Training optimization of multiple lines in a vectored system using a prepared-to-join group

Abstract

A method for initialization of a group of customer premises equipment devices (CPEs) during a training that registers capabilities of the CPEs is disclosed, wherein at least one CPE registers late to the training and cannot be registered. The method includes determining capabilities of the CPEs during a joining phase of the training, wherein it is determined whether a CPE device is capable of employing vectoring, and placing in a hold status the at least one CPE that registers late by keeping a line active that is coupled to the at least one CPE. The method further includes providing another joining phase after the joining phase to register the at least one CPE that registers late.

Claims

1. An apparatus for initialization of a group of customer premises equipment devices (CPEs) to a vectored group during a joining phase, wherein at least one CPE registers after the joining phase is finished, the apparatus comprising: a vector control entity (VCE) configured to register the at least one CPE that registers after the joining phase is finished in a hold status that assigns the at least one CPE to a prepared-to-join group; and a transmitter configured to transmit signals to the at least one CPE.

2. The apparatus according to claim 1, wherein to register the at least one CPE in a hold status comprises to keep a line coupled to the at least one CPE active.

3. The apparatus according to claim 2, wherein keeping a line active includes: to send a signal to the at least one CPE without impacting performance of a joining line connection, or to send a signal to the at least one CPE without impacting performance of a line in Showtime.

4. The apparatus according to claim 3, wherein the signal includes a Flag Tone.

5. The apparatus according to claim 3, wherein to send a signal to the at least one CPE is applied to a first joining phase following system startup.

6. The apparatus according to claim 2, wherein to keep the line active comprises to keep the line coupled to the at least one CPE device.

7. The apparatus according to claim 6, wherein the VCE is configured to provide another joining phase after the joining phase to register the at least one CPE that registers after the joining phase is finished.

8. The apparatus according to claim 1, wherein the VCE is configured to generate signals to the at least one CPE that indicates that the at least one CPE will be registered at a later time.

9. The apparatus according to claim 1, wherein the VCE is configured to provide another joining phase after the joining phase to register the at least one CPE that registers after the joining phase is finished.

10. The apparatus according to claim 1, wherein either one, multiple, or all members of the prepared-to-join group are transferred to a joining group and trained in parallel when an ongoing joining phase is finished.

11. The apparatus according to claim 1, wherein initialization procedures of the CPEs are aligned.

12. The apparatus according to claim 1, wherein the group of CPEs includes initializing CPEs and vectored CPEs.

13. The apparatus according to claim 1, wherein a line is kept active by sending a signal to at least one of CPE devices which are collected in the prepared-to-join group.

14. The apparatus according to claim 13, wherein the signal does not corrupt an orthogonal sequence of sync symbols.

15. The apparatus according to claim 13, wherein the signal does not corrupt user data of data symbols.

16. The apparatus according to claim 13, wherein a new state is defined between state O-P-QUIET and O-P-VECTOR-1, wherein the line is kept active during the new state.

17. The apparatus according to claim 1, wherein all CPEs which have started to setup a connection are grouped for a joining phase in a configurable time interval.

18. The apparatus according to claim 17, wherein the configurable time interval comprises an acceptance window.

19. The apparatus according to claim 18, wherein CPEs which finish handshake after a first acceptance window time interval are neglected for a current joining phase.

20. The apparatus according to claim 19, wherein CPEs which finish handshake after an acceptance window time interval are neglected for a current joining phase.

21. The apparatus according to claim 20, wherein lines which terminate handshake within a sum interval of a TSAmin time and a TSAplus time after the CPEs of the first acceptance window have reached Showtime are accepted for a next joining phase, a second acceptance window.

22. The apparatus according to claim 21 wherein the TSAmin time is a minimum time for crosstalk adaptation in Showtime, and wherein the TSAplus time is an additional time for crosstalk adaptation in Showtime.

23. The apparatus according to claim 1, wherein the at least one CPE joins in one of multiple joining phases.

24. The apparatus according to claim 1, the CPE in the hold status is not in handshake.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The system may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views.

