Serial connection g shdsl or. Digital transmission systems: from HDSL to G.shdsl. Model A, high level of xDSL adoption

Chepusov Evgeniy, employee of SvyazKomplekt company

More recently, high-speed digital transmission systems over copper subscriber lines have been a curiosity. Only a decade has passed - and not every telecommunications specialist is confidently guided by the variety of their characteristics. Another technology has recently appeared - G.shdsl. It was born for a long time, but immediately appeared in the form of the world standard ITU-T (G.991.2) - everyone has long been tired of the chaos of incompatible equipment from different manufacturers.

A bit of background

In the early 1990s, the development of digital signal processing led to the creation of HDSL. This technology combined 2B1Q line coding and sophisticated echo cancellation algorithms. The first two-pair variants were created in the USA and quickly replaced the old ANSI T1 (1544 Mbps) digital transmission systems, which had a working range of just over a kilometer. All this happened due to the fact that HDSL, providing a long range (3.5 km on a 0.4 mm wire), made it possible to abandon the regenerators and significantly reduce the costs of installing and operating newly commissioned lines.

A similar picture developed at that time in Europe - HDSL variants became widespread, which provide the transmission of the E1 ETSI stream (2048 Kbps). First, a variant appeared that used three pairs to obtain higher speed at the same range. The transmission speed for each of the pairs was the same as that of the American version (748 Kbps). Then, a two-pair version was standardized, in which the speed for each of the pairs was higher (1168 Kbit / s) with a shorter operating range (about 3 km on a 0.4 mm wire). But even in this case, the range turned out to be higher than that of equipment with HDB3 line code (Fig. 1).

Figure: 1. Evolution of transmission systems.

Through all the operating experience, HDSL has proven its high performance characteristics. In the vast majority of cases, HDSL equipment is installed without additional steam selection or line conditioning. Thanks to this, today most of the E1 lines are connected using HDSL equipment. Moreover, the very fact of the emergence of the technology that provided the possibility of cost-effective solutions for organizing digital connections for subscribers has led to the fact that the number of such connections began to grow rapidly. In other words, it was the emergence of HDSL that became a kind of catalyst for the development of digital networks.

In turn, the development of digital networks has created a demand for digital xDSL transmission systems with different characteristics. This is how the relatively low-speed IDSL technology appeared, the main advantages of which were operation on one pair and low cost, due to the use of standard components produced for subscriber ISDN equipment. This is how high-speed and asymmetric ADSL, VDSL with all its varieties were born, created to connect individual residential subscribers via their existing telephone line and without abandoning the use of this line for analog or digital (ISDN BRI) telephony. Finally, HDSL variants with other line coding methods (CAP) and adaptive HDSL variants with the ability to change the transmission rate in the line, adjusting it to the characteristics of the line, were developed to provide increased range of operation.

Manufacturers, each in their own way, began to think about implementing variants of HDSL systems that would operate one pair at a time at full speed. The fact is that in parallel with the development of xDSL technologies, the number of lines used by them also grew. Because of this, most operators all over the world already today note an acute shortage of copper on the subscriber section - almost all of it is "eaten" by xDSL lines. But digitalization is not over yet. Around 1996, single pair HDSL variants appeared. But they could not solve the problem due to incompatibility with ADSL - the signal spectrum of such systems partially overlapped with the spectrum of the ADSL signal from the PBX to the client.

The US operators were the first to sound the alarm, and already at the beginning of 1996, the ANSI committee (T1E1.4) was tasked with selecting for further development a technology that, with symmetric data streams and the use of one pair, would provide:

. working range is not less than HDSL;

. resistance to the same physical characteristics of the line as HDSL (attenuation, mutual influence, reflections from discontinuities and branches);

. use to provide the same types of services as HDSL;

. reliable and stable transmission on real lines with all their inherent defects;

. "Coexistence" with other technologies (HDSL, ISDN, ADSL);

. lower operating costs compared to HDSL.

The new technology, which appeared as a result of a huge three-year work, was named HDSL2 (it should be noted that the work on its standardization has not yet been completed due to some disagreements between the main manufacturers and the standard exists as a working version of T1 .418-2000). Symmetric transmission with echo cancellation (SEC) and frequency division multiplexing (FDM) were originally considered as the basis for HDSL2 implementation, but both were rejected due to their inherent disadvantages. The former has serious limitations in the presence of near-end interference, making it unsuitable for mass deployments. The second, although free from the shortcomings of the first, requires the use of a wider spectrum and does not provide requirements for mutual influence with systems for transferring other technologies.

