1 No.7 Signaling System-ISUP
In this course, there are three sections. In the first section, mainly introduce the signaling in ISDN and the
applications of ISUP. Then make explanation about the ISUP call control and call flow. In the last part, give a brief introduction to the ISUP message format. After this course, you will be able to master the call control and call flow of ISUP; understand the signaling in ISDN and the ISUP message format.
1.1 General
Ok, the first section: a brief introduction to the signaling in ISDN and the applications of ISUP.
The purpose of the signaling in an ISDN network is to deliver control information to the switching nodes for call establishment and call control through the ISDN network. The signaling in ISDN is, compared with the relatively simple signaling in the PSTN, a lot more comprehensive and powerful. For example, ISDN's capability to handle many different services puts new requirements on the signaling capacity.
The signaling in ISDN can be divided into two types. The first type is used between a subscriber terminal and the local ISDN exchange. This signaling utilizes the D -channel on the digital subscriber line and is called Digital Subscriber Signaling System No. 1 (DSS 1). The second type of signaling is used between exchanges when more than one exchange is involved in an ISDN call. Here the CCITT No. 7 Signaling System is used to deliver the control information to all involved exchanges. Please observe that, although the control information has the highest priority on the D -channel, there is capacity left for user-to-user information to be transported the same way. This means that also the No. 7 Signaling System must have the capacity to take care of user-to-user information. In the second section, it will be introduced briefly.
ISUP is used for signaling between exchanges in ISDN network. The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls. Here, please pay attention: calls that originate and terminate at the same switch do not use ISUP signaling. The channel which carries the signaling information can be one of the channels in an ordinary traffic PCM system or a dedicated signaling link. The signaling path is separated from the speech path and it serves a great number of speech circuits (common channel signaling).
1.2 Basic ISUP Call Control
Ok, after the brief introduction to the ISUP, the message flow for one call in ISDN is followed. At first, the introduction to the ISUP signaling between the exchanges. Because Both D -channel and ISUP signaling use messages as information carries, so the cooperation between DSS1 and ISUP will be introduced. End-to-End Signaling transmission is very important feature of ISUP. In the third part of this section, it will be introduced to you briefly. At last, brief introduction to the user-to-user signaling.
Ok, let's study the basic call control signaling process in ISDN. The connection between the ISDN users consists of two parts: the connection between user terminal and local exchange (network), and the connection between two exchanges.
First, let's see ISUP signaling between exchanges.
The first message: IAM (initial address message) When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch, it is marked by 1a in this figure.
The IAM includes the following contents:
Originating point code (OPC),
Destination point code (DPC),
Circuit identification code (CIC),
circuit "5" is occupied in this figure,
Dialed digits, that is, the called party number.
Optionally, the calling party number and name.
In the third section of this course, the message format will be introduced.
In this example, the IAM is routed via the home STP of the originating switch to the destination switch, it is marked by 1b.
The second message: ACM (address complete message)
After the IAM is received by the destination, the destination switch then checks the dialed number, determines that it serves the called party, and that the line is available for ringing. The destination switch rings the called party line and transmits an ISUP address complete message (ACM) to the originating switch via its home STP, it is marked by 2a in this figure. The message ACM indicates that the remote end of the trunk circuit has been reserved. The STP transfers the ACM to the originating switch, which is marked by 2b. It rings the calling party's line and connects it to the trunk to complete the voice circuit from the calling party to the called party.
In the example shown, the originating and destination switches are directly connected with trunks. If the originating and destination switches are not directly
connected with trunks, the originating switch transmits an IAM to reserve a trunk circuit to an intermediate switch. The intermediate switch sends an ACM to acknowledge the circuit reservation request and then transmits an IAM to reserve a trunk circuit to another switch. This process continues until all trunks required to complete the voice circuit from the originating switch to the destination switch are reserved.
The third message: ANM (answer message)
When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP. The STP routes the ANM to the originating switch. They are marked by 3a and 3b respectively. The message ANM verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.
The fourth message: REL (release message)
If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches (4a). The STP routes the REL to the destination switch (4b). If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).
The fifth message: RLC (release complete message)
Upon receiving the REL, the destination switch disconnects the trunk from the called party's line, sets the trunk state to idle, and transmits an ISUP release complete message (RLC) to the originating switch (5a) to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC (5b), it terminates the billing cycle and sets the trunk state to idle in preparation for the next call.
ISUP messages may also be transmitted during the connection phase of the call (i.e., between the ISUP Answer (ANM) and Release (REL) messages.
Please note that the same signaling link(s) are used for the duration of the call unless a link failure condition forces a switch to use an alternate signaling link.
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This slide shows another common procedure in ISUP. When a call is placed to an out-of-switch number, the originating SPA transmits an ISUP initial address message (IAM) to the destination switch SPB.
After the IAM is received by the SPB, it then checks message. If the contents which the SPB wants are not included in IAM, for example, the calling number, the SPB will then send message INR (information request) to the originating SPA for the calling number.
The SPA returns the calling party information using message INF (information). The latter message flow is similar to the previous example. Here won't say more about it.
