Transcript Mobile Communications

Gunawan Wibisono
Dept electrical engineering
university of indonesia
Agenda


Cable Modem
DSL
Broadband Network
CABLE TV FOR DATA TRANSFER
Cable companies are now competing with telephone
companies for the residential customer who wants
high-speed data transfer. In this section, we briefly
discuss this technology.
Topics discussed in this section:
Bandwidth
Sharing
CM and CMTS
Data Transmission Schemes: DOCSIS
9.4
Division of coaxial cable band by CATV
9.5
Downstream data are modulated using
the 64-QAM modulation technique.
The theoretical downstream data rate
is 30 Mbps.
Upstream data are modulated using the
QPSK modulation technique.
The theoretical upstream data rate
is 12 Mbps.
9.6
Changes in the Cable Network
• The cable network was designed to deliver TV
signals in one direction from the Head-End to the
subscribers homes
• To provide TV services Cable Operators had to
recreate a portion of the over-the-air radio
frequency (RF) spectrum within a sealed coaxial
cable line
• Operators had to upgrade the cable network so that
signals could flow in both directions
CEENET #8 8.2002
7
Changes in the Cable Network
• Cable Operators assign a spectrum of signal
frequencies to the cable network
• One spectrum is used for the signals that move
from the Head-End towards the cable subscriber
• Another spectrum of signal frequencies are used
for the signals that move from the cable subscriber
towards the Head-End
CEENET #8 8.2002
8
Changes in the Cable Network
• By replacing existing one way amplifiers with two
way amplifiers Cable Operators are able to
separate the upstream and downstream signals and
amplify each direction separately in the right
frequency range
CEENET #8 8.2002
9
Changes in the Cable Network
A Traditional Cable network
CEENET #8 8.2002
10
Changes in the Cable Network
CEENET #8 8.2002
A Modern Cable network
11
What is a Cable Modem?
CEENET #8 8.2002
12
Figure 9.17 Cable modem (CM)
9.13
Figure 9.18 Cable modem transmission system (CMTS)
9.14
How Fast is a Cable Modem?
• Cable modem speeds vary widely
– Depends on the cable modem system
– Cable network architecture
– Traffic load.
• In the downstream direction (from the network to
the computer), network speeds can be up to 27
Mbps
– BUT, this is an aggregate amount of bandwidth that is
shared by users.
CEENET #8 8.2002
15
How Fast is a Cable Modem?
• Few computers will be capable of connecting at
such high speeds or have exclusive access to the
network
– A more realistic number is 1 to 3 Mbps.
• In the upstream direction (from computer to
network), speeds can be up to 10 Mbps.
– However, most modem producers have selected a more
optimum speed between 500 Kbps and 2.5 Mbps
– AND, many cable operators limit the upstream
bandwidth to 128 or 384kbs
CEENET #8 8.2002
16
How Fast is a Cable Modem?
• An asymmetric cable modem scheme is most
common. The downstream channel has a much
higher bandwidth allocation (faster data rate) than
the upstream,
• primarily because Internet applications tend to be
asymmetric in nature.
• Activities such as World Wide Web (http)
navigating and newsgroups reading (nntp) send
much more data down to the computer than to the
network.
CEENET #8 8.2002
17
How Fast is a Cable Modem?
• Mouse clicks (URL requests) and e-mail messages
are not bandwidth intensive in the upstream
direction.
• Image files and streaming media (audio and video)
are very bandwidth intensive in the downstream
direction.
CEENET #8 8.2002
18
What is a Cable Modem &
how does it work?

A Cable Modem is a digital modem that uses
a coaxial cable connection for the data
transmission.
 This data connection is received by a cable
modem that decodes the signal into your PC.
MORE INFO...
http://www.cable-modems.org/tutorial/01.htm
http://www.cable-modems.org/tutorial/02.htm
How fast is a Cable Modem?



Cable modems are up to 10-20Mbps downloads. Typical
downloads are over 300Kbps, or close to 600Kbps, but the
speed of the cable modem depends on a few things.
First it depends on how many users are on the system
since the cable technology is a "shared" bandwidth. Too
many users using too much throughput can drain this
“shared” technology.
The second factor to cable modem speed is a limit on the
cable modem itself. Some cable providers will limit the
upload or download speed on the cable modem, and this
could affect your connection speed.
How secure is a Cable Modem?

