TELECOMMUNICATIONS STANDARDS ADVISORY COMMITTEE TSAC WORKING GROUP ON NEW STANDARDS AND POLICY (NSP)

Transcription

1 TELECOMMUNICATIONS STANDARDS ADVISORY COMMITTEE TSAC WORKING GROUP ON NEW STANDARDS AND POLICY (NSP) Introduction Development of Digital Subscriber Line (DSL) Technology This paper introduces the development of Digital Subscriber Line (DSL) technology including its technical concepts, applications, standards development, system deployment and the way forward for standards adoption in Hong Kong. Major Types of Digital Subscriber Line (DSL) Technology 2. DSL refers to a family of copper based transmission technologies which enable high speed data transmission over existing telephone copper wires provided to business and residential premises. Since the use of DSL technology eliminates the need to re-build/upgrade telecommunications access networks, DSL is seen as a good supplement or alternative for access to broadband services. 3. There are variations of DSL technology which provide either symmetrical (i.e. same bandwidth for transmitting and receiving data) or asymmetrical (different bandwidth for transmitting and receiving data) communication link over the local copper loop. The family of DSL technology (commonly to as xdsl) includes the following major types: (a) High-bit-rate Digital Subscriber Line (HDSL) HDSL is designed to provide symmetrical data application with speeds up to T1 (1.5 Mbit/s) or E1 (2 Mbit/s). HDSL requires two pairs of copper cable to form a T1 connection and two to three pairs for E1 connection, and the maximum distance coverage is up to about 4 km. (b) Single-line Digital Subscriber Line (SDSL) SDSL is similar to HDSL except that it requires the use of a single copper pair only. SDSL also supports symmetrical data transmission up to T1/E1 but the distance coverage is slightly shorter (3-4 km). NSP WG Paper No. 30/98 - Page 1 -

2 (c) Asymmetric Digital Subscriber Line (ADSL) ADSL optimises the transmission performance of the telephone lines by taking advantage of their asymmetric attributes which generally allow a high speed signal to be transmitted from the exchange to the remote user location (downstream direction) and a lower speed signal to be transmitted in the reverse direction (upstream direction). The downstream transmission speed of ADSL ranges from 1.5 Mbit/s to 8 Mbit/s for a distance coverage of 2 to 6 km, while the upstream speed ranges from 16 kbit/s to 640 kbit/s. The data channels of ADSL operate in frequency bands above the existing Plain Old Telephone Service (POTS), thus allowing both telephone and broadband services to be delivered over the same copper loop. (d) Rate Adaptive Digital Subscriber Line (RADSL) RADSL is similar to ADSL except that it allows the option of adapting the line to a variety of data rates according to the required services and achievable over a given copper loop. In addition to the capability of rate adaptation, certain RADSL implementations also support a much higher speed upstream channel than ADSL. (e) Very-high-rate Digital Subscriber Line (VDSL) VDSL is designed for a fibre-to-the-neighbourhood (FTTN) access network with a combination of fibre cables feeding optical network units (ONU) and last leg connection to customer premises by means of short reaches of copper telephone lines. VDSL operates over a short distance of copper loop and transmit high speed data with a range of speeds depending upon actual line length. The downstream data rate under consideration ranges from 55 Mbit/s to 13 Mbits for a distance of 300m to 1500m. The upstream rate for initial implementation will be asymmetric like ADSL, at speeds from 1.6 Mbit/s to 2.3 Mbit/s. The data channels of VDSL are also separated in frequency from bands used for existing narrowband POTS and ISDN services. DSL Modulation Techniques 4. DSL uses advanced line coding algorithms to effectively utilise the spectrum on copper telephone wire for high speed data transmission. Common modulation schemes employed by DSL systems include: NSP WG Paper No. 30/98 - Page 2 -

3 2 Binary 1 Quatenary (2B1Q) - used by HDSL systems Carrierless Amplitude and Phase (CAP) - used by HDSL and ADSL/RADSL systems Discrete Multi-Tone (DMT) - used by ADSL and VDSL systems Quadrature Amplitude Modulation (QAM) - proposed for use by RADSL systems 5. For ADSL/VDSL systems, Frequency Division Multiplex (FDM) is used to separate frequency bands for the POTS/ISDN services, upstream data channel and downstream data channel. In addition, some DSL technologies also provide echo cancellation of the upstream and downstream channels to minimise the use of higher frequencies and maximise distance coverage. Typical frequency spectra for DSL systems are as shown in Figures 1 and 2. Figure 1 Typical HDSL frequency spectra Figure 2 Typical ADSL/VDSL frequency spectrum NSP WG Paper No. 30/98 - Page 3 -

