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System Operation and Performance with a Direct Connect Configuration

System Operation and Performance with a Direct Connect Configuration

This chapter describes basic system operation and the performance metrics captured as a result of system operation in a Direct Connect configuration. Understanding performance statistics becomes easier if you understand the symmetrical digital subscriber line (SDSL) and asymmetric digital subscriber line (ADSL) connection processes. These processes are explained in the
following sections.

10.1 Bit Error Rate and Noise Margin

Bit error rate and noise margin are primary factors in achieving subscriber designated parameters. SDSL and ADSL transceivers seek an upstream and downstream data rate that can be maintained as long as the amount of line noise does not cause a bit error rate (BER) in excess of 1x10-7. Line noise is a function of reach and disturbers. As reach or noise in the loop increases, upstream and downstream payload rates decrease. Also, the thinner the copper wire, the more susceptible it is to noise. Line noise is not constant, and a certain amount of line noise fluctuation is tolerated without the trained rates being affected. This fluctuation is known as noise margin. Noise margin must be supplied to the transceivers when you are using the multiband method of training.

10.2 SDSL Connection Process

STU-C modules are designed to operate within narrow parameters, using a relatively low, 1 Mbps
of upstream and 1 Mbps of downstream and a fixed baud rate of 8 kbps with 16-point constellations. Such a reduction from the ATU-C 7 Mbps downstream and 1.5 Mbps upstream rates presents the
end user, usually a single office/home office subscriber (SOHO), with less cost and less
transport capacity.

Configuring an STU-C module is similar to configuring an ATU-C module.

10.2.1 SDSL Upstream and Downstream Bit Rates

The STU-C module upstream and downstream rates are the same. They range from 144 kbps to  1168 kbps. The seven fixed upstream and downstream rates for the STU-C module are

The baud rate is fixed at 8 kbps. Bit rates are determined by the number of carriers and the number of bits per carrier.

Note The STU-C module is supported by the Cisco 6130 system only.

10.2.2 STU-C Training Rates

If you are familiar with training ATU-C modules, you will note the extended training time
needed for STU-C modules. STU-C modules require at least 35 seconds to train and can take up to 3 minutes. This time frame contrasts sharply with the 60 seconds or less needed by an
ATU-C module.

10.3 STU-C Train Parameters

This section includes trained line attributes such as receive signal quality, receiver gain, and transmit power where it applies to noise margins on the Cisco 6100 Series systems. (See Section 5.4.3, "Per-Subscriber Power Settings Transmittal.") Each of these trained line attributes is displayed in ViewRunner in the STU-C Module Properties---Port Status dialog box.

As a Cisco 6100 Series system operator, you can change the noise margin (upstream and downstream) on an STU-C module for each subscriber line. Increasing this margin could cause a line that trained previously to fail to train.

Figure 10-1 shows an STU-C Module Properties---Session in Progress dialog box. The various train parameters are identified here. See the "Viewing ATU-C or STU-C Port Status Information" section for more information on the fields represented here.


Figure 10-1: STU-C Properties Dialog Box---Session in Progress


10.4 ADSL Connection Process

The Carrierless Amplitude and Phase Modulation (CAP) rate adaptive DSL (RADSL) transceivers used in Cisco ATU-Cs and ATU-Rs can train at a number of different discrete settings within each of three baud rate ranges. Because the upstream and downstream data paths are transmitted in different frequencies, they are trained independently.

The transceivers used in Cisco's Discrete Multi-Tone---Issue 2 (DMT-2) ATU-C modules train at one of two settings within the fixed baud rate. DMT-2 refers to the use of a single or discrete frequency. Both upstream and downstream are transmitted on the same frequency and trained
in tandem.

10.4.1 Payload Transmission Rates

Upstream and downstream payloads are transmitted in different frequency ranges, therefore, the two payload rates are established independently. Layer 2 protocol data units (PDUs) are encoded into Lay 1 (xDSL) transmission frequencies by means of baud rates (also known as symbol rates) and constellations. Different baud rates are used to achieve payload rates.

