ETSI I-ETS 300 353 ed.2 (1998-10)

SLOVENSKI STANDARD
01-december-2003
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Broadband Integrated Services Digital Network (B-ISDN); Asynchronous Transfer Mode
(ATM); Adaptation Layer (AAL) specification - type 1
Ta slovenski standard je istoveten z: I-ETS 300 353 Edition 2
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERIM
EUROPEAN I-ETS 300 353
TELECOMMUNICATION October 1998
STANDARD Second Edition
Source: NA Reference: RI/NA-052626
ICS: 33.020
Key words: B-ISDN, ATM, AAL
Broadband Integrated Services Digital Network (B-ISDN);
Asynchronous Transfer Mode (ATM);
Adaptation Layer (AAL) specification - type 1
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Internet: secretariat@etsi.fr - http://www.etsi.org
Tel.: +33 4 92 94 42 00 - Fax: +33 4 93 65 47 16
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1998. All rights reserved.

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I-ETS 300 353: October 1998
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Standards Making Support Dept." at the address shown on the title page.

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I-ETS 300 353: October 1998
Contents
Foreword .5
Introduction.5
1 Scope .7
2 Normative references.7
3 Definitions and abbreviations .7
3.1 Definitions .7
3.2 Abbreviations .8
4 AAL type 1.9
4.1 Service primitives provided by AAL type 1.9
4.1.1 AAL-UNITDATA-REQUEST.9
4.1.2 AAL-UNITDATA-INDICATION .9
4.1.3 Definition of parameters .10
4.1.3.1 DATA parameter.10
4.1.3.2 STRUCTURE parameter.10
4.1.3.3 STATUS parameter.10
4.2 Information flow across the ATM-AAL boundary .10
4.3 Primitives between the SAR sublayer and the CS.11
4.3.1 General.11
4.3.1.1 SAR-UNITDATA-INVOKE .11
4.3.1.2 SAR-UNITDATA-SIGNAL.11
4.4 Interaction with the management and control planes .11
4.4.1 Management plane.11
4.4.2 Control plane .12
4.5 Functions of AAL type 1.12
4.6 SAR sublayer .12
4.6.1 Functions of the SAR sublayer.12
4.6.2 SAR protocol .13
4.6.2.1 SN field .13
4.6.2.2 SNP field.13
4.7 Convergence Sublayer (CS) .15
4.7.1 Functions of the CS.15
4.7.1.1 Functions of the CS for circuit transport .16
4.7.1.2 Functions of the CS for video signal transport.17
4.7.1.3 Functions of the CS for voice-band signal transport.18
4.7.2 CS protocol.19
4.7.2.1 SC operations.19
4.7.2.1.1 SC operations at the transmitting end .19
4.7.2.1.2 SC operations at the receiving end.19
4.7.2.2 Source clock frequency recovery method .20
4.7.2.2.1 Adaptive clock method.20
4.7.2.2.2 SRTS method .20
4.7.2.2.3 Combination of SRTS and adaptive
clock method.23
4.7.2.3 SDT method .23
4.7.2.4 Correction methods for bit errors and/or cell losses.25
4.7.2.4.1 Correction method for bit errors.25
4.7.2.4.2 Correction method for bit errors and
cell losses without delay restrictions.26
4.7.2.4.3 Correction method for bit errors and
cell losses with delay restrictions.27
4.7.2.5 Partially filled cell method for control of SAR-PDU payload
assembly delay .30

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Annex A (informative): Illustration of the data unit naming convention. 32
Annex B (informative): Encoding and information transfer principles. 33
B.1 Cell payload field encoding. 33
B.2 AAL user information transfer. 33
Annex C (informative): Functional model for the circuit transport with AAL type 1. 35
C.1 Functional model of the SAR. 35
C.2 SDL of the SAR . 36
Annex D (informative): Parameters for AAL type 1 protocol. 38
D.1 Circuit transport . 38
D.1.1 Transport of digital channel supported by 64 kbit/s-based ISDN. 38
D.1.2 Transport of G.702 PDH circuit. 38
D.1.3 Transport of G.707 SDH circuit. 39
D.2 Video signal transport. 39
D.3 Voiceband signal transport . 40
Annex E (informative): Algorithm to detect lost and misinserted cells in AAL 1. 41
E.1 General. 41
E.2 Indications from the SAR sublayer . 41
E.3 Capabilities of the algorithms. 41
