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Historically, CATV has been a unidirectional medium designed to carry broadcast analog video channels to the maximum number of customers at the lowest possible cost. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported. The technology to provide high-margin, two-way services remained elusive to the operator.
During the 1990s, with the introduction of direct broadcast satellite (DBS) and digital subscriber line (DSL), the cable operators experienced a serious challenge to their existence by competing technologies threatening to erode market share of their single product.
The DOCSIS 1.0 standard prescribes multivendor interoperability and promotes a retail model for the consumer's direct purchase of a cable modem (CM) of choice. To ensure multivendor interoperability, CableLabs subjects all products offered to rigorous testing. Equipment successfully passing all tests will be CableLabs Qualified for head-end Cable Modem Terminating System (CMTS), and CableLabs Certified for CM devices.
To date, the DOCSIS 1.0 standard is proving to be a universal success, with deployments now in operation worldwide.
CableLabs, in conjunction with the vendor and user communities, is now in the process of defining DOCSIS 1.1 for the purpose of supporting Voice Over Internet Protocol (VoIP) and advanced security, and is also paving the way for advanced future multimedia services.
A CATV network consists of a head-end location where all incoming signals are received and, regardless of their source, frequency-division multiplexing (FDM) is applied, amplified, and transmitted downstream for distribution to the complete cable plant.
Original CATV networks, as shown in Figure 22-1, were exclusively one-way, comprised of diverse amplifiers in cascade to compensate for the intrinsic signal loss of the coaxial cable in series with taps to couple video signal from the main trunks to subscriber homes via drop cables.
Besides being unidirectional, the long amplifier cascades resulted in a system with high noise that was inherently unreliable and failure-prone, in addition to being susceptible to lightning strikes and ingress noise from foreign radio frequency (RF) signals.
The first significant improvement to the CATV plant was the introduction of fiber-optic technology and the advent of the HFC plant (see Figure 22-2).
Portions of the coaxial cable and supporting amplification elements are replaced with multifiber optic cable from a head end or hub location. The aggregated video signal is used to modulate a downstream laser, which transmits the optical signal to an optical node, which in turn converts the signal from an optical to an electrical signal that can then be propagated downstream to the entire customer serving area.
Two-way operation is achieved by the addition of requisite upstream amplifiers in the amplifier housings, the addition of a narrow-band upstream laser in the optical node, a dedicated upstream fiber to the head end, and a compatible optical receiver to convert any upstream information to an electrical signal. When all components are in place, proper return path alignment is required.
By means of adding an optical RING topography, the cable network affords greater reliability, supports greater bandwidth with the capability to transport more information, and is ready to support two-way operation by the simple addition of requisite components, as illustrated in Figure 22-3.
Network robustness, scalability, and flexibility is further improved by the introduction of the intermediate hub from which advanced services can ultimately be launched.
The HFC network and topography as outlined become the basic building blocks for developing access transport capabilities needed by the MSOs to compete in the dynamic communication environment.
The historical broadcast video channel assignments limit the upstream or reverse direction from the customer to the spectrum between 5 to 42 MHz. This upstream spectrum is frequently hostile to return path connectivity due to the ingress of foreign interfering signals such as ham radio citizen band (CB), among other legitimate RF emissions.
Table 22-1 summarizes the specifications for the downstream direction, and Table 22-2 summarizes the specifications for the upstream direction.
| Downstream Parameter | Assumes nominal analog video carrier level (peak envelope power) in a 6-MHz channel with all conditions present concurrently and referenced to frequencies greater than 88 MHz |
RF channel spacing (BW) | 6 MHz |
Transit delay, CMTS to most distant customer | Less than or equal to 0.800 ms |
CNR in a 6-MHz band | Not less than 35 dB (analog video level) |
C/I ratio for total power (discrete and broadband ingress signals) | Not less than 35 dB within the design BW |
Composite triple-beat distortion for analog-modulated carriers | Not greater than -50 dBc within the design BW |
Composite second-order distortion for analog-modulated carriers | Not greater than -50 dBc within the design BW |
Cross-modulation level | Not greater than -40 dBc within the design BW |
Amplitude ripple | 0.5 dB within the design BW |
Group delay ripple in the spectrum occupied by the CMTS | 75 ns within the design BW |
Microreflections bound for dominant echo | -10 dBc at less than or equal to 0.5 ms -15 dBc at less than or equal to 1.0 ms -20 dBc at less than or equal to 1.5 ms -30 dBc at less than or equal to 1.5 ms |
Carrier hum modulation | Not greater than -26 dBc (5 percent) |
Burst noise | Less than 25 ms at a 10 Hz average rate |
Seasonal and diurnal signal level variation | 8 dB |
Signal level slope (50 to 750 MHz) | 16 dB |
Maximum analog video carrier level at the CM input, inclusive of above signal level variations | 17 dBmV |
Lowest analog video carrier level at the CM input, inclusive of above signal level variation | -5 dBmV |
The greater challenge for the operator is to realize sufficient usable upstream bandwidth to achieve the systems throughput requirements for data or other services. The limited upstream bandwidth must often be shared with other services, ranging from impulse pay-per-view (IPPV), telemetry, and alarm gathering information from the active elements in the cable plant, as well as having to compete with interfering signals that radiate into the lower frequency range.
