cblm-atm1etti.poly.ro/cursuri/anul iv/cblm/cblm documentatie 2008... · 2012-04-04 · cblm-atm1-...

CBLM-ATM1- Prof. E.Borcoci- UPB -2006 2

CBLM-ATM1 Eugen Borcoci-UPB

2006


Table of contents 1 ATM TECHNOLOGY 4

1.1 ATM INTRODUCTION 4 1.1.1 Broadband Networks and Services 4 1.1.2 Transfer Mode Evolution 5 1.1.3 ATM Main Characteristics 5

1.1.3.1 ATM general functions 6 1.1.3.2 General ATM protocol architecture 6

1.1.3.2.1 BISDN lower layers architecture 7 1.1.3.2.2 ATM cell format 8 1.1.3.2.3 ATM cell length choice 8

1.1.3.3 ATM Channels, Paths, Connections 9 1.1.3.4 Multiplexing and Switching 11 1.1.3.5 Signalling 13 1.1.3.6 Traffic Control 14

1.1.4 Reference Models and Protocols 15 1.1.4.1 BISDN Protocol Stack. Reference configurations 15

1.1.4.1.1 ATM protocol stack 15 1.1.4.1.2 BISDN Reference Configurations 16 1.1.4.1.3 Lower Layer Functions 18

1.1.4.2 Physical Layer 19 1.1.4.2.1 Physical Medium dependent 19 1.1.4.2.2 Transmission Convergence Sublayer (TC) 20 1.1.4.2.3 Cell Rate Decoupling 20 1.1.4.2.4 HEC generation and verification 21 1.1.4.2.5 Cell delineation 21 1.1.4.2.6 Cell payload scrambling 21 1.1.4.2.7 Transmission frame adaptation (mapping) of ATM cells 21 1.1.4.2.8 ATM inverse multiplexing (AIM) 23

1.1.4.3 ATM Layer 24 1.1.4.3.1 ATM cell format 24 1.1.4.3.2 ATM layer functions 26

1.1.4.4 ATM Forum Services Categories 28 1.1.4.4.1 Real-time Services 28 1.1.4.4.2 Non-real-time Services 28

1.1.4.5 ATM Adaptation Layer (AAL) 29 1.1.4.5.1 AAL services 29 1.1.4.5.2 AAL protocols 30 1.1.4.5.3 AAL1 30 1.1.4.5.4 AAL2 36 1.1.4.5.5 AAL3/4, AAL5 39

1.1.5 References 42


1 ATM TECHNOLOGY

1.1 ATM Introduction

1.1.1 Broadband Networks and Services

Why broadband networks and services?

• conventional current networks: specialised (voice, audio, data video), limited capabilities; non sufficient WAN bandwidth for current applications ; bandwidth waste in circuit-based networks (ex. PSTN, ISDN); non-sufficient QoS guarantees in data networks

• significant growth of global traffic volume

• new and combined services requirements - for audio/voice/text/data/ fixed or moving images /video - ( multiservices)

10 K b/s

100 K b/s

10 M b/s

100 M b/s

1 G b/s

1 M b/s

Fax C A D

V oice

G raph ics

L A N , M A N

Inter L A N / PB X C om m unication s

V ideoconferen ce

V ideo

T V , H D T V

B roadcastV oice

V ideo

D ata

Figure 1-1 Bandwidth requirements for different applications

• services classification (ITU-T)

- interactive : - conversational (real time); - retrieval (Video-on Demand (VOD), WWW); - messages (data, voice, video); - distribution (broadcast)

• broadband services (ITU-T: required bandwidth ≥ 2Mbit/s, 1.5 Mbit/s)

• service integration ⇒ multiservices networks

• information types to be integrated: voice, data, text, fixed images, video, multimedia

• multiservices networks architectures

- ITU-T, ETSI: Recommendations (standards) : ISDN, BISDN, 3GPP

- IETF, ATM Forum, IEEE, etc.

- manufacturers proprietary solutions

• transport infrastructures

-long distance: , WWDM, DWDM, SONET/SDH, TDM, ATM

-short distance: TDM, Gigabit Ethernet, ATM, FDDI, etc.


- WLANs, WMANs

Network intelligence

transport trunk

network

Access

Access

FAX

Figure 1-2 Typical model of a multiservices network

1.1.2 Transfer Mode Evolution

•circuit switching: constant rate, multi-rate, fast circuit switching

• packet switching

- connection oriented (CO): X.25, frame switching, frame relay

- connectionless: datagrams (IP)

• cell switching (ATM)

ATM

Circuit switching(constant rate)

Fast Circuitswitching

Circuit switching(multi-rate)

Frame switching

Frame Relay

Packet switching

Flexibilityincrease

Simplified,Speedincrease

Figure 1-3 Switching techniques evolution to ATM

1.1.3 ATM Main Characteristics

Organisations involved in producing ATM specifications (Standards, Recommendations, RFCs))

• International Telecommunication Union (ITU-T) : Recommendations Series I, Q

• ATM Forum - ATM specifications

• Internet Engineering Task Force - ATM related RFC documents


• Others: Frame Relay Forum, Switched Multimegabit Data Services (SMDS) Interest Group

• Proprietary solutions for implementations

ATM : initially selected as multiplexing and switching technology for BISDN wide area networks

- current use - BISDN but also non-BISDN environment, e.g. - broadband private networks ( ATM Forum is promoting such applications)

- the exact relationship: BISDN creates the possibility of integrating various services on top of the ATM infrastructure

- non ISDN environment ( private, campus, etc.) – ATM technology is used in a protocol stack different from that of BISDN

1.1.3.1 ATM general functions

• Design philososphy ( for high speed):

- minimal transport functions executed in the network nodes

- service specific functions executed en-to-end

• Result: universal dynamic multiplexing and switching technique

based on small packet - fix size (cell = 5 bytes-header + 48 bytes –info)

- asynchronous: no corelation between source and network clock

• Advantages

- virtually accept any type of traffic (data, voice, audio, video, multimedia)- if adding a new type of traffic, one have to adapt only the interfaces; the network itself is the same

- connection oriented (CO), cell relay –sequencing guarantees

- bandwidth efficiency (dynamic multiplexing)

- constant cell length - hardware based switching (very fast - GBit/s)

- covers LAN, MAN and WAN private and public area

- interoperability with fast transmission media (SDH/SONET, etc)

- transparency of ATM w.r.t. user information semantics

• Drawbacks/limits/problems

- high complexity

- cost (higher than for other technologies)

- significant overheads in certain applications ( cell segmentation)

- source clock is lost at receiver ( waiting queues for cells) – no clock transparency ⇒ special protocols for source clock recovery at reception

- no flow control at ATM layer ⇒ congestion possible

- no guarantees on integrity of the user data carried by the cells

- error in header ⇒ cell loss or miss-insertion

- congestion in network nodes ⇒ cell loss

1.1.3.2 General ATM protocol architecture


TE

ATM

PHY

ATM

PHY

NE NETE

AAL

ATM

PHY

Higherlayers

AAL

ATM

PHY

Higherlayers

UNI UNI

TETE

Figure 1-4 General ATM protocol stack

TE - terminal equipment NE - network element (ATM switch)

PHY – Physical Layer ATM – Asynchronous Transfer Mode

AAL - ATM Adaptation Layer - AAL - adaptation layer needed for different services (voice, data, video); different

services ⇒ different AAL types - AAL – end-to-end oriented; AAL information is transparent to the ATM transport

network

1.1.3.2.1 BISDN lower layers architecture • Plane separation( the same principle as in ISDN):

- User plane ( protocols for user information transport) - Control plane (protocols for signalling information transport) - Management plane ( layer management, plane management)

-different services for upper layer ⇒ adaptation layer required

Higher layers

ATM AdaptationLayer (AAL) – different

types, for different services

Control plane

Asynchronous Transfer Mode

Physical Layer

ATM

Layer managementPlane management

User plane

Figure 1-5 ATM protocol stack ( BISDN ITU-T, I.320)


1.1.3.2.2 ATM cell format

Fields of ATM cell at User Network Interface:

