dual latency
TRANSCRIPT
All Rights Reserved © Alcatel-Lucent 2006, #####
Dual latency discussion (ADSL2+)
All Rights Reserved © Alcatel-Lucent 20072 | UPC DSL technology | July 2007
Index
1. Impulse noise and its protection
2. Problem statement and dual latency solution
3. Customer examples
4. Implementation complexities and problems
5. Performance (HSI/video/VoIP/gaming)
6. Final conclusion
7. Artificial noise
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1 Impulse noise and its protection
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Impulse noise problem
0 2 4 6 8 10 12 14 16 18 20-0.015
-0.01
-0.005
0
0.005
0.01
0.015
Time [DMT Symbols]
Volta
ge o
n 10
0 Oh
ms
[volt
s]
-2 -1 0 1 2 3 4 5 6 7
x 10-3
-1.5
-1
-0.5
0
0.5
1
1.5Neon 6
Neon lamps and economic lamps:e.g. turn on of TL lamp
Longest burst observed
28 DMT symbols
0.2 DMT symbols
ERRORS
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INP & Delay in ADSL2+
Protection (INP) = combination of interleaving and RS overhead.
Complex formula for data rate - can be simplified to
INP bigger => net data rate smallerMax delay smaller => net data rate smaller
Big issue – both driving factors (more protection, less delay) drive to less net data rate
maximum achievable bit rate also capped by interleaver memory size and maximum 1/S
Net_data_rate/Total_data_rate = 1-(INP / (2 delay[ms]))
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2 Problem statement and dual latency solution
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Problem statement
What is the optimal INP_min/max_Delay combination for triple play (ADSL2+) with single latency?
Is there even a reasonable solution?
As a consequence, is dual latency really needed or not?
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3 Customer examples
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Dual latencyIn practice
all 3-play deployment today using ALU equipment is single latency (on ADSLx and VDSL2) operators that initially put dual latency as a requirement finally decided to
deploy single latency after consideration of all aspects
lack of CPE support for dual latency today ; no dual latency IOP today no IOP tests have been done at UNH plugfests with dual latency
All 3-play over xDSL using ALU equipment is offered successfully without dual latency today
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INP_min/delay_max for triple play in real life Examples (information has been summarized because of confidentiality):
VDSL2 customers: INP ranges from INP=1 to INP=2 with delay=8ms and some type of higher layer retransmission is used in all (or most of the) cases
ADSL2+ customers: INP ranges from INP=1 to INP=4 with delay=8ms in all cases one of the customers uses INP=1, delay=8ms for both upstream and
downstream with no higher layer retransmission Only one of the customers uses higher layer retransmission with settings
INP=2,delay=8ms downstream
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4 Implementation complexities and problems
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Implementation complexities and problems (1) Need for selection mechanism to associate incoming traffic with a
bearer on DSL line Dual latency is standardized at the physical layer but there is no correct
standard specification on how the traffic should be split or aggregated above these interfaces and this can result in interoperability and other deployment issues.
If Dynamic Rate Repartitioning (DRR) is not well defined, there is no bandwidth sharing between bearers, meaning that bandwidth is wasted if one of the services is not being used.
Dual latency risks big interoperability issues for each DSL line, different queues may be required per bearer if different
QoS classes are mixed over same bearer. Also, a scheduler resource or instance is required per bearer on each DSL line (complexity of scheduler depends on number of service types that can be mixed on single bearer).
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Implementation complexities and problems (2) The most attractive solution (QOS model) consists in associating each of
the QOS queues to one of both latency channels. This way the selection of the QOS queue (based on Priority bit) automatically results in the selection of the latency channel BUT..
..to offer a QoS-based solution, we would need: agreements from the CPE suppliers to adopt the same model. Most ATM CPE's are expected to use separate PVC's across the different
latency channels. confidence that the Priority bits are well controlled through the network. confirmation that such implementation suits the different customers. the acceptance of or a solution to the technically feasible but controversial
implementation for ATM where a PVC channel can by principle not be split on two bearers.