Because of the limited and often-hostile upstream bandwidth, the hardware design must implement diverse countermeasures to mitigate the effects of both fixed and transient harmful noise. In addition, the network designer must choose from the available remaining spectrum and often must implement bandwidth compromises for a DOCSIS deployment.
The DOCSIS interface specifications enabled the development and deployment of data-over-cable systems on a nonproprietary, multivendor, interoperable basis for transparent bidirectional transfer of Internet Protocol (IP) traffic between the cable system head end and customer locations over an all-coaxial or hybrid-fiber/coax (HFC) cable network.
The system consists of a CMTS located at the head end, a coaxial or HFC medium, and a CM located at the premises of the customer, in conjunction with DOCSIS-defined layers that support interoperability and evolutionary feature capabilities to permit future value-added services.
DOCSIS layer definitions are as follows:
In addition, the specification defines means by which a CM can self-discover the appropriate upstream and downstream frequencies, bit rates, modulation format, error correction, and power levels. To maintain equitable service levels, individual CMs are not allowed to transmit except under defined and controlled conditions.
The DOCSIS layers are represented by Figure 22-4 and are compared with the classic OSI layer.
The DOCSIS physical layer permits considerable flexibility to ensure quality transmission can be achieved over cable plants of varying quality. Of significance are the optional upstream channel bandwidths and modulation choices available for both the upstream and downstream signal flows.
Based upon bandwidth and modulation options, in addition to DOCSIS-specified symbol rates, the total and effective data rates of DOCSIS facilities are summarized in Tables 22-3 through 22-5. The overhead generated by FEC inefficiency represents the difference between the respective rates.
Modulation type | 64 QAM | 256 QAM |
Symbol rate | 5.057 MSs | 5.360 MSs |
Total data rate | 30.34 Mbps | 42.9 Mbps |
Effective data rate | 27 Mbps | 38 Mbps |
Bandwidth | 200 kHz | 400 kHz | 800 kHz | 1600 kHz | 3200 kHz |
Symbol rate | 0.16 MSs | 0.32 MSs | 0.64 MSs | 1.28 MSs | 2.56 MSs |
Total data rate | 0.32 Mbps | 0.64 Mbps | 1.28 Mbps | 2.56 Mbps | 5.12 Mbps |
Effective data rate | 0.3 Mbps | 0.6 Mbps | 1.2 Mbps | 2.3 Mbps | 4.6 Mbps |
Bandwidth | 200 kHz | 400 kHz | 800 kHz | 1600 kHz | 3200 kHz |
Symbol rate | 0.16 MSs | 0.32 MSs | 0.64 MSs | 1.28 MSs | 2.56 MSs |
Total data rate | 0.64 Mbps | 1.28 Mbps | 2.56 Mbps | 5.12 Mbps | 10.24 Mbps |
Effective Data Rate | 0.6 Mbps | 1.2 Mbps | 2.3 Mbps | 4.5 Mbps | 9 Mbps |
DOCSIS further specifies that for a system to become functional and operational, mandatory servers must interface the CMTS and CM deployments. These servers include the following:
For large-scale deployments, it is recommended that these servers be supported by dedicated hardware platforms to ensure rapid system response and scalability.
The CMTS periodically transmits upstream bandwidth allocation maps (henceforth referred to as MAP) in shared time slots in the DS direction.
The CMTS assigns a temporary service identifier (SID) (typically SID = 0) to the CM, which begins a coarse power ranging (R1 using 3 dB increments) and time synchronization process between itself and the CMTS on a contention basis using shared time slots.
The CMTS periodically sends keepalive messages to verify link continuity between itself and all CM units in the same domain. When a CM receives its first keepalive message, it reverts to a fine power ranging (R2 using 0.25 dB increments).
Following the R2 process, a CM is considered to have established a link between itself and the CMTS, but the link will be broken if 16 consecutive keepalive messages are lost.
On a contention basis in shared time slots, using a temporary SID, a CM forwards a bandwidth request to the CMTS, which in turn forwards a grant to the CM, permitting
it to forward upstream information in allocated time slots. The CM subsequently
makes a DHCP discovery followed by a DHCP request. The CMTS forwards a DHCP acknowledgment from the DHCP server containing an IP address, a default gateway, the addresses of a TFTP and TOD server, and a TFTP configuration file name.
The CM subsequently initiates the TOD and TFTP process. From the TFTP server, the CM receives a configuration file containing QoS, security, applicable frequency assignments, and any new software images.