As a result, a transmission system with overlapping but asymmetric spectral density distribution of the signal transmitted in different directions was adopted as a basis, using 16-level PAM (Pulse Amplitude Modulation) modulation. The chosen modulation method PAM-16 provides three bits of payload and an additional bit (error protection coding) in one symbol. PAM modulation alone is nothing new. The well-known 2B1Q is also a PAM modulation, but at four levels. The use of trellis codes, which, due to the introduction of redundancy in the transmitted data, made it possible to reduce the probability of errors, gave a gain of 5 dB. The resulting system was named TC-PAM (Trellis coded PAM). The receiver uses the highly efficient Viterbi algorithm for decoding. An additional benefit is obtained through the use of Tomlinson pre-coding - signal distortion in the transmitter based on knowledge of the channel impulse response. The total gain due to the use of such a rather complex signal coding technology is up to 30% compared to previously used HDSL / SDSL systems.

Figure: 2. Spectral density of G.shdsl signal.

But still, a key element of the success of the new technology is the idea of \u200b\u200basymmetric spectrum allocation, called OPTIS (Overlapped PAM Transmission with Interlocking Spectra) and served as the basis for HDSL2 and, subsequently, G.shdsl. When choosing the spectral density distribution for OPTIS, several problems were solved simultaneously (Fig. 2). In the first region of the frequency range (0-200 kHz), where the transient effect is minimal, the spectral densities of signals transmitted in both directions are the same. In the second frequency range (200-250 kHz), the signal spectral density from the LTU (site equipment) to the NTU (user equipment) is reduced to reduce its impact on the signal in the opposite direction in this frequency range. This ensures that the near-end transient effects are the same in both frequency ranges. In turn, the signal power from NTU to LTU in the second frequency range is reduced, which further improves the signal-to-noise ratio in this frequency range. It should be noted that this decrease does not degrade the signal-to-noise ratio at the NTU input for two reasons: first, the signal bandwidth from the LTU to the NTU is increased compared to the signal bandwidth in the opposite direction, and secondly, the NTU subscriber modems spatially separated, which also reduces the level of crosstalk. In the third frequency range, the spectral density of the signal from LTU to NTU is maximum, since there is almost no signal in the opposite direction in this region, and the signal-to-noise ratio for the signal at the NTU input is high. The selected spectrum shape is optimal not only in the case when only HDSL2 systems operate in the cable. It will also be optimal when working with ADSL, since the HDSL2 signal from NTU to LTU above 250 kHz, where the main power of the ADSL downstream components is concentrated, is practically suppressed. Preliminary calculations have shown that the interference from HDSL2 in the ADSL downlink (LTU to NTU) is less interference from HDSL operating in two pairs, and significantly less interference from HDSL using the 2B1Q code and operating on one pair at full speed. ...

G.shdsl enters the arena

In 1998, the rest of the world took up the ANSI initiative. ITU-T has begun work on the worldwide G.shdsl standard (the G.991.2 standard was approved in February 2001), and ETSI is also working on the European version of this standard (now it is formalized in the TS 101524 specification).

G.shdsl was based on the basic ideas of HDSL2, which were further developed. The goal was to use line coding techniques and HDSL2 modulation technology to reduce the mutual influence on adjacent ADSL lines at transmission rates above 784 kbps.

Since the new system uses a more efficient line code compared to 2B1Q, at any rate the G.shdsl signal occupies a narrower bandwidth than the corresponding 2B1Q signal. Therefore, interference from G.shdsl systems to other xDSL systems is less powerful than interference from HDSL type 2B1Q. Moreover, the spectral density of the G.shdsl signal is shaped to provide near perfect spectral compatibility with ADSL signals.

The noted properties of G.shdsl are extremely important for ensuring stable operation in the context of widespread adoption of xDSL technologies in the future. The results of the analysis of the stability of work, which were carried out on the basis of previously used noise models (including those described in the standards), may turn out to be unreliable. Thus, a telecom operator, while deploying transmission systems today, will not have a guarantee that they will remain stable in the future, when other systems start working on neighboring pairs.