(2) FIG. 1 illustrates the various types of crosstalk typically experienced in a DSL system;

(3) FIG. 2 is a flowchart giving an overview of the ITU-T G.993.5 initialization procedure;

(4) FIG. 3 is a flowchart illustrating the mechanism responsible for a CPE missing the Joining Phase;

(5) FIG. 4 is a flow chart illustrating one improved initialization method in accordance with one example of the invention and the embedding of the method in the standardized VDSL2 training including vectoring;

(6) FIG. 5 illustrates a typical sequence of multiple joining phases during the start-up of a vectoring capable VDSL2 system comprising multiple lines;

(7) FIG. 6 represents an exemplary VDSL2 communication system in accordance to the present invention.

DETAILED DESCRIPTION

(8) FIG. 1 illustrates the various types of crosstalk typically experienced in a DSL system. For simplicity, central office (CO) 110 comprises two transceivers 102, 106 communicating over two subscriber lines to two sets of customer premises equipment (CPE) devices 104, 108. Transceiver 102 communicates with ORE device 104, and transceiver 106 communicates with ORE device 108. As an illustrative example, the crosstalk from CO transceiver 106 and ORE device 108 to either CO transceiver 102 or ORE device 104 is described. However, it should be understood that interference may also be between the transmitter and receiver on the same subscriber line in both the upstream and downstream paths, which is the near-end echo of the transmit signal.

(9) The term “far-end” refers to scenarios in which the source of interference is away from the receiving side, and the term “near-end” refers to scenarios in which the source of interference is close to the receiving side. For example, interference shown by arrow 112 illustrates noise generated by transceiver 106 coupled into the downstream communications and received by ORE device 104. The term “victim” of “victim user” refers to the line or the circuit being examined for crosstalk, and the term “disturber” describes the source of the crosstalk. Since the noise is generated away from the receiving side, this is referred to as downstream far-end crosstalk (FEXT). Likewise, interference shown by arrow 114 illustrates upstream near-end crosstalk (NEXT). Interference shown by arrow 116 illustrates upstream FEXT, and interference shown by arrow 118 illustrates downstream NEXT. In particular, FEXT is a ubiquitous source of noise in VDSL. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies, such as mitigating FEXT. Vectoring is a transmission method that employs the coordination of line signals for reduction of crosstalk levels and improvement of performance. The degree of improvement depends on the channel characteristics. Vectoring may be for a single user or for multiple-users' benefit.

(10) The ITU recommendation G.993.5 covers self-FEXT (far-end crosstalk) cancellation in the downstream and upstream directions. It defines a single method of self-FEXT cancellation, in which FEXT generated by a group of near-end transceivers and interfering with the far-end transceivers of that same group is cancelled. This cancellation takes place between VDSL2 transceivers, not necessarily of the same profile. The recommendation G.933.5 is intended to be implemented in conjunction with ITU-T G.993.2. Multi-pair digital subscriber line (DSL) bonding (see b-ITU-T G.998.1, b-ITU-T G.998.2, and b-ITU-T G.998.3) may be implemented in conjunction with vectoring.

(11) The techniques described in the recommendation G.993.5 provide means of reducing self-FEXT generated by the transceivers in a multi-pair cable or cable binder. Self-FEXT cancellation techniques are particularly beneficial with short cable lengths (<1 km) and limited near-end crosstalk (NEXT), background noise, and FEXT from systems which are not a part of the vectored group (alien noise). The level of non-self-FEXT noise sources relative to that of self-FEXT sources determines the degree to which self-FEXT reduction can improve performance. Another significant factor is the degree to which the self-FEXT cancelling system has access to the disturbing pairs of the cable. Maximum gains are achieved when the self-FEXT cancelling system has access to all of the pairs of a cable carrying broadband signals. For multi-binder cables, significant gains are possible when the self-FEXT cancelling system has access to all of the pairs of the binder group(s) in which it is deployed and has the ability to can-eel at least the majority of dominant self-FEXT disturbers within the binder. When multiple self-FEXT cancelling systems are deployed in a multi-binder cable without binder management, gains may be significantly reduced.

