Cisco Systems UBR900 ,

Voice Over IP Operations

Configuring the Cisco uBR900 Series Cable Access Routers 13

The Cisco uBR924 cable access router supports the following service classes:

•The first CoS in the router’s configuration file is configured as the “Tiered Best Effort Type

Class” used by the router as the primary QoS for all regular data traffic. The class has no

minimum upstream rate specified for the channel.

This service class results in the assignment of a primary SID for the router. In addition to being

used as a data SID, the router uses this SID for all MAC message exchanges with the CMTS. Any

SNMP management traffic from the network to the Cisco uBR924 will also use this SID.

While this class is strictly “best effort,” data traffic within this class can be prioritized into eight

different priority levels. The CMTS system administrator, however, must define the supported

upstream traffic priority levels and include the traffic priority fields in the configuration file

downloaded to the Cisco uBR924.

•When creating a configuration for the Cisco uBR924, the CMTS system administrator typically

configures extra classes of service. These secondary classes of service are expected to be higher

QoS classes and are used by higher priority traffic such as voice. These classes have a minimum

upstream rate specified for the channel.

The multiple SID-per-router feature enables the Cisco uBR924 to use multiple SID queues for

differentiated services. The Cisco uBR924 diverts voice call traffic to the higher QoS secondary SID,

while forwarding “best effort” data from the Ethernet interface and MAC messages on the primary

SID.

H.323 is an International Telecommunications Union (ITU) standard that specifies call signaling and

control protocols for a shared IP data network. The Cisco uBR924 cable access router acts as an

H.323 gateway. In architectures using the VoIP H.323 protocol stack, the session application

manages two call legs for each call: (1) a telephony leg managed by the voice telephony service

provider; (2) the VoIP leg managed by the cable system operator—the VoIP service provider. Use of

the H.323 protocol typically requires a dial plan and mapper at the headend or other server location

to map IP addresses to telephone numbers.

When both legs of the call have been set up, the session application creates a conference between

them. The opposite leg’s transmit routine for voice packets is given to each provider. The CMTS

router passes data to the gateway and gatekeeper. The H.323 stack provides signalling via H.225 and

feature negotiation via H.245.

To make and receive H.323 calls, the Cisco uBR924 cable access router must know:

•The IP address of the gateway for the destination dialed. You can configure these IP addresses

statically using the voip dial peer group CLI commands, or you can obtain these addresses

dynamically from the gatekeeper using Registration, Admission, and Status (RAS).

•The telephone numbers of the attached devices. You can configure the telephone numbers

attached to the Cisco uBR924 by configuring the IP addresses statically using the pots port CLI

commands. When using Cisco Network Registrar (CNR) version 3.0 or higher with the relay.tcl

and setrouter.tcl scripts, you can obtain these addresses dynamically from CNR. The telephone

numbers of attached devices are then sent in DHCP response messages. When the Cisco uBR924

processes the DHCP response, it automatically creates the pots dial peer for each port, creates

the voip dial peer for the RAS target, and starts the H.323 RAS gateway support.