Dual Latency ADSL
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Transcript of Dual Latency ADSL
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Dual latency discussion (ADSL2+)
Victor Cortijo Technical Presales Expert Group
November, 2007
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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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1Impulse 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]
Voltageon100Ohm
s[volts]
-2 -1 0 1 2 3 4 5 6 7
x 10-3
-1.5
-1
-0.5
0
0.5
1
1.5
Neon 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 smaller
Max 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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2Problem 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 reallyneeded or not?
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3Customer examples
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Dual latency
In 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 duallatency 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
http://www.swisscom.com/GHQ/?lang=fr -
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4Implementation 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 QoSclasses 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 QOSqueues 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 channelBUT..
..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 latencychannels.
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 controversialimplementation for ATM where a PVC channel can by principle not be split on twobearers.
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 ASICsneed to support a double number of physical port addresses
Number 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 ofbandwidth on each of its bearers
Dual latency increases operational complexity
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5Performance (HSI/video/VoIP/gaming)
ALU investigation
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Performance summary
Service Packet loss sensitivity Delay sensitivityVideo without correction at
higher layers
High
packet loss 107to 109
Very low
(dejittering buffer of seconds)
Video with correction at higher
layers
Low
packet loss 5%
Very low
(dejittering buffer of seconds)
Web browsing Medium
packet loss 0.1%
High (if RTT is low)
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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 latency
Example: maximum achievable bit rates in function of INP & Delay
26
Upper DS performance limits for Amd1 ADSL2+ standard
40248112144552224426042278092955663
40248112144552224426042278092955632
40248112144552224426042278092955616
0811214455222442604227809295568
00761621092257182761229556400076162092825718295562
000000295561
1684210.50
INP_min
d
elay_
max(ms)
CPE has to be compliant !
= INP 2, 8 ms delay
Limitations: (1/S)max=16, Dmax =511, Max Interleaver Memory=16k
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Dual latency
Example: maximum achievable bit rates in function of INP & Delay
32
Upper DS performance limits for amd.3 ADSL2+ standard
539310844190922470327217283942955663
539310844190922470327217283942955632
402410844190922470327217283942955616
0811219092247032721728394295568
00761621092257182761229556400076162092825718295562
000000295561
1684210.50
INP_min
d
elay_
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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6Final 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 singlelatency
Dual latency (if implemented) increases
dramatically the interop and operationalcomplexity of the solution, leading to other type
of errors/limitations/compromises.
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7Artificial noise
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0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.0016.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
1stNoise increase
(neighbor modem)
2nd Noise increase
(strong radio signal)
Service interruptions:resyncs result in minutes
of downtime
Service degradation:lower bandwidth due to
higher noiseBandwidth(Mbp
s)
time
Line instability cause and visible effects
A closer look at a DSL line during prime time (8pm-midnight):
noise
stable DSLvideo affected by
packet loss
Excessive transmission errorsSpontaneous DSL line resynchronizations
stable DSLvideo 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
Bandwidth
(Mbps)
time
noise
noise
noise
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 0:00
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
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 lowfor maximum bandwidth availability
Neighbour switches
on DSL modem,
generating crosstalk
Dynamic noise
(crosstalk) exceeds
configured margin
Noise margin adapts
to accomodate virtual
noise
Dynamic noise will not
exceed noise margin(on top of A/V noise)
no resync
Noise margin can
remain low, for max.
bandwidth
No resyncs
Stable line
No interruptions
PSD
(dBm/Hz
)
PSD
(dBm/Hz
)
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Artificial Noise / Virtual Noise field results
Transmission errors (CVs)
2 ~3 day monitoring>3500~1 day monitoring
problem line
3500 errors/day
14.9Mbps
Multiple resyncs
Same line with Artificial Noise
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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)48pVDSL2
48pMulti-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 OperationsOptimizations (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 theINP/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 interleavingMessagevector
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 error
per 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)delay4
DS
N
RDS0,5INP
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Step 1: protection for 1RS / 1DMT symbol
NOinterleaving 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 & NOinterleaving
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 depth
N = number of bytes per RS word
incoming
outgoing
Max. 255
Bytes
..
N
B1 B1 B1 B1 B2 B2 B2 B2 Bx Bx Bx Bx 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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conclusions
INP = S x D x R / 2 x N
How 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 interleaving
Messagevector
Checkbytes 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 Byteerror
per bloc!
1 DMT symbol in error:
5 lost bytes
CheckCorrection CheckCorrection CheckCorrection CheckCorrection
D=31
N=q*I=15
K=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 minr net data rate delay max f
delay_maxnis in milliseconds
fsis the data symbol rate in ksymbols/s
INP_min bigger => net data rate smaller
Max 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 often
use interleavedmode rather than fast mode
file size
TCP does not get out of slow start before file transfer is over
use fastmode rather than interleaved mode
overall: interleavedis preferred for file download (onnoisy lines), fastis better for web browsing (on very highcapacity XDSL lines)
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Video performance
Video quality is determined by
bit rateuse 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 loss
use interleavedmode to protect against packet loss
use FEC on packets or a retransmission scheme to protect against remaining packet
loss
Video can tolerate some delay
additional DSL bit pipe delay will have almost no impact on overall zapping time
overall: interleavedis recommended but can work infastmode 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
fastmode is fine; but additional delay of interleavedmode (e.g. 8 or 16 ms) is notdramatic
packet loss
tolerable amount of packet loss is a few percent
interleavedmode is fine; but normally also no problem in fastmode
overall: slight preference for (medium) interleavedmode but works fine in fastmode 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 adversarys) seems to negatively impact
gaming performance
fastmode is fine; but additional delay of interleavedmode (e.g. 8 or 16 ms) is notdramatic
packet loss
does not seem to be crucial
no problem in interleavedand fastmode
overall: slight preference for fast mode but works fine in(medium) interleavedmode too
Dual latency
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Dual latency
Example: maximum achievable bit rates in function of INP & Delay (Amd 1)
x 4000 symbols/sec = bps
Total Data Rate (bits/symbol)
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Increasing DSL stability for IPTV StableDSL R1.0
DSL line stability is critical
Errorspe
rday
1
10
100
1000
IPTV Errors visible
>> complaints
InternetErrors hardly visible
CLEC
HSI/ADSL2+
ILEC
HSI/ADSL
ILEC
IPTV/ADSL2+
CLEC
IPTV/ADSL2+
ILEC
HSI(512k)/ADSL
Up to 25% of DSL lines potentially unstable
Stable lines
Potentially unstable: crosstalk
Solution: 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
performanceThroughput
Delay Error
Correction (INP)
BALANCE
2 4 8 16
2 13056 0 0 0
4 37632 13056 0 0
8 60242 37632 13056 0
16 60242 39168 24084 13056
32 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.
Dual latency
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Dual latency
Example: 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]
bitrate[Mb
/s]
fast down
fast up
INP=2, delay=8ms downINP=2, delay=8ms up
INP=4, delay=16ms down
INP=4, delay=16ms up
INP=8, delay=63ms down
INP=8, delay=63ms up
NVLT-A
measurementconditions NVLT-A (R3.2) profile 12a PSD mask: 998-M2x-A -140 dBm/Hz AWGN loop TP150
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