(12) FIG. 2 is taken from the recommendation G.993.5 describing the initialization of a connection supporting Vectoring. The initialization procedure described in in FIG. 2 is based on ITU-T G.993.2 initialization with additional steps for FEXT channel estimation. The final mode of vectored operation (Le, downstream and upstream vectoring, or downstream only vectoring) is determined during the ITU-T G.994.1 Phase of initialization. FIG. 2 provides an overview of the initialization procedure for both upstream and downstream directions. For recommendation G.933.5, the ITU-T G.993.2 initialization phases are adopted with some modifications to the SOC messages and addition of initialization signals for FEXT channel estimation. The initialization signals added to the ITU-T G.993.2 Channel Discovery phase and Training phase are highlighted in FIG. 2 by bold ones.

(13) If several lines are initialized simultaneously, the initialization procedures of these lines have to be aligned in time, so that all lines pass the vectoring-related phases simultaneously as described in clauses 10.3.3.6 and 10.4.3.9 of the recommendation G.933.5.

(14) In the downstream direction, at the beginning of the Channel Discovery phase, the VTU-O of the initializing line transmits O-P-VECTOR 1 signal which comprises only sync symbols modulated by the pilot sequence and which is aligned with sync symbols of vectored lines. The O-P-VECTOR 1 signal allows the VCE (see also FIG. 6) to estimate FEXT channels from the initializing lines into the vectored lines. The VCE estimates these FEXT channels based on the reported clipped error samples from the VTU-Rs of the vectored lines and enables the pre-coding in the VTU-Os of these vectored lines to cancel FEXT from the initializing lines into these vectored lines during the remainder of the initialization of the initializing lines. At the beginning of the Training phase, the initializing VTU-O will transmit O-P-VECTOR 1-1 signal, which is the same as O-P-VECTOR 1 and allows the VCE to update the downstream FEXT channel estimates from the initializing lines into the vectored lines, prior to transitioning into the ITU-T G.993.2 Training phase.

(15) After the ITU-T G.993.2 Training phase, the VTU-O transmits the O-P-VECTOR 2 signal, followed by the O-P-VECTOR 2-1 signal, which both comprise sync symbols modulated by the pilot sequence and regular symbols carrying the SOC. During the transmission of O-P-VECTOR 2-1, the VCE estimates FEXT channels from all vectored lines into each initializing line and vice versa. Finally, at the end of the transmission of O-P-VECTOR 2-1, the whole FEXT channel matrix, including FEXT coefficients from the initializing line into the vectored lines and FEXT coefficients from the vectored lines into each initializing line, is estimated by the VCE. At this point the initialization process is complete and the initializing lines may be included in the precoding operation. After O-P-VECTOR 2-1 transmission is complete, the VTU-O of the initializing line enters the Channel Analysis and Exchange phase for estimation of the SNR and determination of the bit loading to be used during Showtime.

(16) In the upstream direction, in order to avoid excessive FEXT into vectored lines, the VTU-R of an initializing line, after detection of the O-SIGNATURE message in the Channel Discovery phase, starts transmitting an R-P-VECTOR 1 signal, which has the same format as O-P-VECTOR 1. During transmission of the R-P-VECTOR 1, the VCE estimates the FEXT channels from the initializing lines into all vectored lines, and enables the VTU-Os of the vectored lines to cancel FEXT from the initializing lines during the remainder of the initialization of the initializing lines. The time position of the upstream sync symbols and the upstream pilot sequence are assigned by the VCE and are indicated to the VTU-R in the O-SIGNATURE message and by special markers added to the O-P-CHANNEL DISCOVERY V1 signal.

(17) Furthermore, other optional parameters may be added to the O-P-SIGNATURE message for upstream transmit power reduction during the initial upstream phase (R-P-VECTOR 1). The upstream transmit power reduction can be used to reduce the crosstalk of the R-P-VECTOR 1 signals into non-vectored lines operating in the same binder and provides a flat attenuation of the upstream transmit PSD of R-P-VECTOR 1 in addition to the standard upstream power back-off as defined in ITU-T G.993.2.

(18) At the beginning of the Training phase, the initializing VTU-R will transmit the R-P-VECTOR 1-1 signal, which is the same as R-P-VECTOR 1 and allows the VCE to update the upstream FEXT channel estimates from the initializing lines into the vectored lines, prior to transitioning into the ITU-T G.993.2 Training phase. The VTU-O transmits the O-P-VECTOR 1-1 signal as a time fill signal while the VTU-R transmits R-P-VECTOR 1-1.

