SIST ETS 300 481 E1:2003

SLOVENSKI STANDARD
01-december-2003
Terminalska oprema (TE) – Enota za združevanje kanalov (CAU) B v digitalnem
omrežju z integriranimi storitvami (ISDN) – Postopki in terminalske zahteve
Terminal Equipment (TE); Integrated Services Digital Network (ISDN); B-Channel
Aggregation Unit (CAU); Procedures and terminal requirements
Ta slovenski standard je istoveten z: ETS 300 481 Edition 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
35.180 Terminalska in druga IT Terminal and other
periferna oprema IT peripheral equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 481
TELECOMMUNICATION July 1996
STANDARD
Source: ETSI TC-TE Reference: DE/TE-04029
ICS: 33.080
Key words: ISDN, channel aggregation
Terminal Equipment (TE);
Integrated Services Digital Network (ISDN)
B-Channel Aggregation Unit (CAU);
Procedures and terminal requirements
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
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.fr
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 1996. All rights reserved.

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ETS 300 481: July 1996
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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Contents
Foreword .5
1 Scope .7
2 Normative references.7
3 Definitions.7
4 Symbols and abbreviations .8
4.1 Symbols .8
4.2 Abbreviations .9
5 General.10
5.1 Application of this ETS.10
5.1.1 Intercommunication of audiovisual equipments .10
5.1.1.1 Case A.10
5.1.1.2 Case B.11
5.1.1.3 Case C.11
5.1.2 Intercommunication of non-audiovisual equipments - Case D.12
5.2 Definition of the modes of transmission on the Multiple Channel (MC) side .12
5.3 General description of the aggregation process.15
5.4 CAU functionality .16
6 Choice of ISO Aggregation or H.244 Aggregation procedure .16
7 H.244 Aggregation procedure .19
7.1 Call control.19
7.1.1 Initial channel.19
7.1.2 Additional channels .19
7.2 Channel synchronisation.19
7.3 In-band procedure using ITU-T Recommendation H.221 BAS codes.19
7.3.1 Control of transfer rate .19
7.3.2 Determination of the transfer-rate capabilities to be transmitted on the MC
side.20
7.3.3 Determination of the transfer-rate capabilities to be transmitted on the SC
side.20
7.3.4 Capability sets transmitted from a CAU .20
7.3.4.1 Modification of SCE capset for forwarding towards MC
side .20
7.3.4.2 Modification of capset incoming from MC side for
forwarding towards SCE.21
7.3.5 Notification of change of N .21
a
7.3.6 Commands transmitted from a CAU.22
7.3.6.1 Substitution of transfer rate commands.22
7.3.6.2 Insertion and removal of [SM-comp] and [6B-H0-comp] .22
7.3.6.3 Insertion of [capex] .22
7.3.6.4 Insertion of [AggIN]*.22
7.4 Transmission of user information by a CAU .22
8 Initialisation, mode/rate changing and fault recovery .23
8.1 Initialisation .23
8.1.1 Single-channel side .23
8.1.2 Multi-channel Side, initial channel .23
8.1.3 Multi-channel side, additional channels.24
8.2 Change of transmission rate during a session.25
8.3 Recovery from fault conditions .25

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8.4 Further mode changes. 25
9 ISO Aggregation procedure. 25
Annex A (informative): Examples . 26
Annex B (informative): Example of SDL diagram for H.244 Aggregation initialisation. 30
Annex C (informative): Comments on the modification of capsets and commands. 31
Annex D (informative): Bibliography . 32
History. 33

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Foreword
This European Telecommunication Standard (ETS) has been produced by the Terminal Equipment (TE)
Technical Committee of the European Telecommunications Standards Institute (ETSI).
Transposition dates
Date of adoption of this ETS: 17 May 1996
Date of latest announcement of this ETS (doa): 31 October 1996
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 30 April 1997
Date of withdrawal of any conflicting National Standard (dow): 30 April 1997