E.4 The algorithms. 41
E.4.1 Robust SN Algorithm. 41
E.4.2 Fast SN Algorithm . 43
Annex F (normative): Protocol Implementation Conformance Statement (PICS) proforma for the
AAL type 1. 46
F.1 Introduction. 46
F.2 PICS . 46
F.2.1 SAR sublayer . 46
F.2.1.1 Capabilities . 46
F.2.1.2 PDUs and PDU parameters . 47
F.2.2 CS major capabilities . 47
F.2.2.1 CS for circuit transport. 47
F.2.2.1.1 Capabilities. 47
F.2.2.1.2 Protocol parameters. 48
F.2.2.2 CS for video signal transport . 49
F.2.2.2.1 Capabilities. 49
F.2.2.2.2 Protocol parameters. 49
F.2.2.3 CS for voice band signal transport. 50
F.2.2.3.1 Capabilities. 50
F.2.2.3.2 Protocol parameters. 50
History. 51

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Foreword
This Interim European Telecommunication Standard (I-ETS) has been produced by the Network Aspects
(NA) Technical Committee of the European Telecommunications Standards Institute (ETSI).
Announcement date
Date of adoption of this I-ETS: 9 October 1998
Date of latest announcement of this I-ETS (doa): 3 months after ETSI publication
Introduction
The content of this I-ETS is derived from ITU-T Recommendation I.363.1 [8]. This I-ETS is one of a set of
I-ETSs describing different Asynchronous Transfer Mode (ATM) Adaptation Layer (AAL) types.
The AAL uses the ATM layer service and offers its layer service to the higher layers. The
connection-oriented transmission methods which provide timing relation between sending and receiving
AAL service users, are described in ITU-T Recommendation I.363.1 [8], clause 2. These methods form
the AAL type 1. They check the validity of the cell sequence count, transmit and utilize time stamp
information for source clock recovery at the receiver as a user option, optionally correct data by using
Forward Error Correction (FEC) and offer utilities to transfer structured data. Subtypes are defined for
"circuit transport", "video signal transport" and "voice-band signal transport".

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1 Scope
As ITU-T Recommendation I.363.1 [8] contains options and describes methods which can be used in
different combinations, this Interim European Telecommunication Standard (I-ETS) minimizes the options
and methods and describes a subset of the Asynchronous Transfer Mode (ATM) Adaptation Layer (AAL)
type 1 to be used in Europe.
This I-ETS describes the interactions between the AAL and the next higher layer, and the AAL and the
ATM layer, as well as AAL peer-to-peer operations.
2 Normative references
This I-ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this I-ETS only when incorporated in it by amendment or revision. For undated references the
latest edition of the publication referred to applies.
[1] ITU-T Recommendation G.702: "Digital hierarchy bit rates".
[2] ITU-T Recommendation G.707: "Network node interface for the synchronous
digital hierarchy (SDH)".
[3] ITU-T Recommendation G.711: "Pulse code modulation (PCM) of voice
frequencies".
[4] ITU-T Recommendation G.823: "The control of jitter and wander within digital
networks which are based on the 2048 kbit/s hierarchy".
[5] ITU-T Recommendation G.824: "The control of jitter and wander within digital
networks which are based on the 1544 kbit/s hierarchy".
[6] ITU-T Recommendation I.231: "Circuit-mode bearer service categories".
[7] ITU-T Recommendation I.361 (1993): "B-ISDN ATM layer specification".
[8] ITU-T Recommendation I.363.1 (1996): "B-ISDN ATM Adaptation: Type 1 AAL".
[9] ITU-T Recommendation J.82 (1995): "Transport of MPEG-2 constant bit rate
television signals in B-ISDN".
[10] ITU-T Recommendation H.310 (1995): "Broadband audiovisual communication
systems and terminals".
[11] ITU-T Recommendation H.320 (1993): "Narrow-band visual telephone systems
and terminal equipment".
[12] ITU-T Recommendation H.321: "Adaptation of H.320 visual telephone terminals
to B-ISDN environments".