GFC - Generic Flow Control ; VPI - Virtual Path Identifier (8 bits)

VCI - Virtual Channel Identifier (16 bits); PT - Payload Type

CLP - Cell Loss Priority ( used in congestion cases) ; HEC - Header Error Check

Fields of ATM cell at Network Network Interface (NNI)

- no GFC field , VPI- 12 bits

7 6 5 4 3 2 1 0GFC VPIVPI VCI

VCIVCI PT CLP

HEC

Payload 48bytes

5bytes

trs

bit

Figure 1-6 ATM cell format at User network Interface (UNI)

- allocation of cells to source traffic:

periodical – circuit emulation; at request – packet emulation

1.1.3.2.3 ATM cell length choice

- hardware switching requirements ( 32 – 64 bytes/cell acceptable value ) - trade-off: efficiency/packetisation delay for continuous stream (e.g.voice)

- cell efficiency : ec = P/(P +H) (6-1)

P= 48, H = 5 ⇒ ec = 0.906 ( good) - efficiency of SDU segmentation to cell - process:

ef = I / [ ⎡I/P⎤ (P+H)] (6-2)

I – service data unit (SDU) length, P – Cell Payload length

H – Cell Header length ⎡x⎤ - least integer ≥ x

- monotonic increase with I length

- maximum value is ef → 0.905 if I is very large (P= 48, H = 5)

transfer delay through the network - components: - packetisation – proportional to the cell length ( if constant rate stream) - depacketisation - transmission and propagation - switching and waiting in the switch queues

ITU-T limits of transfer delay for a voice circuit without echo-canceller: 24 ms - delay for 64 Kbit/s PCM uncompressed voice stream :


pd = 48* 0.125 µs = 6 ms – ok! - Note: compressed voice stream at 8 Kbit/s : pd = 48 ms!!

1.1.3.3 ATM Channels, Paths, Connections

Terminology:

• ATM – connection oriented : setup, data transfer, release – phases

• transfer: user – user, user-network, network – network

• preserve cell sequence integrity

Virtual Channel (VC) – generic term for unidirectional transport of ATM cells associated by a identified by a common unique identifier VCI

Virtual Channel Link – unidirectional transport means between a point where a VCI value is assigned and the point where that value is translated or terminated

Virtual Channel Connection (VCC) – concatenation of VC links:

Virtual Path (VP) - generic term for unidirectional transport of ATM cells belonging to VCs, associated by a identified by a common unique identifier VPI,

- VPI, VCI – label is constant during a connection

Virtual Path Link – a group of VC links, identified by a common value VPI between a point where a VPI value is assigned and the point where that value is translated or terminated

Virtual Path Connection (VPC) – concatenation of VP links that extends between the points where VCI values are assigned and the point where those values are translated or removed, extending the length of a bundle of VC links sharing the same VPI

V P I1

V P I2

P h ys ic a llin k

V C I1

V C I2V C I1

V C I2

V C I3

Figure 1-7Virtual channels and virtual channels concepts

• Why - virtual path concept ? - trend in high speed networking : control cost of the network is increasing - reduces the number of network components to be managed by grouping channels in

bundles and manipulate them as units - advantages:

simplified network architecture

increased network performance and reliability

reduced processing and short connection setup time: by reserving capacity on a VPC in anticipating call arrivals, new VCC can be setup by simply executing control functions at the end of VPCs. No call processing is required at transit nodes.

enhanced network service

• Virtual Path, Virtual Channel Characteristics - quality of service (QoS) control –possible (e.g. in fig.1.1.4-1 assign VPI1 to CBR and VPI2 to VBR connections)

- switched and semi-permanent VCCs


- cell sequence integrity - traffic parameter negotiation and usage monitoring - VCI restrictions within a VPC (reserved identifiers) - VCI, VPI – local significance on each link - point-to-point and multipoint connections

ATM network

a

ATM networkb

A

D

C

B

D

A

B

CEchivalent meshnetwork of VCs

sender

Figure 1-8 a. Multicast ATM connection b. Virtual private ATM network

- multipoint (multicast connection) – one cell sent by the source, multiplied by the ATM switches

- virtual private networks – based on permanent virtual circuits (PVCs) - equivalent mesh network between user sites

• Modification of VPI, VCI values

- VPI – in crossconectors /multiplexers ( VP switches) - VCI – in VC switches (ATM switches); VP values can also be changed in VC switches

VCI2VCI1

VCI2VCI1

VCI3VCI2VCI1

VCI2VCI1

VPI2 VPI4

VPI3VPI1

VCI3

VCI2VCI1

VCI2VCI1

VPI2 VPI4

VPI3VPI1

VCI3

VCI2VCI1

VCI3

VCI2VCI1VCI2VCI1

VCI4VCI3VCI2

VCI1

VCI4VCI3

VPI5

VPI6VPI7

Terminationpoint for

VPC

VC switch

VP switch

a. VP switching(transit nodes)

b. VP, VCswitching

Figure 1-9 VP and VC switching


Layers:(VC) Channel

(VP) Path

(PHY) Physical

NT2

NT1

TM TMTMTM

VPI1

VCI1

VPI2 VPI3 VPI4

NT1

NT2VCI2

VCI3

XC/MXXC / MX XC / MX

SW

VCI1 VCI2 VCI3

VCCa

VPI3VPI1 VPI2 VPI4

VPCa VPCb VPCc

VC links

VC connections

VP links

VP connections

1 link VC = (n≥1) VP connections

SW

Figure 1-10 VP, VC, VPC and VPC relationship

TM – Optical Fiber or other Physical Transmission medium

XC/MX – Crossconector/Multiplexer ( VP switch) NT1, NT2 – Network Terminals SW - VC switch (VC switch)

1.1.3.4 Multiplexing and Switching

ATM switch

Terminal

Multiplexer

Shared accessat UNI

Figure 1-11 Multiplexing and switching in networks

UNI – User Network Interface

• ATM – cell multiplexing and switching functions


•••

••

•••

••

••

••

Ok

On

In

IkInfo1 H1

Info2 H2

Info1 H4Info2 H3

Figure 1-12 Spatial switching, label switching and multiplexing

Ik (VPI1, VCI1) → On( VPI2, VCI2)

•••

•••

OPn

OP1

IPn

IP1

IC

InterconnectionNetwork (IC)

IC

Outputports

Controller

TranslationTable (TT)

OC

OC

Inputports IC, OC - Input, Output

Controllers

IP, OP – Input, OutputPorts

Figure 1-13 ATM switch functions

• translation table (switching table ):

(IPin, VPIin, VCIin) → (IPout, VPIout, VCIout)

- each cell label is translated via TT

- queuing structures on each output – variable transfer delay!

• controller -connection setup and release ( routing, resource allocation, synchronisation, maintenance)

• interconnection network - cell transfer, impact on performances: total maximum rate; mean transfer delay and delay jitter; loss rate for different types of traffic • ATM switch architectures elementar switch:

- based on time division - shared transfer medium, shared memory - based on space division - matrix architecture, knock out

high capacity switches – use elementar switches interconnected

- collision problem: - two or more cells targeting the same output or the same internal switching element – collision resolution algorithm required

- interconnection network taxonomy:

switching elements – space or time division – based

memory - at inputs (input queue switch - IQ)

- at outputs (output queue switch - OQ)


- shared ( shared queue switch - SQ)

- combined : ISQ, SOQ, IOQ, IOSQ

single-path, multi-path between input and output

internal blocking or non-blocking structures

different methods of collision resolution - interconnection network structures - Delta, Clos, Banyan, Batcher-Banyan ( sorting

and forwarding fabric)

1.1.3.5 Signalling

signalling (ITU-T): info exchange for connection control and management

- ATM general principle: out of band signalling – separate protocol stack, within the control plane

- interfaces with different signalling: User Network Interface (UNI) – between user terminal and network point of access

Network Network Interface (NNI) – between network nodes

- SVC – Signalling Virtual Channels – VCC for common channel signalling (Out-of band) - possible in-band signalling ( special cells transmitted in a previous established connection

– VPC, or VCC for requesting/releasing resources)