(19) The initial value of timing advance is assigned by the VTU-O and is communicated in O-SIGNATURE, based on the provisional knowledge on the length of the line. If the timing advance is further re-adjusted during the Training phase, then the FEXT channel estimate in the upstream direction will be updated at the end of the Training phase to account for any resulting change in the FEXT channel (signal R-P-VECTOR 1-2 in FIG. 2). The VTU-O transmits the O-P-VECTOR 2 signal as a time fill signal while the VTU-R transmits R-P-VECTOR 1-2.

(20) At the end of the Training phase, the VTU-R transmits R-P-VECTOR 2, which comprises Sync symbols modulated by the pilot sequence and regular symbols carrying the SOC. During the transmission of R-P-VECTOR 2, the VCE estimates the FEXT channels from all vectored lines into the initializing lines and vice versa. Finally, at the end of the R-P-VECTOR 2 transmission, the whole FEXT channel matrix, including FEXT coefficients from the initializing lines into the vectored lines and FEXT coefficients from vectored lines into the initializing ones, are estimated by the VCE. At this point the initialization process is complete and the initializing lines become active members of the vectored group. After R-P-VECTOR 2 transmission is complete, the VTU-R enters the Channel Analysis and Exchange phase for estimation of the SNR and determination of the bit loading to be used during Showtime. During the transmission of R-P-VECTOR 2, the SOC parameters may be set to provide higher speed SOC, necessary to convey cupped error samples from the VTU-R to the VTU-O. Since both VTU-O and VTU-R already passed the Training phase, the number of repetitions in the SOC may be reduced (similarly to ITU-T G.993.2 during the Channel Analysis and Exchange phase). This will provide a fast backchannel which is necessary for quick estimation of FEXT channels from vectored lines into the initializing line.

(21) As mentioned in the introductory part, the weights of the upstream and downstream crosstalk transfer functions are estimated in the sync symbols. The data carded in these sync symbols has to be orthogonal from line to line. This orthogonality of the sync symbols should not be corrupted in certain phases of an ongoing training by lines which try to setup a connection after just having finished handshake. To avoid corruption of the orthogonality in the sync symbols while a vectoring training is already ongoing (Joining Phase), the CO stops sending any tone to those CPE devices which try to setup a connection as soon as handshake has been finished (Quiet Phase) whereupon these CPE devices interrupt theft connection attempt and thus have to start another training attempt after an undefined time. The described behavior, which is depicted in FIG. 3, leads to long and unpredictable training times in case that multiple lines want to join a vectoring system, i.e. if multiple CPE devices start establishing a connection with slightly different starting times. FIG. 3 describes the system behavior in case that a vectoring training is already ongoing (Joining Group) while an additional CPE device has started to connect (Joining Phase Missed).

(22) In accordance with one example of the invention, lines (i.e. CPEs connected to the CO via those lines) which attempt to setup a connection while a joining process is already ongoing (and thus could not be considered for the current Joining Phase) are collected in the “Prepared-To-Join”-group for the Joining Phase following the ongoing one. Collecting in the “Prepared-To-Join” group places the CPE device(s) in a hold status. According to the capability of the vectoring engine (i.e. the vectoring control entity VCE, see FIG. 6) either one, multiple, or even all of the members of the “Prepared-To-Join”-group are transferred to the Joining Group and trained in parallel as soon as the ongoing Joining Phase has been finished. As needed, other lines can, from now on, be again collected in the “Prepared-To-Join”-group for the next joining phase.

(23) To keep the connection active, the CO sends the “O-P-Pre-VECTOR 1” signal to those CPE devices which are collected in the “Prepared-To-Join”-group. The “O-P-Pre-VECTOR 1” signal shall neither corrupt the orthogonal sequence of the sync symbols nor the user data of the data symbols.

