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INTERNATIONAL  TELECOMMUNICATION  UNION







CCITT    H.221

THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE











LINE  TRANSMISSION

OF  NON-TELEPHONE SIGNALS





FRAME  STRUCTURE  FOR
A  64  TO  1920  kbit/s  CHANNEL IN
AUDIOVISUAL  TELESERVICES










Recommendation  H.221







Geneva, 1990













































































Printed in Switzerland





FOREWORD

    The CCITT (the International Telegraph and Telephone Consultative
Committee) is a permanent organ of the International Telecommuni-
cation Union (ITU). CCITT is responsible for studying technical,
operating and tariff questions and issuing Recommendations on them
with a view to standardizing telecommunications on a worldwide
basis.

    The Plenary Assembly of CCITT which meets every four years,
establishes the topics for study and approves Recommendations pre-
pared by its Study Groups. The approval of Recommendations by the
members of CCITT between Plenary Assemblies is covered by the
procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).

    Recommendation H.221 was prepared by Study Group XV and was
approved under the Resolution No. 2 procedure on the 14 of December
1990.





___________________





CCITT  NOTE

    In this Recommendation, the expression ôAdministrationö is used for
conciseness to indicate both a telecommunication Administration and
a recognized private operating agency.



















πITU1990

All rights reserved. No part of this publication may be reproduced or uti-
lized in any form or by any means, electronic or mechanical, including pho-
tocopying and microfilm, without permission in writing from the ITU.

PAGE BLANCHE

Recommendation H.221

Recommendation H.221

FRAME  STRUCTURE  FOR  A  64  TO  1920  kbit/s  CHANNEL
IN  AUDIOVISUAL  TELESERVICES1)


(revised 1990)


    Introduction

    The purpose of this Recommendation is to define a frame structure for
audiovisual teleservices in single or multiple B or H0 channels or a single
H11 or H12 channel which makes the best use of the characteristics and
properties of the audio and video encoding algorithms, of the transmission
frame structure and of the existing Recommendations. It offers several
advantages:

û    It takes into account Recommendations such as G.704, X.30/I.461,
etc. It may allow the use of existing hardware or software.

û    It is simple, economic and flexible. It may be implemented on a sim-
ple microprocessor using well-known hardware principles.

û    It is a synchronous procedure. The exact time of a configuration
change is the same in the transmitter and the receiver. Configura-
tions can be changed at 20ms intervals.

û    It needs no return link for audiovisual signal transmission, since a
configuration is signalled by repeatedly transmitted codewords.

û    It is very secure in case of transmission errors, since the code con-
trolling the multiplex is protected by a double-error correcting
code.

û    It allows the synchronization of multiple 64 kbit/s or 384 kbit/s con-
nections and the control of the multiplexing of audio, video, data
and other signals within the synchronized multiconnection struc-
ture in the case of multimedia services such as videoconference.

û    It can be used to derive octet synchronization in networks where this
is not provided by other means.

û    It can be used in multipoint configurations, where no dialogue is
needed to negotiate the use of a data channel.

û    It provides a variety of data bit-rates (from 300 bit/s up to almost 2
Mbit/s) to the user.

1    Basic principle

    This Recommendation provides for dynamically subdividing an over-
all transmission channel of 64 to 1920kbit/s into lower rates suitable for
audio, video, data and telematics purposes. The overall transmission chan-
nel is derived by synchronizing and ordering transmissions over from1 to
6B-connections, from1 to 5H0-connections, or an H11 or H12 connection.
The first connection established is the initial connection and carries the ini-
tial channel in each direction. The additional connections carry additional
channels.

    The total rate of transmitted information is called the ôtransfer rateö; it
is possible to fix the transfer rate less than the capacity of the overall trans-
mission channel (values listed in AnnexA).

    A single 64 kbit/s channel is structured into octets transmitted at
8kHz. Each bit position of the octets may be regarded as a sub-channel of
8kbit/s (see Figure1a/H.221). The eighth sub-channel is called the Service
Channel (SC), consisting of several parts as described in ºº1.1 to1.4
below.

    An H0, H11 or H12 channel may be regarded as consisting of a num-
ber of 64 kbit/s time-slots (TS) (see Figure1b/H.221). The lowest numbered
time-slot is structured exactly as described for a single 64kbit/s channel,
while the other TS have no such structure. In the case of multiple B or H0
channels, all channels have a frame structure; that in the initial channel con-
trols most functions across the overall transmission, while the frame struc-
ture in the additional channels is used for synchronization, channel
numbering and related controls.

    The term ôI-channelö is applied to the initial or only B channel, to
TS1 of initial or only H0 channel, and to TS1 of H11, H12 channels.



1.1    Frame alignment signal (FAS)

    This signal structures the I-channel and other framed 64 kbit/s chan-
nels into frames of 80 octets each and multiframes (MF) of 16frames each.
Each multiframe is divided into eight 2-frame sub-multiframes (SMF). The
term ôframe alignment signalö (FAS) refers to bits1-8 of the SC in each
frame. In addition to framing and multiframing information, control and
alarm information may be inserted in the FAS, as well as error check infor-
mation to control end-to-end error performance and to check frame align-
ment validity. Other time-slots are aligned to the first.

    The bits are transmitted to line in order, bit 1 first.

    When an 8 kHz network clock is provided, FAS is transmitted and
received in the least significant bit of the octet within each
125microsecond, e.g.in an ISDN basic or primary rate interface.

    The FAS can be used to derive receive octet timing when it is not pro-
vided by the network. However, in the latter case, the terminal cannot trans-
mit FAS with correct alignment into the octet timed part of the network and
cannot intercommunicate with terminals which rely only on network timing
for octet alignment.



