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:
•
IP network layer
•Data link layer comprised of:
–Logical Link Control (LLC) sublayer conforming to Ethernet standards
–Link security sublayer for basic privacy, authorization, and authentication
–Media Access Control (MAC) sublayer for operation supporting variable-length protocol data units (PDU) and featuring:
CMTS control of contention and reservation transmission opportunities
A stream of minislots in the upstream
Bandwidth efficiency through variable-length packets
Extensions for the future support of Asynchronous Transfer Mode (ATM) or other types of PDU
Support for multiple grade of service and wide range of data rates
•Physical (PHY) layer comprised of:
–Downstream convergence layer conforming to MPEG-2 (Rec. H.222.0)
–Physical Media Dependent (PMD) sublayer for:
Downstream based on ITU-T Rec J.83 Annex B with either 64 or 256 quadrature amplitude modulation (QAM), concatenation of Reed-Solomon and Trellis forward error correction (FEC), in addition to variable-depth interleaving
Upstream, employing:
Quadrature phase shift keying (QPSK) or 16 QAM
Support for multiple symbol rates
CM controlled and programmable from the CMTS
Frequency agility
Support for fixed-frame and variable-length PDU formats
Time-division multiple access (TDMA)
Programmable Reed-Solomon FEC and preambles
Capability to support future physical layer technologies
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.
Figure 22-4 DOCSIS and OSI Protocol Layers
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 |
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:
•Dynamic Host Configuration Protocol (DHCP) server, as defined by RFC 2181. This server provides needed IP addresses for both the CM and subsequent PC devices that follow.
•Time of Day (TOD) server, as defined by RFC 868 for the purpose of time-stamping operational system events.
•Trivial File Transfer Protocol (TFTP) server, as defined by RFC 1350 for the purpose of registering and downloading CM configuration files for individual customer service. These configurations could include quality of service (QoS) parameters, baseline privacy (BPI) implementation, operating frequency assignments, the number of host devices, and so on.
For large-scale deployments, it is recommended that these servers be supported by dedicated hardware platforms to ensure rapid system response and scalability.
The DOCSIS specifications dictate a CM registration process as represented by Figure 22-5. In an environment equipped with a CMTS and supported with the required servers, a CM scans the downstream spectrum when it is initially powered on for a compatible RF channel carrying data adhering to DOCSIS physical layer characteristics. The CMTS periodically broadcasts upstream channel descriptors (UCD) over the DS channel, from which a CM will learn its assigned upstream operational frequency. The CM has now established both a US and a DS frequency.
Figure 22-5 Cable Modem Registration Sequence
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.
Following registration, the CM optionally initiates the activation of the 56-bit DES encryption algorithm to provide security between the CMTS and itself over the cable plant.
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.
DOCSIS prescribes that data forwarding through the CMTS may be transparent bridging or, as an alternate, may employ network layer routing or IP switching. It also specifies that data forwarding through the CM is link layer transparent bridging with modifications allowing the support of multiple network layers.
In addition, DOCSIS defines generic CMTS and CM hardware specifications to ensure multivendor interoperability in field deployments. These are summarized in Table 22-7.
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 |
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