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Dipartimento di Informatica e Sistemistica, Università di Napoli Federico II Computer Networks II – a.a. 20092010

Computer Networks II

Asynchronous Transfer Mode

Giorgio Ventre

COMICS LAB

Dipartimento di Informatica e Sistemistica

Università di Napoli Federico II


Nota di Copyright

Quest’insieme di trasparenze è stato ideato e realizzato dai

ricercatori del Gruppo di Ricerca sull’Informatica Distribuita del

Dipartimento di Informatica e Sistemistica dell’Università di

Napoli e del Laboratorio Nazionale per la Informatica e la

Telematica Multimediali. Esse possono essere impiegate

liberamente per fini didattici esclusivamente senza fini di lucro,

a meno di un esplicito consenso scritto degli Autori. Nell’uso

dovrà essere esplicitamente riportata la fonte e gli Autori. Gli

Autori non sono responsabili per eventuali imprecisioni

contenute in tali trasparenze né per eventuali problemi, danni o

malfunzionamenti derivanti dal loro uso o applicazione.

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TOPICS

» The ATM header

» The ATM protocol stack

» The physical layer

» ATM switch architectures

» ATM adaptation layers

» IP over ATM

ATM Networks


Asynchronous Transfer Mode (ATM)

The word Asynchronous in ATM is in

contrast to Synchronous Transfer Mode

(STM) that was proposed earlier on,

which was based on the SONET/SDH

hierarchy.

Transfer Mode refers to a

telecommunication technique

3


ATM was standardized by ITU-T (CCITT) in

1988 as the transfer mode of B-ISDN

It can carry a variety of different types of traffic,

such as

» Voice

» Video

» Data

At speeds varying from fractional T1 to 2.4

Gbps



These different types of traffic have

different Quality-of-Service (QoS)

requirements, such as:

» Packet loss

» End-to-end delay

ATM, unlike IP networks, can provide

each traffic connection a different type of

quality of service.


4


ATM: The Grand Unifier?

What is changing:

Due to fiber, better copper technology, and improved hardware bandwidth is no longer the problem. How can we use it?

CATV is moving to digital too.

500 D-CATV channels

New services: video-on-demand, interactive TV.

Data communication is changing as well.

New applications: multimedia over LANs/WANs, distributed computing, telecontrolling, virtual reality.


ATM Architecture

Assumptions

ATM is a cell-based, connection-oriented transfer methodology

ATM can dynamically allocate bandwidth

ATM can dynamically manage QoS specifications

ATM works on fiber optic fabric and High Quality TP with extremely low error rates

The devices to be connected to ATM networks might be very simple, like a telephone

ATM is organized in a hierarchy, like today’s phone network

5


ATM Architecture

Packets vs Cells

A Cell is a data entity of small, constant

size

Scheduling is more efficient and manageable


ATM Architecture

From Packets to Cells

The transition from packets to cells

can be inefficient

Cell losses can have a critical impact

6


ATM Architecture

Cell Information: why 48 bytes ?

48 Bytes = 384 bits

= 6 msec of PCM Audio @ 64 Kbps

With 48 bytes of payload, a very

efficient and small addressing scheme

must be used.

Only 5 additional bytes are reserved for

addressing, control and error check

header fields.

Cell Size = 5 + 48 = 53 bytes


ATM Architecture

Switching is performed on a per-connection

basis

» Connection ID in the cell header

» Cut-through switching

» hardware-based switching

Connections are identified by two elements

» Virtual Channel Identifier (VCI)

“A concept to describe unidirectional transport of

cells”

» Virtual Path Identifier (VPI)

“A concept to describe a set of virtual channels”

Both identifiers have only “local” significance

7


Global addressing vs. Local addressing

Global addressing:

» Unique identifiers @ World

» Large addressing info

» Global management

» Global knowledge

Local addressing

» Local identifiers

» Small addressing info

» Local management

» Translation needed in local-to-local transit


ATM Architecture

Private UNI

Private NNI

Public UNI Public NNI

ATM

ATM

Router/Bridge

ATM Switch

LANE, NHRP, MPOA

8


Some features of ATM

Connection-oriented packet-switched network

Fixed cell (packet) size of 48+5 bytes

No error protection on a link-by-link

No flow control on a link-by-link

Delivers cells in the order in which they were

transmitted

Header Payload

5 bytes 48 bytes


The structure of the ATM cell

GFC VPI

VPI VCI

VCI

VCI

HEC

PTI CLP

Information

payload

1 2 3 4 5 6 7 81

2

3

4

5

.

