Core Activation

Core activation (Figure 6.26) begins with the customer premises (CP) unit transmitting sequence C_r

for a period of t_cr seconds. The value of t_cr depends on the bit rate: If the payload bit rate is 768 kb/s or less, then the nominal value of t_cr is 2 seconds; for the higher bit rates, the nominal value is 1 second. The CO unit may use the received sequence to perform timing recovery and train its equalizer. The CP unit may use this sequence to train it echo canceller.

Figure 6.26. SHDSL core activation sequence.

Upon the completion of sequence C_r, the CO unit begins sending sequence S_c one-half second later.

Sequence S_c is the 2-level PAM signal formed by inputting ones into the scrambler of the reference startup transmitter. Recall that the transmit power is set to the value specified in the preactivation frame sent during the handshake sequence exchanges. For a duration t_crsr–t_crsc seconds, S_c is the only signal on the line. The duration of t_crsrdepends on the bit rate: for payload rates of 768 kb/s and below, the nominal value is 3 seconds; otherwise the value of t_crsr is 1.5 seconds. During this time the CP unit may train its equalizer and timing recovery circuit from the received signal; during this same time, the CO unit may train its echo canceler. The CP unit begins transmitting Sequence S_r at t_crsr seconds after completion of C_r, and now there is simultaneous transmission of both upstream and downstream data. The sequence S_c is transmitted for a minimum of 5 seconds, which is the time required (timer T_PLL) for the CP unit to synchronize its phase locked loop.

Sequence S_r is also a two-level PAM signal resulting from an input of ones into the scrambler of the reference startup transmitter, and the transmit power is that specified in the handshake sequence exchange. The CP unit begins transmitting at t_crsr seconds after it concludes transmitting C_r. During the period of simultaneous S_c and S_r transmission, the transceivers continue training their equalizers, echo canceler and other necessary functions. If the transceiver functions have not converged by conclusion of S_c and S_r, then the transceiver enters an exception state. At that point the initialization process would need to be restarted.

After the CO unit transceiver has converged and it has been sending the S_c signal for at least 5 seconds (i.e., the value of the T_PLL timer), it transitions to sending signal T_c. During the transmission of T_c, the channel precoder coder coefficients and other system information is sent to the CP unit from the CO unit. Once the CP unit has converged and has begun detecting the T_c signal, it begins transmitting the T_r signal to the CO unit. As with T_c, the T_r signal passes the channel precoder coefficients and other signal parameters to the CO transceiver. The information transferred in the T_c and T_r signals are contained in a core activation frame.

Once the CO unit has detected the T_r signal and has completed transmission of the core activation

SL Advances - Prentice Hall.chm::/images/btn_prev.gif" alt="Previous section">

s="docText">NAK-NR 0

0 1 0 0 0 0 1

NAK-NS 0

0 1 0 0 0 1 0

NAK-CD 0

frame, it begins sending signal F_c. Signal F_c sends the core activation frame of T_c except that the frame sync word is reversed, and all of the remaining information bits are set to arbitrary values.

Two of these frames are sent during F_c, and this can serve as an acknowledgment that the CO unit received T_r.

Upon conclusion of S_c transmission, the CO unit begins sending data and the CP unit begins sending data upon completion of T_r.

Top Top

0 1 0 0 0 1 1

REQ-MS 0

0 1 1 0 1 0 0

REQ-MR 0

0 1 1 0 1 1 1

REQ-CLR 0

0 1 1 0 1 1 1

The Par(1) parameters in the standard information field contain one NPar(1) octet, three SPar(1) octets, and 20 NPar(2) octets defining the subparameter values in support of the parameters

identified in the three SPar(1) octets. The only parameter defined in the NPar(1) octet is to identify use of the nonstandard information field. The SPar(1) octets identify parameters such as net

upstream and downstream data rates, upstream and downstream data flow characteristics, and central office and customer premises splitter information. The 20 NPar(2) octets in support of the SPar(1) parameters are distributed as follows:

z Net upstream data rate—3 octets

z Net downstream data rate—3 octets

z Upstream data flow characteristics—2 octets

z Downstream data flow characteristics—2 octets

z Customer premises splitter information—1 octet

z Central office splitter information—1 octet

z Relative power level/carrier for upstream carrier set A43—1 octet

z Relative power level/carrier for downstream carrier set A43—1 octet

z Relative power level/carrier for upstream carrier set B43—1 octet

z Relative power level/carrier for downstream carrier set B43—1 octet

z Relative power level/carrier for upstream carrier set C43—1 octet

z Relative power level/carrier for downstream carrier set C43—1 octet

z Relative power level/carrier for upstream carrier set B4—1 octet

z Relative power level/carrier for downstream carrier set B4—1 octet

padding="0">Topd of responding with ACK(1) as in basic transaction A, the CO unit requests that the capabilities list be exchanged. Once the two units exchange capabilities and negotiate mode of operation, a follow-up transaction is needed for mode selection.

