Course 8-9 DSL type digital access techniques (Digital Subscriber Line)

Transcription

1 Course 8-9 DSL type digital aess tehniques (Digital Subsriber Line) o The term refers to the tehnologies and equipments used in a telephone network to ensure the aess to a high speed digital network on a twisted wire line; there are two basi ategories namely: SDSL Symmetri DSL and ADSL - Asymmetri DSL; o SDSL ensures the same transfer rate in both diretions, that is in: upstream subsriber exhange downstream exhange subsriber due to the attenuation and rosstalk these systems an work only at medium frequenies; the symmetri DSL variants inlude: SDSL, SHDSL, MSDSL, HDSL, HDSL-, IDSL ; SDSL is ideal for LAN, bidiretional-video, web servers. o ADSL ensures in downstream a large bandwidth hannel, situated at high frequenies and a more narrow band hannel in upstream situated at low frequenies; o this division of the frequeny bands has two reasons: the information quantity transmitted in downstream is larger and it is redued the rosstalk, whih is higher at high frequenies; o ADSL variants inlude: ADSL, ADSL G.lite, RADSL and VDSL (an work both in symmetri and asymmetri mode). o The used transmission mode for different DSL tehniques and the maximum length of the subsriber loop are presented in tab. 1 and fig. 1; it is speified if is possible the use of telephone servie and the number of neessary pairs of wire.

2 Tab. 1 Fig. 1

3 o for a ondensed presentation of the main issues related to SDSL transmissions, in fig. are presented a HDSL system, the distant power supply and the omparison with a simple bidiretional transmission of a primary PCM frame. Fig..1 Fig.. Fig..3

4 o Types of SDSL tehniques: o HDSL (High data rate Digital Subsriber Line) older version of symmetri DSL reated as an alternative to the T1 and E1 servies. transmits a rate of 1,544Mbps on two pairs of twisted wire, on eah pair a 784kbps rate is transmitted in full-duplex mode using the eho ompensation tehnique; allows the use of normal lines (0,5 mm preonditioning is not neessary) with a 1000ft (3700m) maximum length without the use of repeaters; does not allows the standard telephone servie on these lines. o HDSL (Seond generation HDSL) ensures a rate of 1,5Mbps in both diretions on a single pair of twisted wire; does not allow the standard telephone servie on this line. o IDSL (Integrated Servies Digital Network (ISDN) DSL) ensures symmetrial rates up to 144kbps using existing telephone lines and ISDN terminal (ISDN modem); differs from the ISDN by the fat that is ontinuously available; it is used for WAN (Wide Area Network) type appliations; does not allow the standard telephone servie. o SDSL (Symmetri Digital Subsriber Line) ensures high transfer rates on a single twisted pair for T1 and E1 appliations; the maximum transfer rate is,3mbps; allows Ethernet interfae between the SDSL modem and the user equipment. o SHDSL (Symmetri High bit rate Digital Subsriber Line) allows a 0% higher overage than SDSL; allows the use of one or two pairs of twisted wire for ex. 1,Mbps transfer rate an be transmitted at 0000ft (6100m) on two normal (0.4 mm) pairs. o MSDSL (Multi-rate Symmetri Digital Subsriber Line) allows the adaptive hange of the rate aording to the type of the line; for ex. using the CAP modulation (Carrierless Amplitude & Phase Modulation) are available 8 disrete rates between 64kbps/18kbps (9000ft 8900m 0,5mm) and Mbps (15000ft 4600m).

