Installation guide

1-7
Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 1 Overview
Operating Standards for Cisco IOS Software
The data path for DOCSIS networks is divided into the downstream (traffic sent from the CMTS to the
CM) and the upstream (traffic sent from the CM to the CMTS). Because 90% of traffic in a typical
Internet session is sent from the Internet to the user, the downstream is assigned the majority of the
bandwidth allocated to each user.
A single downstream can support thousands of users, depending on their particular service needs. All of
the users on a downstream might share the same upstream (as would be the case with a Cisco uBR7111
installation), or they might be split across several upstreams (as would be the case with a Cisco uBR7114
installation). For installations that have not been upgraded to two-way operations, the upstream can be
provided through a telco-return connection, where the cable modem directs the upstream traffic through
a modem that connects to the headend through the public telephone switched network (PTSN).
The following sections describe the downstream and upstream signals in more detail.
Downstream Signals
Downstream signals are modulated using QAM-64 or QAM-256 quadrature amplitude modulation,
based on the cable interface card used, your cable plant, and the significance of the data. DOCSIS
defines the messages and data types for CMTS to CM (or CM in an STB) 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 that each CM supports.
The Cisco uBR7100 series CMTS 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 CM or STB, or a multicast routing peer.
The Cisco uBR7100 series software periodically sends MAC (Media Access Control) 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 milliseconds.
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 CM, corresponding to the
classes of service the CM 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.
Note Cisco uBR7100 series software supports extensions to DOCSIS 1.0 to operate with DOCSIS 1.0-based
CMs or cable RF CPE devices (such as Cisco uBR924 cable access routers or Cisco uBR910 cable data
service units) that also support DOCSIS 1.0 extensions.
Tip DOCSIS 1.0 extensions address the problem of QoS for VoIP until the DOCSIS 1.1 specification is
finalized. Currently, only certain vendors offer products that support DOCSIS 1.0 extensions.
DOCSIS 1.0 extensions build intelligence into the MAP file, which the CMTS sends to voice-enabled
CMs to address jitter and delay. The extensions support unsolicited grants which are used to create a
constant bit rate-like stream between the CMTS and the CM, in contrast to typical data applications
where CMs request grants from the CMTS before they can transmit upstream.