Noise models that more accurately reflect the current state of the introduction of digital transmission technologies on the subscriber network were proposed by the international initiative organization FSAN (Full Service Access Networks), which since 1995 has been developing requirements and seeking consensus between the interests of operators and various manufacturers of telecommunications equipment working in the field of construction of multiservice networks of narrowband and broadband subscriber access. FSAN has developed four noise assessment models, differing in the number and composition of transmission systems operated in one cable (Table 1). Calculations for the new models are rather complicated, but they can give an idea of \u200b\u200bthe real efficiency of xDSL technologies at the stage of mass deployment of digital subscriber access. With this in mind, it is worthwhile to be very critical about the results of assessing the stability of work, if they use, though stipulated by the standards, but obsolete noise models.

Table 1. Models for assessing the impact of noise, proposed by FSAN.

In order to assess the discrepancy between the results obtained for the old and new models and to verify the advantages of the G.shdsl technology described above, you can use the results published by Schmid Telecom in its presentation on the launch of the Watson 5 family, implemented on the basis of G. shdsl (Table 2). Since among the equipment previously produced by this company, almost all major types of xDSL technologies have been used, the result is very clear. Wherever the noise margins are negative, the equipment in question will not operate in the given noise model of the situation. The advantage that G.shdsl has over other technologies is very noticeable. Attention should also be paid to significant discrepancies between the results obtained using the new FSAN model and the old, generally accepted, evaluation methodology according to ETSI. Of course, the results of evaluations of equipment from other manufacturers may differ from those presented by Schmid Telecom, but given the well-known quality of Watson modems, the differences are likely to be insignificant.

Table 2. Comparison of the noise margin of Schmid Telecom equipment based on the calculation using FSAN noise models.

Notes:
The comparison was made for a speed of 2.032 Mbit / s with a line length of 2400 m, wire D \u003d 0.4 mm in PE insulation.
* For increased NT transfer rate.
** Downstream using PAM8.
*** For comparison, equipment from another manufacturer was used.

There are other advantages to G.shdsl. Compared to two-pair versions, single-pair versions provide a significant improvement in hardware costs and, accordingly, product reliability. The cost reduction resource is up to 30% for modems and up to 40% for regenerators - after all, each of the pairs requires an HDSL transceiver, line circuits, protection elements, etc.

In order to support clients of various levels, G.shdsl decided to provide the ability to select the speed in the range of 192 Kbps - 2320 Kbps with an increment of 8 Kbps. By expanding the set of transmission speeds, the operator can build a marketing policy more closely approximated to the needs of customers. In addition, by reducing the speed, you can achieve an increase in the range in cases where the installation of regenerators is impossible. So, if at the maximum speed the working range is about 2 km (for a wire of 0.4 mm), then at the minimum speed it is over 6 km (Fig. 3). But that is not all. G.shdsl provides the ability to use two pairs for data transmission simultaneously, which allows increasing the maximum transmission rate to 4624 Kbps. But, most importantly, you can double the maximum speed that you can get on a real cable through which the subscriber is connected.

Figure: 3. Capabilities of G.shdsl transmission systems.

To ensure mutual compatibility of equipment from different manufacturers, the G.hs.bis standard (G.844.1) was incorporated into the G.shdsl standard, which describes the connection initialization procedure. There are two options for the procedure. In the first, the LTU equipment (installed on the PBX) dictates the connection parameters to the NTU (to the client's equipment), in the second, both devices "negotiate" the transmission rate, taking into account the state of the line. Given the unknown initial conditions, the communication during initialization uses a low bit rate and one of the classic modulation techniques (DPSK) to ensure connection establishment.

In addition to setting the speed, G.hs also describes the procedure for selecting a protocol during connection establishment. To ensure compatibility with all services used today, the G.shdsl modem frame must implement the ability to work with such protocols as E1, ATM, IP, PCM, ISDN. To ensure guaranteed performance of real-time applications, the G.shdsl standard limits the maximum data delay in the transmission channel (no more than 500 ms). The most used applications of this kind for G.shdsl are VoDSL voice in all its varieties (PCM - conventional digital telephony, VoIP - voice over IP and VoATM - voice over ATM) and video conferencing.

The optimal choice of the protocol during initialization in G.shdsl can further reduce the delay in the transmission channel. For example, for IP traffic, an appropriate protocol is established, which allows you to refuse the transfer of redundant information, compared to IP packets encapsulated in ATM cells. And for the transmission of digital telephone channels in PCM format, part of the DSL channel bandwidth is directly allocated.

