SIST I-ETS 300 353 E1:2003

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 1
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 April 1995
STANDARD
Source: ETSI TC-NA Reference: DI/NA-052617
ICS: 33.080
B-ISDN, ATM
Key words:
Broadband Integrated Services Digital Network (B-ISDN);
Asynchronous Transfer Mode (ATM)
Adaptation Layer (AAL) specification - type 1
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
F-06921 Sophia Antipolis CEDEX - FRANCE
Postal address:
650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Office address:
c=fr, a=atlas, p=etsi, s=secretariat - secretariat@etsi.fr
X.400: Internet:
Tel.: +33 92 94 42 00 - Fax: +33 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 1995. All rights reserved.
New presentation - see History box

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I-ETS 300 353: April 1995
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 Editing and Committee Support Dept." at the address shown on the title page.

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I-ETS 300 353: April 1995
Contents
Foreword .5
Introduction.5
1 Scope .7
2 Normative references.7
3 Definitions and abbreviations .8
3.1 Definitions .8
3.2 Abbreviations .9
4 AAL type 1.9
4.1 Service primitives provided by AAL type 1.10
4.1.1 AAL-UNITDATA-REQUEST.10
4.1.2 AAL-UNITDATA-INDICATION .10
4.1.3 Definition of parameters .10
4.1.3.1 DATA parameter.10
4.1.3.2 STRUCTURE parameter.11
4.1.3.3 STATUS parameter.11
4.2 Interaction with the management and control planes .11
4.2.1 Management plane.11
4.2.2 Control plane .11
4.3 Functions of AAL type 1.12
4.4 SAR sublayer .12
4.4.1 Functions of the SAR sublayer.12
4.4.2 SAR protocol .13
4.4.2.1 SN field .13
4.4.2.2 SNP field.13
4.5 CS.15
4.5.1 Functions of the CS.15
4.5.1.1 Functions of the CS for circuit transport .16
4.5.1.2 Functions of the CS for video signal transport.17
4.5.1.3 Functions of the CS for voice-band signal transport.18
4.5.2 CS protocol.18
4.5.2.1 SC operations.18
4.5.2.1.1 SC operations at the transmitting end .18
4.5.2.1.2 SC operations at the receiving end.19
4.5.2.2 Source clock frequency recovery method .19
4.5.2.2.1 SRTS method .19
4.5.2.2.1.1 General .19
4.5.2.2.1.2 Choice of parameter .21
4.5.2.2.1.3 Network clocks.21
4.5.2.2.1.4 Transport of the RTS .21
4.5.2.2.1.5 Plesiochronous network operation.22
4.5.2.2.2 Adaptive clock method.22
4.5.2.2.3 Combination of SRTS and adaptive
clock method.22
4.5.2.3 SDT method .22
4.5.2.4 Correction method for bit errors and cell losses for
unidirectional video services.23
4.5.2.5 Partially filled cells.24
Annex A (informative): Illustration of the data unit naming convention .25

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Annex B (informative): Functional model for the circuit transport with AAL type 1. 26
B.1 Transmitter . 26
B.2 Receiver. 26
Annex C (informative): Algorithm to detect lost and misinserted cells in AAL 1. 29
C.1 Side effects. 29
C.2 Indications from the SAR sublayer . 29
C.3 Limits of the algorithm . 29
C.4 The algorithm. 29
Annex D (normative): Protocol Implementation Conformance Statement (PICS) proforma for the
AAL type 1. 32
D.1 Introduction. 32
D.2 PICS . 33
D.2.1 SAR sublayer . 33
D.2.1.1 Capabilities . 33
D.2.1.2 PDUs and PDU parameters . 33
D.2.2 CS major capabilities . 33
D.2.2.1 CS for circuit transport. 34
D.2.2.1.1 Capabilities. 34
D.2.2.1.2 Protocol parameters. 35
D.2.2.2 CS for video signal transport . 36
D.2.2.2.1 Capabilities. 36
D.2.2.2.2 Protocol parameters. 36
D.2.2.3 CS for voice band signal transport. 37
D.2.2.3.1 Capabilities. 37
D.2.2.3.2 Protocol parameters. 37
History. 38

