The SCCP provides two categories of service for data transfer: connection-oriented services and connectionless services. Within each service category, two classes of service are defined as follows:
- Class 0— Basic connectionless class
- Class 1— In-sequence delivery connectionless class
- Class 2— Basic connection-oriented class
- Class 3— Flow control connection-oriented class
Connection-oriented Versus Connectionless Services
The analogy of sending letters and postcards best explains the difference between the connection-oriented and the connectionless services. The postal service carries out the physical transfer and is therefore analogous to MTP. Connection-oriented service is much like the exchange of formal letters. When you send a formal letter, you might assign a reference to it—"Our Reference X." When the receiving party responds, they might also assign their own reference to the letter and also copy the sender's reference—"Your Reference X." From that point on, both parties state their own and each other's assigned reference. SCCP connection-oriented service uses the same principles; the "Our Reference" is known as the Source Local Reference (SLR), and the "Your Reference" is known as the Destination Local Reference (DLR). This is similar in principle to Transmission Control Protocol (TCP): data is sent only when a virtual connection has been established through the initial exchange of identifiers. Figure 9-4 illustrates this principle.
Figure 9-4 Analogy of Connection-oriented Service with Official Mail Correspondence
Connectionless service is like sending postcards, where the sender and recipient do not establish references. In principle, it is similar to User Datagram Protocol (UDP): data is sent without first establishing a virtual connection using identifiers.
User Data and Segmentation
The data (from subsystems) is sent in information elements called Network Service Data Units (NSDUs). SCCP provides the capability to segment or reassemble an NSDU that is too large to fit in a single MTP message (MSU) so that it can be transmitted over a number of MSUs (16 maximum). When using the connectionless classes, if an NSDU is greater than 255 octets when using a UDT message or 254 when using a XUDT message, the originating node splits the NSDU into a number of XUDT messages. For a description of UDT and XUDT messages, see section "Message Types" and refer to Appendix C, "SCCP Messages (ANSI/ETSI/ITU-T)." If an NSDU is greater than 255 octets when using the connection-oriented classes, the originating node splits the NSDU into several DT messages. The receiving node reassembles the NSDU. For a description of the DT message, see the section on "Message Types" and refer to Appendix C. Theoretically, the maximum amount of user data is 3952 [1] octets in ITU-T SCCP [58-61] and [2] 3904 octets in ANSI SCCP. This excludes optional parameters and global titles, which will appear to be repeated in each message. The ITU-T recommends using 2560 as the maximum NSDU size as a safe implementation value [16] because it allows for the largest global title and numerous optional parameters. The section on "SCCP Routing Control (SCRC)" covers global titles.
The parameter Protocol Class within each SCCP message specifies the protocol class. Before giving a further explanation of connectionless and connection-orientated procedures the following sections discuss the four classes of data transfer that SCCP provides.
Connectionless Protocol Classes
Class 0 provides a basic connectionless service and has no sequencing control. It does not impose any conditions on the Signaling Link Selection (SLS) values that MTP3 inserts; therefore, SCCP messages can be delivered out of sequence. Class 0 can be considered a pure connectionless service. See Chapter 7, "Message Transfer Part 3 (MTP3)," for information about the SLS field.
Class 1 service adds sequence control to the Class 0 service by requiring the SCCP to insert the same SLS field for all NSDUs that have the same Sequence Control parameter. The higher layers indicate to SCCP whether or not a stream of NSDUs should be delivered in sequence. Therefore Class 1 is an enhanced connectionless service that provides basic in sequence delivery of NSDUs. Failures at the MTP level can still result in messages being delivered out of sequence.
TCAP is the typical user of SCCP connectionless services. The other user is Base Station Subsystem Application Part (BSSAP), which is used solely for GSM cellular radio related signaling. See Chapter 3, "The Role of SS7," for a brief description of BSSAP. Although the applications (subsystems) use TCAP directly, they are considered SCCP users because TCAP is considered transparent. See Chapter 10 for more information about TCAP.
Table 9-1 shows the connectionless service protocol classes and features.
Table 9-1. Connectionless Service Protocol Classes
Protocol Class and Name
|
Features
|
Example Use
|
Protocol Class 0: Basic Connectionless
|
Independent message transport, no sequencing
|
Some BSSMAP messages (Paging), TCAP
|
Protocol Class 1: Connectionless Service
|
Independent message transport, limited sequencing
|
TCAP
|
Connection-oriented Protocol Classes
Class 2 provides a basic connection-oriented service by assigning local reference numbers to create a logical connection. Messages that belong to the same connection are assigned the same SLS value to ensure sequencing. Class 2 does not provide flow control, loss, or missequence detection.