Dual latency puts end-to-end requirements to ensure that different services are mapped on proper latency bearers
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Implementation complexities and problems (3)
Each bearer has to be addressable on the DSL line DSL modem ASICs and interface between network processor and DSL
modem ASICs need to support a double number of physical port addressesNumber of objects to be managed doubles
multiple bearers on a same DSL line have to be managed (configuration, fault and performance mgmt) as different physical lines
with some dependencies between managed objects (bearers), e.g. maximum aggregate bandwidth on a physical DSL line determines possible provisioning of bandwidth on each of its bearers
Dual latency increases operational complexity
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5 Performance (HSI/video/VoIP/gaming)ALU investigation
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Performance summary
Service Packet loss sensitivity Delay sensitivityVideo without correction at higher layers
Highpacket loss 10–7 to 10–9
Very low(dejittering buffer of
seconds)Video with correction at higher layers
Lowpacket loss 5%
Very low(dejittering buffer of
seconds)Web browsing Medium
packet loss 0.1%High (if RTT is low)
<<RTTFile download Medium
packet loss 0.1%Medium (if RTT is low)
<<RTTVoIP Low
few % packet loss acceptable
Medium (total budget is 150 ms but requirements for DSL may
be order less)Multiplayer shooting game Low
few % packet loss acceptable
Medium/high (total budget is 100 ms but user perception may require
low delay)MMORPG (Massively Multiplayer Online Games)
Low packet loss 10%
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Performance conclusion guaranteed correction of 2 successively corrupted DMT symbols
(INP=2) improves video quality and large file downloads in environments with strong impulse noise
medium interleaving delay (8 ms) is fine for gaming, VoIP and web browsing (HTTP)
From performance point of view, single latency is enough INP=2 in combination with delay of 8 ms is good combination for
downstream. good Reed Solomon efficiency (R/N = 1/8) upstream delay and INP values can be less than for downstream
But, is the data rate in this case sufficient for triple play?
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Dual latencyExample: maximum achievable bit rates in function of INP & Delay
26
Upper DS performance limits for Amd1 ADSL2+ standard
402481121445522244260422780929556634024811214455222442604227809295563240248112144552224426042278092955616
0811214455222442604227809295568007616210922571827612295564000761620928257182955620000002955611684210.50
INP_minde
lay_
max
(ms)
CPE has to be compliant !
= INP 2, 8 ms delayLimitations: (1/S)max=16, Dmax=511, Max Interleaver Memory=16k
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Dual latencyExample: maximum achievable bit rates in function of INP & Delay
32
Upper DS performance limits for amd.3 ADSL2+ standard
539310844190922470327217283942955663539310844190922470327217283942955632402410844190922470327217283942955616
0811219092247032721728394295568007616210922571827612295564000761620928257182955620000002955611684210.50
INP_minde
lay_
max
(ms)
CPE has to be compliant !
Limitations: (1/S)max=16, Dmax=511, Max Interleaver Memory=24k
= INP 2, 8 ms delay
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Result with CT562plus INP=2 with Delay 16/8/4ms vs INP=0/Delay=16ms
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Result with ST716 INP=2 with Delay 16/8ms vs INP=0/Delay=16ms
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6 Final conclusion
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Final conclusion
Dual latency is not needed in the current triple play (ADSL2+) scenario Single latency performance is guaranteed with INP=2 and
max_delay = 8 ms Single latency data rate is enough to deploy triple play Other operators deploying triple play successfully with
single latency
Dual latency (if implemented) increases dramatically the interop and operational complexity of the solution, leading to other type of errors/limitations/compromises.