1.2    Bit-rate allocation signal (BAS)

    Bits 9-16 of the SC in each frame are referred to as BAS. This signal
allows the transmission of codewords to describe the capability of a terminal
to structure the capacity of the channel or synchronized multiple channels in
various ways, and to command a receiver to demultiplex and make use of
the constituent signals in such structures. This signal is also used for con-
trols and indications.

    NoteûFor some countries having 56 kbit/s channels, the net available
bit rates will be 8 kbit/s less. Interworking between a 64kbit/s terminal and
a 56kbit/s terminal is established according to the frame structure in
AnnexB.

1.3    Encryption control signal (ECS)

    A future encryption capability may require a dedicated transmission
channel. It is anticipated that 800 bit/s should be provided when required by
allocating the bits17-24 of the service channel. This reduces variable data
and video transmission rates herein by 800bit/s. The 800bit/s is referred to
as the ECS channel.

1.4    Remaining capacity

    The remaining capacity (including the rest of the service channel),
carried in bits 1-8 of each octet in the case of a single 64kbit/s connection,
may convey a variety of signals within the framework of a multimedia ser-
vice, under the control of the BAS. Some examples follow:

û    voice encoded at 56 kbit/s using a truncated form of PCM of Recom-
mendation G.711 (A-law or ╡-law);

û    voice encoded at 16 kbit/s and video at 46.4 kbit/s;

û    voice encoded at 56 kbit/s with a bandwidth 50 to 7000 Hz (subband
ADPCM according to RecommendationG.722); the coding algo-
rithm is also able to work at 48kbit/sûdata can then be dynami-
cally inserted at up to 14.4kbit/s;

û    still pictures coded at 56 kbit/s;

û    data at 56 kbit/s inside an audiovisual session (e.g., file transfer for
communicating between personal computers).

2    Frame alignment

2.1    General

    An 80-octet frame length produces an 80-bit word in the service chan-
nel. These 80 bits are numbered 1-80. Bits1-8 of the service channel in
every frame constitute the FAS (see Figure2/H.221), whose content is as
follows:

û    multiframe structure (see º 2.2);

û    Frame Alignment Word (FAW);

û    A-bit;

û    E-and C-bits (see º 2.6).

    The FAW consists of ô0011011ö in bits 2-8 of the FAS in even frames, com-
plemented by an ô1ö in bit 2 of the succeeding odd frame.

    The ôA-bitö of the I-channel is set to zero whenever the receiver is in multi-
frame alignment, and is set to ô1ö otherwise (see º2.3); for additional chan-
nels, see º2.7.1.



2.2    Multiframe structure

    See Table 1/H.221.

    Each multiframe contains 16 consecutive frames numbered 0 to 15
divided into eight sub-multiframes of two frames each (see Figure3/H.221).
The multiframe alignment signal is located in bit 1 of frames1-3-5-7-9-11
and has the form 001011. Bit1 of frame15 remains reserved for future use.
The value is fixed at0.

    Bit 1 of frames 0-2-4-6 may be used for a modulo 16 counter to num-
ber multiframes in descending order. The least significant bit is transmitted
in frame0, and the most significant bit in frame6. The receiver uses the
multiframe numbering to equalize out the differential delay of separate con-
nections, and to synchronize the received signals.

    Bit 1 of frame 8 is set to 1 when multiframes are numbered and is set
to 0 when they are not.

    Bit 1 of frames 10-12-13 must be used to number each channel in a
multiconnection structure so that the distant receiver can place the octets
received in each 125microseconds in the correct order.

    Information bits in the multiframe should be validated by, for exam-
ple, being received consistently for three multiframes.



2.3    Loss and recovery of frame alignment

    Frame alignment is defined to have been lost when three consecutive
frame alignment words have been received with an error.

    Frame alignment is defined to have been recovered when the follow-
ing sequence is detected:

û    for the first time, the presence of the correct first seven bits of the
frame alignment word;

û    the eighth bit of the frame alignment word in the following frame is
detected by verifying that bit 2 isa1;

û    for the second time, the presence of the correct first seven bits of the
frame alignment word in the next frame.

    If frame alignment is achieved but multiframe alignment cannot be
achieved, then frame alignment should be sought at another position.

    When the frame alignment is lost, A-bit of the next odd frame is set to 1 in
the transmit direction.

2.4    Loss and recovery of multiframe alignment

    Multiframe alignment is needed to number and synchronize two or
more channels, and possibly also for encryption. Terminals such as those
having only single-channel capabilities which have no use for the multi-
frame structure must transmit the multiframe structure, but need not check
for multiframe alignment on the incoming signal: they may transmit outgo-
ing A=0 when frame alignment is recovered.

    NoteûSuch a terminal cannot transmit TEA (see Figure3/H.221).

    After multiframe alignment has been validated the other functions
represented by bit 1 of the service channel can be used. When multiframe
alignment of the distant terminal has been signalled (A=0 received) the dis-
tant terminal is expected to have validated BAS codes and to be able to
interpret BAS codes.

    Multiframe alignment is defined to have been lost when three consec-
utive multiframe alignment signals have been received with an error. It is
defined to have been recovered when the multiframe alignment signal has
been received with no error in the next multiframe. When multiframe align-
ment is lost, even when an unframed mode is received, the A-bit of the next
odd frame is set to 1 in the transmit direction. It is reset to 0 when multi-
frame alignment is regained. It is reset in additional channels when multi-
frame alignment and synchronism with the initial channel is regained.

2.5    Procedure to recover octet timing from frame alignment

    When the network does not provide octet timing, the terminal may
recover octet timing in the receive direction from bit timing and from the
frame alignment. The octet timing in the transmit direction may be derived
from the network bit timing and an internal octet timing.

