SS7 Protocol Stack
SS7 MTP – Message Transfer Part Components
SS7 Message Transfer Part
MTP 1 – Layer 1
MTP 1 is the physical interface of SS7 Protocol Stack, here we control the physical and electrical characteristics that make a link between two signaling points.
Physical Layer use TDM over E1/T1 Links to transfer the bits converted into electric signals over the network and it controls the link status.
CCITT recommend 64 Kbps and ANSI recommend 56 Kbps.
- E1 Link have 32 time slots ( 64kb/s per channel * 32 slots = 2048 kb/s )
- T1 Link have 24 time slots
MTP 1 Standards
Narrowband – ITU-T Recommendation Q.702
- Europe – ETSI EN 300 008-1
- USA – ANSI
- China – CHN
SS7 physical Cards
Quad Span T1 Card E1 Card ISDN PRI Digital ss7 Voice Board with 4 E1/T1 Ports
- Quad Span ISDN Pri Card
- Supports Freepbx,Issabel Freeswitch AsteriskNow…
- E1 Card,T1 Card asterisk card te420
- Voice Board 4 Port E1 / T1 Card
- with PCI Express (PCI-E) Connector
- E1 circuits are used in Europe, Asia, Middle East and South America
- T1 circuits are used in USA, Canada
- J1 circuits are used in Japan
MTP 2 – Layer 2 Data Link
Ensure end-to-end transmission of signalling units across the signalling links and for that implement message sequencing validation, flow control, message retransmission and error checking.
Monitor and test the links status and reports
MTP 2 Message Format
The MTP2 controls the functions of link and is responsible for error free transport of signal units send or received.
Flag – Start Flag
The start flag indicates the beginning of a new signal unit.
Binary Value : 0111 1110
BSN – Backward Sequence Number
BIB – Backward Indicator Bit
FSN – Forward Sequence Number
FIB – Forward Indicator Bit
LI – Length Indicator
Used to identify the type of signal unit, stores values between 0 and 63
SIO – Service Information Octet
SIF – Signalling Information Field
CRC Checksum – Error validation
Flag – End Flag
279 octets Max Packet Size =
273 octets (data) + 1 octet (flag) + 1 octet (BSN + BIB) + 1 octet (FSN + FIB) + 1 octet (LI + 2 bits spare) + 2 octets (CRC)
Signalling Units in SS7
Signalling units in SS7 can be divided in three different units:
FISU – Fill In Signal Unit
Only carry out layer two information, it can check the link status, all units are send one a time base.
LSSU – Link Status Signal Unit
MSU – Message Signal Unit
Signalling Link functionalities / Error Handling
MTP 3 – Layer 3 Network
This layer is responsible to route the messages between signalling points using PC’s (Point Codes) , it controls the traffic when a congestion or a broken link occurs.
MTP 3 is divided in two parts one responsible from messages and the other responsible from the network.
SMH Signalling Message Handling
SNM Signalling Network Management
SS7 User and Application Parts
SCCP – Signalling Connection Control Part
SCCP is used when the routing with Global Titles (GT) is a request , an SCCP provide GTT (Global Title Translation) capabilities to convert into PCs (Point Codes) and SSNs (Subsystem Numbers). It relies on MTP3 to
it is a protocol that can be used as oriented connection or connectionless protocol.
When using TCAP services is the protocol responsible for transport application data.