Identifying the Problem

Today’s business world offers many more technologically advanced options than ever before. Information is sent to employers over e-mail instead of letters in mailboxes. Employees have the ability to telecommute instead of living in the city or state that their employer is located. Consumers shop over the Internet instead of physically going to Wal-Mart or K-Mart and buying what they need. So what kind of affect does this have on the business world? One affect is that we need a faster means for accomplishing these tasks.

Shortage of Copper

Today’s business world is demanding higher speed communications. Businesses have been using HDSL (High bit-rate Digital Subscriber Line) to deliver T1 (Trunk Level 1) circuits over two pairs of twisted copper wire. HDSL is defined as "a technology that provides high-speed digital transmission over existing copper telephone lines" [9]. HDSL will be described later in the History of HDSL2 section of the paper. T1 is "a digital transmission link with a total signaling of 1.544 Mbps. T1 is the standard for digital transmission in North America [13]. The problem is these increasing demands for T1 services and speeds are causing a copper shortage. The solution is HDSL2 (High bit-rate Digital Subscriber Line – 2^nd Generation), which allows T1 access over a single pair of copper wire [4]. LECs (Local Exchange Carriers) are happy with the performance of HDSL, but this shortage of copper and the growing demand for T1 capabilities is causing the need for a more efficient way of using their copper resources. Another reason why there is a copper shortage is because businesses and consumers are installing multiple lines to the business/home. These lines are generally used for fax, Internet access, and separate voice lines (6).

Interference/Noise

Another problem with existing DSL (Digital Subscriber Line) is interference. DSL is "a generic name for digital services provided by local telephone companies to their subscribers. DSL can carry both voice and data at the same time, in both directions, as well as the signaling data used for call information and customer data. These services include different technologies such as HDSL2, ADSL (Asymmetrical Digital Subscriber Line), HDSL, SDSL (Single Pair Symmetrical Services), and VDSL (Very high-bit-rate Digital Subscriber Line)" [13]. Interference comes from crosstalk between signals and crosstalk, a radiation of signal from one circuit to another, results in noise [2]. Crosstalk noise can exist at the receiver end as either Near End Crosstalk (NEXT) or at the far end as Far-End Crosstalk (FEXT). Noise will degrade the overall performance of a line and mixed crosstalk (self-crosstalk + T1 crosstalk) might cause HDSL2 circuits to go down. Self-crosstalk is "crosstalk from the transmitters of other ISDN (Integrated Services Digital Network) or HDSL lines into the receiver of the desired ISDN or HDSL line" [19]. Also, it is more difficult to test the downfalls of HDSL2. DSL products can be tested with similar noise and therefore a good idea of worst case scenario can be decided, HDSL2, on the other hand, must be put through many test environments before worst case scenario can be decided [18]

Brief Overview of HDSL2

Although HDSL2 is classified as "2^nd Generation", it is not second-generation HDSL. Instead it is more of a complement to existing HDSL [11]. HDSL2 offers the same 1.544 Mbps capacity that HDSL offers, but does it on one pair of copper wires rather than two pair. HDSL2 offers three great things: (1) full T1 transmission rates over a single copper pair, with a reach of up to 12,000 feet, (2) equal or better spectral compatibility than traditional HDSL, and (3) interoperability. Being "spectrally compatible" means that HDSL2 has to maintain its compatibility with other services, which may be in the same bundle: HDSL, ADSL, SDSL, ISDN, etc. This will be described in more detail in the How HDSL2 Works section of this paper. Interoperability is defined as "the ability to operate multi-vendor components together using a common set of protocols or standards" [13]. This also will be discussed in greater detail in the How HDSL2 Works section of this paper. HDSL2 was developed in responsiveness to customers. Customers want:

• An advanced single-pair T1 system should go CSA (Carrier Service Area) reach to have real value.
• It should tolerate all the normal loop disturbances (crosstalk, bridged taps, etc.) that HDSL tolerates.
• It should not interfere more with existing services than existing HDSL does.
• It should be as cost-effective as conventional HDSL.
• It needs to work in the real-world!

