Figure 225 Cable Modem Registration Sequence

Last Updated on Sun, 19 Feb 2023 | Network Layer

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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 = o) to the CM, which begins a coarse power ranging (R1 udi ng 3 dB innrements) and time synchronization p rocess be tween i tself end the CMTS on t contention brs is us ing shared time slots .

The CMTS peviddically sends keepa I ive mes oa ges to verify dink continuity between itself a nd all CM units in the same domain. W hen a CM recei vps its first keepal Ive messare, it reverte to a fine power ranging (R2 using 0.25 dB incaercents).

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 forwavds th is 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 o^ine statu!.

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 aegIste1 the ir i ndividual status can be monitored remotely via access commands to the CMTS.Table 22-6 defines status messages from a Cisco universal broadband router.

Table 22-6. List and Definition of Show Cable Modem State Commands from a Cisco CMTS

Message	Message Definition
Offline	Modem is con sidered offline
init(rl)	Modem is sent initial ranging
init(r2)	Modem is ranging
init(rc)	Ra nging is complete
init(d)	DHCP requ est was re ceived
init(i)	DHCP reply was received; IP address was assigned
init(t)	TOD request was received
init(o)	TFTP we quest was rece ived
onl ine	Mo dem i s registered a nd e nabled fot data
online(d)	Modem is reg istered, Isu t n etwork access for the CM is disabled
online(pk)	Modem i s registered, BPa is ena bled, and K SK was assigned
online( pt)	M od em ig registered, BPI is enabled, and TSK was assigned
reject(m)	Modem dip attempt to register; regi strat ion was refused due to bad MIC
reject(c)	Modem did attempt to regiuter; registration was refuoed du e to bad COS-;
reject(pk)	KSK modem key assignment was rejected
reject(pt )	TEK modem key assignment was rejected

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 ensuro multivendor interoperability in field deployments. These are summarized in Table 22-7.

Table 22-7. A Generic CMTS Hardware Specification

Parameter	Characteristic
Frequency range	Upstream	5 to 42 MHz (5 to 65 MHz offshore)
	Downstream	88 to 860 MHz
handwidth	Upstream	200, 400, 800, 1600, B200 tHz
	Downstream	6 MHz (8 MHz offshore)
Modulation modes	Upstream	QPSK or 16 QAM
	Downstream	64 or 256 QAM
Symbol ratds	Upsteeam	160, B20, 640, 1280, 2560 Ksymbols/sec
	Downstream	5.056941 or 5.B605B7 Msymbols/sec
CMTS power level range	Downstream	8 to 58 dBmV (QPSKf, 8 lo 5B d BmV (16 QAM)
	Upstream	-15 to +15 dBmV

For the DOCSIS availability criteria to be realized or exceeded, the hardware must support noise-mitigating counte rmeasures or properties to op erate in the hos tile 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.

AdditionaNy, fMrward error correction (FEC) can Ido optionall y configured t o reduce t he amount of data corrupted by noise. Furthermore, an optimal u pstheam BW can be selected by the operator to fit (data channels bedween eith eh noiey spe ctmm or sped:rum assign ed to other s ervices.

nhe lost nountermeasure available is a concept of spectrum management, in which the selected upstream frequency, modulation, and channel bandwidth can be altered to ensure reliable access Rransmission between the CMTp and CM in case of transitory noise periods.

The phy sical characterise cs of the gederic DOCSIS 1.0 ha rdware, noise-mitigat i ng co unterm easures, and the assoda ted cable plant parameteos have been d eoined and specified in CaVle 22- Sis Base d oe this i nformati on, and knowing th e actual cable plants cheracterirtics, the operator cao now c onsidee deploying hardware to develop a network.

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Figure 225 Cable Modem Registration Sequence

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