2.5.1    General rule


    The receive octet timing is normally determined from the FAS posi-
tion. But at the start of the call and before the frame alignment is gained, the
receive octet timing may be taken to be the same as the internal transmit
octet timing. As soon as a first frame alignment is gained, the receive octet
timing is initialized at the new bit position, but it is not yet validated. It will
be validated only when frame alignment is not lost during the next 16
frames.

2.5.2    Particular cases


a)    When, at the initiation of a call, the terminal is in a forced recep-
tion mode, or when the frame alignment has not yet been gained,
the terminal may temporarily use the transmit octet timing.

b)    When frame alignment is lost after being gained, the receive octet
timing should not change until frame alignment is recovered.

c)    As soon as frame and multiframe alignment have been gained once,
the octet timing is considered as valid for the rest of the call, unless
frame alignment is lost and a new frame alignment is gained at
another bit position.

d)    When the terminal switches from a framed mode to an unframed
mode (by means of the BAS), the octet timing previously gained
must be kept.

e)    When a new frame alignment is gained on a new position, different
from that previously validated, the receive octet timing is reinitial-
ized to the new position but not yet validated and the previous bit
position is stored. If no loss of frame alignment occurs in the next
16 frames, the new position is validated, otherwise the stored old
bit position is reutilized.

2.5.3    Search for frame alignment signal (FAS)


    Two methods may be used: sequential or parallel. In the sequential
method, each of the eight possible bit positions for the FAS is tried. When
FAS is lost after being validated, the search must resume starting from the
previously validated bit position. In the parallel method, a sliding window,
shifting one bit for each bit period, may be used. In that case, when frame
alignment is lost, the search must resume starting from the bit position next
to the previously validated one.

2.6    Description of the CRC4 procedure


    In order to provide an end-to-end quality monitoring of the connec-
tion, a 4-bit Cyclic Redundancy Check (CRC4) procedure may be used and
the four bitsC1, C2, C3 and C4 computed at the source location are inserted
in bit positions 5 to 8 of the odd frames. In addition, bit 4 of the odd frames,
the E-bit, is used to transmit an indication as to whether the most recent
CRC block, received in the incoming direction, contained errors or not.

    When the CRC4 procedure is not used, bit E shall be set to 0, and bits
C1, C2, C3 and C4 shall be set to 1 by the transmitter. Provisionally, the
receiver may disable reporting of CRC errors after receiving eight consecu-
tive CRCs set to all 1s, and it may enable reporting of CRC errors after
receiving two consecutive CRCs each containing a 0 bit.

2.6.1    Computation of the CRC4 bits


    The CRC4 bits C1, C2, C3 and C4 are computed for each B/H0/H11/H12
channel2) , for a block made of two frames: one even frame (containing
the first seven bits of FAW) followed by one odd frame (containing the
eighth bit of FAW). The CRC4 block size is then 160/960/3840/4800 octets
for a B/H0/H11/H12 channel2) and the computation is performed 50times
per second.

    Note û This is still valid for the case of H0/H11 in restricted networks, the
stuffed bits being included in the computation. For restricted B, see
AnnexB.


2.6.1.1    Multiplication-division process

    A given C1-C4 word located in block N is the remainder after multi-
plication by x4 and then division (modulo2) by the generator polynomial
x4+x+1 of the polynomial representation of block (N-1).

    When representing contents of a block as a polynomial, the first bit in
the block should be taken as being the most significant bit. Similarly C1 is
defined to be the most significant bit of the remainder and C4 the least sig-
nificant bit of the remainder.

    This process can be realized with a four-stage register and two exclu-
sive-ORs.


2.6.1.2    Encoding procedure

i)    The CRC bit positions in the odd frame are initially set at zero, i.e.
C1=C2=C3=C4=0.

ii)    The block is the acted upon by the multiplication-division process
referred to above in º 2.6.1.1.

iii)    The remainder resulting from the multiplication-division process is
stored ready for insertion into the respective CRC locations of the
next odd frame.

    Note û These CRC bits do not affect the computation of the CRC bits of the
next block, since the corresponding locations are set at zero before the com-
putation.


2.6.1.3    Decoding procedure

i)    A received block is acted upon by the multiplication-division pro-
cess, referred to above in º 2.6.1.1, after having its CRC bits
extracted and replaced by zeros.

ii)    The remainder resulting from this multiplication-division process is
then stored and subsequently compared on a bit-by-bit basis with
the CRC bits received in the next block.

iii)    If the decoded calculated remainder exactly corresponds to the
CRC bits sent from the encoder, it is assumed that the checked
block is error-free.


2.6.2    Consequent actions


2.6.2.1    Action on bit E

    Bit E of block N is set to 1 in the transmitting direction if bits C1-C4
detected in the most recent block in the opposite direction have been found
in error (at least one bit in error). In the opposite case it is set to zero.

2.6.2.2    Monitoring for incorrect frame alignment (see Note)

    In the case of a long simulation of the FAW, the CRC4 information
can be used to re-invite a search for frame alignment. For such a purpose it
is possible to count the number of CRC blocks in error within two seconds
(100blocks) and to compare this number with 89. If the number of CRC
blocks in error is greater than or equal to 89, a search for frame alignment
should be reinitiated.

    These values 100 and 89 have been chosen in order that:

û    For a random transmission error rate of 10-3, the probability of
incorrectly reinitiating a search for frame alignment, because of 89
or more blocks in error, should be less than 10-4.

û    In case of simulation of frame alignment, the probability of not rein-
itiating a search of frame alignment after a two-second period
should be less than 2.5%.

