digital subscriber line technologies (xdsl)

© 2005 Petr Grygarek, Advanced Computer Networks Technologies 1

Digital Subscriber LineDigital Subscriber LineTechnologies (xDSL) Technologies (xDSL)

and their usage for data transferand their usage for data transfer

Petr GrygPetr Grygáárekrek

2© 2005 Petr Grygarek, Advanced Computer Networks Technologies

Basic Characteristics of DSLBasic Characteristics of DSL• Uses existing (symmetric) telco last-mile copper Uses existing (symmetric) telco last-mile copper

pairspairs• Utilizes frequency band up to Utilizes frequency band up to 1MHz1MHz (or more) (or more)

• may span hundreds of meters or a few kilometersmay span hundreds of meters or a few kilometers• Allows for high-rate transfersAllows for high-rate transfers

• up to tens ofup to tens of Mb Mbpsps

• Widespread usage allowed by development of Widespread usage allowed by development of new modulation techniques and digital signal new modulation techniques and digital signal processing techniquesprocessing techniques • mass production of cheap DSPsmass production of cheap DSPs


DSL Technology ClassificationDSL Technology Classification1.1. Baseband transmissionBaseband transmission

• HDSL, SDSLHDSL, SDSL2.2. Broadband transmissionBroadband transmission: :

• ADSL, VDSLADSL, VDSL• allows for simultaneous operation withallows for simultaneous operation with POTS POTS oror ISDN ISDN

4.4. Symmetric - the same Symmetric - the same upstream / downstreamupstream / downstream transfer ratetransfer rate

• suitable for interconnection of central offices, attachment suitable for interconnection of central offices, attachment of service providers, …of service providers, …

5.5. Asymmetric - differentAsymmetric - different upstream / downstream upstream / downstream transfer ratetransfer rate

• Suitable for traffic with asymmetric nature like Suitable for traffic with asymmetric nature like video on video on demanddemand (MPEG2) or (MPEG2) or WWW WWW clients clients


Digital Subscriber LineDigital Subscriber Line

• DSL modem at customer premiseDSL modem at customer premise• Subscriber lines terminated at central office at Subscriber lines terminated at central office at

DSL Access Multiplexer (DSLAM)DSL Access Multiplexer (DSLAM)• DSLAMDSLAM acts as data streams concentrator acts as data streams concentrator• extracts data stream from individual subscriber lines extracts data stream from individual subscriber lines

and forwards it to ISP data networkand forwards it to ISP data network• using ATM or Ethernet backboneusing ATM or Ethernet backbone


Asymmetric Digital Subscriber LineAsymmetric Digital Subscriber Line(ADSL)(ADSL)


ADSL – Basic CharacteristicsADSL – Basic Characteristics• Downstream channel bitrate differs from upstream channel Downstream channel bitrate differs from upstream channel

(asymmetric bitrates)(asymmetric bitrates)• Uses single pair for both directionUses single pair for both direction• Separated uSeparated upstreampstream/downstream frequency bands and voice band/downstream frequency bands and voice band

• boundary most commonly at 138 kHzboundary most commonly at 138 kHz• upstreamupstream/downstream bands may even overlap to obtain additional bandwidth /downstream bands may even overlap to obtain additional bandwidth

for downstream channelfor downstream channel• uses echo-cancellation method to mix both directions on single pairuses echo-cancellation method to mix both directions on single pair

• Compatible with POTS telephones (FDM)Compatible with POTS telephones (FDM)• or with ISDN BRIor with ISDN BRI• Voice/ISDN band separated by splitterVoice/ISDN band separated by splitter

• Downstream channel up to 8 Mbps, upstream channel up to 1 MbpsDownstream channel up to 8 Mbps, upstream channel up to 1 Mbps• Theoretical maximum Theoretical maximum of of 15.3 Mbps downstream and 1.8 Mbps upstream15.3 Mbps downstream and 1.8 Mbps upstream

