DOCSIS WFQ Scheduler on the Cisco CMTS Routers

The DOCSIS WFQ Scheduler is an output packet scheduler that provides output scheduling services on both WAN uplink interfaces and DOCSIS downstream interfaces.

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Contents

Hardware Compatibility Matrix for Cisco cBR Series Routers


Note

The hardware components introduced in a given Cisco IOS-XE Release are supported in all subsequent releases unless otherwise specified.

Table 1 Hardware Compatibility Matrix for the Cisco cBR Series Routers

Cisco CMTS Platform

Processor Engine

Interface Cards

Cisco cBR-8 Converged Broadband Router

Cisco IOS-XE Release 3.15.0S and Later Releases

Cisco cBR-8 Supervisor:

  • PID—CBR-CCAP-SUP-160G

  • PID—CBR-CCAP-SUP-60G1

  • PID—CBR-SUP-8X10G-PIC

Cisco IOS-XE Release 3.15.0S and Later Releases

Cisco cBR-8 CCAP Line Cards:

  • PID—CBR-LC-8D30-16U30

  • PID—CBR-LC-8D31-16U30

  • PID—CBR-RF-PIC

  • PID—CBR-RF-PROT-PIC

Cisco cBR-8 Downstream PHY Modules:

  • PID—CBR-D30-DS-MOD

  • PID—CBR-D31-DS-MOD

Cisco cBR-8 Upstream PHY Modules:

  • PID—CBR-D30-US-MOD

1 Effective with Cisco IOS-XE Release 3.17.0S, CBR-CCAP-SUP-60G supports 8 cable line cards. The total traffic rate is limited to 60Gbps, the total number of downstream service flow is limited to 72268, and downstream unicast low-latency flow does not count against the limits.

Prerequisites for DOCSIS WFQ Scheduler

No specialequipment or softwareis needed to use the DOCSIS WFQ Scheduler feature.

Restrictions for DOCSIS WFQ Scheduler

  • The DBS feature is only applicable to DOCSIS 3.0 downstream channel bonding.

Information About DOCSIS WFQ Scheduler

The DOCSIS WFQ scheduling engine is used to provide output packet scheduling services, including absolute priority queueing, weighted fair queueing, minimum rate guarantee, traffic shaping, and DOCSIS bonding group dynamic bandwidth sharing on the Cisco cBR-8 converged broadband router.

The DOCSIS WFQ Scheduler provides services on both WAN uplink interfaces and DOCSIS downstream interfaces. The scheduling parameters on WAN uplink interfaces are configured through the Modular QoS CLI (MQC). On cable downstream interfaces, queues are created for DOCSIS service flows with parameters configured by DOCSIS downstream QoS type, length, values (TLVs).

The default queue size for the DOCSIS service flows (with bandwidth greater than 150 Mbps) is based on the bandwidth on the cable downstream interfaces (see Table below). Additionally, the queue limit for all service flows can also be adjusted using the cable queue-limit command, buffer size in service class or downstream buffer control TLVs.


Note
The default queue size change, and the cable queue-limit command do not affect the DOCSIS high priority queues.

Table below is an example of the queue size based on Annex B 256 QAM channels.

Table 2  Bandwidth, Queue Sizes, and Queue Limits

Channel

Bandwidth (Mbps)

Default Queue Size

Queue Size

1 ms

20 ms

30 ms

40 ms

200 ms

1

37.5

63

63

63

92

123

617

2

75

255

63

123

185

247

1235

3

112.5

255

63

185

277

370

1852

4

150

255

63

247

370

494

2470

5

187.5

319

63

308

463

617

3087

6

225

383

63

370

555

741

3705

7

262.5

447

63

432

648

864

4323

8

300

511

63

494

741

988

4940

12

450

767

63

741

1111

1482

7411

14

525

895

63

864

1296

1729

8646

16

600

1023

63

988

1482

1976

9881

The DOCSIS WFQ Scheduler also allows significant enhancement to the queue scaling limits.