.

.

53

B

y

t

e

UNI cell format

VPI

VPI VCI

VCI

VCI

HEC

PTI CLP

Information

payload

1 2 3 4 5 6 7 81

2

3

4

5

.

.

.

53

B

y

t

e

NNI cell format

9


Fields in the ATM cell header

GFC: (Generic Flow Control)

Connection identifier: VPI/VCI,

Payload type indicator: (PTI)

Cell loss priority (CLP)

Head error control (HEC)


ATM connections

Identified by the combined fields

» virtual path identification (VPI), and

» virtual channel identification (VCI)

VPI field:

» 256 virtual paths at the UNI interface, and

» 4096 virtual paths at the NNI interface.

VCI field:

» a maximum of 65,536 VCIs.

10


VPI/VCI values have local significance.

That is, they are only valid for a single

hop.

A connection over many hops, is

associated with a different VPI/VCI

value on each hop.

Each switch maintains a switching table.

For each connection, it keeps the

incoming and outgoing VPI/VCI values

and the input and output ports.

ATM connections


ATM Switching

VPI 4VPI 4VCI 1

VCI 2

VCI 1

VCI 2

VPI 5VPI 5VCI 1

VCI 2

VCI 1

VCI 2

Transmission Path

ATM Switch ATM Switch ATM Switch

Virtual Path

Connection (VPC)

Virtual Channel

Connection (VCC)

Virtual Path

Link

Virtual Channel

Link

11


ATM Switching

Virtual Path Switching

End-to-end Virtual Channel Connection (VCC) set up No matter of routing across the network-

VCs associated with VP are globally switched without processing

the individual VC or changing VCI number

Virtual Circuit can be permanent or switched

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VC2

VC1

VC3

VC4

VC5

VC6

VC7

VC8

VP1

VP2

ATM Switch VP Switch ATM Switch

VP1

VP2


ATM Switching

Virtual Circuit Switching

In this case both VPI and VCI are transformed by the switch

In theory, mere VPI switching can also be performed

VC Switch

VP Switch

VPI 5VPI 4VCI 1

VCI 2

VCI 1

VCI 2

VPI 1VCI 1

VCI 2

VCI 4

VCI 3

VPI

3’

VPI 1’VPI 2

VPI 3

VPI 2’

VC 1

VC 2VC 4

VC 3

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Label swapping

VPI=10

VCI=89

VPI=30

VCI=53

VPI=100

VCI=53

VPI=50

VCI=77

VPI=30

VCI=41

30 53 4 100 53 530 41 1 30 53 4

40 62 2 10 89 3

10 89 1 50 77 6

ATM

switch 1ATM

switch 2

ATM

switch 3

VPI=40

VCI=62A

B

C

D2

1

3

44

1

5

6


PVCs and SVCs

Depending on how a connection is set-up, it may be

» Permanent virtual circuit (PVC)

» Switched Virtual circuit (SVC)

PVCs are set-up administratively. They remain up for a long time.

SVCs are set-up in real-time using ATM signalling. Their duration is arbitrary.

13


Call & Connection Control

ATM Connections on Demand

Poin-to-Point & Point-to-Multipoint

Symmetric & Asymmetric bndwdt allocation

Single Connection calls

Procedures for» Call setup

» Request

» Answer

» Clearing

» Out-of-band signaling



ATM Connections on Demand (cont.)

Non-negotiation of QoS among users

Support for A, C, and X traffic classes

Specification of VPI/VCI ranges

Guidelines for Addressing Formats

Designation of OOB Signaling Channel

Error Recovery

Client Address Registration Procedures

Non-support of Multicasting Operations

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Connection Control Messages

Call establishment» SETUP, CALL PROCEEDING, CONNECT,

CONNECT ACK

Call Clearing» RELEASE, RELEASE COMPLETE

Point-to-multipoint» ADD PARTY, ADD PARTY ACK, ADD PARTY

REJ, DROP PARTY, DROP PARTY ACK

Misc» RESTART (VC), STATUS ENQUIRY, STATUS



USER USERATM

Network

UNI UNI

SETUP

SETUP

CALL PROC

CONNECT

CONNECT

CONN ACK

Call Ref.

