Signaling Connection Control Part (SCCP) [Under SS7]
MTP provide routing based on point code, but when the message is to be addressed based on address other than point code, MTP will not able to do it. SCCP comes into picture there. SCCP does the "global title" translation. In addition, with the help of "sub-system number", SCCP identifies the user of SCCP within the entity. Let's have a look at the pizza (800) number example (SS7 -1) for more clarity.
SSP would like to know the actual/routable number corresponding to (global) pizza number dialed. In this case, SSP can inject a database query into STP network without knowing the SCP point code and database that is to be queried. SCCP on in-between STPs will insert correct SCP point code/sub-system number and so the message will be routed to appropriate SCP. This is known as "global title translation".
SCCP allow routing based on (one, two or all) GT, SSN, and Point Code. Examples of GT are 800 number (US public switched telephone network), Mobile number (GSM network). Example values of SSN are SCCP management, HLR, VLR, MSC, MAP (last 4 related to GSM network).
MTP provide node-to-node connectivity, SCCP can provide origination node-to-destination node connectivity. SCCP provide both connection-oriented (CO) and connection-less (CL) services. Let us be clear about difference between these two services. In CO, SCCP user will (explicitly) create a connection (with primitive N-CONNECT-REQUEST) giving Called Address. This "connection" - identified by Connection Identification parameter - will be used by SCCP user for data transfer (using primitive N-DATA-REQUEST). In CL, SCCP user do not have to request a connection and can directly send data (using primitive N-UNITDATA-REQUEST).
SCCP provide following services - known as protocol class:
Class 0 - Basic CL
Class 1 - Sequenced CL
Class 2 - Basic CO
Class
3 - Flow control CO
Class 4 - Class 3 + Error recovery CO
SCCP CL services are popular and mostly used. Popular usage is for TCAP queries - used for Database queries (for 800 number calling, Local Number Portability) and MAP (Mobile Application Part) transactions in GSM networks. BSSAP layer - part of "A" interface between Access network (BSC node) and Core network (MSC node) - in GSM uses SCCP CO services.
In case the message (SIF) exceed MTP SDU limit, SCCP segment the message and peer SCCP does the reassembly.
Below is a diagram showing internal architecture of SCCP.
SCMG handles signaling route management (similar to MTP link functions but between end-to-end point), Flow control. It may even restart the transmission. SCMG function are as per request protocol class.
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Flow Control
Flow Control is one of the important concepts in telecom/networking world. As the name suggests, Flow Control controls the in-flow and out-flow of data. A simple example would be two nodes doing data transfer, but support different transfer rate. Node supporting low data rate may not be able to process (consume) all the incoming data and its incoming data "queue" is likely to get full.
A simple way to avoid this scenario would be: receiving node may signal to transmitting node to "suspend" the transmission and then signal to "resume" once its ready. This may be called "closed loop" as there is feedback from receiver.
Standard specify ways for flow control. |
Various SCCP primitives are:
CO service:
N-CONNECT (Ind/Res/Cnf)
N-DATA
(Req/Ind)
N-EXPEDITED DATA (Req/Ind)
N-DISCONNECT
(Req/Ind)
N-RESET (Req/Ind/Res/Cnf)
N-INFORM
(Req/Ind)
REQUEST type 1
REQUEST type 2
REPLY
CL service:
N-UNITDATA (Req/Ind)
N-NOTICE (Ind)
SCCP management:
N-COORD (Req/Ind/Res/Cnf)
N-STATE (Req/Ind)
N-PCSTATE
(Ind)
Below diagram shows SCCP message format.
Some of important parameters are Called Party Address (GT,SSN,PC), (SCCP) Protocol Class etc.
In next article, we will continue with TCAP.
References: Q.700 (SS7), Q.711 (Functions/Services of SCCP), Q.714 (SCCP procedures), Q.713 (SCCP messages), SS7 book by Russell, GSM Networks book by Heine.
© Copyright Samir Amberkar 2010
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