It is worth noting that the above-mentioned voice and video conferencing require symmetric data streams to be transmitted in both directions. Symmetric transmission is also necessary for connecting local networks of corporate users who use remote access to servers with information. Therefore, unlike other high-speed technologies (ADSL and VDSL), G.shdsl is the best suited for organizing the last mile. So, at maximum speed, it provides transmission of 36 standard voice channels. Whereas ADSL, where the limiting factor is the low transfer rate from the subscriber to the network (640 Kbps), allows organizing only 9 voice channels, leaving no room for data transmission.

Another task that has been successfully solved in G.shdsl is to reduce power consumption. Since one pair is used for remote power supply, the importance of this task cannot be overemphasized. Another positive side - the reduction in power dissipation - paves the way for highly integrated plant equipment.

New equipment options - freedom of choice for operators

As follows from the above, G.shdsl has a number of advantages over other xDSL technologies. In terms of the main indicators, we can say that G.shdsl, in comparison with the single-pair version of the 2B1Q HDSL, can increase the transmission speed by 35-45% at the same range or increase the range by 15-20% at the same speed. In addition, G.shdsl has the basic capabilities for its use at the last mile in PCM (PCM), ATM, IP, FR networks from the very beginning. Due to this, G.shdsl has the widest range of applications (Fig. 4).

Figure: 4. Examples of using G.shdsl equipment.

It would seem that the new technology will become a panacea, and the demand for all other symmetric xDSL technologies will disappear, and for asymmetric technologies it will significantly decrease. However, both the majority of equipment operators and most of the G.shdsl equipment manufacturers note that the new technology cannot be considered as a complete replacement for the HDSL / SDSL / MSDSL families. They all agree that it cannot serve as a substitute for them, but is a supplement. Therefore, in the near future, hardware platforms will begin to win, which realize the possibility of using all the main technologies within a single system (Fig. 5). They will allow the operator to choose the xDSL technology for connecting a subscriber that is optimal for existing conditions and tasks to be solved.

Figure: 5. An example of using a universal xDSL platform.

Confirmation of this concept is confirmed by the serious success of WATSON equipment from SchmidTelecom, which is well known in the Russian market. This versatile platform has always included components that take all major line coding technologies (2B1Q and CAP) to the limit. It now includes the WATSON5 family, which fully implements all the requirements of the G.shdsl standard, including G.hs.bis. Such a short development time can be easily explained - Schmid Telecom works closely with component manufacturers and participated in the development of prototype equipment at all the final stages of the G.shdsl standard. It should be noted that only a few xDSL equipment manufacturers, who are leaders in this area, can afford such a degree of awareness and participation in the development process. Only such companies will be able to offer G.shdsl equipment to the market in the near foreseeable future.

However, even today even small companies offer G.shdsl equipment. The explanation for this fact is simple - we are talking about equipment that partially meets the requirements of the G.shdsl standard. Due to the fact that it does not implement all the functions described in the standard or implements them using simplified non-standard algorithms, it is very inexpensive. Typically, in such devices, compatibility with the standard is limited to the use of TC-PAM line coding. The field of application of these devices abroad is limited to the point-to-point application, which is used to integrate PBXs and segments of local networks of institutions. It is easy to distinguish such devices - they do not have options with a high density of equipment (several modems on one module), focused on installation at communication centers.

In conclusion, I would like to draw your attention to the fact that one of the fundamental points in the G.shdsl standard, which will determine the success of this technology in the telecommunications equipment market, is the compatibility of equipment from different manufacturers. This capability will allow operators in the future to easily change suppliers or purchase subscriber and station equipment from different suppliers, which is already widely practiced for ADSL today. The IOL laboratory (IterOperability Lab, University of New Hampshire), specially created by the leading manufacturers, is engaged in checking compatibility, working in cooperation with the DSL Forum - the founder of the xDSL "fashion". Validation is a very expensive process, so only serious suppliers can reasonably ensure that their equipment is fully compliant with G.shdsl and G.hs.bis standards. It is on their equipment that we recommend you to opt for.

The technology provides symmetric duplex data transmission over a pair of copper conductors. It is mainly used to connect subscribers with the provider's access point (the so-called last mile). The main ideas are taken from HDSL2 technology.