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7 Artificial noise
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0.002.004.006.008.00
10.0012.0014.0016.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
RESYNC
1st Noise increase (neighbor modem)
2nd Noise increase (strong radio signal)
Service interruptions: resyncs result in minutes of downtime Service degradation: lower bandwidth due to higher noise
Band
widt
h (M
bps)
time
Line instability – cause and visible effectsA closer look at a DSL line during prime time (8pm-midnight):
RESYNC
noise
stable DSL video affected by packet loss
Excessive transmission errorsSpontaneous DSL line resynchronizations
stable DSL video affected by line resync
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Stabilizing an unstable line with Artificial/Virtual Noise
Original situation – high bandwidth but unstable
Traditional solution: High Noise Margin – stable but reduced bandwidth
The Alcatel-Lucent solution: Artificial/Virtual Noise – stable and high bandwidth
Band
widt
h (M
bps)
time
noise
noise
noise
0.002.004.006.008.00
10.0012.0014.0016.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
0.002.004.006.008.00
10.0012.0014.0016.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
0.002.004.006.008.00
10.0012.0014.0016.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
stable @11.6 Mb/s
stable @4.7 Mb/s
2 resyncstable @ 9.7 Mb/s
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Margin
frequencies ()
Receiver Noise
Artificial Noise (ADSL) / Virtual Noise (VDSL)
frequencies ()
MarginMargin
Receiver Noise
Artificial / Virtual Noise
•resyncs•Unstable line•Service interruptions
Artificial/Virtual noise guarantees DSL stability whilst keeping Noise Margin low
for maximum bandwidth availability
Neighbour switches on DSL modem, generating crosstalkDynamic noise (crosstalk) exceeds configured margin
Noise margin adapts to accomodate virtual noiseDynamic noise will not exceed noise margin (on top of A/V noise) – no resyncNoise margin can remain low, for max. bandwidth
•No resyncs•Stable line•No interruptions
PS
D (d
Bm
/Hz)
PS
D (d
Bm
/Hz)
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Artificial Noise / Virtual Noise – field results
Transmission errors (CV’s)2
~3 day monitoring>3500
~1 day monitoring
problem line3500 errors/day14.9MbpsMultiple resyncs
Same line with Artificial Noise<1 error/day12.0MbpsNo resyncs
Preliminary field results are excellent
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StableDSL R1.0
ADSL – Artificial Noise Unique Alcatel-Lucent solution invented by ALU – patent pending Works with all deployed CPEs
VDSL – Virtual Noise Invented by ALU – patent pending Included in standard (optional)
48p
VDSL
248
p M
ulti
-DSL
Artificial/Virtual noise in ISAM Network Analyzer
Alcatel-Lucent consultancy groups help operators stabilize their lines
1 2
3
Premium Package – never included in base price DSL line troubleshooting
Automated Artificial / Virtual Noise configuration
Automated line analysis
Access Network Design & Transformation (AND&T)Logical and physical network design: introduction of new DSL flavours introduction of new Triple Play services
Access Network Operations Optimizations (ANOO) Operational optimization of DSL networks: Troubleshooting & Tuning of networks 5520 AMS, 5580 HNM and 5530 NA
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StableDSL R1.0 : Practical Information
Availability: today Virtual Noise: ISAM R3.1 ETSI, Artificial Noise: ISAM R3.3 (DR5
Aug07) Network Analyser support: AN/VN analysis R5.2 (Jul07)
Virtual/Artificial noise should be implemented independently of the INP/Delay settings Helps with resynchronizations and line stability Helps in conditions of high repetitive noise
