Assuming HFC CATV topography as shown in Figure 22-6, CMTS equipment could be deployed at both the hub and the head end locations. For the purpose of this application, the Cisco universal broadband router is considered. The uBR7246 is an integrated router with a capacity of up to four CMTS units, with CMTS units available with one downstream port and from one to six upstream ports. In addition, the universal broadband router can be equipped for backbone connectivity from a large selection of port adapters, ranging from T1/E1 serial to Packet Over SONET (POS), to Dynamic Packet Transport (DPT) and from 10BaseT Ethernet to High-Speed Serial Interface (HSSI).
Figure 22-6 CMTS Deployment Possibilities in a Typical HFC CATV Plant
When selecting the backbone connection option, an assessment of the total backbone traffic and the available medium must be considered. In all likelihood, for our example, the backbone from the hub location would be transported optically to the head end, where all traffic would be aggregated by either a router or an IP switch before being forwarded to
the Internet or to the public switched telephone network (PSTN). Often, the MSO will provision a cache engine at the head end to reduce the bandwidth to the Internet and consequently reduce the facility lease cost.
Connectivity to the PSTN is often required to support either dialup Internet service, voice, or Telco return data service.
Telco return, if available from the MSO, is as an interim service offering because its plant topography has not been fully upgraded to two-way operation. In such applications, a high-speed downstream connection is established over the HFC plant, whereas the upstream connection is established via a Point-to-Point Protocol (PPP) connection over the telephone network, with a dialup modem at the customer premise and an access server located electrically before connecting to the Internet.
To emphasize the engineering considerations evaluated for establishing a DOCSIS network, a simple business case model will be evaluated. The relevant business parameters are summarized in Table 22-9. The analysis considers only residential and limited business customers supported by the head end location.
The business plan indicates that the DOCSIS service is for an existing serving area that will experience moderate growth, probably limited to new home construction, over the plan period. The operator intends to offer a single data product to each of the residential and business users within the serving area.
Penetration rate is the percentage of total homes passed in the serving area and represents the number of customers who buy the service.
The activity factor represents the percentage of subscribers who are actively online either uploading or downloading information.
The peak factor represents the relationship between the perceived or apparent bandwidth load of a system compared to the actual finite bandwidth available from the physical CMTS deployment. Peaking reflects the fact that data transfer to an individual user is typically during intervals of instantaneous duration.
The cable plant infrastructure (head end serving area) that is considered for this deployment has characteristics, assigned spectrum, and selected modulation as summarized in Table
22-10.
The head end supports a local serving area of 25,000 homes passed, distributed among 25 optical nodes with upstream CNR ranging from 30 to 36 dB. The CNR is a significant parameter because it dictates the number of nodes that can be combined into a single receive port. DOCSIS requires a CNR of 25 dB, irrespective of the upstream modulation chosen for certified operation.
The selection of QPSK and bandwidth of 800 kHz will impact the return path data throughput rate.
From the business case variables, a five-year customer and traffic profile summary is prepared and summarized as in Table 22-11.
The table indicates that the number of homes passed and the penetration rates have increased considerably over the evaluation, with the resultant perceived bandwidth to be processed by the CMTS equipment at the head end.
The number of CMTS units to support the perceived load must be determined considering the use of the Cisco uBR-MC16C consisting of one downstream and six upstream ports. First, however, a valid upstream aggregation scenario must be established.
Consider combining three nodes, each having a CNR of 36 dB, resulting in an aggregated CNR of approximately 27 dB that comfortably exceeds the DOCSIS criteria.
We must now determine the quantity of CMTS units to satisfy this application:
Considering the 800 kHz QPSK upstream limitations, the hardware selection must be validated against the traffic analysis for the business plan, as summarized in Table 22-11.