Bridge taps are defined as "the same cable pair appearing multiple times at several distribution points. Bridged taps are sections of cable that are not on the direct path between a user’s location and the central office" [13]. The next section of this paper will discuss HDSL2 in greater detail.

History of HDSL2

HDSL2 was designed from HDSL technology - so what is HDSL? HDSL is a technology that provides high-speed digital transmission over existing copper telephone lines. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions [9]. Bandwidth is defined in Carr and Snyder’s book, The Management of Telecommunications: Business Solutions for Business Problems as: "the data throughput capability of a channel. In digital circuits it is measured in bits per second, denoting the speed of transmission. For analog channels, it is the difference between the highest and lowest analog frequencies, measured in hertz (Hz), of a transmission channel." HDSL allows signals to be transported over distances of up to 12,000 feet on copper cable without line repeaters. The difference between HDSL and HDSL2 is that HDSL uses two pair of copper wires while HDSL2 uses a single pair of copper wire.

Pre-HDSL

Before there was HDSL, network providers delivered T1 lines to businesses and organizations using repeaters. Repeaters are defined as "an electronic device used to regenerate digital signals and restore signal quality across a certain distance of cable. In some cases repeaters must be used to improve the quality of strength of a signal over long distances to combat noise and line degradation" [13]. T1 signals have the capacity to go a maximum of 3,000 feet before a repeater needs to be used. Also, the lines have to be specially conditioned by removing bridge taps. This could be very expensive if the consumer is a further distance away from the Central Office than 3,000 feet. [3]

HDSL

HDSL was developed to eliminate the need for repeaters and line conditioning and because it was cheaper and easier to deploy. As mentioned earlier, HDSL has the capacity to run up to 12,000 feet but it has no need for repeaters or line conditioning. HDSL uses two pair of copper wires, but instead of a transmit pair and a receive pair, it creates two bidirectional pairs, each running at 784 Kbps [3].

Figure 1 demonstrates geographically how HDSL works. DS-1 (Digital Signal 1) is used as a signal in the T1 carrier. The digital signal is what is carried inside the carrier signal. HDSL was widely accepted because of its resilience to crosstalk. It is the most used method of network providers in North America [3]. About 50% of T1 circuits deployed today use HDSL.

Two modulation methods for current HDSL have been specified. The method mostly used by the United States if four-level Pulse Amplitude Modulation, baseband, commonly referred to as 2B1Q because it maps two bits into one quaternary symbol [16]. Baseband is defined as a "narrow band circuit, generally not subdivided into channels, it often describes a digital circuit" [2]. PAM is defined as "a technique for placing binary information on a carrier to transmit that information. It is a technique for analog multiplexing. The amplitude of information being modulated controls the amplitude of the modulated pulses. Samples of each input voltage are placed between voltage samples from other channels. The cycle is repeated fast enough so the sampling rate of any one channel is more than twice the highest frequency transmitted" [13]. PAM was largely used because of its previous use of basic rate ISDN. The other modulation method is CAP (Carrierless Amplitude Modulation) which is widely used in Europe, South America and Central America for two-pair E1 HDSL. CAP passes pairs of symbols through a "Hilbert pair" o passband filters, that is, two filters with the same amplitude response and phase responses that differ by 90 degrees over the frequency band. It is the equivalent of two passband PAM systems superimposed on each other [16].

There are several issues driving the development of HDSL technology for high-speed capacity, T1 service over copper:

• There is fierce competition among service providers in metropolitan areas.
• Most customers are impatient about the long provisioning intervals for services that typically can be provided over conditioned copper T1 lines.
• The majority of these customers need only access and transport because the high-capacity services come to them from an interexchange carrier through the LEC.
• The major service providers have tight budgets and shrinking human resources to manage and deliver service.
• The current medium in the local loop is copper, and it is likely to remain that way for some time.
• Neither customers nor regulators will allow carriers to pass on, with any significance, the additional expense of duplicating an already existing copper network with fiber.

