Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TELECOMMUNICATIONS SYSTEM
This invention relates to telecommunications networks and in particular to
a system and method for asynchronous transfer mode (ATM) transmission
of traffic.
BACKGROUND OF' THE INVENTION'
A recent development in telecommunications technology has been the
introduction of the asynchronous transfer mode (ATM) transmission
technique. The asynchronous transfer mode (ATM) technology is a
flexible form of transmission which allows various types of service traffic,
1o e.g. voice, video or data, to be multiplexed together on to a common
means of transmission, the traffic being carried in cells each having a
header indicating its destination. The service traffic is adapted typically
into 53 byte cells comprising 5 byte headers and 48 byte payloads such
that the original traffic can be reconstituted at the far end of the ATM
network. This form of adaptation is performed in the ATM adaptation layer
(AAL). The technique allows large volumes of traffic to be handled reliably
and efficiently.
Specification No EP-A1-0,225,714 describes a communications network
2 0 comprising a number of nodes and in which short delay limits are met by
creating composite packets carrying information for more than one call. A
call is allocated one or more octets at a given location in packets having a
given connection number. A description of voice transport in an ATM
broad band network is given by W O Covington in "Communications
2 5 Technology for the 1990's and Beyond, Dallas, November 27-30, 1989,
Volume 3, 27 November 1989, IEEE, pages 1921-1925.
A limiting factor in the introduction of ATM is the difficulty of interfacing
new
broad band ATM networks not only with existing narrow band networks,
3 o commonly referred to as legacy networks, but also with the newly
emerging narrow band cellular and wireless networks. In an attempt to
address the latter-mentioned part of this problem, ANSI committee T1 S1.5
has recently issued a baseline document containing proposals for a new
ATM Adaptation Layer (AAL) that encapsulates and transports short user
3 5 packets (called AAL-SDUs) inside an ATM cell stream. The new AAL is
intended to be applicable to both fixed and variable length short packets
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and has been named 'Small Multiplexed ATM Adaptation Layer' (SMAAL).
SMAAL is intended to support applications such as low bit rate-
compressed voice, both with and without silence suppression, leading to
both constant bit rate and variable bit rate user information. A
consequence of these applications can be the unacceptably long voice
packetisation delay that can occur in' filling a complete ATM cell with the
resulting user information from a single source. The purpose of SMAAL is
to permit multiple short packets of user information from one or more users
to be multiplexed inside a single ATM cell, thereby alleviating the
1o packetisation delay problems. While this facilitates the transmission of
short packets within the ATM network, it does not provide an effective
means of interfacing with existing legacy networks, neither basic 64 kbit/s
narrow band networks nor narrow band voice networks employing
compression and silence suppression techniques.
SUMMARY OF THE INVENTION
The object of the invention is to minimise or to overcome these
disadvantages.
2 0 A further object of the invention is to provide an improve apparatus and
method for the transport of narrow band communications traffic.
According to the invention there is provided a method of transporting traffic
between first and second narrow band networks across an asynchronous
2 5 transfer mode (ATM) network, the method including multiplexing traffic
from users of the first narrow band network into structured blocks within
ATM cells, providing each cell with a respective single control information
field (CIF) and a header, transmitting the cells across the ATM network to
the second narrow band network, and demultiplexing the user traffic from
3 0 the transmitted cells, characterised in that said control information
field
incorporates a length indicator (LI) indicative of the size of the structured
blocks within that cell whereby to define the demultiplexing process, that
changes in structured block size are signalled via a change indicator (CI)
contained in the control information field, and that transport of user
35 information or of signalling information is denoted by a single bit
information/signalling (I/S) indicator contained in the control information
field.
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According to another aspect of the invention there is provided an
arrangement for transporting traffic between first and second narrow band
networks across an ATM network, the arrangement including means for
multiplexing traffic from users of the first narrow band network into
structured blocks within ATM cells, means for providing each cell with a
respective single control information field and a header, means for
transmitting the cells across the ATM network to the second narrow band
network, and means for demultiplexing the user traffic from the transmitted
cells, characterised in that said control information field incorporates a
length indicator indicative of the size of the structured blocks within that
cell whereby to define the demultiplexing process, that changes in
structured block size are signalled via a change indicator (CI) contained in
the control information field, and that transport of user information or of
signalling information is denoted by a single bit information/signalling (I/S)
indicator contained in the control information field.
The method and arrangement provide for the transport across ATM
networks of narrow band 64 kbit/s channels as required for interworking
2 o between legacy networks. A single unified transport capability is made
available to fulfil the requirements for the ATM trunking of narrow band
services, whether constant bit rate or piece wise constant bit rate (as for
compressed voice with silence suppression) in character.