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Backup
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Impulse noise protection
Reed Solomon plus interleavingMessage vector Ctrl Data to be transmitted
Transmitted Data
Bloc 0 Bloc 1 Bloc 2
CtrlCorrection CtrlCorrection CtrlCorrection CtrlCorrection CtrlCorrection
Bloc 3 Bloc 4
Bloc 0 Bloc 1 Bloc 2 Bloc 3
Burst errors
6 lost bytes
1 Byte errorper bloc!
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Impulse Noise Protection (INP) in ADSL2(+)
Impulse noise protection How much of the DMT symbol is protected? Protection via Reed Solomon and extended via interleaving
Which parameters influence the INP S = # DMT symbols per RS word D = interleaving depth (# of combined RS words used) N = Number of bytes per RS word (1 255 bytes) R = Number of RS overhead bytes (0 16 bytes)
(ms)delay 4
DS
NRDS0,5INP
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Step 1: protection for 1RS / 1DMT symbol
NO interleaving introduced R=overhead bytes N=Total bytes K= payload bytes Correction on payload = R/2
What part of the DMT symbol is protected? Number of correctable bytes over number of bytes in DMT symbol INP = DMT protection = payload correction / N = R / (2xN)
K R
DMT symbol
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Assume 1 RS word / 4 DMT symbols & NO interleaving S = # DMT symbols per RS word = 4 We have seen before that RS correction = R/2
How much of the DMT symbol is protected? RS word is now spread over 4 DMT symbols
With R=16 you have 8 correctable bytes over 4 DMT symbols
INP = (# correctable bytes) / (#bytes in a DMT symbol)= = (R/2) / (N/S) = (S x R) /( 2 x N)
INP increases with a factor S
Step 2: protection for 1RS / S DMT symbols
DMT DMT
RS
DMT DMT
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...
1 2 3 4 5 6
Step 3: introducing interleaving
Correction has improved by a factor D Errorred bytes are spread over “D” RS words Payload correction = D x R/2
DMT protection has as such also increased = # correctable bytes / N = (DxR)/(2xN)
BufferD
D = interleaving depthN = number of bytes per RS word
incoming
outgoing
Max. 255 Bytes
..
N
B1B1B1B1B2B2B2B2 BxBxBxBx Bz Bz BN BN BN BN...
Assume 1 interleaved RS word / DMT symbol
Size N
Max. 64
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Step 4: all together
RS introduces a correction = R/2 RS correction presented by parameter R
Interleaving introduces an improvement on the number of correctable bytes Interleaving represented by parameter D
S factor introduces an impact on the number of correctable bytes per DMT symbol INP = (S x # correctable bytes) / N
= S x R x D / (2 x N)
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conclusionsINP = S x D x R / 2 x NHow to increase the INP
Increase S > increases the introduced delay Increase D > increases the introduced delay Increase R > Decreases the available bitrate Decrease N > Decreases the available bitrate
When configuring a DSL port a max delay needs to be given and a minimum INP This will impact the max. possible bitrate
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Impulse noise protection
Reed Solomon plus interleavingMessag
e vector
Check bytes Data to be transmitted
Transmitted Data
RS word 0 RS word 1 RS word 2
Received Data
CheckCorrection
RS word 3 RS word 4
RS word 31 Byte error
per bloc!
1 DMT symbol in error:5 lost bytes
CheckCorrection CheckCorrection CheckCorrection CheckCorrection
D=31
N=q*I=15K=9 R=6
I=5
S=5/15
RS word 4RS word 0 RS word 1 RS word 2
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INP & Delay in VDSL2
Protection (INP) = combination of interleaving depth and RS overhead.
