Provisioning Cable Modems

Cable modem provisioning can seem a bit daunting when compared with other technologies. There are several steps involved in the process. The headend CMTS must have operational provisioning servers such as DHCP and TFTP in order for IP addressing and configuration files to be provided. The steps defined by DOCSIS are as follows:

Step 1 Downstream setup—At power-on, the cable modem scans and locks the downstream path for the allocated RF data channel in order for physical and data link layers to be established.

Step 2 Upstream setup—The cable modem listens to the management messages arriving via the downstream path. These include information regarding how and when to communicate in the upstream path. These are used to establish the upstream physical and data link layers.

Step 3 Layer 1 and 2 establishment—Connection established from CM to CMTS to build physical and data link layers.

Step 4 IP address allocation—After Layer 1 and 2 are established, Layer 3 can be allocated as well. This is done by the DHCP server.

Step 5 Getting DOCSIS configuration—The CM requests the DOCSIS

configuration file from the TFTP server. This is an ASCII file created by DOCSIS editors. A DOCSIS configuration file is a "binary file" and has the parameters for cable modems to come online in accordance to what the ISP is provisioning, such as maximum downstream and upstream rates, maximum upstream burst rate, class of service or baseline privacy, management information bases (MIBs), and many other parameters. This file can be loaded on the CM via TFTP or the CM can be manually configured.

Step 6 Register QoS with CMTS—The CM negotiates traffic types and QoS settings with the CMTS.

Step 7 IP network initialization—Once Layers 1, 2, and 3 are established and the configuration file is pulled from the TFTP server, the CM provides routing services for hosts on the subscriber side of the CM. It also performs some Network Address Translation (NAT) functions so that multiple hosts might be represented by a single public IP address.

As part of the initialization phase, the CM makes contact with a DHCP server on the provider's network. The DHCP server provides the following information to the CM:

■ Default gateway

■ DHCP relay agent

■ The complete name of the DOCSIS configuration file

■ Address of ToD server

■ Syslog server address

Once this information is obtained, the CM can issue a request to the ToD server to set its clock to the correct time. This facilitates syslog timestamps. At this point, also, it can issue a TFTP request to the TFTP server for its DOCSIS configuration file.

To facilitate standardization of router software on client CMs, the Cisco IOS images desired for use with the CMs can be stored on the TFTP server. The Cisco IOS version and filename can be specified in the DOCSIS configuration file to be downloaded at each power-on of the router. This takes several minutes, but does provide some degree of control on the part of the service provider.

Additionally, the router configuration(s) can be stored on the TFTP server to be downloaded at each power-on as well.

These are additional steps, as the Cisco IOS image and configuration can be stored on the router as traditionally done in most routing environments. This makes the power-on sequence a much shorter process in the event of a router reload.

Critical information for the configuration of CM hosts is carried in the DOCSIS configuration file. This is a file that contains information pertinent to all CM hosts attaching to the provider network. All DOCSIS-compliant configuration files include the following information elements:

■ Radio frequency information

— Downstream frequency

— Upstream channel ID

— Network access configuration

■ Class of service information

— Maximum downstream rate

— Maximum upstream rate

— Upstream channel priority

— Minimum upstream rate

— Maximum upstream channel burst

— Class of service privacy enable

■ Vendor-specific options

— Vendor-specific options

■ SNMP management

— SNMP write-access control and SNMP MIB objects

■ Baseline privacy interface configuration

— Authorize wait timeout

— Reauthorize wait timeout

— Authorization grace timeout

— Operational wait timeout

— Rekey wait timeout

— Authorize reject wait timeout

■ Customer premises equipment

— Maximum number of CPEs

— CPE Ethernet MAC address

■ Software upgrade

— TFTP software server IP address

— Software image filename

■ Miscellaneous

— Concatenation support

— CMTS authentication

After the CM has downloaded its configuration file, it can begin to communicate further on the network. Many options in the DOCSIS configuration file are unused for the bulk of CM provisioning.

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