Cable connections are not 100% secure in any
instance like many other connections on the
Internet. Even though most cable providers block
ports 137-139, cable modems are likely to be
generated in any case where a user has file and
print sharing turned on, or possibly other
services like SMTP (Simple mail transfer
protocol), Web Servers and Telnet services. A
general rule is to keep passwords long and turn
off any service that you don't absolutely need
running. A firewall type application should be
used to keep a network as secure as possible.
Real-world performance
• The theoretical performance of a Cable Modem is
based upon all other devices being able to work at
the same speed and performance as the Cable
Modem
• However, in a similar way that the actual usable
bandwidth on a 10Mbps Ethernet connection
reduces to a 4Mbps, so too will the performance of
a Cable Modem connection be reduced
CEENET #8 8.2002
22
Real-world performance
• The Cable network itself will suffer the same
problems of Internet performance as any other
Internet Service Provider (ISP)
• Although performance to services on the cable
network itself can be amazingly fast, access to 'the
outside world' will be slowed down by the
performance of other connections on the way.
CEENET #8 8.2002
23
Real-world performance
• As usage on your segment grows (as more
customers are added) the bandwidth must be
shared by more people
– Adding more cable network segments is very expensive
for the cable operator
• If you connect to a remote Internet site that itself
has a connection speed equivalent to a T1
connection (1.5Mbps), then that is as fast as the
data can be served to you, no matter how fast your
receiving equipment is
CEENET #8 8.2002
24
Who Makes Cable Modems?
• 3Com, Cisco Systems, Com21, General
Instrument, Motorola, Nortel Networks,
Phasecom, Samsung, Terayon, Toshiba, Zenith
• And many others
CEENET #8 8.2002
25
Cable Modem Technology
• It MOdulates and DEModulates signals
• Much more complicated than their telephone
counterparts
• Cable modems can be part modem, part tuner, part
encryption/decryption device, part bridge, part
router, part network interface card, part SNMP
agent, and part Ethernet hub
CEENET #8 8.2002
26
Cable Modem Technology
• Typically, a cable modem sends and receives data
in two slightly different fashions
– In the downstream direction
• he digital data is modulated and then placed on a typical 6
MHz television channel, somewhere between 50 MHz and 750
MHz
• 64 QAM is the preferred downstream modulation technique,
offering up to 27 Mbps per 6 MHz channel
• This signal can be placed in a 6 MHz channel adjacent to TV
signals on either side without disturbing the cable television
video signals.
CEENET #8 8.2002
27
Cable Modem Technology
– The upstream channel is more tricky
CEENET #8 8.2002
• Typically, in a two-way activated cable network, the upstream
(also known as the reverse path) is transmitted between 5 and
42 MHz
• This tends to be a noisy environment, with RF interference and
impulse noise. Additionally, interference is easily introduced in
the home, due to loose connectors or poor cabling
• Since cable networks are tree and branch networks, all this
noise gets added together as the signals travel upstream,
combining and increasing
• Due to this problem, most manufacturers use QPSK or a
similar modulation scheme in the upstream direction, because
QPSK is more robust scheme than higher order modulation
techniques in a noisy environment
• The drawback is that QPSK is "slower" than QAM.
28
Cable Modem Services
• The dominant service is high-speed Internet access
– This enables the typical array of Internet services to be
delivered at speeds far faster than those offered by dialup telephone modems
– Other services will include
– access to streaming audio and video servers, local
content (community information and services)
– access to CD-ROM servers
– a wide variety of other service offerings. New service
ideas are being developed daily.
CEENET #8 8.2002
29
Cost of Cable Modem Service
• In North America, cable operators are packaging
high-speed data services much like they do basic
cable television service
• Typically charging $40 - $60 per month for an
Internet service package
– Includes software, unlimited Internet access,
specialized content and rental of a cable modem
CEENET #8 8.2002
30
Cost of Cable Modem Service
• At the low end of this pricing scale, a very robust
Internet service is available to consumers for
about the cost of a dial-up account with a local
Internet service provider and a second telephone
line
• Even at $60 per month, cable is a far better value
than ISDN.
CEENET #8 8.2002
31
"Telco-Return" Modems
• Not really a cable technology
• Used more often with Direct Satellite video
systems
• Satellite down link is used for fast downstream
transmission
• A telephone modem handles upstream
communication over the public telephone network.
CEENET #8 8.2002
32
Support for Multiple PCs
• A cable modem can provide Intenet access to
multiple PCs, if they are connected via a local area
network (LAN)
• Cable modems typically have an Ethernet output,
so they can connect to the LAN with a standard
Ethernet hub or router
• Each PC must have an assigned IP address
– The cable ISP usually sells at a premium of $5-$10 a
month per PC
– NAT (Network Address Translation) can allow
multiple PCs to "hide" behind a single IP Address
CEENET #8 8.2002
33
Cable Modems vs. ADSL
There is one major advantage that ADSL has over
cable modems. Cable modems use a shared
networking technology where all the cable modems
share a single pipe to the Internet. This pipe speed
will fluctuate depending on the number of subscribers
on the network.
When ADSL is used, the pipe to the Internet is solely
"yours", and is not shared along the way to a central
office. This allows for a more consistent speed, and
this speed does not typically fluctuate like cable
modem networks.
MORE INFO...
http://www.whatis.com/adsl.htm
ADSL vs. cable modem

Pro:
– Secure. “Point to point
connectivity” of ADSL
ensures the security of
the service. Cable, by
contrast, is shared media
and is not secure at all.
– Bigger coverage area.
– Cheap. ADSL uses
existing twisted pair,
hence is cheap in
installation and also
cheap in monthly
payment.

Cons:
– Bandwidth. ADSL has
about 1.1MHz BW due to
loop limitations, while
cable modem has about
745MHz BW.
– Bridge taps, DLCs, load
coils can lead to
problems.
– Mutual noise among
different DSL lines, T1
lines.
Introduction
to
DSL
Yaakov J. Stein
Chief Scientist
RAD Data Communications
Stein Intro DSL 36
PSTN
Stein Intro DSL 37
Original PSTN
UTP
UTP
Manual switching directly connected two local loops
Due to microphone technology, audio BW was 4 kHz
Stein Intro DSL 38
Analog switched PSTN
Invention of tube amplifier enabled long distance
Between central offices used FDM spaced at 4 kHz
(each cable carrying 1 group = 12 channels)
Developed into hierarchical network of automatic switches
(with supergroups, master groups, supermaster groups)
Stein Intro DSL 39
Data supported via
voice-grade modems
UTP
modem
modem
To send data, it is converted into 4 kHz audio (modem)
Data rate is determined by Shannon's capacity theorem
•there is a maximum data rate (bps) called the "capacity"
that can be reliably sent through the communications channel
•the capacity depends on the BW and SNR
In Shannon's days it worked out to about 25 kbps
today it is about 35 kbps (V.34 modem - 33.6 kbps)
Stein Intro DSL 40
Digital PSTN
CO SWITCH
“last mile”
TDM
analog
“last mile”
Subscriber Line
digital
PSTN
TDM
CO SWITCH
LP filter to 4 kHz at input to CO switch (before A/D)
Stein Intro DSL 41
Digital PSTN
Sample 4 kHz audio at 8 kHz (Nyquist)
Need 8 bits per sample = 64 kbps
Multiplexing 64 kbps channels leads to higher and higher rates
Only the subscriber line (local loop) remains analog
(too expensive to replace)
Can switch (cross connect) large number of channels
Noise and distortion could be eliminated due to
Shannon's theorems
1. Separation theorem
2. Source coding theorem
3. Channel capacity theorem
Stein Intro DSL 42
Voice-grade modems
still work over new PSTN
CO SWITCH
PSTN
UTP subscriber line
modem
CO SWITCH
modem
network/
ISP
Internet
But data rates do not increase !
Simulate analog channel so can achieve
Shannon rate < native 64 kbps rate
router
Stein Intro DSL 43
Where is the limitation ?
The digital network was developed incrementally
No forklift upgrades to telephones, subscriber lines, etc.
Evolutionary deployment meant that the new network
needed to simulate pre-existing analog network
So a 4 kHz analog channel is presented to subscriber
The 4 kHz limitation is enforced by LP filter
at input to CO switch (before 8 kHz sampling)
The actual subscriber line is not limited to 4 kHz
Is there a better way
to use the subscriber line for digital transmissions ?
Stein Intro DSL 44
UTP
Stein Intro DSL 45
What is UTP?
The achievable data rate is limited by physics of the subscriber line
The subscriber line is an Unshielded Twisted Pair of copper wires