5 o CAP (Carrierless Amplitude&Phase Modulation) variant of the QAM modulation (Quadrature Amplitude Modulation). o QAM one of the best options for data transmissions allows the use in the same frequeny band of two orthogonal sine arriers a sine arrier and a osine arrier on eah arrier it is possible to transmit different signals in the same bandwidth Q X MODULATOR os(πf t) bits bit assignment on modulation onstellation mapping Fig. 3.1 QAM modulator I I Q LPF FTJ LPF FTJ BPF sin(πf t) TX s QAM (t) DEMODULATOR os(πf t+ f t+ϕ) s QAM (t) arrier reovery BPF Fig. 3. QAM demodulator LPF FTJ FTJ LPF I Q lok reovery blok + deision blok + demapping blok bits sin(πf t+ f t+ϕ) s QAM ( I h ) os( ω t) + ( Q h ) sin( ω t) = I' os( ω t) + Q' sin( ω t) = (1) LPF LPF where h LPF (t) is the impulse response of low pass shaping filters. I (t) şi Q (t) signals obtained after the QAM demodulation using a loal arrier with f + f frequeny and phase ϕ : I" ( ) ( ) = I' os( ω t) + Q' sin( ω t) = sqam os( ωt + ω t +ϕ) hlpf t I' [ os ( ω t + ω t +ϕ) + os( ω t +ϕ) ] = I' = os ( ω t +ϕ) Q' sin ( ω t +ϕ) h + (( ) os( ωt + ω t +ϕ) ) hlpf Q' [ sin( ω t + ω t +ϕ) sin( ω t +ϕ) ] h = LPF LPF () =

6 Q" = ( ) ( ) = I' os( ω t) + Q' sin( ω t) = sqam sin( ωt + ωt + ϕ) hftj t I' [ sin( ω t + ω t + ϕ) + sin( ω t + ϕ) ] I' = sin ( ω t + ϕ) Q' + os Q' + ( ω t + ϕ) h (( ) sin( ωt + ω t + ϕ) ) hftj [ os( ω t + ϕ) os( ω t + ω t + ϕ) ] h = FTJ (3) o if the arrier reovery is perfet, meaning ω =0 and ϕ=0, then I (t) and Q (t) demodulated signals are: I" = I' h ; Q" = Q' h (4) o the use of QAM type modulations in the subsriber loop implies the following problems (see fig. 4 ): FTJ the bandwidth of the QAM signal is twie of the I(t) and Q(t) modulator signals bandwidth. the spetrum of the QAM signal is entered on the arrier frequeny. arrier reovery is neessary H h a n n e l ( f) ; S Q A M ( f ) H s u b s r ib e r - lo o p ( f) FTJ FTJ = f B Q A M B I = B Q F ig. 4 S p e tr u m o f Q A M m o d u la te d s ig n a l; B P F a n d L P F h a n n e l tr a n s fe r h a r a te r is ti s o the use of a base-band transmission with high spetral effiieny is the best idea. o the CAP modulation ensures the transmission of two different base-band signals in the same bandwidth. f Q X MODULATOR bits Fig. 5.1 CAP modulator I bit assignment on modulation onstellation mapping I Q filtru in phase emisie în filter fază h ef (t) ef (t) filtru emisie în in uadratură quadrature filter h eq h(t) eq (t) I Q s CAP (t)

7 DEMODULATOR s CAP (t) Fig. 5. CAP demodulator in filtru phase de I reepţie reeption în fază h rf (t) filter h rf (t) filtru in de quadrature reepţie în uadratură reeption h rq (t) filter h Q rq (t) lok reovery blok+ deision blok bits o Conditions for the transmission and reeption filters transfer funtions H ϕ ef ef ( ω) = H ( ω) π ( ω) = ϕ ( ω) + if ω > 0 ; ϕ ( ω) = ϕ ( ω) if ω < 0 eq eq h ω ef eq π (5) h 0 (6) ef eq = ϕ ϕ rf rq π π ( ω) = ϕ ( ω) + daa ω > 0 ; ϕ ( ω) = ϕ ( ω) eq π π daa ω < 0 ( ω) = ϕ ( ω) + daa ω > 0 ; ϕ ( ω) = ϕ ( ω) daa ω < 0 ef rf rq eq ef (7) o basi relations with desribe the modulation and demodulation s = I h + Q h (8) I" = CAP = scap hrf = I hef + Q heq I ( h h ) ef rf ef ( ) h = I h h eq ( ) ( ) + Q h h ( ) ( ) ( ) ( ( ) ( )) ( ) ( ( ) ( )) Q" = scap hrq = I hef + Q heq hrq t = I t hef t hrq t + Q t heq t hrq t = Q ( heq hrq ) (9) o the bandwidth of the CAP modulated signal is half of the QAM modulated signal bandwidth. o rf the spetrum is entered at low frequenies where the subsriber loop harateristi has low attenuation distortions. ef rf eq rf = =

