ISUP

1 Overview

I. Basic Concepts
ISUP is one kind of UPs of the No.7 public channel signaling system. It provides the signaling function required for supporting voice and non-vocie basic bearer services and supplementary services in the integrated services digital network (ISDN).
ISUP is applied to the hybrid digital/analog network, telephony network, and dedicated circuit-switched data network. ISUP meets the requirements of CCITT for international semi-automatic and automatic telephony services and circuit-switched data services.
II. Protocol stack architecture
The ISUP uses services provided by the MTP to transfer information between ISUPs. Figure 6-7 shows the protocol stack of the ISUP. ISUP information is carried to the MTP or from MTP to ISUP by primitives in the form of parameters.
Figure 6-7 ISUP protocol stack
Primitives used between MTP and ISUP include the transfer primitive, recovery primitive, suspension primitive, and status primitive.
The MTP transfer primitive receives and sends ISUP singaling messages by encapsulating the messages.
When MTP sends the MTP suspension primitive, it means MTP cannot send messages to a specific destination as parameters.
When MTP sends the MTP recovery primitive, it means MTP can be recovered to parameters and can send messages to a specific destination in an unrestricted manner.
When MTP sends the MTP status primitive, it means that the signaling route to a specific destination is congested or that there is no ISUP on the destination. This may be because ISUP is not installed or ISUP cannot be accessed. Other unkown factors may cause this problem too.
III. Application in NGN
ISUP has three application modes in NGN solutions, as shown in Figure 6-8, Figure 6-9 and Figure 6-10.
1) Application of ISUP in NGN (MTP-MTP)
Figure 6-8 Application of ISUP in NGN (MTP-MTP)
A MTP link goes directly from the SoftX3000 to connect a STP, thus realizing the interoperation of SS7 with a PSTN switch through an SS7 network. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
2) Application of ISUP in NGN (M2UA-MTP)
Figure 6-9 Application of ISUP in NGN (M2UA-MTP)
The SoftX3000 provides an M2UA link to connect with the TMG8010, and exchanges SS7 with a PSTN switch through the TMG8010, which has the function of an embedded signaling gateway. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
3) Application of ISUP in NGN (M3UA-MTP)
Figure 6-10 Application of ISUP in NGN (M3UA-MTP)
The SoftX3000 provides an M3UA link to connect with the SG7000, and exchanges SS7 with a PSTN switch through the SG7000. In the voice channel, the SoftX3000 controls the TMG8010 to implement the interconnection with the PSTN.
6.3.2 Singnaling Message
ISUP messages are transferred on the signaling link through the MSU. The messages are encapsulated in the SIF of the MSU. An ISUP message consists of six parts: routing tag, circuit identification code (CIC), message type code, mandatory fixed part, mandatory variable part, and optional part, as shown in Figure 6-11.
For details of the routing tag and CIC, refer to 6.2.2 Singnaling Message. The following introduces other parts of the ISUP message.
Figure 6-11 Structure of the ISUP message
I. Message Type Code
A message type code is an SIO field, which is required for all messages. The message type code defines the function and format of each kind of ISUP message. See Table 6-4.
Table 6-4 ISUP message code
Code
Abbreviation
Meaning
00000001
IAM
Initial address message: A message sent in the forward direction to initiate occupancy of an outgoing circuit and to transmit number and other information relating to the routing and handling of a call.
00000010
SAM
Subsequent address message: A message that may be sent in the forward direction following an initial address message, to convey additional called number information.
00000011
INR
Information request: A message sent by a switch to request information in association with a call.
00000100
INF
Information: A message sent to convey information in association with a call, which may have been requested in an information request message.
00000101
COT
Continuity message: A message indicating whether or not there is continuity on the preceding circuit as well as of the selected circuit to the following switch, including verification of the communication path across the switch with the specified degree of reliability.
00000110
ACM
Address complete message: A message indicating that all the address signals required for routing the call to the called party have been received.