•
Downstream Validation
Two uBR-MC16C resulting in 2 ¥ 27 = 54 Mbps, compared to Year 5 requirement of 144/8 = 18.1 Mbps (where 144 MBps is the Year 5 apparent BW and 8 is the data peaking factor)
•Upstream Validation
Two uBR-MC16C with 9 active receivers configured for QPSK and 800 kHz BW, resulting in 9 ¥ 1.2 = 10.8 Mbps, compared to Year 5 requirement of 72/8 = 9 Mbps (where 72 MBps is the Year 5 apparent BW and 8 is the data peaking factor)
•Subscriber Limit Validation
The total number of Year 5 subscribers is 2207 + 5 = 2212 which is well within the suggested limit of 1200 subscribers per CMTS.
Based on the analysis of this simple business case, the initial deployment of CMTS hardware will meet the needs of the entire five-year plan and beyond, without compelling the operator to upgrade the configuration.
Future DOCSIS Applications
This chapter describes the DOCSIS 1.0 product definition intended to support high-speed data over a cable network. The standard is evolving to DOCSIS 1.1 to support additional services and future applications, which coincide with product enhancements to support the needs of the market to ensure network reliability, and high system availability.
Planned future services and applications include telephony based upon Voice over Internet Protocol (VoIP), video over IP using Motion Picture Expert Group (MPEG) frame format, quality of service (QoS), and enhanced security definitions. At the same time, CM and set top box (STB) devices capable of supporting these and other services are being introduced.
When considering the simultaneous support of these new services and applications, a more extensive planning concept must be considered.
Summary
Historical coaxial broadcast networking was described in this chapter, and its inherent detriments to advanced services were identified. HFC networking was included, with a brief description of its advantages and benefits capable of supporting high-speed data connectivity.
The limitations of prevailing HFC designs, DOCSIS availability criteria, and requisite cable plant specifications and terminology were addressed as well.
In addition, this chapter summarized the DOCSIS standard, signaling protocol, requisite supporting servers, generic product specifications, and applications. Representative CM status messages as viewed at the CMTS were provided to reflect parameters and tools critical for the operational aspects of a DOCSIS system.
Finally, future services and applications were identified to coincide with the evolution to DOCSIS 1.1.
Review Questions
Q—Describe the advantages or benefits offered by an HFC network.
A—HFC networks provide increased bandwidth, increased reliability, ready support for two-way operation, improved noise immunity, and reduced operation and maintenance costs.
Q—Identify the process of providing two-way operation of an HFC cable plant.
A—Two-way operation can be established on an HFC cable plant by installing the narrow-band upstream amplifiers in the amplifier housings, adding a narrow-band return laser at the optical node, providing an optical return path, and placing an optical receiver at the head end or hub location. Proper alignment procedure of the return path is also required.
Q—Describe the upstream and downstream bandwidths associated with the DOCSIS standard.
A—The DOCSIS bandwidth limitations are 5 to 42 MHz for the upstream direction, and 54 to 860 MHz for the downstream direction.
Q—Summarize the DOCSIS availability criteria.
A—A DOCSIS system must provide greater than 99 percent availability when forwarding 1500-byte packets at a rate of 100 packets per second when the cable plant meets the published DOCSIS system specifications.
Q—Identify the DOCSIS-defined networking layers.
A—The DOCSIS-defined layers consist of the IP network Layer, the data link layer, and the physical (PHY) layer.
Q—Identify the DOCSIS 1.0 servers, and describe their respective purposes in the network.
A—DOCSIS servers include the DHCP server (RFC 2181), which provides IP addresses to both the CM and PC devices; the TFTP server (RFC 1350), which registers and downloads CM configuration files; and the TOD server (RFC 868), which provides a time stamp to operational system events.
Q—What are the facilities in which an MSO might deploy the universal broadband router?
A—The universal broadband router can be deployed as needed in both the head end and hub locations.
Q—Define Telco return and tell when this application might be considered.
A—Telco return describes a data service that provides high-speed downstream connectivity over the coax plant, and low-speed connectivity over the PSTN. This application is typically used in rural networks, where the upgrade cost is prohibitive, or as an interim networking solution permitting the MSO to offer service while the cable plant is being upgraded for two-way service.
Q—List a few of the properties and future applications associated with DOCSIS 1.1.
A—DOCSIS 1.1 will support VoIP, enhanced security, packet concatenation and fragmentation, as well as QoS. Service applications include telephony and video.
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