8 o ADSL and VDSL tehniques o ADSL (Asymmetri Digital Subsriber Line) an ensure rates between 6 and 9 Mbps in downstream and up to 1Mbps in upstream ; DSL was developed in the late 1980s, its original funtion being video delivery over opper (twisted pairs). theoretially an deliver three VHS hannels or one MPEG hannel with digital stereo sound; early ADSL systems were not apable ensure quality video servies, and the emphasis was put on high bit rate data ommuniations high speed internet aess; works on a single pair of twisted wire and allows the standard telephone servie; o ADSL G.lite is a simplified variant of ADSL for domesti users whih an deliver in downstream up to 1,5Mbps and up to 500kbps in upstream ; the onnetion to the telephone line is simpler. o RADSL (Rate Adaptive Digital Subsriber Line) allows an adaptation of the transmission rate up to 7Mbps in downstream and up to 1Mbps in upstream ; The transeiver adapts automatially the bandwidth assigned to upstream and downstream transmission in order to obtain the highest possible effetive rate (small files an be transmitted at a higher rate than large files); Allows both symmetrial and asymmetrial appliations; o VDSL (Very high bit rate Digital Subsriber Line) transfer rates between 5Mbps and 50Mbps an be obtained over low distanes se pot (ten meters, maximum hundred meters), that is from the opti able to the user; it an be onfigured also for symmetri transmissions; it is intended for university ampuses, industrial parks where it is a small distane to the fiber opti terminal; VDSL standardization ativities started in 1995; in 1997 an assoiation led by British Teleom developed the first VDSL requirements speifiations; the standardization ativity was delayed due to debates related of whih modulation tehnique should be used, DMT or QAM. Different VDSL tehnial assoiations supported different transmission tehnologies VDSL Alliane, a partnership between Alatel, Texas Instruments and other supported DMT tehnology, VDSL Coalition, led by Luent and Broadom, supported QAM-CAP tehnology. Finally DMT tehnology won out, and in

9 G standard (003) (VDSL1), the main transmission tehnique is DMT, the QAM tehnique being one alternative possibility; o In the table are presented some parameters onerning the ADSL and VDSL tehniques. Tab. o ADSL aess and main harateristis loal exhange distribution frame; onentrator subsriber ADSL FTTNode fiber opper opper FTTCurb FTTHomee FTTBuilding Fig. 6 ADSL aess senarios Fig. 7 ADSL bit rates and overage

10 TF exhange (POTS) ADSL modem omputer telephone (POTS) or Dial-Up modem splitter-r Fig. 8 ADSL aess arhiteture DSL HPF subsriber loop abonat splitter-l server/digital paket swithing equipment HPF DSL POTS LPF LPF POTS entral offie Fig. 8.1 ADSL aess arhiteture o Bandwidth alloation of ADSL transmissions subsriber PSD RX filter spetral distribution seen by the exhange PSD TX filter spetral distribution seen by the subsriber TX TX RX POTS upstream downstream f RX POTS upstream downstream f Fig. 9.1 Bandwidth alloation of ADSL transmissions if the transmission diretions are separated using eho ompensation PSD spetral distribution seen by the exhange PSD spetral distribution seen by the subsriber RX TX POTS upstream downstream f POTS upstream downstream f Fig. 9. Bandwidth alloation of ADSL transmissions if the transmission diretions are separated using frequeny multiplexing TX RX

11 FILT. ECOU digital DSP omponents D/A A/D upstream AMP TX FILT. onverters, analog filters and amplifiers +nonlinear distortions downstream hybrid +eho upstream FILT. ECOU D/A A/D downstream AMP RX FILT. +nonlinear distortions hybrid upstream +eho downstream Fig ADSL modem line interfae transmission diretion separation using eho ompensation digital DSP omponents D/A AMP TX FILT. A/D wanted RX signal AMP onverters, analog filters and amplifiers driver FILT. RX internal+ external TX signal eho TX + wanted RX external passive filter with high dynami range hybrid Fig. 10. ADSL modem line interfae transmission diretion separation using frequeny multiplexing H(f) (db) o Transmission -60 parameters distortions harateristi to ADSL transmissions Fig Cable frequeny transfer harateristi ,4 mm diameter and 4,5km length able f (khz)