The following sections explain the DOCSIS WFQ Scheduler features:

Queue Types

The DOCSIS WFQ Scheduler feature supports the following types of queues:

  • Priority queues
  • CIR queues
  • Best Effort queues

Priority Queues

Priority queues are serviced with absolute priority over all the other queues. On DOCSIS downstream interfaces, the priority queues are configured by DOCSIS applications that request a priority service flow, for example, a packet cable voice service flow. On WAN uplink interfaces, the priority queues are configured by the MQC policy maps.

The following restrictions apply to priority queues:

  • Only one priority queue is allowed per WAN uplink interface.
  • Only one priority queue is allowed for low latency service flows created for each DOCSIS downstream interface.

  • All low latency flows on a DOCSIS downstream are aggregated to the single priority queue.

CIR Queues

A CIR queue is guaranteed to be serviced with at least the Committed Information Rate (CIR). CIR queues are used to service DOCSIS service flows with non-zero minimum reserved rates. If the offered load to a CIR queue exceeds its CIR value, the excess traffic is serviced as best effort traffic.

Best Effort Queues

The Best Effort (BE) queues share the interface bandwidth not used by the priority queue and the CIR queues. The sharing is in proportion to each queue’s excess ratio.

The following conditions apply to BE queues:

  • On DOCSIS downstream interfaces, BE queues are created by DOCSIS service flows that do not request a minimum reserved rate.
  • Each DOCSIS flow without a minimum reserved rate uses its own BE queue.

DOCSIS QoS Support

DOCSIS defines a set of quality of service (QoS) parameters, including traffic priority, maximum sustained traffic rate, minimum reserved traffic rate, maximum traffic burst, maximum downstream latency, and peak traffic rate.

The downstream service flows use the QoS parameters to specify the desired QoS. The downstream policer and scheduler provides services such as traffic shaping, bandwidth provisioning, traffic prioritization, and bandwidth guarantee.

The DOCSIS service flow parameters are mapped to the packet queue parameters and provided with appropriate QoS support for the packet queues to support the DOCSIS parameters

The following DOCSIS QoS parameters are supported:

  • Traffic priority
  • Maximum sustained traffic rate
  • Minimum reserved traffic rate

Note
The maximum traffic burst size and the peak traffic rate are supported as described in the http:/​/​www.cisco.com/​c/​en/​us/​td/​docs/​cable/​cbr/​configuration/​guide/​b_cmts_quality_of_services/​docsis_​wfq_​scheduler.html#con_​1085732.

Traffic Priority

The downstream channel bandwidth available to the best effort traffic, namely the channel bandwidth minus the amount consumed by the priority traffic and the CIR traffic, is allocated to the best effort service flows in proportion to their DOCSIS traffic priorities. For example, if there are three service flows sending packets at a particular moment over the same downstream channel, and their DOCSIS traffic priorities are 0, 1 and 3, respectively, their share of the channel bandwidth will be 1:2:4. To achieve this bandwidth allocation, each service flow is assigned a value known as its excess ratio which is derived from its DOCSIS priority. Table below shows the default mappings of DOCSIS priority to excess ratio.


Note
When traffic priority for a flow is not explicitly specified, a default priority value of 0 is used as per the DOCSIS specification.
Table 3 DOCSIS Priority to Excess Ratio Mapping

DOCSIS Traffic Priority

Excess Ratio

0

4

1

8

2

12

3

16

4

20

5

24

6

28

7

32

Custom DOCSIS Priority to Excess Ratio Mappings

This option is introduced to configure custom priority to excess ratio mappings for downstream service flows that override the default mappings listed in the above Table.


Note
The configured values are used only for new service flows that are created after the configuration has been applied. All the existing service flows maintain their previous excess ratio values.

The option to configure priority to excess ratio mappings is available on a per downstream forwarding interface basis and is applicable to legacy cable, wideband and modular cable, and integrated cable interfaces.

The cable downstream qos wfq weights command is used to configure the mappings.