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I-ETS 300 353: April 1995
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).
NOTE: This I-ETS was submitted to the Public Enquiry phase of the ETSI standards approval
procedure as a draft ETS. Following resolution of the comments received, the
document was converted into an I-ETS. The annex containing Specification and
Description Language (SDL) diagrams for the circuit transport with AAL type 1 has
been removed, and may be re-inserted in a later version of this standard.
An ETSI standard may be given I-ETS status either because it is regarded as a provisional solution ahead
of a more advanced standard, or because it is immature and requires a "trial period". The life of an I-ETS
is limited to three years after which it can be converted into an ETS, have it's life extended for a further
two years, be replaced by a new version, or be withdrawn.
Announcement date
Date of latest announcement of this I-ETS (doa): 31 July 1995
Introduction
The content of this I-ETS is derived from ITU-T Recommendation I.363 [10]. This I-ETS is one of a set of
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 [10], §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 [10] 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.
In addition, a functional model for the circuit transport transfer of asynchronous data, i.e. Synchronous
Residual Time Stamp (SRTS) method in combination with Structured Data Transfer (SDT), is given in
annex B.
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. This I-ETS is based on the classification and the AAL
functional organization described in ITU-T Recommendation I.362 [9].
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.709: "Synchronous multiplexing structure".
[3] ITU-T Recommendation G.711: "Pulse code modulation (PCM) of voice
frequencies".
[4] ITU-T Recommendation G.722: "7 kHz audio-coding within 64 kbit/s".
[5] ITU-T Recommendation G.823: "The control of jitter and wander within digital
networks which are based on the 2048 kbit/s hierarchy".
[6] ITU-T Recommendation G.824: "The control of jitter and wander within digital
networks which are based on the 1544 kbit/s hierarchy".
[7] ITU-T Recommendation I.231: "Circuit-transport-mode bearer service
categories".
[8] ITU-T Recommendation I.361 (1993): "B-ISDN ATM layer specification".
[9] ITU-T Recommendation I.362 (1993): "B-ISDN ATM adaptation layer functional
description".
[10] ITU-T Recommendation I.363 (1993): "B-ISDN ATM adaptation layer
specification".
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3 Definitions and abbreviations
3.1 Definitions
For the purposes of this I-ETS, the following definitions apply:
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.

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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 Transfer Mode - 1
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. This I-ETS is based on the classification and the AAL
functional organization described in ITU-T Recommendation I.362 [9].
Different combinations of SAR sublayers and CS provide different Service Access Points (SAPs) to the
layer above the AAL. In some applications the SAR and/or CS may be empty.
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. See ITU-T
Recommendation I.361 [8] for definition of ATM layer services and description of primitives provided by
the ATM layer.
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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],
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.
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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 Interaction with the management and control planes
Currently no interactions are standardized.
4.2.1 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.
4.2.2 Control plane
Currently no interactions are standardized.

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4.3 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) blocking and deblocking of user information;
b) handling of cell delay variation;
c) handling of cell payload assembly delay;
d) handling of lost and misinserted cells;
e) source clock frequency recovery at the receiver;
f) recovery of the source data structure at the receiver;
g) monitoring of AAL-PCI for bit errors;
h) handling of AAL-PCI bit errors;
i) monitoring of user information field for bit errors and possible corrective action.
4.4 SAR sublayer
4.4.1 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 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 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 from the CS and conveys it
to the peer CS entity. The use of this indication by the CS is optional;
c) sequence numbering:
- associated with each SAR-PDU payload, the SAR sublayer receives a SC 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:
- the SAR sublayer protects the SC value and the CS indication against bit errors. It informs
the receiving CS when the SC value and the CS indication are errored and can not be
corrected.
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4.4.2 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
4.4.2.1 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
4.4.2.2 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 check bit.
Even parity
CRC field (3 bits)
bit
SNP field (4 bits)
Figure 3: SNP field format
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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;
- 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.
4.5 CS
4.5.1 Functions of the CS
Depending on the specific AAL service provided, the CS may include the following functions. To perform
some of these functions, the CS will need a clock. The CS can utilize the CS indication provided by the
SAR sublayer to support CS functions for some AAL users:
a) handling of cell delay variation for delivery of AAL-SDUs to an AAL user at a constant bit rate;
b) 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;
c) for AAL users requiring recovery of source clock frequency at the destination end, the AAL can
provide a mechanism for a timing information transfer;
d) transfer of structure information between source and destination;
e) 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 give more secure protection
against errors.
The following subclauses identify the functions of the CS for individual layer services of AAL type 1.