12 Central offie Feeder able Distribution able Bridged taps Slie Fig. 11. Typial ooper loop onfiguration Fig o The modulation tehniques used in ADSL/VDSL tehnologies are CAP and DMT (Disrete Multi-Tone) o The priniples of DMT modulation are explained in the following figures:

13 X 1 g(t) j f1t e π j ft e π Q I X i X 1 e e j π j π 1 N N n n X g(t) digital implementation X X N/ g(t) j fn / t e π Fig. 1 Priniples of DMT modulation o the sub-arriers are orthogonal; subarrier separation=4,315khz=dmt symbol period = sub-hannel symbol period; 55 sub-arriers are used in the ase of ADSL systems; maximum number of bits / tone 15. o The frequeny bands oupied by the downstream and upstream hannels when frequeny multiplexing is used for transmission diretion separation 5 tones for upstream; 15 tones for downstream. Power spetral density X N/ e j π N / N n Fig. 13 Tone and frequeny alloation for ADSL transmissions frequeny multiplexing used for transmission diretions separation o RADSL assigns adaptively the frequeny bands (and tones) for upstream and downstream. o Adaptation of DMT modulation to the hannel frequeny transfer harateristi

14 bits / sub-arrier for ideal hannel line frequeny transfer harateristi + distortions bits / sub-arrier for real hannel a) b) ) interferene Fig. 14 Adaptation of the DMT modulation to the hannel transfer harateristi o DMT has superior performanes omparatively to the CAP modulation - the seletive assignment of the number of bits/tone ensures an equalization of the hannel and it is possible to do a fine adaptation of the transmission rate to the hannel distortions binary rate D M T overage C AP C A P D M T subsriber loop length Fig. 15 Comparison between DMT and CAP in what onern the possibility to adapt the transmission rate and the maximum length of the subsriber line o VDSL frequeny alloation plan Fig. 16

15 o VDSL spetral mask and spetral ompatibility with xdsl Fig. 17 Fig. 18

16 o VDSL performanes Fig. 19. Fig. 19.1

17 o New DSL tehnologies o ADSL o Extra failities provided by the ADSL standard higher transmission bit rate; larger overage area; more flexible adaptation of the transfer rate to the hannel harateristis; hannel diagnosti failities; stand-by funtioning mode; o ADSL ensures a bit rate up to 1Mbps in downstream and up to 1Mbps in upstream ; these performanes are obtained by the inrease of the modulation effiieny, the derease of framing overhead, by larger oding gain (ADSL uses a 4 dimensional trellis oded modulation with 8 states ADSL uses a 4 dimensional trellis oded modulation with 16 states), by improvement of the initialization proess and by more omplex signal proessing: there are used modulations with 1 bit per symbol important for long lines; reordering of the tones based on data provided by the reeiver allows the spreading of the interferenes; variable overhead (fixed overhead in ADSL standard 3kbps; variable in ADSL standard, overhead between 4kbps and 3kbps); more flexible RS ode; o The initialization of the ADSL has a lot of improvements: apabilities to derease the transmission power at both ends to redue the rosstalk; dynami alloation of the pilot tones to avoid spetral zeros inserted by the bridged taps and radio interferenes; adaptive hange of the initialization period for optimal training of different omponents; adaptive hoose of tones (subarriers) used in to transmit the initialization messages to avoid the spetral zeros inserted by bridged taps and radio interferenes; suppression of non used tones during the initialization period to allow the measurement and suppression of radio interferenes; o fig. 0 shows a omparison between the bit rates are overage area of the ADSL and ADSL systems Fig. 0 Comparison between the bit rate and overage area of ADSL and ADSL tehniques

