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DSL Services

Gregory F. Wetzel

Chief Network Architect

February 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

Covad Confidential

Section Overview


Terminology


Network Elements


CPE


DSLAM


ATM switch


BRAS


ADSL Service Architectures


In a CLEC environment


Wholesale


Retail


Service Matrix

Covad Confidential

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

Covad Confidential

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

Covad Confidential

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

Covad Confidential

IP Termination of DSL Services


Redback

SMS
-
1800


(#2)


ATM

Switch




BRAS


IP

Router

Ether Switch

RADIUS

Proxy

DNS


5

IP

Network

CLEC Collocation

BRAS

DS
-
3/

OC
-
3

IP

Network

IP

Router

DS
-
3/

OC
-
3

DS
-
3/

OC
-
3

Covad Confidential

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)

Covad Confidential

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

Loop

Voice

Data

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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 5

Voice

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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


Line

Voice

Data

Class 5

Covad Confidential

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.

Covad Confidential

Line Sharing

MDF

DSLAM



ILEC Class 5 Switch

Voice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

Data

Customer

Covad Space

Covad
Networ
k

Internet

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

Covad Confidential

Line Splitting (UNE
-
P)

MDF

DSLAM



ILEC Class 5 Switch

Voice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

Data

Customer

Covad Space

Covad
Networ
k

Internet

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

Covad Confidential

Loop Splitting (UNE
-
L)

MDF

DSLAM



CLEC Class 5

Switch

Voice 2 wire connection

Splitter

Voice 2 wire connection

Combined voice and Data

OSP Loop

Data

Customer

Covad Space

Covad
Networ
k

Internet

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

Covad Confidential

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


Covad Confidential

IP Service Model

ISP

...

ATM

Switch

Router

CO

CO

DSLAM

CO

DSLAM

ATM

Switch

DSLAM

ATM

Switch

Internet

ISP

...

ATM

Switch

Router

CO

CO

DSLAM

CO

DSLAM

ATM

Switch

DSLAM

ATM

Switch

Internet

ISP

Router

ISP

Router

DNS

Auth

ISP

Router

ISP

Router

Email

Web

Host

Covad Confidential

IP POP Detail


Redback

SMS
-
1800


(#2)


ATM

switch




BRAS


OC
-
3/DS
-
3

F E/GE

F E/GE

Gateway

Router

FE

FE

Cisco 19xx (Hub2)

RADIUS

Proxy

DNS

Resolver


5

Backbone

BRAS

Ethernet

Switch

OC
-
3/DS
-
3

OC
-
3/DS
-
3

Covad Confidential

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?