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4.5.1.1 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 Mbit/s, 2,048 Mbit/s, 6,312 Mbit/s, 8,448 Mbit/s, 32,064 Mbit/s, 34,368 Mbit/s and 44,736 Mbit/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 [7] or conveyance of Synchronous Digital
Hierarchy (SDH) signals described in ITU-T Recommendation G.709 [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 4.1.1.3.2 and the SDT method described in subclause
4.5.2.3 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 4.5.2.2.1;
b) handling of cell delay variation:
- a buffer is used to support this function. The size of this buffer is dependent upon ATM
1)
performance specifications . 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;
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 C;
- 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;
- the handling of timing relation for asynchronous circuit transport is referred to as source clock
frequency recovery. Recovered source clock shall have satisfactory jitter performance. The
jitter performance for ITU-T Recommendation G.702 [1] signals is specified in ITU-T
Recommendations G.823 [5] and G.824 [6], for which the CS procedure to be used (the
SRTS method) is described in subclause 4.5.2.2.1. 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.

1)
The ATM performance specifications are out of the scope of this I-ETS.

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4.5.1.2 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;
- for those AAL users which require transfer of structured data, the STRUCTURE parameter
option of primitives defined in subclause 4.1.1.3.2 and the SDT method described in
subclause 4.5.2.3 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 4.1.1.3.3 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
2)
performance specifications . 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.
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 C;
- 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
recovery methods currently identified are; SRTS, adaptive clock method;
e) correction of bit errors and lost cells:
- this is an optional function provided for those AAL users requiring bit error and cell loss
performance better than that provided by the ATM layer. Examples are unidirectional video
services for contribution and distribution. When this function is performed, the CS procedure
described in subclause 4.5.2.4 shall be applied.

2)
The ATM performance specifications are out of the scope of this I-ETS.

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4.5.1.3 Functions of the CS for voice-band signal transport
The following functions support transport of voice-band signals, e.g. 64 kbit/s A-law and μ-law coded ITU-
T Recommendation G.711 [3] signals, and 64 kbit/s ITU-T Recommendation G.722 [4] signals:
NOTE: For transporting signals of speech and 3,1 kHz audio bearer services as specified in
64 kbit/s ISDN, the need for A-law to μ-law conversion is identified. This conversion
function is outside the scope of this AAL.
a) handling of AAL user information:
- the length of the AAL-SDU is one octet;
b) handling of cell delay variation:
- a buffer is used to support this function. The size of this buffer is dependent upon ATM
3)
performance specifications . 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". In the event of
buffer overflow, the CS drops the appropriate number of bits;
c) handling of lost and misinserted cells:
- the detection of lost and misinserted cells, if needed, may be provided by processing the SC
values. The monitoring of the buffer fill level can also provide an indication of 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 C.
d) handling of timing relation:
- some AAL users may require source clock frequency recovery. A possible method currently
identified is the adaptive clock method.
4.5.2 CS protocol
The following subclauses describe CS procedures to be provided for implementing CS functions. The use
of each procedure depends on the required CS functions and is given in subclauses 4.5.1.1 to 4.5.1.3.
4.5.2.1 SC operations
4.5.2.1.1 SC operations at the transmitting end
At the transmitting end, the CS shall provide the SAR with a SC value and a CS indication associated with
each SAR-PDU payload. The count value shall start with 0, be incremented sequentially and be numbered
modulo 8.
3)
The ATM performance specifications are out of the scope of this I-ETS.

