Forschungszentrum Telekommunikation Wien ADSL im Detail Entnommen aus Seminar xdsl-übertragungstechnik Hollabrunn 2002 Werner Henkel ftw. (jetzt IUB)
Einleitung A D S L Asymmetrical Digital Subscriber Line Hochratenkanal Niederratenkanal
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/s Upstreamrate (Duplex): max. 640 kbit/s Rateneinstellung in 32-kBit-Schritten Anpassung an Kanalcharakteristika sehr flexibel
DMT (Discrete MultiTone) am Beispiel von ADSL (G.dmt) Das Prinzip
DMT (Discrete MultiTone) am Beispiel von ADSL (G.dmt) Die DMT-Parameter von ADSL LDS in db(mw/hz) -30-40 -50-60 -70-80 -90-100 -110-120 0 1 2 3 4 f in MHz Downstreamrate: max. 8,192 Mbit/s Upstreamrate (Duplex): max. 640 kbit/s N = 512 N akt < N/2 = 256 Trägerabstand: 4,3125 khz POTS - Belegung: Downstream: 6-255 Upstream: 6-31 ISDN - Belegung: Downstream: 32(33) - 255 Upstream: 32(33) - 63 Nutzband: 21,5625/138 khz 1,104 MHz
ADSL auf der Basis von DMT DMT-ADSL Start-up Aufgaben 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
ADSL auf der Basis von DMT DMT-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
ADSL auf der Basis von DMT DMT-ADSL Start-up Die Schritte der ADSL-DMT-Initialisierung: 2.) Adaption der Übertragungseinrichtungen Dauer: mindestens 7808 Symbole = 1,81 s
ADSL auf der Basis von DMT DMT-ADSL Start-up Die Schritte der ADSL-DMT-Initialisierung: 2.) Adaption der Übertragungseinrichtungen Dauer: mindestens 19022 Symbole = 4,69 s
10 4.) Austausch von Parametern 80-6000 10 120 2 6 2 80-4000 10 2 1 2 64-4000 10 4 2 1 2 Dauer: mindestens 769 Symbole = 0,19 s 2 62 10 4 2 1 2 4000 Übergabe der Bit- und Leistungsverteilung 227-4000 10 510 2 10 4000 10
Transporting over DSL by T. Nordström Ethernet 10base-T ATM 25 NT Line LT DSLAM ATM / STM1
Outline Historic Perspective Current Solutions Future Outlook Conclusions
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
Service Taxonomy PVCs End-to-End ATM SVCs ATU-R Subscriber
End-to-End ATM Solution DSLAM / CO DSL Remote DSL AAL5 AAL2 AAL5 AAL2 ATM25 VTOA ATM ATM xdsl ATM xdsl
ATM vs Customers However, customers use /Ethernet Thus, there was a need to transport on ADSL
DSL Architecture Video Servers User PC(s) ATM Access Network DSLAM ISP Internet Service Aggregator ATU-C ATU-R CPE NSP NAP Service Users
Service Taxonomy Sub. 1/2 Bridging Bridging 1/2 Bridging ATU-R Subscriber
Bridging Service Aggregator DSLAM/ATU-C Remote DSL/ATU-R Ethernet Ethernet Ethernet Ethernet Ethernet 1483/B 1483/B 1483/B AAL5 AAL5 AAL5 ATM ATM ATM DS3 xdsl xdsl
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, hijacking possible) Partly solved with subscriber half bridging & bridge groups - The number of possible bridge groups are limited
Service Taxonomy Termination ATU-R Subscriber Non-VPN Policy Routing ISP Contexts PPP L2TP VPN Portals MPLS esc
PPP over ATM (PPPoA) Service Aggregator DSLAM/ATU-C Remote DSL/ATU-R Etherne t PPP L2TP PPP PPP 1483 1483 Etherne t Etherne t 1483R AAL5 AAL5 ATM xdsl AAL5 ATM xdsl ISP ATM DS3 VC-mux or LLC/SNAP
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
PPP over Ethernet (PPPoE) Service Aggregator DSLAM/ATU-C Remote DSL/ATU-R PPP PPP PPP L2TP PPPoE PPPoE PPPoE PPPoE AAL5 Ethernet Ethernet Ethernet Ethernet ATM 1483/B 1483/B DS3 AAL5 AAL5 ATM ATM xdsl xdsl Note: modified since handout