TransitAccess Access

SPSP SP

STP

SVC SVC

(2)

TETE

ATM connections

SVC(VPI =0,VCI =5)

SVC(VPI =0,VCI =5)

SVCs

(1)

Figure 1-14 ATM signalling architecture

TE – tyerminal equipment SP – signalling point

STP – signaling transfer point SVC – Signalling Virtual Channel

VCI = 5 – reserved value for SVC

(1) – associated signalling (2) – quasi-associated signalling

- each ATM interface has a default SVC ( VPI=0, VCI=5) – opened permanently; each

VPC has a SVC on VCI=5 - other SVC – established at request, by meta-signalling - meta-signalling channel – opened by default (for path VPI=0, VCI=1) - other VPC meta-signalling channels are established when opening VPCs; a terminal

equipment can have more signalling entities


CEQ1

Meta-signalling entity (MSE) Signalling entity (SE)CEQ – Customer Equipment CRF – Connection Related Functions

a

CEQ2

b

VP

VP

LocalCRF

VC

CEQ3

Network

Figure 1-15 ATM meta-signalling and signalling cases

a. [MSE(CEQ1) ↔ MSE(CRF) ] → open SVCs for SEs

[SE(CEQ1) ↔ SE(CRF)] → open VCC to other users

b. [MSE(CEQ1) ↔ MSE(CRF) SVC] → open SVCs

[SE(CEQ1) ↔ SE(CRF)] → open VPC to other user CEQ2

on VPC(CEQ1-CEQ2) - (transparent to the network):

[MSE(CEQ1) ↔ MSE(CEQ2) ] → open SVCs for user SEs

[SE(CEQ1) ↔ SE(CEQ2)] → open VCCs

1.1.3.6 Traffic Control

• ATM technology accepts any type of traffic : traffic control is very important function

- traffic control - still open subject for research

no current consensus for a full-blown traffic and congestion control strategy

a. Requirements for ATM traffic and congestion control

• conventional packet networks transfer non-real time traffic:

- no need to recover the source timing at reception

- flow control – possible to avoid congestion

• ATM – specific conditions (different of packet and frame relay networks) - part of traffic not amenable to reactive flow control (voice, video, MM): bandwidth-delay product is high – feedback flow control is slow; reactive policies in flow control do not fit the high speed of switching and transmission - wide range of rates for applications ( kbits/s – Mbits/s) – no simple schemes for flow

control possible - different traffic patterns (Constant Bit Rate-CBR, Variable Bit Rate-VBR) – makes

difficult to apply conventional congestion control techniques


- different application ⇒ different services (different QoS)

b. QoS requirements

• principles ( ITU-T, I-356) - ATM offers a basic set of services - QoS at request negotiable at UNI and renegotiable during the session - upper layer can enhance the QoS

• QoS parameters

• Cell Error Rate (CER) • Cell Loss Rate (CLR)

• Cell Misinsertion Rate (CMR) • Mean Cell Transfer Delay

• Cell Delay Variation

c. Statistical multiplexing

- deterministic multiplexing: RRN

ii =∑

=1

Ri is the peak rate of source i and R is the output rate of the multiplexer.

- statistical multiplexing gain is obtained if

RRN

ii ≥∑

=1

effect : degradation of QoS in certain time intervals - main problem of traffic control :

statistical multiplexing + QoS guarantees for certain connections

d. Traffic control functions • Connection Admission Control (CAC)

• Usage Parameter Control (UPC) •Congestion Control - congestion control - mechanisms to minimise intensity and duration of congestion status

1.1.4 Reference Models and Protocols

1.1.4.1 BISDN Protocol Stack. Reference configurations

1.1.4.1.1 ATM protocol stack


P lan m an age m en t ( M )

C on tro l(C )l

U se r p lan e ( U )

H ig her la yer s

S G N( V B R )

C B R V B R C O C L (V B R )( ex.D S 1 ,D S 3 ,vo ic e )

(ex. v ideo ) (V B R )( ex.FR ) ( ex. S M D S )

3

2

1

L a yerm a nager

L aye r 2 +A A L

A T M

P h ysica l (S O N E T /S D H ) E qu iva le ntO S I la yers

P la nem a nager

S A A L

Fig.1.2.1-1 BISDN/ATM protocol stack- refinement

SAAL – Signalling AAL SGN – Signalling protocols

CBR – Constant Bit Rate VBR- Variable Bit Rate

CO Connection Oriented Services CL – Connectionless Services

SMDS – Switched Multimegabit Data Services

1.1.4.1.2 BISDN Reference Configurations

- ATM – transport ( switching and multiplexing) technology chosen for BISDN wide area networks

- functional grouping – defined in ITU-T I.371

T E 2 /B - T E 2

B - T E 1

B - T E 1

B - T E 1

( B - T E 1 ) + ( B - N T 2 )

( B - N T 2 ) + ( B - N T 1 )

B - N T 1

B - N T 1

B - N T 1B - N T 2

T B

S B

S B

S B

S B = T B

B - T AR

b

c

d

B - N T 2 is m is s in g

a

Figure 1-16 BISDN Reference configurations at UNI

TE1- narrow band Terminal Equipment with ISDN interface TE2 – non ISDN terminal equipment B-TE1, B-TE-2 – broadband terminal equipment with ISDN I/F B-NT1, B-NT2 – broadband network terminals of type 1, 2 B-TA – broadband terminal adapter ( translation from/to

R to/from S I/F) SB, TB, - BISDN reference points ( identical interfaces) R – non-ISDN interfaces


a. – basic reference configurations b., c., d. – configuration grouping variants B-TE functions: user applications and protocols, interface terminations, signalling. B-NT1 functions: ( ≈ OSI level 1) : line termination, transmission interface management,

Operation and Maintenance functions (OAM). B-NT2 functions: ( ≈ OSI level ≥ 1) : cell framing, different topologies and transmission

media adaptation ( coaxial, FO, UTP/STP, etc.,) concentration, multiplexing/demultiplexing, storage, resource allocation, local switching, OAM.

- B-NT2 is not mandatory to exist ( see d. configuration); not always possible to drop B-NT2; e.g.: B-NT1 is Synchronous Digital Hierarchy (SDH) based and B-TE1 is using simply multiplexed cells without an envelope frame.

- B-NT2 implementation: centralised or distributed ( medium adapter – MA distributed function exists)

TB

SB

B-TE1 MA

B-TE1 MA

B-TE1 MA

SB

SB

M A B-NT1

TB

SBB-TE’

SSB

SSB

B-TE’

B-TE’B-NT1SB

B-NT2

a b

Figure 1-17 a. Distributed B-NT2 b. Shared –medium B-NT2

MA – medium adapter a. B-NT2 is implemented as MAs – for adaptation to medium b. multiple access configuration with B-NT2 included in a LAN. B-TE’ contains B-TE

functions and also medium access control functions -for shared medium MAC functions are required.

Implementation:

standard interfaces or special interfaces (Interworking Units -IWU)

B-NT2 B-NT1B-TE

:

:star - ( B - NT2 )

B-NT2 B-NT1

B-NT2 B-NT1

. . . .

