Docsis-Compliant Upstream Signals - Cisco 7246VXR - uBR Router Software Configuration Manual

Universal broadband router

Table of Contents

Chapter 1

Overview of Cisco uBR7200 Series Software

DOCSIS and CMTS Interoperability

communications. All CMs listen to all frames transmitted on the downstream channel on which they are

registered and accept those where the destinations match the units themselves or the devices each

supports.

The Cisco uBR7200 series supports multicast groups using standard protocols such as Protocol

Independent Multicast (PIM), Distance Vector Multicast Routing Protocol (DVMRP), and Internet

Group Management Protocol (IGMP) to determine if multicast streams are to be forwarded to a

prescribed downstream cable modem or STB, or a multicast routing peer.

The Cisco uBR7200 series software periodically sends MAC allocation and management

messages—known as MAPs—to all CMs on the network, defining the transmission availability of

channels for specific periods of time. The MAP rate is fixed—every 2 msec.

Different transmission intervals are defined that associate an interval with a Service Identifier (SID).

SIDs define the devices allowed to transmit and provide device identification and class of service

management. Software defines what type of transmission is allowed during the interval.

The CMTS system administrator typically assigns one or more SIDs to each cable modem,

corresponding to the classes of service the cable modem requires. Each MAP is associated with a

particular upstream channel. The SID concept supports multiple data flows and use of protocols such as

Resource Reservation Protocol (RSVP) that allows IP backbone QoS features to be extended to the

CMTS. The CMTS schedules the times granted for sending and receiving packets, and if defined,

manipulates the Type-of-Service (ToS) field in the IP packet header to accommodate QoS.

Cisco uBR7200 Series software supports extensions to DOCSIS 1.0 to operate with DOCSIS 1.0-based

Note

CMs or cable RF CPE devices (such as Cisco uBR924 cable access routers or Cisco uBR910 series cable

data service units) that also support DOCSIS 1.0 extensions.

Tip

DOCSIS 1.0 extensions address the problem of QoS for VoIP until DOCSIS 1.1 is solidified. Currently,

only certain vendors offer products that support DOCSIS 1.0 extensions.

DOCSIS 1.0 extensions build intelligence into the MAP file the CMTS sends to voice-enabled CMs to

address jitter and delay. The extensions support unsolicited grants, allowing a portion of bandwidth to

be dedicated to a voice call as soon as a subscriber initiates a call until that call is terminated. Unsolicited

grants are used to create a constant bit rate-like stream between the CMTS and the cable modem, in

contrast to typical data applications where CMs request grants from the CMTS before they can transmit

upstream. Refer to the

"This section summarizes Cisco uBR7200 series router software features for all

supported Cisco IOS Release trains, and directs you to additional configuration information for each

feature." section on page 1-22

for feature descriptions and links to configuration information.