We have studied that the connection between the ISDN users consists of two parts: the connection between user terminal and local exchange (network), and the connection between two exchanges. In the first section, we have studied that the signaling between a subscriber terminal and the local ISDN exchange utilizes the D -channel on the digital subscriber line and is called Digital Subscriber Signaling System No. 1 (DSS 1). Both D -channel and ISUP signaling use messages as information carries. A simple D -channel signaling sequence to set up a B -channel connection is shown in the figure. Ok, let's see the signaling cooperation between DSS1 and ISUP. The call request information in a SETUP message is transmitted between ISDN nodes by an Initial Address Message (IAM). The D -channel message ALERT corresponds, in this case, to an Address Complete Message (ACM), and CONNECT is converted to an Answer Message (ANM).
End-to-End Signaling transmission is very important feature of ISUP. In this part, you will study what is the end-to-end signaling and its applications at first, let’s study the transmission of end-to-end signaling.
End-to-end signaling is defined as the capability to transfer signaling information of end points significance directly between signaling end points. The end point is the originating and terminating exchanges of call. This kind of signaling is used, for instance, to request additional call related information, to invoke a supplementary service or to transfer user-to-user information transparently through the network. The typical application of end-to-end signaling is transmission of information about a certain supplementary service related with the call. End-to-end signaling is related with the call, but not directly with the circuit control. So this is a kind of non-circuit relative message in nature.
There are two end-to-end signaling methods supported in ISDN: Pass-along and SCCP methods.
In the Pass-along method a special message type (Pass-along Message-PAM) is used to inform the intermediate exchanges that end-to-end signaling is used.
The intermediate exchange is in charge of modification of the routing label, that is, CIC and DPC, and sends the message to the next point. The intermediate exchange doesn't analyze the contents of message, in other words, the message is transferred transparently via them to the destination.
This method can be used when the information to be transferred relates to an existing call for which a physical connection has been established. This method is mainly used when the exchange doesn't install SCCP.
In the SCCP method the ISDN User Part is using the services provided by the Signaling Connection Control Part (SCCP) to establish a signaling connection for the end-to-end signaling. The procedure is shown in this slide.
Now we will study the User-to-User signaling. User-to-user information carried by call establishment D-channel messages are also carried by call establishment messages over the ISDN network. It provides the direct communication between the ISDN users through the D-channel and No. 7 signaling network. All network nodes don’t analyze and process it. During the call, D- channel messages can be transmitted over the same signaling path that was established during the call setup. The called number analysis is not needed as the signaling path still exists in the processor memory. It can be used during the call setup, call connection and call release. During the call setup and release, the user-to-user information is transferred which is put in the message IAM, ACM, ANM, or REL. During the call setup, it can be transferred through the message USR between the ACM and ANM. During the talk or data communication, it is transferred using the message USR. User-to-user signaling without B -channel connection is done by establishing a virtual signaling connection. This can be solved in two ways:
1. A complement, called Signaling Connection Control Part (SCCP), is added between MTP and the user part.
2. One bit (called the M -bit) indicates that the Circuit Identification Code, CIC, does not refer to a physical B -channel circuit. The SCCP solution is more flexible and it can also be used for other purposes. For the user-to-user signaling and end-to-end signaling, they are not necessary to be mastered. Just understand it.
1.3 ISUP Message Format
Ok, let's see the last section for this course: ISUP message format. First, an introduction to the ISUP message format, then, take the message IAM as the example, explain it in detail.
ISUP information is carried in the Signaling Information Field (SIF) of MSU The field SI(Service Indicator) is equal to 0101, it indicates the ISUP message in the SIF.
In this course, introduce it according to the ITU-T standard.
The SIF contains the routing label followed 12-bit circuit identification code (CIC).
The CIC indicates the trunk circuit reserved by the originating switch to carry the call.
The CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC) which defines the contents of the remainder of the message. Each ISUP message contains a mandatory fixed part containing mandatory fixed-length parameters. Sometimes the mandatory fixed part is comprised only of the message type field.
The mandatory fixed part may be followed by the mandatory variable part and/ or the optional part.
The mandatory variable part contains mandatory variable-length parameters. The optional part contains optional parameter which is identified by a one-octet parameter code followed by a length indicator field. Optional parameters may occur in any order. If optional parameters are included, the end of the optional parameters is indicated by an octet containing all zeros. From above, you can find the ISUP message format is very similar to the SCCP. The description method of the parameters is also same to the SCCP.
The first part of ISUP message is routing label. It consists of DPC, OPC, and SLS. It is same to the SCCP.The first part of ISUP message is routing label. It consists of DPC, OPC, and SLS. It is same to the SCCP.
CIC follows the routing label. It occupies 2 octet in which 4 bits are spare.
Ok, now take the message IAM as example to explain the message format. The routing label and CIC are followed by the contents of five mandatory fixed length parameters which are message type, Nature of Connection Indicator, Forward Call Indicator Bits, Calling Party Category and Transmission Medium Requirement in sequence. Their length is shown in the slide. There is only one mandatory variable length parameter: called party number. So two pointers to it and optional part follow the mandatory fixed length part. After the mandatory variable part, the optional part follows. Suppose the calling party number is selected. 0A is its parameter name code. At last, the end of the optional parameters is indicated by an octet containing all zeros.
The message IAM parameters are shown in this slide.
Now, we have finished this course.
At last, let's make a review about it.
In the first section, it’s about the signaling in ISDN and the applications of ISUP.
Then, it is the explanation of the ISUP call control and call flow. In the last part, a brief introduction to the ISUP message format is given.
The second part is more important than the other two. After this course, you will be able to master the call control and call flow of ISUP; understand the signaling in ISDN and the ISUP message format.
Thank you
