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Voice and Multimedia Over ATMLoop Emulation Service Using AAL2
Solution Focus for Loop Emulation
Packet voice over broadband access networksDriven by progress in Digital Subscriber Line technologyMultiple phone channels plus high speed data on a single copper pair or “loop”Phone channels provide access to Public Switched Telephone Network (PSTN) or PBXVoice over DSL offers greatly improvedaccess network economics for voice
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
Introduction to Voice over DSLDSL = Digital Subscriber LineAsymmetric DSL (ADSL) supports ATM plus one baseband phone channel or ISDN BRI on a copper pairOther varieties of DSL support ATM but not baseband phone channelAdditional “derived” phone channels use packet voice over ATMThis is the focus of the Loop Emulation Service Using AAL2
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Telephones
Local PSTN and DSL Access Today
LocalLocalExchangeExchange
SwitchSwitchDigitalLoop
Carrier
Trunks
DSLAccess
MuxATM RouterDSL
ModemTo ISPs
X XADSL
PassiveSplitters
Analog LoopLoop
T1/E1AnalogLoop
BasebandPhoneConnection
VoDSL for Incumbents
TelephonesLocal
ExchangeSwitchDigital
LoopCarrier
Trunks
DSLAccess
MuxATM RouterIAD
To ISPs
X XADSLADSL
Analog LoopLoop
T1/E1/E1AnalogLoop
PSTNGateway
T1/E1T1/E1
VoDSLPath
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VoDSL for Challengers
TelephonesTelephonesLocalLocal
ExchangeExchangeSwitchDigital
LoopCarrier
Trunks
DSLAccess
MuxATM RouterIAD
To ISPs
X XADSL
Analog Loop
T1/E1
AnalogAnalogLoop
PSTNGateway
T1/E1
VoDSLPath
LocalExchange
Switch Trunks
VoDSL Value Proposition
Incumbent Local Service Providers– Deliver additional lines in areas suffering from
copper pair shortages– Defend market position against competitors, for
example those using Cable TelephonyChallengers
– Improve business model for using unbundledloops to deliver local telephony
– Leverage broadband access infrastructure for voice as well as data
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Packet Voice Technology Choices
Most DSL systems are based on ATMPacket voice access could be VoIP / ATM or Voice over ATMVoice over ATM is preferred method
– Better bandwidth efficiency– QoS can guarantee low delay and low loss– Inherently more secure, particularly with
PVCs
Driven by service and network requirementsDriven by service and network requirements
Voice over ATM Choices
Dynamic bandwidth allocation– Voice and data sharing DSL connection
Mix of encoding schemes on one VCC– Compressed speech, uncompressed modem/fax– Change of encoding on-the-fly for auto fax detect
Silence suppression– For additional bandwidth savings
Driven by service and network requirementsDriven by service and network requirements
AAL2AAL2
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
Loop Emulation Service Using AAL2
Reference ModelReference ModelATM
Network
CO-IWF
CP-IWF
CP-IWF
ServiceNode
Customer Customer Premises Premises
Interworking Interworking FunctionFunction
CentralCentralOffice Office
Interworking Interworking FunctionFunctionLoop Loop
EmulationEmulationServiceService
ServiceNode
Interface
UserSide
Interface
UserUserSideSide
InterfaceInterface
CustomerTelephonyEquipment
CustomerTelephonyEquipment
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Elements of Reference ModelCP-IWF function resides in Integrated Access Device (IAD)User Side Interfaces to phone terminals
– Analog POTS (Plain Old Telephone Service)– ISDN BRI (Basic Rate Interface)
CO-IWF function resides in PSTN Gateway or in Local Exchange switchService Node Interfaces to LE switch
– GR-303, TR-008 (North American standards)– V5.1, V5.2 (ITU-T standards)
AAL2 Trunking and Loop Emulation
AAL2 TrunkingSymmetric, peer-to-peer interworking functionsCAS and CCS signaling passed transparentlyIWFs are located in network operations or switching centers
Loop EmulationHighly asymmetric interworking functionsSignaling for POTS is terminated inCP-IWFOnly the CO-IWF may be located in network operations or switching center
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Loop Emulation Specific Requirements
Highly asymmetric interworking functionsSignaling for POTS is terminated inCP-IWFCP-IWF is located on a customer premises
