Twisted-Pair

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:

• Category 1 Cat 1 cable was originally designed for voice telephony only, but thanks to some new techniques, long-range Ethernet and DSL, operating at 10Mbps and even faster, can be deployed over Cat 1.
• Category 2 Cat 2 cable can accommodate up to 4Mbps and is associated with token-ring LANs.
• Category 3 Cat 3 cable operates over a bandwidth of 16MHz on UTP and supports up to 10Mbps over a range of 330 feet (100 m). Key LAN applications include 10Mbps Ethernet and 4Mbps token-ring LANs.
• Category 4 Cat 4 cable operates over a bandwidth of 20MHz on UTP and can carry up to 16Mbps over a range of 330 feet (100 m). The key LAN application is 16Mbps token ring.
• Category 5 Cat 5 cable operates over a bandwidth of 100MHz on UTP and can handle up to 100Mbps over a range of 330 feet (100m). Cat 5 cable is typically used for Ethernet networks running at 10Mbps or 100Mbps. Key LAN applications include 100BASE-TX, ATM, CDDI, and 1000BASE-T. It is no longer supported, having been replaced by Cat 5e.
• Category 5e Cat 5e (enhanced) operates over a bandwidth of 100MHz on UTP, with a range of 330 feet (100 m). The key LAN application is 1000BASE-T. The Cat 5e standard is largely the same as Category 5, except that it is made to somewhat more stringent standards. Category 5e is recommended for all new installations and was designed for transmission speeds of up to 1Gbps (Gigabit Ethernet). Although Cat 5e can support Gigabit Ethernet, it is not currently certified to do so.
• Category 6 Cat 6, specified under ANSI/TIA/EIA-568-B.2-1, operates over a bandwidth of up to 400MHz and supports up to 1Gbps over a range of 330 feet (100 m). It is a cable standard for Gigabit Ethernet and other network protocols that is backward compatible with the Cat 5/5e and Cat 3 cable standards. Cat 6 features more stringent specifications for crosstalk and system noise. Cat 6 is suitable for 10BASE-T/100BASE-TX and 1000BASE-T (Gigabit Ethernet) connections.
• Category 7 Cat 7 is specified in the frequency range of 1MHz to 600MHz. ISO/IEC11801:2002 Category 7/Class F is a cable standard for Ultra Fast Ethernet and other interconnect technologies that can be made backward compatible with traditional Cat 5 and Cat 6 Ethernet cable. Cat 7, which is based on four twisted copper pairs, features even more stringent specifications for crosstalk and system noise than Cat 6. To achieve this, shielding has been added for individual wire pairs and the cable as a whole.

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).

ISDN

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:

• Basic Rate Interface (BRI) Also referred to as Basic Rate Access (BRA), BRI includes two B-channels and one D-channel (often called 2B+D). The B-channels are the bearer channels, which, for example, carry voice, data, or fax transmissions. The D-channel is the delta channel, where signaling takes place. Because signaling doesn't occur over long periods of time, where allowed by the service provider, the D-channel can also be used to carry low-speed packet-switched data. Each B-channel offers 64Kbps, and the D-channel provides 16Kbps. So, in total, 2B+D offers 144Kbps, delivered over a single twisted-pair with a maximum loop length of about 3.5 miles (5.5 km). BRI is used in residences, in small businesses that need only a couple lines, and for centrex customers. (A centrex customer leases extensions from the local exchange rather than acquiring its own PBX for the customer premise. Thus, the local exchange pretends to be a private PBX that performs connections among the internal extensions and between the internal extensions and the outside network.)
• Primary Rate Interface (PRI) Also referred to as Primary Rate Access (PRA), PRI is used for business systems. It terminates on an intelligent system (e.g., a PBX, a multiplexer, an automatic call distribution system such as those with menus). There are two different PRI standards, each deployed over two twisted-pair: The North American and Japanese infrastructure uses 23B+D (T-1), and other countries use 30B+D (E-1). As with BRI, in PRI each of the B-channels is 64Kbps. With PRI, the D-channel is 64Kbps. So, 23B+D provides 23 64Kbps B-channels for information and 1 64Kbps D-channel for signaling and additional packet data. And 30B+D provides 30 64Kbps channels and 1 64Kbps D-channel.

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.")

xDSL

The DSL family includes the following:

• High-Bit-Rate DSL (HDSL)
• Asymmetrical DSL (ADSL, ADSL2, ADSL2+)
• Symmetrical (or Single-Line) DSL (SDSL)
• Symmetric High-Bit-Rate DSL (SHDSL)
• Rate-Adaptive DSL (RADSL)
• Very-High-Bit-Rate DSL (VDSL, VDSL2)

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.

HDSL

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

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

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

SHDSL, the standardized version of SDSL, is a symmetric service that supports up to 5.6Mbps in both the downstream and upstream directions.

RADSL

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

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:

• High availability More than 1 billion telephone subscriber lines based on twisted-pair have been deployed, and because it's already in the ground, the telcos will use it. Some say that the telcos are trapped in their copper cages; rather than build an infrastructure truly designed for tomorrow's applications, they hang on to protecting their existing investment. It is a huge investment: More than US$250 billion in terms of book value is associated with the twisted-pair deployed worldwide. This can be construed as both an advantage and a disadvantage.
• Low cost of installation on premises The cost of installing twisted-pair on premises is very low.
• Low cost for local moves, adds, and changes in places An individual can simply pull out the twisted-pair terminating on a modular plug and replace it in another jack in the enterprise, without requiring the intervention of a technician. Of course, this assumes that the wiring is already in place; otherwise, there is the additional cost of a new installation.

Twisted-pair has the following disadvantages:

• Limited frequency spectrum The total usable frequency spectrum of twisted-pair copper cable is about 1MHz.
• Limited data rates The longer a signal has to travel over twisted-pair, the lower the data rate. At 30 feet (100 m), twisted-pair can carry 100Mbps, but at 3.5 miles (5.5 km), the data rate drops to 2Mbps or less.
• Short distances required between repeaters More components need to be maintained, and those components are places where trouble can arise, which leads to higher long-term operational costs.
• High error rate Twisted-pair is highly susceptibility to signal interference such as EMI and RFI.

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.