ADSL im Detail

Entnommen aus Seminar „xDSL-Übertragungstechnik“Hollabrunn 2002

Werner Henkel

ftw. (jetzt IUB)

Forschungszentrum Telekommunikation Wien

© W. Henkel, 2003

A D S LAsymmetrical Digital Subscriber Line

Einleitung

Hoch-raten-kanal

Nieder-raten-kanal

© W. Henkel, 2003

DMT (Discrete MultiTone)am Beispiel von ADSL (G.dmt)

Standardisiertes ADSL-Verfahren:ANSI: T1.413 (POTS-ADSL), ETSI: DTS/TM-06006 (ISDN-ADSL),ITU: G.dmt (G.992.1), G.lite (G.992.2)Downstreamrate: max. 8,192 Mbit/sUpstreamrate (Duplex): max. 640 kbit/sRateneinstellung in 32-kBit-SchrittenAnpassung an Kanalcharakteristika

ñ sehr flexibel

© W. Henkel, 2003

DMT (Discrete MultiTone)am Beispiel von ADSL (G.dmt)Das Prinzip

© W. Henkel, 2003

DMT (Discrete MultiTone)am Beispiel von ADSL (G.dmt)Die DMT-Parameter von ADSL

N = 512Nakt < N/2 = 256Trägerabstand: 4,3125 kHzPOTS - Belegung:

Downstream: 6 - 255Upstream: 6 - 31

ISDN - Belegung: Downstream: 32(33) - 255Upstream: 32(33) - 63

Nutzband: 21,5625/138 kHz –1,104 MHz

0 1 2 3 4-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

f in MHz

LDS

in d

B(m

W/H

z)

Downstreamrate: max. 8,192 Mbit/s

Upstreamrate (Duplex): max. 640 kbit/s

© W. Henkel, 2003

ADSL auf der Basis von DMTDMT-ADSL Start-upAufgaben der Initialisierung:

� Bitsynchronisation mittels Pilotträgern

� Rahmen- und Bitsynchronisation mittels Sync-Rahmen

� Grobe Leistungseinstellung und AGC zur optimalen Nutzung des Aussteuerbereichs der A/D-Wandler

� Adaption des Zeitbereichsentzerrers mit SNR-Bestimmung für die einzelnen Träger

� Bitbelegung und Verteilung der Sendeleistung auf die einzelnen Träger, Anpassung der Gesamtausgangsleistung an den Sollwert

� Mitteilen der Bitbelegung an den Sender

� Einstellung des Entzerrers im DFT-Bereich, entspricht einer AGC für jeden Träger

© W. Henkel, 2003

ADSL auf der Basis von DMTDMT-ADSL Start-up

Die Schritte der ADSL-DMT-Initialisierung:

1) Aktivierung und Quittierung (activation and acknowledgement),2) Adaption der Übertragungseinrichtungen (transceiver training),3) Kanalanalyse (channel analysis),4) Austausch von Parametern (exchange).

1.) Aktivierung und Quittierung

Dauer: mindestens 3 x 128 Symbole

= 89 ms

© W. Henkel, 2003

ADSL auf der Basis von DMTDMT-ADSL Start-upDie Schritte der ADSL-DMT-Initialisierung:

2.) Adaption der Übertragungseinrichtungen

Dauer: mindestens 7808 Symbole

= 1,81 s

© W. Henkel, 2003

ADSL auf der Basis von DMTDMT-ADSL Start-upDie Schritte der ADSL-DMT-Initialisierung:

2.) Adaption der Übertragungseinrichtungen

Dauer: mindestens 19022 Symbole

= 4,69 s

© W. Henkel, 2003

80-6000

10

26

2

80-4000

10421

262

2

≤ 4000

10

10

1021

2

64-4000

1042

12

227-4000

105102

≤ 4000

10

Übergabeder Bit- undLeistungs-verteilung

1204.) Austausch vonParametern

Dauer: mindestens 769 Symbole

= 0,19 s

© W. Henkel, 2003

Transporting IP over DSLby T. Nordström

NT LT Line

Ethernet10base-T

ATM 25ATM / STM1

DS

LAM

© W. Henkel, 2003

Outline

Historic PerspectiveCurrent SolutionsFuture OutlookConclusions

© W. Henkel, 2003

Historic Perspective

In the beginning there was...- T1/E1 bit stream- HDSL was later designed to be a more efficient