Two plastic insulated copper wires

Two directions over single pair

Twisted to reduce crosstalk

Supplies DC power and audio signal

Physically, UTP is
– distributed resistances in series
– distributed inductances in series
– distributed capacitances in parallel
so the attenuation increases with frequency

Various other problems exist (splices, loading coils, etc.)
Stein Intro DSL 46
UTP characteristics

Resistance per unit distance

Capacitance per unit distance

Inductance per unit distance

Cross-admittance (assume pure reactive) per unit distance
X
R
L
G
C
Stein Intro DSL 47
UTP resistance
Influenced by gauge, copper purity, temperature
Resistance is per unit distance

24 gauge 0.15 W/kft

26 gauge 0.195 W/kft
Skin effect: Resistance increases with frequency
Theoretical result
R~f
1/2
In practice this is a good approximation
Stein Intro DSL 48
UTP capacitance
Capacitance depends on interconductor insulation
About 15.7 nF per kft
Only weakly dependent on gauge
Independent of frequency to high degree
Stein Intro DSL 49
UTP inductance
Higher for higher gauge
24 gauge 0.188 mH per kft
26 gauge 0.205 mH per kft
Constant below about 10 kHz
Drops slowly above
Stein Intro DSL 50
UTP admittance
Insulation good so no resistive admittance
Admittance due to capacitive and inductive coupling
Self-admittance can usually be neglected
Cross admittance causes cross-talk!
Stein Intro DSL 51
Propagation loss
Voltage decreases as travel along cable
Each new section of cable reduces voltage by a factor
1v
1/2 v
1/4 v
So the decrease is exponential
Va / Vb = e
-g x
= H(f,x)
where x is distance between points a and b
We can calculate g, and hence loss,
directly from RCLG model
Stein Intro DSL 52
Attenuation vs. frequency
24 AWG
26 AWG
Stein Intro DSL 53
Why twisted?
from Alexander Graham Bell’s 1881 patent
To place the direct and return lines close together.
To twist the direct and return lines around one another so that they
should be absolutely equidistant from the disturbing wires
n
a
V = (a+n) - (b+n)
b
Stein Intro DSL 54
Why twisted? - continued
So don't need shielding, at least for audio (low) frequencies
But at higher frequencies UTP has cross-talk
George Cambell was the first to model
(see BSTJ 14(4) Oct 1935)
a
b
Lbc
Cbc
Lad
Cbd
c
d
Cross-talk due to capacitive and/or inductive mismatch
|I2| = Q f V1 where
Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)
Stein Intro DSL 55
Loading coils
Long loops have loading coils to prevent voice distortion
What does a loading coil do?
Flattens response in voice band
Attenuates strongly above voice frequencies
loops longer than 18 kft need loading coils
88 mH every 6kft starting 3kft
Stein Intro DSL 56
Bridge taps
There may also be bridged taps
Parallel run of unterminated UTP


unused piece left over from old installation
placed for subscriber flexibility
High frequency signals are reflected from the open end
A bridged tap can act like a notch filter!
Stein Intro DSL 57
Other problems
Splices
Subscriber lines are seldom single runs of cable
In the US, UTP usually comes in 500 ft lengths
So splices must be made every 500 ft
Average line has >20 splices
Splices are pressure connections that add to attenuation
Over time they corrode and may spark, become intermittent, etc.
Gauge changes
US binder groups typically start off at 26 AWG
Change to 24 AWG after 10 kft
In rural areas they may change to 19 AWG after that
Stein Intro DSL 58
Binder groups
UTP are not placed under/over ground individually
In central offices they are in cable bundles
with 100s of other UTP
In the outside plant they are in binder groups
with 25 or 50 pairs per group
We will see that these pairs interfere with each other
a phenomenon called cross-talk (XTALK)
Stein Intro DSL 59
CSA guidelines
1981 AT&T Carrier Service Area guidelines
advise as follows for new deployments
 No loading coils
 Maximum of 9 kft of 26 gauge (including bridged taps)
 Maximum of 12 kft of 24 gauge (including bridged taps)
 Maximum of 2.5 kft bridged taps
 Maximum single bridged tap 2 kft
 Suggested: no more than 2 gauges
In 1991 more than 60% of US lines met CSA requirements
Stein Intro DSL 60
Present US PSTN
UTP only in the last mile (subscriber line)



70% unloaded < 18kft
15% loaded > 18kft
15% optical or digital to remote terminal + DA (distribution area)
PIC, 19, 22, 24, 26 gauge
Built for 2W 4 KHz audio bandwidth
DC used for powering
Above 100KHz:



severe attenuation
cross-talk in binder groups (25 - 1000 UTP)
lack of intermanufacturer consistency
Stein Intro DSL 61
Present US PSTN - continued
We will see, that for DSL - basically four cases

Resistance design > 18Kft loaded line - no DSL possible

Resistance design unloaded <18 Kft <1300 W - ADSL

CSA reach - HDSL

DA (distribution area) 3-5 kft - VDSL
Higher rate - lower reach
(because of attenuation and noise!)
Stein Intro DSL 62
ADSL — What is it?