    NoteûValues in this and the next section exemplify the case of a 64 kbit/s
channel. For H0, H11 or H12 channels the details will differ but the princi-
ples are still applicable.

2.6.2.3    Monitoring for error performance

    The quality of the 64 kbit/s connection can be monitored by counting
the number of CRC blocks in error within a period of one second
(50blocks). For instance, a good evaluation of the proportion of seconds
without errors as defined in RecommendationG.821 can be provided.

    For information purposes, Table 1/H.221 gives the proportions of
CRC block in error can be computed for randomly distributed errors of error
rate Pe.

    By counting the received E-bits, it is possible to monitor the quality of
the connection in the opposite direction.



2.7    Synchronization of multiple connections

    Some audiovisual terminals will be able to communicate over multi-
ple B or H0 connections (see Note). In this case, a single B or H0 initial con-
nection is established, the possibility for more connections is determined
from the transfer rate capability BAS of AnnexA and the additional connec-
tions are then established and synchronized by the terminal using the multi-
frame structure.

    NoteûA connection is an individual call between the terminals. A
channel is the transmission in one direction over the connection.

2.7.1    Multiple B-connections

    FAS and BAS are transmitted in each B-channel.

    FAS operation is as follows:

û    multiframe numbering is used to determine relative transmission
delay between B-channels as described in º2.2;

û    the channel numbers are transmitted as described in º 2.2 with the
channel of the initial connection being numbered 1 and there being
up to five additional connections;

û    the outgoing A-bit is set to 1 in the additional B-channel of the same
connection whenever the received additional channel is not syn-
chronized to the initial channel;

û    when receive synchronization is achieved between the initial and
additional channels by introducing delay to align their respective
multiframe signals, the transmitted A-bit is set to0;

û    the E-bit for each additional B-channel is transmitted in the addi-
tional B-channel in the same connection, because it relates to a
physical condition of the transmission path.

    The BAS operation in additional connections is restricted to the transmis-
sion of the additional channel number (thus the channel numbering must be
sent both in BAS according to AnnexA and in the FAS as in º2.2).

    The distant terminal, upon receiving the A-bit set to zero with respect to
sequentially numbered channels, can add their capacity to the initial connec-
tion by sending the transfer rate BAS in AnnexA. The order of the bits
transmitted in the channels is in accordance with the examples given in
Figure4/H.221.

2.7.2    Multiple H0 connections

    FAS and BAS are transmitted in the first time-slot of each H0.

    FAS operation is as in º 2.7.1 except that the channel number is used
to order the six octets received each 125 microseconds with respect to the
six octet groups received in other channels.

    The BAS operation in additional channels is as specified in º 2.7.1.

3    Bit-rate allocation signal

3.1    Encoding of the BAS

    The bit-rate allocation signal (BAS) occupies bits 9-16 of the service
channel in every frame. An eight bit BAS code (b0, b1, b2, b3, b4, b5, b6,
b7) is complemented by eight error correction bits (p0, p1, p2, p3, p4, p5,
p6, p7) to implement a (16,8) double error correcting code. This error cor-
recting code is obtained by shortening the (17,9) cyclic code with generator
polynomial:

    g(x)=x8+x7+x6+x4+x2+x+1

    The error correction bits are calculated as coefficients of the remain-
der polynomial in the following equation:

    p0x7+p1x6=p2x5+p3x4+p4x3+p5x2+p6x+p7

    =
RESg(x)[b0x15+b1x14+b2x13+b3x12+b4x11+b5x10+b6x9+b7x
8]

where RESg(x)[f(x)] represents the residue obtained by dividing f(x) by
g(x).

    The BAS code is sent in the even-numbered frame, while the associ-
ated error correction bits are sent in the subsequent odd-numbered frame.
The bits of the BAS code or the error correction are transmitted in the order
shown in Table2/H.221 to avoid emulation of the frame alignment word.



    The decoded BAS value is valid if:

û    the receiver is in frame and multiframe alignment, and

û    the FAW in the same sub-multiframe was received with two or fewer
bits in error.

    Otherwise the decoded BAS value is ignored.

    When the receiver actually looses frame alignment, it may be advisable to
undo any changes caused by the three previously decoded values as they
may well have been erroneous even after correction.

3.2    Values of the BAS

    The encoding of BAS is made according to a hierarchical attribute
method. This consists of attribute class (8classes), attribute family
(8families), attribute (8 attributes) and value (32values). The first three bits
of an attribute represent its number describing the general command or
capability, and the other five bits identify the ôvalueöûthe specific com-
mand or capability.

    The following attributes are defined in the Class (000) and
Family(000):



    The values of these attributes are listed and defined in Annex A. They
provide for the following facilities:

û    transmission at various total rates and on single and multiple chan-
nels, on clear channels and on networks subject to restrictions to
56 kbit/s and its multiples;

û    audio transmission, digitally encoded to various recommended algo-
rithms;

û    video transmission, digitally encoded to a recommended algorithm,
with provision for future recommended improvement;

û    Low-Speed Data (LSD) within the I-channel, or TS1 of a higher rate
initial channel;

û    High-Speed data (HSD) in the highest-numbered 64 kbit/s channel
or time-slots (excluding the I-channel);

û    data transmission within a multilayer protocol, either in the I-chan-
nel (MLP) or in capacity other than the I-channel (H-MLP);

û    an encryption control signal;

û    loopback towards the network for maintenance purposes;

û    signalling for control and indications;

û    a message system for, inter alia, conveying information concerning
equipment manufacturer and type.

    The command BAS attributes have the following significance: on receipt of
a BAS command code in one (even) frame and its error-correcting code in
the next (odd), the receiver prepares to accept the stated mode change begin-
ning from the subsequent (even) frame; thus a mode change can be effected
in 20milliseconds. The command remains in force until countermanded
(see RecommendationH.242, º12). The bit positions occupied by combina-
tions of BAS commands are exemplified in Figures4a/H.221 to 4g/H.221.















