6.10 Initialization
| Initialization in SHDSL consists of two phases: the preactivation phase and the core activation phase. Figure 6.24 shows the total activation sequence for SHDSL link initialization. The preactivation phase consists of two handshake sequences and a line probe session, whose total goal is to negotiate system parameters and learn the best SHDSL configuration that can be supported on the given channel. Once the system configuration parameters are determined from the preactivation phase, the core activation phase trains the modem for optimum performance when transitioning to data mode. Figure 6.24. SHDSL total activation sequence. As seen in Figure 6.24, there are numerous timer values associated with the total activation time. The total activation time (t Act_Global ) is the sum of the preactivation and core activation times. The line probing session has a maximum duration time (t p-total ) of 10 seconds. The core activation time (t Act ) has a maximum duration of 30 seconds. Note that in the preactivation phase, the handshake mechanism does not have a maximum time-out associated with it, in that the handshake mechanism is outside the scope of the SHDSL specification. However, reasonable designs of the handshake can limit the total time to less than 5 seconds for typical SHDSL configurations. In most cases, we can expect the total activation time to be less than 30 seconds total. 6.10.1 Preactivation PhaseAs we mentioned earlier, the goal of the preactivation phase is to negotiate the selection of the transceiver parameter and determine (if necessary) the optimum transmission configuration using the optional line probe capability. Figure 6.25 shows the preactivation sequence of SHDSL. Figure 6.25. SHDSL preactivation sequence. In the first handshake (G.994.1) session, the transceivers negotiate the configuration of line probe sequences. The sequences P r1 “P rN represent the line probe symbol sequence generated by the customer premises unit (STU-R). Similarly, the line probe sequences generated from the CO (STU-C) unit are labeled P c1 “P cN . During this preactivation phase, the transceiver operates using 2-PAM. The transmitter components active during line probe are the scrambler, bit-to-symbol mapper, and the spectral shaper. The symbol rate, spectral shape, duration, and power backoff level are selected during the first handshake session. 6.10.2 Core ActivationCore 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 crsr depends 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 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 . |
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