SIGNALLING BETWEEN CIRCUIT MULTIPLICATION EQUIPMENTS (CME) AND
INTERNATIONAL SWITCHING CENTRES (ISC)
1. Introduction
This Recommendation contains principles and examples of signalling
between ISC (exchanges) and their associated circuit multiplication
equipments.
Circuit multiplication equipments may have integral echo control and
A/µ law converter functions. The information in this Recommendation is
compatible with the control procedures for such devices.
2. Definitions relating to CME
For a complete description of additional definitions see
Recommendation Q.dcme.
2.1 Digital circuit multiplication equipment (DCME) and CME
DCME and CME constitute a general class of equipment which permits
concentration of a number of trunks on a reduced number of transmission
channels. DCME in particular permits concentration of a number of 64 kbit/s PCM
encoded trunks on a reduced number of digital transmission channels.
2.2 Speech interpolation; digital speech interpolation (DSI)
A method of profiting from the time instants when a speaker is not
active, which is indicated by a speech detector. The channel is then used by
another active connection. The signals carried by a transmission channel
therefore represent interleaved bursts of speech signals derived from a number
of different trunks.
2.3 Low rate encoding (LRE)
Speech coding methods with bit rates less than 64 kbit/s, e.g. the
32 kbit/s transcoding process defined in G.721 applied to speech coded
according to G.711.
2.4 Speech activity
The ratio of the time speech and corresponding hangover occupies the
trunk to the total measuring time, averaged over the total number of trunks
carrying speech.
2.5 CME gain
The trunk channel to transmission channel multiplication ratio, which
is achieved through application of CME, including LRE and/or speech
interpolation (DSI).
FIGURE 1/Q.50
CME gain
2.6 Trunk
A bidirectional connection consisting of a forward channel and a
backward channel between the ISC and CME not subject to LRE or DSI operation.
2.7 Transmission channel - bearer channel
One channel of the connection between the transmit unit and receive
unit of corresponding CME.
2.8 Freeze-out
The temporary condition when a trunk channel becomes active and cannot
immediately be assigned to a transmission channel, due to lack of available
transmission capacity.
2.9 Freeze-out fraction
The ratio of the sum of the individual channel freeze-outs to the sum
of the active signals and their corresponding hangover times and front end
delays, for all trunk channels over a fixed interval of time, e.g. one minute.
2.10 Transmission overload
The condition when the freeze-out fraction or average bits per sample
goes beyond the value set in accordance with speech quality requirements.
2.11 Operating modes
2.11.1 Point-to-point mode (see Figures 2a/Q.50 and 2b/Q.50)
Point-to-point - Using Figure 2a/Q.50 for reference, the transmit side
CME concentrates N trunks into N/G transmission channels, where G is the CME
gain.
At the receive side, the receiving CME simply reconstitutes the
N trunks from the N/G transmission channels.
FIGURE 2a/Q.50
Point-to-point unidirectional
FIGURE 2b/Q.50
Point-to-point two origins unidirectional
FIGURE 2/Q.50
Multi-clique for two origins and two destinations unidirectional
The example in Figure 2b/Q.50 also shows a point-to-point mode. From
the switching point of view there could be a difference between the
configurations in Figures 2a/Q.50 and 2b/Q.50.
For transmission of alarms it has also to be considered, that different
exchanges may be connected to one CME.
2.11.2 Multi-clique mode (see Figure 3/Q.50)
Multi-clique mode - in this mode the pool of transmission channels is
sub-divided into several independent pools (cliques) of fixed capacity, each
destination specific. If a part of the cliques capacity is not used, it cannot be
used for another destination.
FIGURE 3/Q.50
Multi-clique mode (only one direction shown)
2.11.3 Multi-destination mode
A DCME operational mode where input trunk channel traffic is
interpolated over a pool of available transmission channels for all destinations having
traffic in the pool. The transmit trunk channels are designated to receive trunk
channels at corresponding locations.
Figure 4/Q.50 shows a unidirectional system block diagram for a multi-
destination mode with two transmit and two receive DCME units.
FIGURE 4/Q.50
Multi-destination mode (only one direction shown)
3. Requirements for control
3.1 Reasons for use of circuit multiplication equipments (CME)
Circuit multiplication equipments are used in order to reduce the
bandwith required for transmission of a given set of calls. This can be achieved
by reducing the redundancy which is inherent in speech communications. CME gains
of up to 5:1 can be achieved using DSI + LRE with subjectively acceptable quality.
Thus, the amount of line plant required between switching points and hence the cost
of provision can be minimized.
3.2 Integration of CMEs into the telephone network
Normally, when an exchange needs an outgoing circuit, the circuit selection
is based on circuit availability. In this example, the call may be blocked if all
of the circuits are unavailable due to traffic or maintenance. If the same call
encounters a CME, the possible outcomes are more complex.
