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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)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
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
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
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