• all subchannels utilized at maximum capacityall subchannels utilized at maximum capacity• upstream and downstream channels overlap upstream and downstream channels overlap • no frequency band reserved for analog telephone/ISDNno frequency band reserved for analog telephone/ISDN


Discrete Tone Modulation (DMT)Discrete Tone Modulation (DMT)Frequency band divided into subchannelsFrequency band divided into subchannels• 256 subchannels of 4 kHz bandwidth256 subchannels of 4 kHz bandwidth

• one channel reserved for pilot frequencyone channel reserved for pilot frequency• different different pilot pilot channelchannelss to indicate coexistence with to indicate coexistence with POTS / ISDNPOTS / ISDN

• each subchannel uses independent QAM modulation with variable each subchannel uses independent QAM modulation with variable number of states adaptively adjustable according to conditions at number of states adaptively adjustable according to conditions at subchannel’s frequency bandsubchannel’s frequency band• attenuation, noiseattenuation, noise• link conditions measured upon initialization and sometimes even during link conditions measured upon initialization and sometimes even during

data transferdata transfer


ADSL Subscriber Line in PracticeADSL Subscriber Line in Practice


ADSL FiltersADSL Filters• Splitter – passive filter, separates frequency bandsSplitter – passive filter, separates frequency bands

• high-pass + low-pass filter high-pass + low-pass filter • (3.4 kHz POTS (Annex A), 80 kHz ISDN (Annex B))(3.4 kHz POTS (Annex A), 80 kHz ISDN (Annex B))

• no need for filter poweringno need for filter powering• MicrofiltersMicrofilters

• cheaper optioncheaper option• installed just before telephone setsinstalled just before telephone sets

• chooses 4kHz voice frequency bandchooses 4kHz voice frequency band• sometimes implemented in telephone outletssometimes implemented in telephone outlets

• Useful for customers with multiple telephone sets connected in parallelUseful for customers with multiple telephone sets connected in parallel• ADSL modems connected directly to lineADSL modems connected directly to line

• ADSL LiteADSL Lite• Omits filters completelyOmits filters completely• Signal reflections due to impedance mismatchesSignal reflections due to impedance mismatches• Increased error rate during pulse dialing and ringingIncreased error rate during pulse dialing and ringing

Filters may be external or integrated with ADSL modemsFilters may be external or integrated with ADSL modems


ADSL ModemsADSL Modems

• ATU-C: ADSL Termination Unit-Central OfficeATU-C: ADSL Termination Unit-Central Office• many ATU-Cs integrated inside DSLAMmany ATU-Cs integrated inside DSLAM

• ATU-R: ADSL Termination Unit-RemoteATU-R: ADSL Termination Unit-Remote• at customer sideat customer side

May be configured for synchronous data transfer May be configured for synchronous data transfer or transfer of ATM cellsor transfer of ATM cells• both modes may operate simultaneouslyboth modes may operate simultaneously• in ATM mode, only one (P)VC is commonly usedin ATM mode, only one (P)VC is commonly used


Error Error DDetection and etection and EError rror CCorrection orrection in ADSLin ADSL

Because poorBecause poor--quality physical lines are expected, quality physical lines are expected, measurements must be taken to avoid high bit error measurements must be taken to avoid high bit error ratesrates

• CRC CRC providesprovides error detection error detection• Forward error correction (FEC) using Reed-Solomon Forward error correction (FEC) using Reed-Solomon

codecode• Interleaving Interleaving

• allows error burst caused by impulse noise to be corrected by allows error burst caused by impulse noise to be corrected by FEC by spreading neighboring data bits into multiple frames FEC by spreading neighboring data bits into multiple frames with their own FEC checksumswith their own FEC checksums• Disadvantage: increases latencyDisadvantage: increases latency


ADSL frameADSL frame• Complex structureComplex structure• SynchronousSynchronous bitstream bitstream• Carries user data, service Carries user data, service EOCEOC channel channel and CRCand CRC

• EOC = EOC = Embedded Operation ChannelEmbedded Operation Channel• EOC channel allows to exchange service messages between ATU-R and EOC channel allows to exchange service messages between ATU-R and