00000111
CON
Connect message: A message indicating that all the address signals required for routing the call to the called party have been received and that the call has been answered.
00001000
FOT
Forward transfer message: A message sent in the forward direction on semi-automatic calls when the outgoing international switch operator wants the help of an operator at the incoming international switch. The message will normally serve to bring an assistance operator into the circuit if the call is automatically set up at the switch. When the call is completed through an operator at the incoming international switch, the message should preferably cause this operator to be recalled.
00001001
ANM
Answer message: it indicates that the call has been answered. In semi-automatic working, this message has a supervisory function. In automatic working, this message is used in conjunction with charging information.
00001100
REL
Release message: A message sent in either direction to indicate that the circuit is being released due to the cause supplied and is ready to be put into the idle state on receipt of the release complete message. When the call is redirected, the redirection message will also carry the redirection number.
00001110
RES
Resume message: A message sent in either direction indicating that the calling or called party, after having been suspended, is reconnected.
00010000
RLC
Release complete message: A message sent in either direction in response to the receipt of a release message, or if appropriate to a reset circuit message, when the circuit concerned has been brought into the idle condition.
00010001
CCR
Continuity check request message: A message sent by a switch for a circuit on which a continuity check is to be performed, to the switch at the other end of the circuit, requesting continuity checking equipment to be attached.
00010010
RSC
Reset circuit message: A message sent to release a circuit, due to memory broken or other causes.
0010011
BLO
Blocking: A message sent only for maintenance purposes to the switch at the other end of a circuit, to cause an engaged condition of that circuit for subsequent calls outgoing from that switch. When a circuit is used in the dual-circuit mode of operation, a switch receiving the blocking message must be capable of accepting incoming calls on the concerned circuit unless it has also sent a blocking message. Under certain conditions, a blocking message is also a proper response to a reset circuit message.
00010101
BLA
Blocking acknowledgement: A message sent in response to a blocking message, indicating that the circuit has been blocked.
00010111
GRS
Circuit group reset: A message sent to release an identified group of circuits.
00011000
CGB
Circuit group blocking message: A message sent to the switch at the other end, indicating the specified circuit group has been blocked.
00011001
CGU
Circuit group unblocking: A message sent to the switch at the other end of an identified group of circuits to cause cancellation in that group of circuits of an engaged condition activated earlier by a blocking or circuit group blocking message.
00011010
CGBA
Circuit group blocking acknowledgement: A message sent in response to a circuit group blocking message to indicate that the requested group of circuits has been blocked.
00011011
CGUA
Circuit group unblocking acknowledgement: A message sent in response to a circuit group unblocking message to indicate that the requested group of circuits has been unblocked.
00011111
FAR
Facility request: A message sent from a switch to another switch to request activation of a facility.
00100000
FAA
Facility accepted: A message sent in response to a facility request message, indicating that the requested facility has been activated.
00100001
FRJ
Facility rejected: A message sent in response to a facility request message to indicate that the facility request has been rejected.
00100100
LPA
Loop-back acknowledgement message: A message sent in the backward direction in response to a continuity check request message indicating that a loop (or transceiver in the case of a 2-wire circuit) has been connected.
00101000
PAM
Pass-along message
00101001
GRA
Circuit group reset acknowledgement: A message sent in response to a circuit group reset message and indicating that the requested group of circuits has been reset. The message also indicates the maintenance blocking state of each circuit.
00101010
CQM
Circuit group query message: A message sent on a routine or demand basis to request the far-end switch to give the states of all circuits in a particular range.
00101011
CQR
Circuit group query response: A message sent in response to a circuit group query message to indicate the states of all circuits in a particular range.
00101100
CPG
Call progress: A message sent in either direction during the set-up or active phase of the call, indicating that an event, which is of significance, and should be relayed to the originating or terminating access, has occurred.
00101111
CFN
Confusion message: A message sent in response to any message (other than a confusion message) if the switch does not recognize the message or detects a part of the message as being unrecognized.