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1 Scope
This European Telecommunication Standard (ETS) defines protocols for the synchronisation and
aggregation of multiple ISDN channels at 64 kbit/s, these channels being with a differential delay in the
range of ± one second and with or without octet integrity structure. It also addresses the case of
interworking with a digital network restricted to a rate of 56 kbit/s. It is applicable to a wide range of user
data transport up to 63 channels using ISO/IEC 13871 [1], but special provision is made for audiovisual
systems according to ETS 300 144 [2], up to 24 channels; the aggregation may be network-based or
associated with Customer-Premises Equipment. The available operating modes (B1, B2, B3, H2) provide
options with/without transmission-management overhead and user-data rates at or less than the exact
multiples of 64 kbit/s or 56 kbit/s.
The number of aggregated channels may be varied dynamically during a session. Procedures are given
for dealing with faults, including loss of channels and slip.
Call control is outside the scope of this ETS.
2 Normative references
This ETS incorporates by dated or undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications listed hereafter.
For dated references, subsequent amendments to, or revisions of any of these publications apply to this
ETS only when incorporated in it by amendment or revision. For undated references the latest edition of
the publication referred to applies.
[1] ISO/IEC 13871: "Information Technology - Telecommunications and information
exchange between systems - Private telecommunications networks - Digital
channel aggregation".
[2] ETS 300 144: "Integrated Services Digital Network (ISDN); Audiovisual services;
Frame structure for a 64 to 1 920 kbit/s channel and associated syntax for
inband signalling".
[3] ETS 300 143: "Integrated Services Digital Network (ISDN); Audiovisual services;
Inband signalling procedures for audiovisual terminals using digital channels up
to 2 048 kbit/s".
[4] ITU-T Recommendation H.320 (1993): "Narrow-band visual telephone systems
and terminal equipment".
[5] ETS 300 145: "Integrated Services Digital Network (ISDN); Audiovisual services;
Videotelephone systems and terminal equipment operating on one or two
64 kbit/s channels".
[6] ITU-T Recommendation H.242 (1993): "System for establishing communication
between audiovisual terminals using digital channels up to 2 Mbit/s".
3 Definitions
For the purposes of this ETS, the following definitions apply:
called end: The Channel Aggregation Unit (CAU) or Multiple Channel Equipment (MCE) which accepts
the first 64/56 channel connection request of the whole session, resulting in the establishment of the
"initial channel". If during fault-recovery procedures the initial channel is transferred to another connection,
this does not affect the definition (see note 1).
calling end: The CAU or MCE which requests the first 64/56 channel connection of the whole session,
resulting in the establishment of the "initial channel". If during fault-recovery procedures the initial channel
is transferred to another connection, this does not affect the definition (see note 1).

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ETS 300 481: July 1996
Channel Aggregation Unit (CAU): A device having both multiple 64 kbit/s accesses and a single access
at a higher bitrate. Reference in this ETS to "CAU" or "aggregator" means equipment conforming to this
ETS.
compatibility bits: Bit 8 of the first sixteen octets in every timeslot of a single channel except Time-slot 1.
external setting: Term used to express operations outside the scope of this ETS which determine the
behaviour of a CAU as to choice of option or timing of its action.
H.221 frame structure, H.221 framing: Frame structure according to ITU-T Recommendation H.221, as
set out in ETS 300 144 [2].
H.320 terminal: A terminal that conforms to ITU-T Recommendation H.320 [4] (note 2).
Mode H2: Modes of aggregation by the method given in clauses 7 and 8 of this ETS.
Modes B1, B2, B3: Modes of aggregation by the method of ISO/IEC 13871 [1], see subclause 5.2.
Multiple Channel Equipment (MCE): A terminal or other unit (such as a Local Area Network (LAN)
gateway) directly attached to an ISDN (through one or more ISDN accesses, whether basic accesses or
primary rate access(es)).
No Terminal Aggregator Communication case (ISO Aggregation case): No in-band management
communication between Single Channel Equipment (SCE) and CAU takes place - the single channel is
only a clear path for the data which will be wholly transmitted to the remote party; any communication
between the terminal and the CAU is by means not specified in this ETS (see clause 9).
redundant command: A Bit Allocation Signal (BAS) command which repeats, unchanged, a previously
transmitted mode value which is still in force.
Single Channel Equipment (SCE): A terminal or other unit (such as a LAN interface) having a single
serial bi-directional digital interface.
H.244 Aggregation case: In-band communication between SCE and CAU takes place using ITU-T
Recommendation H.221 BAS codes (see clauses 7 and 8 of this ETS).
user information: The application data streams passing (in both directions) between CAU and SCE
(note 3).
NOTE 1: The significance of "calling" and "called" end-points is mentioned in subclause 8.1.3.
NOTE 2: ETS 300 145 [5] is the ETSI equivalent to ITU-T Recommendation H. 320 [4]. The
scope of ETS 300 145 [5] is, however, restricted to systems supporting 1B or 2B
access.
NOTE 3: In the audiovisual cases, the application data stream includes the ITU-T
Recommendation H.221 frame structure.
4 Symbols and abbreviations
4.1 Symbols
For the purposes of this ETS, the following symbols apply:
NOTE: For readability, the capability and command values listed in ETS 300 144 [2] are
referred to in this ETS by their names rather than codepoint byte values; the use of
{name} for capabilities and [name] for commands serves to distinguish between these.
N an integer, applied to 64/56 channels to define a parallel (un-aggregated)
bitstream of rate N×64 kbit/s.