Addresses

QoS

Traffic Char.

•Allocate Res.

•Path Disc.

•Build VC

Call Ref.

Addresses

QoS

Traffic Char.

VPI/VCI

CALL PROC

Call Ref.

Call Ref.

•Complete VC Creation

Call Ref.

Call Ref.

CONN ACK

Call Ref.

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Payload Type Indicator

PTI Meaning

000 User data cell, congestion not experienced, SDU type=0

001 User data cell, congestion not experienced, SDU type=1

010 User data cell, congestion experienced, SDU type=0

011 User data cell, congestion experienced, SDU type=1

100 Segment OAM flow-related cell

101 End-to-end OAM flow-related cell

110 RM cell (resource management)

111 Reserved


The ATM protocol stack

ATM adaptation layer

ATM layer

Physical layer

voice Video Data

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Protocol Architecture

I.121 Protocol Reference Model

U-PlaneThe User Plane provides for the transfer of user application information.

C-PlaneThe Control Plane protocols deals with call and connection control for switching.

M-PlaneThe Management Plane provides management function and exchange information between U-Plane and C-Plane

Plane Management

Layer Management

Control Plane User Plane

Higher Layer

Protocols

Higher Layer

Protocols

ATM Adaptation Layer

ATM Layer

Physical Layer


Headers and Trailers

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ATM Networks

User Network Interface (UNI) Protocol adapted to define connection between ATM

users (end-station) and ATM network switch

Network Network Interface (NNI) Subnet of UNI defines interface between network nodes

(switches)

Higher Layer Protocol

Convergence CS

AALSegmentation & Reassembly SAR

Generic Flow Control

Cell Header Generation & Extraction

Cell VPI/VCI Routing/Translation

Cell Multiplex/Demultiplex

ATM

PHY

HEC Generation & Verification

Cell Delineation

Frame Multiplexing

Transmission Frame Gener. & Recov.

TC

PMD

ATM

PHY


Convergence CS






ATM

PHY


Cell DelineationTC

PMD

ATM

PHY

UNI UNINNI

Frame Multiplexing



ATM Networks

For OAM operations we need to invoke AAL Therefore we must have AAL capabilities also in the

switches for dealing with C-plane and M-plane commands

For some OAM messages SAR is not required This is the case of messages contained in just one ATM

cell


Convergence CS






ATM

PHY


Cell Delineation

Frame Multiplexing


TC

PMD

ATM

PHY


Convergence CS






ATM

PHY


Cell DelineationTC

PMD

ATM

PHY

UNI UNINNI

Frame Multiplexing


AAL AAL

HLP HLP

18


ATM and the OSI model

ATM does not map directly with the OSI layers

ATM layer performs operations typically found in OSI layers 2 and 3

AAL combines features of layers 4, 5, and 7 of the OSI model

Consequentely, it is difficult to separate functions so that internetworking can be performed efficiently

E.g.: TCP/IP on top of ATM


Layering and delay

End to end delay is made of different contributions C/S: Convergence /SAR 6000 s variable

PD: Propagation Delay 2000 s e2e 500 km

SD: Switching Delay 24 s /swicth variable

QD: Queueing Delay 225 s/swicth variable

Another important factor: link speed However it contributes only to constant delay

ATM ATM ATM

C/S C/S

PD PD PD PD

QD & SD QD & SD QD & SD

19


ATM Layers and Sublayers - Physical Layer

Physical Medium Dependent (PMD) sublayer

Defines actual speed at which ATM traffic can be

transmitted across a given physical medium

SONET/SDH

» Synchronous Transfer Signal. STS-n: basic unit for

SONET n=1-3-12 n=1 51.84 Mbps

» Synchronous Transfer Module. Basic unit for SDH

n=1-2-4 n=1 155.52 Mbps

Other Interfaces : E1(2.048 Mbps), T1(1.544 Mbps), T3

(DS-3 44.736 Mbps), etc.