DMT-2 transceivers support upstream payload rates ranging from 864 kbps to 32 kbps, and downstream payload rates ranging from 8032 kbps to 32 kbps.

CAP RADSL transceivers support one baud rate for establishing upstream payload rate
(136 kilobaud), and three baud rates for establishing downstream payload rate (340, 680, and
952 kilobaud). These baud rates enable CAP RADSL implementations to support downstream payload rates ranging from 7168 to 640 kbps, and upstream data rates ranging from 1088 kbps to
91 kbps.

10.4.2 Changing Noise Margins

You can change the noise margins for a subscriber line. This means that a line that was training successfully might not train once you set a new margin. A warning appears that alerts you to use caution when changing the margin.

Cisco recommends that the margin be set for 6 dB upstream and 3 dB downstream to provide performance similar to that offered by the Cisco 6100 Series system releases before Release 2.3.5. The default values for margin are 0 upstream and 0 downstream.

For example, if you train a unit provisioned with 3 dB downstream margin and 6 dB upstream margin against 24-ISDN NEXT set to the 0db Reference level (-52.6 dBm), you can turn the noise up to -49.6 dBm at the CPE and -46.6 dBm at the CO and still maintain a bit error rate less than 1 bit in error for every 10 million sent (10-7). Within each baud rate, transceivers use constellation s to encode data into a frequency spectrum, and subsequently enable the discrete payload options shown in Table 10-1 and Table 10-2.

Note The DMT-2 module default downstream margin setting is 6 dB.

Cisco DMT-2 ATU-C module functionality supports a number of new training options to
measure and regulate subscriber traffic. These options can be configured from the Subscriber Properties DMT-2 dialog box. See "Service Provisioning for Direct Connect," for
more information.

10.4.3 DMT-2 Training Rates

DMT-2 transceivers support

10.4.4 DMT-2 Training Mode

The training mode featuring the StandardTrain option supports Direct Connect lines only and only DMT-2 ATU-C modules. One setting is used for both upstream and downstream. One option is available for the training mode: StandardTrain. StandardTrain is the default.

If you train a DMT-2 module provisioned with 3 dB downstream margin and 6 dB upstream margin against 24-ISDN NEXT set to the 0 dB Reference level (-52.6 dBm), you will be able to turn the noise up to -49.6 dBm at the CPE and -46.6 dBm at the central office (CO) and still maintain a bit error rate less than 1 bit in error for every 10 million sent (10-7).

10.4.5 CAP RADSL Training Rates

CAP RADSL transceivers support

These baud rates enable CAP RADSL implementations to support

Within each baud rate range, transceivers use constellations to encode data into a frequency spectrum, and subsequently enable the discrete payload options shown in Table 10-1 and Table 10-2.

Cisco's CAP RADSL implementation supports a number of ADSL training options that can be used to control subscriber traffic. The following background information provides an overview of the training process.

10.4.6 CAP RADSL Upstream and Downstream Constellation Combinations

Constellation combinations refers to the way discrete subscriber line chip sets communicate with one another. Different constellations provide different levels of data transmission accuracy.

Valid CAP RADSL constellations are 256 UC, 256, 128, 64, 32, 16, 8, and 8 extended range (ER). The 128, 32, 8, and 8ER constellations are not supported with downstream baud rates of 952 and 680. Because of this, some upstream-downstream payload combinations cannot be achieved.

Table 10-1 shows valid upstream and downstream constellation combinations.