ADSL — Asymmetric Digital Subscriber Line
– High speed communications over twisted pair.
– Concurrent with POTS (plain old telephone
service).
– Secure way of Internet access.
– Originally standardized in ANSI (American
National Standards Institute) T1.231-1993.
– Currently standardized in ANSI T1.413-1998.
– Growing really fast.
ADSL is an asymmetric communication
technology designed for residential
users; it is not suitable for businesses.
The existing local loops can handle
bandwidths up to 1.1 MHz.
ADSL is an adaptive technology.
The system uses a data rate
based on the condition of
the local loop line.
9.64
Why asymmetry?
NEXT is the worst interferer stops HDSL from achieving higher rates
FEXT much less (attenuated by line)
FDD eliminates NEXT
All modems must transmit in the SAME direction
A reversal would bring all ADSL modems down
Upstream(US) at lower frequencies and power density
Downstream (DS) at high frequencies and power
Stein Intro DSL 65
Why ADSL?

Over the past 15 years, a thousand-fold
transmission rate is realized. But it still does
not meet today’s need.
– Viewing a full-motion movie requires about 5Mbps.
– Downloading Netscape requires 10 minutes.

ADSL:
– 20 fold faster
Figure 9.10 Discrete multitone technique
9.67
Figure 9.11 Bandwidth division in ADSL
9.68
Figure 9.12 ADSL modem
9.69
Figure 9.13 DSLAM
9.70
Table 9.2 Summary of DSL technologies
9.71
ADSL Duplexing

US uses low DMT tones (e.g. 8 - 32)

If over POTS / ISDN lowest frequencies reserved

DS uses higher tones
– If FDD no overlap
– If ECH DS overlaps US
P
O
T
S
US
8
DS
32
G.992.1 FDD mode
256
* 4.3125 kHz
Stein Intro DSL 72
Why asymmetry? - continued
PSD (dBm/Hz)
US
DS
F(MHz)
Stein Intro DSL 73
Echo cancelled ADSL
FDD gives sweet low frequencies to US only
and the sharp filters enhance ISI
By overlapping DS on US
we can use low frequencies and so increase reach
Power spectral density chart
Stein Intro DSL 74
ADSL - continued
ADSL system design criterion BER 10-12
(1 error every 2 days at 6 Mbps)
Raw modem can not attain this low a BER!
For video on demand:
 RS and interleaving can deliver (error bursts of 500 msec)
 but add 17 msec delay
For Internet:
 TCP can deliver
 high raw delay problematic
So the G.992.1 standard defines TWO framers
fast (noninterleaved ) and slow (interleaved) buffers
Stein Intro DSL 75
ADSL standard
ITU (G.dmt) G.992.1, ANSI T1.413i2 standard
DS - 6.144 Mbps (minimum)
US- 640 kbps
First ADSL data implementations were CAP (QAM)
ITU/ANSI/ETSI standards are DMT with spacing of 4.3125 kHz




DMT allows approaching water pouring capacity
DMT is robust
DMT requires more complex processing
DMT may require more power
Stein Intro DSL 76
Broadband
Network
(Internet)
Wiring
Distribution
Frame)
Customer
Premises
Wiring
... once more...
Stein Intro DSL 77
ADSL
= Assymmetric Digital Subscriber Line
- inmodulation band (not baseband)
- ANSI standards (T1.413 of T1E1.4 group), ETSI (european requirement added
to T1.413), ITU (groups of standards ITU-T G.991, 992, 995 etc. – they are
downloadable from : ITU - publications – ITU-T)
Specifications:
• high bit rate transmission + telephone (and also analog) connection, or ISDN
• max. downstream from 1,5 to 8 Mbps / max. upstream from 16 to 832 kbps
(basic ADSL system) – various data speeds in dependence from user distance
• freq.band up to 1,1 MHz, DMT modulation scheme (Discrete Multitone
Transmission), max. 256 DMT channels, each is 4 kHz wide
• for analog teleph.- lower 4 kHz, for ISDN up to 80 kHz (if there is ISDN
transmission, the band for digital data is reduced)
• reach - 5,5 km
• frame transmission by means Cu- lines
• Full / Lite
versions
Stein Intro DSL 78
ISDN-BRA
Frequency
Analog
teleph.signal
Fig. 1 ADSL spectrum with various variants [2]
ADSL
variant
number of
subchannels
from
to
speed
number of
subchannels
from
to
speed
only data
Tab.1 Comparison of ADSL variants
Stein Intro DSL 79
POTS
Downstream
Upstream
Frequency
Fig. ADSL spectrum in frequency multiplex
POTS
Upstream
Downstream
Frequency
Fig. ADSL spectrum with echo compensation
Stein Intro DSL 80
ADSL and ISDN
Upstream
Downstream
Frequency
Basic Access (4B3T link code)
Frequency
Basic Access (2B1Q link code)
Frequency
Stein Intro DSL 81
ADSL
Stein Intro DSL 82
btw., relation between bandwidth and data speed:
Shannon-Hartley theorema for information capacity of channel with both digital
signal with mean power S and additive Gauss noise with mean power N. Bandwidth
of channel is B [Hz].
S