00110000
OLM
Overload message: A message sent in the backward direction, on non-priority calls in response to an IAM, to invoke temporary trunk blocking of the circuit concerned when the switch generating the message is subject to load control.
00110001
CRG
Charging information: Information sent in either direction for accounting and/or call charging purposes.
00110010
NRM
Network resource management message: A message sent in order to modify network resources associated with a certain call. The message is sent along an established path in any phase of the call.
00110011
FAC
Facility: A message sent in either direction at any phase of the call to request an action at another switch. The message is also used to carry the results, error or rejection of a previously requested action.
00110110
IDR
Identification request
00110111
IDS
Identification response
00111000
SGM
Segmentation message: A message that may be sent in either direction to convey an additional message segement of an extremely long message.
00011101
Reserved.
00011100
00011110
00100111

Each kind of message is composed of a message type code and several parameters. Each parameter has a name that is coded by a single octet. The length of a parameter can be fixed or variable, and each parameter has an LI, the length of which is one octet.
II. Mandatory Fixed Part
For a specified message type, those mandatory parameters with fixed lengths are contained in the mandatory fixed part. The locations, lengths and order of them are defined by the message type. In this case, the message does not include the name and LI of its parameter.
III. Mandatory Variable Part
Those mandatory parameters with variable lengths are contained in the mandatory variable part. A pointer is used to indicate the start of each parameter, and it is coded based on a single octet. The parameter name and pointer transmitting order are implicit in the message type, and the numbers of parameters and pointers are defined by the message type.
A pointer also can be used to indicate the start of an optional part. If a message type specifies that the optional part is not allowed, this pointer does not exist. If a message type specifies that there may be optional part, but this specific message does not include optional part, the pointer field is all 0.
All pointers are transmitted consecutively at the start of mandatory variable part. Each parameter includes parameter LI and contents.
IV. Optional Part
Optional part is composed of parameters, which may appear or not appear in any specific message type. Parameters may be of fixed or variable lengths, and optional parameters can be transmitted in any order. Each optional parameter should contain a parameter name (one octet), a LI (one octet) and the parameter content.
"Optional parameter end" octet: If there are optional parameters, the "optional parameter end" octet will be transmitted after all optional parameters are sent out, and the octet is all 0.
6.3.3 Basic Signaling Flow
Figure 6-12 shows the call setup and release flow originated by an IP trunk media gateway. When an IP trunk media gateway originates a call, the media gateway is connected with subscribers through the circuit trunk of a circuit-switched network, and call signaling enters the SoftX3000 through an SS7 gateway.
This flow example is based on the following conventions:
l The caller is controlled by MG1 and SG1.
l The callee is controlled by MG2 and SG2.
l ISUP is taken as an example of SS7.
l MG1 and MG2 are controlled by the same SoftX3000.
Figure 6-12 ISUP call setup and release flow originated by a trunk media gateway
1) After the caller dials the number of the callee, an IAM is sent to the SoftX3000 through SG1.
2) A context is created in MG1. TDM termination and RTP termination are added in the context. [Mode] is set to “SendReceive”. The jitter buffer and voice compression algorithm are also set. MG1 returns its RTP port number and voice compression algorithm through the Reply command.
3) A context is created in MG2. TDM termination and RTP termination are added in the context. [Mode] is set to “SendReceive”. The jitter buffer and voice compression algorithm are also set. MG2 returns its RTP port number and voice compression algorithm through the Reply command.
4) The SoftX3000 sends an IAM to the circuit-switched network through SG2. The circuit-switched network returns an ACM. The phone set of the callee rings.
5) The SoftX3000 sends the Modify command to MG1 and reports the remote RTP port number.
6) The SoftX3000 sends an ACM to SG1.
7) The callee hooks off. SG2 sends an ANM to the SoftX3000.
8) The SoftX3000 sends an ANM to SG1.
9) The callee hooks off. SG2 sends an REL to the SoftX3000. The SoftX3000 sends an REL to SG1.
10) The SoftX3000 sends the Subtract command to MG1 and MG2.