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High Bit-Rate Digital Subscriber Line (HDSL)

High Bit-Rate Digital Subscriber Line (HDSL) was also developed by Bellcore at the request of the RBOCs. It is intended as a more cost-effective means of providing T-1 local loop circuits over existing UTP.

HDSL Technology

HDSL eliminates repeaters in the T1 local loop for distances up to 12,000 feet, using the same 2B1Q (2 Binary, 1 Quaternary) coding scheme used in ISDN. Conventional T1 uses 2 pairs, each of which operates in simplex mode (one transmitting and one receiving) in a different direction at the full T1 rate of 1.544 Mbps. At such high frequencies, interference can be significant between the T1 pairs and adjacent pairs in the cable; additionally, signal attenuation is considerable. Therefore, it is necessary that the repeaters be spaced approximately 6,000 feet apart in order to adjust for distortion and signal loss.

HDSL, while it also uses two pairs of UTP, splits the pairs into full-duplex (FDX) channels. Each pair operates at 784 Kbps, (see Figure 10.2) half the T-1 rate plus some additional overhead. As the transmission rate per pair is halved, the frequency level is roughly halved; therefore, signal loss and interference are much less. The yield is that of longer transmission distances (up to 9,000 feet) without repeaters. As one might expect, the UTP cable plant must be in good condition and free of bridged taps.


Figure 10.2  HDSL configuration in example business application.

The real advantage of HDSL is that T1 service can be provisioned at much lower cost and in a much shorter interval of time. While HDSL equipment currently costs about $2,000 per circuit [10-12], that cost will come down quickly as the technology gains market share. As repeaters are eliminated or reduced in number, the incremental cost of HDSL is mitigated to some extent. Additionally, the cable plant supporting HDSL does not require special conditioning or engineering, further reducing costs, as well as allowing service to be provisioned much more quickly. Finally, HDSL offers error performance of approximately 10-10, as compared to the 10-7 level offered by repeatered T1 over twisted-pair, according to PairGain Technologies Inc., the leading supplier of HDSL equipment [10-13], with over 300,000 units sold.

Wireless Local Loop (WLL)

Wireless Local Loop (WLL) technologies, which will be discussed in more detail in Chapter 12, involve a relatively low-power, omnidirectional digital radio transmitter capable of supporting bidirectional communications. A number of manufacturers have released such products for voice applications—AT&T, DIVA, Ericsson, and Motorola are notable. Such a system is designed to support voice and low-speed data in a relatively small geographic area, with a radio transceiver located at the customer premise and serving multiple terminal devices. Spread spectrum radio provides improved security, as well as improved bandwidth utilization.

While its Quality of Service (QoS) generally does not compare favorably with cable alternatives, WLL does offer the advantages of rapid deployment and immunity to many issues of topography. WLL is not limited to narrowband voice. Broadband WLL operates much like ADSL, although on a wireless basis. Bellcore standard systems for Broadband WLL include ADML and LMDS. Asymmetric Digital Microcell Link (ADML) provides up to 1 Gpbs aggregate bandwidth at each radio site; [ge]1 Mbps can be provided to each premise. Local Multipoint Distribution Services (LMDS) is similar to ADML, but with a greater area of coverage.

Hybrid Local Loops

Hybrid local loop technologies include some combination of media and transmission technologies. Generally discussed in the context of a convergence scenario (Chapter 15), hybrid local loops typically involve Fiber-To-The-Neighborhood (FTTN), terminating in a remote shelf comprising a TDM MUX and an optoelectric conversion device. The signal is carried the last few hundred meters or so over coax, UTP (ADSL), or radio (WLL). This approach provides maximum bandwidth and signal quality to a remote unit, which contains all of the expensive optoelectric conversion equipment. The relatively short connection from the remote unit to the premise can easily be supported by conventional cable systems or WLL technology.

SONET/SDH

SONET/SDH is a set of international standards for broadband communications over single-mode fiber optic transmission systems, allowing manufacturers to build equipment to support full interconnectivity and interoperability. SONET/SDH uses a transfer mode which defines the switching and multiplexing aspects of a transmission protocol, supporting both asynchronous and synchronous traffic in any form. Intended primarily for the carrier networks, SONET/SDH can also be deployed to the user premise, although such implementation currently is unusual. The Network-to-Network Interface (NNI), also known as Network Node Interface, was specified by the CCITT in order that national and regional networks can be blended into a cohesive global network. The User Network Interface (UNI) was also specified in order that users might connect on a standard basis.

SONET/SDH describes the characteristics of a fiber optic physical infrastructure (OSI Layer 1), rather than a set of services. However, a number of recently developed services depend on bandwidth, error performance, flexibility, and scalability which can be provided best over a SONET infrastructure. Examples of such services certainly include Frame Relay, SMDS, ATM, and B-ISDN, all of which will be discussed in Chapter 11. Additionally, T-carrier, DDS, N-ISDN, and X.25 networks traffic all benefit from the performance characteristics of SONET infrastructure. For that matter, even voice traffic gains advantage.

According to Northern Business Information, the SDH/SONET transmission equipment market is expected to reach $8.5 billion by 1999, out of a total transmission market of $37.4 billion. Growing at 25% per year, the SDH/SONET market is strongest in the Asia/Pacific region, where huge network upgrades are taking place in China, Indonesia, Malaysia, and Thailand [10-14]. With the exception of definition and initial discussion of SDH, this chapter will focus of SONET, the original and U.S. standard.


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