The historical foundation of the public switched telephone network (PSTN) lies in twisted-pair, and even today, most people who have access to networks access them through a local loop built on twisted-pair. Although twisted-pair has contributed a great deal to the evolution of communications, advanced applications on the horizon require larger amounts of bandwidth than twisted-pair can deliver, so the future of twisted-pair is diminishing. Figure 2.1 shows an example of four-pair UTP.
Figure 2.1. Twisted-pair
Characteristics of Twisted-Pair
The total usable frequency spectrum of telephony twisted-pair copper cable is about 1MHz (i.e., 1 million cycles per second). Newer standards for broadband DSL, also based on twisted-pair, use up to 2.2MHz of spectrum. Loosely translated into bits per second (bps)a measurement of the amount of data being transported, or capacity of the channeltwisted-pair cable offers about 2Mbps to 3Mbps over 1MHz of spectrum. But there's an inverse relationship between distance and the data rate that can be realized. The longer the distance, the greater the impact of errors and impairments, which diminish the data rate. In order to achieve higher data rates, two techniques are commonly used: The distance of the loop can be shortened, and advanced modulation schemes can be applied, which means we can encode more bits per cycle. A good example of this is Short Reach VDSL2 (discussed in Chapter 12, "Broadband Access Alternatives"), which is based on twisted copper pair but can support up to 100Mbps, but over a maximum loop length of only 330 feet (100 m). New developments continue to allow more efficient use of twisted-pair and enable the higher data rates that are needed for Internet access and Web surfing, but each of these new solutions specifies a shorter distance over which the twisted-pair is used, and more sophisticated modulation and error control techniques are used as well.
Another characteristic of twisted-pair is that it requires short distances between repeaters. Again, this means that more components need to be maintained and there are more points where trouble can arise, which leads to higher costs in terms of long-term operation.
Twisted-pair is also highly susceptible to interference and distortion, including electromagnetic interference (EMI), radio frequency interference (RFI), and the effects of moisture and corrosion. Therefore, the age and health of twisted-pair cable are important factors.
The greatest use of twisted-pair in the future is likely to be in enterprise premises, for desktop wiring. Eventually, enterprise premises will migrate to fiber and forms of wireless, but in the near future, they will continue to use twisted-pair internally.
Categories of Twisted-Pair
There are two types of twisted-pair: UTP and STP. In STP, a metallic shield around the wire pairs minimizes the impact of outside interference. Most implementations today use UTP.
Twisted-pair is divided into categories that specify the maximum data rate possible. In general, the cable category term refers to ANSI/TIA/EIA 568-A: Commercial Building Telecommunications Cabling Standards. The purpose of EIA/TIA 568-A was to create a multiproduct, multivendor standard for connectivity. Other standards bodiesincluding the ISO/IEC, NEMA, and ICEAare also working on specifying Category 6 and above cable.
The following are the cable types specified in ANSI/TIA/EIA 568-A:
The predominant cable categories in use today are Cat 3 (due to widespread deployment in support of 10Mbps Ethernetalthough it is no longer being deployed) and Cat 5e. Cat 4 and Cat 5 are largely defunct.
Applications of Twisted-Pair
The primary applications of twisted-pair are in premises distribution systems, telephony, private branch exchanges (PBXs) between telephone sets and switching cabinets, LANs, and local loops, including both analog telephone lines and broadband DSL.
Analog and Digital Twisted-Pair
Twisted-pair is used in traditional analog subscriber lines, also known as the telephony channel or 4KHz channel. Digital twisted-pair takes the form of Integrated Services Digital Network (ISDN) and the new-generation family of DSL standards, collectively referred to as xDSL (see Chapter 12).
Narrowband ISDN (N-ISDN) was introduced in 1983 as a network architecture and set of standards for an all-digital network. It was intended to provide end-to-end digital service using public telephone networks worldwide and to provide high-quality, error-free transmission. N-ISDN entails two different specifications:
Given today's interest in Internet access and Web surfing, as well as the availability of other high-speed options, BRI is no longer the most appropriate specification. We all want quicker download times. Most people are willing to tolerate a 5-second download of a Web page, and just 1 second can make a difference in customer loyalty. As we experience more rapid information access, our brains become somewhat synchronized to that, and we want it faster and faster and faster. Therefore, N-ISDN has seen better days, and other broadband access solutions are gaining ground. (ISDN is discussed further in Chapter 7, "Wide Area Networking.")
The DSL family includes the following:
Some of the members of the DSL family are symmetrical and some are asymmetrical, and each member has other unique characteristics.