. . . .:

B-TE

B-TE star of buses- ( B - NT2 )

ring- ( B - NT2 )


Figure 1-18 Implementation configuration variants

1.1.4.1.3 Lower Layer Functions Lower Layers: PHY, ATM, AAL (ITU-T I.150, I.321, I.363, I.413, I.432)

Higher layers

Convergence sublayer (adaptation) CS

Segmentation/reassembling SAR

AAL

Translating (switching) - VPI, VCI

Multiplexing/demultiplexing

Generic flow control

Cell rate decoupling (ATM Forum- unassigned cells)

QoS management

A

T

M

Cell rate decoupling (ITU-T – IDLE cells)

HEC generation / verification

Cell delineation (using HEC)

Cell scrambling/descrambling Path signal identification

BISDN

functions

Multiplexing

Frequency justification(pointer processing)

Frame scrambling /descrambling

Frame generation and reception

Transmission

Convergence

(TC)

M

A

N

A

G

E

M

E

N

T

Bit sequencing

Line coding

Electrical/optical interface

SDH/

SONET

functions

Physical Medium

Dependent

(PMD)

P

H

Y

S

I

C

A

L

Table 1-1 Lower layer ATM stack functions

• AAL

- adaptation of ATM services to higher layer requirements - convergence sublayer (CS)– adaptation functions to specific services , can include flow

and error control, or can be void, - segmentation/reassembling (SAR)– main AAL sub-layer ( cuts service data units SDUs in

ATM cells and re-assembles the SDUs at the receiving end) - AAL1, AAL2, AAL3/4, AAL5

• ATM - header generation and extraction, switching, multiplexing / demultiplexing, generic flow control ( only at User Network Interface) , QoS management ( (re)negotiation, monitoring, adjusting parameters ) • PHY - transmission medium independent (convergence - TC) and physical medium dependent - PMD


ATMlayer

Physicallayer

VCC

VCI1

link VC

VCI2 VCIn

VPC

VPI1

link VP

VPI2 VPIm

transmission path

VC

Layers

VP

transmissionpath

digital section

regenerator section

digitalsection

Figure 1-19 ATM and physical connections hierarchy

Note: the transmission path is detailed for SDH physical layer • transmission path – between network elements where the payload info of the cell is

assembled/disassembled • digital section - between network elements where bit or byte strings are

assembled/disassembled • regenerator section - between two regenerators

1.1.4.2 Physical Layer

Transmission Convergence Syblayer (TC) – transforms the flow of cells into a steady flow of bits and bytes for transmission

Physical Medium Dependent Subayer (PMD) – provides the actual transmission of bits

1.1.4.2.1 Physical Medium dependent

Standard bodies: ANSI, ITU-T, ATM Forum

-transmission media:

single mode fiber SMF, multimode fiber MMF

twisted pair : Shielded (STP) , or unshielded (UTP) –type 3, 5

Standard

body

Interface Rates

[Mbits/s]

ANSI

T1.624

SONET for ATM UNI:

Single mode optical I/F STS-1, STS-3c, STS-12c

DS3

51.84 155.22 622.08

44.736, using Physical Layer convergence Protocol of IEEE 802.6 DQDB standard


ITU-T

I.432

SDH

STM-1, STM-16

DS1, DS2, DS3

E1, E3, E4

155.52 622.08

1.544 6.312 44.736

2.048, 34.368 139.264

ATM Forum Public networks:

DS3

STS-3c (SMF, MMF)

Private networks:

FDDI based –TAXI – MMF

Fiber channel based - MMF

Shielded Twisted Pair

Unshielded Twisted Pair

44.736

155.52

100

155.52

155.52

155.52

Table 1-2 Standard ATM Physical Interfaces

Other ATM Forum interfaces : - Frame Relay UNI ( FUNI); - 25 Mbit/s

1.1.4.2.2 Transmission Convergence Sublayer (TC)

TC – conversion: bits clocked to the physical layer – ATM cells

- transmission – maps cells in the TDM frame format

- reception – cell delineation ( from the TDM frame or via HEC-based method)

functions:

• specific to the TDM frames used

- frame generation and reception and cell mapping

• non- specific to the TDM frames used

- cell rate decoupling, - cell delineation, HEC generation and verification

1.1.4.2.3 Cell Rate Decoupling - ATM rate ≠ transmission rate – rate decoupling is necessary • ATM Forum solution : insertion/extraction of unassigned ATM Cells at the ATM layer!

• ITU-T solution: insertion of idle Cells at the Physical Layer

- possible incompatibility if different systems use different solutions!

Types of cells :

ATM layer: assigned or unassigned cells

PHY layer: ATM layer cells, idle cells and OAM cells


VPI/VCI

+Queue

Insert idle orunassignedcells

ATM transmitter

- Distribute

Extract idleor unassignedcells

ATM receiver

VPI/VCI

Figure 1-20 Cell rate decoupling using unassigned or idle cells

- three methods: - continuous cell flow (in SDH or PDH envelopes) - discontinuous cell flow ( in ATM – LANs) – without SDH or PDH envelope – insertion of

“idle” line symbols between cells ( e.g. ATM-FDDI I/F) - combined method - grouping of a constant number of cells in blocks ( inserting

idle/unassigned cells) and insertion of idle symbols between blocks to get periodicity of 125 µs

- preassigned H values for PHY cells: H = 00, 00, 00, 01, HEC (hex notation) – idle cell

H = 00, 00, 00, 09, HEC (hex notation) - OAM cell

H = PPPP 0000, 0000 0000, 0000 0000, 0000PPP1, HEC (binary notation) – PHY reserved cells

1.1.4.2.4 HEC generation and verification

VPI, VCI – used in routing at ATM layer ⇒ need of HEC generation/ verification at PHY layer

Method : CRC code protection

1.1.4.2.5 Cell delineation

- cell extraction: from TDM frame( if any) or from the bytes flow - no synchronising dedicated byte in the ATM cell

HEC method: 4 + 1 bytes satisfy the CRC HEC relationship

- synchronising automaton based on the above relationship

1.1.4.2.6 Cell payload scrambling

- to increase the robustness of delineation process ⇒ scrambling of cell info field

SDH based PHY – self-synchronising scrambler ( X43 +1 – polynomial). The header is not scrambled

Cell based PHY – distribute sample scarambler ( adding modulo 2 a pseudo-random sequence)

1.1.4.2.7 Transmission frame adaptation (mapping) of ATM cells

interface types: SDH, PDH and cell based

• SDH based interfaces (BISDN specifications)


ITU-T I.432: STM-1( 155 Mbit/s), STM-4 (622 Mbit/s) • Rate-STM-1 = ,155,52 Mbit/s. Rate-VC4 = 9 * 260 bytes/125 µs = 2340* 8 bits * 8KHz

= 149,76 Mbit/s.

SOH

AU-4 PTR

SOHH4

ATM cell

.........................9

9

261

VC4 - POH

VC4

9

261

149,76 Mbit/s

VC-4

155,52 Mbit/s

STM-1

9

260

offset

pointer

6

Figure 1-21 STM-1 payload for SDH-based ATM cell transmission

- 2340 bytes is not multiple of 53 bytes ⇒ number of cells/frame is not integer - cell delineation: HEC mechanism + additional pointer-based mechanism - pointer (H4) – first cell offset ( on the same row) - advantages of SDH approach: - frames can carry ATM cells or STM payloads ( initially one can develop the SDH

infrastructure and circuit switching and later migrate to ATM - some specific connections (e.g.CBR) can be embedded in exclusive payloads and

circuit switched ( nore efficient than ATM) - several ATM streams can be combined into higher rate stream than supported by one

ATM layer ( e.g. ATM/155 Mbit/s mapped in STM-1 can be combined in one 622 Mbit/s STM-4 SDH stream – cheaper than one ATM stream at 622 Mbit/s

• Rate-STM-4 = 4 * 155,52 = 622,08 Mb/s.

• PDH based interfaces (legacy transmission systems) ITU-T : G 804 Recommendation for multiplex streams defined in G 703: DS1, E1, DS3, E3,

E4 - mapping: - ATM cells embedded in PDH frame payload - OAM information included in special bytes - cell delineation – HEC method

- cell payload is scrambled to avoid false HEC-based delineation


OAM bytes

FA1, FA2 – Frame Alignment

FA2

ATM cell

59

9

FA1

Figure 1-22 ATM to E3 G.832 mapping

Example: E3 – Rate: 59 x 9 + 6 = 537 bytes; 537*8 bits * 8kHz = 34.368 Mbits/s

• Other PDH-based interfaces : ATM-Forum I/F for E3 ( different from UTU-T) - mapping method: PLCP ( Physical Layer Convergence Protocol – used in IEEE 802.6 –

DQDB ( Distributed Queue Dual Bus) - frame = 9 lines x (4 + 53) bytes G.751 E3 frame + 18-20 byte trailer; first four bytes –

framing and management: line structure: A1, A2, POI, POH, cell

• ATM-Forum I/F for DS3 - PLCP based mapping ( older method, 1990 – UNI3.0/3.1, ANSI T.624) - PLCP frame contains an integer number of cells and is not directly related to the transport

frame - DS3 (45 Mbits/s) ( like E3 PLCP mapping) -direct mapping of ATM cells into the DS3 payload ( ANSI T1.646 – 1995) - the logical structure of info is an array 7 lines x 680 bits, each line having interleaved framing and OAM bits and cell nibbles E1 mapping (ITU-T, 1991)