(24) In order to be able to train as many lines as possible in parallel right at the beginning, the O-P-Pre-VECTOR 1 phase can also be applied to the very first Joining Phase following the system startup. Following this procedure, the training time in case of multi-line joining can be significantly reduced without impacting the performance of those VDSL2 connections, which are already in Showtime. The training of a line which attempts to setup a connection while a joining phase is already ongoing has no longer to be interrupted and thus a restart including handshake (see G.994.1) is no longer needed what in case of multiline joining additionally leads to a more robust and reproducible training. As CO stands for Central Office, it may designate any component of Central Office Equipment such as a Digital Subscriber Line Access Multiplexer (DSLAM) or a linecard of a DSLAM. In fact, the vectoring control entity may be arranged on a line card used in a DSLAM or in a module of a DSLAM servicing more than one linecard.

(25) The Joining Phases may include a handshake procedure, which is compliant with the ITU-T G.994.1 standard. The mentioned training (i.e. the Training Phase) is accomplished in compliance with the ITU-T G.992.3 and 993.5 standards. The CPE device(s) of the “Prepared-to-Join” Group are placed in a hold status by keeping the line(s) active that is coupled to the respective CPE device(s). Keeping a line active is accomplished by sending a signal to each CPE device, wherein the signal does neither impact the joining lines nor the lines (CPE devices) in Showtime. For example, the signal includes only Flag Tones in its sync symbols, e.g. when all Pilot Tones are used for the crosstalk adaption. However, in cases in which not all Pilot Tones are used for crosstalk adaption, also the unused Pilot Tones may be used instead of or in addition to the Rag Tones.

(26) The embedding of this procedure in the standardized VDSL2 training including vectoring is depicted in FIG. 4. As compared to the existing standard the following extensions of the vectoring training are proposed: (1) definition of the “Pre-pared-To-Join”-group, (2) definition of a new state (O-P-Pre-VECTOR 1) between state O-P-QUIET and O-P-VECTOR-1, and (3) definition of the O-P-Pre-VECTOR 1 signal (flag tones only) from the CO to the CPE(s).

(27) Accordingly, the improved procedure described herein provides a method for initialization of a group of CPE devices (short: CPEs) during a training that in part registers capabilities of the CPEs, wherein at least one CPE registers late to the training and thus cannot be registered for the reasons described above. In accordance with the example described herein, the method comprises: determining capabilities of the CPEs during a Joining Phase of the training, wherein it is determined whether a CPE device is capable of employing vectoring. The method further comprises placing in a hold status (i.e. assign to the “Prepared to Join”-group) the at least one CPE that registers late by keeping a line active that is coupled to the at least one CPE. Another Joining Phase is provided after the Joining Phase in order to register the at least one CPE which has registered late. The step of placing in a hold status may maintain the line as active by sending signals to the at least one CPE that indicates that the at least one CPE will be registered at a later time.

(28) The method described above may be implemented in an apparatus which includes a vector engine that is configured to place the at least one CRE, that registers late, in a hold status (Le, assign to the “Prepared to Join”-group) by keeping a line active that is coupled to the at least one CPE. The vector engine provides another Joining Phase after the Joining Phase to register the at least one CPE that registers late.

(29) FIG. 5 depicts a typical sequence of multiple joining phases during the start-up of a vectoring capable VDSL2 system comprising multiple lines. The Acceptance Window n corresponds to a configurable time interval right after reset. In this time interval all lines which have started to setup a connection are collected for the first Joining Phase (Line 1 to Line k). The Acceptance Window n+1 is opened for collecting those lines for the second Joining Phase which have started to setup a connection while Line 1 to Line k are being trained (Line k+1 to Line k+m). In this example the last line (Line k+m+1) tries to set-up a connection while the second joining group is being trained (Acceptance Window n+2). The timings outlined FIG. 5 are for controlling the size of the Acceptance Windows. In the timing diagrams of FIG. 5 TDmin is the minimum time for O-P-Pre-VEC-TOR-1. This configurable time interval Is re-started with every “GHS_COMPLETE” message Issued In Acceptance Window n. TDmax denotes the maximum time for O-P-Pre-VECTOR-1. Lines finishing G.hs after this configurable time interval are neglected for the current joining Phase. Lines which terminate G.hs within TDmax after the previous line has reached OP-Pre-VECTOR-1 and within TDmax after the first line of the current Joining Phase has reached O-P-Pre-VECTOR-1 are accepted for the current Joining Phase (Acceptance Window n). The time TSAmin is the minimum time for xTalk adaptation in Showtime. This time interval is hard-coded in the firmware. TSA.sub.PIUS is an additional time for xTalk adaptation In Showtime. This configurable time interval Is for further fine-tuning of the xTalk coefficients. Lines which terminate G.hs within TsAmin+TSA.sub.PIUS after the lines of Acceptance Window n have reached Showtime are accepted for the next Joining Phase (Acceptance Window n+1).

