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System Operation and Performance with a Digital Off-Hook Configuration

System Operation and Performance with a Digital Off-Hook Configuration

This chapter describes basic system operation and the performance metrics captured as a result of system operation in a Digital Off-Hook (DOH) configuration. Understanding performance statistics is easier if you understand the process that the Cisco 6100 Series system goes through to establish digital off-hook connections.

Note  Digital Off-Hook provisioning and operation is not supported in Release 2.4.0.

9.1 DOH Operation

The following events occur when a Cisco 6100 Series system initiates a connection request to the customer premises equipment (CPE) product:

    1. The ATU-R modem sends an asymmetric digital subscriber line (ADSL) start tone through the remote POTS splitter across the subscriber loop and into the central office (CO) POTS splitter.

    2. The CO POTS splitter routes the start tone sequence to the subscriber's line interface module (LIM) port.

    3. Each subscriber loop is hardwired to a specific POTS module port, and subsequently to a specific LIM port. The LIM notifies the LIM controller module in the same chassis as the LIM that one of its ports is requesting connection to a CAP ATU-C modem.

    4. The LIM controller module signals the system controller (SC) module in the multiplexer chassis (MC) that a CAP ATU-C modem connection is requested.

    5. Provided that a modem is available in the logical pool, the SC module uses a round robin distribution algorithm to decide which CAP ATU-C modem will handle the connection. (The modem must be in the logical pool to which the subscriber is assigned.) The SC module then instructs the LIM controller module accordingly.

    6. The LIM port (subscriber line with ATU-R modem at the far end) and the designated CAP ATU-C modem are then connected, and a successful connection counter is incremented for the logical pool, the line, and the subscriber ID. At this point, the modem training sequence begins.

9.1.1 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.

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.

9.1.2 Bit Error Rate and Noise Margin

Bit error rate and noise margin are primary factors in achieving subscriber designated parameters.

Because 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 and/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. Since line noise is not constant, 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 (multiband is discussed further below).

9.1.2.1 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 Layer 1 (ADSL) transmission frequencies by means of baud rates (also known as symbol rates) and constellations. Different baud rates are used to achieve different payload rates.

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 kbps to 640 kbps, and upstream data rates ranging from 1088 kbps to
91 kbps.

9.1.2.2 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. You will see a warning alerting you to use caution when you are 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 Cisco 6100 Series releases prior to Release 2.4.0. 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 0 db 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 constellations to encode data into a frequency spectrum, and subsequently enable the discrete payload options shown in Table 9-1.

9.1.3 CAP RADSL Upstream and Downstream Constellation Combinations

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

Table 9-1 shows valid upstream and downstream constellation combinations.


Table 9-1: 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 9-1 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 this payload rate, because it supports a slightly better reach based on 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 this payload rate, 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, then the Allow 136K Baud toggle in 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 similar data-rate settings for two different baud rates; consequently, Table 9-2 simplifies the selection process slightly. The table lists valid upstream and downstream rate combinations.

.
Table 9-2: 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 in Table 9-2.

9.1.4 Training Procedure

The following rules apply to the training sequence:

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

With the above rules, the CAP ATU-C has complete control over upstream and downstream data rate selection. Valid upstream and downstream data rate combinations are selected from within ViewRunner and are communicated to the CAP ATU-C through the following procedure:


Figure 9-1: Subscriber Properties Dialog Box---Data Rate Selection



Figure 9-2: Module Properties Dialog Box---Data Rate Combinations


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.

9.2 CAP ATU-C Train Parameters

This section includes information about receive signal quality, receiver gain, and transmit power where transmit power applies to noise margins on the Cisco 6100 Series systems. (See the "Service Provisioning for Digital Off-Hook" section.) Each of these trained line attributes is displayed in ViewRunner in the CAP 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 9-3 shows the CAP ATU-C Module Properties---Port Status dialog box for a session in progress. The various train parameters are identified in the CAP ATU-C Module Properties
dialog box.


Figure 9-3: ATU-C Module Properties Dialog Box


9.2.1 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.

9.2.1.1 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 dB range of 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.

9.2.1.2 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.

9.2.1.3 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 Systems supported 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 into) to 3 dB might, however, enable the full
7 x 1 service to be provided.

9.2.2 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 9-3 lists documented reach and receiver gains.