-TS0 – framing, TS1 – signalling, TS1-15, 16-31 for ATM cell bytes

- ATM cells are byte-aligned

- no relationship between ATM cell and E1 frame

- no superframe structure N x 64 kbit/s - variant of standard UNI for low rate flows on E1/Ds1 ATM link: - each 64 kbit/s channel is assigned by management to non ATM CBR or to ATM data

1.1.4.2.8 ATM inverse multiplexing (AIM) - used to fill the gap between E1/DS1 rates (too low) and E3/DS3 (too high) - combining multiple physical connections into a higher bandwidth logical link - AIM allows a single Nx64 kbit/s ATM trunk to be split among multiple physical connections - preserving QoS and cell order - ability to add/delete DS1/E1 physical links on demand (e.g. – if load changes) - new type cells Sequence Number OAM cell – used to synchronise the links and to identify

the added or deleted connections. SN OAM cells are used by the receiver to reconstruct the original cell stream


- AIM function compensate for differential link delays and eliminates links not meeting the QoS requirements

- during connection establishment the AIM first check the link quality and then qualifies the links through the use of Sn-OAM cells.

6 5 4 3 2 1

14

6 5 4 3 2 125

36

AIMUX AIMUX

Figure 1-23 ATM inverse multiplexing (AIMUX)

• Cell based interfaces ( no temporal frames structures) - streams of 53 byte cells without a framing envelope - synchronisation – HEC-based method - OAM cells inserted (VPI=0, VCI = 9) specific for each of the three layers: F1 -regenerator

section, F2 - digital section, F3- transmission path.

ATM layer cell PHY layer cell(OAM/idle)

Figure 1-24 ATM cell based stream

• FDDI based interface

- ATM Forum – 125 Mbaud, MMF interface for private UNI, based on FDDI physical layer; Physical Medium Dependent (PMD) sublayer – 4B/5B line code

1.1.4.3 ATM Layer

1.1.4.3.1 ATM cell format

• Fields of ATM cell header at User Network Interface:

GFC - Generic Flow Control VPI - Virtual Path Identifier (8 bits)

VCI - Virtual Channel Identifier (16 bits)

PT I- Payload Type Identifier CLP - Cell Loss Priority

HEC - Header Error Check

• Fields of ATM cell header at Network Network Interface (NNI)

- no GFC field , VPI- 12 bits

• VPI, VCI – cell label, avoid the fixed resource reservation ( circuit switching)

- the field length – results from the number of simultaneous virtual channels:

(622Mbit/s) / (min-channel rate = 1cell/s) ≈ 5.2 * 106 channels


VPI – 8 bits, VCI - 16 bits ⇒ 224 > 5.2 * 106

• PTI (Payload Type Identifier)

EFCI – Explicit Forward Congestion Indication – indicates to the destination if the cell encountered or not a network element congested (used in congestion control)

PTI0 (in user cells) transports user-to-user indication

Example: the segmentation process – PTI0 marks the last cell of segmented upper layer SDU – start of reassembling machine )

- PTI0 – used also in ATM switches to discard ( if need) the cells of the same upper layer SDU (avoid discarding cells from different SDUs)

PTI2 PTI1 PTI0

0 0 (EFCI=0, no congestion

encountered) 1

0

0

(user cell)

1 (EFCI=1, congestion

encountered) 1

ATM User-to-ATM User indication

0 (F5 – link of VCC) 0 (OAM cell)

1 (VCC end –to-end)

1 0 (resource management)

1

(network

cell )

1 1 (reserved)

Table 1-3 PTI field of the ATM cell header

• Cell Loss Priority (CLP)

CLP – set to 0/1 by the user or by the network; CLP =1 marks the cells that can be first discarded if congestion occurs

User setting: video coded by sub-bands with two QoS parameters; CLP can mark one or another QoS

Network setting: during the cell transfer the User Parameter Control (UPC) mechanisms detects that a flow does not meet the traffic contract constraints; the non-conformant cells are CLP marked

- problems: non standard usage of CLP ⇒ difficulties when passing through different networks having different usage of CLP

• Generic Flow Control (GFC) – (at UNI only)

- to control the fairness at UNI when multiple traffic sources exist

• Header Error Control (HEC)

- detection and correction of errors in the first four bytes of header

- cell delineation ( HEC correlation search)

- HEC verification at lower layer (Physical) to avoid miss-routing due to errors in (VPI, VCI)


pair

- multiplication error effect if errors occurs in the header

Multiplication factor :

- error in H detected but not corrected : M = (h+1)/(1 + h/i)

- error in H detected and corrected : M = 1/(1 + h/i) – no multiplication

h= length of header [bits], i = payload length [bits]

1.1.4.3.2 ATM layer functions

• VPI, VCI translation in ATM switches( VPI, VCI), or cross-connectors (VPI)

• Multiplexing/demultiplexing – basis of service integration by multiplexing different flows on the same physical layer

• Header generation/extraction – basis of service integration by multiplexing different flows on the same physical layer

- header is generated for each cell, except the HEC which is processed at physical layer

• Generic flow control ( at UNI only)

- necessary when different connections with different QoS share the same UNI - typical case: several terminal on the same UNI - GFC does not control output network traffic - two procedures : controlled – in shared medium configuration

uncontrolled ( GFC field = 0000)

- GFC is still in study

• Operation and Maintenance (OAM) - F4, F5 – bi-directional flows of OAM cells for VP layer and respectively to VC layer

Cell type VPI

(hex)

VCI

(hex)

PTI

(binary)

CLP

bit

Unassigned cells 00 00 00 - 0

Meta-signalling cells XX 00 01 0A0 B

General broadcast cells XX 00 02 0AA B

Point-to-point signalling cells XX 00 05 0AA B

Segment OAM flow F4 cells YY 00 03 0A0 A

End-to-end OAM flow F4 cells YY 00 04 0A0 A

Segment OAM flow F5 cells YY ZZ ZZ 100 A

End-to-end OAM flow F5 cells YY ZZ ZZ 101 A

Resource management cells YY ZZ ZZ 110 A

User information cells YY VV VV 0CU L

Table 1-4 The pre-assigned value header values at ATM layer by ITU-T

A – bit available for use at the ATM layer


B – bit set to 0 but the network can change the value

C – Explicit Forward Congestion Indication (EFCI)

L – Cell loss Priority bit

U – ATM layer user- to – ATM layer user indication bit

X – any VPI value ( for VPI =0, VCI value is valid for signalling with local exchange)

Y – any VPI value

Z – any VCI value other than 0

V - any VCI value above 31H

Notes:

• ITU-T makes distinction between IDLE cells and unassigned cells:

- IDLE cells - visible at PHY layer, not passed to ATM layer. They are used for stuffing unused bandwidth. Idle cells cannot use GFC field because GFC is not a PHY function.

- unassigned cells – visible at ATM and PHY layers, mark the unused positions at ATM level. PHY treats them as any ATM layer cell. They can be

• Meta-signalling cells – used to negotiate on signalling VCI and signalling resources

• General broadcast cells – carries info to be sent to all terminals at UNI

• Point-to-point signalling cells - at UNI or NNI ; the network sees only one signalling entity at the other side

• F4 cells have the same VPI as the user cells except the VCI which is 03H for the flow of a segment and 04H for the flow end-to-end

• F5 cells of a VCC have the same VPI, VCI as the user cells except the PTI field ( see also the Table 1.2.3-1) which is coded 4H and 5H for segment and end-to-end

• ATM Forum differences (w.r.t ITU-T). ILMI protocol - deviates slightly from this Recommendation – by making PTI and CLP bits of metasignalling and general broadcast headers – available for use at ATM layer

- defines an additional pre-assigned header value at the ATM layer for a VCI allocated to Interim Local Management Interface (ILMI)

- ILMI – allows user to get status and control info about VPCs, VCCs at its UNI interface. ILMI is based on the Simple Network Management Protocol (SNMP) and a standard Management Information Base (MIB).