    The capability BAS attributes have the following significance: they
indicate the ability of a terminal to receive and properly treat the various
types of signal. It follows that having received a set of capability values
from the remote terminal Y, terminal X must not transmit signals lying out-
side that declared range.

    Values [0-7] of the attribute (111) are reserved for setting the class,
and [8-15] for setting the family; the default value is (000) for both.

    The next eight attribute values of the attribute (111) are temporary
escape BAS codes of Single Byte Extension (SBE). The last three bits of the
temporary escape BAS form a pointer to one of eight possible escape BAS
tables of 224 entries each (codes beginning with 111 are not used in the
escape BAS tables). Then the next received BAS indicates the specific entry
in the escape BAS table.

    The value (111)[24] is the capability marker (see Recommendation
H.242, º 2) which is followed by normal BAS codes, not by any escape val-
ues.

    The last seven attribute values of the attribute (111) are of Multiple
Byte Extension (MBE) and are used to send messages as specified in the
Notes to the table in AnnexA.


3.3    Procedures for the use of BAS

    The use of BAS codes is specified in Recommendation H.242.



ANNEX A

(to Recommendation H.221)

Definitions and tables of BAS values

    The definitions of BAS values are given below, and the corresponding
numerical values are listed in TablesA-1/H.221 and A-2/H.221.


A.1    Audio command values(000)

    For bit position illustrations see Figure 4/H.221. Abbreviations
ôG.711ö and ôG.722ö refer to Recom-mendations.


Neutral                    Neutralized I-channel, containing only FAS and BAS;
all other bits are to be ignored at the receiver.

Au-off, U                No audio signal, no frame (mode 10); all the I-channel is
available for use under other commands3).

Au-off, F                    No audio signal, FAS and BAS in use (mode 9); 62.4
kbit/s available for use under other commands.

A-law, OU                G.711 audio at 64 kbit/s, A-law, no framing (mode OU)3).

A-law, OF                G.711 audio at 56 kbit/s, A-law, truncated to 7 bits in bits 1-7,
with FAS and BAS in bit 8; bit 8 is set to zero at the
PCM audio decoder (mode OF).

╡-law, OU                G.711 audio at 64 kbit/s, ╡-law, no framing (mode OU)3).

╡-law, OF                G.711 audio at 56 kbit/s, ╡-law, truncated to 7 bits in bits 1-7,
with FAS and BAS in bit 8; bit8 is set to zero at the
PCM audio decoder (mode OF).

G.722, m1                G.722 7 kHz audio at 64 kbit/s, no framing (mode 1)3).

G.722, m2                G.722 7 kHz audio at 56 kbit/s, in bits 1-7 (mode 2).

G.722, m3                G.722 7 kHz audio at 48 kbit/s, in bits 1-6 (mode 3).

Au-40k                    Reserved for audio at less than 48 kbit/s (for example
40 kbit/s in bits 1-5).

Au-32k                    Reserved for audio at less than 48 kbit/s (for example 32
kbit/s in bits 1-4): the algorithm of ôAu-16kö below
may be extended to code a wider speech bandwidth at
32 kbit/s as a result of further studies.

Au-24k                    Reserved for audio at less than 48 kbit/s (for example
24 kbit/s in bits 1-3).

Au-16k                    Audio at 16 kbit/s to Recommendation H.200/AV.254
in bits 1 and 2 (mode 7).

Au-<16k                    Reserved for audio at less than 48 kbit/s (for example
8 kbit/s in bit 1).

Au-ISO-64/128/192/256    Audio to ISO standard at 64/128/192/256 kbit/s, in
the lowest-numbered time-slots (other than TS1) of an
H0 or greater channel.

Au-ISO-384                Audio to ISO standard at 384 kbit/s in time-slots 2-7 of a
channel greater than H0.

A.2    Transfer-rate command values (001)

    NoteûIf the transfer-rate command is less than the available con-
nected capacity, the information occupies the lowest-numbered channel(s)/
time-slot(s).

64                    Signal occupies one 64 kbit/s channel.

2┤64                    Signal occupies two 64 kbit/s channels, with FAS and
BAS in each.

3 to 6┤64                Signal occupies three to six 64 kbit/s channels, with FAS and
BAS in each.

384                    Signal occupies 384 kbit/s, with FAS and BAS in the first
64kbit/s time-slot; the effective channel may be the
whole of an H0 channel or the lowest numbered time-
slots of an H11 or H12 channel.

2┤384                    Signal occupies two channels of 384 kbit/s, with FAS
and BAS in each.

3 to 5┤384                Signal occupies three to five 384 kbit/s channels, with FAS
and BAS in each.

1536                    Signal occupies 1536 kbit/s, with FAS and BAS in the first
64kbit/s time-slot. The effective channel occupies the
whole of an H11 channel or the lowest numbered
time-slots of an H12channel.

1920                    Signal occupies 1920 kbit/s, with FAS and BAS in the first
64kbit/s time-slot. The effective channel occupies the
whole of an H12channel.

128/192/256                Signal occupies 128/192/256 kbit/s, with FAS and BAS in the
first 64 kbit/s time-slot. The effective channel occu-
pies the lowest numbered time-slots of an H0 or larger
channel.

512/768/1152/1472        Signal occupies 512/768/1152/1472 kbit/s, with FAS
and BAS in the first 64 kbit/s time-slot. The effective
channel occupies the lowest numbered time-slots of
an H11 or H12channel.

