Juniper Networks G10 CMTS manual MAC Layer Functions

DOCSIS Module

•MAC Layer Functions

•	The DOCSIS Module provides the following MAC layer functions:
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•	! Classifier—Classifies upstream data frames into higher layer packet flows; classifies
•	downstream frames into corresponding service flows using service flow IDs (SFIDs).
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•	! Frame generator—Encapsulates downstream packets into DOCSIS frames.
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•	! Encryption—Encrypts downstream data frames in accordance with the DOCSIS Baseline
•	Privacy and Baseline Privacy Plus standards.
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•	! Decryption—Decrypts upstream data.
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•	! Fragmentation/concatenation—Reassembles upstream fragmented MAC frames and
•	deconcatenates concatenated MAC frames.
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•	! Frame parser—Parses DOCSIS MAC header, identifies packet as data or management,
•	and routes accordingly. Verifies header checksum (HCS) and cyclical redundancy
•	checking (CRC).
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•	! MAC management—Provides cable modem, service flow, and RF management
•	functions. Performs resource allocation scheduling of requests, service flows, QoS, and
•	other items. Handles cable modem and service flow admission control.
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•Physical Layer Functions

•	The DOCSIS Module provides the following physical layer functions:
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•	! Downstream transmission convergence (DTC) sublayer:
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•	! Manages the use of internal or external clock in MPEG transport stream; inserts
•	timestamp.
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•	! Examines packets for DOCSIS PID (packet identifier) and MPEG null PID and
•	multiplexes queued data packets into available MPEG packets.
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•	! Re-stamps DOCSIS PID with MPEG null PID if no data is queued for transmission.
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•	! Physical media dependent (PMD) sublayer:
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•	! Frames downstream MPEG packets by substituting synchronization byte with parity
•	checksum.
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•	! Implements FEC (forward error correction) and interleaving downstream;
•	descrambles data and decodes FEC upstream.
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•	! Modulates to IF baseband and upconverts to RF for downstream traffic;
•	demodulates upstream traffic.
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•	! Monitors upstream performance characteristics such as timing, frequency offset,
•	BER (bit error rate), and RF spectrum.
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34 JUNOSg 3.0 G10 CMTS Hardware Guide