- ILMI uses AAL5 to encapsulate SNMP messages into ATM - ATM level MIB contents: info about PHY, ATM layer, ATM layer statistics, VPCs and VCCs

Cell type VPI

(hex)

VCI

(hex)

PTI

(binary)

CLP

bit

ILMI cells 00 00 10 0AA B

Table 1-5 The pre-assigned value header values at ATM layer by the ATM Forum

• QoS provisioning and negotiation

- providing the user of a VPC or VCC with one QoS class from those available. The negotiation is performed at connection setup.

- QoS not modifiable during the connection life


- several QoS classes are possible for one service - UIT : no negotiation possible for specific QoS user defined; the user have to select between the QoS offered by the network - if a VPC multiplexes several VCCs then VPC is offering the most exigent QoS set of parameters from those required by VCCs

- two different QoS can be negotiated for the same connection ( the cells for each QopS are identified by CLP =0, CLP =1)

1.1.4.4 ATM Forum Services Categories

Real-time service

• Constant Bit Rate (CBR)

• Real-time Variable Bit Rate (rt-VBR)

Non-real-time service

• Non-real-time Variable Bit Rate (nrt-VBR)

• Unspecified Bit Rate (UBR)

• Available Bit Rate (ABR)

1.1.4.4.1 Real-time Services

- real-time services for video, audio, voice flows with constraints on transfer delay, jitter delay, error probabilities, cotinuity, etc.

• Constant Bit Rate (CBR)

- fixed data rate continuously available during the connection lifetime and a relative tight upper bound on transfer delay, common use is for uncompressed info

Examples:

Videoconference, interactive audio(e.g. telephony),

audio/video distribution (TV, distance learning, pay-per-view)

audio/video retrieval (e.g. video-on-demand, audio library)

- relative less sensible to bit errors

-

• Real-time Variable Bit Rate (rt-VBR)

- for time sensitive applications ( tightly constrained delay and delay variation) but variable bit rate flow (e.g. due to compression: MPEG video- I,B,P frames)

- allow more flexibility in the network than CBR ( statistical multiplexing is possible), but still meet the QoS requirements

- sensible to bit errors ( decompression multiplies the error rate)

1.1.4.4.2 Non-real-time Services

- applications with bursty traffic but not tight time related constraints

- greater use of statistical multiplexing is possible, greater efficiency

• Variable Bit Rate (nrt-VBR)

- application in which is possible to characterise the expected traffic flow, so the network can improve QoS w.r.t. loss and delay

- the end system specifies: Peak Cell Rate (PCR), sustainable or average rate, and burstiness measures; the network can properly allocate the resources to provide relatively low delay and loss


- applications: airline or hotel reservations, banking transactions, process monitoring

• Unspecified Bit Rate (UBR) – best effort service

-use of resources not consummed by the CBR and VBR application. The amount of resources varies with time, so no guarantees can be met

- application that can tolerate variable delay and some loss ( e.g. TCP based traffic)

- FIFO service for cells is applied using the available capacity

- initial commitment is made, no feedback on congestion provided

• Available Bit Rate (ABR)

- bursty applications that use reliable end-to-end protocol like TCP can detect network congestion by observing increased round trip delays and packet loss

- but TCP has no mechanism to assure a fair resources sharing among TCP connections; TCP does not minimise congestion as is possible using EFCI

- to improve UBR service ABR is introduced

ABR:

- an application using ABR service specifies: PCR and Minimum Cell Rate (MCR) it requires

- the network allocates resources to guarantee at least MCR

- any unused capacity is shared in a fair and controlled way among all ABR sources

- ABR uses explicit feedback to sources to assure that capacity is fairly allocated. Any capacity not used by ABR is available for UBR

Example: LAN interconnection via ATM; sources of info are the routers

Constant Bit Rate

Variable Bit Rate

Available Bit RateUnspecified Bit Rate

time

Percentageof line

capacity

0 %

100 %

Figure 1-25 ATM bit rate services

1.1.4.5 ATM Adaptation Layer (AAL)

AAL – adaptation of ATM services to support transfer protocols not based on ATM; different types of AAL are necessary

- support for U, C, M planes communications

1.1.4.5.1 AAL services - handling of transmission errors - segmentation and reassembly to map larger SDUs to ATM cells - handling of lost and misinserted cells - flow control and timing control


ITU-T : four classes of service to cover a broad range of requirements

Criterion Class A Class B Class C Class D

Bit rate CBR VBR

Source-destination

timing relations

yes no

Mode CO/CL Connection Oriented Connectionless

AAL protocols AAL1 AAL2 AAL3/4

AAL5

AAL3/4

AAL5

Table 1-6 Service classification for AAL

Examples:

A - circuit emulation, CBR, timing relations, CO(e.g. 2Mb/s, 45Mb/s, etc.).

B – compressed video VBR, (e.g.. MPEG) - videoconference. Timing is important

C – high volum VBR data transfer, CO –mode, (e.g. FTP, X.25, FR)

D - transactional data transfer, VBR, CL ( e.g. SMDS/CBDS services)

1.1.4.5.2 AAL protocols

- AAL consists in end–to–end protocols

- CS - service dependent, CS = SSCS + CPCS

SSCS – Service Specific Convergence Sublayer ( can be void)

CPCS – Common Part Convergence Sublayer

- CPCS –encapsulation of higher layer info into CPCS-PDUs

- SAR – packs and unpacks the information : CPCS-PDU ↔ SAR-PDU, each SAR-PDU having 48 bytes

CPCS

SSCS (can be void)

SAR

AAL primitives AAL -SAP

ATM -SAP ATM primitives

AAlprimitives

primitives

CS

Figure 1-26 AAL sublayers

ATM-SAP, AAL-SAP - ATM and AAL Service Access Points

1.1.4.5.3 AAL1


- AAL1 - used for CBR flows

- SAR resonsibility to pack the bits into cells (transmission) and unpack them at destination

- sequence numbers are used to track errors in PDUs. The sequence itself number is protected

Service provided

• CBR transfer of SDUs •Transfer of timing information between source and destination •Transfer of info structure between source and destination

• Indication of lost and errored info not recovered by AAL1

1.1.4.5.3.1 Overall functions

• Segmentation/Reassembly if SDU length is greater than 47 bytes or Grouping/Degrouping if SDU length is less than 47 bytes ( 1 byte is AAL1 overhead)

• Handling of cell delay variation

• Handling of cell payload assembly delay

• Handling of lost or misinserted cells

• Source clock recovery at destination

• Transfer of source data structure to the receiver

• Monitoring and handling of PCI errors

• Monitoring of user information for bit errors and possible correction actions

1.1.4.5.3.2 SAR functions

• Mapping between CS-PDU and SAR-PDU • Indication of existence of CS functions • Sequence numbering • Error protection

1.1.4.5.3.3 CS functions

• Handling of cell delay variation • Handling of lost or misinserted cells

• Source clock recovery at destination

• Transfer of source data structure to the receiver

• FEC for high quality audio and video • Reporting of end-to-end performance status

Interaction with control and Management planes

- AAL1 sends to M info on: transmission errors, loss and misinsertion of cells, errors in PCI information, loss of synchronisation, buffer overflow and underflow

- AAL1 interaction with C plane – further study

1.1.4.5.3.4 SAR sublayer


CS-PDU(47 bytes)CSI + SN +

CSI(1)

CS

SARSN(3)

CRC(3)

P (1)

CS-PDU(47 bytes)

SAR-PDU-H = PCI SAR-PDU-info

ATMATM (payload)AT M (header)

Figure 1-27 SAR-PDU structure for AAL1

PCI - Protocol Control Information

SN (Squence Number) and SNP (Seq. Number Protection)

SN : CSI bit (Convergence Sublayer Indication) which contains info about the upper layer. This bit can also, be used for:

- RTS (Residual Time Stamp) for source clock recovery at reception

- data block delimiter

SNC (Sequence Number Counter) - 3 bits sequence numbering the cells, allows detection of lost cells or misinserted