[13] ITU-T Recommendation H.221 (1995): "Frame structure for a 64 to 1 920 kbit/s
channel in audiovisual teleservices".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of this I-ETS, the following definitions apply:

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ATM Adaptation Layer (AAL): The AAL uses the ATM layer service and includes multiple protocols to fit
the need of the different AAL service users. In AAL type 1 source timing recovery is provided at the
receiver.
Convergence Sublayer Indication (CSI): The CSI is a part of the Protocol Control Information (PCI) in
the SAR sublayer; it indicates a special event in the sending Convergence Sublayer (CS) entity in
combination with the Sequence Count (SC) and depending on the AAL subtype used: it supports source
clock timing recovery using the SRTS method, data structure indication using the SDT method and bit
error and cell loss recovery using Forward Error Correction (FEC).
Forward Error Correction (FEC): The FEC method is adapted to the error conditions at the ATM layer.
non-P format: Format of the SAR-PDU (Protocol Data Unit) payload, which does not carry a pointer of
the SDT method.
P format: Format of the SAR-PDU payload which carries a pointer of the SDT method.
Residual Time Stamp (RTS): The SRTS method uses the RTS value to measure and convey information
about the frequency differences between a common reference clock (derived from the network) and a
service clock. The same derived network clock is assumed to be available at both the transmitter and the
receiver.
Sequence Count (SC): This 3-bit field counts the SAR-PDUs from 0 to 7 (modulo 8).
Sequence Number (SN): The SN field consists of the 1-bit indication called CSI and a 3-bit SC in the
SAR-PDUs.
Sequence Number Protection (SNP): The SNP protects the SN by Cyclic Redundancy Check (CRC)
and parity check.
Structured Data Transfer (SDT): The SDT method supports the transmission of structured data (blocks
of user data organized in octets) by using a pointer to the start of a block.
Synchronous Residual Time Stamp (method) (SRTS): This method uses the RTS values (transferred
peer-to-peer) to recover the source service clock at the receiver side.
3.2 Abbreviations
For the purposes of this I-ETS, the following abbreviations apply:
AAL ATM Adaptation Layer
ATM Asynchronous Transfer Mode
CRC Cyclic Redundancy Check
CS Convergence Sublayer
CSI Convergence Sublayer Indication
FEC Forward Error Correction
PCI Protocol Control Information
PDU Protocol Data Unit
PICS Protocol Implementation Conformance Statement
RPOA Recognized Private Operating Agency
RTS Residual Time Stamp
SAP Service Access Point
SAR Segmentation And Reassembly (sublayer)
SC Sequence Count
SDH Synchronous Digital Hierarchy
SDL Specification and Description Language
SDT Structured Data Transfer
SDU Service Data Unit
SN Sequence Number
SNP Sequence Number Protection
SRTS Synchronous Residual Time Stamp (method)
STM-1 Synchronous Transport Module - 1

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4 AAL type 1
The AAL enhances the service provided by the ATM layer to support functions required by the next higher
layer. The AAL performs functions required by the user, control and management planes and supports the
mapping between the ATM layer and the next higher layer. The functions performed in the AAL depend
upon the higher layer requirements.
The AAL supports multiple protocols to fit the needs of the different AAL service users. The service
provided by the AAL type 1 protocol to the higher layer and the functions performed are specified in this
I-ETS.
Details of the data unit naming convention used in this I-ETS can be found in annex A.
This I-ETS describes the interactions between the AAL and the next higher layer, and the AAL and the
ATM layer, as well as AAL peer-to-peer operations.
Different combinations of SAR sublayers and CS provide different Service Access Points (SAPs) to the
layer above the AAL.
4.1 Service primitives provided by AAL type 1
The layer service capabilities provided by AAL type 1 to the AAL user are:
- transfer of service data units with a constant source bit rate and the delivery of them with the same
bit rate;
- transfer of timing information between source and destination;
- transfer of structure information between source and destination;
- indication of lost or errored information which is not recovered by AAL type 1, if needed.