The technology involved with HDSL2 is OPTIS (Overlapped Pulse Amplitude Modulation with Interlocking Spectra), which is provided by specialized sets as a 16-PAM line code, an advanced 2BIQ line code. HDSL2 also uses advanced spectral shaping and error-correction. OPTIS is the standard that ANSI (American National Standards Institute) has adopted for HDSL2 technology. This standard supports a loop reach of 12,000 feet and has a 5 dB (decibel) noise margin in worst-case scenarios. HDSL2 is to be deployed using CSA guidelines, which are the guidelines, that HDSL technology uses today. This will allow for the easy transition to new HDSL2 technology in respect to circuit design, loop qualifications, and facilities troubleshooting [11].

The table on the following page compares the deployment guidelines for HDSL and HDSL2.

 
 

Deployment Guideline	HDSL	HDSL2
Payload	T1 @ 1.544 Mbps	T1 @ 1.544 Mbps
Pairs	2-pairs	1-pair
Load Coils Non-loaded loops	Non-loaded loops
Maximum Loop Length  26-AWG* cable	9 kft	9 kft
Maximum Loop Length 24-AWG* cable	12 kft	12 kft
Maximum Single Bridged Tap Length	2 kft	2 kft
Maximum Total Bridged Tap Length	2.5 kft	2.5 kft
Noise Margin on  CSA Loops 1-8	6 dB	5 dB

*AWG (American Wire Gauge) – Name of the diameter of a conductor. AWG is the U.S. standard measuring gauge for certain conductors, including copper. The higher the AWG number the thinner the wire [13].

Spectral Compatibility

HDSL2 must be compatible with existing technologies that already exist in the network such as ISDN, T1, and HDSL. This will enable service providers to be confident that the integrity of the services of the network will be maintained. This is also part of the standards issue that ANSI is working on [11]. In order to obtain spectral compatibility, vendors need to work together to develop the technology behind their HDSL2 products that will not interfere with existing technologies. This leads to the next step in making HDSL2 work, interoperability.

Interoperability

Interoperability between vendors and products will accelerate the acceptance of HDSL2 by the service providers. By defining the bits and bytes of the start-up sequence, the embedded operations channel, and the span of powering schemes for HDSL2, ANSI is ensuring the ability to have interoperable products from multiple vendors [11]. In order to ensure interoperability, ANSI has adopted OPTIS as the standard code for HDSL2. Dr. George Zimmerman of PairGain Technologies, Inc designed this line code. Vendors have to use OPTIS in developing their HDSL2 products if they want to state that they are "ANSI Compliant".

Standards

The American National Standards Institute (ANSI) submitted a draft proposal to set up the standards for HDSL2. This draft was presented at meeting in Ottawa, Ontario and took place on June 7-11, 1999. During the course of my research the draft was resubmitted with modifications at a meeting in Baltimore, Maryland on August 23-27, 1999. For a more detailed explanation of the electrical characteristics behind HDSL2 technology, I recommend reading this draft for standards approval. The Accredited Standards Committee on Telecommunications, T1 have set up the specifications for developing this standard.

The goal of the standards committee is to develop the criteria for interoperability between vendors of HDSL2 products. The standard will provide the minimal set of requirements for satisfactory transmission between the network and the remote installation. Developing a standard guarantees interoperability as longs as the vendors follow the standard. The standard will discuss line code, spectral shaping, system performance, and forward error correction [8].

Companies Involved

Companies such as ADC Telecommunications, Inc., ADTRAN, Inc., Conexant Systems, Inc., GlobeSpan, Inc., Metalink Ltd, PairGain Technologies Inc, Teltrend Inc., and Level One Communications, Inc. have all come together to form the HDSL2 Consortium at the University of New Hampshire’s InterOperability Lab. This lab provides an independent testing environment where members of the HDSL2 Consortium can conduct extensive testing and research of HDSL2 equipment with one another. This means that all of these companies are to share information and mutually benefit from the test results, as well as educate the students at the University of New Hampshire who are interested in the HDSL2 technology [10].