Loss-i.c.                    Designated ôInitial channelö, especially used following loss
of the channel previously so designated (see H.242,
º7.2.3)

Channel No. 2-6            Numbering of additional channelsûsee º 2.7.1.

A.3    Video, encryption, loop and other commands (010)

Video-off                No video; video switched off.

H.261                    Video on, to Recommendation H.261: video occupies all
capacity not otherwise allocated by other commands;
video cannot be inserted in the I-channel when var-
LSD or var-MLP is in force; examples are given in
Figure4e/H.221.

                    Specifically, the video rate in initial B-channel (framed) or
TS1 is: 62.4 kbit/s û audio rateû{800 bit/s if ECS is
ON} û {MLP rate if ON} û {LSD rate if ON}.

Vid-imp.(R)                Reserved for video on, to improved recommended algo-
rithm.

Video-ISO                Video on, to ISO standard: video occupies the same capacity as
stipulated above for the case of H.261 video.

AV-ISO                    Composite audio/video to ISO standard: the composite sig-
nal occupies the same capacity as stipulated above for
the case of H.261 video.

Freeze-pic.                Freeze-picture request (see Recommendation H.230, VCF).

Fast-update                Fast-update request (see Recommendation H.230, VCU).

Encryp-on                ECS Channel active.

                    NoteûWhen encryption is active, it applies to all informa-
tion bits in all channels of the connection, except bits
1-24 of the SC in the I-channel and the FAS and BAS
positions of the other channels; use of encryption in
conjunction with MLP is for further study.

Encryp-off                ECS channel off.

Au-loop                    Audio loop request (see Recommendation H.230,
LCA).

Vid-loop                    Video loop request (see Recommendation H.230,
LCV).

Dig-loop                    Digital loop request (see Recommendation H.230,
LCD).

Loop-off                    Loop off request (see Recommendation H.230, LCO).

                    NoteûLoopback requests are intended for use by
maintenance staff.

6B-H0-comp                To provide for compatibility between terminals connected to
single H0 channel and six B-channel accesses, the
least significant bits of the first 16 octets of all time-
slots of the H0 channel, except TS1, are not used; the
H0 terminal must discard these bits from the incom-
ing signal on receipt of this code, and must set the
same bits to ô1ö in the outgoing signal.

Not-6B-H0                Negates the command ô6B-H0-compö.

                    NoteûUsed, for example, in testing.

Restrict                    To provide for operation on a restricted network, and for
interconnection between a terminal on restricted and
unrestricted networks: on receipt of this code, a termi-
nal must treat the SC as being in bit 7 of the I-channel,
and discard bit 8 of every other channel and/or time-
slot; in the outgoing direction these bits are set to ô1ö.

Derestrict                On receipt of this code, a terminal must revert to ôunrestricted
networkö operation, treating the SC as being in bit 8
of the I-channel.

A.4    LSD/MLP commands (011)

    For bit position illustrations see Figure 4/H.221.

#                    These LSD rates are not allowed if ECS channel is in
use.

*                    In restricted cases, the starred bit numbers are reduced
by one.

LSD off                    LSD switched off.

300                    Low-speed data at 300 bit/s in SC, octets 38-40.

1200                    Low-speed data at 1200 bit/s in SC, octets 29-40.

4800                    Low-speed data at 4800 bit/s in SC, octets 33-80.

6400                    Low-speed data at 6400 bit/s in SC, octets 17-80#.

8000                    Low-speed data at 8000 bit/s in bit 7*.

9600                    Low-speed data at 9600 bit/s in bit 7* and octets 25-
40 of SC.

14400                    Low-speed data at 14400 bit/s in bit 7* and octets 17-
80 of SC#.

16k                    Low-speed data at 16 kbit/s in bit 6* and bit 7*.

24k                    Low-speed data at 24 kbit/s in bits 5*, 6* and 7*.

32k                    Low-speed data at 32 kbit/s in bits 4*-7*.

40k                    Low-speed data at 40 kbit/s in bits 3*-7*.

48k                    Low-speed data at 48 kbit/s in bits 2*-7*.

56k                    Low-speed data at 56 kbit/s in bits 1-7 (no framing in
restricted case).

62.4k                    Low-speed data at 62.4 kbit/s in bits 1-7 and octets 17-80 of
SC. If ECS channel i in use the data rate is reduced to
61.6kbit/s, but returns to 62.4kbit/s if ECS channel is
closed.

64k                    Low-speed data at 64 kbit/s in bits 1-8, no framing.

Var-LSD                    Low-speed data occupying all I-channel capacity not allo-
cated under other fixed-rate commands; cannot be
invoked when other LSD is on, or when variable-MLP
is on (may also be impractical when video is on in I-
channel alone).

                    Exact var-LSD rate: 62.4 kbit/sûaudio rateû{800 bit/s if
ECS in ON}û{fixed-MLP if ON}.

DTI(R)                    Three codes reserved for communicating the status of the
data terminal equipment interfaces.

MLP-off                    MLP off in all channels.

MLP-4k                    MLP on at 4 kbit/s in octets 41-80 of SC.

MLP-6.4k                MLP on at 6.4 kbit/s in octets 17-80 of SC; if ECS channel is in
use, the data rate is reduced to 5.6 kbit/s in octets25-
80, but returns to 6.4kbit/s if ECS channel is closed.

Var-MLP                    MLP occupying all I-channel capacity not allocated under
other fixed-rate commands: cannot be invoked when
other MLP is on, or when variable-LSD is on (may
also be impractical when video is on in I-channel
alone).

                    Exact var-MLP rate: 62.4 kbit/sûaudio rateû{800 bit/s if
ECS is ON} û {fixed-LSD if ON}.