At the AAL-SAP, the following primitives are used between the AAL type 1 and the AAL user. They
represent an abstract model of the interface and they are not intended to constrain implementations:
- from an AAL user to the AAL,
AAL-UNITDATA-REQUEST;
- from the AAL to an AAL user,
AAL-UNITDATA-INDICATION.
An AAL-UNITDATA-REQUEST primitive at the local AAL-SAP results in an AAL-UNITDATA-INDICATION
primitive at its peer AAL-SAP.
4.1.1 AAL-UNITDATA-REQUEST
AAL-UNITDATA-REQUEST:
- (DATA [mandatory];
- STRUCTURE [optional]).
The AAL-UNITDATA-REQUEST primitive requests the transfer of the AAL-SDU, i.e. contents of the DATA
parameter, from the local AAL entity to its peer entity. The length of the AAL-SDU is constant and the time
interval between two consecutive primitives is constant. These two constants depend upon the specific
AAL service provided to the AAL user.
4.1.2 AAL-UNITDATA-INDICATION
AAL-UNITDATA-INDICATION:
- (DATA [mandatory];
- STRUCTURE [optional];
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- STATUS [optional]).
An AAL user is notified by the AAL that the AAL-SDU from its peer is available (i.e. via the contents of the
DATA parameter). The length of the AAL-SDU shall be constant and the time interval between two
consecutive primitives shall be constant. These two constants depend upon the specific AAL service
provided to the AAL user.
4.1.3 Definition of parameters
4.1.3.1 DATA parameter
(Mandatory).
The DATA parameter carries the AAL-SDU to be sent or delivered. Its size depends on the specific AAL
service used.
4.1.3.2 STRUCTURE parameter
(Optional use).
The STRUCTURE parameter can be used when the user data stream to be transferred to the peer AAL
entity is organized into groups of octets. The length of the structured block is fixed for each instance of the
AAL service. The length is an integer multiple of one octet. An example of the use of this parameter is to
support circuit mode bearer services of the 64 kbit/s based ISDN. If the optional parameter is present, the
two values of the STRUCTURE parameter are:
- START; and
- CONTINUATION.
The value START is used when the DATA is the first part of a structured block which can be composed of
consecutive DATA. In other cases, the STRUCTURE parameter is set to CONTINUATION. The use of the
STRUCTURE parameter depends upon the specific AAL service provided. The use of this parameter is
agreed prior to or at the connection establishment between the AAL user and the AAL.
4.1.3.3 STATUS parameter
(Local optional use).
The STATUS parameter identifies that the DATA is judged to be non-errored or errored. The STATUS
parameter has two values:
- VALID; and
- INVALID.
The INVALID status can also indicate that the DATA is a dummy value. The use of the STATUS
parameter and the choice of dummy value depend upon the specific AAL service provided. The use of this
parameter is agreed prior to or at the connection establishment between the AAL user and the AAL.
4.2 Information flow across the ATM-AAL boundary
ITU-T Recommendation I.361 [7] describes the primitives exchanged between the ATM layer and the
AAL. This subclause describes the usage of these primitives for AAL type 1.
The AAL receives from the ATM layer the information in the form of a 48 octet ATM Service Data Unit
(ATM-SDU). The AAL passes to the ATM layer information in the form of a 48 octet ATM SDU.
The submitted CLP (Cell Loss Priority) in the request primitive is set to the high priority by the AAL
transmitter. The value of the receive loss priority in the indication primitive is ignored by the AAL receiver.
The AUU (ATM-user-to-ATM-user) parameter is set to "0" in the request primitive. Future procedures may
require that the AUU parameter can be set to "0" or "1". Such usage is reserved for future standardization.

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The congestion indication is ignored by the AAL receiver.
The encoding principles for mapping information between the ATM layer and AAL type 1 are given in
annex B.
4.3 Primitives between the SAR sublayer and the CS
4.3.1 General
These primitives model the exchange of information between the SAR sublayer and the Convergence
Sublayer (CS). As there exists no Service Access Point (SAP) between the sublayers of the AAL type 1,
the primitives are called "invoke" and "signal" instead of the conventional "request" and "indication" to
highlight the absence of the SAP. Functional model and SDL is given in annex C.
4.3.1.1 SAR-UNITDATA-INVOKE
SAR-UNITDATA-INVOKE at the AAL type 1 transmitter has the following parameters:
- Interface data: This parameter specifies the interface data unit passed from the CS to the SAR entity.