Need to distinguish between CP-IWF and CO-IWF functionalityPOTS signaling protocols must be specifiedNeed embedded management channel between CO-IWF and CP-IWF
Highlights of LES / AAL2 SpecBased on ITU-T I.366.2 “SSCS for NarrowbandServices over AAL2”Supports derived POTS and ISDN servicesSpecifies details of signaling
– Both CAS and CCS are covered
Specifies management of AAL2 channels– Allocation of channels to voice, signaling, management– Channel activation / de-activation procedures
Specifies in-band management for CP-IWF
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
Status of ITU-T AAL2 Specifications
AAL2 consists of a Common Part Sublayer(CPS) and a Service Specific Convergence Sublayer (SSCS)The AAL2 CPS specification, ITU-T I.363.2 was approved in September 1997The AAL2 SSCS consists of two elements:
– Segmentation and Reassembly (I.366.1) approved in June 1998
– Trunking (I.366.2) approved in Feb 1999
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AAL2 Protocol OverviewUser 1 User 3User 2 User 1 User 2
AAL2 Header(3 Octets)ATM Cell Header
(5 Octets)
ATM Cell (53 Octets)AAL2Payload
User Layer
ATM AdaptationLayer (AAL2)
ATM Layer
STF(1 Octet)
AAL2 CPS Packet Format
CID:LI:
UUI:HEC:
CPS INFO:
CID LI UUI HEC CPS INFO
CPSCPSPacket HeaderPacket Header
(3 Octets)(3 Octets)CPS Packet PayloadCPS Packet Payload
(Variable Length)(Variable Length)
CPS PacketCPS Packet
Channel Identifier (8 bits)Length Indicator (6 bits)User-to-User Indication (5 bits)Header Error Control (5 bits)Information (1-45/64 Octets)
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UUI, LI packet contentpacket content
UUI, LI packet contentpacket content CRCCRC--1010MessageMessageTypeType
Packet Format Type 1 - unprotected
UUI, LI
Packet Format Type 2 - partially protected
Packet Format Type 3 - fully protected
I.366.2 Packet Formats
packet contentpacket contentCRCCRCHeader InformationHeader Information
Used for voice andvoiceband data payload in LES
Not used in LES
Used for CAS, DTMF transport etc. in LES
ProfileA set of entries, usually encodersAn entry is indicated with UUI and LengthUUI is used as sequence numberOnce a profile is adopted, the transmitter can change to any encoder in the profile during a call
UUI Codepoint Range
Packet Length (octets) Description of Algorithm Packet Time
(ms)
0-15
0-15
0-15
0-15
40
20
10
2
PCM, G.711-64, generic
CS-ACELP, G.729-8
CS-ACELP, G.729-8
G.729 SID
5
20
10
10
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Profiles for LESLES can use any of the ITU-T I.366.2 pre-defined profilesMandatory support for ITU-T profile #1
– PCM 64 kbps with 40-octet packetsAdditional profiles defined in the LES spec for optimum performance in LES application
– PCM 64 kbps, ADPCM 32 kbps, silence removal– 44 octet packets for maximum bandwidth
efficiency
Profile Entry Selection in LESAAL2 permits the transmitter to determine which profile entry (voice encoding) to useIt is not always appropriate to let theCP-IWF determine its own profile entry
– Choice of entry impacts bandwidth usage– Service provider may want to be in control
LES defines two modes of CP-IWF operation– Independent Mode: CP-IWF determines profile
entry to use on its own– Master/slave Mode: CP-IWF sets its profile entry to
match that chosen by CO-IWF
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“CPS Lite” Option in LESUser 1User 1 User 2User 2
AAL2 HeaderAAL2 Header(3 Octets)(3 Octets)
ATM Cell HeaderATM Cell Header(5 Octets)(5 Octets)
ATM Cell (53 Octets)ATM Cell (53 Octets)AAL2AAL2PayloadPayload
User LayerUser Layer
ATM AdaptationATM AdaptationLayer (AAL2)Layer (AAL2)
ATM LayerATM Layer
STFSTF(1 Octet)(1 Octet)
“CPS Lite” Option in LESEvery ATM cell contains a single AAL2 packetAAL2 packet always follows after Start FieldIf payload is less than 44 octets, remainder of cell is padded with zerosEquivalent to setting the “CU_timer” defined in I.363.2 to a value of zeroSimplifies implementationOptimizes performance for small number of voice channels on a VCC
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
Signaling for Analog POTS: CAS
CAS = Channel-Associated SignalingUsed by North American standard PSTN interfaces, e.g. GR-303Line states represented by 4-bit codeword
