Troubleshooting guide

5-2
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
DOCSIS 1.0+
When CMTS receives the REG-REQ, it creates a local database entry for the MTA. A static SID is
immediately assigned to the MTA for the data service. For the phone line service, the CMTS only
creates 2 deferred service flows (for subsequent activation) in the MTA's database entry. No SIDs
are assigned for the phone line service during registration.
Whenever an MTA wants to get a Voice/Fax channel with real time CBR service, it sends a
DSA-REQ MAC message to the CMTS, specifying its special CBR scheduling requirements like
grant-size, grant-interval (grant-size, grant-interval depend on the codec type G.711/G.729 being
used on the MTA).
When the CMTS receives the DSA-REQ, it first checks in that MTA's database entry if any deferred
service flow is available. If a deferred service flow is available, the CMTS assigns a new dynamic
SDI for that MTA and triggers unsolicited grants (CBR slots) on that newly assigned dynamic SID.
The CMTS informs the MTA of the newly assigned dynamic SDI, using the DSA-RSP.
Given that the CMTS can accommodate the new CBR connection, that MTA will keep getting
unsolicited grants of the correct size (enough to fit the periodic Voice/Fax) packet at correct periodic
intervals. The MTA does not have to contend with any other MTA on the upstream for sending these
real-time packets. It has a dedicated TDM sub-channel on the upstream in the form of the unsolicited
grants. The delay/jitter is well bounded and good voice quality is thus maintained on the upstream
path from the MTA to the uBR.
The MTA colors the precedence bits in the IP header of these voice packets with the predefined value
of 0x05 for propagating the preferential local access QoS eventually into the IP backbone.
When the Voice packets arrive at the CMTS in the CBR slots, they are either switched into the WAN
(IP cloud), or forwarded to some other MTA on the downstream channel.
In the former case, the backbone routers like the Cisco GSR 12000 need to be configured to
recognize and give preferential treatment for these Voice transport packets (precedence value 0x05),
as compared to signaling/regular best effort data packets with precedence 0x3, 0x0 respectively.
In the later case, where the upstream packets are switched to the downstream channel of the same
uBR, the Voice packets (0x05) are separately handled for rate limiting, as compared to signaling,
data packets based on their precedence values.
Even if at the time of the call, the destination MTA is doing a large downstream file transfer, the
Voice packets forwarded to it on the same downstream will be unaffected by FTP on the same MTA,
due to the use of ip-precedence values in doing downstream bandwidth accounting.
When the call is torn down, the MTA sends a DSD-REQ to the CMTS to release the dynamic SDI.
The CMTS stops the CBR grants, destroys the dynamic SDI indicated in DSD-REQ, frees up one
deferred flow for the MTA, and sends a DSD-RSP to the MTA confirming the same.
The CMTS ensures that a subscriber MTA provisioned for 2 virtual phone lines can only get up to
2 high quality dynamic CBR QoS SIDs at runtime. Every time the MTA sends a DSA-REQ
requesting a new dynamic SID, the CMTS first checks to see if that MTA has any unused deferred
service flow available before creating a new dynamic SID. If the MTA is already using 2 dynamic
SIDs, both its deferred service flows will show up as in-use at the CMTS. So long as a dynamic SID
is using the service flow, the service flow will be unavailable for creation of any new dynamic SID
from this MTA.
Note Refer to “DOCS-IF-MIB.my” and “CISCO-DOCS-EXT-MIB.my” in Chapter 9 for information on
DOCSIS-related SNMP MIBs for monitoring MTA performance.