A.5    Audio capabilities (100)

Neutral                    Neutral capability: no change in the current capabili-
ties of the terminal.

A-law                    Capable of decoding audio to Recommendation
G.711, A-law.

╡-law                    Capable of decoding audio to Recommendation
G.711, ╡-law.

G.725-T1                Terminal type 1 defined in Recommendation G.725, º 2.

G.725-T2                Terminal type 2 defined in Recommendation G.725, º 2.

Au-16k                    Capable of decoding audio, both to Recommendation H.200/
AV.254 and RecommendationG.711.

Au-ISO                    Capable of decoding audio to ISO standard at all rates
up to 384kbit/s.

A.6    Video, MBE and encryption capabilities (101)

QCIF                    Can decode video to QCIF picture format, but not CIF (see
RecommendationH.261)ûthis code must be fol-
lowed by one of the four minimum picture interval
(MPI) values below.

CIF                    Can decode video to CIF and QCIF formats (see Recom-
mendation H.261) û this code must be followed by
two MPI values, the first applicable to QCIF and the
other to CIF format.

                    Minimum picture interval (MPI) codes are as follows:

1/29.97                    Can decode video, having a minimum picture interval of 1/
29.97seconds, to RecommendationH.261.

2/29.97                    Can decode video, having a minimum picture interval of 2/
29.97seconds, to RecommendationH.261.

3/29.97                    Can decode video, having a minimum picture interval of 3/
29.97seconds, to RecommendationH.261.

4/29.97                    Can decode video, having a minimum picture interval of 4/
29.97seconds, to RecommendationH.261.

Vid-imp(R)                Reserved for future improved recommended video algo-
rithm.

Video-ISO                Can decode video to ISO standard.

AV-ISO                    Can decode composite audio/video signal to ISO
standard.

MBE-cap                Can handle multiple-byte extensions messages in the BAS posi-
tion, those beginning with codes in the range
(111)[25-31], in addition to other values.

Esc-CF(R)                Reserved for capability to accept non-zero class/family
escape codes.

Encryp.                    Capable of handling signals on the ECS channel.

A.7    Transfer-rate capabilities (100)

64, 384                    Can accept signals only on one 64 kbit/s channel, one
384 kbit/s channel.

2┤64                    Can accept signals on one or two 64 kbit/s channels,
and synchronize them.

...                    ...

6┤64                    Can accept signals on one to six 64 kbit/s channels,
and synchronize them.

2┤384                    Can accept signals on one or two 384 kbit/s channels,
and synchronize them.

...                    ...

5┤384                    Can accept signals on one to five 384 kbit/s channels,
and synchronize them.

1536/1920                Can accept signals on a 1536 kbit/s channel, a 1920 kbit/s
channel.

Restrict                    Can work only at p┤56 kbit/s, rate-adapted to p┤64 kbit/s
by moving the SC to bit position 7 and setting bit8 to
ôoneö in every channel or time-slot; a constant ôoneö,
however, may be set in bit8 if it is known by out-of-
band signalling prior to the connection that the
restriction exists; this code has the effect of forcing
the remote terminal to work in the p┤56kbit/s mode
(see AnnexB).

6B-H0-comp                Capable of acting upon the corresponding command.

128/192/256                Capable of accepting the transfer rate specified by the corre-
sponding command.

512/768/1152/1472        Capable of accepting the transfer rate specified by the
corresponding command.


A.8    LSD/MLP capabilities (101)

300 (to 64k)                Can accept LSD at 300 bit/s (to 64 kbit/s) in the bit positions
specified against the corresponding commands.

Var-LSD                    Can accept LSD variable rate in the bit positions specified
against the corresponding command.

MLP-4k                    Can accept MLP at 4 kbit/s in the SC.

MLP-6.4k                Can accept MLP at up to 6.4 kbit/s in the SC.

Var-MLP                    Can accept MLP at up to 64 kbit/s in the I-channel.


A.9    Escape table values (111)

HSD                    High-speed data: a 32-code table containing HSD
capabilities and commands.

H.230                    Control and indications: a 32-code table with definitions in
RecommendationH.230.

Start-MBE                First byte of (N+2) octet BAS message; the message format
is:

                        start-MBE//value of N (max=255)//N bytes.

NS-cap                    First byte of non-CCITT capabilities message; the
message format is:

                        NS-cap//value of N (max=255)//country
code4)//manufacturer code*//(N-4) bytes.

NS-comm                First byte of non-CCITT command message; the message
format is:

                        NS-comm//value of N (max=255)//country
code4) //manufacturer code*//(N-4) bytes.

Cap-mark                Capability markerûthe first item in a capability setûsee
RecommendationH.242, º2.

Data-apps                Applications within LSD/HSD channels: a 32-code tableû
see TableA-3/H.221.

    Note1ûThe value of N is coded by its binary representation.

    Note2ûThe most significant bit of each MBE message byte is trans-
mitted as the b0 bit of BAS.


A.10    HSD/H-MLP capabilities (111)[10000]-(101)

64k to 1536k                Can accept HSD at the specified rate in the bit positions speci-
fied against the corresponding commands.

HSD-other                Reserved for other HSD rates.

Var-HSD                    Can accept HSD variable rate in the bit positions specified
against the corresponding command.

H-MLP-62.4k                Can accept MLP at 62.4 kbit/s in the bit positions specified
against the corresponding command.

H-MLP-r                    Can accept MLP at r=64/128/192/256/320/384 kbit/s in the
bit positions specified against the corresponding com-
mand.

Var-H-MLP                Reserved for capability to accept H-MLP variable rate in the bit
positions specified against the corresponding com-
mand.


