What dsl technology provides equal bandwidth in both directions?

9.5.15 Digital subscriber line (DSL)

As interest in narrow band ISDN faded, and cable and satellites promise speedier Internet access, telephone companies turned to broadband services such as DSL to provide high-speed Internet access for the home. Digital subscriber lines and other advanced forms of multiplexing are designed to use as the transport medium the billions of dollars' worth of conventional copper telephone lines which the local telephone companies own, without requiring any new wires into the home. Telephone and other telecommunication companies desire to give their networks a packet-switch orientation and are trying to convert current voice-oriented networks that also carry data into more efficient data-oriented networks that will also carry voice. One such service over a single pair of twisted copper wires is referred to as voice-over DSL. The difference between DSL and traditional telephone service is that DSL converts voice into digital 0 and 1’s and sends them in packets over copper wire. Packets from several conversations as well as bits of email and other data travel together in seeming random order. For voice service over DSL, the trick is to deliver the voice packets to the right destination at the right time in the appropriate order, so that the “reassembled” conversations sound natural.

The demand for more network capacity, or bandwidth, closer to the home customer, which is causing telephone companies to deploy DSL, is also causing increased installations of optical fibers across the country, increasing the network's backbone capacity and bringing it closer to the neighborhoods. This is important for DSL which is a copper-based telephone line, high-speed but short-distance service in which the customer can be no more than a few miles from a telephone switching station. Fig. 9.14 clarifies this restriction and shows that copper lines attenuate a 1 MHz signal by 9 dB in a length of 1 km. At the present, to run broadband fiberoptic lines into homes is expensive, thus the “last mile” copper wire link between the telephone company's central office and home remains in place.

DSL, which accommodates simultaneous Internet and voice traffic on the same line, can relieve the bottlenecks in the last mile to the home. In DSL, the 1 MHz bandwidth is divided into two greatly unequal parts: the low end of the spectrum, 4 kHz, is used for voice traffic and acts as an ordinary telephone connection52 (POTS), while the high end, which is practically the entire spectrum, is used for data, typically Internet traffic. The 1 MHz of bandwidth which is available for DSL translates into high-speed data rates of up to 10 Mbps. Of course the high frequencies introduce problems such as high noise and high attenuation that did not exist at 4 kHz; hence sophisticated software and hardware techniques have to be applied at the central office to counter these. To reduce the effects of noise, line-coding techniques are applied to control the frequency band and digital signal processors (DSPs) are applied to restore the original signals from distorted and noisy ones (Fig. 9.33).

There are various flavors of DSL (also referred to as xDSL), for example, asymmetric and very high-speed. But they have one universal characteristic: the higher the data speed, the shorter the distance between home and switching station must be. In addition all are equipped with modem pairs, with one modem located at a central office and the other at the customer site. Before we give a list of the various types of DSL, let us define a few terms.

Symmetrical. A service in which data travel at the same speed in both directions. Downloads and uploads have the same bandwidth.

Asymmetrical. A service that transmits at different rates in different directions. Downloads move faster than uploads.

Downstream. Traffic is from the network to the customer.

Upstream. Traffic from the customer to the network operating center.

Available types of DSL are:

ADSL. Asymmetric digital subscriber lines deliver traffic at different speeds, depending on its direction, and support a wide range of data services, especially interactive video. ADSL provides three information channels: an ordinary telephone (POTS) channel, an upstream channel, and a higher-capacity downstream channel. These are independent, i.e., voice conversation can exist simultaneously with data traffic. These channels can be separated by frequency-division multiplexing. Downstream speed 1.5–7 Mbps; upstream 16–640 kbps; range 2–3.4 miles.

ADSL Lite. A slower version of ADSL designed to run over digital loop carrier systems and over lengths of more than 3 miles. Downstream 384 kbps − 1.5 Mbps; upstream 384–512 kbps.

HDSL. High-bit-rate digital subscriber lines provide Tl service in both directions for applications that require communications symmetry, such as voice, corporate intranets, and high-volume email. Typical use is between corporate sites. 1 Mbps up- and downstream; range 2–3.4 miles.

IDSL. Uses ISDN-ready local loops. An international communications standard for sending voice, video, and data over digital telephone lines. Up to 144 kbps up- and downstream; range 3.4–4.5 miles.

SDSL. Single-pair symmetric high-bit-rate digital subscriber lines operate on a single copper twisted pair. The advantage is a reduction from two wire pairs to just one. 128 kbps–2 Mbps up- and downstream; range 2 miles.

