Below is SS7 layers diagram.
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Umbrella specification for SS7 is Q.700. MTP (Message Transfer Part) - Q.701 - Q.709 SCCP (Signaling Connection Control Part) Q.711 - Q.716 TUP (Telephone User Part) - Q.721 - Q.725 ISUP (ISDN User Part) - Q.761 - Q.766 TCAP (Transaction Capabilities Application Part) - Q.771 - Q.775 Test - Q.780 - Q.783 |
SS7 recommend Primitive (messages) based approach for interlayer communication as shown below. 3GPP (GSM) standard follow the same practice.
MTP (Message Transfer Part)
MTP 1 correspond to OSI Physical layer. SS7 do not specify the physical interface to be used, but due to existing deployment, DS0A or V.35 interface. Both these interfaces provide 54/64 kpbs data rate. In practice, high speed links are used DS1/DS3. These link may get migrated to ATM and then SONET. This migration will also require replacing MTP 1, and MTP 2, and part of MTP 3 with an adaptation layer.
MTP 2 correspond to OSI Data link layer. "Reliable" transfer from "node to node" is aim here. To do this, it incorporates Link management, Error detection and checking techniques (like CRC - Cyclic Redundancy Check), Acknowledgment techniques (Sequence numbering). Link management is about bringing the link to service, detecting failed links etc. Error detection and checking techniques typically puts additional data to transmission data before transmitting. The additional data is calculated in such a way from transmission data that it would be possible to detect the error and may be possible to correct it. Sequence numbering allows to make sure that the data has been received by the receiver. It gives sequence number to the packet of data. And receiver acknowledges these sequence numbers. If sender do not receive acknowledgment for some sequence numbers, sender does the retransmission.
MTP 3 is layer 3 (handling network layer functions of OSI model) of SS7 stack. As mentioned point code is used by MTP 3 for routing. The point code is be provided by ISUP, SCCP, or TUP in routing label (part of message passed to MTP 3). Once MTP 3 receives a message from MTP 2, it checks if it is intended for the current entity with the help of point code. If so, it passes the message to upper layer removing MTP related elements from message. Otherwise, the message is routed to next point (SS7 entity) based on routing tables.
Below is block diagram of MTP functions given by Q.701.
Below is a diagram showing MTP Signaling unit (message).
SU - Signaliing Unit
FCS - Frame Check Sequence (Error correction)
LI - Length Indicator (indicate type of SU)
FIB - Forward Indicator Bit
FSN - Forward Sequence Number
BIB - Backward Indicator Bit
BSN - Backward Sequence Number
SIF - Signaling Information Field (International or National and
Priority(2 bits) )
RL - Routing Label (Contain Originating Point Code, Destination
Pointe Code, etc.)
SIO - Signaling Information Octet
SSF - SubService Field
SI - Service Indicator (indicate (service) whether ISUP, SCCP, TUP
etc.)
MTP primitives are
MTP-TRANSFER (Req/Ind),
MTP-PAUSE (Ind),
MTP-RESUME
(Ind),
MTP-STATUS (Ind)
In next article, we will continue with SCCP.
References: Q.700, Q.701, SS7 book by Russell, GSM Networks book by Heine, SS7 tutorial from Performance Technologies, Wikipage on SS7.
© Copyright Samir Amberkar 2010
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