C  B log 2 1  
 N
[bps] ...
channel information capacity
B ... bandwidth [Hz]
S ...power of signal in the given band B [V2 or W]
N...power of noise in the given band B [V2 or W]
S/N . .. signal–to-noise ratio [-]
(we know already SNR[dB] =10 log (S/N) )
Stein Intro DSL 83
LF
symmetrical pair
HF
Fig.2 Typical termination of ADSL line on the user side
Fig.3 ADSL line configuration with splitters
user
Provider
user line
Data network
ATU-C = ADSL transceiver unit at the central office,
Stein Intro DSL 84
ATU-R .....at the Remote home or business
Splitter
Stein Intro DSL 85
xDSL
Stein Intro DSL 86
Alternatives for data services
Fiber, coax, HFC
COST: $10k-$20k / mile
TIME: months to install
T1/E1
COST: >$5k/mile for conditioning
TIME: weeks to install
DSL
COST: @ 0 (just equipment price)
TIME: @ 0 (just setup time)
Stein Intro DSL 87
xDSL
Need higher speed digital connection to subscribers
Not feasible to replace UTP in the last mile
Older voice grade modems assume 4kHz analog line
Newer (V.90) modems assume 64kbps digital line
DSL modems don’t assume anything
Use whatever the physics of the UTP allows
Stein Intro DSL 88
xDSL System Reference Model
Analog
modem
CO SWITCH
PSTN
POTS-C
network/
ISP
POTS
SPLITTER
router
WAN
POTS-R
UTP
POTS
SPLITTER
DSLAM
xTU-C
PDN
xTU-R
x = H, A, V, ...
POTS
xDSL
frequency
DC 4 kHz
Stein Intro DSL 89
Splitter
Splitter separates POTS from DSL signals



Must guarantee lifeline POTS services!
Hence usually passive filter
Must block impulse noise (e.g. ring) from phone into DSL
ADSLforum/T1E1.4 specified that splitter be separate from modem
No interface specification (but can buy splitter and modem from different vendors)
Splitter requires installation
 Costly technician visit is the major impediment to deployment
 ADSL has splitterless versions to facilitate residential deployment
Stein Intro DSL 90
Why is DSL better
than a voice-grade modem?
Analog telephony modems are limited to 4 KHz bandwidth
Shannon’s channel capacity theorem
gives the maximum transfer rate
N
S
for SNR >> 1
C = BW log2 ( SNR + 1 )
C(bits/Hz)  SNR(dB) / 3
So by using more BW we can get higher transfer rates!
But what is the BW of UTP?
Stein Intro DSL 91
Maximum reach
To use Shannon's capacity theorem
we need to know how much noise there is
One type of noise that is always present
(above absolute zero temperature) is thermal noise
Maximum reach is the length of cable for reliable communications
ASSUMING ONLY THERMAL NOISE
Bellcore study in residential areas (NJ) found


-140 dBm / Hz
white (i.e. independent of frequency)
is a good approximation
We can compute the maximum reach from known UTP attenuation
Stein Intro DSL 92
xDSL - Maximum Reach
Stein Intro DSL 93
Other sources of noise
But real systems have other sources of noise,
and thus the SNR will be lower
and thus will have lower reach
There are three other commonly encountered types of noise

RF ingress

Near End Cross Talk (NEXT)

Far End Cross Talk (FEXT)
Stein Intro DSL 94
Sources of Interference
XMTR
RCVR
RCVR
XMTR
FEXT
NEXT
RCVR
XMTR
THERMAL
NOISE
XMTR
RCVR
RF INGRESS
Stein Intro DSL 95
Unger’s discovery
What happens with multiple sources of cross-talk?
Unger (Bellcore) : 1% worst case NEXT



(T1D1.3 185-244)
50 pair binders
22 gauge PIC
18 Kft
Found empirically that cross-talk only increases as N0.6
This is because extra interferers must be further away
Stein Intro DSL 96
Channel Modeling
(characteristic impedance, propagation constant,
channel attenuation)
R( f )  sL( f )
Z ( s) 
G( f )  sC ( f )
20
LdB (d , f )  20 log10 H (d , f ) 
d ( f )  8.686 d ( f )
ln 10
H (d , s )  e
 d g ( s )
e
 d  ( f )  j d  ( f )
e
Noise
There are three main types of noise that affect
DSL system performance:



NEXT (Near End Crosstalk)
FEXT (Far End Crosstalk)
Impulse Noise
NEXT

When a transceiver sends a signal and a
nearby transceiver at the same end “hears”
the signal, it’s NEXT.
A simplified NEXT model for N disturbers:
3
N 0.6
1
2
NEXTN  ( )
f
13
49 1.134  10
NEXT
Only close points are important

Distant points are twice attenuated by line attenuation |H(f,x)|2
Unger dependence on number of interferers
Frequency dependence

Transfer function ~ I2Campbell / R ~ f 2 / f 1/2 = f 3/2

Power spectrum of transmission
Total NEXT interference (noise power)
KNEXT N0.6 f 3/2 PSD(f)
Stein Intro DSL 100
FEXT

When a transceiver sends a signal and a
transceiver at the far end “hears” the signal,
FEXT occurs.
A simplified FEXT model for N disturbers:
N 0.6
2
2
FEXTN  ( ) k  f  d  H ( f )
49
FEXT
Entire parallel distance important

Thus there will be a linear dependence on L
Unger dependence on number of interferers
Frequency dependence

Transfer function ~ I2Campbell ~ f 2

Power spectrum of transmission
Total FEXT interference (noise power)
KFEXT N0.6 L f2 |Hchannel(f)|2 PSD(f)
Stein Intro DSL 102
Impulse Noise


Impulse noises are large surges of noise with
short duration. The sources of impulse noises
are not well understood yet. It is a very
devastating noise if not handled well.
A concatenated code, using a 2-dimensional
8-state trellis code and a 4-error-correcting
Reed-Solomon code with an interleaving
depth of 18 symbols, was found to be suitable
for eliminating impulse noise.
Multiple Access


FDM (Frequency Division Multiplexing)
ECH (Echo Canceller with Hybrid)
Line Code
Two main contenders:
 DMT — Discrete MultiTone
– A multi-carrier system using Discrete Fourier
Transforms to create and demodulate individual
carriers.

CAP — Carrierless Amplitude and Phase
– A version of suppressed carrier QAM.
DMT


Existing ANSI and ETSI standards
Consists of up to 256 sub-channels, (also
called tones or bins), of 4.3125KHz
– upstream use 25-163KHz (bins 6 to 38)
– downstream use 142KHz-1.1MHz (bins 33 to 255)
– bins 16 (69KHz) and 64 (276KHz) are pilot tones.


Outperforms CAP in field trials
More expensive and complex
DMT Line Code
Observations
Three Channels:
 POTS channel
– POTS channel is split off from the digital modem
by filters, thus guaranteeing uninterrupted POTS.