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n an integer, applied to 64 kbit/s to define a serial (or otherwise aggregated)
bitstream of rate n*64 kbit/s.
N the number of active 64/56 channels between CAU and MCE or between two
a
CAUs.
N the maximum number of 64/56 channels which can be accepted by a CAU
m
transmitting the value {N ×B}.
m
N' used to denote the incoming value of N from the remote CAU, or equivalent
m m
from an MCE.
N the number of connections requested by the calling end, the lower of N and
d m
N' .
m
{N×64k} capabilities in the series {1B}, {2B}, {3B}. corresponding to N=1, 2, 3.,
(applies to Mode H2 only).
{n*64k} capabilities in the series {64k}, {128k}, {192k}. corresponding to n=1, 2, 3.
n such that {n *64k} is the highest transfer-rate capability incoming from the SCE
m m
for which the continuous series of rate capabilities {(n -1)*64k}, {(n -
m m
2)*64k},.{2*64k} is also present in the capset.
[N×64] Multiple-channel commands (as defined in ETS 300 144 [2]).
[n*64k] Single-channel commands (as defined in ETS 300 144 [2]).
[capex] command issued by a CAU (see subclause 7.3.6.3) to an SCE to stimulate a
capability exchange.
[AggIN]* a double SBE symbol indicating the number n as determined by the process of
subclause 7.3.4.2; see the procedure described in clause 8 (see ETS 300 144
[2]).
{null} capability having no significance other than as a filler; only ever transmitted by a
CAU, and therefore it identifies the capset as having last come from, or been
forwarded by, a CAU; SCE ignores any number of these in an incoming capset
(see ETS 300 144 [2]).
{SM-comp} capability to transmit and receive user information without using any
compatibility bits, setting these to 1 (see ETS 300 144 [2]).
[SM-comp] user information is not contained in the compatibility bits (and is not sent in the
compatibility bits in the reverse direction either). See ETS 300 144 [2].
Σ Audible signal (to be defined), which would be recognised by a human user as a
call progress tone (he would not hang up).
4.2 Abbreviations
For the purposes of this ETS, the following abbreviations apply:
BAS Bit Allocation Signal
Capset Capability set
CAU Channel Aggregation Unit
FAS Frame Alignment Signal
H.221 See subclause 3.1, H.221 frame structure, H.221 framing
MC (side) Multiple Channel (side, of a CAU)
MCE Multiple-Channel Equipment
NII Network Indicate Incompatible - aggregators
ISO Aggregation No Terminal-Aggregator Communication