SNP: 3 bits CRC to protect SN ( simple error correction) P parity bit computed for the other 7 bits of PCI (double error detection)

1.1.4.5.3.5 Convergence sublayer (CS)

- service dependent sublayer

Functions:

- sequence number management – allows processing of lost or misinserted cells. CS can discard the misinserted cells or can introduce stuffing instead of lost cells to maintain the synchronism valid

- CSI bit carries clock recovery information or can indicate that AAL1 will consume one more byte

- cell delay variation smoothing ( with memory buffers)

- source clock recovery at reception :

- SRTS -Synchronous Residual Time Stamps method

- receiver buffer fill driven method

- data structure recovery at reception

- error correction (optional):

- bit errors, bit errors plus lost cells without delay restrictions


- bit errors plus lost cells with delay restrictions

- end-to-end status reports

ITU-T AAL1 (I.363.1): CS Circuit Emulation Service, CBR video transport, HiFi voice transport, HiFi audio transport

a. Handling the cell delay jitter

- Suppose that the receiver knows the source clock; it observes jitter of arrival instants

- delay jitter ⇒ buffer fill or buffer empty. If the buffer length is enough long then it can absorb the jitter (no buffer fill or buffer empty status)

- Question: what is the minimum delivery delay in order to absorb the jitter? ( note that if the receiver buffer length is enough large the problem is solved but the transfer delay can be excessive)

cells end ofsendinginstants

idealreceptioninstants

kT

C0 delivery

t0 C0

r0

r0d

ri0ri0-δm

ri0-δM

Ck

rkd

Ck delivery

tk

rik=ri0 +kT

jitter range

?

Figure 1-28 Delaying the cells delivery instants

Notations

t0, t1, . . . – sending instants (at intervals T)

r0, r1, . . . – reception instants

ri0, ri1, . . . - ideal cell reception instants

ri0-δm , ri0+δM – jitter range for cell 0, (δm , δM ≥ 0)

r0d, r1d, . . . – cell delivery instants to the user

If ∆ is the delivery delay measured from the receiving instant of the first cell to the delivery instant. The condition of absorbtion is

∆ ≥ δM + δm

• Maximum waiting delay of a cell in the rceiver buffer is

Dmax= 2 ∆


• Minimum receiver buffer length = Bmin = ⎡2∆/d⎤ , where ⎡ x⎤ is the least natural number greater than x and d = source rate in [bit/s].

b. Source clock recovery

- the reciver must recover the source clock. Methods:

• Adaptive clock adjustment, controlled by the current buffer fill

ATM

PLLExtractioncontroller

UserBuffer

AAL receiverbuffer

current fill level

Figure 1-29 Adaptive clock adjustment controlled by the current buffer fill

• Explicit transmitssion of time stamp - SRTS (Synchronous Residual Time Stamp)

Hypotheses:

- there exist a reference clock for the emmision and reception, obtained from a network clock

- one use a time-stamp (RTS) represnting the source local variation w.r.t. reference instants

- RTS ( coded by 4 bits) is sent to the receiver as the difference between emmision local frequency and a common frequency. RTS is carried using CSI bit of 4 consecutive SAR-PDUs ( having odd sequence numbers)

- these information drive the receiver PLL

Notation:

fs – source clock frequency

fn – clock frequency derived from the network clock

T – measure period, negotiated at connection setup, corresponding to N clock cycles Ts = 1/ fs . We have N = fsT.

Principle of the method:

- the source measures the fn cycle count in each T ( that is in N cycles of fs

- let M = measured cycle count; M = fnT

- we got the value needed for receiver fs = fnN/M, possible to be computed if one knows apriori fnN and the rceiver receives the M value fs = fnN/M


ATM

PLL ExtractionController

UserBuffer

AAL Buffer

Timestamps

Figure 1-30 Synchronous Residual Time Stamp principle

- M is binary coded and sent using CSI bits from AAL header. The nominal value Mnom can be negotiated initially between source- destination. So, only the difference ∆M = M - Mnom is sent, coded with signed number of p bits.

Example: ITU-T G 702, consider 8 cells SAR-PDU. The total number of bits are

N = 8 celule x 47 [oct.] x 8 [biţi] = 3008

let 1 < fnx / fs < 2, with fn = 155,52 MHz, iar fnx = fn*2-k cu k = 0, 1, 2, . . .

For ε = 2x10-4 we get p = 4, so the RTS info can be sent in CSI bits of 8 consecutive cells ( having odd sequence number)

c. Transfer of structure information between source-destination

- necessary when the original info is structured in blocks and this structure must be recovered at reception ( e.g. CES – Circuit Emulation Service)

- structured blocks are included in the payload as they are

- problem: the different sizes: block/cell

Solution:

- in cells having even SN one introduce a 7 bit pointer P indicating the block offset - the presence of the pointer is indicated by CSI bit ( CSI is used in odd cells for SRTS)

Block i Block i+1 . . .. . .

cell flow

flow structured in blocks

a

b

H-SARH-cell

pointer 46 oct 47 oct

block block start

Figure 1-31 Transfer of block structured data

a. – mapping principle of blocks into the cell flow


b. – pointer indicating the block start

d. AAL1 error processing

CS –AAL1 can optionally process the errors:

- simple or multiple in the payload

- loss or misinsertion of cells ( signalled by SAR)

- case examples in which the errors are not treated at AAL level:

-data circuit emulation ( if bit error rate is sufficiently small)

- voice service –commercial quality

- the methods are adapted to real-time characteristic of the flow

• Interpolation

- one sample loss in a continuous media sampled flow can be recovered by interpolation

- cell loss ⇒ loss of 47 consecutive samples. Solution: mapping of p consecutive samples in p consecutive cells ( one cell loss affects samples situated at a distance of p). The interpolation can be applied.

• Forward Error Correction (FEC) - used for enhanced quality in audio or video applications

e.g.: ITU-T proposes Reed Solomon codes ( 128, 124)

- 124 bytes info + 4 bytes redundancy = 128

- correction capacity ( up to 2 bytes on a 128 bytes line)

- addistionally SAR indicates missing cells

• Combined method ( octet interleaving and FEC)

• Parity protection method - group of p cells ( each with 47 octets) is protected by an additional cell which contains 47 parity bytes for each byte line of the group

1.1.4.5.4 AAL2

-for analog application VBR real time flow ( video, audio) which require timing information

- initial proposal for AAL2 CS + SAR has been withdrawn

- new proposal ITU-T I.362.2 – draft exists

- proposals exist to supplement AAL1 and AAL5 to support video, audio

1.1.4.5.4.1 Service provided

• VBR transfer of SDUs • Transfer of timing information between source and destination • Indication of lost and errored info not recovered by AAL1

1.1.4.5.4.2 Overall functions

• Segmentation/Reassembly

• Handling of cell delay variation and cell payload assembly delay


• Handling of lost or misinserted cells

• Source clock frequency recovery at destination

• Recovery of source data structure at the receiver

• Monitoring and handling of header and trailer bit errors

• Monitoring of user information for bit errors and possible correction actions

1.1.4.5.4.3 AAL2 protocols

- ITU-T : draft Rec. for AAL2 I.362.2

- mechanism for sending small packets ( e.g. voice) over ATM network in manner to ensure small delays:

e.g. voice compressed at 8kbit/s ⇒ 48 ms delay to packetise in an ATM cell !

- support multiplexing of multiple connections on one cell ( avoid pad filling of not complete with info cells)

- connections called LLC ( Logical Link Connections)

- support multiplexing of variable length packets ( accommodate variable bit coders and silence compression)

1 oct.CPS packet -fixed length

CPS-SDUpayload

CPSheader

PADCPS-SDUpayload

CPSheader

OSF(6)

SN(1)

P(1)

START field

HEC(5)

CID(8)

LI(6)

UUI(5)

3 oct.

48 oct.

