transcript
FY07 Q1 Cisco Presentation Template White VersionCisco
Confidential
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Cisco Confidential
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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Cisco Confidential
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.
Cisco Confidential
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.
Cisco Confidential