4 DSL System Configuration 6. A typical HDSL connection (see Figure 3) consists of a modem pair with a central unit at the exchange and a remote unit at the customer premises. The customer premises equipment (CPE) is connected to the remote unit via a standard T1/E1 interface, such that the use of HDSL is transparent to the user. Exchange Side Customer Premises Data Network HDSL Central Unit 2/4/6- wire copper loop HDSL Remote Unit T1/E1 interface CPE Figure 3 Typical HDSL system configuration 7. A typical ADSL connection (see Figure 4) similarly requires a ADSL transceiver unit at the exchange (ATU-C) and a similar unit at the remote customer premises (ATU-R). Since ADSL can support POTS and data services on the same copper loop, a splitter can be optionally provided to separate the high frequency data signals from the low frequency POTS signals. The splitter may be integrated into the ATU, physically separated from the ATU. The ATU-R connection with the CPE is typically by standard data interface like Ethernet (10baseT), ATM-25 or T1/E1. Exchange Side Customer Premises Data Network ATU-C (optional) POTS Splitter 2-wire copper loop POTS Splitter (optional) ATU-R data interface (10baseT, ATM-25, T1/E1 etc) PC or Set-top Box PSTN Telephone Figure 4 Typical ADSL connection NSP WG Paper No. 30/98 - Page 4 -

5 8. Recently with the strong support of major computer and networking vendors, a simplified version of ADSL is being developed as a universal ADSL standard for delivering to consumers a moderately high speed data transmission of up to 1.5 Mbit/s downstream over existing telephone lines. The difference form the configuration in Figure 4 is to have a splitterless connection at the customer premises, while still maintaining simultaneous POTS and high speed data communication in both on-hook and off-hook conditions in order to simplify user installation and operation. 9. In a typical VDSL connection (see Figure 5), the exchange is connected via optical fibre to an optical network unit (ONU) located in the neighbourhood of the customer premises. The ONU is then extended to the customer premises by VDSL modem pairs over copper loop with optional splitter if narrowband POTS/ISDN services are provided on the same copper pair. The VDSL remote unit is in turn connected to the CPE by a standard data interface. Exchange Side Customer Premises Data Network fibre ONU VDSL (optional) POTS Splitter 2-wire copper loop POTS Splitter (optional) VDSL data interface (10baseT, ATM-25, T1/E1 etc) PC or Set-top Box PSTN Telephone Figure 5 Typical VDSL connection DSL Application 10. By offering broadband transmission capacity over existing telephone infrastructure, DSL can be used to support broadband services like: (a) High Speed Data Services With the use of DSL, traditional high speed data services like T1/E1, fractional T1/E1, Frame Relay and ATM services can be provided to customers over ordinary copper loop instead of requiring special screen cable or fibre cable. In addition, common LAN interface (eg Ethernet, token NSP WG Paper No. 30/98 - Page 5 -

6 ring) is usually integrated into the remote unit of DSL equipment and this provides easy support of applications like LAN-to-LAN interconnection and remote LAN access. (b) Broadband Internet Access With the increasing popularity of Internet services and the multimedia applications supported, the bandwidth demand for internet access by the end user is ever rising. At present, dial-up modem used over ordinary telephone line can achieve a maximum of 56 kbit/s. Owing to the asymmetric traffic nature of normal Internet access traffic, such as for web based applications, ADSL is considered a very suitable technology to overcome bandwidth limitation in the last mile of the access network between the exchange and customer premises. Compared with dial-up modem, the use of ADSL allows by-pass of the public switched telephone network (PSTN) and provides a high speed direct connection to Internet Service Providers (ISPs). (c) Digital Video Transmission DSL can support the transmission of digital video signals which has bandwidth requirement of 1.5 Mbit/s and above, and this makes possible interesting applications like Video On Demand and Interactive TV. By using ADSL, video signals can be directly transmitted from the exchange to the customer premises over a distance of 2-3 km. If fibre-to-the-neighbourhood is implemented such that buildings are connected to the exchange by optical fibre, VDSL can be used for the transmission over copper telephone cables in the last reach of typically m within the same building. DSL Standards Development 11. Standards development for DSL is actively undergoing in international, regional and industry standard bodies. At present, standards are available for HDSL and ADSL systems, while standards for VDSL are under definition. American National Standards Institute (ANSI) 12. The T1E1.4 Working Group of the T1 Committee accredited by ANSI is responsible for the standardisation work of DSL in the USA. The current status is as follows: NSP WG Paper No. 30/98 - Page 6 -

7 A HDSL standard ANSI T1.403 Network-to-Customer Installation - DS1 Metallic Interface is available and a new HDSL2 standard is being developed to support T1/E1 over a single copper pair (i.e. SDSL). An ADSL standard ANSI T1.413 "Network and Customer Installation Interfaces - Asymmetric Digital Subscriber Line (ADSL) Metallic Interface is available which specifies the use of DMT line code for up to 6 Mbit/s downstream and 640 kbit/s upstream transmission. A new Issue 2 has just been adopted which includes modifications necessary to operate with different networks from Europe and Japan, add versatility and provide test and management functions. A draft RADSL standard T1E1/97-104R2a Draft Proposed American National Standard - Interface Between Networks and Customer Installation - Rate Adaptive Digital Subscriber Line (RADSL) Metallic Interface is under development No VDSL standard has been defined yet and the current plan is to agree the system requirements and to develop one specification that will include the major proposals received. European Telecommunications Standards Institute (ETSI) 13. The ETSI TM6 Working Group is responsible for the standardisation work within ETSI on DSL systems and the current status is as follows: A HDSL standard TS v1.4.1 Transmission and Multiplexing (TM); High bit rate Digital Subscriber Line (HDSL) transmission system on metallic local lines; HDSL core specification and applications for 2048 kbit/s based access digital sections has been adopted which specifies the use of two line code options 2B1Q and CAP for E1 transmission over one to three twisted copper pairs. Technical requirements for ADSL has been defined in ETR 328 Transmission and Multiplexing (TM); Asymmetric Digital Subscriber Line (ADSL); Requirements and Performance which is based on ANSI T1.413 Issue 1. Further study is being carried out to extend the bit rate to 8 Mbit/s downstream and to combine ADSL with ISDN BRA on the same copper pair. Work on RADSL is foreseen in the future. Draft functional requirements for VDSL have been proposed in a document DTS/TM Transmission and Multiplexing (TM); Very High Speed Subscriber Lines (VDSL); Part 1 - Functional Requirements. NSP WG Paper No. 30/98 - Page 7 -

