| OSI layer | SS7 protocols |
|---|---|
| Application | INAP, MAP, IS-41... |
| Network | MTP Level 3 + SCCP |
| Data link | MTP Level 2 |
| Physical | MTP Level 1 |
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks. It is specified by the ITU-T as part of the Q.76x series,[1] ANSI (T1.113-YEAR) and Telcordia Technologies formerly Bellcore Generic Requirements document GR-246-CORE.
When a telephone call is set up from one subscriber to another, many telephone exchanges will be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called or calling party number to the next switch in the network using ISUP messages.
The telephone exchanges are connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Each timeslot between two switches is uniquely identified by a Circuit Identification Code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signalling information (especially the Called Party Number) to determine which inbound CICs and outbound CICs should be connected together to provide an end to end speech path.
In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available timeslots. In the case of no outbound CIC being available on a particular exchange, a release message is sent back to the preceding switches in the chain so a new route can be tried.
Contents |
Different ISUP variants exist. The main specification task is performed by the ITU-T. In Europe ETSI releases its own ISUP specification which is very close to the ITU-T ISUP.[2] The ITU-T and ETSI ISUP are used for international connections and they are the base for national ISUP variants. Most of the countries have their own ISUP variant to cover national specific requirements. ANSI specifies the ISUP variant utilized in the USA and Canada.
According to ITU-T Q.761 section 2.4.1 ISUP interworking ISUP'92 is backwards compatible with ISUP Blue Book and Q.767[3] for basic call procedures and supplementary services except for some procedures (eg. number portability).[4] Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions.
An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by a fixed-length part and optional variable-length part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part, or, less often, the Signalling Connection Control Part. These messages are transmitted in various stages of call setup and teardown. The most common messages are:[5]
This is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding.
A subscriber telco switch A telco switch B B subscriber
Off hook
Dial digits --->
-- IAM -->
-Ringing ->
<-- ACM --
Off hook
<-- ANM --
----------------------- Conversation -----------------------
On hook
-- REL --> On hook
<-- RLC --
Detailed call flows are provided in ITU-T Recommendation Q.784.1.
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| Routing Label
... |
|||||||
| CIC Least Significant 8 Bits | |||||||
| Padding | CIC Most Sig. 4 Bits | ||||||
| Message type | |||||||
| Mandatory fixed part
... |
|||||||
| Mandatory variable part
... |
|||||||
| Optional part
... |
|||||||
The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain the following components:[6]
The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.
The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.[7]
The Message Type indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type. The Mandatory fixed part, when present, contains the mandatory, fixed-length parameters associated with the message type. The Mandatory variable part, when present, contains the mandatory, variable-length parameters associated with the message type. The Optional part, when present, contains the optional parameters permitted to be included in the message type.
When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components (Message Type, Mandatory fixed part, Mandatory variable part, Optional part""). The routing label and circuit identification code are not included in the user data passed to SCCP.[8]