Archives

By Ron Hranac, CT Senior Technology Editor

In part two of our tutorial on cable Internet access, Ron Hranac explains cable modem initialization, takes a closer look at DOCSIS and discusses the challenges of running a data service.

When a Data Over Cable Service Interface Specification (DOCSIS) cable modem is installed at a subscriber location, the modem must go through several steps in order to be initialized for operation. These steps are for the most part transparent to the user and fully automatic. The following description is a simplified overview of the initialization process. More detailed information may be found in the Data-Over-Cable Service Interface Specifications, Radio Frequency Interface Specification SP-RFI-I05-991105 at CableLabs' Web site (www.cablemodem.com/SP_RFI_I05_991105.pdf).

After power-up, the modem begins to scan the downstream radio frequency (RF) spectrum for a 64- or 256-QAM (quadrature amplitude modulation) digitally modulated carrier that contains cable modem-specific data. Once synchronization--sometimes called QAM lock--with the appropriate carrier is established, the modem looks for an upstream channel descriptor (UCD) message in the downstream data. The UCD provides broadcast instructions to cable modems about where to set their upstream transmit frequency.

The modem begins to transmit on the assigned upstream frequency at its lowest transmit power level, gradually increasing the power until the cable modem termination system (CMTS) "hears" the new modem. At this point, the CMTS and modem begin two-way communication, a ranging process where cable modem upstream RF operating levels are fine-tuned, and timing offsets are established to avoid data collisions with other cable modems.

The next step is to establish Internet protocol (IP) connectivity. The cable modem sends the CMTS a dynamic host configuration protocol (DHCP) request to obtain an IP address and other parameters necessary to establish IP connectivity. After IP connectivity is established, the modem requests the current date and time from the time-of-day (TOD) server. Accurate timing is necessary for time-stamping logged events.

After DHCP and TOD, the CMTS downloads to the modem operational parameters using trivial file transfer protocol (TFTP). When the operational parameters have been transferred to the modem, it completes a registration process. If the cable operator is using DOCSIS baseline privacy (BP, a type of data encryption for cable modem traffic) on the network, the cable modem must be provisioned for this and BP initialized.

Assuming all of the cable modem initialization steps are successful, the modem is now considered operational. The data rates shown in Table 1 and Table 2 are supported by DOCSIS.

A closer look at DOCSIS

As mentioned in part one of this series (see May issue, page 140), the DOCSIS standard is intended to ensure interoperability among different manufacturers' cable modems and CMTSs. Interoperability plays a critical role in moving cable modems to the retail market. Indeed, CableLabs-certified cable modems are available at retail outlets in a limited number of markets where the incumbent cable company is operating a DOCSIS cable modem platform. As high-speed data services become more widely available, expect cable modem retail availability to grow.

In addition to being an interoperable cable modem standard, DOCSIS includes recommended parameters for cable network performance. The DOCSIS Radio Frequency Interface Specification summarizes minimum recommended performance criteria for what may be considered a DOCSIS-compliant cable network. Reliable high-speed data transmission over the cable network can be expected if the network meets or exceeds these recommendations. As well, the cable network--headend, distribution network and subscriber drops--should comply with relevant government technical specifications, such as those found in Part 76 of the Federal Communications Commission's (FCC) Rules and Regulations.

Table 3, Table 4, Table 5 and Table 6 are from the previously mentioned DOCSIS 1.0 Radio Frequency Interface Specification. Parameters in the DOCSIS 1.1 Radio Frequency Interface Specification (www.cablemodem.com/RFI-I06-001215.pdf) are similar, although are not included here.

Additional DOCSIS operating recommendations of interest include 64- and 256-QAM downstream digitally modulated carrier transmission levels and post-FEC (forward error correction) bit error rate (BER). The average power level of each digitally modulated carrier should be set in the -10 decibel-carrier (dBc) to -6 dBc range relative to analog TV channel visual carrier levels. The recommended post-FEC BER is less than or equal to 10^-8 at specified carrier-to-noise (C/N) ratios and modem input levels.

Putting it all together

Installing, connecting and configuring cable modem equipment--the CMTS, router, Ethernet switch, access server and other components--are perhaps the easiest parts of deploying high-speed data. Operating the service is where the real challenges occur.

Once a cable operator ventures down the path of non-traditional services, the business quickly changes from an entertainment delivery service to a full-fledged telecommunications service. Some common challenges include:

• Cable modem installations--Many cable companies have found, at least in the early stages of a high-speed data service rollout, that cable modem installations require two persons to complete. One installs a new cable outlet in the cable modem location (and upgrades or replaces the existing in-home cable wiring if necessary), while the other installs the network interface card (NIC), related software and the cable modem itself. As cable companies have become more experienced with high-speed data, the two job functions have been successfully combined in many instances.
• Training--Training programs are critical to the success of a high-speed data service. All personnel, from customer service representatives to installers and technicians, must be thoroughly trained in areas and skills related to the new business. Indeed, field personnel will require skills beyond traditional cable television RF technology. The high-speed data generation of installers and technicians must understand Ethernet, data networks, personal computers (PCs) and software.
• Capacity management--One area of extreme importance is network capacity management. As more cable modem subscribers use the shared-medium service, data throughput will slow down significantly if capacity is not managed effectively. If there are plans to implement data rate limiting [for instance, 2 megabits per second (Mbps) downstream and 128 kilobits per second (kbps) upstream], it should be done from the first day of cable modem service--not after early subscribers have grown accustomed to what they may perceive as unlimited data rates. Data throughput must be monitored and bottlenecks identified and eliminated before they become visible to subscribers. This includes Internet backbone connectivity, because high-speed circuits sometimes take several months to be installed and provisioned.
• Network maintenance practices--Two-way cable system maintenance practices are more involved than those for one-way cable networks. To help mitigate upstream ingress problems, the network must be kept much tighter from a downstream signal leakage perspective. Simply maintaining leakage to the FCC 20 microvolt per meter limit is not good enough. Network maintenance that results in service interruptions must be performed at times that will affect the fewest subscribers. This often means scheduling network downtime between midnight and 6 a.m. Even routine maintenance such as system sweeping requires that the sweep transmitter be set up so as not to interfere with the digitally modulated carriers in the downstream and upstream spectrums. Forward and reverse sweeping is recommended to maintain proper system operation.
• Security--The always-on nature of two-way cable modem service means subscribers may be at risk for denial-of-service attacks and similar unwanted hacking. Subscribers should be educated about these risks and advised of what they can do to minimize problems. My column in the February 2000 issue of CT includes several suggestions. As well, the cable operator should consider activating DOCSIS baseline privacy, and other proactive measures.

Summary

Cable Internet access using cable modems is well beyond the trial stage and represents a viable way for the cable TV industry to remain successful in today's competitive environment. Interoperable DOCSIS technology is making it even easier for cable operators to deploy high-speed data, and is a key factor in moving cable modems to the retail consumer marketplace.

Cable operators today can choose to develop and operate a stand-alone high-speed data service, or partner with third party high-speed Internet service providers that offer content and backbone access, and in some instances full turnkey solutions.

Ron Hranac is a consulting systems engineer for Cisco Systems, and senior technical editor for Communications Technology. You may reach him at .

 Back to June 2001 Issue

Access Intelligence's CABLE GROUP
Communications Technology | CableFAX Daily | CableFAX's CableWORLD | CT's Pipeline
CableFAX Magazine | CableFAX databriefs | Broadband Leaders Retreat | CableFAX Leaders Retreat