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ADSL Systems - An Overview

ADSL Systems - An Overview. Praveen Reguraman InterOperability Lab University of New Hampshire 1999. Contents. ADSL - An Overview ADSL Network Architecture ADSL Protocol Architecture Typical ADSL Implementations RFC1483 PPP Over ATM Over ADSL PPP over Ethernet. ADSL. An Overview.

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ADSL Systems - An Overview

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  1. ADSL Systems - An Overview Praveen Reguraman InterOperability Lab University of New Hampshire 1999

  2. Contents • ADSL - An Overview • ADSL Network Architecture • ADSL Protocol Architecture • Typical ADSL Implementations • RFC1483 • PPP Over ATM Over ADSL • PPP over Ethernet.

  3. ADSL An Overview

  4. ADSL • ADSL - Asymmetric Digital Subscriber Line • Copper based High speed network access technology • rapidly growing broadband access solution for home networking and small business systems • uses multi-carrier modulation over unused frequency bands in phone lines • supports data rates up to 6144 Kbps downstream and 640 Kbps upstream

  5. ADSL Network Architecture • ATU-R - ADSL Transceiver Unit at remote end. • DSLAM - Digital Subscriber Line Access Multiplexer ATU-R DSLAM ATM Switch Network Termination Broadband network

  6. ADSL Protocol Architecture Upper layer Protocols Upper layer Protocols IP IP PPP PPP AAL5 AAL5 ATM ATM ADSL Internet or outside n/w session management TelcoCO DSL client

  7. ADSL Implementations • RFC 1483 • PPP Over ATM Over ADSL • PPP Over Ethernet

  8. RFC 1483 • RFC 1483 - Multi-Protocol Over AAL5 • creation and use of PVCs for data transfer. • uses LLC/SNAP Encapsulation for routed/bridged PDUs. • Supports IEEE 802.3/802.2, Ethernet II Bridged PDUs and PPP protocol units for ADSL.

  9. LLC/SNAP Encapsulation A Brief overview

  10. Payload format for routed ISO PDUS • The routed/bridged PDUs are encapsulated in the IEEE 802.2 LLC header. • The value 0xFE-FE-03 indicates a routed ISO PDU. • Routed ISO protocol is identified by one octet NLPID field which is a part of protocol data • IP, though has a defined NLPID, should be encapsulated using SNAP. • LLC Header is followed by a SNAP header for all non-ISO protocols and for bridged PDUs. LLC 0xFE-FE-03 ISO PDUs

  11. SNAP Header OUI PID • The Value 0xAA-AA-03 in LLC header indicates the presence of a SNAP header. • OUI - Organizationally Unique Identifier ( 3 octets) identifies an organization which administers the protocol encapsulated. • PID - Protocol Identifier ( 2 octets) identifies the protocol encapsulated.

  12. Payload format for Routed Non-ISO PDUs • The OUI Value 0x00-00-00 indicates that the following PID is an EtherType. • The EtherType value is 0x08-00 for IP. LLC 0xAA-AA-03 OUI EtherType (2 octets) Non-ISO PDU

  13. Payload format for Bridged PDUs LLC 0xAA-AA-03 • The type of the bridged media is specified by the PID field( 2 octets). • Padding is added to align the user information filed of the bridged PDU to a 4 octet boundary. OUI 0x00-80-C2 PID PAD MAC destination address Remainder of MAC frame LANFCS

  14. VC Multiplexing • VC Multiplexing is an alternative approach to LLC/SNAP encapsulation. • VC Multiplexing is used when a large number of VCs could be dynamically created and used. • VC Multiplexing assigns a particular VC for a particular protocol and all PDUs of that protocol use the assigned VC. • VC multiplexing is preferred in a SVC environment and LLC/SNAP is used for PVCs.

  15. PPP over ATM over ADSL

  16. PPP Over AAL5 • Uses LLC/SNAP encapsulation for PPP PDUs. • The NLPID value is 0xCF for PPP. • The PPP PDU contains the PPP -Protocol Identifier followed by information and the padding.

  17. PPP - An Introduction

  18. Protocol summary • PPP -designed for simple links to transport packets between two peers. • PPP encapsulation provides for multiplexing of different network-layer protocols simultaneously over the same link. • PPP provides a Link Control Protocol (LCP) which negotiates the establishment and termination of a PPP link. • LCP also negotiates the options for encapsulation format,authentication and link quality monitoring. • The network layer protocols that are supported by the PPP link are managed by a family of network control protocols defined.

  19. PPP Encapsulation • The Protocol field identifies the datagram encapsulated in the information field. • Information filed contains the datagram and could be zero or more octets up to a Maximum Receive Unit (MRU). • The default MRU value is 1500 octets ,though it could vary by negotiation. • The Information field may be padded up to the MRU number of octets. Protocol (8/16 bits) Information Padding

  20. PPP Link Operation. LCP PAP/CHAP UP Dead Establishment Authentication fail fail LCP closing NCP Termination Network down

  21. Link Establishment • Link establishment phase uses the Link control protocol. • Link Configuration Options - the Maximum Receive Unit size. - Authentication and protocol to be used for authentication - Protocol Field Compression. - Link quality monitoring - Magic number option for detecting looped back links. - Address and Control field compression.

  22. Link Establishment Process Configure request Peer • The configure-request message is sent to request a link establishment and it contains the various options requested. • This request is responded with a Configure-Ack if the negotiation is accepted. • A Configure-Nak is sent if the negotiation is not acceptable and it suggests an acceptable negotiation. Peer Configure Ack/Nak

  23. Authentication • Authentication Option uses Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP). • The protocol used depends on negotiation. • CHAP uses a one-way hashing algorithm which is known only to the user, to respond to a challenge sent by the authenticator. • CHAP is more secure than PAP.

  24. Magic Number • This option detects looped back links. • The magic number is a unique number chosen in the most random way possible. • This number enabled in a configure request is compared with the last request sent and if equal indicate a looped back link. • Link quality monitoring also uses this option to verify proper link communication.

  25. PPP Over Ethernet A Brief Introduction.

  26. PPPoE - How it works • PPPoE - enables establishing PPP sessions and encapsulating PPP packets over Ethernet. • It has two distinct stages - A discovery stage and a Session stage. • The discovery stage identifies the Ethernet MAC address of the peer with which the PPP session is to be established. • The session stage gets a PPPoE session ID for the PPP session and encapsulates PPP packets.

  27. Discovery stage • The discovery stage works similar to the ARP (Address resolution) request-response mechanism. • The host which is required to establish a PPP session broadcasts a session Initiation packet. • The Initiation packet is responded with offer packets from the Access Concentrators accepting the request. • The host follows with a unicast session request packet to a particular Access concentrator. • The Access concentrator replies with a confirmation and a session ID is obtained and the session established.

  28. EthernetPayload for PPPoE VER TYPE CODE SESSION_ID • The EtherType is set to 0x8863 for discovery stage and 0x8864 for PPP session stage. • PPPoE provides the ability to connect a network of hosts over a bridging access device to a Remote access concentrator. LENGTH payload

  29. Conclusion • ADSL is rapidly gaining widespread attention due to its low cost broadband access solution . • Various methods are developed to enable the mass deployment of ADSL systems . • With QoS metrics gaining more attention day by day, the impact of QoS implementations on the cost effectiveness of ADSL solutions is an interesting thought. • How the existing solutions adapt themselves to QoS metrics or how they should be modified will lead to interesting research.

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