RDSL. Rate-adaptive digital subscriber lines offer adjustable downstream and upstream rates. This service can adapt its bit rates according to line conditions or customer desires. For example, if a line is noisy, the bit rate can be decreased, making this service more robust. Downstream 40 kbps–7 Mbps; upstream up to 768 kbps; range 2–3.4 miles.

VDSL. Very high-bit-rate asymmetric digital subscriber lines provide very high bandwidth downstream, but have distance limitations and require fiberoptic cable. Originally developed to provide video-on-demand over copper phone lines. Downstream 13–52 Mbps; range 1000 ft.

Internet technology is still rapidly evolving. Internet speeds have gone from just 56 kbps to 500 Mbps in just a few decades and will likely not stop there. For instance, a new DSL technology, G.fast, may bring old phone lines and copper technology back into the internet speed race with 1 Gbps speeds. Consumers will be able to buy a G. fast modem, attach it to their land-line phone connection, and receive 1 Gbps speeds, sufficient for 4K video. However, copper lines, when used in “last mile” connection are limited by their rapid attenuation of signals with distance (see Fig. 9.14).

8.2.3.1.2 Digital Communication Technologies

For surface monitoring applications (e.g., open pit mines, regional monitoring, etc.) wireless communications is the standard solution. Underground environments however, pose a unique set of challenges to the communications infrastructure mainly owing to the limited access and harsh conditions. Wireless communication is difficult underground due to the reduced range of radio communications without line of sight, so large numbers of repeaters are needed, with each requiring their own power source.

Today ethernet technology is ubiquitous in both consumer and industrial applications. Ethernet has the lowest cost/speed ratio of any digital communications technology, and its various media, speed, and distance options means it is possible to run high-speed Gigabit (1000 Mbit/s) ethernet networks throughout even the most expansive/deep mines.

But this wasn't always the case, especially in the gold mines of South Africa, where deep shafts and extensive tunnels mean that communication distances of more than 10 km are common. At these distances the only practical options to achieve the kind of data rates required to send all digitized data are ethernet over optical fiber or digital subscriber line (DSL) technologies. Fiber optic cable is expensive and fragile and requires specialized skills and equipment to install and maintain, so is considered impractical for many mines. Copper twisted pair cables are cheaper, easier to install and maintain without specialized equipment or skills, and in many cases already exist due to legacy telephone networks. So even today, in some of the most advanced mines around the world, copper twisted pair is still the communications medium of choice.

DSL technology, which relies on complex digital signal processing techniques, is a relatively recent development. Therefore since the first digital seismic monitoring systems of the 1980s until about a decade ago, we were left with no choice but to limit the amount of digital seismogram data transmitted over legacy communications networks using copper cable.4

Introduction

High speed digital subscriber line (HDSL) interfaces support full-duplex data rates up to 1.544Mbps over 12,000 feet using two standard 135Ω twisted-pair telephone wires. The high data rate is achieved with a combination of encoding 2 bits per symbol using two-binary, one-quaternary (2B1Q) modulation, and sophisticated digital signal processing to extract the received signal. This performance is possible only with low distortion line drivers and receivers. In addition, the power dissipation of the transceiver circuitry is critical because it may be loop-powered from the central office over the twisted pair: Lower power dissipation also increases the number of transceivers that can placed in a single, non-forced-air enclosure. Single-pair HDSL requires the same performance as two-pair HDSL over a single twisted pair and operates at twice the fundamental 2B1Q symbol rate. In HDSL systems that use 2B1Q line coding, the signal passband necessary to carry a data rate of 1.544Mbps is 392kHz. This signal rate will be used to quantify the performance of the LT1497 in this article.

5.11 Digital subscriber lines (xDSL)

xDSL is a family of technologies developed to send high-speed data over copper access lines. It is primarily a telecommunications technology and application but interaction with LAN and premises structured cabling systems is inevitable. Access to the telecommunications environment, such as the internet, over twisted pair telephone lines is currently achieved by using analogue modems with a maximum speed of 56.6kb/s or by integrated services digital network (ISDN) which offers a data rate of 64kb/s. A modem (modulator/demodulator) encodes data to fit within the standard 3.3kHz analogue transmission band of telephone lines. DSL technology aims to significantly improve on that transmission performance over twisted pair cables.

xDSL can be symmetric or asymmetric. Symmetric means that the data rate in both directions is the same. Asymmetric means that the data rate from the exchange to the subscriber is much larger than the rate from the subscriber back to the exchange. The physical separation or distance from the transmitter to the receiver determines the maximum achievable data rate.

