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

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

ADSL Network Architecture

• ATU-R - ADSL Transceiver Unit at remote end.

• DSLAM - Digital Subscriber Line Access Multiplexer

ATU-R DSLAM ATM SwitchNetwork

Termination

Broadband network

ADSL

ATM

AAL5

PPP

IP

Upper layerProtocols

ATM

AAL5

PPP

IP

Upper layer Protocols

ADSL Protocol Architecture

DSL client Telco COsession

management

Internet

or outside n/w

ADSL Implementations

• RFC 1483

• PPP Over ATM Over ADSL

• PPP Over Ethernet

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.

LLC/SNAP Encapsulation

A Brief overview

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

SNAP Header

• 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.

OUI PID

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

Payload format for Bridged PDUs

• 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.

LLC 0xAA-AA-03

OUI 0x00-80-C2

PID

PAD

MAC destination address

Remainder of MAC frame

LAN FCS

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.

PPP over ATM over ADSL

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.

PPP - An Introduction

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.

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

PPP Link Operation.

•

Dead Establishment Authentication

Termination Network

failfail

closing

LCP PAP/CHAP

NCPLCP

UP

down

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.

Link Establishment Process

• 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 Peer

Configure request

Configure Ack/Nak

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.

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.

PPP Over Ethernet

A Brief Introduction.

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.

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.

Ethernet Payload for PPPoE

• 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.

VER TYPE CODE SESSION_ID

LENGTH payload

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.