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4.5.2.1.2 SC operations at the receiving end
At the receiving end, the CS receives from the SAR the following information associated with each
SAR-PDU payload:
- SC;
- Convergence Sublayer Indication (CSI);
- check status of the SN.
The use of SC values and CS indications will be specified on a service specific basis. See subclause 4.4.2
for details about the calculation of the check status.
The CS processing at the receiving end shall identify lost and misinserted SAR-PDU payloads.
CS processing shall identify the following conditions:
- SAR-PDU payload sequence normal (i.e. in correct sequence);
- SAR-PDU payload loss;
- SAR-PDU payload misinsertion.
Processing of SC values may provide information to CS layer management entities, as required. Some
examples are:
- location of lost SAR-PDU payload in the incoming SAR-PDU stream;
- number of consecutive SAR-PDU payloads lost;
- identification of misinserted SAR-PDU payloads.
NOTE: Processing of SC values may be subject to performance specifications. The
performance specifications will be applied on an AAL service specific basis.
4.5.2.2 Source clock frequency recovery method
4.5.2.2.1 SRTS method
4.5.2.2.1.1 General
The SRTS method uses the RTS to measure and convey information about the frequency difference
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. If the common network
reference clock is unavailable (e.g. when working between different networks which are not synchronized),
then the asynchronous clock recovery method is in a mode of operation associated with "Plesiochronous
network operation" which is described in subclause 4.5.2.2.1.5. The SRTS method is capable of meeting
the jitter specifications of the 2,048 Mbit/s hierarchy in ITU-T Recommendation G.823 [5] and the 1,544
Mbit/s hierarchy in ITU-T Recommendation G.824 [6].
The following is a description of the SRTS method. The description uses the notation below:
fs service clock frequency;
fn network clock frequency, e.g. 155,52 MHz;
fnx derived network clock frequency, fnx=fn/x, where x is an integer defined by an
inequality (see formula {1});
N period of RTS in cycles of the service clock of frequency fs;
T period of the RTS in seconds;

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M(Mnom, Mmax, Mmin) number of fnx cycles within a (nominal, maximum, minimum) RTS period;
Mq largest integer smaller than or equal to M.
The SRTS concept is illustrated in figure 5. In a fixed duration T measured by N service clock cycles, the
number of derived network clock cycles Mq is obtained at the transmitter. If Mq is transmitted to the
receiver, the service clock of the source can be reconstructed by the receiver, since it has the necessary
information: fnx, Mq and N. However, Mq is actually made up of a nominal part and a residual part. The
nominal part Mnom corresponds to the nominal number of fnx cycles in T seconds and is fixed for the
service. The residual part conveys the frequency difference information as well as the effect of the
quantization and, therefore, can vary. Since the nominal part is a constant, it can be assumed that the
nominal part of Mq is available at the receiver. Only the residual part of Mq is transmitted to the receiver.
tolerance
N cycles T seconds
fs
t
Mq
Mm in Mnom Mm ax
fnx
t
yy
p
Figure 5: The concept of SRTS
A simple way of representing the residual part of Mq is by means of the RTS, whose generation is shown
in figure 6. Counter Ct is a p-bit counter which is continuously clocked by the derived network clock. The
output of counter Ct is sampled every N service clock cycles. This p-bit sample is the RTS.
T
Counter A
fs RTS
Latch
divided by N
p-bit
fn fnx
1 / x
counter Ct
Figure 6: Generation of RTS
With a knowledge of the RTS and the nominal part of Mq at the receiver, Mq is completely specified. Mq is
used to produce a reference timing signal for a Phase-Locked Loop to obtain the service clock.

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