High speed downstream channel
– Its data rate depends on length of the copper line,
its wire gauge, presence of bridged taps, cross
talk, etc.

Medium speed upstream channel
DMT Features



Discretely divides the available
frequencies into 256 sub-channels or
tones.
Incoming data is broken down into a
variety of bits and distributed to a specific
combination of sub-channels.
To rise above noise, more data resides in
the lower frequencies and less in the
upper frequencies.
DMT Transmission Parameters

Downstream
–
–
–
–
symbol rate: 4KHz
FFT size: 512
Cyclic prefix: 32
Sampling rate:
2.208MHz
– Transmit power:
20dBm
– Highpass filter:
62.5kHz

Upstream
–
–
–
–
Symbol rate: 4kHz
FFT size: 64
Cyclic prefix: 4
Sampling rate:
276kHz
– Transmit
Power:7dBm
– Lowpass filter:
43.875kHz
DMT Block Diagram
PSD of DMT
PSD is useful for finding received signal power,
thus useful for analyzing NEXT and FEXT
noises.
Upstream and downstream PSD models are:





2V  sin(fT ) 
1
f8
 8




3 8 
f
ZT  f
 (20  10 ) 
8  f
 1  (
)

1.104  106 

2
PSDADSL, DS
PSDADSL,US 
2
2V
ZT
 sin(fT ) 
2

 HUS ( f )
f


2
DMT
Discrete Multitone is a form of FDM (Frequency Domain Multiplexing)
Discrete Multitone is a form of MCM (MultiCarrier Modulation)
It uses many different carriers, each modulated QAM
Each tone is narrow


low baud rate (long frame)
channel characteristics are constant over tone
Number of bits per tone chosen according to water pouring
Put more bits where SNR is good
Stein Intro xDSL 3.113
DMT - continued
DMT is OFDM (Orthogonalized FDM)



Carrier spacing is precisely baud rate
Center of tone is precisely the zero of all other sincs
ICI minimized
ISI minimized by having a long interframe guard time
DMT modem can be efficiently implemented using FFT
DFT is mathematically equivalent to a bank of filters
Filtering is equivalent to cyclic convolution
So use cyclic prefix rather than guard time
Stein Intro xDSL 3.114
DMT - continued
frequency
time
Stein Intro xDSL 3.115
ADSL DMT
Baud rate (and channel spacing) is 4.3125 KHz
US uses tones 8 - 32 (below 30 KHz reserved)
DS uses 256 tones (FDM from tone 33, EC from tone 8)
P
O
T
S
US
8
DS
32
256
Stein Intro xDSL 3.116
DMT misc.
bit handling ((de)framer, CRC, (de)scrambler, RS, (de)interleaver)
tone handling (bit load, gain scaling, tone ordering, bit swapping)
QAM modem (symbolizer, slicer)
signal handling (cyclic prefix insertion/deletion, (I)FFT,
interpolation, PAR reduction)
synchronization (clock recovery)
channel handling
(probing and training, echo cancelling, FEQ, TEQ)
Stein Intro xDSL 3.117
Splitterless ADSL
Splitterless ADSL, UAWG, G.lite, G.992.2, G.992.4
Splitterless operation
 fast retrain
 power management to eliminate clipping
 initialization includes probing telephone sets for power level
 microfilters
 modems usually store environment parameters
G.992.2 - cost reduction features




G.992.1 compatible DMT compatible using only 128 tones
512 Kbps US / 1.5 Mbps DS (still >> V.34 or V.90 modems)
features removed for simplicity
simpler implementation (only 500 MIPS < 2000 MIPS for full rate)
Stein Intro DSL 118
Frame Structure
Frame Structure (cont.)
A super frame is defined for every 68 IFFT/FFT
operations.The super frame has a time duration
of 68/4k=17ms for baud rate of 4kHz.
CAP





Initial ADSL implementations were done
using CAP
1996 - 90% of world-wide ADSL
implementation based on CAP
Variant of QAM - widely understood
Not yet incorporated in ANSI standards
T1.413 or ETSI
Supported by GlobeSpan Technologies
CAP Transmission Parameters

Downstream
–
–
–
–

Constellation size: 64
Baud rate: 266.67KHz
Throughput: 1.6 Mbps
Sampling
rate:1.0667MHz
– Transmit power: 12dBm
– Signal spectrum:
170 ~ 410KHz
Upstream
–
–
–
–
Constellation size: 16
Baud rate: 6KHz
Throughput: 24Kbps
Transmit power:4.8dBm
– Signal spectrum:
96 ~ 102KHz
Example - Interference spectrum
Stein Intro DSL 123
Examples of Realistic Reach
More realistic design goals (splices, some xtalk)

1.5 Mbps
18 Kft
5.5 km
(80% US loops)

2 Mbps
16 Kft
5 km

6 Mbps
12 Kft
3.5 km

10 Mbps
7 Kft

13 Mbps 4.5 Kft
1.4 km

26 Mbps
3 Kft
900 m

52 Mbps
1 Kft
300 m (SONET
(CSA 50% US loops)
2 km
STS-1 = 1/3 STM-1)
Stein Intro DSL 124
ADSL Speed Comparison
Pure Fibre
Hybrid Fibre/Copper
FTTH
Enhanced
Copper
FTTx,
VDSL2,
ADSL2plus
ADSL
ISDN
Voice band
Modem
Stein Intro DSL 125
ADSL Range
In general, the maximum range for DSL without a repeater
is 5.5 km
As distance decreases toward the telephone company o
office, the data rate increases
Data Rate
Wire gauge Wire size
o
Distance
1.5 or 2 Mbps 24 AWG
0.5 mm
5.5 km
1.5 or 2 Mbps 26 AWG
0.4 mm
4.6 km
6.1 Mbps
24 AWG
0.5 mm
3.7 km
1.5 or 2 Mbps 26 AWG
0.4 mm
2.7
For larger distances, you may be able to have DSL if your
phone company has extended the local loop with optical
fiber cable

Stein Intro DSL 126
ADSL Speed Factors
The distance from the local exchange

The type and thickness of wires used

The number and type of joins in the wire

The proximity of the wire to other wires carrying
ADSL, ISDN and other non-voice signals
The proximity of the wires to radio transmitters.