From the point of view of call set-up, two CME aspects may necessitate
information transfer between the exchange and the CME.
a) Transmission Capacity - The circuit multiplication characteristics of
a CME result in a lower total transmission capacity for the CME as compared to the
transmission capacity of all of the input trunks. A call may find a free (unseized)
circuit from the exchange to the CME but no available transmission channels between
two CMEs. For systems employing speech interpolation, allowing additional calls
could lead to unacceptable speech quality degradation due to freeze-out. The
probability of freeze-out can be reduced by the creation of overload channels using bit-stealing techniques. Additional quality control is achieved if the exchange knows,
through a Transmission Resource Management System, if the CME has available
capacity to complete a new call.
b) Call Set-Up/Release - Depending on the bearer service type of the call
to be set-up, and on whether or not the CME is able by itself to establish the
inter-CME connections, the seizing/releasing actions in the exchange may need to be
extended to the CME by means of out-of-band information transfer. For example, in DSI
systems, speech connections are made dynamically on detection of channel activity
performed by built-in speech detectors. For 64 kbit/s unrestricted on-demand
connections (and for 3.1 kHz audio, if appropriate) through DSI systems (i.e., not
through internal pre-assignment), the establishment and disestablishment of connections
between the CMEs have to be initiated from the outgoing exchange.
In general, these two aspects are strictly independent from each other as
each serves a different purpose. However, depending on the design criteria in the
CME and the call set-up procedures in the exchange and the CME associated with one
aspect may be related to that of the other.
3.3 Factors for signalling functions determination
The functional requirements for signalling between CMEs and exchanges are
determined by the type of CME with its capabilities and limitations, and by the
types of bearer services it supports.
The remote control of echo control devices and A/µ-law converters, if they
are integrated into the CME, is accomplished either by the terminal or test
equipment or directly from the ISC (based on call set up information/signalling
information).
Requirements and actions for control of ECD are described in CCITT
Recommendation Q.115.
3.3.1 Circuit multiplication equipment and physical location
There are different types of CME which are being used or will most likely
be used in the international telephone network, each with its own capabilities and
limitations:
a) 32 kbit/s low rate encoding (LRE);
b) analogue speech interpolation equipment;
c) digital speech interpolation (DSI) with 64 kbit/s PCM;
d) combined 32 kbit/s LRE and DSI
e) 16 kbit/s LRE.
The location of certain types of CME relative to the exchange
determines the choice of signalling interface. These CMEs can be
located at the ISC or remote from the ISC (e.g., at an earth station).
Certain types of signalling interfaces may be more practical when these CMEs
are co-located with the ISC, and others may be more practical when they
are remote from the ISC. Therefore, the location of the CME needs to be
considered when choosing the signalling between ISC and CME.
When the CME is remote from the ISC, the link between the ISC and CME
could be composed of digital or analogue transmission path. Both conditions
have different equipment configurations and different signalling requirements
(see section 7).
3.3.2 Bearer services supported on CME links
Up to four basic bearer service types are supported or will likely be
supported by CMEs in the international network:
- speech bearer service (full duplex, analogue or digital);
- 3.1 kHz audio bearer service (full duplex);
- 64 kbit/s unrestricted bearer service (full duplex);
- alternate speech/64 kbit/s unrestricted bearer service (full
duplex) (in call modification is for further study).
Each CME type supports one or more bearer services depending on special
facilities or functional options built in the equipment.
Different LRE algorithms will also have different levels of performance,
for instance, in terms of voiceband data. Since certain speech optimized
algorithms have limited transparency to voice band data, the CME has internal facilities
(e.g., data detectors combined with route around mechanisms and/or special
algorithms) to overcome its inherent limitations. This approach clearly separates the
CME transmission problems from the ISC switching functions as much as possible to
allow independent developments.
TABLE 1/Q.50
Bearer services supported in CMEs in relation to CME-exchange signalling
List of abbreviations:
TRM = transmission resource management
CSM = call set-up messages between CME and ISC
NS = bearer service not supported
NX = bearer service supported without message exchange
FS = further study
1) message exchange not necessarily implemented
2) supported through pre-assignments (e.g., Recommendation G.761
transcoder DNI)
3) supported in a limited fashion (e.g., Recommendation G.761)
4) CSM not needed with internal CME special handling facilities
4. Bearer services and CME techniques in the context of signalling
Table 1/Q.50 gives the relationship between CME techniques and the four
bearer services identified in 3.3.2 with regard to their supportability and the
need for CME-exchange message transfer.
The signalling function requirements are categorized on the basis of
bearer services supported by the different CME techniques. For speech bearer
services, transmission resource management (TRM) information alone is adequate especially
for CMEs employing speech interpolation. The objective of this provision is to
maintain the reduction of transmission quality within tolerable limits. In addition
to TRM information, external call set-up message (CSM) exchange is needed for
bearer services involving on-demand 64 kbit/s unrestricted service in contemporary
digital circuit multiplication equipment (32 kbit/s LRE and DSI).