ATU-C and also remote management of customer’s ATU-RATU-C and also remote management of customer’s ATU-R• multiple frames form (logical) multiframemultiple frames form (logical) multiframe

• 68 frames + one synchronization frame, 17ms in total68 frames + one synchronization frame, 17ms in total• Each frame divided into two partsEach frame divided into two parts: : interleaved an non-interleaved an non-

interleaved (fast) channelinterleaved (fast) channel• allows better immunity against impulse noise or lower latencyallows better immunity against impulse noise or lower latency• no fixed boundary, allocated to meet particular application needsno fixed boundary, allocated to meet particular application needs• Interleaving depth – controls level of data spreadinInterleaving depth – controls level of data spreadingg

• Bits of interleaved channel mapped to carriers (subchannels) Bits of interleaved channel mapped to carriers (subchannels) with more QAM states (and thus more susceptiblewith more QAM states (and thus more susceptible to noise to noise))• better chance to correct errors caused by impulse noise with FECbetter chance to correct errors caused by impulse noise with FEC


ADSL Modem OperationADSL Modem Operation• Most of functions implemented digitally (DSP)Most of functions implemented digitally (DSP)

• implement of QAM 256 times is ineffective in analog circuitryimplement of QAM 256 times is ineffective in analog circuitry• signal created in frequency domain for individual channels, summed-up, signal created in frequency domain for individual channels, summed-up,

converted to time domain using inverse FFT and passed to D/A converterconverted to time domain using inverse FFT and passed to D/A converter• Data assembled into ADSL frameData assembled into ADSL frame

• ScramblingScrambling of user data of user data• Forward Error Correction Forward Error Correction checksum generated using checksum generated using Reed-Solomon codeReed-Solomon code

• Data interleavData interleaved across multiple framesed across multiple frames• Frame divided into subblocks and mapped Frame divided into subblocks and mapped toto individual subchannels individual subchannels• Each subchannel carries 2 to 15 bits per symbolEach subchannel carries 2 to 15 bits per symbol

• number of bits per symbol at each subchannel calculated with regard to noise number of bits per symbol at each subchannel calculated with regard to noise and attenuation to reach 10and attenuation to reach 10-7-7 bit error rate and 6dB noise margin bit error rate and 6dB noise margin• 15 bits per symbol (15 bits per symbol (32768 QAM states32768 QAM states)) givesgives 60kbps for 4 kBd 60kbps for 4 kBd

• Trellis code modulation improves QAM state detectionTrellis code modulation improves QAM state detection• Special signal (Cyclic Prefix) between symbols added to limit intersymbol and Special signal (Cyclic Prefix) between symbols added to limit intersymbol and

inter-subchannel interference and provide synchronizationinter-subchannel interference and provide synchronization• results toresults to pseudoperiodic signal pseudoperiodic signal


Options of Frequency Band UsageOptions of Frequency Band Usage• Lowest part of frequency band reserved for PSTN/ISDN BRILowest part of frequency band reserved for PSTN/ISDN BRI• Full ADSL (ITU/T G.922.1=g.dmt)Full ADSL (ITU/T G.922.1=g.dmt)

• Uses frequency band up to 1104 kHzUses frequency band up to 1104 kHz• ADSL Lite (ITU/T G.922.2=g.lite)ADSL Lite (ITU/T G.922.2=g.lite)

• Uses only half of Full-ADSL frequency band (up to 552 kHz)Uses only half of Full-ADSL frequency band (up to 552 kHz)• provides lower downstream bitratesprovides lower downstream bitrates

• Support for POTS coexistence (G.922.2 Annex A)Support for POTS coexistence (G.922.2 Annex A)• Support for overlapped upstream/downstream channels (G.922.2 Annex B)Support for overlapped upstream/downstream channels (G.922.2 Annex B)

