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Chapter 7Performance optimisation for the support of TCIS pminislot can be used either for contention, contention/data, initial maintenance or stationmaintenance. A CM with data to send issues a request, using contention minislots. Inthe case of a collision, this protocol applies the exponential backoff algorithm and theinitial and truncated backoff values of this algorithm are indicated in each signallingframe called as MAP. This frame also describes the usage of the upstream bandwidth.7.4.1.2 PHY layer comparisonThe basic characteristics at the physical layer are summarised in Table 7.3. In this tablethe downstream and upstream spectrum allocations and the slight variations in thefrequency ranges for the DVB/DAVIC and DOCSIS standards are shown. Thedownstream channels support the legacy analogue broadcast television (frequency rangeare ≈ 80-450 MHz) and multiples of 1-6 MHz (for DOCSIS) or 1-8 MHz channels (forDVB/DAVIC and EuroDOCSIS) in the 450-860 MHz region for transmission of digitaldata. The upstream channels are also divided into 1-6 MHz but poor SNR limits the datacapacity to 1-10 Mbps per channel for DOCSIS and 1-6 Mbps for DVB/DAVIC. This isa low data rate compared to the downstream channel where 28-40 Mbps and 42-52Mbps are available for DOCSIS and DVB/DAVIC, respectively.In terms of modulation schemes, most manufacturers have implemented 64 and 256-QAM for the downstream and QPSK for the upstream channels. DOCSIS also supportsa 16-QAM modulation in the upstream direction.For upstream transmission at the physical layer, DOCSIS MAC PDUs (which arecomposed of Ethernet packets) are segmented into PHY codewords, which are ofTable 7.3 – Frequency allocation and modulation characteristics.UpstreamDownstreamFeature DVB/DAVIC DOCSISModulation QPSK, 16-QAM under development QPSK, 16-QAMFrequency 5 – 65 MHz DOCSIS: 5–42MHz, EuroDOCSIS: 5-65MHzSpacing 200 kHz, 1 MHz, 2 MHz, 4 MHz 200, 400, 800 kHz, 1.6, 3.2MHzData rates Mandatory: 3.088 Mbps,320 kbps, 640 kbps,Optional: 256 kbps, 1.54 kbps, 6.176 Mbps 1.28 Mbps, 2.56 Mbps, 5.12 Mbps, 10.24 kbpsModulation IB: QPSK, OOB: 16, 32, 64, 256-QAM 64-QAM or 256-QAMFrequency OOB: 70-130 MHz / 300-862 MHz,80 – 860MHzIB: 300-862 MHzSpacingOOB: 1 MHz, 2 MHz,DOCSIS: 6MHz, EuroDOCSIS: 8 MHzIB: 7/8 MHzData rates 52 Mbps (256-QAM), 42 Mbps (64-QAM) 42 Mbps (256-QAM), 30 Mbps (64-QAM)7-26