18 o in the ase of ADSL modems are provided diagnosti and monitoring failities; an be measured the bakground noise, the loop attenuation, signal to noise ratio; the diagnosti information are sent to the entral offie and are used for servie quality monitoring. o another signifiant improvement brought by ADSL tehnique is related to power management the ADSL modems from first generation operate ontinuously in the fullpower mode, the so alled L0 mode (even when nothing is transmitted) signifiant onsumption of energy whih is a problem espeially in the ase of abinet based ADSL there are problems related to power supply and heat dissipation; the ADSL tehnique brings two new power management tehniques, namely: (see fig. 1): L low-power mode : dediated to the ATU-C modem loated in the CO (or abinet) it is allowed the fast entering and exiting of the low-power mode based on the data traffi on the onnetion if the traffi is large the modem works ontinuously in the full power mode (L0) to maximize the transmission speed; in the moment when the traffi dereases the modem enters the L operation mode and a redued bit rate is used to transmit the swithing between the two funtioning modes is realized instantaneously and without affeting the error probability or the servie quality; L3 low-power mode : dediated both to modems ATU-C and ATU-R the modem enters a sleep mode in the moment when no traffi is deteted on the onnetion for a longer time interval; transition to normal operation requires approximately 3s Fig. 1 Power management apabilities harateristi to ADSL tehnique o another important faility of the ADSL tehnique is the so alled seamless rate adaptation (SRA), meaning a ontinuous adaptation of the bit rate it is possible to modify ontinuously the bit rate during the transmission, without any interruption of the servie or modifiation of the bit error probability (the standard ADSL systems allows the modifiation of the bit rate only at the beginning of the transmission) hanges in the hannel state are deteted and the bit rate is adapted to the new onditions it is based on the separation of modulation and framing proessing the bit rate is modified without affeting the framing (synhronization) the bit lok synhronization is maintained in this proess it is used the so alled OLR proedure online reonfiguration shortly this proedure ats in the following way: the reeiver monitors the SNR on the hannel and deide if it is neessary a hange of the transmission rate on the hannel to ompensate the hannel modifiations;

19 the reeiver transmits a message to the transmitter to initiate the bit rate hanging proedure. This message inludes all the data neessary for the transmission of the new bit rate and speifies the number of bits per symbol and the power on eah tone (subarrier); the transmitter sends a Syn Flag signal whih is used as a marker and identifies exatly the moment of the bit rate hange. the marker is deteted by the reeiver and both ends of the ommuniation hange the bit rate transparently to the provided servie; o The inrease of the bit rate an be ahieved by oupling several telephone lines in a single ADSL onnetion (bonded ADSL) it is an important faility of the ADSL tehnique; it is neessary the insertion of a multiplexing/demultiplexing layer whih allows the distribution of a larger flow on different physial onnetions bonded ADSL+ was introdued in 005. o ADSL offers the possibility to split the bandwidth in hannels with different harateristis for different appliations. For ex. ADSL allows simultaneous support for voie appliations higher allowed bit error probability but smaller delay and data appliations smaller allowed bit error probability but larger delay. This apaity for hannelization provides support for CVoDSL Channelized Voie over DSL transparent transmission method of TDM voie hannels with DSL tehnique. 64kbps bit rate hannels are reserved for transmission of DS0 to CO or to a multiplexer see fig.. Fig. CVoDSL tehnique for transmission of TDM voie hannels using the DSL tehnique o Additional benefits offered by the ADSL tehnique: Improved interoperability additions to the initialization proedures offer a better interoperability between equipments with hips from different suppliers; Fast initialization ( startup ) the initialization time an be redued from 10s (standard ADSL) to 3s (ADSL); All-digital mode optional mode in whih the standard telephone band is used for data transmissions the bit rate an be inreased with 56kbps; Support for paket based servies it is provisioned a speial onvergene level whih allows paket based servies (for ex. Ethernet) to be transported over ADSL; o ADSL+ It is an extension of the ADSL standard; ADSL speifies a downstream band of 1.1MHz or 55kHz (ADSL G.lite.bis), and ADSL+ speifies a band of.mhz signifiant inrease of the downstream bit rate for lines shorter than approximately 5000 feet; the upstream bit rate is around 1Mbps, aording to the transmission hannel parameters (length of the line); see fig. 3 for frequeny bands alloation for ADSL+;