(30) FIG. 6 illustrates a DSLAM (Digital Subscriber Line Access Multiplexer) 100 located at a CO (central office) or at a remote location closer to subscriber premises, and comprising G.993.8 compliant transceiver units 101 (or VTUC1), 102 (or VTUC2) and 103 (or VTUC3), a line initialization controller 111 (or CTRL), and the above-mentioned vectoring engine or vectoring control entity 112 (or VCE). The line initialization controller 111 as we as the vectoring control entity 112 are coupled to the transceivers units 101, 102 and 103. Regardless of the actual location of the DLSAM it is regarded as part of Central Office Equipment (COE).

(31) The DSLAM 100 is coupled to CPE devices via Unshielded Twisted Pairs (UTP), such as CAT3 cables. The first transceiver unit 101 is coupled to a first remote transceiver unit 201 (or VTUR1) via a first subscriber line L1; the second transceiver unit 102 is coupled to a second remote transceiver unit 202 (or VTUR2) via a second subscriber line L2; and the third transceiver unit 103 is coupled to a third remote transceiver unit 203 (or VTUR3) via a third subscriber line L3. The remote transceiver units 201, 202 and 203 form part of e.g. a modem, a gateway, a router, a set top box, a laptop, etc.

(32) The subscriber lines L1, L2 and L3 are bundled in a cable binder 301 together with further subscriber lines, and induce crosstalk into each other as they are in close vicinity over whole or part of theft length. In DMT based DSL systems, crosstalk mostly reduces to FEXT (Far-End Crosstalk); some substantial amount of the signal transmitted by a transceiver unit (the disturber) couples into a neighboring line and impairs reception of the direct signal transmitted over that neighboring line at a remote transceiver unit (the victim). For instance, the downstream signal transmitted by the VTUC 101 over line L1 couples into line L2 and is detected as noise by the VTUR 202. Also, the upstream signal transmitted by the VTUR 203 over line L3 couples into line L1 and is detected as noise by the VTUC 101.

(33) The DSLAM 100 further includes a precoder to mitigate FEXT in downstream direction and/or a FEXT canceller to mitigate FEXT in upstream direction. Presently, the subscriber lines L1, L2 and L3 form part of the same vectoring group, which may comprises further subscriber lines, and the precoder and/or FEXT canceller are configured to mitigate downstream and/or upstream crosstalk between lines of the vectoring group.

(34) Typically, the frequency samples of each downstream data symbol of each subscriber line are forwarded to the precoder by the transceiver units, and cross-talk-compensated samples are returned by the precoder to the transceiver units for Inverse Discrete Fourier Transform (IDFT), Digital to Analog Conversion (DAC) and further transmission over the subscriber line. Similarly, the frequency samples of each received upstream data symbol are forwarded to the crosstalk canceller by each transceiver unit, and (almost) crosstalk-free samples are re-turned by the crosstalk canceller to each transceiver unit for detection and demodulation. Primarily, the transceiver units 101, 102 and 103 are configured to terminate the subscriber lines L1, L2 and L3 respectively, and to initialize and operate DSL communication channels CH1, CH2 and CH3, respectively. So are the remote transceiver units 201, 202 and 203.

(35) The transceiver units 101, 102 and 103 are further configured to notify the line initialization controller 111 about a new line starting up (joining line), being on behalf of the transceiver unit at the central office or the transceiver unit at the customer premises, and further to carry out the DSL initialization procedure after approval from the line initialization controller 111.

(36) The communication channels CH1, CH2 and CH3 comprise a downstream data communication path and an upstream data communication path using distinct downstream and upstream frequency bands (frequency division multiplexing). Respective bit loadings and gains for downstream and upstream carriers are determined and agreed upon during line initialization, thereby yielding a total downstream data rate and a total upstream data rate. The DSL initialization procedure comprises a handshake phase, multiple crosstalk acquisition phases, a channel discovery phase, a channel training phase, and a channel analysis and exchange phase.