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SC (side) Single Channel (side, of a CAU)
SCE Single-Channel Equipment
SM-comp Single-Multiple compatibility
H.244 Aggregation Terminal-Aggregator Communication
UD Unspecified Data
5 General
5.1 Application of this ETS
On the single-channel connections, which may be very short if the terminal and CAU are co-located, or
much longer if a telecommunications link is involved, two options are covered by this ETS:
a) no in-band management communication on the single channel, this being only a clear path for the
data which will be wholly transmitted to the remote party; any communication between the terminal
and the CAU is by means not specified in this ETS; this is referred to as the "ISO Aggregation" case
- No Terminal-Aggregator Communication;
b) in-band communication as specified in this ETS, using the BAS codes defined in ETS 300 144 [2]
and the procedures of ETS 300 143 [3]; this is referred to as the "H.244 Aggregation" case -
Terminal-Aggregator Communication. This corresponds to the use of Mode H2 (see subclause 5.2,
figure 8 and associated text).
5.1.1 Intercommunication of audiovisual equipments
For audiovisual terminals conforming to ETS 300 144 [2] and ETS 300 143 [3], this ETS provides for
synchronisation and aggregation of up to 24 channels of 64 kbit/s or 56 kbit/s, each being with or without
octet integrity and relative transmission delay in the range ± 1 second.
Provision is made for the following intercommunication Cases A to C.
5.1.1.1 Case A
For interconnection of a Single-Channel Equipment (SCE) and a Multiple-Channel Equipment (MCE)
audiovisual end-point, both conforming to ETS 300 144 [2] and ETS 300 143 [3], only the H.244
Aggregation approach can be followed (see figure 1); this involves only one CAU, which may be within the
network or close to the single-channel end-point.
CAU ISDN
SCE
MCE
TAC
Figure 1: Case A
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5.1.1.2 Case B
This case provides for interconnection between two single-channel audiovisual end-points which see the
aggregated link purely as a clear data channel; each has a CAU associated with it, but there is no in-band
control communication between the end-point and its associated CAU (see figure 2); this is the ISO
Aggregation approach. The system is exactly the same as for unspecified data applications (see
subclause 5.1.2); the CAU-ISDN-CAU combination provides a clear channel at one of the transfer rates
specified in ETS 300 144 [2], and, therefore, shall use Mode B1 or B3 (see subclause 5.2).
CAU ISDN CAU
SCE SCE
(NoTAC) (NoTAC)
optional external communication
Figure 2: Cases B and D
5.1.1.3 Case C
This provides for the interconnection of two single-channel end-points both conforming to ETS 300 144 [2]
and ETS 300 143 [3] and capable of the operation described in clauses 7 to 8, the H.244 Aggregation
approach (see figure 3); this involves two CAUs, which may be within the network or close to the
single-channel end-points.
NOTE: H.244 Aggregation cannot be used at one end and ISO Aggregation at the other, see
clause 6.
CAU ISDN CAU
SCE SCE
TAC TAC
Figure 3: Case C
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5.1.2 Intercommunication of non-audiovisual equipments - Case D
This ETS provides for synchronisation and aggregation of up to 63 channels of 64 kbit/s or 56 kbit/s, each
being with or without octet integrity and relative transmission delay in the range ± one second. It is
applicable to a wide range of user information transport, including LAN-LAN interconnect, private circuit
back-up, and other "unspecified data" applications. No in-band control communication takes place on the
single-channel side. The equipment see the aggregated link purely as a clear data channel, and the
structure or content of the data streams is of no concern to the aggregating equipment.
The ISO Aggregation approach is appropriate (see figure 2); the CAU-ISDN-CAU combination provides a
clear channel at one of the transfer rates specified in ISO/IEC 13871 [1] using Mode B1, B2 or B3 (see
subclause 5.2).
5.2 Definition of the modes of transmission on the Multiple Channel (MC) side
The four modes of transmission are defined below, the accompanying figures being given by way of
example for the case of about 192 kbit/s on the Single Channel (SC) side.
Mode B1: the user information occupies an integral number of 64k/56k channels (usually the N available
and aligned channels); only unframed signals are transmitted (see figure 4), alignment of the channels
has previously been achieved using a framed mode, and it is assumed that no slip or other network fault
has occurred since then. There are no management bits multiplexed into the stream on either Multiple
Channel (MC) or SC sides, and the total bitrates are equal; the mode is therefore confined to ISO
Aggregation cases, and rate change cannot occur.
CAU
192 kbit/s
64 kbit/s 64 kbit/s 64 kbit/s
Figure 4: Mode B1 - applicable to the ISO Aggregation case only
Audiovisual signals according to ETS 300 144 [2] can be transmitted, but the ITU-T Recommendation
H.221 framing plays no part in the aggregation process, and shall appear at a random position within one
of the B-channels as shown in black in figure 5.

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CAU
192 kbit/s
64 kbit/s 64 kbit/s
64 kbit/s
Figure 5: Mode B1 - framed audiovisual signal carried as Unspecified Data (UD)
Mode B2: signals are transmitted with framing according to ISO/IEC 13871 [1] (depicted as white in
figure 6), the user information rate being about 1,5 % less than an integral multiple of 64 kbit/s or 56 kbit/s
due to the overhead of the frame structure. There is no provision for management bits on the SC side, so
the use of this Mode is confined to the ISO Aggregation case.
CAU
(192-3x) kbit/s
(64-x) kbit/s (64-x) kbit/s (64-x) kbit/s
Figure 6: Mode B2 - applicable to the ISO Aggregation case only
Mode B3
: signals are transmitted with framing in all N channels according to ISO/IEC 13871 [1], the user
information rate being an integral multiple (such as N-1) of 64 kbit/s or 56 kbit/s; thus there is a
management overhead of about 1,5 % and generally some unused capacity, as illustrated in figure 7. The
exact bit positions occupied by user information are defined in ISO/IEC 13871 [1].