– Meanings defined by GR-303– Upstream: on-hook = 0101, off-hook = 1111– Downstream: ringing = 0000, idle = 0101
Signaling for each voice channel carried in CAS packets on same CID as voice
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Signaling for Analog POTS: CCS
CCS = Common Channel SignalingUsed by standard PSTN interfaces outside North America, e.g. V5.1, V5.2Line states and line control conveyed via messages
– E.g. [steady signal: off-hook]Messages for all voice channels are carried over a single common channel (CID = 8)Requires link layer offering reliable transport
Protocol Stack for CCS
PSTN
LAPV5-DL
SSTED
SSSAR
AAL2
ATM
PhysicalPhysical
Analogto
PSTNConversion
PSTN
LAPV5-DL
SSTED
SSSAR
AAL2
ATM
Physical
ETSI V5.1 EN 300 324-1
ITU-TI.366.1
CPCP--IWFIWF COCO--IWFIWF
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Signaling for ISDN BRIBRI = Basic Rate InterfaceComprises 2 x B-channels, 1 x D-channel
– B-channel is 64 kbps bearer– D-channel carries signaling messages
LES provides transparent message relay of all D-channel messages
– Signaling and layer managementD-channel signaling is not terminated byCP-IWF, but passed on to terminal equipment
Protocol Stack for D-channel Relay
PhysicalATMAAL2
SSSARSSTED
PhysicalHDLC-F
Mapping
Physical PhysicalHDLC-F
AAL2SSSARSSTED
PhysicalATM
Mapping
Physical
Layer 2Signaling
Layer 3Signaling
Layer 2Signaling
Layer 3Signaling
ISDNTerminal
ISDNService Node
CP-IWF
CO-IWFATM
User SideUser SideInterfaceInterface
Service NodeService NodeInterfaceInterface
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
AAL2 Channel Management
Specifies procedures for allocating voice and signaling channels to AAL2 CIDsSpecifies procedures for activating and de-activating AAL2 channelsTwo options specified for channel activation:
– Implicit– Explicit
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Implicit Channel Activation
Physical ports of CP-IWF statically mapped to AAL2 CIDs
– Analog POTS: CID range 16 – 127– ISDN B-channels: CID range 160 – 223– ISDN D-channels: CID range 128 – 159
CO-IWF activates a CID by commencing AAL2 transmissions on that CIDCP-IWF considers a CID active if it is receiving AAL2 packets on that CID
Explicit Channel Activation
Requires support of “Emulated Loop Control Protocol” (ELCP)Based on messages sent over common signaling channel (CID = 8)Messages explicitly assign CP-IWF ports to AAL2 CIDs, and activate CIDsMessages explicitly de-assign and de-activate AAL2 CIDsMessages also used for port control
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Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
Remote Management Operations
ATM OAM CellsMandatory support of F5 OAM loopback cell
– Enables end-to-end VCC continuity checking– Can be used for round-trip delay measurement
LES Embedded Operations ChannelOptional remote management channel
– Can support remote configuration, inventory retrieval, alarms and performance reporting
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LES Embedded Operations Channel
Similar concept to Integrated Local Management Interface (ILMI)SNMP messages sent end-to-end over AAL2 CID = 9SNMP messages segmented using SSTED / SSSAR layers of I.366.1CP-IWF implements SNMP agent, CO-IWF implements management applicationMIB definitions outside scope of LES spec
Topics
Introduction to Voice over DSLOverview of Loop EmulationUsage of ITU-T AAL2 RecommendationsTelephony Signaling in Loop EmulationAAL2 Channel ManagementRemote Management OperationsLES Futures
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Possible LES Future WorkLoop Emulation Service enhancements
– Improved specification of SVC usage– Support for multiple physical connections, for
improved capacity and fault toleranceEvolution to “Next Gen Network”
– Support of Megaco / H.248– Establishment of direct packet voice paths
between IADs– Support for direct connections between IADs and
trunking gateways
SummaryLoop Emulation Service has been driven by specific needs of VoDSLCan be applied to other broadband access technologies that support ATMFully utilizes flexibility of AAL2 and itsSSCS layers to support voiceband bearers, signaling and managementOffers range of options to support ISDN and region-specific POTS variations
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