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SCCP Segmentation

Overview

SCCP segmentation provides the capability of breaking down larger packets of data into smaller ones in order to achieve compatibility with any network protocol that requires the smaller packet size. This is necessary whenever large blocks of data need to be transmitted across a network Segmenting the data helps offset problems with both time delays and error correction that could lead to traffic congestion. Segmentation in this way conserves critical network resources.

Specifically, the process of SCCP Segmentation gives an SCCP-user the ability to configure and send user data without having to perform any segmentation at the user level. SCCP handles all segmentation requirements.

Size of Packetized Data

The size of data sent to the host is currently limited by the ICB length. The ICB length is a maximum of 255 bytes in API messages. Connectionless transfer of data using SCCP requires the use of unitdata and the extended unitdata structures. These message structures provide all the information necessary for data to be transferred to a remote entity and to be processed by that remote entity. The segmentation parameter is included with these messages in an SCCP connectionless service:

• Extended UnitData (XUDT, (maximum of 16)

• Extended UnitData Service (XUDTS)

For an SCCP user resident in the host, the size of the total data sent between the SS7 PQ card and the host will be limited by the maximum length of the ICB, which is 255 bytes, until the system level segmentation has been implemented.

No segmentation occurs at the user level; therefore, an SCCP user is able to send up to 2048 bytes of data for ITU protocol and up to 3092 bytes of user data for the ANSI protocol.

Related Specifications

• ITU-T Q.711 - Q.714: Signaling Connection Control Part, White Book

• ANSI T1.112 - 1996

Managing User Data in SCCP

Data sent by an SCCP user can be limited by the constraints inherent in the MTP layer, that is, the maximum length of the Message Signal Unit (MSU). Through segmentation, the user is able to transmit through MTP efficiently because the data is segmented into manageable units called XUDT messages. When SCCP receives the XUDT messages, it reassembles all data into an N_UNITDATA primitive and forwards that to the SCCP user/receiver.

SCLC, SCRC and SSI Modules

It is the SCCP SCLC module that segments the N_UNITDATA primitive into XUDT messages and which also performs the reassembly of data. The SCRC module performs the routing of these connectionless messages. The SCCP SUSI module receives the reassembled message and sends a larger UNITDATA PPL Event Indication to the SCCP user (whether on the CSP or resident on the host).

Error Conditions

If the SCCP cannot process the XUDT message because an error condition has occurred on the network, and the return option is set, then it gets an XUDTS message and forwards notification of the error to the SCCP user at the sending end using an N_ NOTICE primitive. (See the ITU-T Q.711 - Q.714: Signaling Connection Control Part, White Book and ANSI T1.112 - 1996).

Configuration

Configuration bytes for the TCAP module in the PPL Configure message must be set for each component ID. With SCCP Segmentation implemented, these config bytes allow the TCAP layer to send larger blocks of data to the SCCP layer if desired. SCCP can send or receive messages directly with the host or through the TCAP layer. The configuration bytes are discussed further in the SS7 PPL Information chapter, and include the following:

• SCCP Component SCLC--Bytes 22 and 26 (a 4-byte value)

• SCCP Component SUSI--Byte 20

• TCAP Component CCO--Bytes 1 and 2

• TCAP Component TUSI--Bytes 4 and 5

Limitations

SS7 Segmentation allows a maximum number of 300 reassembly processes simultaneously.

Redundancy

SCCP software is redundant.

See Information Control Block.
See Signaling System Number Seven.
International Telecommunications Union (UN)
American National Standards Institute. ANSI administers and coordinates the U.S. voluntary standardization and conformity assessment system for the
telecommunications industry. More details on these standards may be
viewed at www.ansi.org.
International Telephone Union – Telecommunications sector (UN)
ITU-TSS Publication on Functional Description of SCCP
ITU-TSS Publication on SCCP Procedures
SCCP. The signaling connection and control part of the SS7 signaling protocol. For example, performs Global Title Translations (GTT), Subsystem Management.
Digital communications link - voice, data, and video at a 1.544 Mbps rate (24 64 Kbps channels)
See Message Transfer Part.
See Programmable Protocol Language.
ANSI Publication on SCCP
See Transaction Capability Application Part.