Maximum Sustained Traffic Rate

The maximum sustained traffic rate (MSR) specifies the peak information rate of a service flow. The MSR of a service flow is mapped to the shape rate of the packet queue. When the maximum sustained traffic rate is not specified or set to zero, its traffic rate becomes limited only by the physical channel capacity set by DOCSIS specifications.

Minimum Reserved Traffic Rate

The minimum reserved traffic rate (MRR) specifies the minimum rate reserved for a service flow. The MRR of a service flow is mapped to the CIR of the packet queue, which ensures the minimum amount of bandwidth a queue gets under congestion. When the MRR is not specified, the CIR is set to zero as per DOCSIS specifications.

High Priority Traffic

High priority traffic flows are mapped to a Low Latency Queue (LLQ) on the data forwarding interface. The packets in LLQ are serviced with absolute priority over other queues on the same interface.

The following service flows require high priority service:

  • Service flows with DOCSIS downstream latency TLV set to a value above zero. For example, PacketCable Multimedia Specification (PCMM) voice calls.
  • PacketCable downstream service flows.
  • Service flows with Unsolicited Grant Service (UGS) type—non-PacketCable voice calls—upstream flows.

Enhanced Rate Bandwidth Allocation

The DOCSIS WFQ Scheduler supports the Enhanced Rate Bandwidth Allocation (ERBA) feature for service flows. The ERBA feature allows cable modems (CMs) to burst their temporary transmission rates up to the full line rate for short durations of time. This capability provides higher bandwidth for instantaneous bandwidth requests without having to make changes to existing service levels in the QoS profile.

The DOCSIS WFQ Scheduler allows each service flow to have one dedicated queue. When ERBA is enabled for the service flow, the peak rate is implemented as the queue shape rate within the scheduler, while the maximum sustained rate is set as the token bucket refill rate. When ERBA is turned off, the burst size and the peak rate value are not used.

The maximum traffic burst parameter is used to control a service flow burst duration, to burst up to the channel line rate or a configured peak rate, when it is within its maximum burst size allowance. On the Cisco cBR-8 Converged Broadband Router, the cable ds-max-burst command is used to control this behavior explicitly.


Note
The ERBA feature is not applicable for high priority service flows and multicast service flows.

Table below summarizes the ERBA support for the Cisco cBR-8 router.

Table 4 Enhanced Rate Bandwidth Allocation Support for the Cisco cBR-8 Router

Policer Rate

Policer Exceed Action

Policer Token Bucket Size

Queue Shape Rate

Traditional Service Flow

Maximum Sustained Traffic Rate (unused)

Transmit

A value computed internally by CMTS (unused)

Maximum Sustained Traffic Rate

ERBA-Enabled Service Flow

Maximum Sustained Traffic Rate

Drop

Maximum Traffic Burst TLV

Peak Traffic Rate

For information about ERBA support on the Cisco CMTS routers, refer to Using Enhanced Bandwidth Rate Allocation (ERBA) Support for DOCSIS 1.0 Cable Modems at the following location: DOCSIS 1.1 for the Cisco CMTS Routers

Peak Traffic Rate

The peak-rate option of the cable ds-max-burst command allows you to specify the peak rate an ERBA-enabled service flow can use. The peak-rate value is a global value and is applied to all service flows created after the configuration of the cable ds-max-burst command. The default value of the peak-rate is zero.

If the DOCSIS 3.0 TLV 25.27 is specified for a service flow, the peak-rate value is set as the TLV value. However, if ERBA is not turned on for a service flow, the peak-rate value is ignored.

The peak-rate value can also be configured through cable service class command which forms part of the service class template. During modem registration or Dynamic Service Addition (DSA) operation, the service class name TLV 25.4 is sent to create the static or dynamic downstream service flow that matches the service class template. These downstream service flows are created with a specific peak-rate . If the peak-rate is not specified, then the value specified by the cable ds-max-burst command is used.

If a service flow has both service class and TLV 25.27 defined peak-rate , then the peak-rate value specified in the TLV is used.

