Twisted-pair

Twisted-pair

The historical foundation of the public switched telephone network (PSTN) lies in the twisted-pair, and even today most people who have access to networks access those networks through a local loop built on twisted-pair. So, 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 3.1 shows an example of four-pair UTP.

Figure 3.1. Twisted-pair

The total usable frequency spectrum of twisted-pair copper cable is about 1MHz (that is, one million cycles per second). Loosely translated into bits per second (bps) a measurement of the amount of information being transported over a distance you see about 2Mbps to 3Mbps over 1MHz of spectrum. But there's an inverse relationship between distance and loop length, and hence the available bandwidth, so as you shorten the distances, you can actually exercise higher data rates over twisted copper pairs. For instance, in local area networks (LANs), you can use Ethernet, which offers 100Mbps over twisted-pair, but the distance end-to-end on that network can be no more than 330 feet (100 meters). New developments will likely allow more efficient use of twisted-pair and enable the higher data rates that are needed for Web surfing and Internet access, but it will be important to limit the distance of the twisted-pair from the access point.

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 the 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 the outside interference. Most implementations today use UTP. Twisted-pair is also divided into categories that specify the maximum data rate possible. In general, the cable category term refers to the ANSI/TIA/EIA 568-A Commercial Building Telecommunications Cabling Standards. Other standards bodies including ISO/IEC, NEMA, and ICEA are also working on specifying Category 6 and above cable.

The following are the cable types specified in the ANSI/TIA/EIA Cabling Standards:

         Category 1 Cat 1 cable is for voice telephony only; it does not support data.

         Category 2 Cat 2 cable can accommodate up to 4Mbps and is associated with Token Ring LANs.

         Category 3 Cat 3 cable operates over 16MHz on UTP and supports up to 10Mbps over a range of 330 feet (100 meters). Key LAN applications include 10Mbps Ethernet and 4Mbps Token Ring LANs.

         Category 4 Cat 4 cable operates over 20MHz on UTP and can carry up to 16Mbps over a range of 330 feet (100 meters). The key LAN application is 16Mbps Token Ring.

         Category 5 Cat 5 cable operates over 100MHz on UTP and can handle up to 100Mbps over a range of 330 feet (100 meters). Key LAN applications include 100BaseTX, ATM, CDDI, and 1000BaseT.

         Category 5E Cat 5E (enhanced) operates over 100MHz on UTP, with a range of 330 feet (100 meters). The key LAN application is 1000BaseT.

Advanced cable types, such as the following, are being developed all the time:

         Category 6 Cat 6 cable operates over 250MHz on UTP, over a range of 330 feet (100 meters). Category 6 is expected to support 1Gbps, but only over short distances. There are currently no applications for Cat 6.

         Category 7 Cat 7 cable will operate over 600MHz over a range of 330 feet (100 meters) and will use STP or screened twisted-pair (ScTP).

The predominant cable categories in use today are Cat 3 and Cat 5E. Cat 4 and Cat 5 are largely defunct, and Cat 6 and Cat 7 are not yet standards in the United States.

Applications of Twisted-pair

The primary applications of twisted-pair are in premises distribution systems, telephony, private branch exchanges (PBXs) between the telephone sets and the switching cabinets, LANs, and local loops.

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 Digital Subscriber Line (DSL) standards, collectively referred to as xDSL.

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 the public telephone networks worldwide and, therefore, 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, which is 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 kilometers). 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 (that is, a PBX, a multiplexer, an automatic call distribution system such as the sequencers you encounter when you dial an airline). There are two different PRI standards, each deployed over two twisted-pairs: The North American and Japanese infrastructure uses 23B+D, and other countries use 30B+D. As with BRI, in PRI each of the B channels is 64Kbps. With PRI, the D channel is 64Kbps. So, 23B+D provides twenty-three 64Kbps B-channels for information and one 64Kbps D-channel for signaling and additional packet data. And 30B+D provides thirty 64Kbps channels and one 64Kbps D-channel.