A.11    HSD/H-MLP commands (111)[10000]-(011)

    NoteûIn the cases of multiple channels, the term ôhighest-numbered
time-slotö refers to the highest-numbered channel.

HSD-off                    HSD switched off; FAS and BAS restored in addi-
tional channels.

64k                    HSD on, in highest numbered channel/time-slot; FAS and
BAS are removed in the case of multiple B-channels.

128/192/256k                HSD on in highest-numbered time-slots of an H0 or greater
channel.

320k                    HSD on in highest-numbered time-slots of an H0 or
greater channel.

384k                    HSD on in highest-numbered H0 channel, or highest-num-
bered time-slots of a greater channel; FAS and BAS
are removed in the case of multiple-H0 channels.

HSD-other                Reserved for other HSD rates.

Var-HSD                    Reserved for high-speed data occupying all capacity, other
than in the I-channel, not allocated under other com-
mands: cannot be invoked when other HSD is on, or
when var-H-MLP is on (may also be impractical
when video is on, the latter then being confined to the
I-channel).

H-MLP-off                H-MLP switched off (this does not affect I-channel MLP).

H-MLP-62.4                H-MLP on at 62.4 kbit/s, occupying second 64 kbit/s channel
except FAS and BAS positions.



H-MLP on at 64/128/192/256/320 kbit/s in the low-
est-numbered time-slots, (otherthan TSI) of an H0 or
greater channel.

H-MLP-384k                 H-MLP on at 384 kbit/s in time-slots 2-7 of a greater chan-
nel than H0.

Var-H-MLP                Reserved for MLP occupying all capacity, other than in the I-
channel, not allocated under other commands: cannot
be invoked when other MLP is on, or when var-HSD
is on.

    NoteûWhen the ôrestrictö command is in force the least significant
bit of all octets covered by the HSD and H-MLP commands is set to
ô1ö, so the effective data rate is less than that indicated by the com-
mand.

A.12    Applications within LSD/HSD channelsûcapabilities (111)[10010]-
(101)

ISO-SP baseline on on LSD    Can accept ISO-still picture (SP) baseline mode
on specified LSD rate.

ISO-SP baseline on HSD    Can accept ISO-still picture baseline mode on
specified HSD rate.

ISO-SP spatial            Can accept ISO-still picture baseline and spatial modes.

ISO-SP progressive        Can accept ISO-still picture baseline and progressive
modes.

ISO-SP arithmetic            Can accept ISO-still picture baseline and arithmetic
modes.

Graphics cursor            Can handle graphics cursor data.

Group3 Fax                Can accept group 3 Fax.

Group4 Fax                Can accept group 4 Fax.

V.120 LSD                Can accept V.120 terminal adaptation within an LSD chan-
nel.

V.120 HSD                Can accept V.120 terminal adaptation within an HSD chan-
nel.

A.13    Applications within LSD/HSD channels û commands (111)[10010]-
(011)

ISO-SP on in LSD            ISO-still picture switched on in specified LSD.

ISO-SP on in HSD            ISO-still picture switched on in specified HSD.

Cursor data on in LSD        Cursor data switched on in specified LSD.

Fax on in LSD            Fax switched on in specified LSD.

Fax on in HSD            Fax switched on in specified HSD.

V.120 LSD                V.120 switched on in specified LSD.

V.120 HSD                V.120 switched on in specified HSD.





ANNEX B

(to Recommendation H.221)

Frame structure for interworking between a 64 kbit/s terminal
and a 56 kbit/s terminal

B.1    Sub-channel arrangement

    The sub-channel arrangement is given in TableB-1/H.221.



B.2    Operation of the 64 kbit/s terminal

    The transmitter fills the eighth sub-channel with ô1ö, while the
receiver searches FAS at every sub-channel.

B.3    Restriction against some communication modes

    Since the interworking bit rate becomes 56 kbit/s, the transmission
modes using more than 56 kbit/s are forbidden (receivers ignore these com-
mand BAS codes). Facilities using the original seventh sub-channel move to
the sixth sub-channel.

B.4    Audio Command Codes (000)

    The following are applicable instead of those in Annex A.

Neutral                Neutralized I-channel, containing only FAS and BAS; all other
bits are to be ignored at the receiver.

Au-off, U            No audio signal, no framing; bits 1-7 of the I-channel are avail-
able.

Au-off, F                No audio signal, FAS and BAS in use; 54.4 kbit/s available for
use under other commands.

A-law, U7            G.711 audio at 56 bit/s, A-law truncated to 7 bits, no framing
(mode OU).

A-law, F6            G.711 audio at 48 kbit/s, A-law truncated to 6 bits, with FAS
and BAS in bit 7.

m-law, U7            G.711 audio at 56 kbit/s, m-law truncated to 7 bits, no framing
(mode OU).

m-law, F6            G.711 audio at 48 kbit/s, m-law truncated to 6 bits, with FAS
and BAS in bit 7.

G.722, U8            not possible to transmit 8 bits per octet.

G.722, U7            G.722 7 kHz audio in bits 1-7, 56 kbit/s (unframed).

G.722, F6            G.722 7 kHz audio at 48 kbit/s, in bits 1-6 (mode 3).

Au-16 kbit/s            Audio at 16 kbit/s to Recommendation H.200/AV.254 in bits
1,2 (mode 7).

[Other]                All other values reserved.

    The following (000) values are assigned maintaining the same number
of audio bits per octet between the 64kbit/s and 56 kbit/s environments:

[0]    Neutral

[6]    not possible

[7]    Au-off, U

[18]    A-law, U7

[19]    ╡-law, U7

[20]    A-law, F6

[21]    ╡-law, F6

[24]    G.722, U7

[25]    G.722, F6

[29]    Au-16 kbit/s

[31]    Au-off, F