The interface data is 47 octets, and represents a SAR-PDU payload;
- CSI: The Convergence Sublayer Indication (CSI), either "0" or "1", is passed from the CS to the SAR
entity;
- Sequence count: The sequence count value is passed from the CS to the SAR entity. The value of
sequence count starts with 0, is incremented sequentially and is numbered modulo 8.
4.3.1.2 SAR-UNITDATA-SIGNAL
SAR-UNITDATA-SIGNAL at the AAL type 1 receiver has the following parameters:
- Interface data: This parameter specifies the interface data unit passed from the SAR to the CS entity.
The interface data is 47 octets, and represents a SAR-PDU payload;
- CSI: The CSI is passed from the SAR to CS entity, regardless of the check status (valid or invalid);
- Sequence count: The sequence count value is passed from the SAR to CS entity, regardless of the
check status (valid or invalid);
- Check status: This parameter specifies the status of the sequence count and CSI, and has the value
of either valid or invalid.
4.4 Interaction with the management and control planes
Currently no interactions are standardized.
4.4.1 Management plane
For example, the following indications may be passed from the user plane to the management plane:
- errors in the transmission of user information;
- lost and misinserted cells;
- cells with errored AAL-PCI;
- loss of timing and synchronization;
- buffer underflow and overflow.

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4.4.2 Control plane
Currently no interactions are standardized.
4.5 Functions of AAL type 1
The following functions may be performed in the AAL type 1 in order to enhance the ATM layer service:
a) segmentation and reassembly of user information;
b) blocking and deblocking of user information;
c) handling of cell delay variation;
d) handling of cell payload assembly delay;
e) handling of lost and misinserted cells;
f) source clock frequency recovery at the receiver;
g) recovery of the source data structure at the receiver;
h) monitoring of AAL-PCI for bit errors;
i) handling of AAL-PCI bit errors;
j) monitoring of user information field for bit errors and possible corrective action.
NOTE: For some AAL users, the end-to-end QoS may be monitored. This may be achieved by
calculating a CRC for the CS-PDU payload, carried in one or more cells, and
transmitting the CRC results in the CS-PDU or by the use of OAM cells.
4.6 SAR sublayer
4.6.1 Functions of the SAR sublayer
The SAR sublayer functions are performed on an ATM-SDU basis:
a) mapping between CS-PDU and SAR-PDU:
- the SAR sublayer at the transmitting end accepts a 47 octet block of interface data from the
CS, and then prepends a one octet SAR-PDU header to each block to form the SAR-PDU;
- the SAR sublayer at the receiving end receives the 48 octet block of data from the ATM layer,
and then separates the SAR-PDU header. The 47 octet block of SAR-PDU payload (interface
data) is passed to the CS;
b) indication of existence of CS function:
- the SAR sublayer has the capability to indicate the existence of a CS function. Associated
with each 47 octet SAR-PDU payload, it receives this indication (CSI) from the CS and
conveys it to the peer CS entity;
c) sequence numbering:
- associated with each SAR-PDU payload, the SAR sublayer receives a sequence count value
from the CS. At the receiving end, it passes the SC value to the CS. The CS may use these
SC values to detect lost or misinserted SAR-PDU payloads (corresponding to lost or
misinserted ATM cells);
d) error protection:
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- the SAR sublayer protects the SC value and the CS indication against bit errors. It informs
the receiving CS by the value of check status whether the SC value and/or the CS indication
are errored.
4.6.2 SAR protocol
The SAR-PDU header together with the 47 octets of the SAR-PDU payload comprises the 48 octet
ATM-SDU (cell information field). The size and positions of the fields in the SAR-PDU are given in
figure 1.
Cell header SN field SNP field SAR-PDU payload
4 bits 4 bits 47 octets
SAR-PDU header
SAR-PDU (48 octets)
Figure 1: SAR-PDU format of AAL type 1
4.6.2.1 SN field
The SN field is divided into two subfields as shown in figure 2. The SC field carries the SC value provided
by the CS. The CSI bit carries the CS indication provided by the CS. The default value of the CSI bit is "0".
