subscriber Line (HDSL), and Wireless Local Loop (WLL).

Asymmetric Digital Subscriber Line (ADSL)

Asymmetric Digital Subscriber Line (ADSL) is an advanced, high-bandwidth local-loop technology designed to extend the life of existing UTP loops for the transmission of broadband signals. Developed by Bellcore at the request of the regional Bells, ADSL provides for very high-capacity transmission over relatively short local loops in the Carrier Serving Area (CSA). ADSL makes use of DMT (Discrete MultiTone) and other compression techniques to accomplish this feat. ADSL protocols and interfaces recently were standardized by ANSI as T1.413. The technology is promoted by the ADSL Forum, which boasts membership of about 60 manufacturers and carriers [10-1].

ADSL Technology

In addition to supporting POTS voice over a separate analog channel, ADSL supports a high-speed downstream channel. The downstream bearer channel is in increments of 1.536 Mbps up to 6.144 Mbps, based on T1 specifications. Additionally, a bidirectional channel is provided in increments of 64 Kbps up to 640 Kbps. Variations of standard ADSL are several, as indicated in Table 10.1. The higher-speed versions ([ge]51.84 Mbps) also are known as Very High Data Rate Subscriber Line (VDSL).

Table 10.1 ADSL variations

DOWNSTREAM CHANNEL (Simplex) BIDIRECTIONAL CHANNEL (HDX) MAXIMUM DISTANCE

6.144 Mbps (US)*640 Kbps 2 Miles (3 Km)
51.84 Mbps (US) 5 Mbps 0.6–1.2 Miles (1–2 Km)
155 Mbps (US) 15 Mbps 500 meters
8 Mbps (European) 608 Kbps 3–4 Km


*4 bearer channels @ 1.536 data rate yields a data rate of 6.144 Mbps; overhead is additional. Further variations exist as specified in ANSI T1E1.4/94-007 Revision 8.


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