• First upstream channel at First upstream channel at • 25.8 kHz if used with POTS (G.911.1 Annex A)25.8 kHz if used with POTS (G.911.1 Annex A)• 138 kHz if used with ISDN BRI (G.911.1 Annex B)138 kHz if used with ISDN BRI (G.911.1 Annex B)

• lowerlower bound of downstream channel also shifted up in this bound of downstream channel also shifted up in this casecase

• Echo cancellation Echo cancellation • Allows upstream and downstream channels to overlapAllows upstream and downstream channels to overlap

• More downstream subchannels allows for higher downstream data rates More downstream subchannels allows for higher downstream data rates (32kbps steps)(32kbps steps)


ADSL initializationADSL initialization (1) (1)ATU-C to ATU-R handshake (ITU-T G.994.1)ATU-C to ATU-R handshake (ITU-T G.994.1)

• ATU-C sends pilot signal (414 kHz)ATU-C sends pilot signal (414 kHz)• ATU-R detects pilot signal and starts to send pseudorandom ATU-R detects pilot signal and starts to send pseudorandom

signalsignal• ATU-C analyzes pseudorandom signal to set receiver’s gain and ATU-C analyzes pseudorandom signal to set receiver’s gain and

transmit powertransmit power• The same process repeats for ATU-RThe same process repeats for ATU-R

• Channel measurements for adjustment of echo-cancellation Channel measurements for adjustment of echo-cancellation circuitrycircuitry

• Measurements of SNR at individual subchannelsMeasurements of SNR at individual subchannels• number of bits to assign to particular carriers determined herenumber of bits to assign to particular carriers determined here


ADSL initializationADSL initialization (2) (2)• Exchange of configuration parameters between ATU-C and Exchange of configuration parameters between ATU-C and

ATU-RATU-R• Echo cancellation usage indicationEcho cancellation usage indication• Frame structure (size of interleaved/non-interleaved part)Frame structure (size of interleaved/non-interleaved part)• Number of bits assigned to each carrierNumber of bits assigned to each carrier• Relative amplification of individual channelsRelative amplification of individual channels

• (related to testing signal power)(related to testing signal power)• ……

• After successful initialization, ADSL Overhead and Control After successful initialization, ADSL Overhead and Control channel (AOC) established to adapt connection without channel (AOC) established to adapt connection without interruption of user data flowinterruption of user data flow• AOC allows to set number of bits allocated for individual subchannels AOC allows to set number of bits allocated for individual subchannels

and subchannels’ transmit powerand subchannels’ transmit power

Initialization shouldn’t last for more than 11.3 secsInitialization shouldn’t last for more than 11.3 secs


xDSL Implementation ScenariosxDSL Implementation Scenarios• Telco operatorTelco operator

• Uses it’s own cables (subscriber lines)Uses it’s own cables (subscriber lines)• DSLAMs with high port density connected together using ATMDSLAMs with high port density connected together using ATM• Provides selection of network service providerProvides selection of network service provider

• Internet Service ProviderInternet Service Provider• Cabling (and commonly also DSL devices) leased from telco operatorCabling (and commonly also DSL devices) leased from telco operator• DSLAMs with medium densityDSLAMs with medium density

• Often avoids ATM, uses Ethernet and Bridged/Routed architecture insteadOften avoids ATM, uses Ethernet and Bridged/Routed architecture instead• Connected to Internet with IP-based access lineConnected to Internet with IP-based access line

• Corporate solutionCorporate solution• Internal cabling infrastructure (campus, hotel, hospital, factory, …)Internal cabling infrastructure (campus, hotel, hospital, factory, …)• Bridged architectureBridged architecture

• Point-to-PointPoint-to-Point• ATU-C+ATU-R modems, DSLAM completely avoidedATU-C+ATU-R modems, DSLAM completely avoided• Most often used with VDSL (symmetrical)Most often used with VDSL (symmetrical)


ADSL FutureADSL Future• ADSL 2, ADSL 2+ADSL 2, ADSL 2+• Doubles frequency band used (up to 2.2 MHz) Doubles frequency band used (up to 2.2 MHz)