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CAU
(3x) kbit/s
(64-x) (64-x) (64-x)
unused
192 kbit/s
capacity
kbit/s kbit/s kbit/s
Figure 7: Mode B3 - applicable to the ISO Aggregation case only
Mode H2: only ITU-T Recommendation H.221-framed signals are transmitted (framing is shown as white
in figure 8), the user information rate being an integral number of 64k/56k channels (usually the N
available and aligned channels), including the Frame Alignment Signal (FAS) and BAS within this rate. On
the MC side all channels are framed; on the SC side only the I-channel (Time-Slot 1) carries H.221
framing, but the corresponding bits (shown with dashed outline) in the other time-slots shall be vacated,
so that framing can be inserted at the CAU in the SC to MC direction (and removed in the other): this is
done under the [SM-comp] command.
CAU
192 kbit/s
64 kbit/s 64 kbit/s 64 kbit/s
Figure 8: Mode H2 - applicable to the H.244 Aggregation case only
Mode properties
The properties of the four modes are summarised in table 1. "Dynamic rate change" refers to dynamic
variation of the number N of aggregated channels during a session: rate changes within available
connection capacity can normally be made in less than 40 ms for Modes H2 and B3; when new
connections shall be added, the response depends on call set-up time; for Mode B1 interruption of the
application is inevitable.
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Table 1: Mode properties
Mode B1 Mode H2 Mode B2 Mode B3
Aggregation overhead None 2,5 % in the additional 1,5 % 64 kbit/s or
channels 56 kbit/s
Dynamic rate changes No Yes Yes Yes
Exact multiples of Yes Yes (H.221 - framed No Yes
64 kbit/s/56 kbit/s only)
Interworking with MCE No Yes not applicable No
(audiovisual)
The applicability of the Modes is summarised in table 2.
Table 2: Mode applicability
Mode AV/non-AV Mode applicable when.
B1 Audiovisual the remote end is not an MCE, and the remote CAU does not support Mode
H2, and Mode B3 is considered too inefficient or is not available; bitstream
treated as UD only; ISO Aggregation case, so external means shall be used
to set bitrate.
Non-audiovisual exact multiple of 64 kbit/s or 56 kbit/s required, absence of dynamic rate
change tolerable, B3 considered too inefficient.
B2 Audiovisual not applicable - B2 does not provide suitable bitrates.
Non-audiovisual exact multiple of 64 kbit/s or 56 kbit/s not essential, dynamic rate change
desired.
B3 Audiovisual the remote end is not an MCE, and the remote CAU does not support Mode
H2; dynamic rate change more important than efficiency; bitstream treated
as UD only; ISO Aggregation case, so external means shall be used to set
bitrate.
Non-audiovisual exact multiple of 64 kbit/s or 56 kbit/s required, dynamic rate change more
important than efficiency.
H2 Audiovisual remote end is an MCE, or a CAU supporting Mode H2; H.244 Aggregation
case, so no external control is needed.
Non-audiovisual not applicable (unless conformant to ETS 300 144 [2]).
5.3 General description of the aggregation process
CAU is a device having, on the one side, a single-channel port and on the other side a multiplicity of 64/56
channel ports. This ETS defines the behaviour of the CAU in respect of the input and output signals at the
two sides.
When new multiple channels are active, the CAU buffers the incoming signals in order to synchronise
them, and then reads the user information to the SC side. In the reverse direction, the CAU takes the
serial input into the SC side and loads it into those parallel channels which it knows to be in synchronism
at the remote end. However, according to the two quite different conditions described at the beginning of
subclause 5.1, the CAU has two different processes, that is, depending on whether or not it is
communicating in-band with the single-channel terminal; selection of the correct process is described in
clause 6.
When there is such communication (this applies only to audiovisual terminals), effectively it is the terminal
which is communicating with the other terminal at the remote end of the connection, by capability sets and
commands; the CAU merely moderates this communication in order to assure satisfactory conditions on
the multiple 64/56 channel subclause. The CAU informs the terminal about the upper limit of transmission
rate, which depends on the number of channels available, or what the other end can accept, whichever is
the lower; it does this by suitably modifying the capability sets exchanged. The synchronisation process is
based on ETS 300 144 [2], which also provides for numbering of the channels. Negotiation of appropriate
conditions is based on ETS 300 143 [3]; the capability sets emanating from the CAU define the range of
signals that could be sent to it, while commands define the actual signal structure being transmitted from
the CAU. See clauses 7 to 8 of this ETS.

