【正文】 ilizes an 8 MHz wide modulated Signal. ? Spectrum Reuse CATV’s most fundamental concept is spectrum reuse. Historically, the overtheair spectrum has been assigned to many other purposes. Much of it is therefore not available for the carriage of TV. The result is an inadequate supply of spectrum to serve viewers needs. Cable can reuse spectrum that is sealed in its aluminum tubes. ? Coaxial Cable The principal physical media over which CATV systems are built. The main drawback of Coax Cable is it’s relatively high loss. Coax signal loss is a function of its diameter, dielectric construction, temperature, and operating frequency(f) 7 Cisco Systems Confidential Terms ? CarriertoNoise C/N (also CNR): The difference in amplitude between the desired RF carrier and the noise in that portion of the spectrum ? SignaltoNoise S/N (also SNR): Similar to C/N but relates to a BaseBand signal ? Ingress Noise Overtheair signals that are inadvertently coupled into the nominally closed coaxial cable distribution system. Difficult to track down and intermittent in nature. ? QPSK Quadrature Phase Shift Keying. A digital modulation method in which the there are 2 data bits represented with each baud symbol. ? QAM Quadrature Amplitude Modulation. A digital modulation method in which the value of a symbol consisting of multiple bits is represented by amplitude and phase states of a carrier. Typical QAM types are 16QAM(4 bits per symbol), 64QAM(6 bits per symbol), and 256QAM(8 bits per symbol) ? Downstream(DS) Signal flow from headend toward subscribers. Also called Forward Path ? Upstream(US) Signal flow from the subscribers to the Headend. Also called the Return or Reverse Path 8 Cisco Systems Confidential Terms ? Carrier or RF carrier. A signal on which another, lowerfrequency signal is modulated in order to transport the lowerfrequency signal to another location. ? FDM Frequency Division Multiplexing. A data transmission method in which a number of transmitters share a transmission medium, each occupying a different frequency. ? Headend The location where the Cable pany aggregates, bines, mixes, and modulates all signals in order to send them downstream and receive upstream signals ? Broadband Refers to the ability to frequency division multiplex(FDM) many signals in a wide RF bandwidth over an HFC work, and the ability to vast amounts of information. ? FEC Forward Error Correction. In data transmission, a process by which additional data is added that is derived from the payload by an assigned algorithm. It allows the receiver to determine if certain classes of errors have occurred in transmission and, in some cases, allows other classes of errors to be corrected. 9 Cisco Systems Confidential Measurement Units ? dB Decibel. A measure of the relative strength of two signals. Multiplication bees addition and Division bees subtraction. ? dBm Decibels with respect to one milliwatt. A unit of RF signal strength used in satellite work and other munications applications ? dBmV Decibels with respect to 1 millivolt in a 75ohm system. The unit of RF power used in CATV industry. ? 0 dBmV = 1 millivolt measured across an impedance of 75 ohms. 10 Cisco Systems Confidential Devices and Symbols Head End Fibre Satellite Antenna VHF/UHF Antenna RF Amplifier BiDirectional Amplifier Splitter 8Way Tap Fibre Transmitter Fibre Node 11 Presentation_ID 169。 16 QAM 64 QAM。 1999, Cisco Systems, Inc. DOCSIS Server Types and Purpose 41 Cisco Systems Confidential DOCSIS Server Requirements Mandatory Servers ? DHCP (Dynamic Host Control Protocol) Server RFC 2131: Dynamically provides and reuses IP addresses to CM and PC ? ToD (Time Of Day) Server RFC 868: Provides Timestamp for Logged events. ? TFTP (Trivial File Transfer Protocol) Server RFC 1350: Provides Modem Configuration files. Optional Servers ? DNS (Domain Name Server) ? Security Server ? Log Server 42 Cisco Systems Confidential Cisco Network Registrar in Headend TFTP Server Cable Modem Termination System Rack DHCP/DNS Network Registrar Security Server Remote Access Server Rack Routers, etc. Int er Time Server Log Server 43 Presentation_ID 169。 Requests (. for new modems with no SID to e online). ? Upstream uses contention based sub channels DOCSIS Protocol and Signal Sequence 46 Cisco Systems Confidential Link Establishment Head End Interface Clock Synch Broadcast (One per msec) Cable Modem Scan DS for QAM Wait for Clock Synch Set signaling clock Fails if no / bad DS Wait for UCD Set US Channel Freq UCD Broadcast US Channel Descriptor, DS Channel ID amp。 registration was refused due to bad mic reject(c) modem did attempt to register。 1999, Cisco Systems, Inc. DOCSIS Protocol and Signaling Sequence Demonstration 53 Cisco Systems Confidential offline modem considered offline init(r1)