Stein Intro DSL 127
ADSL network components
The ADSL modem at the customer premises(ATU-R)
The modem of the central office (ATU-C)
DSL access multiplexer (DSLAM)
Broadband Access Server (BAS)




Splitter - an electronic low pass filter that separates the
analogue voice or ISDN signal from ADSL data
frequencies DSLAM.

Stein Intro DSL 128
Bonding (inverse mux)
If we need more BW than attainable by Shannon bounds
we can use more than one UTP pair (although XT may reduce)
This is called bonding or inverse multiplexing
There are many ways of using multiple pairs:

ATM - extension of IMA (may be different rates per pair)
ATM cells marked with SID and sent on any pair

Ethernet - based on 802.3(EFM)
frames are fragmented, marked with SN, and sent on many pairs



Time division inverse mux
Dynamic Spectral Management (Cioffi)
Ethernet link aggregation
Stein Intro DSL 129
Duplexing
Up to now we assumed that only one side transmits
Bidirectional (full duplex) transmission
requires some form of duplexing
For asymmetric applications we usually speak of
DS downstream and US upstream
Four methods are in common use:




Half duplex mode (4W mode) (as in E1/T1)
Echo cancellation mode (ECH)
Time Domain Duplexing (requires syncing all binder contents)
Frequency Domain Duplexing
POTS
US
DS
frequency
DC 4 kHz
Stein Intro DSL 130
Muxing, inverse muxing, duplexing
inverse
multiplexing
multiplexing
data streams
physical line
data stream
physical lines
Duplexing =
2 data streams in 2 directions on 1 physical line
Multiplexing =
N data streams in 1 direction on 1 physical line
Inverse multiplexing = 1 data stream in 1 direction on N physical lines
duplexing
Stein Intro DSL 131
(Adaptive) echo cancellation
Signal transmitted is known to transmitter
It is delayed, attenuated and distorted in the round-trip
Using adaptive DSP algorithms we can
 find the delay/attenuation/distortion
 subtract
modulator
4W to 2W
HYBRID
demodulator
Stein Intro DSL 132
xDSL types
and
history
Stein Intro DSL 133
DSL Flavors
DSL is often called xDSL
since there are many varieties (different x)
e.g. ADSL, HDSL, SHDSL, VDSL, IDSL, etc.
There were once many unconnected types
but now we divide them into three main families
The differentiation is by means of the application scenario



HDSL (symmetric, mainly business, data + telephony)
ADSL (asymmetric, mainly residential, Internet access)
VDSL (very high rate, but short distance)
Stein Intro DSL 134
PSD(dBm/Hz)
Some xDSL PSDs
T1
IDSL HDSL HDSL2
ADSL
F(MHz)
Stein Intro DSL 135
ITU G.99x standards

G.991 HDSL (G.991.1 HDSL
G.991.2 SHDSL)

G.992 ADSL (G.992.1 ADSL
G.992.2 splitterless ADSL
G.992.4 splitterless ADSL2
G.992.3 ADSL2
G.992.5 ADSL2+)

G.993 VDSL (G.993.1 VDSL

G.994 HANDSHAKE

G.995 GENERAL (INFO)

G.996 TEST

G.997 PLOAM

G.998 bonding (G.998.1 ATM
G.993.2 VDSL2)
G.998.2 Ethernet G.998.3 TDIM)
Stein Intro DSL 136
ITU xDSL layer model

Transport protocol (ATM, STM, PTM)

Transport Protocol Specific - Transmission Convergence (TPS-TC)

Physical Medium Specific - Transmission Convergence (PMS-TC)

Physical Medium Dependent (PMD)

Physical medium
Stein Intro DSL 137
More xDSL flavors
modem
speed
reach
main applications
IDSL
160 (144) Kbps
5.5 km
HDSL
2 Mbps (4-6W)
3.6-4.5 km
HDSL2
2 Mbps (2W)
3 km
POTS
replacement,
videoconferencing,
Internet access
T1/E1 replacement
PBX interconnect,
FR
same as HDSL
SHDSL
2.3 Mbps
3 km
same as HDSL
SHDSLbis
4.6 Mbps
3 km
same as HDSL
Stein Intro DSL 138
More xDSL flavors (cont.)
Not
DSL
modem
speed
reach
main applications
ADSL
6 Mbps DS
640 Kbps US
3.5-5.5 km
residential Internet,
video-on-demand
ADSL2
8 Mbps DS
800 Kbps US
> ADSL
Internet access,
VoIP
ADSL2+
16 Mbps DS
800 Kbps US
< 2 km
“
VDSL
<= 52 Mbps
300m - 1 km
VDSL2
200 Mbps
(aggregate)
up to 1.8 km
LAN interconnect,
HDTV,
combined services
“
cable modem
10-30Mbps DS
shared
50 km
residential Internet
HPNA
1, 10 Mbps
home wiring
residential
networking
Stein Intro DSL 139
HDSL2
With the success of HDSL,
customers requested HDSL service that would :
 require only a single UTP HDSL
 attain at least full CSA reach
 be spectrally compatible w/ HDSL, T1, ADSL, etc.
The result, based on high order PAM, was called