(37) The handshake phase is described in G.994.1, and makes use of one or more predefined set of carriers (so-called signaling family) depending on the one or more specific annex of the recommendation being supported. Those pre-defined carrier sets comprise very few carriers only (typically 2 or 3), thereby causing little interference on neighboring lines. The handshake procedure comprises a first sub-phase A during which peer transceiver units acknowledge their mutual presence by exchanging probe signals comprising a carrier set and acquire clock synchronization for the probe signals, and a second sub-phase B during which peer transceiver units ex-change their respective capabilities and agree on a common mode for training and operation. A successful completion of the handshake phase will lead to the first crosstalk acquisition phase O-P-VECTOR 1.

(38) All messages in the handshake phase are sent with the one or more limited set of carriers. All carrier frequencies within a carrier set, and all carrier sets, are simultaneously modulated with the same data bits using Differential Phase Shift Keying (DPSK). The transmit point is rotated 180° from the previous point if the transmit bit is a 1, and the transmit point is left unchanged if the transmit bit is a 0.

(39) Initially, the VTUR is in state R-SILENT0 transmitting silence, and the VTUC is in state C-SILENT1 transmitting silence.

(40) For duplex mode of operation, and in the event of the VTUR initiating the initialization procedure, the first handshake sub-phase A starts with the VTUR transmitting a R-TONES-REQ signal from one or both of its signaling family with phase reversals every 16 ms. When this has been detected by the VTUC, the VTUC shall respond by transmitting a C-TONES signal from one or both of its signaling family. When this has been detected by the VTUR, the VTUR shall transmit silence (R-SILENT1) for 50 to 500 ms and shall then transmit a R-TONE1 signal from only one signaling family. When the VTUC has detected R-TONE1 signal, it shall respond by transmitting GALE (0x81=one complement of 0x7E) on modulated carriers (C-GALF1). When the VTUR has detected GALE, it shall respond by transmitting FLAGs (0x7E) on modulated carriers (R-FLAG1). When the VTUC has detected FLAGS, it shall respond by transmitting FLAGs (C-FLAG1). When the VTUR has detected FLAGS, it shall enter the sub-phase B by initiating the first message transaction.

(41) In the event of the VTUC initiating the initialization procedure, the first handshake sub-phase A starts with the VTUC directly transmitting C-TONES and keeps on as aforementioned.

(42) Slightly different timing and signals are defined for half-duplex mode of operation.

(43) The second handshake sub-phase B starts with the VTUR sending a CAPABILITY LIST REQUEST CLR message conveying the capabilities of the VTUR (that is to say, a list of possible modes of operation), and whereby the VTUR further requests the VTUC capabilities. The VTUC replies with a CAPABILITY LIST CL message conveying the VTUC capabilities. The VTUR acknowledges the good receipt of the CL message by returning an ACK(1) acknowledgment.

(44) The sub-phase B carries on by either the VTUR or the VTUC selecting a common mode of operation according to the advertised capabilities. This is achieved by issuing a MODE SELECT MS message conveying the selected mode of operation, and by returning an ACK(1) acknowledgment. Typically, the VTUR selects the most appropriate mode of operation at once and issues the MS message. Yet, the VTUR can request the VTUC to select a particular mode of operation by issuing a MODE REQUEST MR message, or can propose a particular mode of operation while leaving the final decision to the VTUC by issuing a MODE PROPOSAL MP message. Once a particular mode of operation is acknowledged, the VTUC and the VTUR enter the O-P-VECTOR 1 and R-P-VECTOR 1 crosstalk acquisition phases respectively. G.994.1 defines a provision for re-iterating through the sub-phase B by allowing the VTUC to respond to the MS message with a REQUEST-CAPABILITY LIST REQUEST REQ-CLR message requesting the VTUR to proceed again with CLR/CL/ACK(1) message exchange and further MS/MR/MP/ACK(1) message exchange, and without returning to the initial transaction state (R-SILENTO). The line initialization controller 111 is further configured to control the DSL initialization procedure over each subscriber line, and more specifically to supply the VTUCs 101, 102 and 103 with a number n of iterations for the second handshake sub-phase B to be carried out, as well as with an additional delay D for answering any message requiring a specific response or acknowledgment from the VTUC during the sub-phase B, such as a CLR or MS or MR or MP message.