Class 3 is an enhanced connection-oriented service that offers detection of both message loss and mis-sequencing (for each connection section). Class 3 also offers flow control using an expedited data transfer function. The ETSI European SCCP standard, ETS 300-009 [10], offers support for Class 3 only from V1.4.2 (November 1999) onwards.
The ITU-T had specified a Class 4, but this was never implemented on live networks and was later removed in White Book editions.
Table 9-2 shows the connection-oriented service protocol classes and features.
Table 9-2. Connection-oriented Service Protocol Classes
Protocol Class and Name
|
Features
|
Example Use
|
Protocol Class 2: Basic Connection-oriented Service
|
Logical signaling connection used for message transport
|
Some BSSMAP messages (Setup)
|
Protocol Class 3: Connection-oriented Service
|
Logical signaling connection used for message transport, and flow control (expedited data transfer)
|
No known current use
|
SCCP Connectionless Control (SCLC)
SCLC is used to provide the capabilities that are necessary to transfer one NSDU in the "data" field of a UDT, Long Unit Data (LUDT), and XUDT message. For a description of SCCP messages, see section "Message Types" and Appendix C. The SCLC routes the message without regard to the route that the messages follow through the network. These services are provided without setting up a logical connection.
SCLC formats the user data into a message of the appropriate protocol class (0 or 1 in the case of connectionless) and transfers it to SCRC for routing. The section on "SCCP Routing Control (SCRC)" describes SCRC. On receiving a message, SCLC is responsible for decoding and distributing the message to the appropriate subsystem. Figure 9-5 shows data transfer using SCLC: data is simply sent without the prior establishment of references at each side.
Figure 9-5 The Transfer of Connectionless Messages from One SCCP User to Another
SCCP Connection-Oriented Control (SCOC)
SCOC is used to route messages through a specific, fixed logical network path. To establish a dedicated logical connection between an originating SCCP user (subsystem) and a terminating SCCP user (subsystem), the SCCP users residing at different nodes throughout the network communicate with each other.
A signaling connection between the SCCP users is established, making both SCCP users aware of the transaction by using the DLR and SLR parameters. The signaling connection is released at the end of the transaction (information transfer). This is similar to SS7 protocol TUP/ISUP, which is used to control telephony calls, in that a connection is setup and released at a later time. However, the connection is virtual; there is not a trunk with user traffic being set up and released—rather, there is a virtual connection over the signaling network for the purpose of data transfer between applications (subsystems).
Connection-oriented procedures can be split into three phases:
- Connection Establishment Phase— The SCCP users set up a logical, fixed path that the data packets will follow. The path might involve only two or three nodes with SCCP capability or, depending on how many intermediate nodes exist between the originator and terminator, it might involve a much larger number.
- Data Transfer Phase— After the connection is established, the data that is to be transferred is converted into an NSDU and sent in a DT1 or DT2 message. For a description of SCCP messages, see the section on "Message Types" and Appendix C. Each NSDU is uniquely identified as belonging to a specific signaling connection. In this way, it is possible for the SCCP to simultaneously handle independent signaling connections.
- Connection Release Phase— After all NSDUs have been transmitted and confirmed, either or both of the user applications that initiated the process release the logical path. A release can also occur if the connection fails.
An example of a connection-oriented data transfer is carried out in Figure 9-6. At the request of the SCCP user, SCCP A establishes a logical connection by sending a Connection Request (CR) message to SCCP B and assigning a SLR to the request. The remote node confirms the connection by sending a Connection Confirm (CC) message and includes its own SLR and a DLR that is equal to SCCP A's SLR. This gives both sides a reference for the connection.
Figure 9-6 The Transfer of Connection-oriented Messages from One SCCP User to Another Using a Temporary Connection
The CR message contains the address of the destination SCCP node and user. The subsequent data message DT1 only needs to send the DLR because the logical connection has been established through the proceeding exchange of SLR and DLR. The clear-down messages contain both SLR and DLR. If intermediate nodes are involved, they make associations between pairs of SLR/DLRs to establish the logical connection. Upon release, the SLR/DLR references are available for further use on other transactions. SCCP nodes can establish multiple simultaneous logical connections through the use of the SLR and DLR.
In Figure 9-5, if SCCP B received a CR message and either the SCCP B or the SCCP A could not establish the connection, a Connection Refused (CREF) message would have been returned.
Classes 2 and 3 (discussed previously) can either establish temporary connections (that is, on demand by SCCP user), as shown inFigure 9-5, or permanent signaling connections that are established by management action. Temporary connections are analogous to dialup connections, and permanent connections are analogous to leased lines. The connection establishment and release services are not required on permanent connections.
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