replacement for T1/E1

Beginning of ’90s the ILEC thought ATM would solve all their problems

- Therefore the new thing called ADSL should be based upon ATM transport

© W. Henkel, 2003

Service Taxonomy

ATU-RSubscriber

End-to-End ATMSVCs

PVCs

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End-to-End ATM Solution

VTOA

AAL5

ATM

xDSL

ATM

xDSL

ATM25

AAL2

Remote DSL

ATM

DSLAM / CO DSL

AAL2

AAL5

© W. Henkel, 2003

ATM vs Customers

However, customers use IP/EthernetThus, there was a need to transport IP on ADSL

© W. Henkel, 2003

DSL Architecture

DSLAM

ATU-R

ATU-C

ATM AccessNetwork

Service Aggregator

User PC(s)

ISP

NSP NAP Service Users

Internet

Video Servers

CPE

© W. Henkel, 2003

Service Taxonomy

ATU-RSubscriber

Bridging

Sub. 1/2 Bridging

1/2 Bridging

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Bridging

Remote DSL/ATU-RService AggregatorDSLAM/ATU-C

IP

AAL5

ATM

xDSL

IP

AAL5

ATM

xDSL

IP

Ethernet

AAL5

ATM

DS3

1483/B1483/B

EthernetEthernet EthernetEthernet

1483/B

© W. Henkel, 2003

Bridging pros and cons

The DSL modem acts as an Ethernet bridge+ Simple to understand; easy to install; minimal

configuration of the CPE+ Multi protocol support+ Ideal for internet access in a single user environment

- Depends heavily on broadcast to establish connectivity (scales badly)

- Insecure (ARP spoofing, IP hijacking possible)Partly solved with subscriber half bridging & bridge groups- The number of possible bridge groups are limited

© W. Henkel, 2003

Service Taxonomy

ATU-RSubscriber

PPP

Policy Routing

Non-VPN

VPN

Termination

L2TP

ISP Contexts

Portals

MPLS

IPesc

© W. Henkel, 2003

PPP over ATM (PPPoA)

IPIP

AAL5

ATM

xDSL

IP

AAL5

ATM

xDSL

IP

Remote DSL/ATU-R

IP

Service AggregatorDSLAM/ATU-C

PPPPPP Ethernet

Ethernet

VC-mux or LLC/SNAP

1483 1483

PPP

AAL5

ATM

DS3

1483R

Ethernet

IP

L2TP

ISP

© W. Henkel, 2003

Dial-in Experience for DSL

ISP/CLEC: we must have PPP from the user- So that DSL is just an extension of the telephone

modem experience

This lead to many PPP suggestions for the customer premises

- PPP over ATM with an xDSL NIC/ATU-R in the PC- PPP over ATM with an ATM NIC (ATM{25} to ATU-R)- PPP over L2TP over Ethernet to ATU-R- PPP over ATM over BMAP over Ethernet to ATU-R- PPP over Ethernet

© W. Henkel, 2003

PPP over Ethernet (PPPoE)

Remote DSL/ATU-RService AggregatorDSLAM/ATU-C

IP

PPP

PPPoE

AAL5

ATM

xDSL

IP

PPP

AAL5

ATM

xDSL

PPPoE

IP

PPP

AAL5

ATM

DS3

1483/B1483/B

L2TP

EthernetEthernet

PPPoE

Ethernet

PPPoE

Ethernet

Note: modified since handout

© W. Henkel, 2003

PPPoE Advantages (for ISP)

Preserve the existing dialup network experienceReuse of ISP infrastructure and administration (e.g. RADIUS server)Allow per session accounting and authentication based on Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP)Requires no configuration of the xDSL modem at the customer site. Work with all existing xDSL modems (that support Ethernet MAC bridging) The NSP can oversubscribe resources by deploying idle and session timeouts

© W. Henkel, 2003

PPPoE User Advantages?