HDSLHigh speed digital subscriber line. Offers up to 2Mb/s in each direction (i.e. symmetric). The first 1993 specification called for three pair operation, though the 1996 version used two pairs. There is currently a draft ANSI specification for a one pair version. This will probably only be used for the American T1 data rate (1.5Mb/s) not the European E1 data rate (2Mb/s). HDSL uses a band from DC to 748kHz and uses 2B1Q or CAP encoding.ADSLAsymmetric digital subscriber line. Offers 6–8Mb/s downstream and 640kb/s-1Mb/s upstream. There is a standard, ANSI T1.413, which specifies discrete multi tone (DMT) encoding but there is also a non-standard scheme using carrierless amplitude phase (CAP) and quadrature amplitude modulation (CAM). ADSL uses a band of 25kHz-1.1MHz.ADSL-LiteADSL-Lite is a ‘stripped’ down version of ADSL to give a cheaper, user-installable high speed delivery system (1.5Mb/s) primarily for internet use. The ITU is working on a standard (G.992.2) to define this technology.VDSLVery high speed digital subscriber line. This emerging technology pushes data transmission over simple copper telephone lines to the limits. It will be able to offer a symmetric service of 26Mb/s and an asymmetric service of 2 and 52Mb/s, but over relatively short distances of around 300m for the higher speeds. VDSL uses a combination of DMT and QAM and requires a band of 300kHz-10MHz.

17.3.1 Wireline Access Technologies and Energy Consumptions

Currently, major wireline access technologies include digital subscriber loop (DSL) as standardized in ITU-T G.922, hybrid fiber coaxial (HFC) as standardized in ITU-T J.112/122, and fiber-to-the-x (FTTx), where x could be home, curb, neighborhood, office, business, premise, user, etc.

17.3.1.1 Digital Subscriber Loop

Figure 17.4 shows the general DSL architecture. DSL is provided through copper pairs originally installed to deliver a fixed-line telephone service. A DSL modem at each customer home connects via a dedicated copper pair to a DSL access multiplexer (DSLAM) at the nearest central office.

Figure 17.4. DSL architecture.

17.3.1.2 Hybrid Fiber Coaxial

Figure 17.5 illustrates the HFC network. HFC was initially deployed to deliver television services. Nowadays, HFC also delivers Internet and telephony services. Typically, the television program material is compiled from national and regional sources at a headend distribution center in each regional city. This material is distributed on radio frequency (RF) modulated optical carriers through an optical fiber to local nodes, where the optical signal is converted into an electrical signal. That electrical signal is then distributed to customers through a tree network of coaxial cables, with electrical amplifiers placed as necessary in the network to maintain signal quality. Hence, these networks are commonly termed hybrid fiber coaxial networks.

Figure 17.5. HFC architecture.

17.3.1.3 Fiber-to-the-x

To realize FTTx solutions, passive optical networks (PONs) have become the most promising technology. As shown in Figure 17.6, a PON is a point-to-multipoint optical access network architecture in which one optical line terminal (OLT) is connected with multiple optical network units (ONUs), and an optical splitter is employed to enable a single optical fiber to serve multiple end users.

Figure 17.6. PON architecture.

The energy consumption of each access network can be split into three components: the energy consumption in the customer premises equipment (i.e., the modem), the remote node or base station (base transceiver station, BTS), and the terminal unit, which is located in the local exchange/central office. The per-customer power consumption can be expressed as pa=pCPE+pRNNRN+1.5pTUNTU, where pCPE,pRN, and pTUare the powers consumed by the customer premises equipment, remote node, and terminal unit, respectively. NRNand NTUare the number of customers or subscribers that share a remote node and the number of customers that share a terminal unit, respectively. Table 17.1 lists the typical power consumption of these three access networks [39].

Table 17.1. Energy Consumption of ADSL, HFC, and PON

pTU (kW)NTUpRNNRNNCPETechnology LimitPer-User Capacity (Mb/s)ADSL1.71008N/AN/A515 Mb/s2HFC0.624805711206.5100 Mb/s0.3PON1.34102403252.4 Gb/s16

It can be seen that PON consumes the smallest energy per transmission bit; this is attributed to the proximity of optical fibers to the end users and the passive nature of the remote node among various wireline access technologies [10]. However, as PON is deployed worldwide, it still consumes a significant amount of energy. It is desirable to further reduce the energy consumption of PON since every single watt saved will end up creating an overall terawatt or even larger power savings. Reducing the energy consumption of PON becomes even more important as the current PON system migrates into next-generation PON systems with increased data rate provisioning [35, 36].