ATM Layers and Sublayers - Physical Layer

Transmission Convergence (TC) Sublayer Deadline protocol for preparing cells for transmission

across the physical medium defined by PMD

Functions differ according to physical medium

Transmission

Convergence

Sublayer

Physical

Medium

Dependent

Sublayer

HEC Generation/Verification

Cell scrambling/descrambling

Cell delineation (H4, HEC)

Path Signal Identification (C2)

Frequency justification/Pointer processing

Multiplexing

Scrambling/Descrambling

Transmission frame generation/recovery

Bit timing, Line coding

Physical medium

B-ISDN specific function

SONET

or

SDH

20


The transmission convergence (TC) sublayer

HEC cell generation and verification» Implements the HEC state machine

Decoupling of cell rate» Maintains a continuous bit stream by inserting idle cells

Transmission frame generation and recovery» Such as SONET frames

Cell delineation


Error detected(cell discarded)

Head Error Control (HEC)

Correction

mode

Detection

modeNo Error detected

(no action)

Single bit error detected

(correction)

Multiple bit error detected

(cell discarded)

ATM HEC: Header Error Correction & Detection

» Based on a 8 bit field in the header

» Allows Detection of Single bit and Multiple bit errors

» Single Error allows Correction -> Cell recovered

» Multiple Error -> Cell discarded

» Based on an Hamming coding: Bose - Chaduri - Hocquengem (BHC)

» 40 bit header needs 6 bits for 1 bit error recovery

» With 8 bits we reach 84% of multiple bits error detection

No Error detected

(no action)

21


The ATM layer

The ATM layer is concerned with the end-to-end

transfer of information, i.e., from the transmitting

end-device to the receiving end-device.

The ATM layer is a connection-oriented point-to

point packet-switched network with fixed-size

packets (known as cells).

Cell switching is performed at the ATM layer.

Cells are delivered to the destination in the order

in which they were transmitted.


No error and flow control on each hop

Low probability of a cell getting lost or delivered to the

destination end-device in error.

The recovery of the data carried by lost or corrupted cells

is expected to be carried out by a higher-level protocol,

such as TCP.

When TCP/IP runs over ATM, the loss or corruption of the

payload of a single cell results in the retransmission of an

entire TCP PDU.

22


Addressing

Each ATM end-device and ATM switch has a

unique ATM address.

Private and public networks use different ATM

addresses. Public networks use E.164

addresses and private networks use the OSI

NSAP format.

ATM addresses are different to IP addresses.


Quality of service in ATM

Each ATM connection is associated with a quality-

of-service category.

Each quality-of-service category is associated with

a set of traffic parameters and a set of quality-of-

service parameters.

The ATM network guarantees the negotiated

quality-of-service for each connection.

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ATM Traffic Classes

Constant Bit Rate Applications:

They require that delay from source to destination be bounded, so that isochrony is respected.

Variable Bit Rate Applications:

They require that delay be bounded but for variable data transmission.

Connection-oriented Data Applications:

ATM should support traditional data communication traffic. E.g. X.25.

Connectionless Data Applications:

Most of “data community” currently uses datagram networking protocols. E.g. TCP/IP.

Different classes of applications require different adaptation

layer protocols:


Service Classes and Resource Allocation

CBR

VB

R

ABRABR

VBR

UBR?

24


ATM Forum Service Categories (in prior. order)» CBR: assured steady supply of bndwdt at PCR values

» VBR: assured supply of bndwdt at ACR with rt and nrt reqs

» ABR: bndwdt to keep the application running

» UBR: bndwdt as available with no assurance

Service Bndwdt Delay Var. Through. Cong. fbck

CBR yes yes yes no

rt-VBR yes yes yes no

nrt-VBR yes no yes no

ABR yes no min. Yes

UBR no no no no

Service Classes and QoS Guarantees


Original Service Classes

ITU-T Traffic Classes

» Class A: CBR - Connection oriented - Timing relationship

» Class B: VBR - Connection oriented - Timing relationship

» Class C: VBR - Connection oriented - No timing req.