Products Relating with HDSL2

Although HDSL2 is being designed to work with existing loop technologies, some companies are going ahead and putting out a whole line of products to co-exist with HDSL2. ADTRAN, Inc. has developed the H2R: 239 HDSL2 Repeater. This repeater doubles the HDSL2 loop length, making it up to 24,000 feet of 24-gauge wire. ADTRAN, Inc. has also developed several different transceiver units to work with this repeater and the HDSL2 circuits. The ADTRAN Total Access H2TU-C is a high-density central office transceiver unit that works with a remote unit to establish T1 data rates over a single pair local loop. This unit allows for 28 HDSL2 circuits in a 6-inch tall space [1]. PairGain Technologies Inc. has developed HiGain Solitaire. HiGain Solitaire is the HDSL2 family from PairGain. Both ILECs and CLECs can benefit greatly from HiGain Solitaire. The copper wire pair gain enables ILECs to expand T1 services into areas with copper shortages, while CLECs get the opportunity to offer an increased number of services over the available lines being leased. These are only examples of products from two of the many companies that I listed earlier that are researching HDSL2 technology [18].

In researching HDSL2, I first started out by searching the Internet for a basic description of what HDSL2 is. I used various search engines and searched on phrases such as: "HDSL", "HDSL2" and "HDSL2 Technology". The results of these searches were thousands upon thousands of web sites with information pertaining to HDSL2 and related topics. I then proceeded to review the most recently dated sites. Through these sites I established the problem the business world is facing today and the need for this type of technology. I also found definitions, history, specifications, standards, and companies involved. After doing research on the Internet, I went to Indiana State’s library and proceeded to search for articles and books on HDSL2. In order to get a better understanding of the need for HDSL2 and the advanced technology involved, I went back to the Internet and Indiana State University Library and searched for other relevant technologies associated with HDSL2. Studying other technologies such as T1, DS-1, and HDSL helped me to better understand the need for developing HDSL2. I did e-mail several of the people who had e-mail addresses in the literature I found and either the e-mail recipient was no longer there, I got no response, or I got directions to web sites that were on the Internet that could hopefully answer my questions.

HDSL2 versus Other Technologies
 

Technology	Downstream Rate	Upstream Rate	Wires	CO* distance
HDSL2	1.544 Mbps	1.544 Mbps	1 Copper Pair	12,000 feet
ISDN	128 Kbps	128 Kbps	1 Copper Pair	18,000 feet
ADSL	1.5 – 8 Mbps	16 – 640 Kbps	2 Copper Pair	18,000 feet
RADSL**	1.5 – 8 Mbps	16 – 640 Kbps	1 Copper Pair	18,000 feet
HDSL	1.544 Mbps	1.544 Mbps	2 Copper Pair	12,000 feet
SDSL	128 – 1,024 Kbps	128 – 1,024 Kbps	1 Copper Pair	22,000 – 11,500 feet
VDSL	12.96 – 55.2 Mbps	1.5 – 6 Mbps	UTP***	4,500 – 1,000 feet

*CO – County Office

**RADSL – Rate Adaptive Digital Subscriber Line

***UTP – Untwisted Pair

SDSL can deliver full T1 capabilities but at less that half the reach of HDSL2. Also, SDSL does not use OPTIS and therefore is not compliant with the ANSI standard. ADSL supports asymmetrical transmissions for Internet and other applications that necessitate a higher bandwidth in one direction, whereas HDSL2 is bi-directional. RADSL works like HDSL2 accept it can adjust transmission speed according to the performance of the copper loop. VDSL is still in development but will most likely resemble ADSL but with a higher downstream capability [13]. HDSL2 is a standard, HDSL is more of a proprietary technology that shared similar characteristics between vendors [4].

