S7 signaling system was develop by CCITT, SS7 is a common channel signaling system, that mean only one channel that used for sending the signaling information, whatever the system has one bearer channel or multiple bearer

MTP-3

Message Transfer Part level 3 or MTP-3, it transfer message between the node of the signaling network ,MTP-3 ensure the reliable transfer of signaling message even the case the failure signaling link and signaling transfer point (STP)

Service indicator

Used for message distribution and some case perform message routing the service indicator using international signaling nework for following purpouse

Subservice field

The subservice field contain the network indicator and two spare bit to discriminate between national and international message

MTP-2

Message Transfer Part-Level 2 is a signaling link witch together MTP-3 provide reliable transfer of signaling message between two endpoint where directly connected signaling point

Routing Table

A standard routing table

Message Type Code

A one octet code which is mandatory for all message, the message type code uniquely define and format of each SCCP Message, existing message code are ;

CR            Connection Request

CC            Connection  Confirm

Creff        Connection Refuse

RLSD        Release

RLC          Release Complete

DT1          Data Form 1

DT2          Data Form 2

AK            Data Acknowledgment

UDT         Unidata

UDTS       Unidata Service

ED            Expedited Data

EA            Expedited Data Acknowledgment

RSR          Reset Request

RSC          Reset Confirm

ERR          Protocol Unit data Error

IT              Inactive Test

XUDT       Extended Unidata

XUDTS     Extended Unitada Service

Mandatory Fixed Part
The part is mandatory and of fixed length for particular message type will be contained in the mandatory fixed part

Mandatory Variable Part
Mandatory parameters of variable length will be include in mandatory variable part
Optional Part

ISUP

ISUP is ISDN user part of SS7, ISUP define the protocol and procedures used to setup, manage and release trunk circuit that carry voice and data call over the Public Switch Telephone Network (PSTN) . ISUP is used for ISDN and non-ISDN calls. Call that origin and terminate at the same switch do not used ISDN signaling, ISDN user part message that carried on the signaling link  by mean signaling units, the signaling information field each of message signal unit contains an ISDN usep Part Message consisting of an integral number of octets