PPPoE Advantages (for ISP) Preserve the existing dialup network experience Reuse 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
PPPoE User Advantages? Preserve the existing dialup network experience Possibility to provide secure access to a corporate gateway without managing end-toend permanent virtual circuits (PVCs) and making use of Layer 3 routing and/or Layer 2 Tunneling Protocol (L2TP) tunnels Users can use low-cost Ethernet NIC to connect their PCs and even allow multiple PCs to share a single xdsl modem
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 attacks Connections are just PPP sessions - not possible to hold on to an number (good or bad?) Problems with TCPs MTU discovery due to reduced payload size
Service Taxonomy PVCs End-to-End ATM SVCs ATU-R Subscriber Bridging PPP Routing Sub. 1/2 Bridging 1/2 Bridging Non-VPN VPN Termination Policy Routing ISP Contexts L2TP Portals MPLS esc
Routing Service Aggregator DSLAM/ATU-C Remote DSL Ethernet 1483/R 1483/R Ethernet Ethernet 1483 AAL5 ATM DS3 AAL5 ATM xdsl AAL5 ATM xdsl
Future Outlook Users: - Still only wants /Ethernet Ethernet @ home - Wants Peer2Peer; Home servers Always on - Needs security, firewalling Security - Use Vo/Multimedia/Video (if price is right) QoS, Multicast ISP/CLEC: - Wants to manage resources Bandwidth Charge per usage numbers Reuse no./nat or v6? Money (costs): HW costs, Admin costs Easy installation
Packet over DSL Very similar to the routing service Service Aggregator DSLAM/ATU-C Remote DSL Ethernet PPP PPP Ethernet Ethernet xdsl xdsl Note that we could also bridge ethernet in PPP
Packet over DSL Useful extensions: Traffic shaping (QoS) Header compression Firewalling, Service restrictions, Virtual LANs Issues to solve: Customer number handling - Static - DHCP - v6
Conclusion PPPoE has some benefits (esp. for ISPs) and will be very common Bridging is relatively common today but will fade away unless the new security and scaling aspects is solved In the future we need a native packet solution for xdsl
- Wählanschluß Arbeitsstation NT Modem DSLAM ADSL ADSL LTs/ Mux ZSS V-Konzept der Telekom Radius Proxy ( 13x ) AAA server RADIUS-Plattform ISP T-Online RADIUS Server T Interconnect ISP MMDP Arbeitsstation NT Modem STM-1 STM-1 ZSS ZSS T-Net ATM ZSS PVC STM-1 BB NAS Radius Proxy Client AAA - CLIENT MMDP BBA POP MMDP Kunden Datenbank K Router CP Firewall Arbeitsstation NT Modem ADSL DSLAM LTs/ Mux ZSS HTML server MMDP Data Center (ISP data network 1) ppp session RADIUS Proxy RADIUS
Sinnvolles Ratenverhältnis DS/UP Übliches Datenformat zunächst: over ATM Frage: Welches ist das sinnvolle Ratenverhältnis? Antwort: Abhängig von der Anwendung, jedoch existieren Randbedingungen: übliche Fragmentierung von - Paketen: 576 Byte minimales 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
Kleiner Exkurs: HERLAN/2 MAC frame MAC frame 2 ms MAC frame Broadcast phase Downlink phase Uplink phase RCH phase Burst Burst BCH 15 bytes FCH ACH 9 bytes SCH SCH LCH LCH Frame type 4 bits Info 52 bits CRC 2 bytes Frame type 2 bits SN 10 bits CL 12 bits Data 48 bytes CRC 3 bytes SCH 9 bytes LCH 54 bytes Figure 12. MAC frame structure in HERLAN/2