» Class D: VBR - Connectionless - No timing req.

ATM Forum Traffic Classes

» Class X: ABR & UBR

Service Classes Class A Class B Class X Class C Class D

Applications Circuit Emulation Compresse Media Cell Relay Bursty data Datagram service

Parameters

Constant Bit Rate Variable Bit Rate

Timing required Timing not required

Connection Oriented Connectionless

AALs AAL1 AAL2 AAL0AAL3/4

AAL5

25


Quality of Service in ATM

Parameter Definition

Cell Loss Ratio Ratio of lost cells to transmitted

cells

Cell Misinsertion Rate Number of misinserted cells

per connection per second

Cell Error Ratio Errored cells to delivered cells

Severely Errored Cell

Block Ratio

Number of errored cells blocks

to total number of cell blocks

Cell Transfer Delay Transfer delay for a single cell

Mean Cell Transfer

Delay

Delay averaged over multiple

cells

Cell Delay Variation Difference between average

delay and a single observation


Cell loss rate

» This is a very popular QoS parameter and

it was the first one to be used extensively

in ATM networks.

» It is easy to quantify, as opposed to other

QoS parameters such as jitter and cell

transfer delay.

It has been used extensively as a guidance to

dimensioning ATM switches, and

in call admission control algorithms.

26


An important QoS parameter for voice and video.

It refers to the variability of the inter-arrival times at the destination

ATM

cloud

Inter-departure gaps

Sender

cell

i-1

Inter-arrival gaps

Receiver

cell

i cell

i+1

cell

i-1

cell

i

cell

i+1

ti-1

ti

si-1

si

Jitter


Cell transfer delay (CTD)

The time it takes to transfer a cell end-to-end,

that is, from the transmitting end-device to the

receiving end-device. It comprises of

» Fixed cell transfer delay

– Propagation delay, fixed delays induced by

transmission systems, and fixed switch

processing times

» Variable cell transfer delay, known as the

peak-to-peak cell delay variation

– Queueing delays in switches

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max CTD

Fixed CTD

pdf1% of the

total area

Cell Delay

variation

cells

delivered

late

This is a statistical upper bound on the end-to-end cell transfer delay.

Maximum Cell transfer delay (max CTD)


Cell error ratio (CER) and Cell misinsertion rate (CMR)

The CER of a connection is the ratio of thenumber of errored cells to the total number ofcells transmitted by the source. An erroredcell is a cell delivered with erroneous payload.

CMR is the rate of cells delivered to a wrongdestination, calculated over a fixed period oftime.

28


Traffic Descriptor

Peak Cell Rate (PCR)

Sustained Cell Rate (SCR)

Minimum Cell Rate (MCR)

Cell Delay Variation Tolerance (CDVT)

Maximum Burst Size (MBS)


Flow Specification

29


Attributes for: CBR, RT-VBR, NRT-VBR, UBR

CBR» Class attributes: PCR, CDVT

» QoS attributes: peak-to-peak CDV, MaxCTD, CLR

rt-VBR» Class attributes: PCR, CDVT, SCR, MBS, CDVT

» QoS attributes: peak-to-peak CDV, MaxCTD, CLR

nrt-VBR» Class attributes: PCR, CDVT, SCR, MBS, CDVT

» QoS attributes: CLR

UBR» PCR is specified, but it may not be subject to CAC and policing

» No QoS parameters are signaled


Attributes for ABR and GFR

ABR» Class attributes: PCR, CDVT, MCR

» QoS attributes: CLR (possible, depends on network)

» Other attributes: feedback messages

GFR» Class attributes: PCR, CDVT, MCR, MBS, MFS,

CDVT

» QoS attributes: CLR (possible, depends on network)

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QoS can be guaranteed only if traffic control mechanisms are enforced» Virtual Path Traffic Management

» Connection Admission Control

» Flow Control Usage Parameter Control (UPC)

Network Parameter Control (NPC)

» Priority Control

» Traffic Shaping

» Fast Resource Management

» Congestion Control



Connection Admission Control (CAC)

» The mechanisms in the call set-up phase to decide if a VC/VP connection can be accepted

» Users specify the traffic characteristics peak cell rate

average cell rate

burstiness

peak duration

» … and the requested QoS

» The network evaluates its capability to offer that QoS given the current network occupancy

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Flow Control

» Can be performed at both UNI and NNI levels

» Checks on the validity of VPI/VCI values

» Traffic volume monitoring

Three basic mechanisms to punish misbehaviors

» Connection release

» Cell discarding

» Cell tagging

Effective if combined with Traffic Shaping


The ATM adaptation layer

The purpose of AAL is to isolate higher

layers from the specific characteristics of the

ATM layer.