• doubles theoretical maximum transfer ratedoubles theoretical maximum transfer rate• Faster link initialization procedureFaster link initialization procedure• ““Standby” modeStandby” mode

• limits physical layer communication during inactivity limits physical layer communication during inactivity periodsperiods

• Possible to remove ATM layer, carries Ethernet Possible to remove ATM layer, carries Ethernet frames insteadframes instead


Very high speed Digital Subscriber Very high speed Digital Subscriber LineLine

(VDSL)(VDSL)


VDSL Basic CharacteristicsVDSL Basic Characteristics• Defined in ITU-T G.933.1 (=g.vdsl)Defined in ITU-T G.933.1 (=g.vdsl)

• CAP or DMT (4096 x 4 kHz) modulationCAP or DMT (4096 x 4 kHz) modulation• DMT simplifies implementations compatible with ADSLDMT simplifies implementations compatible with ADSL

• Uses frequency band up to 30MHzUses frequency band up to 30MHz• shorter reach (1.6 km max)shorter reach (1.6 km max)

• suitable for LANs and FTTC networks (curb to the user)suitable for LANs and FTTC networks (curb to the user)• upstream and downstream channel may not overlap in frequencyupstream and downstream channel may not overlap in frequency

• unacceptable NEXT at high frequenciesunacceptable NEXT at high frequencies• Can coexist with POTS/ISDN (FDM as with ADSL)Can coexist with POTS/ISDN (FDM as with ADSL)

• Both symmetric and asymmetric mode (adaptive transfer rate)Both symmetric and asymmetric mode (adaptive transfer rate)• 26 Mbps in both directions26 Mbps in both directions• 52 Mbps downstream, 6.4 Mbps upstream52 Mbps downstream, 6.4 Mbps upstream

• More complex dedication of parts of frequency band to More complex dedication of parts of frequency band to upstream/downstream channelupstream/downstream channel• Requirement of similar frequency scheme for customers with various Requirement of similar frequency scheme for customers with various

bitrates (or ADSL) in the same multipair cablebitrates (or ADSL) in the same multipair cable• Muliple downstream and upstream channels may be usedMuliple downstream and upstream channels may be used

• Interlaced and non-interlaced mode, FEC, synchronous data / ATMInterlaced and non-interlaced mode, FEC, synchronous data / ATM


Service Providing using DSL-Based Service Providing using DSL-Based Access NetworkAccess Network


Position of ATM in DSLPosition of ATM in DSL• ADSL originally considered that future CPE ADSL originally considered that future CPE

devices will be based on ATMdevices will be based on ATM• ATM channels extracted from individual DSLs ATM channels extracted from individual DSLs

pass through DSLAM with ATM interface and pass through DSLAM with ATM interface and continue across telco’s ATM backbonecontinue across telco’s ATM backbone• terminated at ATM-enabled routerterminated at ATM-enabled router

• Too expensive both for telcos and customerToo expensive both for telcos and customer• Ethernet prevails today (with potential QoS Ethernet prevails today (with potential QoS

support)support)


Structure of Access Network Structure of Access Network • Customer Premise NetworkCustomer Premise Network

• Internal customer’s LAN/WANInternal customer’s LAN/WAN• Customer Premise Equipment (CPE)Customer Premise Equipment (CPE)

• Delimited by xDSL modem/routerDelimited by xDSL modem/router• Network Access Provider (NAP)Network Access Provider (NAP)

• Delimited by DSLAM from Customer Premise NetworkDelimited by DSLAM from Customer Premise Network• DSLAM’s functions and interfaces not standardized, many different options available DSLAM’s functions and interfaces not standardized, many different options available

on the marketon the market• NAP’s backbone ties together DSLAMs and provides connections to NSPsNAP’s backbone ties together DSLAMs and provides connections to NSPs

