Voice and Multimedia Over ATM Loop Emulation Service Using ...

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

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

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

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

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

Topics


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

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

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

Topics


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

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)

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

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

“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

Topics


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

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

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

Topics


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

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

Topics


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

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


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

Voice and Multimedia Over ATM Loop Emulation Service Using ...

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