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ETS 300 481: July 1996
When there is no in-band communication between the CAU and SCE, the CAU shall be totally responsible
for its communication with the remote CAU, presenting on its SC side a clear digital channel at a specified
bit rate, which can be used for any purpose whatsoever (including audiovisual systems). Here again the
CAU shall control the transmission rate from the terminal - this can be done by clocking if the application
rate can be varied in this way (and this does not apply to terminals conforming to ETS 300 145 [5]), or by
other external signalling. In this ("ISO Aggregation") case, the synchronisation and communication
processes are fully described in ISO/IEC 13871 [1] - see clause 9 of this ETS.
5.4 CAU functionality
The CAU functionality can be summarised as follows:
1) all modes
MC-to-SC direction: synchronisation and aggregation of N incoming 64/56 channels into one
outgoing single channel; SC-to-MC direction: partition of incoming user information onto N outgoing
64 kbit/s/56 kbit/s channels, with framing as appropriate.
H2 mode
ignore the compatibility bits incoming on the SC side, putting FAS/BAS into the corresponding
outgoing bits on the MC side; remove FAS/BAS in additional channels incoming on the MC side,
putting "ones" into the corresponding outgoing bits on the SC side.
2) all modes
control the aggregated bitrate according to application requirements and the available MC capacity.
H2 mode
modify capability sets and commands to ensure correct operation of the system; detect changes to
capacity requirements from incoming capability sets.
3) all modes
detect fault conditions and take appropriate action.
4) all modes
(optionally) transfer the call-control signalling between the ISDN and single-channel networks.
The action of the CAU is not completely specified, some matters being left to the choice of the
implementer or service provider. For example, requests for channels will not all mature simultaneously,
and it is a matter of choice whether each should be added in to the communication as soon as available
(increasing the bit rate in a series of many steps), or alternatively held back until all or most are available
(increasing the bit rate in one single step).
6 Choice of ISO Aggregation or H.244 Aggregation procedure
The procedure at the start of the call is as depicted in figure 9 and described as follows. It is mandatory
that all CAUs intended for use with audiovisual terminals conforming to ETS 300 144 [2] shall be able to
operate in Mode H2.
There are two alternatives for the choice between ISO Aggregation and H.244 Aggregation processes at
each CAU:
1) the CAU may be pre-set to operate in the ISO Aggregation procedure or the H.244 Aggregation
procedure only, according to the environment for which the CAU is intended;
2) if 1) has not been applied, a decision shall be taken on the basis of incoming signals on both SC
and MC sides, as described below.
Reference is made to figure 9. The CAU operation is essentially the same whether it is the originating
(calling) end or the destination (called) end (the setting of Flag1 is for possible later use when the
additional channels are to be requested).

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ETS 300 481: July 1996
First, three local conditions are tested, and if any of the three are not met then ISO Aggregation (clause 9)
shall be used. These conditions are:
a) there has been no pre-set to specify ISO Aggregation only; note that if the connected SCE is an
ITU-T Recommendation H.320 [4] terminal, then specifying ISO Aggregation means that only
modes B1 or B3 may be used - B2 does not provide valid bitrates;
b) H.221 framing is detected, coming from the SCE; note that a time-out is involved here;
c) the capset in that H.221 frame contains the value {SM-comp} and/or {6B-H0-comp}.
Only under these three conditions is the H.221-framed signal forwarded from the SCE on the initial
channel, otherwise the CAU generates the signal defined in section 7.1 of ISO/IEC 13871 [1].
Second, the incoming signal is searched for H.221 framing and ISO/IEC 13871 [1] signal. If
ISO/IEC 13871 [1] signal is detected, then ISO Aggregation procedure of clause 9 shall be used - if
previously following clauses 7 to 8 because the above three conditions were met, this means switching to
ISO/IEC 13871 [1] signal on the outgoing transmission. A time-out of 10 seconds is applied to the
detection of the ISO/IEC 13871 [1] signal to allow for the possibility that clauses 7 to 8 is initially being
followed at the remote end. Only if the transmissions in both directions are H.221-framed with {SM-
comp} or {6B-H0-comp} does the procedures of clauses 7 to 8 apply.
Failure states
If no incoming framing, either of ITU-T Recommendation H.221 or ISO/IEC 13871 [1] type, Failure State
1 is reached, it shall be presumed that the remote end-point is neither a CAU nor an audiovisual MCE (in
some applications, speech communication using PCM according to CCITT Recommendation G.711 may
still be possible).
If one end is transmitting according to ISO Aggregation operation (clause 9), one of the above three
conditions not having been fulfilled, and the remote end is sending H.221 framing but without {SM-comp}
or {6B-H0-comp}, Failure State 2 shall be reached, reflecting the technical impossibility of interconnecting
to an MCE at the remote end.
Failure State 3 shall be reached if the CAU is set to operate in the H2 mode but the connected SCE is not
sending H.221 framing.
The action to be taken on reaching any of the failure states shall be an implementation matter. The SBE
symbol NII (Network Indicate Incompatible-aggregators), indicating incompatible aggregators, may be
used in appropriate fault conditions.