• Network Service Provider (NSP)Network Service Provider (NSP)• Connected to multiple NAPsConnected to multiple NAPs• Voice service provider, Internet service provider, Video on Demand, …Voice service provider, Internet service provider, Video on Demand, …• Intranets of huge companies establishes peering with Network Access Providers Intranets of huge companies establishes peering with Network Access Providers

to allow direct access using xDSLto allow direct access using xDSL

Roles of access provider and service provider formally separated to allow Roles of access provider and service provider formally separated to allow for competition, may be bound together in some usage scenariosfor competition, may be bound together in some usage scenarios


Network Access ProviderNetwork Access Provider• Assures physical transfer from CPEs to backbone networkAssures physical transfer from CPEs to backbone network

• using it’s own or leased copper pairsusing it’s own or leased copper pairs• Implements backbone to allow customers to access alternative service Implements backbone to allow customers to access alternative service

providersproviders• Backbone ties together individual DSLAMs together and provide virtual Backbone ties together individual DSLAMs together and provide virtual

circuits from customers to service providerscircuits from customers to service providers• QoS-enabledQoS-enabled

• Traditionally ATM Traditionally ATM • Potential to integrate with ATM-based DSL technology and hard-QoSPotential to integrate with ATM-based DSL technology and hard-QoS• Usage limited to PVCs – no signalization support needed in CPE devicesUsage limited to PVCs – no signalization support needed in CPE devices

• Frame Relay is also an optionFrame Relay is also an option• Probably will be replaced with QoS-enabled IP infrastructure soon (cost)Probably will be replaced with QoS-enabled IP infrastructure soon (cost)

• Customer can be allowed to dynamically choose network service provider it Customer can be allowed to dynamically choose network service provider it wants to be connected to at particular time or based on required servicewants to be connected to at particular time or based on required service• Connection to multiple service providers at the same time may be allowed Connection to multiple service providers at the same time may be allowed

for different traffic classesfor different traffic classes• Provider selection using web-based user interface Provider selection using web-based user interface

• (Service Selection Gateway SSG)(Service Selection Gateway SSG)


Architectures of IP-Based ServicesArchitectures of IP-Based Services


General OptionsGeneral Options• Frames/packets from individual customers passed through DSL Frames/packets from individual customers passed through DSL

and Network Access Provider’s backbone using virtual circuits to and Network Access Provider’s backbone using virtual circuits to the device with bridging/routing capabilitythe device with bridging/routing capability

• PPP sessions used to adhere traditional dial-up modelPPP sessions used to adhere traditional dial-up model• allows for authentication, authorization, accounting, detection of active allows for authentication, authorization, accounting, detection of active

users and other functions common in traditional dial-up networksusers and other functions common in traditional dial-up networks• Advantage of PPP flexibility (LCP/NCPs)Advantage of PPP flexibility (LCP/NCPs)• PPP sessions from xDSL CPE router or from individual customer’s PPP sessions from xDSL CPE router or from individual customer’s

devicesdevices• PPP sessions terminated at Network Access Provider device (aggregator) PPP sessions terminated at Network Access Provider device (aggregator)

or tunneled to chosen Network Service Provideror tunneled to chosen Network Service Provider• PPP frame forwarding using protocols like L2TP or PPP session terminated PPP frame forwarding using protocols like L2TP or PPP session terminated

and data forwarded further using traditional IP routing or VLANsand data forwarded further using traditional IP routing or VLANs• PPP session may be forwarded to different NSPs based on user PPP session may be forwarded to different NSPs based on user

(/application) authenticated(/application) authenticated• IP addresses may be given by both NAP or NSPIP addresses may be given by both NAP or NSP

• PPP session may carry traffic other than IP (L2)PPP session may carry traffic other than IP (L2)• May be tunneled to network service provider/corporate intranetMay be tunneled to network service provider/corporate intranet


RFC 1483 Bridging/RoutingRFC 1483 Bridging/Routing


RFC 1483 Bridging/RoutingRFC 1483 Bridging/Routing• Transfer of plain (Ethernet) frames or (IP) packets in Transfer of plain (Ethernet) frames or (IP) packets in