According to the standard, SHDSL technology provides data transmission at speeds from 192 Kbps to 2.3 Mbit / s (with a step of 8 Kbps) over one pair of wires, respectively, from 384 Kbps to 4.6 Mbps. in two pairs.
By using TC-PAM 128 encoding methods, it became possible to increase the transmission rate up to 15.2 Mbps over one pair and up to 30.4 Mbps over two pairs, respectively. [ ]

Work on G.shdsl began in 1998 in the International Telecommunication Union (ITU-T), and in February 2001 it was adopted as the G.991.2 standard. The European version of this standard is also handled by ETSI, now it is issued in the form of TS 101524 specification.

Technology features

G.shdsl was based on HDSL2 ideas, which were further developed. By using line coding and HDSL2 modulation, it has been possible to reduce the impact on adjacent ADSL lines at speeds over 784 kbps. Since the new system uses more efficient line coding (TC-PAM) compared to 2B1Q, the SHDSL signal occupies a narrower bandwidth at any rate. Therefore, the interference from the new system to other xDSLs is also less powerful than the interference from HDSL 2B1Q. G.shdsl also has a spectral density waveform that provides near-perfect compatibility with ADSL signals.

SHDSL options using a single pair of wires provide a significant improvement in hardware costs and, consequently, product reliability compared to two-pair options. The cost is reduced by 30% for modems and 40% for regenerators, since each of the pairs requires an HDSL transceiver, line circuits, protection elements, etc.

To support customers of different levels, it was decided to make the choice of the signal transmission rate. Thanks to this, operators can build a marketing policy that is closest to customer needs. In addition, it is possible to achieve an increase in the transmission distance without the use of regenerators by reducing the speed. At the maximum speed (for a 0.4 mm wire), the working range is about 3.5 km, and at the minimum speed, over 6 km. It is also possible to use two pairs at the same time, which allows you to double the top speed. Currently, the maximum stable data transfer rate over one copper pair reaches 15296 Kbps.

see also

Notes

Links

• ITU-T Recommendation G.991.2: Single-pair high-speed digital subscriber line (SHDSL) transceivers

Data transmission at speeds from 192 Kbps to 2.3 Mbps (in 8 Kbps steps) over one pair of wires, and 384 - 4.6 Mbps over two pairs.

Technology features

Links

• ITU-T Recommendation G.991.2: Single-pair high-speed digital subscriber line (SHDSL) transceivers
• Sigrand: the fastest SHDSL modem - 15.2 Mbps over one pair

see also

Wikimedia Foundation. 2010.

See what "SHDSL" is in other dictionaries:

- (Single pair High speed Digital Subscriber Line, ITU G.991.2) - one of the xDSL technologies that describes a method for transmitting a signal over a pair of copper conductors. It is used mainly to solve the problem of the "last mile", i.e. connecting subscribers with ... Wikipedia

SHDSL - Saltar a navegación, búsqueda EL SHDSL (Single pair High speed Digital Subscriber Line, Línea digital de abonado de un solo par de alta velocidad) ha sido desarrollada como resultado de la unión de las diferentes tecnologías DSL de conexión Español ... ... Wikipedia

SHDSL - (Single pair High speed Digital Subscriber Line) ou Ligne Numérique d Abonné Symétrique à très haut niveau de transmission sur des distances plus grandes que les autres technologies DSL. Elle permet de relier des utilisateurs situés à plus de ... ... Wikipédia en Français

SHDSL - SDSL (Symmetric Digital Subscriber Line) ist eine DSL Zugangstechnik zu einem öffentlichen digitalen Netzwerk wie beispielsweise dem Telefonnetz über eine Telefonleitung. Im Gegensatz zu ADSL lassen sich Daten mit der gleichen Geschwindigkeit in…… Deutsch Wikipedia

SHDSL - ● en sg. m. NORM Symmetric High bitrate DSL. DSL avec un débit garanti de 2,3 Mbps sur une simple ligne téléphonique ... Dictionnaire d "informatique francophone

G.SHDSL

G.SHDSL - (Global Standard for Single Pair Highspeed Digital Subscriber Line) ist eine symmetrische DSL Übertragungstechnik in digitalen Weitverkehrsnetzen. Bei G.SHDSL werden die gleichen Datenübertragungsraten im Up wie im Downstream über eine oder zwei ... ... Deutsch Wikipedia