In transaction B:C, the CP unit requests that the CO unit select the mode of operation via a MR message, but instead of the expect MS response of basic transaction B, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection.

In transaction D:C, the CP unit proposes a mode of operation via the MP message and requests that the CO unit select the operating mode. Instead of the expected MS message from the CO unit of basic transaction D, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection.

5.11.3 Message Segmentation

Note that in the above transactions, some messages can become rather large when passing messages that contain the identification and standard information parameter fields. Excluding the two FCS octets and any octets inserted for transparency, the maximum message length in a frame is 64 octets.

If a message is longer than 64 octets, then it must be segmented into two or more messages. The message types that can be segmented are those that contain the parameter octets, namely, CL, CLR, MP, and MS.

When a receiving station is parsing a segmented message, the receiving station sends an ACK(2), which indicates to the sending station that it is ready to receive the remainder of the message. Once the complete message is received, the receiving station responds with an ACK(1) or other

appropriate response.

5.11.4 Example Transactions

In this section, we provide some example G.994.1 sessions to demonstrate the handshake process.

Example 1: Table 5.7 shows the use of a transaction sequence that combines basic transaction C with A. The CP unit first initiates a capabilities list request where the two units exchange and negotiate capabilities via the CLR and CL messages. The CP unit then selects the mode of operation via the MS command (basic transaction A).

Example 2: Table 5.8 shows an example transaction that combines extended transaction A:C with basic transaction A. First the CP unit selects a mode of operation and requests that the CO select this

Top

Table 5.7. Example 1—Basic Transaction C Followed by Basic Transaction A

CP Unit CO Unit CP Unit CP Unit CO Unit

CLR_ CL_ ACK(1) MS_ ACK(1)

Table 5.8. Example 2 – Extended Transaction A:C Followed by Basic Transaction A

CP Unit CO Unit CP Unit CO Unit CP Unit CP Unit CO Unit

MS_ REQ-CLR_ CLR_ CL_ ACK(1) MS_ ACK(1)

mode. Instead the CO unit requests the CP unit for a capabilities list request. The two modems then exchange capabilities lists and negotiate operating modes. Once the exchange and negotiation are complete, the CP unit selects the mode of operation via the MS message.

References

[1] ITU-T Recommendation G.991.2, "Single-Pair High-Speed Digital Subscriber Line (SHDSL) Transceivers," February 2001.

[2] ETSI TS 101 524 V1.1.3 (2001-11), "Transmission and Multiplexing (TM); Access Transmission System On Metallic Access Cables; Symmetrical Single Pair High Bitrate Digital Subscriber Line (SDSL)," November 2001.

[3] Committee T1–T1.418-2000, "High Bit Rate Digital Subscriber Line—2nd Generation (HDSL2)."

[4] T1.422, "Single-Pair High-Speed Digital Subscriber Line (SHDSL) Transceivers," October 2001.

[5] Committee T1–T1.403-1999, "Network and Customer Installation Interfaces–DS1–Electrical Interface."

[6] ITU-T Recommendation G.703, "Physical/Electrical Characteristics of Hierarchical Digital Interfaces," November 2001.

[7] ITU-T Recommendation G.704, "Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44,736 kbit/s Hierarchical Levels," October 1998.

[8] ETSI TS 101 135 V1.5.3 (2000–09), "Transmission and Multiplexing (TM); High Bit-Rate Digital Subscriber Line (HDSL) Transmission Systems on Metallic Local Lines; HDSL Core Specification and Applications for Combined ISDN-BA and 2,048 kbit/s Transmission," September 2000.

[9] T1E1.4/2001-006R2, "Draft Standard: High Bit Rate Digital Subscriber Line—2nd Generation (HDSL2/HDSL4) Issue 2," November 2001.

[10] G. Ungerboeck, "Channel Coding with Multilevel/Phase Signals," IEEE Transactions on Information Theory IT-28, no. 1, January 1982.

[11] Pairgain Technologies, "A 512-State PAM TCM Code for HDSL2," T1E1. 4/97-300, September 22, 1997.

[12] Adtran, "Performance and Characteristics of One-Dimensional Codes for HDSL2," T1E1.4/97-337, September 25, 1997.

[13] Adtran, Cicada, Siemens, Tellabs, and Westell, "Proposal to Break the FEC Logjam for HDSL2," T1E1.4/97-443, December 8, 1997.

Top