The least significant bit of the SC value is right justified in the SC field.
CSI bit Sequence count field (3 bits)
SN field (4 bits)
Figure 2: SN field format
4.6.2.2 SNP field
The SNP field provides error detection and correction capabilities over the SAR-PDU header. The format
of this field is given in figure 3. A two step approach is used for the protection:
1) the SN field is protected by a 3 bit CRC code;
2) the resulting 7 bit code word is protected by an even parity bit, i.e. the parity bit is set such that the
8 bit SAR-PDU header has an even parity.
Even parity
CRC field (3 bits)
bit
SNP field (4 bits)
Figure 3: SNP field format
The receiver is capable of either single-bit error correction or multiple-bit error detection:
a) operations at transmitting end:
- the transmitter computes the CRC value across the first 4 bits of the SAR-PDU header and
inserts the result in the CRC field.

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The notation used to describe the CRC is based on the property of cyclic codes. The
elements of an n-element code word are thus the coefficients of a polynomial of order n-1. In
this application, these coefficients can have the value 0 or 1 and the polynomial operations
are performed using modulo 2 operations. For example a code vector such as 1011 can be
represented by the polynomial P(x) = x + x + 1. The polynomial representing the content of
the SN field is generated using the left most bit of the SN field as the coefficient of the
highest order term.
The CRC field consists of three bits. It shall contain the remainder of the division (modulo 2)
3 3
by the generator polynomial x + x + 1 of the product x multiplied by the content of the SN
field. The coefficient of the x term in the remainder polynomial is left justified in the CRC
field;
- after completing the above operations, the transmitter inserts the even parity bit (i.e. the sum
of the eight bits shall be even);
b) operations at receiving end:
The receiver has two different modes of operation: correction mode and detection mode. These
modes are related as shown in figure 4. The default mode is the correction mode, which provides
for single-bit error correction. At initialization, the receiver is set up in this default mode;
No error detected Error detected
(valid SN) (invalid SN)
No error detected (valid SN)
Correction Detection
Mode Mode
Single-bit error detected
(valid SN after correction)
Multi-bit error detected
(invalid SN)
Figure 4: SNP - receiver modes of operation
- the receiver examines each SAR-PDU header by checking the CRC and the even parity. If a
header error is detected, the action taken depends on the state of the receiver. In the
"Correction Mode", only single-bit errors can be corrected and the receiver switches to
"Detection Mode". In "Detection Mode", all SAR-PDU headers with detected errors are
declared to have an invalid SN; however, when a SAR-PDU header is examined and found
not to be in error, the receiver switches to "Correction Mode";
- tables 1 and 2 give the detailed mandatory operations of the receiver in the "Correction
Mode" and "Detection Mode", respectively. The operation is based on the combined validity
of the CRC and the parity bit.
The receiver shall convey the sequence count and the CS indication to the CS together with SN check
status (valid or invalid).
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Table 1: Operations in Correction Mode
CRC syndrome Parity Action on current SN + SNP Reaction for next SN + SNP
Zero No violation No corrective action. Continue in Correction Mode
Declare SN valid.
Non-zero Violation Single bit correction based on syndrome. Switch to Detection Mode
Declare SN valid.
Zero Violation Correct Parity bit. Switch to Detection Mode
Declare SN valid.
Non-zero No violation No corrective action: Switch to Detection Mode
multi-bit errors are uncorrectable.
Declare SN invalid.
Table 2: Operations in Detection Mode
CRC syndrome Parity Action on current SN + SNP Reaction for next SN + SNP
Zero No violation No corrective action. Switch to Correction Mode
Declare SN valid.
Non-zero Violation No corrective action. Continue in Detection Mode
Declare SN invalid.
Zero Violation No corrective action. Continue in Detection Mode
Declare SN invalid.
Non-zero No violation No corrective action. Continue in Detection Mode
Declare SN invalid.
4.7 Convergence Sublayer (CS)
4.7.1 Functions of the CS
Depending on the specific AAL service provided, the CS may include the following functions:
a) blocking of user information to form a 47 octet block of SAR-PDU payload is performed at this
sublayer. If no octet interleaving is applied, the AAL-SDUs are sequentially concatenated. They are
placed left justified in the 47 octet block beginning from the first octet available for user information.
