DSL Services

Gregory F. WetzelChief Network ArchitectFebruary 11, 2004

Covad Confidential

Agenda

• DSL Physical Layers• CPE• ADSL, SDSL, IDSL, T1• DSLAM• ATM Switch

• DSL Logical Layers• ATM connections• IP

• Services Layered on top of DSL• Markets: Consumer, SOHO, Small Business• Subscriber Management• Internet Access• Voice / Video / other?

DSL Physical Layer

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Section Overview

• Terminology• Network Elements

• CPE• DSLAM• ATM switch• BRAS

• ADSL Service Architectures• In a CLEC environment

• Wholesale• Retail

• Service Matrix

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Terminology

• ADSL – Asymmetric DSL• BRAS – Broadband Remote Access Server• CLEC – Competitive Local Exchange Carrier, used in

this presentation to generically reference a Local Exchange Carrier other than the Incumbent. For example, Covad, MCI, AT&T, etc.

• DSL – Digital Subscriber Line• IDSL – ISDN-based DSL• Line – loop plus ILEC-provided dial tone (think

telephone line)

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Terminology

• Loop – copper pair connecting end user premise to central office facility

• Rate/Reach – DSL analog signals are limited by electrical properties of the loop. Rate/Reach is the tradeoff between getting a certain data rate at a certain distance between the transmitter and receiver.

• SDSL – Symmetric DSL (aka, Single Loop DSL)• SMB – Small/Medium size Business• SOHO – Small Office/Home Office

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General DSL Network Architecture

HUB CO

HUB CO

HUB CO

ISP

ISP...

ATMSwitch

Router

CO

CO

DSLAM

CO

DSLAM

ATMSwitch

DSLAM

Router

ISPATM

Switch Router

Internet

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DSL Technology

• Different Markets have Different Requirements• Small-Medium Businesses

• Symmetric bandwidth requirements• VoDSL• Web hosting or Databases on site

• Service Level Agreements• Many PCs

• Consumer• Asymmetric bandwidth requirements• One to a few PCs• Price is a decider

• Wholesale versus Retail• Managing aggregation by wholesale customer• Many wholesale customers want direct PVC to end

user

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DSL Technology

• Symmetric DSL (SDSL)• Today SDSL/2B1Q is common, but not standardized• ITU-T G.shdsl soon• Addresses SMB market

• ISDN DSL (IDSL)• Used today for customers beyond the reach of other

DSL technologies• Distance too great• Behind Digital Loop Carrier

• Asymmetric DSL (ADSL)• Standardized• Focused on consumer offerings

• T1 Access to DSLAM• Standarized• Focused on business offerings

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Customer Premises Equipment (CPE)

• This equipment comes in a variety of models:• Ethernet Bridge, common for consumer• Ethernet Router, common for SOHO and SMB• Can have a variety of DSL interfaces

• ADSL, SDSL, IDSL, T1• Different models of CPE support different DSL architectures

• Use of PPP over Ethernet or over ATM• Static or Dynamic IP address assignments

• Functions• Terminates a copper loop• Terminates the DSL line coding (xTU-R)• Extracts or Encodes the Layer 2 signal

• typically ATM, but can be Ethernet or Frame Relay

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DSL Access Multiplexer

• The DSLAM aggregates multiple DSL lines• Functions

• Terminates copper loops• Terminates the DSL line coding (xTU-C)

• Provides additional control on DSL signals than the CPE• Extracts or Encodes the Layer 2 signal

• typically ATM, but can be Ethernet or Frame Relay• Aggregates Layer 2 input/output from multiple

subscribers onto a “backhaul” or “trunk”• Typically ATM PVCs are the element of aggregation

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ATM Switch

• Grooms ATM PVCs between• The DSLAM on which the subscriber is attached• The handoff port to

• ISP customer or• CLEC’s own IP infrastructure

• ATM Switches are arranged in a network• Regional and/or National• ATM connections from a subscriber can be handed off

in the same or a different region• Regulatory difference between CLEC and ILEC

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IP Termination of DSL Services

RedbackSMS-1800

(#2)

ATMSwitch

BRAS IPRouter

Ether Switch

RADIUSProxy

DNS

5

IPNetwork

CLEC Collocation

BRAS

DS-3/OC-3

IPNetwork

IPRouter

DS-3/OC-3

DS-3/OC-3

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Broadband Access Server

• The BRAS terminates Layer 2• Typically ATM PVCs• Also PPP over Ethernet or PPP over ATM

• Subscriber Management• Uses RADIUS to

• Authenticate userid/password• Configure subscriber parameters

• shaping profile• filtering profile (e.g., restrict email servers or proxy web)

• Assigns IP addresses and DNS resolver addresses

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The Flavors of DSL

• There are many standard and proprietary versions of Digital Subscriber Line technology

• ADSL is the most common and is used mostly with consumers

• Asymmetric bandwidth allocation in the up- and down-stream directions. More in the down-stream.

