Download PacketCable Multimedia architecture and Digital

PacketCable Multimedia architecture and Digital TV testing
How good should it be?
PCMM architecture
Scientific notation
Targeted performance goals
The charts shown below outline performance goals for a typical network. Specific system requirements may require
tighter or less critical performance.
Expected MER & BER results
256 QAM
Pre FEC
BER
Post FEC
BER
35 dB
35 dB
32 dB
0.0 E-00
1.0E-08
1.0E-07
0.0 E-00
0.0E+00
1.0E-08
Excellent
Acceptable
Marginal
34 dB
31 dB
28 dB
35 dB
34 dB
30 dB
0.0 E-00
1.0E-08
1.0E-07
0.0 E-00
0.0 E-00
1.0E-08
Excellent
Acceptable
Marginal
33 dB
30 dB
25 dB
35 dB
32 dB
27 dB
1.0E-09
1.0E-08
1.0E-07
0.0 E-00
1.0E-09
1.0E-08
Excellent
Acceptable
Marginal
32 dB
28 dB
24 dB
35 dB
31 dB
28 dB
1.0E-08
1.0E-07
1.0E-06
0.0 E-00
1.0E-09
1.0E-08
Tap
Excellent
Acceptable
Marginal
32 dB
27 dB
23 dB
35 dB
31 dB
27 dB
1.0E-08
1.0E-07
1.0E-06
0.0 E-0
1.0E-08
1.0E-07
Modem
Node Headend
35 dB
33 dB
30 dB
Excellent
Acceptable
Marginal
35 dB
34 dB
32 dB
35 dB
35 dB
34 dB
0.0 E-00
0.0 E-00
1.0E-08
0.0 E-00
0.0E+00
1.0E-09
Excellent
Acceptable
Marginal
35 dB
33 dB
30 dB
35 dB
34 dB
32 dB
0.0 E-00
1.0E-09
1.0E-08
0.0 E-00
0.0 E-00
1.0E-09
Excellent
Acceptable
Marginal
Excellent
Acceptable
Marginal
Excellent
Acceptable
Marginal
33 dB
31 dB
28 dB
35 dB
33 dB
30 dB
1.0E-09
1.0E-08
1.0E-07
0.0 E-00
0.0 E-00
1.0E-09
33
29
25
32
28
25
35
32
30
35
32
28
1.0E-08
1.0E-07
1.0E-06
1.0E-08
1.0E-07
1.0E-06
0.0 E-00
1.0E-09
1.0E-08
0.0 E-0
1.0E-08
1.0E-07
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
-8
m
m
Application
manager
Excellent
Acceptable
Marginal
MER
64 QAM
Digital data
Headend
Similar to its role in PacketCable 1.x, the PCMM RKS receives event messages pertaining to policy decisions from
the Policy Server and event messages pertaining to QoS resource usage from the CMTS. In the PCMM architecture,
the RKS does not receive messages directly from the Application Manager. However, the AM can embed opaque data
in messages that it sends to the Policy Server, and this data can then be included in event messages that are
subsequently sent to the RKS.
Post FEC
BER
Node
Record Keeping Server (RKS)
Pre FEC
BER
256 QAM
Amp
The AM resides in the Session Control Domain (SCD), a logical grouping of elements that offer applications and
content to service subscribers. The AM also coordinates subscriber initiated requests for application sessions with
access to the resources needed to meet those requests, and maintains application-level state. The AM may reside on
the MSO’s network or it may reside outside this domain and interact with the MSO network via a particular trust
relationship that is typically defined by and enforced on the basis of a Service Level Agreement. The AM may be
controlled by the operator or a third party.
MER
64 QAM
Tap
Digital video
Set-top
Application Manager (AM)
Amp
The PacketCable Multimedia (PCMM) architecture could soon play a big role with a range of IP-based applications,
including video telephony and video streaming. Taking a step beyond the primary line, voice-centric PacketCable 1.x
architecture and layering itself on the DOCSIS 1.1 and 2.0 platforms, PCMM injects quality of service (QoS) into IPbased, latency-sensitive applications.
-7
mm
-4
pkt-
mm
Cable pkt-mm-1
modem
MSO managed
IP network
pkt-
Client
pkt-mm-3
Record
keeping server
-5
mm
Looking beyond
the physical layer
pkt-mm-2
CMTS
Policy server
pkt-mm-6
mm-9
MPEG
single/multi
program
transport
stream
64/256
QAM
RF
signal
mm-10
Cable Modem Termination System (CMTS)
A PCMM-capable CMTS is a generalized version of the PacketCable 1.x CMTS that is also responsible for fulfilling
QoS requests received from one or more Policy Servers. It performs this function by installing Gates, which enable the
customer’s cable modem to request network resources from the CMTS through the creation of dynamic DOCSIS flows
with guaranteed levels of QoS. The CMTS also sends event messages detailing actual usage of QoS resources to the
Record Keeping Server.
Policy Server (PS)
The PS, situated between the Application Manager (AM) and the CMTS, simultaneously plays a dual role as a
“proxy” for AM-initiated session requests and a “sentry” for defining and enforcing Resource Control Domain policy.
This element also serves as a Policy Decision Point (PDP) in relation to the CMTS in that the PS implements MSOdefined authorization and recourse-management procedures. Conversely, the PS plays the role of Policy Enforcement
Point (PEP) in relation to the Application Manager as it proxies Gate Control messages to and from the CMTS. There
are two basic classes of Policy Servers: Stateful and Stateless.
