Telecommunications Industry Association
(TIA)
Portland, OR
TR-30.3/06-09-024
September 25 - 26, 2006
DOCUMENT SUBMITTED TO
TR-30.3 Meeting
The document to which this cover statement is attached is submitted to a Formulating Group or sub-element thereof
of the Telecommunications Industry Association (TIA) in accordance with the provisions of Sections 6.4.1-6.4.6
inclusive of the TIA Engineering Manual dated March 2005, all of which provisions are hereby incorporated by
reference.
SOURCE:
Anonymous
CONTACT:
Jack Douglass
TITLE:
IP Network Characteristics for DOCSIS
PROJECT:
PN-3-0062 (TIA-921) RV1
DISTRIBUTION:
Members of TR-30.3
INTENDED PURPOSE OF
DOCUMENT:
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FOR INCORPORATION INTO A TIA PUBLICATION
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FOR INFORMATION ONLY
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OTHER (please describe):
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ABSTRACT
This document contains information on the IP Network Characteristics for DOCSIS QoS Networks, Well
Managed/Best Effort Networks, Medium Managed Networks and Poorly Managed Networks.
QoS Network
US Latency in DOCSIS networks that are using QOS features such as UGS have very low access latencies <1ms. VoIP services
for example are generally configured with UGS. Since jitter is bounded by the absolute access delay it is also < 1 ms. UGS mode
doesn't rely on a contention mechanism, so it does not have loss of packets unless they originate from noise or other physical
impairments. Operators have significantly improved their physical layer maintenance and they are particularly careful in the
regions were QoS based services such as telephony are provided.
Well Managed/Best Effort Networks
Best Effort US latency on a well managed networks depend on the Request-Grant latency inherent in DOCSIS. Depending on the
configuration, it ranges from 4 ms to 6 ms (Typically 6 ms). This also assumes that the operator is not using the longer
downstream interleaving setting which would add another 2-3 ms (This configuration is not typically used by operators when
providing data services).
The jitter in a well managed best effort network is typically bounded by the position within a downstream DOCSIS MAP interval
which is typically
2 ms. A properly dimensioned network suffers negligible packet loss based on the contention/request for bandwidth best effort
mechanism.
Packet loss in well managed network would only be then due to a physical impairment but in well managed networks that are
taken advantage of error correction, burst robustness mechanisms, tight CM power level tolerances and a suitable modulation
order and channel width for the robustness needed would have a packet loss that is typically significantly < than 0.25%.
Medium Managed Networks.
This is a network that the physical layer is managed well but the operator has not properly dimensioned the network or it is
reaching is capacity limits. Latency is increased by an additional 2ms to 4 ms because potentially due to congestion the scheduler
takes an additional one or two MAP intervals longer to grant a transmission period to the CM. So the total access latency would
range from 6 ms to 10 ms. Since the transmission may no longer for the most part be contained within the same DOCSIS MAP
interval, but the transmission would spill into the next interval or the following one. Jitter in that case may increase by 4 ms so it
would total 6 ms. I think that we can assume that the operator is using appropriate and robust configurations so that the
robustness and error correction mechanisms take care of more of the errors comfortably achieving a PER <0.25%.
Poorly Managed Networks
Best Effort latency on poorly managed networks. Poorly dimensioned networks may have a multitude of simultaneous problems.
An operator may have decided to unnecessarily set their interleaver to the longest burst protection settings that adds an additional
3 ms to the latency. It may also have also have packet loss due to poor dimensioning and insufficient resources. If we assumed
that retransmissions are on the average taxing an additional 8 ms we have a total latency of about 15 ms. The jitter that is incurred
due to the re-transmission is about 10 ms. A poorly managed network would result in a packet loss of 1%.
On the downstream the CATV network is very benign and it a simple environment protocol wise because we have a single source
(the CMTS). We can probably approximate the latency and jitter for all but the poorly configured networks to be <1ms.
Assuming an interleaver configured for the longer burst protection we would have in that scenario a 4 ms latency. DS packet loss
is for the most part negligible.
Service Tiers
Operators service tiers vary widely. 6Mbps down / 768 Kbps up is a popular service tier. Also 4 Mbps down and 512 Kbps up are
also being used. From a competitive perspective you have that operators have began offering 15 Mbps down and 2 Mbps up in
the regions where there is FIOS competition. This is just a small sample of the service tiers. To make matter more complex.
Some operators are incorporating the bursting enhancement capabilities that allow to burst potentially above your maximum
downstream setting. This mechanism allocates a burst allowance.
For example if your burst allowance is set to 10MB, that user can download in a very short period of time the 10 MB allowance
without being bounded by your downstream speed setting. This allowance is replenish at a set rate. Only well managed networks
may take advantage of the bursting enhancement mechanisms.
The amount of capacity available in the upstream and downstream vary depending on the configuration. However operators used
for the most part channel widths of 3.2 MHz and QPSK and 16QAM modulation schemes. 3.2 MHz and 16QAM is now
becoming the most popular configuration. Although the raw bit rate that can be obtained in 3.2MHz/16QAM is 10.24 Mbps the
actual available bandwidth due to the overhead is less than that. The overhead is packet size dependant. For a packet size
distribution encompassing longer and shorter packets we can assume an Ethernet payload efficiency of around 70%.
In the downstream the efficiency variation is much smaller. Assuming a mixed packet size distribution the efficiencies ranges
from 80 to 85% which using 256QAM which is now the most popular configuration mode provides a capacity of about 37 Mbps.
In DOCSIS 2.0 or earlier there is no re-ordering of packets and all DOCSIS system will handle 1518 byte packets.
Likelihood of occurrence
We can assume that QoS Networks and Well Managed networks have the same percentage of occurrence since they represent
potentially customers in the same physical DOCSIS network receiving services that are treated using either QoS mechanisms as
well as services using best effort. Due to the emphasis that operators have placed on VoIP services and the tools that are
becoming available to troubleshoot the networks, there is a significant percentage of these networks as well as well managed
networks. The poorly managed networks are quickly becoming a small percentage. No detail percentages for the different types
of networks at this point.