Some of the DOCSIS 1.x and DOCSIS 2.0 cable modems, which are not fully DOCSIS 1.x or DOCSIS 2.0 compliant, may fail to come online when they receive TLV 25.27 from the Cisco CMTS during registration. In order to overcome this you can configure the cable service attribute withhold-TLVs command with the peak-rate keyword to restrict sending of this TLV to non-DOCSIS 3.0 cable modems.

DOCSIS 3.0 Downstream Bonding Support with Bonding Group Dynamic Bandwidth Sharing

DOCSIS 3.0 introduces the concept of downstream channel bonding. Each Bonding Group (BG) is made up of a collection of downstream channels, which can be used by one or more bonding groups. Each downstream channel can also serve as a primary channel in a MAC domain and carry non-bonded traffic, while being part of a BG.

Prior to DOCSIS 3.0 standards, the downstream service flows were associated with a single downstream interface, which in turn corresponded to a physical downstream on an RF channel. In DOCSIS 3.0, the downstream service flows are associated with the downstream bonding groups. These bonding groups can use multiple downstream RF channels.

DBS is the dynamic allocation of bandwidth for wideband (WB) and integrated cable (IC) interfaces sharing the same downstream channel. Due to the channel sharing nature of the bonding groups, the bandwidth available to bonding groups or non-bonded channels is not fixed. The bandwidth depends on the configuration and the traffic load on the WB or IC.


Note
Bonding groups are implemented as WB interfaces and non-bonded channels as IC interfaces.

In the DBS mode, the bandwidth of the shared RF channels is dynamically allocated among the WB and IC interfaces. The DBS enables efficient use of the underlying RF channel bandwidth even in the presence of high burst traffic. The DBS is configured at the WB or IC interface level. By default, bandwidth for a WB or IC channel is statically allocated (non-DBS).

For information about DBS support on the Cisco CMTS routers, refer to the Dynamic Bandwidth Sharing on the Cisco CMTS Router feature.

How to Configure DOCSIS WFQ Scheduler

You cannot configure the DOCSIS WFQ Scheduler feature as it is automatically loaded. The parameters that the schedule uses include the interface bandwidth and queue parameters.

This section describes the following required and optional procedures:

Mapping DOCSIS Priority to Excess Ratio

This section describes how to map DOCSIS priorities to custom excess ratios for downstream service flows. These custom mappings will override the default mappings.

Procedure
     Command or ActionPurpose
    Step 1enable


    Example: 
    Router> enable
     

    Enables privileged EXEC mode. Enter your password if prompted.

     
    Step 2configure terminal


    Example: 
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3interface wideband-cable slot/subslot/port :wideband-channel or interface integrated-cable slot/subslot/port :rf-channel

    Example: 
    Router(config)# interface wideband-cable 2/0/0:0 or 
Router(config)# interface integrated-cable 1/0/0:0
     

    Enters interface configuration mode for the indicated cable downstream interface.

     
    Step 4cable downstream qos wfq weigthts {weight1...weight8}


    Example: 
    Router(config-if)# cable downstream qos wfq weights 10 20 30 40 50 60 70 80
     

    Configures the custom excess ratios for 8 priorities:

    Note    The custom values are used only for new service flows and not existing ones.
     
    Step 5end


    Example: 
    Router(config-if)# end
     

    Exits interface configuration mode and returns to privileged EXEC mode.

     

    Verifying the Downstream Queues Information

    To verify the downstream queue information for a modem, use the show cable modem [mac-address |ip-address ]service-flow command.

    To check queue stats of all queues on an Integrated-Cable or Wideband-Cable interface, use the show cable dp queue interface command.

    Additional References

    The following sections provide references related to the DOCSIS WFQ Scheduler feature.

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    Feature Information for DOCSIS WFQ Scheduler

    Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http:/​/​tools.cisco.com/​ITDIT/​CFN/​. An account on http:/​/​www.cisco.com/​ is not required.


    Note

    The below table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    Table 5 Feature Information for DOCSIS WFQ Scheduler

    Feature Name

    Releases

    Feature Information

    DOCSIS WFQ Scheduler

    Cisco IOS-XE Release 3.15.0S

    This feature was introduced on the Cisco cBR Series Converged Broadband Routers.