Program
Video ES
Audio ES
Program
Video ES
Audio ES
Program
Video ES
Audio ES
The physical RF layer also contains important piece parts that need to be acounted for.
The digital video world is much more complicated than
its analog predecessor. To perform proper testing and monitoring on the digital signal, technicians and engineers must
look within the physical layer to the underlying MPEG
transport layer to view how the signal is being encapsulated.
The physical layer must be up and running for everything
else to work, but only looking at the physical layer does not
provide much data about the underlying layers and the
applications that are running there. Without testing the
underlying MPEG layer, problems that could be missed
include audio/video ES timing errors, excessive PCR jitter
(which could cause loss of picture), missing PSI/SI tables
and PIDS, as well as low bit-rate of PIDs, which could
degrade the picture.
ES = Elementary Stream
?
BER (bit error rate) measurements are
expressed in terms of errors divided by the
total number of un-errored bits transmitted or
received. Because the number of errors is very
small compared to the number of bits transmitted, the measurement is typically expressed in
scientific notation. For example, one error out
of one million bits would be expressed as
1/1,000,000 or 1.0 E-6. Confusion often arises
when a second measurement is compared. Is
7.0 E-7 better or worse? 7.0 E-7 means seven
errors out of 10 million bits, which is actually a
little better than one in one million. The chart at
right may be helpful in interpreting scientific
notation.
One important note: Many instruments will
read 0 (zero) or 0.0E-0 when no errors have
been detected. E0 or E-0 is equal to 1, but the
leading 0 makes the measurement equal to 0.
SCIENTIFIC NOTATION
You can depend on Acterna.
DTS-330: The only one-box solutions for MPEG, DVB and ATSC testing
- Performs real time analysis, evaluation, and troubleshooting to maintain the
highest level of QoS
- Includes a wide range of standard interfaces: ASI, SPI, SMPTE-310M, COFDM,
8VSB, QPSK, GigE, and QAM
- Allows for flexibility of all required interfaces, mobility and remote troubleshooting
- New! MPEG-4 analysis capabilities
To learn more, call 1-866-ACTERNA or e-mail [email protected].
For all the details, please visit www.acterna.com/tripleplay.
1/1
One
1.00E-01
1/10
One in Ten
1.00E-02
1/100
One in One Hundred
1.00E-03
1.00E-04
1/1,000
1/10,000
One in One Thousand
One in Ten Thousand
1.00E-05
1.00E-06
1/100,000
1/1,000,000
One in One Hundred Thousand
One in One Million
1.00E-07
One in Ten Million
1.00E-08
1/10,000,000
1/100,000,000
One in One Hundred Million
1.00E-09
1.00E-10
1/1,000,000,000
1/10,000,000,000
One in One Billion
One in Ten Billion
1.00E-11
1.00E-12
1/100,000,000,000
1/1,000,000,000,000
One in One Hundred Billion
One in One Trillion
0.00E-00
0x1
Zero (no errors)
MER and BER
When it comes to testing digital QAM
signals, testing experts suggest that cable network operators use their digital video analyzers to test both Modulation Error Ratio (MER)
and Bit Error Rate (BER). That’s because
MER and BER measurements detect different
types of impairments.
MER is the measurement in dB of the RMS
Figure 1
Figure 2. Constellation with “good” MER
error magnitude over the average symbol
magnitude. The higher the error magnitude,
the poorer the MER. MER essentially assigns a value to the “fuzziness” of
the symbol cluster (see Figure 1). So, the larger or fuzzier the cluster
becomes, the poorer the MER. Likewise, the farther the dots move from
their ideal locations, the poorer the MER.
For example, the diagram shown in Figure 2 is a constellation with a
“good” MER of 34 dB, while the diagram on the right (Figure 3) shows a
constellation with a “poor” MER of 21 dB.
Each symbol, or “dot,” on the
constellation is framed by deci-7
-5
-3
-1
1
sion boundaries (see Figure 4).
-1
Figure 4
When the carrier falls inside the
Figure 3. Constellation with “poor” MER
-3
boundaries, the information is
transmitted without errors. In
Correct locations fall
-5
this example, BER testing is not an effective measurement because the BER is
within decision
perfect. But the good news could be hiding problems.
boundaries
-7
Using MER instead, it is clear that while each of the following constellations
have a perfect BER, the constellation in Figure 7 has a much better MER, with
Locations in error fall out
less noise (see Figures 5, 6 and 7).
of decision boundaries
So, why measure BER? Because MER is a poor indicator of fast, intermittent
transients. Examples of these types of impairments include laser clipping (the
most common cause), loose or corroded connections, sweep system interference and microphonics. So, if you have high
MER, but errors are present, they are probably being caused by intermittent interference. This shows up on a constellation
diagram as a lone dot that is away from the main cluster.
-7
-5
-3
-1
-1
1
3
5
-7
7
-5
-3
-1
-1
1
3
-7
-3
-3
Figure 5
-5
-3
-1
-1
-7
Good MER
Perfect BER
Poor MER
Perfect BER
Without a view beyond the physical layer, there is no way to
tell what’s happening inside the “haystack.”
3
5
7
-5
Figure 7
Figure 6
-7
1
-3
-5
-5
®
When ensuring the quality of your Digital TV service is of the utmost importance…
1.00E+00
P.O. Box 266007, Highlands Ranch, CO 80163-6007
© CED magazine, August 2005 • www.cedmagazine.com
Tel.: 303-470-4800 • Fax: 303-470-4890
-7
Best MER
Perfect BER
The publisher gratefully acknowledges
Trilithic Inc., Sunrise Telecom, Acterna and
others for contributing content to this chart.