SHDSL (English Single pair High speed + DSL), G.shdsl, ITU G.991.2 is one of the xDSL technologies that provides symmetric duplex transmission of signal data over a pair of copper conductors. Mainly used to connect subscribers with the node ... ... Wikipedia

G.991.2 - SHDSL SHDSL (Single pair High speed Digital Subscriber Line) ou Ligne Numérique d Abonné Symétrique à très haut niveau de transmission sur des distances plus grandes que les autres technologies DSL. Elle permet de relier des utilisateurs situés à ... Wikipédia en Français

ITU G.991.2 - SHDSL SHDSL (Single pair High speed Digital Subscriber Line) ou Ligne Numérique d Abonné Symétrique à très haut niveau de transmission sur des distances plus grandes que les autres technologies DSL. Elle permet de relier des utilisateurs situés à ... Wikipédia en Français

1. What is SHDSL?

SHDSL (Simmetric High Speed \u200b\u200bDigital Subscriber Line) is a symmetric high-speed digital subscriber line, the most modern type of DSL technology, aimed primarily at ensuring guaranteed quality of service, that is, at a given speed and data transmission distance, ensure an error rate of at least 10 -7 even in the most adverse noise conditions.
This standard is a development of HDSL, since it allows a digital stream to be transmitted over one pair.
SHDSL technology has several important advantages over HDSL. First of all, these are better characteristics (in terms of the limiting line length and noise margin) due to the use of a more efficient code, a precoding mechanism, more advanced correction methods and improved interface parameters. This technology is spectrally compatible with other DSL technologies. Since the new system uses a more efficient line code compared to HDSL, at any rate the SHDSL signal occupies a narrower bandwidth than the corresponding HDSL signal. Therefore, interference from other DSL systems generated by SHDSL is less powerful than interference from HDSL. The spectral density of the SHDSL signal is shaped so that it is spectrally compatible with ADSL signals. As a result, in comparison with the single-pair version of HDSL, SHDSL can increase the transmission speed by 35-45% at the same range or increase the range by 15-20% at the same speed.

2. SHDSL transmission rate

To organize access via SHDSL, a dedicated line (physical two-wire line) is required. The speed of access when connecting via SHDSL is determined by the technical characteristics, the length of a specific communication line and a specific brand of modem, on average, reaching full speed is possible on two-wire lines with a length of 1.5 km. with a copper wire diameter of about 0.4 mm.
SHDSL technology provides symmetric traffic over one twisted pair in the speed range: from 192 kbps to 2.3 Mbps, and over double pair - from 384 kbps to 4.6 Mbps.
SHDSL makes it possible to unite disparate local networks into a single corporate network, which significantly saves time and money when exchanging information between branches of an enterprise, ensuring a sufficient level of information security of the corporate network. SHDSL allows you to organize video conferencing when you need to maintain the same data streams in both directions.
One of the main advantages of SHDSL technology is the ability to use already existing (laid and actually working) copper pairs of wires of subscriber telephone lines, of which there are a huge number all over the world.

3. Differences between SHDSL and SDSL.

SDSL -Symmetric Digital Subscriber Line - symmetric digital subscriber line.
As well as HDSL technology, SDSL technology provides symmetric data transmission at rates corresponding to the rates of T1 / E1 lines, but SDSL technology has two important differences. Firstly, only one twisted pair of wires is used, and secondly, the maximum transmission distance is limited to 3 km. The technology provides the advantages necessary for business representatives: high-speed Internet access, the organization of multi-channel telephone communication (VoDSL technology), etc.
SHDSL - G.shdsl, Singlepair Highspeed Digital Subscriber Line - 1-pair high-speed symmetric digital subscriber line.
This technology makes it possible to extend the DSL line length up to 20 km (with regenerators) compared to the standards used today (according to which the maximum subscriber line length is approximately 5-6 km). It provides data transmission over 1 pair with a speed of 192 Kbps - 2,320 Mbps or over 2 pairs with a speed 2 times higher.

4. What is meant by the SHDSL standard.

Today there are three main categories of SHDSL standards: ANSI (T1E1.4 / 2001-174) for North America, ETSI (TS 101524) for Europe, and ITU-T (G. 991.2) worldwide. All of these standards have been published and are consistent. All standard flavors of ADSL (ITU G.992.1, G.992.2, and ANSI T1.413-I2) use the same technique, Discrete Multi Tone (DMT).