Given today's interest in Internet access and Web surfing, BRI is no longer the most appropriate specification. We all want quicker download times. Most people are willing to tolerate an eight-second download of a Web page, and just one 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)

         ISDN DSL (IDSL)

         Symmetrical (or Single-Line) DSL (SDSL)

         Rate Adaptive DSL (RADSL)

         Very-High-Bit-Rate DSL (VDSL)

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 network and therefore the DSL family member you choose is 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, then you need substantial bandwidth in the upstream direction as well as in the downstream direction. In this case, you need a symmetrical service. Some of the members of the DSL family are symmetrical and some are asymmetrical, and each member has other unique characteristics. The following sections briefly describe each of these DSL family members, and Chapter 13, "Broadband Access Solutions," 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 is a symmetrical service that can be deployed over a distance of about 2.2 miles (3.6 kilometers). HDSL is deployed over two twisted-pairs, and it affords equal bandwidth in both directions (that is, it is symmetrical).

HDSL is deployed as two twisted-pairs, but some homes have only a single pair of wiring running through the walls. Therefore, a form of HDSL called HDSL2 is being promoted for consumer/residential action. HDSL2 provides symmetrical capacities of up to 1.5Mbps or 2Mbps over a single twisted-pair. The projections for deployment of HDSL2 do not suggest large numbers.

ADSL ADSL is an asymmetrical service that is 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 kilometers) from the exchange point. New developments allow the distance to be extended because remote terminals can be placed closer to the customer.

There are two ADSL standards: ADSL1 and ADSL2. The vast majority of the ADSL that is currently deployed and available is ADSL1. ADSL1 supports 1.5Mbps (North American standards) to 2Mbps (International Telecommunication Union [ITU] standards) downstream and 16Kbps to 64Kbps 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, ADSL1 is not sufficient for things such as the digital TV or interactive services that are likely to be available in the near future. For these activities, ADSL2 is preferred. ADSL2 supports 6Mbps (North American standards) to 8Mbps (ITU standards) downstream and 640Kbps to 840Kbps upstream.

We are continuously trying to deploy ADSL over longer distances. We'd like to get up to 7.5 miles (12 kilometers) or so, but predominantly today ADSL operates over distances up to 2.3 miles (3.7 kilometers). Again, the greater the distance, the lower the data rate, and the shorter the distance, the better the throughput.

IDSL IDSL has a maximum loop length of 3.5 miles (5.5 kilometers), and it is deployed as a single twisted-pair that offers 128Kbps in each direction. It is basically ISDN without the voice service. As discussed previously, this data rate is too low to be pursued in the future, but if there are no broadband solutions available, you can use IDSL to get double the rate of a 56Kbps analog connection.

SDSL SDSL is a symmetrical service that has a maximum loop length of 3.5 miles (5.5 kilometers) 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.

RADSL RADSL has a maximum loop length of 3.5 miles (5.5 kilometers) and is deployed as a single twisted-pair. It adapts the data rate dynamically, based on any changes that may be occurring in the line conditions and based 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 kilometers) over a single twisted-pair. Over this distance, you can get up to a rate of 13Mbps downstream. But if you shorten the distance to 1,000 feet (300 meters), you can get up to 52Mbps downstream, which would be enough capacity to facilitate tomorrow's digital TVs. With VDSL you can get up to 1.5Mbps to 2.3Mbps upstream.

Advantages and Disadvantages of Twisted-pair

Twisted-pair has several key advantages:

         High availability More than one billion telephone subscriber lines based on twisted-pair have been deployed, and if it's there, we're going to use it. You may have heard the argument that telcos are trapped in their copper cages, and rather than build out an infrastructure that's 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 actual 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.

The following are the disadvantages of twisted-pair:

         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 meters), twisted-pair can carry 100Mbps, but at 3.5 miles (5.5 kilometers), the data rate drops to 2Mbps or less.

         Short distances required between repeaters More components need to be maintained and where trouble can arise, and this 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 been adapted to some new applications, better media are coming down the pike for tomorrow's broadband world.