The deblocking function is the reverse of the blocking function. It segments the user information
into a stream of AAL-SDUs again;
b) handling of cell delay variation for delivery of AAL-SDUs to an AAL user at a constant bit rate;
c) handling of SAR-PDU payload assembly delay may be performed by partially filling the SAR-PDU
payload;
d) processing of SC. The SC value and its error check status provided by the SAR sublayer can be
used by the CS to detect cell loss and misinsertion. Further handling of lost and misinserted cells is
also performed in this sublayer;
e) the CS can utilize the CS indication provided by the SAR sublayer to support CS functions for some
AAL users. When the CS indication is not used, the CSI bit is set to "0" by the transmitter, and no
further CS action related to that indication is performed at the receiver, i.e., the CS receiver ignores
the received CSI value;
f) for AAL users requiring recovery of source clock frequency at the destination end, the AAL can
provide a mechanism for a timing information transfer;
g) transfer of structure information between source and destination;
h) for video signal transport, FEC may be performed to protect against bit errors. This may be
combined with interleaving of AAL user bits (e.g. octet interleaving) to correct cell losses;

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i) The CS may generate reports giving the status of end-to-end performance as deduced by the AAL.
The performance measures in these reports could be based on:
- events of lost and misinserted cells;
- buffer underflow and overflow;
- bit error events.
AAL type 1 protocol aims at having as many common procedures as possible among various types of
CBR services in an ATM network. As such, AAL type 1 CS protocol is somewhat of a tool kit, whereby a
specific higher layer needs to choose procedures given in this I-ETS, taking account of required service
features (e.g., synchronous or asynchronous transport), required performance (e.g., error and delay
characteristics at the AAL service boundary), and anticipated network performance (e.g. cell losses and
delay variations).
The following subclauses describe CS functions needed for three layer services, i.e., circuit transport,
video signal transport and voiceband signal transport. These subclauses also refer to a specific procedure
which is defined in subclause 4.7.2, where the description of each procedure is independent from CS
functions. These four layer services and associated description of required procedures are general and
not exhaustive. Annex D gives informative and example parameters, i.e., a set of procedures and options,
for some specific AAL type 1 services. Having this structural description, this I-ETS gives the ground for a
generic protocol to support a large number of CBR services.
4.7.1.1 Functions of the CS for circuit transport
The following functions support both asynchronous and synchronous circuit transport.
Asynchronous circuit transport provides transport of signals from constant bit rate sources whose clocks
are not frequency-locked to a network clock. Examples are ITU-T Recommendation G.702 [1] signals at
1 544 kbit/s, 2 048 kbit/s, 6 312 kbit/s, 8 448 kbit/s, 32 064 kbit/s, 34 368 kbit/s and 44 736 kbit/s.
Synchronous circuit transport provides transport of signals from constant bit rate sources whose clocks
are frequency-locked to a network clock. Examples are signals at 64 kbit/s, 384 kbit/s, 1 536 kbit/s and
1 920 kbit/s as described in ITU-T Recommendation I.231 [6] or conveyance of Synchronous Digital
Hierarchy (SDH) signals described in ITU-T Recommendation G.707 [2].
a) handling of AAL user information:
- the default value for the length of the AAL-SDU is one bit;
- for those AAL users which require transfer of structured data, e.g. 8 kHz structured data for
circuit mode bearer services of the 64 kbit/s based ISDN, the STRUCTURE parameter option
of the primitives defined in subclause 4.1.3.2 and the SDT method described in
subclause 4.7.2.3 shall be provided and used. When the STRUCTURE parameter option is
used without the SRTS method the length of the AAL-SDU is one octet. The SRTS method is
described in subclause 4.7.2.2.2;
b) handling of cell delay variation:
- a buffer is used to support this function. The size of this buffer is dependent upon ATM
performance specifications (note). In the event of buffer underflow, the CS maintains bit
count integrity by inserting the appropriate number of dummy bits and dropping the
corresponding number of bits which follow this insertion. The inserted dummy bits are
all "1"s; In the event of buffer overflow, the CS drops the appropriate number of bits;
NOTE: The ATM performance specifications are out of the scope of this I-ETS.
