Broadband Access Networks and Services

1
Broadband Access Networks and Services
EE4541.759 Spring 2004

• Chapter 2
• Twisted-Pair based Access Networks
• Byeong Gi Lee
• Seoul National University

2
2. TP-based Access Network

• Limitation of POTS and ISDN
• Digital Loop Carrier
• DSL Technologies IDSL, HDSL, SDSL, ADSL,
  G.Lite, VDSL, and Comparisons
• ADSL Access Network, Architecture, Components,
  and Protocol Model
• Spectrum Allocations
• Modulation Techniques
• Applications, Challenges, and Penetration

3
Structure of Access Network

• ADSL Asymmetric Digital Subscriber Line
• DBS Direct Broadcast Satellite
• FTTC Fiber To The Curb
• HFC Hybrid Fiber Coaxial
• LMDS Local Multipoint Distribution System
• MMDS Multi-channel Multipoint Distribution
  System
• MSO Multiple Service Operator

TP-based AN
Digital DBS
Fiber or Wireless
MMDS/LMDS
LMDS only
Hub Station
DBS STB/ MMDS/LMDS STB
64Kbps
ISDN Adaptor
ISDN Line
MSO /CO
Analog Modem
56Kbps
ADSL Modem
6Mbps
Home Terminal
640Kbps
30Mbps
Coax
Cable Modem
Fiber
HFC
3Mbps
ONU
Coax or Copper
VDSL Modem
Fiber
52Mbps
FTTC/FTTO
3Mbps
4
Access and Backbone Infrastructure
5
Limitation of POTS and ISDN

• Limitation in existing subscriber lines
• Restrictive for high-speed service
• Limited data-carrying capacity of 56kbps using
  V.90
• Analog mode and suppressing filters above 3,400Hz
• Limitation of POTS
• Customer connected over thin-wire pairs (26guage,
  0.5mm)
• Local loops are bundled in binder groups (feeder
  part)
• Limitation of ISDN services
• Not possible to provision ISDN and POTS in same
  local loop
• Internet Problems for POTS and ISDN
• Both carries data in circuit mode (voice switch)

6
Digital Loop Carrier

• T1 or HDSL base feeder (connecting CO and RT)

7
xDSL

• Advanced modulation technology based bandwidth
  expansion (128kbps to 52Mbps)
• xDSL easier to install than ISDN
• needs inside wiring work and copper line cleaning
• Variations depending on symmetry of
  upward/downward data rates (ADSL vs SDSL) and the
  location of equipment (ADSL vs VDSL)
• Supports POTS using splitter (split flter). But
  G.Lite can eliminate splitter.
• HDSL can replace T1/E1 without repeaters.

8
Configuration of xDSL
End User (POTS)

• Central
• Office
• (PSTN)

TP (2 pairs)
Line Repeater
Line Repeater
CO Repeater
T1/E1
0.31km
11.8 km
0.31km
End User (POTS)
Central Office (PSTN)
TP (2 pairs)
HDSL RT Unit
HDSL CO Unit
HDSL
End User (Video)
Headend (Video)
ADSL CO Unit
TP (1 pair)
ADSL RT Unit
ADSL
PSTN
POTS
End User (Video)
Headend (Video)
Optical Network Unit
TP (1 pair)
VDSL RT Unit
VDSL CO Unit
VDSL
SPF
POTS
PSTN
9
IDSL

• ISDN Digital Subscriber Line is a cross point
  between ISDN and xDSL
• Made available with the introduction of ISDN
• 2B D basic rate access (BRA), 144kbps16kbps
• B 64kbps for voice or data
• D 16kbps for call control or packet data
• Transmission rate of 80 ksps
• 2B1Q modulated
• Travels over 5.4 km over 24 gauge (.5mm)

10
HDSL

• Comparable with T1/E1 lines
• 1.5 - 2 Mbps rate transmission with 2 pairs
• Can replace T1/E1 lines without repeaters
• For T1 rate (1.5Mbps), two TP wires are necessary
• For E1 rate (2Mbps), three TP wires are necessary
• 2B1Q Modulated, CAP, DMT
• 2B1Q 3.6km, CAP5.5km over 24 gauge
• Interference problem when bundled in large size

11
SDSL

• Symmetric (or Single-line) HDSL (SHDSL, SDSL)
• Two-wire (single pair) implementation of 2B1Q or
  CAP
• Data rate of 384kbps2Mbps (Sub-T1/E1 rates
  obtainable)
• HDSL2 (G.SHDSL) stadard
• Symmetric 2.3 Mbps over a single loop pair
• Symmetric 4.6 Mbps over two loop pair
• Echo cancellation and adaptive equalization