5. What is the difference between North American and European standards?

First of all, SHDSL is an international standard. Thus, the standard is defined as a special digital loop condition that corresponds to regionally defined supplementary services (eg T1). However, most equipment will support all international requirements.

6. What is TC-PAM modulation?

TC-PAM technology forms the basis of the first worldwide ITU standard for high-speed symmetric transmission over one pair - G.shdsl. It allows you to select a line speed in the range from 144 Kbps to 2.3 Mbps (8 Kbps steps), has a narrower frequency spectrum than its predecessors - 2B1Q and CAP. Thus, a long range of operation and electromagnetic compatibility with other DSL technologies such as ISDN, ADSL, G.lite and analogue PCM 15x2 systems (as opposed to HDB3 and 2B1Q) are provided.
The TC-PAM coding type has the best range and EMC performance for single-pair subscriber lines today. TC-PAM stands for Trellis Coded Pulse Amplitude Modulation (trellis-coded pulse amplitude modulation). The essence of this coding method is to increase the number of levels (code states) from 4 (as in 2B1Q) to 16 and apply a special error correction mechanism.

7. What is the transmission distance for SHDSL lines?

The data transfer rate using SHDSL equipment can reach 2.3 Mbit / s over one twisted pair (for "normal" DSL connections - 1.5 Mbit / s). Moreover, it is argued that the range for the new connection standard is 30% higher than for DSL, and since the standard supports repeaters, the latency for broadband applications (such as voice and media streaming) should be very low.
This technology makes it possible to increase the DSL line length up to 20 km (with regenerators) compared to the standards used today (according to which the maximum subscriber line length is approximately 5-6 kilometers). SHDSL provides data transmission over one pair at a speed of 192 Kbps - 2,320 Mbps or over 2 pairs at a speed 2 times faster. Echo cancellation ensures full duplex communication at all speeds.

8. Can repeaters be used in SHDSL lines?

Yes. Additional repeaters can be used for both double pair and single pair. The ITU standard supports up to eight retiters per pair, which reduces interference and regenerates signals before they are sent to the next segment, which in turn increases the transmission distance.

9. What is meant by "4-wire mode"?

SHDSL standard supports such an additional function as the ability to increase the speed and distance of data transmission when using two pairs of wires - "4-wire mode". The load is evenly distributed across the two pairs, but data is transmitted simultaneously.

10. Is SHDSL compatible with other DSL standards except ADSL?

Absolutely. "Symmetric" DSL options (SDSL and HDSL) can be used both to connect companies to the Internet and to transfer traffic between Fast Ethernet network segments. Modern xDSL technologies facilitate the solution of those tasks where high-speed connections are required: the creation of an Intranet network in a geographically distributed company, the organization of access to transport data transmission networks, etc.

11. What type of protocol does SHDSL technology support?

SHDSL technology supports such protocols as TDM, ATM, Frame Relay, as well as other network protocols. This allows the construction of geographically distributed corporate networks, as well as used as part of solutions related to ensuring guaranteed bandwidth of the data transmission channel (VoIP, video conferencing, etc.).

12. Can SHDSL technologies be used in PBX?

Using high-frequency channel resources and such packet technologies as ATM or IP, integrated access devices with VoDSL functions allow organizing several (say, 4, 8, 16 or 24) telephone channels simultaneously with a high-speed data stream. VoDSL systems will help traditional operators solve the problem of telephony in the residential sector (as you know, telephone installation is often impossible due to the lack of free copper pair) and form attractive offers for business users.

13. Hardware compatibility

SHSDL technology ensures the mutual compatibility of equipment from different manufacturers. For this purpose, G.hs.bis (G.844.1) standard was included in G.shdsl, which describes the connection initialization procedure. There are two options for the procedure. In the first case, the LTU equipment (installed on the PBX) dictates the connection parameters to the NTU (client equipment), in the second, both devices "negotiate" the transmission rate, taking into account the state of the line. Given the unknown initial conditions, during initialization, data exchange is carried out at a low speed to ensure connection establishment, and transmission is carried out using one of the classic modulation methods (DPSK).

14. Benefits of SHDSL technology

SHDSL technology allows both high-speed Internet access and fast and high-quality transmission of a large amount of information. Also, with the help of SHDSL technology, it is possible not only to receive information from the Internet, but also to use IP-telephony (city, intercity, international communication) and video conferencing.