CPCS + SAR

user1 user2

ATM

Figure 1-32 The new AAL2 type cell format

CPS header:

CID channel identifier LI – Length indicator

UUI – User-user indication HEC – Header Error Control

PAD – padding bytes if needed

Start field:

OSF – offset of the first packet in the cell, reletive to the end of the Start field

SN – Sequence Number for alternate bit numbering

P – Parity bit for start field protection

CPS – Common Part Sublayer: CPS header, CPS payload

Header: CID – Channel ID- identify the user; AAL2 is a bidirectional channel – the same value is used for both directions

LI = length indicator = length(CPS-payload) –1;


max length = 64

UUI – transparently conveys info between CPS-user and layer management to distinguish between SSCS users and management users of the common part CPS

HEC – protection of the header = remainder of division of 19 bits polynomial to generator polynomial x5 + x2 + 1

1.1.4.5.4.4 Example: VoATM gateway

ATM network

VoATMGateway

VoATMGateway

Figure 1-33 AAL2 support of voice traffic in VoATM solution

- VoATM GW accepts analog speech at the sender, digitises it, creates G.729.Avoice packets, and presents them to AAL2.

- G.729.A low bit audio rate coder – 8 kbit/s; each frame has - 10 bytes length

- Real Time Protocol (RTP) encapsulation – 14 bytes packet

- 14 bytes + 3 bytes CPS header = 17 bytes/packet

PAD

1stcell

17

G.729.APH G.729.APHS

48 octets

1

G.729.A

PH

17 13

2nd

cell

17

G.729.APH G.729.APH

1

S G.729.A

17 94

part of G.729.A from first cell

Figure 1-34 The new AAL2 type – fixed packet length format

S – Start field ( containing offset)

PH – packet header


- AAL2 can multiplex voice and data - identifies each stream by CID

- packets wth higher lengths are segmented and mapped on several cells

1.1.4.5.5 AAL3/4, AAL5

- initial AAL3 and AAL4 – similar;merged in AAL3/4

- AAL5 – simplified version of AAL3/4 – more popular; it is recommended for class C - AAL5 - used in LANE, IP/ATM, MPOA, FRoATM - I.362 recommends AAL3/4 for class C (CO) - I.363 recommends AAL3/4 possible for class C(CO) and D (CL)

1.1.4.5.5.1 Service provided by AAL3/4 and AAL5

- similar services, different protocols for AAL3/4 and AAL5

• Connection oriented (CO) or connectionless (CL) mode service

• Message mode service • Streaming mode service • Assured operation or non-assured operation

- different services can use the same SAR + CPCS

- CO mode - is possible to define multiple SAR logical connections over a single ATM connection

- CL mode – each block of data ( SAR-SDU) presented to SAR – treated independently

- Message mode service transfers framed data ( any of the OSI protocols fit into this category (e.g. LAPD or Frame Relay); a single block of data from AAL-user is transferred as one interface data unit through service interface and then mapped int one or more cells.

-Streaming mode – transfer of low speed continuous data with low delay requirements. Data are presented to AAL in fixed-size blocks ( may be as small as one octet)

- message mode –used for transport of AAL-SDU of fixed or variable length

- if AAL-SDU has small and fixed length (framed data), one can group several AAL-SDU into one SSCS-PDU

- variable length AAL-SDU can be segmented in SSCS in several SSCS-PDU

- one AAL-SDU is mapped into one SSCS-PDU

- if void SSCS then AAL-SDU is transferred to SAR as CPCS-SDU

- stream mode – one AAL-SDU is passed through the service I/F as one ore more AAL-IDU (can be transferred at separate time instants). Serrvice is used for variable length AAL-SDU continuous data , low speed.

- segmentation is possible in SSCS : one AAL-SDU transferred via one or more (AAL-IDU) is segmented in m x SSCS-PDU, m≥1

- pipe-line mode possible: SSCS can start down transfer to SAR wthout waiting the end of receive for AAL-SDU from the upper layer


- if no segmentation, then mapping is: 1 x AAL-SDU → 1 x SSCS-PDU

- SSCS can be void : the units AAL-IDU belonging to one AAL-SDU are included into one CPCS-PDU

Example: AAL-user is a router to link an Ethernet LAN to WAN. The router receive frames at 10 Mbit/s. The router can start data transfer to AAL without waiting the end of the frame.

- both message and stream modes can be offered in assured or non-assured mode

• Assured operation – error correction (retransmission) exists for AAL-SDU lost or corrupted, flow control is mandatory; service is offered in point-to-point connections

•Unassured operation – AAL is not concerned in lost info recovery. Flow control is optional

1.1.4.5.5.2 AAL3/4 protocols

Notes:

not used any more in practice

- too much overhead ( 4octets/cell)

- too complex

SAR functions

• Segmentation/Reassembly • Error detection • Sequence integrity

• Multiplexing

CS functions

• Error detection and handling • Indication of buffer allocation size

• SAR Sublayer

Functions:

- assuring the SAR-PDU integrity, including payload, and detection of lost and misinserted SAR-PDUs. CS layer is notified in such cases.

- multiplexing/demultiplexing of AAL-SDUs from different connection, one one ATM connection

- segmentation/reassembly or grouping/degrouping

• Common Part Convergence Sublayer (CPCS)

Services: CO or CL

Functions

• delimiters for CPCS-SDU

• error detection. Corrupted PDUs can be dropped or delivered to the upper layer together with an indication on error

• informs the receiver on buffer size needed to receive the whole CPCS-PDU


1.1.4.5.5.3 AAL5 protocol

- AAL increasingly popular in LAN based application - introduced to provide a streamlined transport facility for CO higher layer protocols

- reduced overhead in comparison with AAL3/4: if higher layer performs connection management and ATM layer introduces minimal errors then most of the SAR fields and CPCS PDU fields of AAL3/4 are not necessary

- for CO communication MID is not needed ( AAL5 supposes that huigher layer makes multiplexing uf necessary)

- reduced transmission overhead

- ensure adaptability to existing transport protocols

CPCS-PDU payload PAD CPCS-T CPCS-PDU

CPI Length CRCCPCS-UU

1 oct. 1 oct. 2 oct. 4 oct.

Figure 1-35 Common Part Convergence Sublayer-PDU for AAL5

CPCS-UU – User-to-user indication – transparent transfer of user

CPI – Common Part Indicator –interpretation of the remaining fields

CRC – protection field for the whole CPCS-PDU

- buffer allocation size negotiation is left for higher layers

- PAD field –s used to make the (CPCS-PDU + PAD) = n x 48 octets - lack of protocol overhead in ATM cells

- because there is no sequence numbers, AAL5 suppose all cells arrive in order. CRC can detects misorder occurences

- the last bit of CPCS-T is the last bit of the last SAR-PDU


CPCS

CPCS-PDU payload PAD CPCS-T

Higher layer PDU

CPCS-PDU

SAR

48

SAR-PDUs48

48

48

CPCS-T

CPCS-TH ATMATM cell

0 0 0 1PTI bit in ATM –Header

to show the last cell ofCPCS-PDU

Figure 1-36 AAL5 segmentation

1.1.5 References [WS97] William Stalling, “Data and Computer Communication”, Prentice Hall, New-

York, 1997, ISBN 0-02-415425-3 [WS98] William Stalling, “High-Speed Networks: TCP/IP and ATM Design

Principles”, Prentice Hall, New York, 1998, ISBN 0-13-525965-7 [KG98] D.Kofman, M.Gagnaire, “Réseaux haut debit”, InterEditions, 2-ème edition,

Dunod Paris, 1998 ISBN2-225-82927-6 [MP95] M.Prycker, “Asynchronous Transfer Mode-Solution for BISDN”, third

edition, Prentice Hall, New-York, 1995, ISBN 0-13-342171-6 [UB99] U.Black, “Voice Over IP”, Prentice Hall, New-York, 1999, ISBN 0-13-

022463-4

[ITU-T, I.327] ITU-T Recommendation I.327, ”BISDN Functional Architecture”, March, 1993

[ITU-T, I.361] ITU-T Recommendation I.361”BISDN ATM Layer Specification”, March,

1993 [ITU-T, I.363] ITU-T Recommendation I.363, ”BISDN ATM Adaptation Layer (AAL)

Specification”, March, 1993

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