c) handling of lost and misinserted cells:
- the SC values are further processed at this sublayer to detect lost and misinserted cells. The
performance specifications for the processing of SC values are not yet defined. Detected
misinserted cells are discarded. An informative example of processing is given in annex E;

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- in order to maintain the bit count integrity of the AAL user information, lost cells detected by
buffer underflow and SC processing are compensated by inserting the appropriate number of
dummy SAR-PDU payloads; the content of this dummy SAR-PDU payload is all "1"s;
d) handling of timing relation:
- this function is required for delivery of AAL-SDUs to an AAL user at a constant bit rate;
- recovered source clock shall have satisfactory jitter and wander performance. The jitter and
wander performance for ITU-T Recommendation G.702 [1] signals is specified in
ITU-T Recommendations G.823 [4] and G.824 [5] for which the CS procedure to be used
(the SRTS method) is described in subclause 4.7.2.2.2. For signals other than
ITU-T Recommendation G.702 [1] signals possible timing recovery methods currently
identified are: SRTS, adaptive clock method and a combination of SRTS and adaptive clock
method.
4.7.1.2 Functions of the CS for video signal transport
The following functions support transport of video signals for interactive and distributive services:
a) handling of AAL user information:
- the length of the AAL-SDU is one octet when utilizing the correction methods described in
subclause 4.7.2.4;
- for those AAL users which require transfer of structured data, the STRUCTURE parameter
option of primitives defined in subclause 4.1.3.2 and the SDT method described in
subclause 4.7.2.3 shall be provided and used. When the STRUCTURE parameter option is
used without the SRTS method the length of the AAL-SDU is one octet;
- depending on the type of AAL service provided (i.e. the interface to the AAL user) the
STATUS parameter defined in subclause 4.1.3.3 is passed to the AAL user to facilitate
further picture processing, e.g. error concealment or not;
b) handling of cell delay variation:
- a buffer is used to support this function. The size of this buffer is dependent upon ATM
performance specifications (note). In the event of buffer underflow, the CS maintains bit
count integrity by inserting the appropriate number of dummy bits and dropping the
corresponding number of bits which follow the insertion. The inserted dummy bits are all "1".
In the event of buffer overflow, the CS drops the appropriate number of bits;
NOTE: The ATM performance specifications are out of the scope of this I-ETS.
c) handling of lost and misinserted cells:
- the SC values are further processed at this sublayer to detect lost and misinserted cells. The
performance specifications for the processing of SC values are not yet defined. Detected
misinserted cells are discarded. An informative example of processing is given in annex E;
- information in lost cells may be recovered by the mechanism described in e);
- in order to maintain the bit count integrity of the AAL user information, lost cells detected by
buffer underflow and SC processing are compensated by inserting the appropriate number of
dummy SAR-PDU payloads. The content of this dummy SAR-PDU payload is all "1"s;
d) handling of timing relation:
- this function is required for delivery of AAL-SDUs to an AAL user at a constant bit rate;
- some AAL users may require source clock frequency recovery, e.g. recovery at the receiving
end of camera clock frequency which is not locked to the network clock. Possible timing

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recovery methods currently identified are: SRTS, adaptive clock method; the CS procedures
available for that purpose are given in subclause 4.7.2.2;
e) correction of bit errors and lost cells:
- this is an optional function provided for those AAL users requiring error correction, i.e., bit
error and/or cell loss performance better than that provided by the ATM and physical layer.
Examples are unidirectional video services for contribution and distribution. When this
function is performed, the CS procedure described in subclause 4.7.2.4 shall be applied.
4.7.1.3 Functions of the CS for voice-band signal transport
The following functions support transport of a single voice-band signal, i.e. one. 64 kbit/s A-law or μ-law
coded ITU-T Recommendation G.711 [3] signal:
a) handling of AAL user information:
- the length of the AAL-SDU is one octet; forty seven consecutive AAL-SDUs constitute one
SAR PDU payload, i.e., partially filled cells are not used. The CS provides structured data
transfer with single octet delineation, i.e., the pointer is not used;
b) handling of cell delay variation:
- a buffer is used to support this function. The size of this buffer is dependent upon ATM
performance sp
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