• Achieved by allocation the spectrum asymmetrically.• Current std has max 6Mbps down and 768Kbps up

• IDSL is used mainly by CLECs• Regulations do not require ILECs to share Remote

Terminals, therefore reach is an issue with CLECs• IDSL bonds an ISDN BRI into a 128 or 144 kbps non-

switched channel

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Flavors of DSL

• SDSL or SHDSL is used mainly by SMB• Symmetric service needed by businesses that, e.g.,

host web sites• Current SDSL and SHDSL supports up to 2Mbps

• In addition, T1s can be attached to DSLAMs, functionally making them a digital access line solution

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CLEC Environment

• What do the ILECs sell?• Unbundled Network Element – Loop (UNE-L)• High Frequency Portion of the Loop (HFPL)• Unbundled Network Element – Platform (UNE-P)

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UNE-L

• Primarily a copper product• CLEC leases the loop and may use the loop to provide voice,

data, or both.• Covad uses UNE-L to provide SDSL and IDSL services, for

example

LoopVoiceData

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HFPL

• Primarily a data product• CLEC leases high frequency portion of the loop to

provide data service. (It only applies to ADSL) • ILEC provides voice service to the end user

Line

Data

Class 5Voice

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UNE-P

• Primarily a voice product• CLEC leases the entire loop plus local circuit switching (class

5 port)• The CLEC may choose to put data services on the loop or

designate an authorized agent (e.g. Covad) to provide the data

LineVoiceData

Class 5

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Service Elements

• How are the elements used to deliver ADSL services?

• Line Sharing (HFPL)• Line Splitting (UNE-P)• Loop Splitting (UNE-L)

• Perspective shown in the following slides is primarily wholesale.

• Covad is used as the provider name to distinguish access provider from CLEC wholesale customers.

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Line Sharing

MDF

DSLAM

ILEC Class 5 SwitchVoice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

DataCustomer

Covad Space Covad Networ

k

Covad Networ

k

InternetInternet

CLEC

ILEC is Customer of Record

Customer gets:• ILEC voice• CLEC data (Covad

provided)

• ILEC provides local voice service to End User• Covad gets access to High Frequency Portion of the Loop (HFPL)

using Covad company codes (Covad billed for HFPL)• CLEC is ISP to End User

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Line Splitting (UNE-P)

MDF

DSLAM

ILEC Class 5 SwitchVoice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

DataCustomer

Covad Space Covad Networ

k

Covad Networ

k

InternetInternet

CLEC

CLEC is Customer of Record

Customer gets:• CLEC voice• CLEC data (Covad provided)

• CLEC provides local voice service to End User using UNE-P• Covad gets access to High Frequency Portion of Loop (HFPL) using

CLEC company codes (CLEC billed for Linesplit loop charges)• CLEC is ISP to End User

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Loop Splitting (UNE-L)

MDF

DSLAM

CLEC Class 5 SwitchVoice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

DataCustomer

Covad Space Covad Networ

k

Covad Networ

k

InternetInternet

CLEC

CLEC is Customer of Record

Customer gets:• CLEC voice • CLEC data (Covad provided)

• CLEC provides local voice service to End User using CLEC owned voice switch

• Covad gets access to High Frequency Portion of Loop (HFPL) using CLEC company codes (CLEC billed for loop charges)

• CLEC is ISP to End User

DSL Logical Layers

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What are the Layers?

• Layers• DSL physical layer• ATM layer• PPP layer (not always present)• IP layer

• ATM Layer rides on DSL Layer and consists of• Typically a single PVC with UBR or VBR-nrt QoS using

AAL5• Possible to have multiple PVCs

• Supporting different QoS per PVC• Architectures using multiple PVCs become complex due

to IP addressing and routing issues• Support ATM OAM flows for troubleshooting

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PPP Layer

• PPP over Ethernet for Multi-protocol Encapsulation• Used for consumers• Terminates on PC (or home gateway)• Provides a single login per subscriber PC• CPE can be a bridge (until recently less expensive than a

router)• Bridge has very little configuration, good for self-install

• PPP over ATM• Used for businesses, terminates on CPE• CPE typically a router

• Interworks with RADIUS on the BRAS• Assigns dynamic or “framed-route” IP address/subnet• Pushes DNS resolver addresses

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IP Layer

• IP Layer spans from CPE to IP POP• Premises LAN through CPE• CPE encapsulates IP in PPP• CPE encapsulates PPP in AAL5/ATM• Transmission across ATM network• BRAS undoes ATM and PPP encapsulations

• BRAS participates in SP’s IP routed network• In retail or Layer 3 wholesale model BRAS is owned by

access provider• In Layer 2 wholesale model BRAS is owned by

wholesale customer

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IP Service Model

ISP...

ATMSwitch

Router

CO

CO

DSLAM

CO

DSLAM

ATMSwitch

DSLAM

ATMSwitch

Internet

ISP...

ATMSwitch

Router

CO

CO

DSLAM

CO

DSLAM

ATMSwitch

DSLAM

ATMSwitch

InternetISP

Router

ISP

Router

DNS Auth

ISP

Router

ISP

Router

Email WebHost

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IP POP Detail

RedbackSMS-1800

(#2)

ATMswitch

BRASOC-3/DS-3F E/GE

F E/GE

GatewayRouter

FE

FE

Cisco 19xx (Hub2)

RADIUSProxy

DNSResolver

5

Backbone

BRASEth

ern

et

Sw

itchOC-3/DS-3

OC-3/DS-3

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Summary

• We’ve discussed overall DSL network architectures• Impacts from market segment• Impacts from government regulations

• Technical stacks/layering• DSL• ATM• PPP• IP

• Implementation of architecture

Questions?