- Signalling system 7
- Message transfer protocol (MTP)
- Databases
- Signalling connection control part (SCCP)
- ISDN user part (ISUP)
- Transaction capabilities application part (TCAP)
Signalling system 7
- OOB signalling in PSTN: SSP, STP, SCP
- point code ≈ IP address
- homologation: certify HW and SW that connects to country’s PSTN
- static routing, based on endpoint type and type of available links
- address signalling
- en-bloc: whole called number in a single message
- overlap: > 1 message
- message transfer point (MTP)
Service switching point (SSP)
- end office switch: originate, terminate calls
- circuit-switching: for voice
- not circuit-switching: DB query with SCP (e.g., 800 number)
- upstream: SS7
- downstream: ISDN PRI, channel-associated switching (CAS)
- tandem switch ≡ SSP + STP
Signal transfer point (STP)
- always in pairs
- signalling routing
- traffic monitoring
- global title translation
- SSP may have no DPC for SCP, offloading it to STP
- STP may return DPC and subsystem number for DB, may route to another STP
- gateway STP
- protocol conversion (national → ITU)
- network interconnect
- security (e.g., screening)
Service control point (SCP)
- DB access for IN application, not DB itself
- upstream: SS7
- downstream: X.25
- subsystem number = DB ID, unique
Linkset
- several links to single endpoint
- ≈ port-channel/ECMP
- combined linkset – to mated STP with different PC
Signalling
- associated
- signalling and voice path are directly connected between endpoints
- non-associated
- signalling through several STP
- quasi-associated
- signalling through 1 STP (endpoints – connected)
Message transfer protocol (MTP)
MTP L1
- OSI L1
- DS0, E1/T1, V.35
MTP L2
- reliable P2P links
- CRC, packet sequencing, link status monitoring
- signal units
- fill-in signal unit (FISU): error detection, no retransmit
- link status signal unit (LSSU): L2 and L3 link status
- message signal unit (MSU): transports SCCP, TUP, ISUP, TCAP
FISU
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Flag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Back Seq Num |B| Fwd Seq Num |F| Len Indic |Rsv| CRC16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flag: 01111110, end of signal unit
Backwards sequence number (BSN): last received FSN, acknowledge reception
Forward sequence number (FSN)
F: forward indicator bit (FIB)
B: backward indicator bit (BIB)
FIB ≠ BIB ≡ retransmit request
Length indicator:
- 0: FISU
- 1-2: LSSU
- 3-63: MSU
LSSU
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag |Back Seq Num |B| Fwd Seq Num |F|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Len Indic |Rsv| Status field (8/16 bits) | CRC16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flag: 01111110, end of signal unit
Backwards sequence number (BSN): last received FSN, acknowledge reception
Forward sequence number (FSN)
F: forward indicator bit (FIB)
B: backward indicator bit (BIB)
FIB ≠ BIB ≡ retransmit request
Length indicator:
- 0: FISU
- 1-2: LSSU
- 3-63: MSU
Status field:
- 0x00: out of alignment ≡ link failed, alignment needed
- 0x01: normal alignment
- 0x02: emergency alignment
- 0x03: out of service
- not the processor failure
- stop MSU, start FISU
- 0x04: processor outage
- processor failed ≡ no connection to upper-layer protocols
- stop MSU, start FISU
- 0x05: busy
- L2 congestion
- stop MSU, start FISU
MSU
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag |Back Seq Num |B| Fwd Seq Num |F| Len Indic |Rsv|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SI | SSF | Service information field (MTP L3) \
+-+-+-+-+-+-+-+-+ (1-272 bytes) +-+-+-+-+-+-+-+-+
\ | CRC16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flag: 01111110, end of signal unit
Backwards sequence number (BSN): last received FSN, acknowledge reception
Forward sequence number (FSN)
F: forward indicator bit (FIB)
B: backward indicator bit (BIB)
FIB ≠ BIB ≡ retransmit request
Length indicator:
- 0: FISU
- 1-2: LSSU
- 3-63: MSU
Service information (SI):
- 0x0: SNM message
- 0x1: maintenance regular message
- 0x2: maintenance special message
- 0x3: SCCP
- 0x4: TUP
- 0x5: ISUP
- 0x6: data user part (DUP), circuit-based message
- 0x7: DUP, facility message
SSF: subservice field
- 0x0: international
- 0x1: reserved
- 0x2: national
- 0x3: reserved
Service information field: ULP data, routing label, control information
MTP L3
- in SIF MSU
- signalling message handling (SMM) ≡ SS7 routing
- signalling network management (SNM) ≡ reroute on failure
- if target user is not available for processing – send User Port Unavailable (UPU) back
- buffer exceeded → Onset of Congestion messages in SNM
- no VOQ ⇒ all dst congested
SMH/SNM MSU SIF
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination point code | Originating code point | SLS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User message /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SLS: signalling link selector
