FY07 Q1 Cisco Presentation Template White VersionCisco Confidential
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Combining Point
We assume (as per DOCSIS Spec) we timestamp the Sync packets in the DS TC sub layer
We assume “zero” delay added by coax cable from CMTS or EQAM to the combining point
CM
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CMTS Timestamp The count state of an incrementing 32 bit binary counter clocked with the
CMTS 10.24 MHz master clock.
The CMTS timestamp represents the count state at the instant that the first byte (or a fixed time offset from the
first byte) of the Time Synchronization MAC Management Message is transferred from the Downstream
Transmission Convergence Sublayer to the Downstream Physical Media Dependent Sublayer as described in
Section 4.3.7. The CMTS MUST NOT allow a SYNC message to cross an MPEG packet boundary.6
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Cable Network Transmission
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The timestamp value in the Sync
Packet is placed at the moment the first byte of the packet is leaving the DS TC Layer
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SAW
BCM3033
TX
UPX
SAW
BCM3033
TX
UPX
DS1
DS0
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Fix or Deterministic delay
Miller DTI FPGA
Console management
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100
100
Sync
(100)
110
MAP
Network Delay (one way) =20
140
180
RNG-REQ
RNG-RSP
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100
120
Sync
(100)
110
MAP
Network Delay (one way) =20
Delay between CMTS and EQAM = 20
CMTS and EQAM have the “same” timeline
140
140
RNG-REQ
RNG-RSP
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Combining Point
We assume (as per DOCSIS Spec) we timestamp is in the TC sub layer
We assume “zero” delay added by coax cable from CMTS or EQAM to the combining point
Cable 5/0/0
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CMTS Timestamp The count state of an incrementing 32 bit binary counter clocked with the
CMTS 10.24 MHz master clock.
The CMTS timestamp represents the count state at the instant that the first byte (or a fixed time offset from the
first byte) of the Time Synchronization MAC Management Message is transferred from the Downstream
Transmission Convergence Sublayer to the Downstream Physical Media Dependent Sublayer as described in
Section 4.3.7. The CMTS MUST NOT allow a SYNC message to cross an MPEG packet boundary.6
© 2008 Cisco Systems, Inc. All rights reserved.
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Conclusions
The network delay between CMTS and EQAM is irrelevant for the timing offset calculation.
It is key the EQAM sets the timestamp of SYNC packets at the moment the first byte of the packets is exiting the DS TS convergence layer.
It is key since the SYNC packet is time stamped to have a “deterministic” delay between the exit of the DS TC later and the exit in the Physical Media Dependent sub-layer.
Whether the CM is receiving the SYNC packets from a Legacy DS or a Remote DS (from EQAM), the timeline for that CM should not have discontinuities at the moment of a DCC#4 operation to guarantee the CM to stay online.
© 2008 Cisco Systems, Inc. All rights reserved.
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