DSL (ATM channels), bridged or routed between DSL (ATM channels), bridged or routed between customers and service provider in broadband access customers and service provider in broadband access serverserver• ATM PVCs from customer DSL modems pass through ATM PVCs from customer DSL modems pass through

DSLAM and terminated at broadband access serverDSLAM and terminated at broadband access server• Bridging/routing functionality may be integrated into Bridging/routing functionality may be integrated into

DSLAM in some low-cost solutionsDSLAM in some low-cost solutions

• RFC1483 standardizes how to encapsulate L2 frames or RFC1483 standardizes how to encapsulate L2 frames or L3 packets on ATM AAL5 PVCL3 packets on ATM AAL5 PVC


RFC 1483 Bridging Pros and ConsCheap solution for corporate DSL implementation or tiny ISPs

• Advantages:Advantages:• SimplicitySimplicity

• ATU-R behaves as pure Ethernet bridge• No CPE configuration (authentication, IP routing)No CPE configuration (authentication, IP routing)

• DSLAM connected to packet network using Ethernet interface• No ATM neededNo ATM needed

• Single DHCP server may support all customers on the “segment”Single DHCP server may support all customers on the “segment”• Multiprotocol supportMultiprotocol support

• Disadvantages:Disadvantages:• Poor scalability (all customers in the same broadcast domain)Poor scalability (all customers in the same broadcast domain)• Security issuesSecurity issues• Problematic accounting (checking for session duration)Problematic accounting (checking for session duration)


VC-MUX and LLC encapsulationVC-MUX and LLC encapsulation

• L3 protocols differentiated with protocol type L3 protocols differentiated with protocol type tag of LLC-SNAP encapsulationtag of LLC-SNAP encapsulation• special value reserved to indicate L2 frame presencespecial value reserved to indicate L2 frame presence

• Separate virtual circuit for every L3 protocol Separate virtual circuit for every L3 protocol when VC-MUX encapsulation is usedwhen VC-MUX encapsulation is used


RFC 1483 Bridging Protocol StackRFC 1483 Bridging Protocol Stack

Picture from: Janovic, R: Picture from: Janovic, R: Koncepcie a architektúry širokopásmového DSL prístupu. Conference presentation, Data-Voice-Video 2001.


RFC 1483 Routing Protocol StackRFC 1483 Routing Protocol Stack

Picture from: Janovic, R: Koncepcie a architektúry širokopásmového DSL prístupu. Conference presentation, Data-Voice-Video 2001.


PPP over ATM AAL5 (PPPoA)PPP over ATM AAL5 (PPPoA)


PPP over ATM AAL5 (PPPoA)PPP over ATM AAL5 (PPPoA)• PPP session between CPE (ATU-R) and aggregation routerPPP session between CPE (ATU-R) and aggregation router

• Aggregation router at NAP or NSPAggregation router at NAP or NSP• routes between virtual interfaces connected with individual customers and routes between virtual interfaces connected with individual customers and

backbone interfacebackbone interface• CPE and aggregating device may use both static and dynamic routingCPE and aggregating device may use both static and dynamic routing

• Relies on RFC 1483 (LLC-SNAP or VC-MUX protocol multiplexing)Relies on RFC 1483 (LLC-SNAP or VC-MUX protocol multiplexing)• Per-session authenticationPer-session authentication

• allows the access provider/service provider to assign only one IP address allows the access provider/service provider to assign only one IP address to CPEto CPE• CPE may implement NATCPE may implement NAT

• Dynamic assignment using PPP (IPCP)Dynamic assignment using PPP (IPCP)• High number of PPP sessions may be terminated at aggregating High number of PPP sessions may be terminated at aggregating

router – good scalabilityrouter – good scalability• Hundreds, thousands to tens of thousandsHundreds, thousands to tens of thousands• Uses AAA server to authenticate users (e.g. RADIUS)Uses AAA server to authenticate users (e.g. RADIUS)


PPPoA Pros and ConsPPPoA Pros and Cons• Advantages:Advantages:

