Transcript ADSL Systems - An Overview

ADSL Systems - An Overview
Praveen Reguraman
InterOperability Lab
University of New Hampshire
1999
Contents
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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
DSLAM
ATM Switch
Network
Termination
Broadband
network
• ATU-R - ADSL Transceiver Unit at remote end.
• DSLAM - Digital Subscriber Line Access Multiplexer
ADSL Protocol Architecture
Upper layer
Protocols
Upper layer
Protocols
IP
IP
PPP
PPP
AAL5
AAL5
ATM
ATM
ADSL
DSL client
Telco CO
session
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
LLC
0xFE-FE-03
ISO PDUs
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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.
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.
Payload format for Routed Non-ISO PDUs
LLC 0xAA-AA-03
OUI
EtherType (2 octets)
Non-ISO PDU
• The OUI Value 0x00-00-00 indicates that the following PID is an
EtherType.
• The EtherType value is 0x08-00 for IP.
Payload format for Bridged PDUs
LLC 0xAA-AA-03
OUI 0x00-80-C2
PID
PAD
MAC destination address
Remainder of MAC frame
LAN FCS
• 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.
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
Protocol
Information
(8/16 bits)
Padding
• 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.
PPP Link Operation.
LCP
UP
Dead
PAP/CHAP
Establishment
fail
LCP
down
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Termination
Authentication
fail
closing
Network
NCP
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
Configure request
Peer
Configure Ack/Nak
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.
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
VER
TYPE
LENGTH
CODE
SESSION_ID
payload
• 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.
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.