17.3.1.4 BPON, GPON, and EPON

Besides the low energy consumption, PON has four other major advantages. First, a PON yields a small fiber deployment cost in the local exchange and local loop. Second, a PON provides higher bandwidth due to the deep fiber penetration [36]. Third, as a point-to-multipoint network, a PON allows for downstream video broadcasting. Fourth, a PON eliminates the necessity of installing multiplexers and demultiplexers in the splitting locations, and thus lowers the operational expenditure [37]. Owing to these advantages, the number of FTTx users has recently surpassed thirty million and is continuing to grow at a rapid rate.

The currently deployed PON systems are TDM PON systems [38]. As shown in Figure 17.7, the downstream traffic is continuously broadcast to all ONUs, and each ONU selects the packets destined to it and discards the packets addressed to other ONUs. In the upstream, each ONU transmits during the time slots that are allocated by the OLT. Upstream signals are combined by using a multiple access protocol, usually time division multiple access (TDMA). The OLTs “range” the ONUs in order to provide time slot assignments for upstream communication. Owing to their burst transmission nature, burst-mode transceivers are required for upstream transmission. There are generally three flavors of TDM PON technologies that have been standardized. They are Broadband PON (BPON), Gigabit PON (GPON), and Ethernet PON (EPON). Table 17.2 compares these three PON technologies.

Figure 17.7. The upstream and downstream transmission in TDM PONs.

Table 17.2. Comparison between BPON, GPON, and EPON

BPONGPONEPONStandardITU G.983ITU G.984IEEE 802.3ahFramingATMGEM/ATMEthernetMax bandwidth622 Mb/s2.488 Gb/s1 Gb/sUsers/PON326416VideoRFRF/IPRF/IP

Both BPON and GPON architectures were standardized by the Full Service Access Network (FSAN), which is an affiliation of network operators and telecom vendors. Since most telecommunications operators have heavily invested in providing legacy TDM services, both BPON and GPON are optimized for TDM traffic and rely on framing structures with very strict timing and synchronization requirements.

EPON is standardized by the IEEE 802 group, and focuses on preserving the architectural model of Ethernet. No explicit framing structure exists in EPON; the Ethernet frames are transmitted in bursts with a standard interframe spacing. The burst sizes and physical layer overhead are large in EPON. As a result, ONUs do not need any protocol and circuitry to adjust the laser power. Also, the laser-on and laser-off times are capped at 512 ns, a significantly higher bound than that of GPON. The relaxed physical overhead values are just a few of many cost-cutting steps taken by EPON. Another cost-cutting step of EPON is the preservation of the Ethernet framing format, which carries variable-length packets without fragmentation. EPON has been rapidly adopted in Japan and is also gaining momentum with carriers in China, Korea, and Taiwan since IEEE ratified EPON as the IEEE802.3ah standard in June 2004.

1.7.1 Rural Broadband Deficit Was a Driver of Interest in TVWS

Wireless technology was a clear alternative to DSL technology, enabling rapid deployment of broadband connectivity and better performance in many cases. Much of the available wireless technology had been developed for frequencies of 2.4 GHz and above (e.g. 5 GHz), where there was already spectrum available on a licence-exempt basis in many locations around the world. The only problem with these frequencies was that the range was limited at the allowed emission power, typically 100 metres or less. Going to lower frequencies enables greater coverage for the same power and helps reduce infrastructure deployment costs.

In the early 2000s, major technology providers had started to look at the possibility of licence-exempt access to sub-1 GHz bands. There was little or no clear spectrum available that was not already assigned and licensed. However, geographical and temporal usage was a very different story. Studies by industry and regulators [1] had demonstrated that only a small fraction of available spectrum was in use at any given place and time.

The UHF bands were a clear example of underuse, and where there was the greatest potential for harmonisation around the world, albeit with some regional variations. The harmonisation potential is key to driving new technology costs down and encouraging more widespread adoption. The unused spectrum was referred to as TV white spaces and it was clear that its use could bring major coverage improvements. Key players in the technology sector then set about the business of convincing regulators.

In the UK, regulatory measures had led to a competitive residential broadband market with choice and value in urban areas. However, many in rural areas (as was also true in the US) were feeling left out of the digital revolution. This was ironic, as rural residents and businesses have arguably more to gain from broadband than their urban counterparts: it can offset the greater travel difficulties in rural areas.