HDSL2 (ANSI)
SDSL Symmetric DSL (ETSI)
and is now called

SHDSL Single pair HDSL (ITU)
Stein Intro DSL 140
SHDSL
Uses Trellis Coded 16-PAM with various shaping options
Does not co-exist with POTS service on UTP
Can uses regenerators for extended reach
single-pair operation
 192 kbps to 2.312 Mbps in steps of 8 kbps
 2.3 Mbps should be achieved for reaches up to 3.5 km
dual-pair operation (4-wire mode)
 384 kbps to 4.608 Mbps in steps of 16 kbps
 line rate is the same on both pairs
Latest standard (G.shdsl.bis - G.991.2 2003 version)
 bonding up to 4 pairs
 rates up to 5696 kbps
 optional 32-PAM (instead of 16-PAM)
 dynamic rate repartitioning
Stein Intro DSL 141
ADSL
Asymmetric - high rate DS, lower rate US
Originally designed for video on demand
New modulation type - Discrete MultiTone
FDD and ECH modes
Almost retired due to lack of interest
…but then came the Internet
Studies - DS:US for both applications can be about 10:1
Some say ADSL could mean
All Data Subscribers Living
Stein Intro DSL 142
ADSL2+
2,2 MHz
1,1 MHz
Up to 18,000 feet (5.5 km)
Up to 25 Mbps down
Up to 1 Mbps Upstream
Fig.6 ADSL 2+ system
Stein Intro DSL 143
ADSL2
- ITU-T G. 992.3, .4
- 2nd generation of ADSL standard
- downstream - up to 12 Mbps
- DMT modulation
- bandwidth - up to 2,2 MHz
- but: shorter reach (only from 1,5 to 2 km) !
- CVoDSL
ADSL2 + (fig.6)
- ITU-T G. 992.5
- downstream - up 24 Mbps
- bandwidth - up to 2,2 MHz (512 subchannels DMT, each 4kHz wide, up to
2,2 MHz)
- full data speed only in reach of max. 1,5 km from DSLAM (!)
Stein Intro DSL 144
• RE-ADSL = Reach Extended ADSL
-ITU-T G.992.3 – Annex L (it is annex to ADSL2 standard)
- optimalized DMT channels with the goal of larger length (manipulation
with PSD of some channels  their higher throughput
- dedicated to long lines (not for short ) – up to 5,5 km with the same
date speed as in ADSL2
• RADSL = Rate Adaptive DSL
- it is in development
- both symetrical and assym.transmission
transmission speed is adaptive (it depends on transmission conditions
and distance)
down 1-12 Mbps / up 128kbps-1Mbps
DMT or CAP (and QAM) are supposed
- for applications without synchronization requirements (IP services,
ATM, Frame Relay)
• Bonded ADSL
- combines (bonds) 2 or more (up to 32) Cu-pairs for higher or extreme
data speeds (for big and reach  companies)
Stein Intro DSL 145
ADSL2
ADSL uses BW from 20 kHz to 1.1 MHz
ADSL2 Increases rate/reach of ADSL by using 20 kHz - 4.4 MHz
Also numerous efficiency improvements
 better modulation
 reduced framing overhead
 more flexible format (see next slide)
 stronger FEC
 reduced power mode
 misc. algorithmic improvements
for given rate, reach improved by 200 m
3 user data types - STM, ATM and packet (Ethernet)
ADSL2+ dramatically increased rate at short distances
Stein Intro DSL 146
More ADSL2 features
Dynamic training features

Bit Swapping (dynamic change of DMT bin bit/power allocations)

Seamless Rate Adaptation (dynamic change of overall rate)
Frame bearers

Multiple (up to 4) frame bearers (data flows)

Multiple latencies for different frame bearers (FEC/interleave lengths)

Dynamic rate repartitioning (between different latencies)
Stein Intro DSL 147
VDSL
Optical network expanding (getting closer to subscriber)
Optical Network Unit ONU at curb or basement cabinet
FTTC (curb), FTTB (building)
These scenarios usually dictates low power
Rates can be very high since required reach is minimal!
Proposed standard has multiple rates and reaches
Stein Intro DSL 148
VDSL - rate goals
Symmetric rates
6.5 4.5Kft (1.4 Km)
13
3 Kft (900 m)
26
1 Kft (300 m)
Asymmetric rates (US/DS)
0.8/ 6.5
1.6/13
3.2/26
6.4/52
6 Kft
4.5 Kft
3 Kft
1 Kft
(1.8 Km)
(1.4Km)
(900 m)
(300 m)
Stein Intro DSL 149
VDSL - Power issues
Basic template is -60 dBm/Hz from 1.1MHz to 20 MHz
Notches reduce certain frequencies to -80 dBm/Hz
Power boost on increase power to -50 dBm/Hz
Power back-off reduces VTU-R power so that won’t block another user
ADSL compatibility off use spectrum down to 300 KHz
Stein Intro DSL 150
VDSL - duplexing
In Japan and campus applications can operate TDD (ping pong)
SDMT Synchronous DMT
(2 KHz frame can be heard in adjacent pairs or hearing aids)
Rest of world PSTN only FDD is allowed
Can divide US and DS into 2 areas (e.g. ADSL) or more
Need guard frequencies because of clock master/slave problems
Zipper - large number of interleaved frequency regions
(even on a bin by bin basis)
Stein Intro xDSL 3.151
VDSL line code wars
VDSL Alliance
DMT
VDSL Coalition
QAM
MORE
LESS
robust to noise
power
capacity
spectral compatibility
IPR
complex
expensive
A/D bits
With no complexity constraints probably equivalent
Stein Intro xDSL 3.152
VDSL2
DMT line code (same 4.3125 kHz spacing as ADSL)
VDSL uses BW of 1.1 MHz - 12 MHz (spectrally compatible with ADSL)
VDSL2 can use 20 kHz - 30 MHz

new band-plans (up to 12 MHz, and 12-30 MHz)

increased DS transmit power
various algorithmic improvements
borrowed improvements from ADSL2
3 user data types - STM, ATM and PTM



Stein Intro DSL 153
VDSL2 band plans
North American bandplan
US0 (if present) starts between 4 kHz - 25 kHz
and ends between 138-276 kHz
Europe - six band plans (2 A and 4 B)
A (998) US0 from 25
DS1 from 138 or 276
US1 3750-5200 DS2 5200-8500
B (997) US0 from 25 or 120 or nonexistent
DS1 from 138 or 276
US1 3000-5100 DS2 5100-7050
Stein Intro DSL 154