AAL consists of the

» convergence sublayer, and the

» segmentation-and-reassembly sublayer.

32


ATM layers and sublayers


Headers and Trailers introduced by AAL

33


This AAL can be used for applications such as:

» Circuit emulation services– It emulates a point-to-point TDM circuit over ATM

» Constant-bit rate audio– Used to provide an interconnection between two

PBXs over a private or public ATM network

ATM Adaptation Layer 1- AAL 1


ATM Adaptation Layer 1- AAL 1

» Constant Bit Rate (CBR); traffic sensitive to cell

loss and delay-Digital voice and video, circuit emulation (transport for E1 link)

» Include mechanisms for recovering the source

timing

» Requires an additional byte of header for

sequence numbering (47 bytes of payload)

User data from higher layer protocol

47-Bytes

CS-PDU CS-PDU CS-PDU

47-Bytes

47-BytesCSI SNPSN

CS

SAR

CSI 1-bit

SN sequen.num.3 bit

SNP protection 4 bit

34


The SAR encapsulation for AAL 1

payloadSAR Header

47 bytesSN SNP

CRC-3

3 bits

Parity

1 bit3 bits1 bit

CSI Sequence. count


The AAL 1 CS functions:

1. Handling of cell variation» Due to queueing delays, inter-arrival times of cells vary

(jitter).

» CS writes received data into a buffer, and then delivers the information to the application at constant bit rate.

ATM

cloud

Inter-departure gaps

Sender

cell

i-1

Inter-arrival gaps

Receiver

cell

i cell

i+1

cell

i-1

cell

i

cell

i+1

ti-1

ti

si-1

si

35


2. Processing of the sequence count» The sequence count values are processed by CS

in order to detect lost or misinserted cells. Detected misinserted cells are discarded. In order to maintain bit count integrity of the AAL user information, it may be necessary to compensate for lost cells by inserting dummy SAR-PDU payloads.

3. Forward error correction» For video and high quality audio, forward error

correction may be performed in order to protect against bit errors. This may be combined with interleaving of AAL user bits to give a more secure protection against errors.



4. Transfer of timing information

a. Synchronous residual time stamp (SRTS):

CS conveys to the receiver in the CSI field the

difference between a common clock derived

from the network and the sender’s clock

b. Adaptive clock method:

The receiver writes the received information into

a buffer and reads out from the buffer. If its clock

is fast/slow the occupancy in the buffer will be

below/over the median


36


5. Structured and unstructured data transfers

Two CS-PDU formats have been defined:

a.CS-PDU non-P format:

Constructed from 47 bytes of informationsupplied by an AAL user

b. CS-PDU P format:

Constructed from a 1-byte header and 46bytes of information supplied by an AALuser.

The header consists of a 7-bit pointer (SDTpointer) and 1 even bit parity.



ATM Adaptation Layer 2 - AAL 2

Variable Bit Rate (VBR); time sensitive traffic.

» Packetized voice and video (compressed)

Allows a cell to be transmitted before the payload is full to accommodate an application's time requirement.

Specifications completed only in 1997.

37


At the sender, AAL 2 multiplexes several

streams onto the same ATM connection

At the receiver, it de-multiplexes the date

from the connection to the individual

streams.