Table 9-3: Reach and Receiver Gain
Reach (Kft) Receiver Gain (dB)

16

42

14.5

39

11

27

9

21

8.7

19

9.2.3 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. 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.

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.

9.3 Understanding Performance Train Counters

The Cisco 6100 Series system has a variety of connection counters, train counters, and threshold counters for capturing DOH system performance statistics. These counters follow:

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

If the CAP ATU-C modem and ATU-R modem train, a successful train counter increments for the CAP ATU-C modem. If the CAP ATU-C modem and ATU-R modem fail to train, a failed train counter increments for the CAP ATU-C modem. You can view train counter statistics on the Performance Management dialog box.

9.3.1 Connection Counters

Once a LIM port and CAP ATU-C modem have been successfully connected, a successful connection counter increments for both the logical pool and the line, allowing the modem training sequence to begin.

If all modems within the logical pool are busy, the SC module sends a busy tone to the ATU-R modem and increments the logical pool blocked-connection and line blocked-connection counters.

9.3.2 Train Counters

After the connection between the LIM port and CAP ATU-C modem has been established, the training sequence described above begins.

If the CAP ATU-C modem and ATU-R modem train, a successful train counter increments for the CAP ATU-C modem. If the CAP ATU-C modem and ATU-R modem fail to train, a failed train counter increments for the CAP ATU-C modem.

9.3.3 Threshold Counters

Two threshold counters are also available for each logical pool: 80 percent and 100 percent modem use. These two threshold counters provide the number of

The sum of the above three statistics is equal to the total connection requests handled by the
logical pool.

Note Counters continue to increment unless the SC module is reset.

In addition to the integer connection counters, connection activity is also represented in terms of percentage connections in each category.

9.4 Performance Management Dialog Boxes

The Logical Pool Performance dialog box provides feedback on the current performance of each logical pool in a DOH system. The dialog box allows you to select a particular physical and logical pool, and provides the following performance statistics for that selection:

These statistics are captured in the views described below. To access any of these views, right-click the MC and select Cisco 6100 Series Performance.

Each Performance Management dialog box has its own set of associated counters. These counters are reset under certain conditions. The specific counters and the conditions under which a counter is or is not reset are described in the following sections.

Counters are not reset when you

9.4.1 Performance Management---Pool Summary

The Performance Management---Pool Summary dialog box provides detailed information about successful and blocked connections, and successful and failed CAP ATU-C trains (see Figure 9-4). Additionally, summary statistics indicating the number of trains at or below 80 percent logical pool use, over 80 percent, and blocked requests for each logical pool are provided.


Figure 9-4: Performance Management---Pool Summary


The Cisco 6100 Series system maintains the following pool summary counters on each defined pool:

Pool summary counters get reset in the following situations:

Table 9-4 describes these counter displays.


Table 9-4: DOH Counters
Counter Description

Pool time

Total number of minutes that the logical modem pool has been in existence (or since the last system reset).

Block time

Total number of blocked minutes experienced by the logical modem pool since its creation (or since the last system reset).

Note A blocked minute is any minute during which one or more subscribers are refused modem port assignments. This refusal is in response to Digital Off-Hook connection requests and 100 percent modem use.

9.4.2 Performance Management---Subscriber

The Performance Management---Subscriber dialog box lists data about all successful and blocked connection activity by a specific subscriber. Additionally, summary statistics indicate the number of connections at, below, or over 80 percent logical pool use. Summary statistics also show blocked requests for each logical pool.

The Performance Management---Subscriber dialog box can be accessed by putting your cursor over the outside area of the appropriate LCC, right-clicking, and selecting Pool Properties
(see Figure 9-5).


Figure 9-5: Performance Management---Subscriber


Table 9-5 describes the fields on the dialog box.


Table 9-5: Logical Pool Performance Data Dialog Box: Subscriber View Fields
Field/Tab Description

Pool

Contains two parts:

  • The physical pool to which provided statistics apply, either A or B.

  • The logical pool within the physical pool to which provided statistics apply, either 1, 2, or 3.

Subscriber ID

The unique identifier by which the subscriber is known.

Successful xConn

The number of successful cross connections achieved for that subscriber ID since the last time the counter was reset.

Blocked xConn

The number of blocked cross-connections realized for that subscriber ID since the last time the counter was reset.