The CM forwards this configuration file to the CMTS and initiates a registration request. If the configuration file is valid, the CMTS assigns the CM a permanent SID and registers the CM to online status.
As CMs register, their individual status can be monitored remotely via access commands to the CMTS. Table 22-6 defines status messages from a Cisco universal broadband router.
| Message | Message Definition |
Offline | Modem is considered offline |
init(r1) | Modem is sent initial ranging |
init(r2) | Modem is ranging |
init(rc) | Ranging is complete |
init(d) | DHCP request was received |
init(i) | DHCP reply was received; IP address was assigned |
init(t) | TOD request was received |
init(o) | TFTP request was received |
online | Modem is registered and enabled for data |
online(d) | Modem is registered, but network access for the CM is disabled |
online(pk) | Modem is registered, BPI is enabled, and KEK was assigned |
online(pt) | Modem is registered, BPI is enabled, and TEK was assigned |
reject(m) | Modem did attempt to register; registration was refused due to bad MIC |
reject(c) | Modem did attempt to register; registration was refused due to bad COS |
reject(pk) | KEK modem key assignment was rejected |
reject(pt) | TEK modem key assignment was rejected |
In addition, DOCSIS defines generic CMTS and CM hardware specifications to ensure multivendor interoperability in field deployments. These are summarized in Table 22-7.
| Parameter | Characteristic | |
Frequency range | Upstream Downstream | 5 to 42 MHz (5 to 65 MHz offshore) 88 to 860 MHz |
Bandwidth | Upstream Downstream | 200, 400, 800, 1600, 3200 kHz 6 MHz (8 MHz offshore) |
Modulation modes | Upstream Downstream | QPSK or 16 QAM 64 or 256 QAM |
Symbol rates | Upstream Downstream | 160, 320, 640, 1280, 2560 Ksymbols/sec 5.056941 or 5.360537 Msymbols/sec |
CMTS power level range upstream downstream |
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For the DOCSIS availability criteria to be realized or exceeded, the hardware must support noise-mitigating countermeasures or properties to operate in the hostile upstream. For the upstream, the operator has a choice of either QPSK or 16 QAM enabling operation within a degraded CNR, but with reduced spectral efficiency.
Additionally, forward error correction (FEC) can be optionally configured to reduce the amount of data corrupted by noise. Furthermore, an optimal upstream BW can be selected by the operator to fit data channels between either noisy spectrum or spectrum assigned to other services.
The last countermeasure available is a concept of spectrum management, in which the selected upstream frequency, modulation, and channel bandwidth can be altered to ensure reliable access transmission between the CMTS and CM in case of transitory noise periods.
The physical characteristics of the generic DOCSIS 1.0 hardware, noise-mitigating countermeasures, and the associated cable plant parameters have been defined and specified in Table 22-8. Based on this information, and knowing the actual cable plants characteristics, the operator can now consider deploying hardware to develop a network.
| Parameter | Characteristic |
CM power level range: Output
Input |
QPSK: 8 to 58 dBmV 16 QAM: 8 to 55 dBmV -15 to 15 dBmV |
Transmission level | -6 to -10 dBc |
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).
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.
| Plant Growth | 0.75 Percent per Annum |
High-speed data service offered: Residential Business |
256 kbps DS 128 kbps US 1.5 Mbps DS 512 kbps US |
Penetration rates: Residential Business |
3 percent in first year with 30 percent CAGR Two in year two; add one per year thereafter |
Analysis assumptions: Residential activity factor Business activity factor Data peak factor |
25 percent 25 percent 8 percent |
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 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.
HFC characteristics | Downstream: 88 to 750 MHz Upstream: 5 to 42 MHz |
Head end serving area | 25,000 homes passed 25 nodes (average of 1,000 homes each) CNR varying between 30 and 36 dB; average of 32 dB |
Available spectrum | Downstream: EIA channel 60 at 439.25 MHz Upstream: 32 MHz, 800 kHz bandwidth |
Modulation | Downstream: 64 QAM Upstream: QPSK |
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:
25 nodes/3 nodes per receiver = 9 receivers, indicating a need for two uBR-MC16C units
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.
| Year 1 | Year 2 | Year 3 | Year 4 | Year 5 |
| Homes passed | 25,000 | 25,188 | 25,376 | 25,666 | 25,758 |
| Residential customer | 750 | 982 | 1286 | 1685 | 2207 |
| Business customer |
| 2 | 3 | 4 | 5 |
| Total traffic | DS 48M US 24M | DS 64M US 32M | DS 84M US 42M | DS 100M US 55M | DS 144M US 72M |
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.
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.
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.
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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Ciciora, Walter, James Farmer, and David Large. Modern Cable Television Technology. Boston: Morgan Kaufmann Publishers Inc., 1998.
Grant, William. Cable Television, Third Edition. New York: GWG Associates, 1997.
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Posted: Wed Feb 20 21:23:47 PST 2002
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