(45) The VTUCs 101, 102 and 103 are further configured to measure the execution times TA and TB of the first and second handshake sub-phases A and B respectively (not including the configured additional delay D, if any), and to report the so-measured execution time to the line initialization controller 111. By so doing, the execution time spread from different CPE manufacturers and/or implementations is accounted for. Although the initialization controller 111 has been depicted as a central unit within the access node 100, it can be partly or wholly distributed across the VTUCs 101, 102 and 103.

(46) Exemplary implementations discussed herein may have various components collocated; however, it is to be appreciated that the various components of the arrangement may be located at distant portions of a distributed network, such as a communications network and/or the Internet, or within a dedicated secure, unsecured and/or encrypted arrangement. Thus, it should be appreciated that the components of the arrangements may be combined into one or more apparatuses, such as a modem, or collocated on a particular node of a distributed network, such as a telecommunications network. Moreover, it should be understood that the components of the described arrangements may be arranged at any location within a distributed network without affecting the operation of the arrangements. For example, the various components can be located in a Central Office modem (CO, ATU-C, VTU-0), a Customer Premises modem (CPE, ATU-R, VTU-R), an xDSL management device, or some combination thereof. Similarly, one or more functional portions of the arrangement may be distributed between a modem and an associated computing device.

(47) The above-described arrangements, apparatuses and methods may be implemented in a software module, a software and/or hardware testing module, a telecommunications test device, a DSL modem, an ADSL modem, an xDSL modem, a VDSL modem, a linecard, a G.hn transceiver, a MoCA® transceiver, a Homeplug transceiver, a powerline modern, a wired or wireless modem, test equipment, a multicarrier transceiver, a wired and/or wireless wide/local area network system, a satellite communication system, network-based communication systems, such as an IP, Ethernet or ATM system, a modem equipped with diagnostic capabilities, or the like, or on a separate programmed general purpose computer having a communications device or in conjunction with any of the following communications protocols: CDSL, ADSL2, ADSL2+, VDSL1, VDSL2, HDSL, DSL Lite, IDSL, RADSL, SDSL, UDSL, MoCA, G.hh, Homeplug or the like.

(48) Additionally, the arrangements, procedures and protocols of the described implementations may be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements), an ASIC or other integrated circuit, a digital signal processor, a flashable device, a hard-wired electronic or logic circuit such as discrete element circuit, a programmable logic device such as PLD, PLA, FPGA, PAL, a modem, a transmitter/receiver, any comparable device, or the like. In general, any apparatus capable of implementing a state machine that is in turn capable of implementing the methodology described and illustrated herein may be used to implement the various communication methods, protocols and techniques according to the implementations.

(49) Furthermore, the disclosed procedures may be readily implemented in software using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation platforms. Alternatively, the disclosed arrangements may be implemented par-tially or fully in hardware using standard logic circuits or VLSI design. The communication arrangements, procedures and protocols described and illustrated herein may be readily implemented in hardware and/or software using any known or later developed systems or structures, devices and/or software by those of ordinary skill in the applicable art from the functional description provided herein and with a general basic knowledge of the computer and telecommunications arts.

(50) Moreover, the disclosed procedures may be readily implemented in software that can be stored on a computer-readable storage medium, executed on programmed general-purpose computer with the cooperation of a controller and memory, a special purpose computer, a microprocessor, or the like. In these instances, the arrangements and procedures of the described implementations may be implemented as program embedded on personal computer such as an applet, JAVA® or CGI script, as a resource residing on a server or computer workstation, as a routine embedded in a dedicated communication arrangement or arrangement component, or the like. The arrangements may also be implemented by physically incorporating the arrangements and/or procedures into a software and/or hardware system, such as the hardware and software systems of a test/modeling device. The implementations herein are described in terms of exemplary embodiments However, it should be appreciated that individual aspects of the implantations may be separately claimed and one or more of the features of the various embodiments may be combined.