DSL Services

Greg WetzelMarch 1, 2005

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Agenda

Today’s talk will not focus on DSL as a technology but will focus on how DSL is used as an element in an end-to-end service

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

DSL Logical Layers ATM connections IP

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Agenda, con’t

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

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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, including ILEC-provided dial

tone, e.g., telephone line

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Terminology, con’t

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.

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Terminology, con’t

SDSL – Symmetric DSL (aka, Single Loop DSL)

SMB – Small/Medium size Business SOHO – Small Office/Home Office

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Spectrum Allocation in ADSL

POTS Voice versus Packet Voice Packet Voice derives voice channels and

carries them in the DSL data stream Packet Voice is NOT carried in the POTS

baseband of ADSL

POTS DSL Band

Derived Voice and

Data

ADSL Frequencies

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Physical Layer Issues

DSL is affected by distance Power output is limited by cross-talk and other

interference issues Physical signaling using Trellis codes and other

techniques yield a high bit/baud ratio Signal attenuation due to line capacitance, etc. Interference due to other DSLs in the same wire

bundle and due to other technologies (e.g., POTS, T1)

DSPs continue to become more powerful allowing a useful signal to be extracted from lower S/N ratios

Power consumption at DSLAM can be an issue, but typically <1 watt per line

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Physical Layer Issues, con’t

DSL is affected by line quality Wire gauge typically AWG 24

The thinner the wire the lower the S/N Rural areas use thicker wire (it improved

POTS distances) Other electronics in path

POTS load coils over 25kft Wire connection defects

Bad splices/connections, corrosion, wet/damp wires

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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 by Market

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

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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

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DSL Technology

Asymmetric DSL (ADSL) Standardized Focused on consumer offerings

T1 Access to DSLAM Standarized Focused on business offerings

Wholesale versus Retail Managing aggregation by wholesale

customer Many wholesale customers want direct

PVC to end user

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Customer Premises Equip (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

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CPE, con’t

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 lines Functions

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

Provides additional control on DSL signals 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 the subscriber is attached to 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

ATMSwitch

BRAS IPRouter

EtherSwitch

RADIUSProxy

DNS

5

IPNetwork

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

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The Flavors of DSL

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

First std has max 6Mbps down and 768Kbps up

More recent stds allow up to 25Mbps down and 1.5Mbps up (e.g., ADSL2+)

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Flavors of DSL

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

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

DSL Logical Layers

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What are the Layers?

Layers DSL physical layer

Already covered ATM layer PPP layer (not always present) IP layer

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ATM Layer

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

nrt QoS using AAL5, VPI=0/VCI=35 determined by industry agreement

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

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

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PPP Termination

Originally PPPoE termination was intended to be on the PC In some cases, this is still true Identified an individual PC Multiple PCs meant multiple PPPoE sessions on

the same ATM PVC and DSL line Home Gateways or DSL Routers have

changed that to some degree PPPoE termination handled by the HG or DSL

Router Hides multiple PC presences behind the

router/gateway

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IP Layer

IP Layer spans from CPE to IP POP PC or 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

DNS Auth

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IP POP Detail

ATMswitch

BRASOC-3/DS-3GE

GE

GatewayRouter

GE

GE

Ether Switch

RADIUSProxy

DNSResolver

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

Questions?

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Resources

Standards The International Telecommunications Union

Telecommunications Sector, www.itu.int/itu-t/publications/recs.html, controls the physical layer standards.

Recommendations G.991 through G.998 apply to DSL The DSL Forum provides most industry

standards for DSL architectures (how DSL is used to provide services)

http://www.dslforum.org/about_dsl.htm?page=aboutdsl/tech_info.html

Backup Slides

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CLEC Environment

What do the ILECs sell to CLEC? Unbundled Network Element – Loop (UNE-L)

High Frequency Portion of the Loop (HFPL) Unbundled Network Element – Platform (UNE-P)

The regulatory environment has changed substantially since 1996, first in favor of competition and the CLECs and most recently in favor of the ILECs

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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. Most CLECs use UNE-L to provide SDSL

and IDSL services, for example

LoopVoiceData

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HFPL

Primarily a data product, no longer required by regulation

CLEC leases high frequency portion of the loop to provide data service. (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, no longer required by regulation

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.

IPSP is used 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 CLEC Networ

k

CLEC Networ

k

InternetInternet

IPSP

ILEC is Customer of Record

Customer gets:•ILEC voice•IPSP data (CLEC provided)

ILEC provides local voice service to End User CLEC gets access to High Frequency Portion of the

Loop (HFPL) using CLEC codes (CLEC billed for HFPL) IPSP is ISP to End User

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Loop Splitting (UNE-L)

MDF

DSLAM

IPSP Class 5 SwitchVoice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

DataCustomer

CLEC Space CLEC Networ

k

CLEC Networ

k

InternetInternet

IPSP

IPSP is Customer of Record

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

IPSP provides local voice service to End User using IPSP owned voice switch

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