12
ADSL

• Up/down asymmetric bandwidth
• For use in Internet, VOD like applications
• One pair - good for residential use
• 1.5/2 6/8 Mbps down, 16820 kbps up
• Up to 5.4km without repeaters (0.5mm)
• DMT (standardized), CAP (market) modulation
• POTS split through split filter (SF)
• Rate-Adaptive DSL (RADSL) - line condition
• Universal ADSL (UADSL) - without split filter

13
ADSL G.Lite

• ITU G.992.2
• Easy and consumer-installable due to no need of
  POTS splitters like in full-rate ADSL (UADSL)
• Offers the services of ADSL at a lower speed
• POTS service possible by reducing the
  transmission power when off-hook
• Fast retrain procedure 12 s cessation of
  service
• Longer reach and much less power than full-rate
  ADSL

14
VDSL

• Very high data rate, (a)symmetric DSL
• Drop line for FTTC application
• 1352 (155) Mbps down depending on distance
• 300 m to 1.5 km distance (from curb to home)
• CAP, DMT,QAM (DAVIC) modulation
• POTS split through split filter (SF)
• Standardization (DAVIC, ANSI T1E1.4, ETSI TM6,
  FSAN)

15
ADSL vs. G.Lite
16
ADSL vs. VDSL
17
Comparison of xDSL (1)
18
Comparison of xDSL (2)
19
Comparison of xDSL (3)
Distance for 0.5 mm (24 gauge) TP wires
20
ADSL-based Access Network
Switch
Splitter
Splitter
04 kHz
PSTN
Subscriber line
DSLAM
Data Comm. Network
ATU-R
ATU-C
T1.413 Issue 2 Down 25 1104 kHz,
Up 25 138 kHz UADSL
Down 138 552 kHz,
Up 25 138 kHz
Subscriber line
UADSL-R
DSLAM Digital Subscriber Lines Access
Multiplexer ATU-C ADSL Transceiver Unit,
Central ATU-R ADSL Transceiver Unit, Remote
Splitter-less in UADSL
21
Generic ADSL Access Network Architecture
22
ADSL Components

• ADSL Transceiver Units
• Physical layer device for frequency allocation,
  echo cancellation, FDM, rate adaptation, etc.
• POTS splitter
• Lowpass/highpass filter that separates analog
  voice from ADSL frequencies
• DSL Access Multiplexer
• Houses a set of ATU-C interfaces, mux/demuxes
  traffic from multiple ATU-Cs onto a single
  high-speed trunk

23
Protocol Model for Data Services over ADSL
24
Typical Protocol Stack for Access Network
25
Bandwidth Requirement
Acceptable response time lt 3.0 sec
26
Concept of Spectrum Allocation
(Numbers for example)
ADSL (Down)
ISDN
Power Spectra
POTS
ADSL (Up)
4
10
80
200
1000 kHs
100
27
Spectrum Allocations for xDSL
FTT Exchange, FTT Cabinet
28
ADSL Operation (1)

• Two services of ADSL
• transparent access to legacy voice service
• high-speed digital service
• Voice Service (1)
• Provision of voice service relatively simple
• In-home configuration
• Frequencies below 3400Hz shunt by POTS splitters
  to POTS wiring at home and to voice switch at CO
• NID (or NT) by telco powering, ATU-R by local
  powering
• Voice service not to be subject to home wiring
  impairments
• Protection of NID from lightening required not to
  damage home electronics.
• So professional assistance needed for splitter
  installation

29
ADSL Operation (2)

• Voice Service (2)
• Three possible locations of POTS splitter
• LPF at NID and HPF at ATU-R
• both filters in ATU-R
• both filters in NID
• Three constraints
• local powering ATU-R
• voice not to be subject to impairments of home
  wiring
• LPF relatively large, so does not fit into NID
• These three constraints suggest POTS splitter and
  ATU-R should be separated

30
ADSL Operation (3)

• One proposal from British Telecom

31
ADSL Operation (4)

• Digital Service (1)
• 1. Determine latency
• Video traffic subject to burst error, so
  interleaving and FEC used.
• ANSI provided 500microsecond burst protection
  against impulse noise, creating 20ms latency.
• But data service has no such requirement as
  higher-layer protocols can perform error
  correction, - delay of only 4ms.
• So ADSL provides two logical data channels fast
  (4ms) and slow (20ms)
• Video on slow channel due to FEC and interleaving
• Data on fast channel only with (de)modulation
  delay
• ATU-R determines which latency type to take at
  start-up

32
ADSL Operation (5)

• Digital Service (2)
• 2. Determine bit rate
• RADSL adapts to the line condition via a
  negotiation process
• When powered on, ATU-R probes the line to
  determine signal carrying capacity.
• Signal-carrying capacity is a function of
  distance, noise, and allowable margins
• Negotiation goes between ATU-C and ATU-R for
  maximum sustainable bit rate (out of four
  start-up rates, ANSI)
• Entire negotiation within 20s.