Table 10-1: CAP RADSL Upstream and Downstream Constellation Combinations

Upstream

Kilobaud 136 136 136 136 136 136 136 136

Const 256UC 256 128 64 32 16 8 8ER
Kilobaud Const Payload 1088 952 816 680 544 408 272 91
952 256UC 7168

X

X

X

X

D 952 256 6272

X

X

X

X

o 952 64 4480

X

X

X

X

w 952 16 2688

X

X

X

X

n 680 256UC 5120

X

X

X

X

s 680 256 4480

X

X

X

X

t 680 64 3200

X

X

X

X

r 680 16 1920

X

X

X

X

e 340 256UC 2560

X

X

X

X

X

X

X

X

a 340 256 2240

X

X

X

X

X

X

X

X

m 340 128 1920

X

X

X

X

X

X

X

X

340 64 1600

X

X

X

X

X

X

X

X

340 32 1280

X

X

X

X

X

X

X

X

340 16 960

X

X

X

X

X

X

X

X

340 8 640

X

X

X

X

X

X

X

X

136 256UC 1024

X

X

X

X

X

X

X

136 256 896

X

X

X

X

X

X

X

136 128 768

X

X

X

X

X

X

X

136 64 640

X

X

X

X

X

X

X

136 32 512

X

X

X

X

X

X

X

136 16 384

X

X

X

X

X

X

X

136 8 256

X

X

X

X

X

X

X

In Table 10-1 there are

The 680 kilobaud with a 256 constellation entry has been selected as the Cisco 6100 Series system and Cisco 675 default for achieving the payload rate of 4480 kbps, because it supports a slightly better reach according to 24 ISDN near-end cross talk (NEXT) disturbers.
The 340 kilobaud with a 128 constellation entry has been selected as the Cisco 6100 Series system and Cisco 675 default for achieving the payload rate of 1920 kbps, because it is compatible with any of the upstream data rate selections. The 680 kilobaud option is limited to the 1088, 952, 680, or 408 kbps upstream payload.

CAP modules support 136 kilobaud training rates, which can be allowed or disallowed when the feature is supported by the SC software revision and the subscriber is locked. If the SC supports per-subscriber provisionable 136 kilobaud or does not support ATUCPARMS, the Allow 136 Kbaud toggle in the Cisco 6100 Series Properties dialog box is disabled. If the SC supports ATUCPARMS but does not support per-subscriber 136 kilobaud, the toggle is enabled.

The service provider is unlikely to be concerned with the technicalities of selecting similar data-rate settings for two different baud rates; consequently, Table 10-2 simplifies the selection process slightly. The table lists valid upstream and downstream rate combinations.

.
Table 10-2: Valid Rate Combinations for Upstream and Downstream Rates
Upstream Rate (kbps)

Downstream Rate (kbps) 1088 952 816 680 544 408 272 91

7168

X1

X

X

X

6272

X

X

X

X

5120

X

X

X

X

4480

X

X

X

X

3200

X

X

X

X

2688

X

X

X

X

2560

X

X

X

X

X

X

X

X

2240

X

X

X

X

X

X

X

X

1920

X

X

X

X

X

X

X

X

1600

X

X

X

X

X

X

X

X

1280

X

X

X

X

X

X

X

X

1024

X

X

X

X

X

X

X

960

X

X

X

X

X

X

X

X

896

X

X

X

X

X

X

X

768

X

X

X

X

X

X

X

640

X

X

X

X

X

X

X

X

512

X

X

X

X

X

X

X

384

X

X

X

X

X

X

X

256

X

X

X

X

X

X

X

1x = valid, empty cell = invalid

The actual training procedure is a function of transceiver-controlled parameter exchange and algorithms designed by Cisco to place parameters around valid data rate selections from Table 10-2.

10.4.7 ADSL Training Process

The following rules apply to the training sequence:

This means that control of the downstream data rate is relinquished to the ATU-C (the ATU-C controls the upstream data rate setting by default).