HDSL2 vs. T1

HDSL2 has many advantages over T1 lines. The major advantage it has is cost. HDSL2 can deliver the same capabilities that T1 lines can, but it will do accomplish this on already existing copper so it will be cheaper. HDSL2 can offer the same capabilities as T1, but at about a 40% cost deduction [6]. Most houses or businesses already have copper going directly to them. There would not be a need to lay fiber to the home, which is not only expensive, but also very time consuming. Because this copper already exits, people can be reached to the "last mile". Another advantage that HDSL2 has over T1 is the distance it can go without having to use a repeater. HDSL2 can travel up to 12,000 feet without the use of a repeater compared to only 4,000 feet for a T1 line [11].

Issues Associated with HDSL2

Trying to establish standards for HDSL2 technology has also caused some issues to arise. Companies like ADTRAN, Inc. and Siemans have supported PAM, while PairGain, ADC, Level One, and Rockwell have supported OPTIS [15]. There is another issue surrounding "standards-compliant" HDSL2 repeaters. PairGain has put out an alert pertaining to this problem. The ANSI HDSL2 standard does not address repeatered HDSL2 technology because of spectral compatibility issues with ADSL. ADTRAN, Inc. has claimed that they have developed an ANSI standard HDSL2 repeater. PairGain says that there are two facts to remember: (1) HDSL2 is a standards driven technology. Repeaters are not part of the standards and therefore, repeaters are not ANSI compliant. (2) Repeatered HDSL2 can kill ADSL in the same binder group.

The need for this emerging technology is out there. The demand for T1 lines is growing approximately 30% each year leading to a shortage of copper. ADTRAN predicts that carriers will buy enough hardware for 500,000 new T1 lines next year (2000) and that between 20% and 25% of that will be HDSL2 gear [6]. Also, the signal-to-noise ratio has been decreased from 6 dB with HDSL to 5 dB with HDSL2.

Disadvantages

A problem associated with HDSL2 is suppliers and existing technologies. Because HDSL2 runs on two copper wires instead of four, it will cost less to set up. Also, DSL specialist Rhythms NetConnections claims "it will use the technology to support T1 services that will cost 40% less than typical T1 offerings". Some carriers are planning on using these lower prices to compete with the prices that RBOCs (Regional Bell Operating Companies). But RBOCs will not be eager to jump on the bandwagon for this technology when they already have customers who are willing to pay the prices currently charged for T1 access [6]. This will be a disadvantage to businesses and consumers.

Use of repeaters will be more limited with HDSL2 than with HDSL for several reasons. Chief among these is a simple consequence of Ohm’s law (Voltage = Current * Resistance). Because power for the HDSL2 repeater must go over a single-pair instead of two pairs, powering losses are higher in one-pair than two-pair HDSL. As a result, one-pair HDSL2 has much less powering capability than 2-pair HDSL.

Advantages

Level One Communications, Inc. refers to HDSL2 as "The Business DSL." Level One says, "Business DSL is specifically designed to fulfill the needs of business applications requiring symmetrical data distribution. HDSL2 technology opens the door for Regional Bell Operating Companies, Competitive Local Exchange Carriers, Internet Service Providers and other service providers to provide true symmetrical high-speed data transport [12]." Level One also suggests that there is no other current technology capable of meeting the demands of high-speed business communications.

HDSL2 uses a regular, two-wire phone line, most likely already in existence to the customer site. There would not be a need for new wires to be installed [6]. Consumers have made it very clear that they do not want to pay more for HDSL2 than they are currently paying for HDSL and they will not have to. When repeaters are eliminated, minimal maintenance and administration is necessary (6). HDSL2 provides a competitive opportunity by doubling the capacity of existing leases and owned copper lines, while providing revenue growth. CLEC’s now get more that half their profits from T1 service. ILECs (Incumbent Local Exchange Carriers) will use HDSL2 against CLECs to provide new high-bandwidth services. +4+

Other advantages already discussed in previous sections of this paper are interoperability, spectral compatibility, advantages of HDSL, and cost advantages over T1 lines.