BRIEF DESC: . TION OF THE DRAWINGS
An embodiment of the invention will now be described with reference to the
accompanying drawings in which:-
Figure 1 is a schematic diagram illustrating the SMAAL fields and
3 0 formats;
Figure 2 shows the control information field employed in the SMAAL
protocol of figure 1;
Figure 3 is a schematic diagram of a telecommunications network
structure according to an embodiment of the invention;
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Figure 4 shows a control information field used in the transmission
. _ protocol of the network structure of figure 3 to facilitate the transport
of multiple multiplexed 64 kbit/s narrow band channels in a single
ATM connection;
Figure 5 is a schematic diagram illustrating the transport of multiple
structured blocks in single ATM cells across the network of figure 3;
Figure 6 is a schematic diagram illustrating the transport of single
structured blocks in multiple ATM cells across the network of figure
3 ; and
Figure 7 shows a control information field used in the transmission
protocol of the network structure of figure 3 to facilitate the transport
of single narrow band channels carrying compressed and/or silence
suppressed voice information per single ATM connection.
DESCRIPTION OF PREFERRED EMBODIMENT
Reference is first made to figures 1 and 2 which are introduced for
2 o comparative purposes and as an aid to the understanding of the invention.
Figure 1 shows the format and composition of the fields of SMAAL. It also
illustrates the multiplexing of short packets from multiple users into a
SMAAL cell stream. The three information packets (AAL-SDUs) from
users A, B and C are appended each to a header called the Control
Information Field (CIF) before being multiplexed into the cell stream of a
single ATM connection. For efficiency, information from User B is split to
straddle two cell payloads; in this case, the second half of the information
loaded into the second cell payload also carries a CIF header. At the end
of the second cell payload, more than two octets remain following the
3 0 information from User C. Since there is no more information available to
send at this time, the remaining octets are filled by a CIF field plus Pad
characters, labelled 'X'. Finally, the ATM cell payloads are encapsulated
into ATM cells.
The details of the control information field or CIF are shown in Figure 2.
The meanings of the individual fields are as follows
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~ the Extension (E) bit is reserved for future use.
~ the Length Indicator (LI) of 6 bits points to the end of the current packer
so that its value is equal to the length of the packet in octets. A value
of LI greater than 46 indicates that the end of the current packet lies in
the next cell, where the leading CIF contains the length of the
remaining part of the packet.
~ the Logical Link Number (LLN) of 4 bits allows user information from up
to 16 sources to be associated with the correct users.
~ the 5 bit Error Correcting Field (ECF) contains a 4-bit CRC plus a Parity
1o bit. The ECF permits either single bit error correction or error detection
to be performed over the CIF.
The rules pertaining to the use of Pad characters at the end of a cell
payload are as follows
~ with only one octet remaining, fill it with Pad character X.
~ with exactly two octets remaining, fill them with CIF with LI = 0.
~ with more than two octets remaining and no user information waiting to
be sent, fill them with CIF with LI = 0 plus Pad characters X.
2 0 The present version of SMAAL as described briefly above permits the
transport of user information in the form of short fixed or variable length
packets, say from 3 to 36 octets in length, between narrow band networks
across ATM trunk networks.
Referring now to figure 3, this illustrates a network structure providing
interworking between narrow band networks via an ATM network, e.g. a
public network. In the network structure of figure 3, narrow band channels
are transported across the public ATM network between pairs of
Interworking Functions (IWF) each providing an interface between the
3 o respective narrow band network and the ATM network. For maximum
efficiency and cost effectiveness, ATM connections carrying multiplexed
information from multiple users may be set up between the IWF pairs for
this purpose.
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If we consider the information typically originating from legacy 64 kbit/s
narrow band networks, this is in the form of short packets of 1 octet in
length. We have found that the transport of such information between 64
kbit/s narrow band networks and other narrow band networks can be
achieved by the transport between IWFs of multiple octets multiplexed on
to a single ATM connection. We have also found that, in order to transport
such information, there is no need to carry the overhead of a CIF per
packet. This is because the octets originating from a 64 kbit/s narrow band
network may be carried in structured blocks. Thus, if 83 channels need to
be transported between IWFs, one octet per channel can be multiplexed
into the ATM cell stream every 125 microseconds to form a repetitive 83
octet structure. By using this structure we require a single control
information field (CIF) per ATM cell to recover the individual channels at
the destination IWF. This control information field is shown in figure 4.
The LI field in our CIF points to the end of the structure block. A value of
all ones in the LI field indicates that the end of the structure was not in
the
current cell.
Operation of structured information transfer in the manner described above
2 o with reference to figures 3 and 4 preferably requires a sequence number to
be associated with each cell payload, so that missing and misinserted cells
can be detected. Since our structured transfer has no need for the
conventional CIF's Logical Link Number field (LLN), 3 bits of the LLN may
be allocated to convey a sequence number (SN).