With the above rules, the ATU-C has complete control over upstream and downstream data rate selection. Valid upstream and downstream data rate combinations are selected from within the ViewRunner management interface and are communicated to the ATU-C through the
following procedure:
Figure 10-2: Subscriber Properties Dialog Box---Data Rate Selection



Figure 10-3: Module Properties Dialog Box---Data Rate Combination


Note If the ATU-R upstream and downstream data rates from the preset maximum settings are modified, the unit can train independently. Independent training depends on operator-defined settings in ViewRunner.
Note The CO side sets the rates at which the CPE trains. The CPE cannot change this rate.

10.5 ATU-C Train Parameters

This section includes trained line attributes such as receive signal quality, receiver gain, and transmit power where it applies to noise margins on the Cisco 6100 Series systems. (See Section 5.4.3, "Per-Subscriber Power Settings Transmittal.") Each of these trained line attributes is displayed in ViewRunner in the ATU-C Module Properties > Port Status dialog box.

You can change the noise margin (upstream and downstream) on an ATU-C module for each subscriber line, but increasing this margin could cause a line that trained previously, to fail to train.

Figure 10-4 shows a (CAP) ATU-C Module Properties---Session in Progress dialog box. The various train parameters are identified here. See the "Viewing the Port Service Configuration" section for more information on the fields represented here.


Figure 10-4: ATU-C Module Properties---Session in Progress


10.5.1 Receive Signal Quality

Receive signal quality (SQ) is a measure of the signal quality of the upstream channel (from the ATU-R in the case of the Cisco 675). It is represented in decibels.

10.5.1.1 SQ Overview

This value is used to estimate the BER or signal-to-noise ratio (SNR) margin for the received data. It takes into account the total signal-to-interference ratio (SIR), where the interference includes background noise, cross talk, residual inter-symbol interference, residual echo from the neighboring upstream or downstream data, and distortion.

The data in the next paragraph should provide a perspective of observed SQs. Stating explicit valid SQ ranges for a given loop reach is, however, not particularly valuable. The number and variety of interferers and wiring (outside plant and in-home) creates so many different scenarios that specific data is not always helpful if you are trying to institute, for example, a corrective action by moving a pair to a different binder group as a remedy.

For 7 kft loops, it is fairly common to observe receive SQ of 37 to 44 dB, with average interference and a 6 dB noise margin setting. For 10 kft loops, an SQ range of 32 to 35 dB is common. As loop length increases, the SQ decreases. Long loops (12 to 15 kft) or loops that have bridge taps could have an SQ in the low 30s decibel range. Very long loops (over 15 kft) can even train with an SQ as low as 20 dB @ dB margin. If noise margin is reduced to zero, even longer loops can train with an SQ as low as 12 dB.

10.5.1.2 Future RADSL Development

One of the great values of RADSL is that it removes the need for the operator to figure out these values for optimum performance. The values are presented here primarily as an indicator that the trained loop is exhibiting expected characteristics, rather than for troubleshooting purposes. In a subsequent Cisco 6100 Series system and ViewRunner release, Layer 1 performance statistics will be added, which will be more valuable from a historical review perspective for loops that are experiencing problems. For a given training session, remember that if RADSL cannot overcome loop characteristics such that even the lowest upstream/downstream data rates are not supportable, these layer 1 attributes will not be displayed anyway.

10.5.1.3 SQ and Data Rate Interaction

The SQ is not a function of the data rates. In fact the opposite is true, data rates are a function of the SQ. For a given requested upstream/downstream data rate combination (as selected in ViewRunner), if the transceivers cannot maintain the data rates to a 10-7 BER (with a 6 dB noise margin insertion), the transceivers seek the next lowest data rate combination where the BER can be preserved.