5. Division of functionality between the ISC and the CME
5.1 CME dynamic load control process
Transmission resource management (TRM) information is based on traffic
load measurements at the local and distant CMEs. Therefore in the multi-
destination and multi-clique mode of operation, TRM information is provided for each
destination/clique separately.
A universal arrangement is used for handling TRM information between CME
and an ISC. The TRM information is dynamically presented to the exchange in one
of two states for each bearer service. The states are called "available" and "not
available". Logic within the CME is used to determine which of the two states
should be indicated to the exchange regardless of any condition at the exchange.
When a CME encounters a "not available" state for a bearer service
(either locally or remotely), it presents this indication to the exchange so it will
stop routing new calls to the CME for that bearer service even if there are free,
unseized circuits available. The exchange will continue to prohibit calls to the
CME until it receives an "available" indication for the bearer service when in
both, local and remote CMEs, there is no overload.
This dynamic load control information is therefore directly influencing
the circuit selection process in the exchange during call set-up for each bearer
service separately. The circuit selection in the exchange is a check whether or
not a free unseized circuit is suitable for a certain bearer service type, for
which a new call is to be accommodated. For example, the exchange would select a free
circuit for a speech call if "speech capacity available" is indicated,
irrespective of the indications for other bearer service types. If the DCME link is unable
to accommodate additional new 64 kbit/s calls, all free unseized circuits within
the exchange will be marked accordingly. Even though the generation of bearer
service related TRM information with DCMEs may be in part mutually dependent (i.e.,
no capacity for speech implies no capacity for any other bearer service types but
not necessarily vice-versa), separate signalling and processing for each bearer
service type are necessary to allow different future CMEs to develop
independently.
5.2 Call set-up process
According to Table 1/Q.50, the contemporary digital circuit
multiplication equipment, having the capability to support on-demand all four identified
bearer services, in addition to providing TRM to the exchange, requires call set-up
messages (CSM) (from the exchange) for selecting bearer services.
For the 64 kbit/s unrestricted bearer service, a circuit is selected if
"unrestricted capacity available" is indicated, and a CSM in the form of
Seizure/Select request is forwarded to the DCME. An acknowledgement (positive or negative)
is sent upon recognition of a 64 kbit/s request even if capacity is available.
The positive acknowledgement can be used by the ISC to initiate the
interexchange signalling to the next ISC (e.g. transmission of the IAM of Signalling
System No. 7). A failure to establish a 64 kbit/s circuit between CMEs must be
reported to the ISC as soon as the condition has been identified by the CME by using
an out-of-service message.
The out-of-service message is considered by the ISC to be equivalent to
the alarm signal defined in Recommendation Q.33. The ISC will take release actions
(if appropriate) as specified in Recommendation Q.33, 4.
The released 64 kbit/s message from the ISC will be positively
acknowledged after proper completion of the DCME circuit disestablishment process. Failure
to complete this process shall be notified to the ISC using an out-of-service
message and the DCME will put the circuit in a blocked condition. After the failure
condition is removed, this circuit will be in idle condition and a back-in-service message shall be sent to the ISC.
Under a 64 kbit/s unrestricted dual seizure situation, the non-
controlling ISC will initiate a release of the DCME connection using procedures defined in
the appropriate inter ISC signalling system protocol. If the DCME is unable to re-establish a remotely released 64 kbit/s duplex connection, it shall indicate
this abnormal situation to the appropriate ISC by an out-of- service message.
The information elements and procedures necessary to support the
alternate 64 kbit/s speech bearer services are for further study.
FIGURE 5/Q.50
Typical ISC/CME information flows
5.3 Inter-dependency between dynamic load control and call set-up process
To allow a standard method of interworking with inter-exchange signalling
systems it is important to adopt the functional interdependency between TRM and
CSM as described above.
6. Control information elements between ISC and CME
The amount of control information elements utilized between the ISC and
the CME depends on the capabilities of the CME and the ISC. Two categories of CME
signalling capabilities are recognized. The first category of CME (Type 1) is
capable of only transmitting signals from the CME to the ISC (e.g. Dynamic Load
Control, see paragraph 6.1). The second category of CME (Type 2) is able to transmit
and receive signals to/from the ISC.
Tables 2/Q.50, 3/Q.50 and 4/Q.50 give a set of information elements and their
flow on the control link between the ISC and the CME for the second category of
CME.
6.1 Information elements for Type 1 CME
Type 1 CME only should use the following types of information elements.
The "m" indicates mandatory use, the "o" optional use.
1. No capacity for speech available (m)
2. Channel(s) available for speech (m)
(speech includes 3.1 kHz audio)
3. Out-of-service (o)
4. Back-in-service (o)
6.2 Information elements for Type 2 CME
TABLE 2/Q.50
Information elements for transmission resource management