Preserve the existing dialup network experiencePossibility to provide secure access to a corporate gateway without managing end-to-end permanent virtual circuits (PVCs) and making use of Layer 3 routing and/or Layer 2 Tunneling Protocol (L2TP) tunnelsUsers can use low-cost Ethernet NIC to connect their PCs and even allow multiple PCs to share a single xDSL modem

© W. Henkel, 2003

PPPoE Disadvantages

PPPoE client software must be installed on all hosts (PCs) connected to the Ethernet segment. Because PPPoE implementation uses RFC1483 bridging, it is susceptible to broadcast storms and possible denial-of-service attacksConnections are just PPP sessions - not possible to hold on to an IP number (good or bad?)Problems with TCPs MTU discovery due to reduced payload size

© W. Henkel, 2003

Service Taxonomy

ATU-RSubscriber

End-to-End ATM

Bridging

PPP

Routing

SVCs

PVCs

Sub. 1/2 Bridging

1/2 BridgingPolicy Routing

Non-VPN

VPN

Termination

L2TP

ISP Contexts

Portals

MPLS

IPesc

© W. Henkel, 2003

Routing

Remote DSLService AggregatorDSLAM/ATU-C

IP

Ethernet

IP

1483/R

AAL5

ATM

xDSL

IP

AAL5

ATM

xDSL

IPIP

Ethernet

AAL5

ATM

1483/R Ethernet

1483

DS3

© W. Henkel, 2003

Future Outlook

Users:- Still only wants IP/Ethernet Ethernet @ home

- Wants Peer2Peer; Home servers Always on

- Needs security, firewalling Security

- Use VoIP/Multimedia/Video (if price is right) QoS, Multicast

ISP/CLEC:- Wants to manage resources

Bandwidth Charge per usageIP numbers Reuse IPno./NAT

or IPv6?Money (costs): HW costs, Admin costs Easy

installation

© W. Henkel, 2003

Packet over DSL

Very similar to the routing service

Note that we could also bridge ethernet in PPP

IP

Ethernet

IP

xDSL

IP

xDSL

IP

Ethernet

IP

Ethernet PPP PPP

Remote DSLService AggregatorDSLAM/ATU-C

© W. Henkel, 2003

Packet over DSL

Useful extensions:Traffic shaping (QoS)Header compressionFirewalling, Service restrictions, Virtual LANs

Issues to solve:Customer IP number handling

- Static- DHCP- IPv6

© W. Henkel, 2003

Conclusion

PPPoE has some benefits (esp. for ISPs) and will be very commonBridging is relatively common today but will fade away unless the new security and scaling aspects is solvedIn the future we need a native packet solution for xDSL

© W. Henkel, 2003

DSLAM

ModemArbeitsstation

ADSL

ModemArbeitsstation

ADSL

STM-1

ppp session RADIUS Proxy

CP Firewall

K RouterSTM-1

DSLAM

PVC

ISP MMDP

MMDPKunden

Datenbank

RadiusProxy( 13x )

AAAserver

RADIUS

(ISP data network 1)

ZSS

T-Net ATM

ModemArbeitsstation

ADSL

ZSS

ZSS

ZSS

ZSS

NT

NT

NTLTs/Mux

LTs/Mux

STM-1

T InterconnectVIP-Konzeptder Telekom

BBNAS

RADIUSServer

MMDPBBA POP

RADIUS-PlattformISP T-Online

RadiusProxyClient

HTMLserver

AAA -CLIENT

MMDPData

Center

IP - Wählanschluß

© W. Henkel, 2003

Sinnvolles RatenverhältnisDS/UP

Übliches Datenformat zunächst: IP over ATM

Frage: Welches ist das sinnvolle Ratenverhältnis ?Antwort: Abhängig von der Anwendung, jedoch existieren

Randbedingungen: übliche Fragmentierung von IP - Paketen: 576 Byteminimales Acknowledge bei TCP: 40 Byte

Es folgt als extremstes Ratenverhältnis bei reinem Abwärtsverkehr:576 / 40 = 14,4

Sinnvolle Wahl bei überwiegend abwärtsgerichtetem Datentransfer:10:1

© W. Henkel, 2003

Kleiner Exkurs: HIPERLAN/2

Frame type 2 bits

SN10 bits

CL 12 bits

CRC3 bytes

Data48 bytes

LCH 54 bytes

SCH SCH LCH LCHBCH15 bytes

FCH ACH9 bytes

Broadcast phase Uplink phase RCH phaseDownlink phaseBurstBurst

Frame type 4 bits

Info52 bits

CRC2 bytes

SCH 9 bytes

MAC frame MAC frame2 ms

MAC frame

Figure 12. MAC frame structure in HIPERLAN/2