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ETS 300 481: July 1996
standby
CONNECT
originating CONN_ACK
destination
terminal received received terminal
Set Flag1 =0 Set Flag1 =1
Flag is used in ISO 13871
aggregation to distinguish
Calling/Answering Endpoints
ISO
Y
aggregation
Condition 1
present?
Search for H.221 framing
incoming on SC side
H.244
N N
H.221 framing
aggregation
Condition 2
present?
preset?
Y
*Look for {SM-comp} from SCE failure
*alternative:
state 3
{6B-H0-comp}
N
{SM-comp}
Condition 3
present*
Near-end Forward framed PCM Send ISO 13871
favourable for from SC side to MC side signal - §9
H.244 aggregation Capset_mod_1 (§7.3.4.1)
Examine signals
Examine signals
incoming on MC side
incoming on MC side
Neither H.221 framing Neither H.221 framing
ISO 13871 signal
H.221 framing ISO 13871 signal H.221 framing
nor ISO 13871 signals nor ISO 13871 signals
detected detected (Note) detected (Note) detected
detected within 10 s detected within 10 s
N
failure Switch transmitted {SM-comp} failure
state 1 state 1
signal to ISO 13871 present*
Await incoming
change to ISO 13871 failure
state 2
signal structure
All conditions met for
ISO 13871 aggregation selected
1 2
H.244 aggregation in all other cases - §9 applies
§7, §8 apply
Figure 9: Procedure for selection between H.244 Aggregation and ISO Aggregation processes

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ETS 300 481: July 1996
7 H.244 Aggregation procedure
7.1 Call control
The signalling for call control is outside the scope of this ETS. It is assumed that external signalling is
available to set up the appropriate paths between the equipments - for example, the "D-channel" in the
case of ISDN. Means are described in subclause 8.1.3 for the in-band transfer of network address
information where this is needed for call control.
7.1.1 Initial channel
The initial 64/56 channel is requested when communication has been established between the calling-end
SCE and its CAU and any necessary checking has been carried out (for example, presence of {SM-comp}
if required; presence of {n*64k} with n>1.). During establishment of the initial channel, the value of N
m
shall be calculated from the SCE capset (see subclause 7.3.2).
SCE terminals conforming to ETS 300 145 [5] should include the 6B-H0 compatibility mode; if this
capability and {SM-comp} are both absent, the CAU shall adopt ISO Aggregation working - see clause 6.
7.1.2 Additional channels
Additional-channel requests are made by a CAU when:
- the total of channels already requested is less than the lower of N and N' , and (in dial-up cases)
m m
the CAU is the calling end.
Additional channels may be dropped by a CAU when the total of channels already requested exceeds the
lower of N and N' .
m m
7.2 Channel synchronisation
On the multiple-channel side, CAUs shall always synchronise all connected channels (in the ISDN case,
those for which CONNECT has been sent or received). The value of N shall be determined according to
a
the channels for which:
- the incoming A-bit is set to zero;
- the channel numbers in BAS position (see annex A) form a continuous series from 1 to N .
a
7.3 In-band procedure using ITU-T Recommendation H.221 BAS codes
In-band signalling shall follow ETS 300 143 [3] to ensure correct operation between the equipments.
H.221 framing is used on all the multiple channels, and also on the single-channel side. All BAS
commands are effective from the start of the following submultiframe, and shall remain in force until
countermanded, see ETS 300 143 [3].
CAUs shall remain responsive to incoming H.221 framing throughout the call, on the MC side on all
channels, and on the SC side. This means that when no framing is detected the equipment will
nevertheless respond rapidly when the incoming signal is switched to a framed mode, and will also
respond quickly (if required) to fault conditions.
7.3.1 Control of transfer rate
The main function of the CAU is to match the effective transfer rates on its two sides. To do this it shall be
able to control the user information which is sent to it: it does this in-band on the MC side, and in the
H.244 Aggregation case on the SC side also. The in-band method is by means of the capability set (see
ETS 300 143 [3]) which it transmits or forwards; this shall always contain transfer-rate capability values no
higher than the available rate on the other side of the CAU.
NOTE: There can be other in-band signalling between SCE and CAU, however, this is outside
the scope of this ETS, and should be treated in the same way as "out-of-band"
signalling - see ISO Aggregation case.