8 International Telecommunications Union (ITU) 14. The ITU-T Study Group 15 Question 4 committee carries out the work on DSL standards in the international standards organisation. Major progress has been achieved in a recent SG15 meeting on 22 October 1998 as follows: One HDSL related Recommendation Draft G High bit rate digital subscriber line (HDSL) transmission system on metallic local lines has been approved and which includes the two options 2B1Q and CAP line coding for T1/E1 transmission The following six ADSL related Recommendations have been approved: a) Draft G ADSL Transceivers (previously G.dmt) - which specifies a maximum data rate of 8 Mbit/s downstream and 640 kbit/s upstream using DMT line code and is based on ANSI T1.413 Issue 2 and ETR 328 b) Draft G Splitterless ADSL Transceivers (previously G.lite) - which is a customer installed, lower data rate version of G and allows a maximum data rate of 1.5 Mbit/s upstream and 512 kbit/s upstream c) Draft G Handshake Procedures for DSL Transceivers (previously G.hs) - which is modelled after Recommendations V.8/V.8bis (used by V.34 and V.90 modems) d) Draft G Overview of DSL Recommendations (previously G.ref) - which provides an overview of the DSL Recommendations and includes the system reference model e) Draft G Test Procedures for DSL Transceivers (previously G.test) - which is modelled after V.56bis/ter modem test Recommendations f) Draft G Physical Layer Management for DSL Transceivers (previously G.ploam) - which defines the physical layer operation, administration and maintenance functions based on the Simple Network Management Protocol (SNMP). Beginning of work on VDSL related Recommendation (G.993 reserved) and a new HDSL Recommendation (G.991.2) has been agreed. NSP WG Paper No. 30/98 - Page 8 -

9 Industry Standardisation Bodies 15. Important work on DSL standards is also being carried out by industry bodies, notably the ADSL Forum, Universal ADSL Working Group (UAWG), Digital audio-visual Council (DAVIC) and ATM Forum. These bodies have close collaboration with the above formal standardisation organisations in the development of commonly accepted DSL standards. DSL Deployment 16. In the USA, rapid rollout of DSL services has been announced by Regional Bell Operating Companies (RBOCs), interexchange carriers, local exchange carriers and internet service providers (ISPs). With keen support of computer and networking vendors, DSL has become a hot technology which is targeted as a mass market solution to provide broadband access to customers for a wide range of online services and applications. In Europe, major incumbent carriers have conducted or planned DSL trials. Telia AB of Sweden has begun commercial services in May 1998 to residential and business customers and it plans to make ADSL available to all Swedish households by In Asia, commercial service deployment of DSL has begun in Korea, Singapore and Hong Kong, while DSL trials have been conducted by Japan and China In Hong Kong, HDSL has already been deployed extensively for the provision of T1/E1 digital circuits. It is understood that ADSL with data rates in the range 1.5 to 8 Mbit/s have been trialled and deployed to provide video on demand and broadband internet access services. VDSL has also been tested with a fibre-to-the-building (FTTB) network topology to provide short range transmission of Mbps downstream and 1.62 Mbps upstream (based on DAVIC 1.0 part 8 setting B) between in-building optical network units (ONU) and customer premises. 18. In view of the rapid standards development progress, the widespread deployment of DSL in the world and the demand for broadband services, it is anticipated that there would be increasing supply of DSL products in the market and the provision of standards based DSL connections to the local customers will become a reality very soon. 1 Information is based on list of ADSL deployments available from the ADSL Forum web site at NSP WG Paper No. 30/98 - Page 9 -

10 Way forward for DSL standards in Hong Kong 19. It is proposed that adoption of DSL standards in Hong Kong should be considered for the following aspects: to encourage deployment of standards based DSL transmission systems in the access networks of fixed network operators for economic efficiency purpose; and to provide a standard user-to-network interface in the case where a DSL connection terminates directly with customer premises equipment (CPE), such as G.lite ADSL modem which will be designed as a consumer product. Advice Sought 20. Members are invited to note the content of this paper and to consider the way forward for DSL standards in Hong Kong proposed in paragraph 18. Office of the Telecommunications Authority NSP WG Paper No. 30/98 - Page 10 -