CPE Signature Rejects

The total number of CPE signatures that were not detected.

Subscriber Statistics

Keeps track of four parameters.

  • Number of Subscribers

  • Number of Successful XConnects

  • Number of Blocked XConnects

  • Number of CPE Signature Rejects

The Cisco 6100 Series system maintains the following subscriber counters:

Subscriber counters get reset if you

9.4.3 Performance Management---Line Ports

The Performance Management---Line Ports dialog box lists data about all successful and failed CAP ATU-C trains by line port (see Figure 9-6).


Figure 9-6: Performance Management---Line Ports


Table 9-6 describes the fields in the dialog box.


Table 9-6: Logical Pool Performance Data---Line Port Headers
Field/Tab Description

Pool

Contains two parts:

  • The physical pool to which provided statistics apply, either A or B

  • The logical pool within the physical pool to which provided statistics apply, either 1, 2, or 3

Line Port

The line port location by chassis, slot and line port number.

Successful Trains

The number of successful trains achieved for that line port since the last time the counter was reset

Failed Trains

The number of failed trains realized for that line port since the last time the counter was reset

Non-timer Failed Trains

The number of failed trains realized for that line port, not regulated by a timer

The Cisco 6100 Series system maintains the following for each line port counter:

The Cisco 6100 Series system maintains the following for each line port counter:

Line port counters are reset if you

9.4.4 Performance Management---CAP ATU-C Ports

The Performance Management---CAP ATU-C Ports dialog box in Figure 9-7 lists data about all successful and failed ATU-C trains by the CAP ATU-C modem port.


Figure 9-7: Performance Management Dialog Box---CAP ATU-C Ports


Table 9-7 describes the fields in the dialog box.


Table 9-7: Logical Pool Performance Data Dialog Box---ATU-C Ports Fields
Field/Tab Description

Pool

Contains two parts:

  • The physical pool to which provided statistics apply, either A or B

  • The logical pool within the physical pool to which provided statistics apply, either 1, 2, or 3

CAP ATU-C Modem Port

CAP ATU-C modem port location by chassis, slot, and line port number

Successful Trains

Number of successful trains achieved for that CAP ATU-C modem port since the last time the counter was reset

Failed Trains

Number of failed trains realized for that CAP ATU-C modem port since the last time the counter was reset

The Cisco 6100 Series system maintains the following counters on each ATU-C port:

CAP ATU-C port counters are reset if you

9.5 Current Connection Activity

Two current connection activity displays are provided.

9.5.1 Connected To Group Box

For CAP ATU-Cs or LIMs in a pooled DOH configuration, the Status dialog box displays a Connected To group box (Figure 9-8). This box identifies the specific LIM and CAP ATU-C ports currently connected to one another, and provides a Properties button that you can use to open the opposing port's module property tab.

This group box is dimmed when the port's Usage state is Idle.


Figure 9-8: Connected To Group Box


9.5.2 Active Connections Dialog Box

The Active Connections dialog box lists a snapshot of all currently active connections in the
Cisco 6100 Series system. To open the Active Connections dialog box, right click the MC and select Active Connections. The dialog box in Figure 9-9 appears, showing the far left position. Figure 9-10 and Figure 9-11 present the Active Connections dialog box scrolled to the right.


Figure 9-9: Active Connections Dialog Box (Left Side)



Figure 9-10: Active Connections Dialog Box (Center)



Figure 9-11: Active Connections Dialog Box (Right Side)


Note 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 9-8 describes the fields in the Active Connections dialog box.


Table 9-8: Active Connections Field Description
Field Description

Pool

Displays the name of the physical and logical pool.

Active Connections

Displays the number of active connections associated with a physical or logical pool.

In Service Modems

Displays the number of in-service modules that you have assigned to a logical pool and which modules and ports are unlocked.

% Usage

The Active Connections field divided by the In Service Modems field.

Subscriber ID

Displays the identifier by which the subscriber is known.

Line Port

Displays the chassis, slot, and port number to which the subscriber ID is associated.

ATU-C Port

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

Pool

Displays the physical or logical pool of which the line port and ATU-C modem port are members.

Modem State

Displays whether the modem is trained, training, or not trained.

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. In addition, the dialog box includes totals for active connections by pool and the current usage level of each pool.


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Posted: Tue Oct 5 13:12:00 PDT 1999
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