Complex formula for data rate - can be simplified to
_ _ 2 _1_ _ _
n nn
n n s
total data rate INP minrnet data rate delay max f
delay_maxn is in milliseconds
fs is the data symbol rate in ksymbols/s
INP_min bigger => net data rate smallerMax delay smaller => net data rate smaller
Big issue – both driving factors (more protection, less delay) drive to less net data rate
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HSI performance
HSI performance is determined by packet loss due to stationary and impulsive noise
if packet loss is too high, TCP goes in “congestion avoidance” too oftenuse interleaved mode rather than fast mode
file size TCP does not get out of “slow start” before file transfer is overuse fast mode rather than interleaved mode
overall: interleaved is preferred for file download (on noisy lines), fast is better for web browsing (on very high capacity XDSL lines)
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Video performance
Video quality is determined by bit rate
use a high enough video bit rate (1.5 Mb/s for SDTV, 8Mb/s for HDTV)use a state-of-the-art codec (e.g. H.264)
packet loss video is very sensitive to packet loss
– every lost packet is visible when MPEG-Transport Stream is used– different (new) transport mechanisms exist that may offer better
robustness (less visual disturbance) against packet lossuse interleaved mode to protect against packet lossuse FEC on packets or a retransmission scheme to protect against remaining
packet lossVideo can tolerate some delay
additional DSL bit pipe delay will have almost no impact on overall zapping time overall: interleaved is recommended but can work in
fast mode too with FEC or retransmission at packet level
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VoIP performance
Voice quality is determined by end-to-end delay
total budget is 150ms without any drop in quality and even 400ms if a slight loss in interactivity is allowed; however, XDSL line requirement will be something less
fast mode is fine; but additional delay of interleaved mode (e.g. 8 or 16 ms) is not dramatic
packet loss tolerable amount of packet loss is a few percentinterleaved mode is fine; but normally also no problem in fast mode
overall: slight preference for (medium) interleaved mode but works fine in fast mode too
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Gaming performance
Gaming performance is determined by (twice) client-server delay (Ping time)
an additional 60-80ms delay (over the adversary’s) seems to negatively impact gaming performance
fast mode is fine; but additional delay of interleaved mode (e.g. 8 or 16 ms) is not dramatic
packet loss does not seem to be crucialno problem in interleaved and fast mode
overall: slight preference for fast mode but works fine in (medium) interleaved mode too
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Dual latencyExample: maximum achievable bit rates in function of INP & Delay (Amd 1)
x 4000 symbols/sec = bpsTotal Data Rate (bits/symbol)
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Increasing DSL stability for IPTV – StableDSL R1.0
DSL line stability is critical
Erro
rs p
er d
ay
1
10
100
1000
IPTV Errors visible>> complaints
InternetErrors hardly visible
CLECHSI/ADSL2+
ILECHSI/ADSL
ILECIPTV/ADSL2+
CLECIPTV/ADSL2+
ILEC HSI(512k)/ADSL
Up to 25% of DSL lines potentially unstable
Stable lines
Potentially unstable: crosstalkSolution: Artificial/Virtual
noise
0% 20% 40% 60% 80% 100%
More complaintsLess qualifying lines Lower take-up rate/higher churn
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Difference with VDSL2
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Difference with VDSL2
Everything stated remains exactly the same but.. Due to different technology, achievable bit rates are
different than in ADSL2+ (see next slides)
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VDSL2 PHY fault correction
Alcatel VDSL2 implementation allows independent configuration of INP and delay on per port basis max interleaving Delay can be configured in steps of 1ms in range
of 0 to 63 ms (delay 2ms for interleaved path) min INP can be configured in steps of 0.1 DMT symbol in range of 0
to 16 DMT symbols max achievable bit rate is function of combined settings for INP and
delay
Example: downstream for profile 12a/b (simulation with estimated null loop performance Throughput
Delay Error Correction (INP)
BALANCE
2 4 8 162 13056 0 0 04 37632 13056 0 08 60242 37632 13056 0
16 60242 39168 24084 1305632 60242 39168 24084 13645
INP_min
delay_max (ms)
Net Data Rates
Note: The bit rates presented in the table are upper limits which might not be practical or feasible in typical VDSL2 deployment scenarios.
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Dual latencyExample: maximum achievable bit rates in function of INP & Delay
0
10
20
30
40
50
60
0 200 400 600 800 1000 1200 1400
TP150 loop length [m]
bit r
ate
[Mb/
s]
fast downfast upINP=2, delay=8ms downINP=2, delay=8ms upINP=4, delay=16ms downINP=4, delay=16ms upINP=8, delay=63ms downINP=8, delay=63ms up
NVLT-A
measurement conditions NVLT-A (R3.2) profile 12a PSD mask: 998-M2x-A -140 dBm/Hz AWGN loop TP150