20 in fig. 4 is presented a omparison between the bit rates ensured by ADSL and ADSL+ tehniques aording to the length of the line. ADSL+ an be used to derease the rosstalk by using only the frequeny band loated between 1.1MHz and.mhz ADSL+ ould be an option for abinet based DSL (shorter lines), and ADSL for entral offie based DSL ; see fig. 5 relatively to the problem of rosstalk. Comparisons between the ADSL, ADSL and ADSL+ downstream bit rates for situations with and without rosstalk are presented in fig. 6.1 and 6.. Fig. 3 Frequeny bands alloation for ADSL and ADSL+ Fig. 4 Downstream bit rates provided by ADSL and ADSL+ tehniques aording to the length of the line Fig. 5 Derease of the rosstalk by appropriate alloation of the ADSL+ downstream frequeny bands

21 Fig. 6.1 Comparison between the downstream bit rates ensured by ADSL, ADSL and ADSL+ tehniques in white noise onditions for different loop lengths Fig. 6. Comparison between the downstream bit rates ensured by ADSL, ADSL and ADSL+ tehniques in rosstalk onditions for different loop lengths o VDSL o Is the newest and most advaned DSL tehnology; was designed to support a wide ategory of servies like voie, video, data, high definition TV (HDTV), interative gamming. o It is speified by the ITU-T G.993. standard and it is an enhanement of the G (VDSL1) standard. It is a omplex standard with numerous profiles and frequeny bands adapted to various bit rate and reah requirements. o Allows symmetrial and asymmetrial full duplex transmissions up to bit rates of 00Mbps on twisted pairs using a bandwidth of 30MHz. The maximum bit rate of 50Mbps at soure deteriorates quikly to 100Mbps at 0.5km and 50Mbps at 1km, but

22 after that the maximum bit rate degrades slowly and at a 1.6 km distane from the soure the performanes are idential with those of ADSL+. o The VDSL standard inludes many of the features and funtionalities ontained in the ADSL+ standard. It is about advaned diagnosti failities, advaned management, ability to maximize the bit rate and the effiieny of bandwidth use VDSL is an ideal tehnology for video servies. o VDSL allows an ADSL like long reah, meaning 1 4 Mpbs at de 4, 5 km distane, in whih ase we have an ADSL+ type funtioning mode. VDSL type systems, unlike VDSL1 type systems, are not limited to short loops. o The transmission profiles defined for the VDSL tehniques are presented in fig. 7. These profiles haraterize the alloated bandwidth, the number of tones (subarriers), used espeially in downstream, the tone separation, the transmit power and the bit rate, used espeially in downstream. Fig. 7 VDSL transmission profiles o Alloation of the frequeny bands is realized aording to the plans presented in fig. 8. Fig. 8 Frequeny band alloation plans for VDSL1 and VDSL tehniques

23 o VDSL features and performanes o The typial reah of VDSL1 systems is around 3kft (3000 feet); one of the VDSL systems basi harateristi is the larger reah, around 9kft some harateristis of ADSL systems are inluded in VDSL. o VDSL is uses ompletely different initialization proedures of those used by VDSL1 (hannel measurement, and modem training); the VDSL system allows the use of the US0 frequeny band (used by ADSL systems) on long hannels. o The VDSL system provides an improved protetion against impulse noise, one of the most important distortions whih affet DSL transmissions. The VDSL system allows the orretion of error pakets generated by impulses with length between 50µs and 3,75ms. o VDSL ensures support for servies based on paket transmissions (for ex. Ethernet or IP). Evolved management failities allow the transmission on the same physial layer of pakets with different priorities and lengths. VDSL ensures also support for transmission on the same physial hannel of servies with ompletely different requirements (bit error probability, delay, protetion against impulse noise). o VDSL ensures evolved hannel diagnosti and fault identifiation. o VDSL ensures improved ompatibility between equipments with different hip-set and ompatibility with ADSL and ADSL tehniques. o In fig. 9 it is presented a omparison between the performanes (bit rate / reah) of VDSL, VDSL1, ADSL+ tehniques. Fig. 9 Comparison between performanes of VDSL, VDSL1, ADSL+ tehniques The VDSL tehnique must provide full duplex onnetions with 100Mbps bit rate at 350m distane and with 30Mbps at km distane.