33
ADSL Operation (6)

• Digital Service (3)
• 3. Perform auto-configuration
• Configuring IP addresses and software filters for
  ATU-R
• Relatively straightforward since each user has a
  dedicated ATU-C
• Manually configured by DSLAM or some form of
  software tunnel between ATU-R and a device
• Method differs depending whether ATU-R or the end
  system is IP-aware
• If ATU-R and PC is IP-aware DHCP client is one
  autoconfiguration technique

34
ADSL Operation (7)

• Digital Service (4)
• 4. Digital Data Transfer
• After startup complete, data transferring
• ATU-R accepts data from home networking over an
  Ethernet, ATM25, or other digital protocols,
  encapsulates, and then sends out.
• MAC protocol is not required unlike HFC
• Dynamic rate adaptation ATU-R and ATU-C
  renegotiation during data transmission by
  periodic testing of line conditioning --
  retraining time will be required in support of
  this.

35
Modulation Techniques

• 2B1Q (2 binary, 1 quaternary)
• DMT (discrete multi-tone modulation)
• CAP (carrier-less amplitude and phase modulation)
• QAM (quadrature amplitude modulation)
• QPSK

36
2B1Q
37
DMT (1)
DMT Sub-channel based data rate allocation
TWISTED-PAIR
Bits/chan
Bits/chan
Atten
Frequency
Frequency
Frequency
TWISTED-PAIR with TAP, AM/RF, and XTALK
Atten
Bits/chan
Bits/chan
AM
TAP
Frequency
Frequency
Frequency
38
DMT (2)

• DMT Transceiver Block Diagram

39
DMT (3)

• Features
• frequency efficiency
• symbol length per sub-channel longer
• frequency selective noise avoided
• Issues
• complexity, power consumption
• timing and synchronization critical
• ANSI standard

40
CAP (1)

• CAP Transceiver Block Diagram

41
CAP (2)

• 16CAP

Qn
11
01
3
10
11
b2 b3
current state
00
10
1
01
00
-1
-3
3
1
In
01
00
00
10
-1
11
10
-3
11
01
42
QAM

• 16QAM Downstream in DAVIC VDSL

In-Phase
Filter

Data
Band
Symbol
Signal
Pass
Encoder
Input
_
Output
Filter
Quadrature
Filter
10
11
01
11
b2 b3
3
increasing time
current
01
00
state
lsb
msb
TC layer
1
byte alignment
00
01
00
10
b1
b2
b0
b3
b0
b1
b2
b3
b0
b1
b2
b3
b2
-3
1
3
-1
quadrant
-1
01
00
00
10
point within quadrant
11
10
-3
symbol n1 (0..3)
01
11
11
10
symbol n (0..3)
43
QPSK

• QPSK Upstream DAVIC VDSL

Transmit
Filter

cos
Data
Band
Symbol
Signal
Pass
Oscillator
Encoder
Input
_
Output
Filter
sin
Transmit
Filter
44
CAP vs. DMT
45
xDSL Applications (1)

• High-speed Internet Access
• residential and business area
• Intranet access to link
• Remote Office and Branch Office (ROBO)
• low cost, high throughput LAN-to-LAN connectivity
  
• Telecommuting
• Small Office / Home Office (SOHO)

46
xDSL Applications (2)

• On-demand type applications
• VOD, home shopping, tele-education, Interactive
  TV
• Frame relay / ATM network access
• Leased line provisioning (HDSL/SDSL)
• replacing conventional T1/E1 leased line
• FTTC using VDSL

47
Technical Challenges of xDSL (1)

• End-to-end loop qualification impacted by
• Loop length, loading coils, quality of E2E
  splicing of wiring segments, multiple changes of
  wire gauge, home wiring, age, corrosion, hostile
  binder groups, crosstalk, and bridged taps
• Crosstalk
• Near-end crosstalk (NEXT)
• Far-end crosstalk (FEXT)
• Bridged Taps
• Y form branch gt source of echo