As in many other areas of telecommunications, with xDSL there is not one perfect solution. One of the main considerations with xDSL is that not every form of xDSL is available in every location from all carriers. The solution also depends on the environment and the prevailing conditions. For example, the amount of bandwidth needed at the endpoint of a networkand therefore the appropriate DSL family memberis determined by the applications in use. If the goal is to surf the Web, you want to be able to download quickly in one direction, but you need only a small channel on the return path to handle mouse clicks. In this case, you can get by with an asymmetrical service. On the other hand, if you're working from home, and you want to transfer images or other files, or if you want to engage in videoconferencing, you need substantial bandwidth in the upstream direction as well as the downstream direction; in this case, you need a symmetrical service.
The following sections briefly describe each of these DSL family members, and Chapter 12 covers xDSL in more detail.
Carriers use HDSL to provision T-1 or E-1 capacities because HDSL deployment costs less than other alternatives when you need to think about customers who are otherwise outside the permitted loop lengths. HDSL can be deployed over a distance of about 2.2 miles (3.6 km). HDSL is deployed over two twisted-pairs, and it affords equal bandwidth in both directions (i.e., it is symmetrical).
HDSL is deployed as two twisted-pairs, but some homes have only a single pair of wires running through the walls. Therefore, a form of HDSL called HDSL2 (for two-pair) has been standardized for consumer/residential action. HDSL2 provides symmetrical capacities of up to 1.5Mbps or 2Mbps over a single twisted-pair.
ADSL is an asymmetrical service deployed over one twisted-pair. With ADSL, the majority of bandwidth is devoted to the downstream direction, from the network to the user, with a small return path that is generally sufficient to enable telephony or simple commands. ADSL is limited to a distance of about 3.5 miles (5.5 km) from the exchange point. With ADSL, the greater the distance, the lower the data rate; the shorter the distance, the better the throughput. New developments allow the distance to be extended because remote terminals can be placed closer to the customer.
There are two main ADSL standards: ADSL and ADSL2. The vast majority of the ADSL that is currently deployed and available is ADSL. ADSL supports up to 7Mbps downstream and up to 800Kbps upstream. This type of bandwidth is sufficient to provide good Web surfing, to carry a low grade of entertainment video, and to conduct upstream activities that don't command a great deal of bandwidth. However, ADSL is not sufficient for things such as digital TV or interactive services. For these activities, ADSL2, which was ratified in 2002, is preferred. ADSL2 supports up to 8Mbps downstream and up to 1Mbps upstream. An additional enhancement, known as ADSL2+, can support up to 24Mbps downstream and up to 1Mbps upstream.
SDSL is a symmetrical service that has a maximum loop length of 3.5 miles (5.5 km) and is deployed as a single twisted-pair. It is a good solution in businesses, residences, small offices, and home offices, and for remote access into corporate facilities. You can deploy variable capacities for SDSL, in multiples of 64Kbps, up to a maximum of 2Mbps in each direction.
SHDSL, the standardized version of SDSL, is a symmetric service that supports up to 5.6Mbps in both the downstream and upstream directions.
RADSL has a maximum loop length of 3.5 miles (5.5 km) and is deployed as a single twisted-pair. It adapts the data rate dynamically, based on any changes occurring in the line conditions and on the loop length. With RADSL, the rates can vary widely, from 600Kbps to 7Mbps downstream and from 128Kbps to 1Mbps upstream. RADSL can be configured to be a symmetrical or an asymmetrical service.
VDSL provides a maximum span of about 1 mile (1.5 km) over a single twisted-pair. Over this distance, you can get a rate of up to 13Mbps downstream. But if you shorten the distance to 1,000 feet (300 m), you can get up to 55Mbps downstream and up to 15Mbps upstream, which is enough capacity to facilitate delivery of several HDTV channels as well as Internet access and VoIP. With VDSL2 you can get up to 100Mbps both downstream and upstream, albeit over very short distances.
Advantages and Disadvantages of Twisted-Pair
Twisted-pair has several key advantages:
Twisted-pair has the following disadvantages:
Although twisted-pair has been deployed widely and adapted to some new applications, better media are available to meet the demands of the broadband world.
Part I: Communications Fundamentals
Telecommunications Technology Fundamentals
Traditional Transmission Media
Establishing Communications Channels
Part II: Data Networking and the Internet
Data Communications Basics
Local Area Networking
Wide Area Networking
The Internet and IP Infrastructures
Part III: The New Generation of Networks
Broadband Access Alternatives
Part IV: Wireless Communications
Wireless Communications Basics
WMANs, WLANs, and WPANs
Emerging Wireless Applications