AAL2: The SSCS and CPS sublayers

The AAL 2 services are provided by the

convergence sublayer, which is subdivided into

the

» Service Specific Convergence Sublayer (SSCS)

» Common part sublayer (CPS). Sometimes called Common Part Convergence Sublayer (CPCS)

38


Functional model of AAL 2 (sender side)

Each stream is served by a separate SSCS

which is associated with a CID

SSCS

AAL-SAP

ATM-SAP

SSCS

SSCSCID=X

CID=Y

CID=Z

CPS


AAL2 CPS-PDU

A CPS-PDU consists of 48-bytes and forms

the payload of the ATM Cell

The CPS-PDU has a 1 byte header (called

Start Field)

The remaining 47 bytes are filled by multiple

CPS-packets and by padding bits

39


AAL2 CPS-PDU: An example with only 1 CPS-Packet

OSF: Offset - Bits to Payload

SN: Sequence Number

P: Odd Parity Check for the Start Field

CID: Channel ID - Identifies User Traffic

LI: Length Indicator - Bytes in the Payload

UUI: User-to-User Indicator - User Defined Field

HEC: error check on the 3 byte CPS-packet header

OSF SN P CID LI UUI HEC PAD

48 bytes

Start Field CPS-Packet Header

6 1 1 8 6 5 5 Variable

CPS-Packet Payload


AAL2: The Offset Field (OSF)

Used to identify the beginning of a CPS-packet. It points to the first new CPS-packet in the CPS-PDU payload

In the absence of a new CPS-packet, it points to the beginning of the pad

The value of 47 indicates that there is no beginning of a CPS-packet in the CPS-PDU.

40


AAL2: The CPS-Packet header fields

Channel identifier (CID) - 8 bits:» Identifies a channel. Same value is used for both directions.

» CIDs are allocated using the AAL negotiation procedures (ANP)

Length indicator (LI) - 6 bits:

» Default maximum length of the CPS-Packet payload is 45

bytes.

Header error control (HEC) - 5 bits:» Pattern is: x5+x2+1.

User-to-user-indication (UUI) - 3 bits:

» Used to transfer information transparently between the

peers.


AAL2 example: 2 PDUs containing multiple CPS-Packets

Example: G.729A coded voice with RTP timing» 4 bytes compressed RTP header

» 10 bytes G.729A for 8 kbps coding

» AAL fills the cells until there is room

» The third G.729A packet is splitted in two cells

» The S (OSF) field in the second cell points to the PH (CPS-Packet Header) field of the fourth packet

G.729A PHS PH PH G.729A

G.729AS PH PH

G.729A

G.729A G.729A PAD

PDU 1

PDU 2

1 3 14 14 103 3

48

41


ATM Adaptation Layers 3/4 - AAL 3/4

» Bursty (VBR) connection-oriented traffic

(AAL3).-Error messages, large file transfer like CAD or Data Backup-

» Bursty (VBR) connectionless traffic

(AAL4)(short but bursty transfer).

-LANs, Frame Relay-

» Error detection on each cell.

» Support cell multiplexing -AAL3/4 have same

SAR-


AAL 3/4: The CS-PDU format

42


ATM 3/4: The SAR-PDU Format

CPS-PDU payload

47-Bytes

TrailerHeader

SAR-PDU payloadSN CRCST RES

MIDLI

•ST segment type, 2bits

•SN sequence number, 4 bits

•RES MID reserved field or multiplexing identifier, 10 bits

•LI length indicator, 6 bits

•CRC cyclic redundancy check, 10 bits

Begin, cont., end,

single message

CPS-PDU

SAR-PDUs

SAR-PDU



» Simple and efficient adaptation layer (simplified version of AAL 3/4, uses 5

bytes header).

-For connectionless or connection-oriented VBR traffic-

» High speed LANs

» Assumes that higher layer protocol provide for error recovery

-Simplifies SAR-

» Assumes that only one message is crossing the ATM UNI at a time

-No support for cell multiplexing-


CPS-PDU payload


SAR PDU Payload

Header Trailer

CS

SAR

CPS-PDU payloadHeader Trailer

43



» Simple and efficient adaptation layer (simplified version of AAL 3/4, uses 5

bytes header).

-For connectionless or connection-oriented VBR traffic-

» High speed LANs

» Assumes that higher layer protocol provide for error recovery

-Simplifies SAR-

» Assumes that only one message is crossing the ATM UNI at a time

-No support for cell multiplexing-


CPS-PDU payload


SAR PDU Payload

Header Trailer

CS

SAR

CPS-PDU payloadHeader Trailer

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