SNM link management
- link activation
- LSSU SLTM (signalling link test message)
- SLTA (SLTM acknowledgement)
- link restoration
- LSSU, link alignment
- link deactivation
SNM route managmement
- transfer prohibited (TFP): if route to endpoint is lost, notify directly connected nodes
- transfer allowed (TFA): cancel TFP
- transfer restricted (TFR): notify adjacent nodes to use other link, if possible
- transfer controlled (TFC): notify sender about congestion on adjacent node
- signalling route set test: ≈ ping on control plane level
- congestion test: update congestion status
- cluster router set test
- changeover order (COO): between signalling points, notify about link being unavailable
- changeover ack (COA): after switching to alternate link, retransmit lost data over FSN/BSN
- changeback declaration (CBD): intention to switch back over, all messages → into changeback buffer (CBB)
- changeback ack (CBA): after CBA is received, messages in CBB can be sent over old link again
Databases
800 DB
- services numbers 8**, 9** – returns real number
- 800 are not routed as global title address
Line information DB (LIDB)
- user info: PIN, calling card (for billing)
Local number portability DB (LNPDB)
- 10 digits location routing number (LRN) SSP, that services the number
Home location register (HLR)
- cellular
- current phone location, billing
Visitor location register (VLR)
- cellular
- info about roaming subscriber
Signalling connection control part (SCCP)
- transport for TCAP and ISUP
- unitdata message (UDT): query DB, DB response (via SCP)
- unitdata service message (UDTS): error notification
- connection-oriented (not used) and connectionless service
SCCP UDT
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Message type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol class | Called Party Addresss (CDA) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calling Party Address (CGA) | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
\ Subsystem data \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message type:
- 0x09: UDT
- 0x0a: UDTS
SCCP UDTS
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Message type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Return cause | Called Party Addresss (CDA) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calling Party Address (CGA) | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
\ Subsystem data \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message type:
- 0x09: UDT
- 0x0a: UDTS
SCCP address indicator parameter
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PCI |SSNI |Global title indicator | RI | Rsv |
+-----+-----+-----+-----+-----+-----+-----+-----+
PCI: point code indicator, 1 ≡ address contains signalling point code
SSNI: SSN indicator, 1 ≡ address = subsystem number
RI: raising indicator
- 0 ≡ GTT needed
- 1 ≡ route using SSN
ISDN user part (ISUP)
- connect/disconnect calls
- setup/teardown circuit
- switch-to-switch
- messages:
- initial address message (IAM): initiate call setup
- address completion message (ACM): notify caller about ringing
- answer message (ANM): notify caller about off-hook event on called
- call progress (CPG): ≈ ACM, strictly after ACM
- info request (INR): request extra info
- information (INF)
- suspend (SUS): pause call without releasing resources
- release (REL): end call and release resources
- release complete (RLC): trunk is free
- continuity test (COT): test trunk
- resume (RES): resume suspended call
- forward transfer (FOT)
ISUP call setup & teardown
sub1 originating intermediate terminating sub2
switch switch switch
off-hook
----------->
dial tone
<-----------
digits
----------->
IAM
------>
ACM
<------ IAM
------>
ACM
<------
not busy
ANM ANM off-hook
<------ <------ <-----------
voice
<=========================================================================>
on-hook
<-----------
SUS
SUS <------
<------
on-hook
-----------> REL
------> REL
------>
RLC
RLC <------
<------
ISUP user message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Message type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message type:
- 0x01: IAM
- 0x03: INR
- 0x04: INF
- 0x05: COT
- 0x06: ACM
- 0x08: FOT
- 0x09: ANM
- 0x0c: REL
- 0x0d: SUS
- 0x0e: RES
- 0x10: RLC
- 0x2c: CPG
Transaction capabilities application part (TCAP)
- non-circuit based messages, DB query (via SCP)
- end-to-end
TCAP user message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message type | Total length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ TLV /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message type:
- 0x61: unidirectional
- 0x62: begin
- 0x64: end
- 0x65: continue
- 0x67: abort
Total length: bytes
Networking and IT 