High-Speed Digital Services

The CSA Plan is fully compatible with the digital subscriber line for ISDN basic-rate access, while the Resistance Design plan is compatible up to 18 kft. The standard ISDN two-wire loop carries the two “B” channels, the “D” channel, and a maintenance channel at 12 kb/s, for a total of 160 kb/s. The line signal is quaternary, resulting from 2B1Q coding (two binary pulses recoded into one quaternary). A midspan repeater, and removal of loading coils, is sometimes used for range extension beyond 18 kft. Most versions of the DSL family (see below) are intended for use up to the 12–18 kft maximum zone.

14.4.1 Broadband Forum

The current BBF was formed in 2009 from the merger of the DSL Forum and the IP/MPLS Forum. The DSL Forum, founded in 1994, worked on standards related to broadband subscriber access using DSL technology. Their primary focus was on provisioning, creating reference models, network architectures, and best practices for carrying certain types of services. As its work expanded to include other broadband access technologies such as fiber, it changed its name to BBF in 2008. The predecessors of the IP/MPLS Forum are the ATM Forum, Frame Relay Forum, MFA Forum, and MPLS Forum. The focus of the IP/MPLS Forum was the promotion of MPLS and frame relay technologies.

The BBF is composed of 200 member organizations representing service providers, equipment manufacturers, and electronic chip suppliers, defines and facilitates next-generation broadband standards, best practices and solutions.

The BBF’s more recent work includes topics related to PON, Ethernet-based DSL (xDSL), and auto-configuration for CPE such as set-top box and Network Attached Storage units. In its 2014 work program, FTTdp (Fiber to the Distribution Point), xDSL interoperability, and NFV became central themes in the evolution of MultiService Broadband Network (MSBN) architecture and nodal requirements. Its technical recommendations (TRs) provide an integrated framework for managing different types of broadband access technologies and marketing reports (MRs) present technology overviews in support of specifications published in BBF TRs. The work of the BBF is performed under two main working committees: the Technical Committee and the Marketing Committee, a joint technical and marketing WG: Service Innovation and Market Requirements (SIMR); and a special advisory group: the Service Provider Action Council (SPAC). The Technical Committee consists of six WGs, as described in Table 14.7.

Table 14.7. Key Current Working Groups in BBF Technical Committee

WGTitleDescription of ScopeBBHomeBroadbandHomeAll aspects of in-premises device management; TR-069 (CPE WAN Protocol) updates with data model expansion to cover broader range of devices; functional requirements for residential and Internet gateway devicesE2EAEnd-to-End ArchitectureAll aspects of MSBN architecture and requirements; current projects on NFV, FTTdp, hybrid access, and IPv6 transitionFANFiber Access NetworkAll aspects of fiber-based MSBN requirements, including PON optical layer management; current projects on FTTdp functional requirements. Earlier projects included the global industry’s first GPON interoperability program developmentIP/MPLS and CoreIP/MPLS and CoreCurrent projects on IP network integration with optical transport aimed at packet network optimization by using DWDM interfacesO&NMOperations and Network ManagementAll aspects of protocol-independent management model development for MSBN; current projects on fiber infrastructure management (joint with FAN WG), FTTdp management architecture, and DSL quality managementMTMetallic TransmissionAll aspects of functional requirements and test methodology development for metallic wireline MSBN; current projects on FAST (G.Fast) certification test plan development, MSBN copper cable models, in-premises PLCs systemsSIMRService Innovation and Market RequirementsThis group is responsible for expedited exploratory work on emerging MSBN technologies and description/development of use cases and market requirements. Current projects on high-level requirements and framework on NFV and software defined networking (SDN) in MSBN

1.4.1 FMT/FBMC

FMT/FBMC is an interesting solution for very high-speed digital subscriber line transmission, which is intermediate to other proposed single-carrier and multicarrier methods, as well as providing some unusual advantages in terms of spectrum management, unbundling, and duplexing. Fig. 1.6 presents the structure of an FMT/ FBMC multicarrier system.

The complex-valued modulation symbols xk(mT), k = 0, 1, …, K − 1 are obtained from quadrature amplitude modulated (QAM) constellations, where 1/T is the symbol rate. After upsampling by a factor of M, each symbol stream is filtered by a baseband filter with frequency characteristic H(ej2πf) and impulse response h(t). The transmitted signal s(tT/M) is then obtained at the transmission rate of M/T by adding the signals on all K subcarriers. At the receiver, matched filtering (where * denotes complex conjugation) is employed, followed by downsampling by a factor of M. When M = K(M > K), the filter bank is said to be critically (noncritically) sampled.

Which type of DSL technology has equal download?

Which connection technology uses what is known as CMTS?

Which of the following three are Internet service options?

Which solution eliminates the need for dedicated high speed WAN connections between sites?