48
Technical Challenges of xDSL (2)

• Powering remote terminals
• Local power used to power DSL equipment battery
  backup
• Spectral masking
• Long-term narrowband interference such as, AM
  radio and amateur radio ingress
• Notching out specific frequencies
• Impulse noise
• Short-term interference with duration of 100ms
  with peak power of 10mv
• ART-R maintenance
• Software upgrading such as, algorithms

49
xDSL Penetration Worldwide (1)

• Sources from Point-Topic
• 77 million broadband lines worldwide at June 2003
• 60 of all broadband lines are xDSL (46.7
  million)
• DSL lines grew by 30 in the first half of 2003,
  cable modems by 16.4
• Fast increase in Japan and China
• Growth appears to be slowing in Korea and USA
• Expected 60 million xDSL subscribers by end of
  2003

50
xDSL Penetration Worldwide (2)
51
xDSL Penetration Worldwide (3)
52
xDSL Penetration Worldwide (4)
53
xDSL Penetration Worldwide (5)

• USA

54
xDSL Penetration Worldwide (6)

• China (1)

55
xDSL Penetration Worldwide (7)

• China (2)

56
xDSL Penetration Worldwide (8)

• Japan (1)

57
xDSL Penetration in Korea (1)

• 11 million broadband access subscribers (Jun.
  2003)
• 6.2 million xDSL (56.5), 3.8 million Cable
  (34.5)

Source ????? ???LAN LAN based Broadband
Internet Service
58
xDSL Penetration in Korea (2)

• Subscribers for Service Providers

Source ?????
59
xDSL Penetration in Korea (3)

• Service Providers and Base Technologies

Source ?????
60
Access Network Architecture (2)

• Typical technology-independent protocol stack

61
Service Architecture over ADSL (1)

• Basically a p2p switched transmission technology
• IP over PPP over HDLC over ADSL or
• IP over ATM over ADSL
• ADSL intended to be used as access technology for
  Internet or LAN access in conjunction with analog
  telephony service
• Various architectural models possible to meet the
  specific service offerings, interconnection to
  the backbone networks, and strategic direction of
  a network provider

62
Service Architecture over ADSL (2)

• A plausible communications model for data over
  ADSL

63
Service Architecture over ADSL (3)

• ATU-C encoding and modulating the DS data and
  combining the analog POTS signal in US,
  demodulating the signal received from the
  customer and outputting a data stream and POTS
  signal
• Three main architecture options
• Bit-synchronous mode
• ATM cell mode
• Packet mode
• Type of mode is independent of underlying line
  encoding method used

64
Service Architecture over ADSL (4)

• Generic reference architecture at logical model

65
Service Architecture over ADSL (5)

• Bit-Synchronous Mode
• Traditional circuit-switched network digital
  hierarchy
• Interface with either a backbone voice or data
  network via existing CBR circuit-switched
  services
• Plesiochronous digital hierarchy supported in
  ADSL
• DS1 (1.544 Mbps) E1 (2.048 Mbps) DS2 (6.312
  Mbps) E2 (8.448 Mbps)
• An application will typically use the appropriate
  bandwidth from the digital hierarchy meeting the
  bandwidth demand
• Data applications such as Internet and LAN
  interconnect supported in this mode

66
Service Architecture over ADSL (6)

• Packet-based Transport over ADSL
• MUX in the access node adapts the protocol
  carried over ADSL to one of the commonly used
  packet transport services
• Switching and/or routing of variable-length layer
  2 frames and/or layer 3 packets over an ADSL link
• PPP and FR are the most popular L2 protocols
  (HDLC-like) supporting any of L3 protocols (IP,
  IPS,..)
• PPP encapsulation allows many different
  protocols to be transported over p2p links
• FUNI encapsulation variable-size frame to be
  transmitted is encapsulated in FUNI frame

67
Service Architecture over ADSL (7)

• ATM-based Transport over ADSL
• ATU-R provides a native ATM interface to the user
  device or a legacy LAN interface with
  segmentation and reassembly (SAR) functionality
• Edge switch
• ATM multiplexer in access node
• Concentration and ATM layer routing function
• Alternatively, SAR and network layer routing can
  be provided in CO such as DSLAM
• On access side multiplicity of service protocols
  (TCP/IP, PPP, FR, ATM cell relay, ATM circuit
  emulation) over a variety of physical layer
  protocols (HDSL, ADSL, DS1)
• On backbone side standard ATM user-network
  interface for transport to ATM switch, which can
  connect to ATM backbone