Beginning with Cisco 6100 Series Release 2.2.1, the node supports selectable noise margins. The Cisco 6100 Series supports a hardcoded 3 dB down/6 dB up noise margin setting. This is valuable if, for example, a subscriber requests 7.168 Mbps x 1.088 Mbps, but the loop quality is not sufficient to hold the BER to less than or equal to 10-7. The subscriber's RADSL train could perhaps settle to 6.272 Mbps x 1.088 Mbps. In this scenario it could be that interference is causing the downstream channel to have just enough noise to prevent the 7.168 Mbps data rate from being achieved with the 6 dB margin hit. Reducing the noise margin (that the transceivers must take into account for determining the best data rates to lock onto) to 3 dB might, however, enable the full 7 x 1 service to be provided.

10.5.2 Receiver Gain

Receiver gain is a measure of loop attenuation over the entire DSL frequency spectrum. The ATU-C or STU-C has an algorithm that enables it to boost receiver gain such that attenuation can be corrected for proper support of a given receive data rate.

The ATU-C or STU-C algorithm attempts to keep gain to a minimum to prevent near-end cross talk (NEXT). However, if loop conditions warrant, the algorithm must boost gain enough to ensure the minimum signal level required is received.

CAP modules support 136 kilobaud training rates, which can be allowed or disallowed when the feature is supported by the SC software revision and the subscriber is locked. If the SC supports per-subscriber provisionable 136 kilobaud or does not support ATUCPARMS, then the Allow 136 Kbaud toggle on the Cisco 6100 Series Properties dialog box is disabled. If the SC supports ATUCPARMS but does not support per-subscriber 136 kilobaud, then the toggle is enabled.

The service provider is unlikely to be concerned with the technicalities of selecting a similar data-rate settings for different baud rates.

Factors that tend to boost receiver gain are associated with loop impairment. Two primary factors are

Table 10-3 lists documented reach and receiver gains.


Table 10-3: Reach and Receiver Gain
Reach (Kft) Receiver Gain (dB)

16

42

14.5

39

11

27

9

21

8.7

19

10.5.3 Transmit Power

Transmit power is a measure of the downstream power spectral density (PSD) mask. T1E1/97-104R2a states that the PSD for the downstream channel shall have an upper limit of -40 dBm/Hz in the nominal passband region with no variation exceeding -37 dBm/Hz.

Caution Cisco advises you to set the transmit power to -40 dBm/Hz in Release 2.4.0 of ViewRunner for Windows. To avoid data transmission errors, do not change the PSD from
-40 dBm/Hz.

Each network provider chooses the maximum transmit power. As power is boosted, reach can be extended for a given data rate. At the same time, however, the boosted signal can disturb
other services.

Although not currently displayed in the ViewRunner interface, T1E1/97-104R2a also specifies that the PSD for the upstream channel must have an upper limit of -38 dBm/Hz nominal with no variation exceeding -35 dBm/Hz.

Note PSD transmit power cannot be set for DMT-2 upstream in this release.

Beginning with Cisco 6100 Series Release 2.1.1, an operator can control transmit power. This is helpful in cases where the network provider desires to boost or reduce the power of a given loop.

The ATU-R transceiver determines its transmit power during the training process as it tries to fulfill the upstream data rate request. It tries to support the requested data rate at the lowest possible transmit power so that near cross talk in the Cisco 6100 Series system is minimized.

10.6 Understanding Performance Train Counters

The Cisco 6100 Series system has train counters for capturing system performance statistics in a Direct Connect configuration. These counters are listed below.

Counters are used to display individual subscriber and CAP ATU-C, DMT-2 ATU-C, and STU-C modem port statistics.

After the connection has been established, the training sequence as described above begins.

If the CAP ATU-C, DMT-2 ATU-C, STU-C and ATU-R modems train, a successful train counter increments for these modems. If the xTU-C modem and ATU-R modem fail to train, a failed train counter increments for the xTU-C modem. You can view train counter statistics in the Performance Management dialog box.

10.7 Performance Management Dialog Boxes

To check the performance of the ATU-Cs in a dedicated configuration, right-click the multiplexer chassis (MC) and select Cisco 6100 Series Performance. The Performance Management dialog box appears (see Figure 10-5).