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ETS 300 481: July 1996
7.3.2 Determination of the transfer-rate capabilities to be transmitted on the MC side
Reference is made to the in-band streams (1) and (2) in figure 10.
Transfer-rate capabilities transmitted on the MC side (2) are decided on the basis of the capset received
from the SCE (1).
If n >1, an {N×64k} type value, with N = n shall be sent - this is required for communication with an
m m
MCE, and also provides for communication with an SCE with fallback to all lesser multiples of
64 kbit/s/56 kbit/s.
If an SCE-transmitted capset contains values of {n*64k} for which n is higher than n , these also shall be
m
included (see subclause 7.3.4.2) for potential communication with another SCE, though fallback to lesser
rates shall be limited.
in-band information
(initial B-channel)
(3) (2)
in-band information (TAC case)
CAU
(1) (4)
Figure 10
7.3.3 Determination of the transfer-rate capabilities to be transmitted on the SC side
Reference is made to the in-band streams (3) and (4) in figure 10.
Transfer-rate capabilities transmitted on the SC side (4) are decided on the basis of the capset received
from the MC side (3) and the number of channels currently available, N .
a
The incoming capability set on the MC side contains a single transfer-rate value of {N' ×64k} type: this
m
value shall be replaced by a {n*64k}-type transfer-rate value, corresponding either to the incoming value
(i.e. such that n = N' ), or to the available number of channels (i.e. such that n = N ), whichever is the
m a
lesser. In general, the highest {n*64k}-type transfer-rate capability value transmitted from CAU on the SC
side (4) is that which indicates the maximum bitrate that can be accepted, including both user information
and management.
7.3.4 Capability sets transmitted from a CAU
Capsets are not generated by the CAU itself; they are generated only in the terminals, and forwarded, in
both SC and MC directions, by the CAUs with modifications as described in subclauses 7.3.4.1 and
7.3.4.2 below. No changes in the length of a capset are made by a CAU, the value {null} being used to fill
any unused positions.
The capsets outgoing from CAUs shall not violate the rules set out in ETS 300 143 [3]; in particular, all
capsets which follow one another in succession without intervening commands shall be identical. Thus
when circumstances require that a new modification be made, this shall always be done on a new capset,
which shall be stimulated by the procedure of subclause 7.3.5.
7.3.4.1 Modification of SCE capset for forwarding towards MC side
The CAU modifies the SCE capset changing the lowest transfer-rate capability value to {N ×64k}; here
m
N is determined from subclause 7.3.2 above, except that if the SCE capset has only one transfer-rate
m
capability value {n*64k} then this is sent unchanged.

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ETS 300 481: July 1996
This may be understood with reference to the following examples:
Original transfer rate capabilities External setting Modified transfer rate capabilities
512, 384, 320, 256, 192, 128, 64 none
512, 384, 320, 256, 192, 128, 6B
384, 256, 192, 128, 64 6B 384, 256, 192, 128, 4B (note 1)
384, 320, 256, 192, 128 none
384, 320, 256, 192, 6B
384, 320, 256, 192, 128 4B 384, 320, 256, 192, 4B
384 none 384 (note 2)
NOTE 1: 6B cannot be used here since the rate 320 is absent from the list of original
capabilities.
NOTE 2: See Annex D for further information.
Modification of the lowest transfer-rate capability guarantees that the transfer-rate capability for just
64 kbit/s, which is the same as 1B, shall never be present in the same set as a higher rate {N×B}-type.
The remaining transfer rate and all other Capability codes are left unchanged. A remote MCE shall receive
the {N×B}-type and ignore the {n*64k}-type values. The MCE shall use the received A bits to determine
the rate at which it can transmit data.
The CAU shall conform to ITU-T Recommendation H.242 [6] at all times: in particular, it shall not cause a
change (in content or even order) in forwarded capsets without the insertion of a command - this may
require the omission of a complete capset.
7.3.4.2 Modification
...