• Network access provider knows time of session Network access provider knows time of session establishment and terminationestablishment and termination

• Authentication, authorization and accounting is possibleAuthentication, authorization and accounting is possible• Disadvantage:Disadvantage:

• Single session using one VC from CPESingle session using one VC from CPE• no support for service selection without using multiple VCs, which no support for service selection without using multiple VCs, which

increases configuration complexity increases configuration complexity • increases bug probability, more knowledgeable helpdesk neededincreases bug probability, more knowledgeable helpdesk needed

• Username/password must be configured to CPEUsername/password must be configured to CPE• Increases setup complexityIncreases setup complexity• Service provider must maintain user account databaseService provider must maintain user account database

• Problems with NAT traversal (across CPE ADSL router) in Problems with NAT traversal (across CPE ADSL router) in some applicationssome applications


PPP over Ethernet (PPPoE)PPP over Ethernet (PPPoE)


PPPoE Basic ArchitecturePPPoE Basic Architecture• Client-based authentication and access controlClient-based authentication and access control

• not CPE-router based as with PPPoAnot CPE-router based as with PPPoA• PPP sessions between individual clients and aggregation routerPPP sessions between individual clients and aggregation router

• PPPoE software client needed (alternate protocol stack)PPPoE software client needed (alternate protocol stack)• MMaintainaintainss the the traditional per-user dialup model familiar to both traditional per-user dialup model familiar to both

users and service providersusers and service providers• PPPoE provides PPPoE provides method for establishing PPP connections method for establishing PPP connections

through Ethernethrough Ethernett• PPP Frames carried in Ethernet PPP Frames carried in Ethernet frameframes s

• frameframe types 8863 types 8863,,8864 8864 • Runs over RFC 1483 bridgingRuns over RFC 1483 bridging

• PPPoE defined in PPPoE defined in RFC 2516RFC 2516• Discovery and Session stagesDiscovery and Session stages


PPPoE ClientPPPoE Client• Software client at user sideSoftware client at user side

• Built-in in Windows XPBuilt-in in Windows XP• pppoe client in Linuxpppoe client in Linux• ……

• Only inserts additional layer between IP and Only inserts additional layer between IP and EthernetEthernet

• Allows users to handle broadband connection Allows users to handle broadband connection the same way they were used to use dialup the same way they were used to use dialup connectionconnection


PPPoE Discovery StagePPPoE Discovery Stage• HHost ost (PPPoE Client) (PPPoE Client) broadcasts a PADI broadcasts a PADI frame frame to to

discover adiscover any PPPoE servers (access concentrator)ny PPPoE servers (access concentrator)• PADI=PADI=PPPoE Active Discovery InitiationPPPoE Active Discovery Initiation

• PPPoE server PPPoE server replrepliesies with PADO packet, announcing with PADO packet, announcing it’sit’s presence and the services presence and the services itit offer offerss• PADO=PADO=PPPoE Active Discovery OfferPPPoE Active Discovery Offer

• The host picks one of the The host picks one of the PPPoE serversPPPoE servers and transmits and transmits a PADR packet, asking for sessiona PADR packet, asking for session establishment establishment• PADR=PADR=PPPoE Active Discovery RequestPPPoE Active Discovery Request

• PPPoE server PPPoE server replies with a PADS packet. replies with a PADS packet. • PADS=PADS=PPPoE Active Discovery Session-ConfirmationPPPoE Active Discovery Session-Confirmation

The protocol then The protocol then advancesadvances to session stage. to session stage.


PPPoE Session StagePPPoE Session Stage

• host and access concentrator exchange PPP host and access concentrator exchange PPP frames embedded in Ethernet framesframes embedded in Ethernet frames

• Session shutdownSession shutdown• special PADT packet special PADT packet sent by whichever sidesent by whichever side

• PPPoE Active Discovery TerminatePPPoE Active Discovery Terminate

• All features of PPP may be usedAll features of PPP may be used

digital subscriber line technologies (xdsl)

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