T number of channels being transported may be varied dynamically. This
is effected via a signalling exchange between IWFs in order to agree on
the parameters of the new structure, following which an in band change
indicator synchronises the exact instant of structure change. To achieve
3 o this, the fourth bit of the LLN field can be used as a change indicator, a
change in the sense of this bit being used to convey the time of structure
change.
Figure 5 shows an example of the use of our transmission technique to
transport multiple 64 kbit/s-based block structures per ATM cell, with the
size of the block structure equal to 6 in this case. Every 125
microseconds, a block of 6 octets is collected, one from each of the 6
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channels to be transported, and grouped in sequence in the ATM cell
payload. In the figure, only channels 1 and 6 are shown, for clarity. As
shov~in in the example, the cell payload thus holds 7 full blocks of 6 octets,
plus octets from the 8th block belonging to channels 1 to 4. The LI value in
the CIF indicates that the end of the first structure block occurs at octet
number 6. Since four octets of the 8th structure block appear in the
payload shown in figure 5, only two octets of the 8th structure block will
appear in the next cell payload so that an LI value of 2 would be indicated.
1o Figure 6 shows an example of the use of our transmission technique to
transport a single 64 kbit/s-based block structure in more than one ATM
cell, with the size of the block structure equal to 57 in this case. Every 125
microseconds, a block of 57 octets is collected, one from each of the 57
channels to be transported, and grouped in sequence in the payloads of
either two or three ATM cells. In the figure, only channels 1 and 57 are
shown, for clarity. As shown in the example, the first cell payload holds 46
octets of the 57 octet structure block. The LI value in the CIF is set to 64
indicating that the end of the structure block does not occur in this cell.
Since the remaining eleven octets of the structure block appear in the
2 o following cell payload, an LI value of 11 indicates where the end of the
57
octet block structure occurs.
The method described above may be adapted to handle traffic originating
from narrow band voice networks which employ optional compression and
silence suppression techniques. Typically, such networks operate in a
default mode at 64 kbit/s but when using compression, rates of 32, 24 and
16 kbit/s are common. The resulting compressed voice traffic is of
constant bit rate in character but becomes piece-wise constant bit rate
when silence suppression is invoked. Often, these networks arrange for
3 0 an in band negotiation procedure between voice terminals subsequent to
the connection set-up phase of a call, in order to agree the characteristics
of any compression and/or silence suppression to be used during voice
information transfer. Occasionally, the network may arrange to vary these
characteristics on existing calls automatically to reflect the current
congestion within the network. We have found that the transport of such
variable constant bit rate and piece-wise constant bit rate information
between IWFs can be achieved conveniently by the transport of a single
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narrow band channel per ATM connection. With silence suppressiory in
which transmission is halted during the gaps between speech bursts, the
extent of the fill of a given cell payload may be variable and unpredictable.
in addition, the amount of time required to fill the payload of an ATM cell
with information from a single voice channel will vary depending on the
degree of compression being used and to limit this variability, partial cell
fill
may be required in certain circumstances. To achieve this capability we
provide a single control information field (CIF) per ATM cell to recover the
information from individual channels at the destination IWF. This control
1o information field is shown in figure 7. The LI field indicates the end of
the
voice information being carried by the cell payload.
Operation of single channel information transfer in the manner described
above with reference to figures 4 and 7 preferably requires a sequence
number to be associated with each cell payload, so that missing and
misinserted cells can be detected. Since our single channel transfer has
no need for the conventional CIF's Logical Link Number field (LLN), 3 bits
of the LLN may be allocated to convey a sequence number (SN).
2 o In a further modification, the cell payload being transported between IWFs
may carry either voice information or in band signalling. In band signalling
may be used as described previously at the beginning of a call to agree the
compression characteristics far the call. It may also be used during the call
to alter the call characteristics or to convey background noise levels to be
used at the c_ .nation IWF during periods of silence suppression or for
other purposes. This alternate cell payload use is effected by means of an
indicator carried within the control information field which indicates whether
a cell payload carries voice information or inbound signalling information.
To achieve this, the fourth bit of the LLN field can be used as an
3 o information/signalling indicator, a value of zero indicating voice
information
and a value of one indicating signalling information.
The transmission techniques described above make it possible to use the
same basic ATM Adaptation Layer for the transport across an ATM trunk
3 5 network of multiple narrow band services, including
~ constant bit rate compressed voice services at multiple bit rates
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~ piece wise constant bit rate voice services with silence suppression,
again with multiple bit rates
~ constant bit rate 64 kbit/s and n x 64 kbit/s narrow band services.
Although the transmission technique has been described with reference to
the transport of traffic between narrow band legacy networks, it will be
appreciated that it is not limited to that particular application but is of
general application in the ATM transmission field.
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