Note The STU-C Performance Management dialog box lists the names of the types of modules to which ports are associated.

Figure 10-5: Cisco 6100 Series System Performance---Direct Connect ATU-C


Table 10-4 describes the Performance Management dialog box fields.


Table 10-4: Performance Management Field Descriptions
Field Description

xTU-C Port

Lists the chassis, slot and port number of the ATU-C or STU-C unit.

Type

Names the type of module: ATU-C (CAP or DMT-2) or STU-C (2B1Q)

Note This field is applicable only on an STU-C performance management screen.

Successful Trains

Lists the number of times the associated module has
trained successfully.

Failed Trains

Displays the number of times the associated module has failed to
train successfully.

Training Statistics

Allows you to display ATU-C training data. This option is unavailable unless a line within the statistics table is selected.

If a module fails to train, you may have provisioned it incorrectly. See "Service Provisioning for Direct Connect," for more information on provisioning.

The existing table that shows performance statistics, such as successful trains and failed trains, has been modified to allow individual lines of the table to be selected. Once you have selected a line of the table for a DMT-2 or ATU-C module, a button on the performance dialog box allows you to display a graphical representation of the bit allocation, signal-to-noise ratio, and receiver gain for that module. The graph can be refreshed or closed using available buttons on the display. The range of the training statistics display is the same as that specified in the MIB.

In ViewRunner for HP OpenView, the user can open as many ATU-C graphs as are allowed by the dialog boxes system limit.

10.8 Current Connection Activity

Two current connection activity displays are provided.

10.8.1 ATU-C Module Properties Port Status Tab

For ATU-C modules in a Direct Connect configuration, the Port Status tab displays state information, as shown in Figure 10-6.


Figure 10-6: ATU-C Port Properties Status Dialog Box


10.8.2 Active Connections Dialog Box

The Active Connections dialog box displays a snapshot of all currently active connections in the
Cisco 6100 Series system. To open the Active Connections dialog box, right click on the MC and select Active Connections. The dialog box in Figure 10-7 appears, showing the far left position. Figure 10-8 and Figure 10-9 present the Active Connections dialog box scrolled to the right.


Figure 10-7: Active Connections Dialog Box (Left Side)



Figure 10-8: Active Connections Dialog Box (Center)



Figure 10-9: Active Connections Dialog Box (Right Side)


You can use the logical service-oriented navigation to go directly to the desired entity by clicking any blue hyperlink in the dialog box. Table 10-5 describes the fields in the Active Connections dialog box.


Table 10-5: Active Connections Field Descriptions
Field Description

Subscriber ID

Displays the identifier by which the subscriber is known.

ATU-C Port

Displays the chassis, slot, and port number to which the line port is connected.

Modem State

Displays whether the modem is trained or training.

Actual Up

Displays the actual upstream train rate at which the subscriber is training. This rate may never be higher than the Provisioned Up rate.

Actual Down

Displays the actual downstream train rate at which the subscriber is training. This rate may never be higher than the Provisioned Down rate.

Rx SNR

Signal-to-noise ratio for ADSL upstream (receive side) data channel.

Provisioned Up

Displays the upstream rate set by the CO, the maximum upstream rate at which the subscriber can train.

Provisioned Down

Displays the downstream rate set by the CO, the maximum downstream rate at which the subscriber can train.

Provisioned Up Margin

Displays the upstream noise margin provisioned by the operator.

Provisioned Down Margin

Displays the downstream noise margin provisioned by the operator.

Actual Up Margin

Displays the upstream noise margin actually occurring within that margin provisioned by the operator.

Actual Down Margin

Displays the downstream noise margin actually occurring within that margin provisioned by the operator.

The Active Connections dialog box includes fields for provisioned upstream and downstream bit rates, actual trained upstream and downstream bit rates, and received signal-to-noise ratio.


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Posted: Tue Oct 5 13:11:56 PDT 1999
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