Signaling Systems

Common channel signaling (CCS)

Common channel signaling carries the signaling of many speech circuits as digital messages over a separate shared signaling link, instead of sending signals on each speech circuit. Each message has a label that identifies the circuit it belongs to. SS7 (Signaling System No. 7) of ITU-T is the standard CCS system; it uses 64 kbps links (e.g. TS16 of an E1) and its own packet network of signaling points.

Basic call set-up using SS7 (ISUP)

Network: subscriber A on exchange SSP-A, subscriber B on exchange SSP-B; signaling messages pass through one or more STPs; speech goes over a trunk between the exchanges.

 SSP-A          STP          SSP-B
   |---- IAM ---->|---- IAM ---->|   seize trunk, dial info
   |<--- ACM -----|<--- ACM -----|   B is ringing
   |<--- ANM -----|<--- ANM -----|   B answers
   |====== conversation on trunk =====|
   |---- REL ---->|---- REL ---->|   A hangs up
   |<--- RLC -----|<--- RLC -----|   trunk released

1. Off-hook and digits: A lifts the handset; SSP-A gives dial tone, collects the digits and analyses them. It finds that B is on SSP-B and selects a free trunk (circuit) to SSP-B.
2. IAM (Initial Address Message): SSP-A sends IAM with called and calling numbers, the circuit identification code (CIC) of the chosen trunk and call type. The STP routes it to SSP-B using the destination point code.
3. Called side check: SSP-B checks that B is free, reserves the trunk and rings B.
4. ACM (Address Complete Message): SSP-B returns ACM; SSP-A connects ring-back tone to A (or B's exchange sends it over the trunk).
5. ANM (Answer Message): when B answers, SSP-B sends ANM; both exchanges complete the speech path and charging starts.
6. Conversation takes place on the trunk; no signaling uses the speech channel.
7. REL (Release): when either party hangs up, its exchange sends REL.
8. RLC (Release Complete): the other exchange frees the trunk and replies with RLC; the circuit is free for other calls.

If B is busy, SSP-B sends REL with cause "user busy" at once and no trunk is held, which saves trunk capacity. For services like toll-free numbers, an SSP may first send a TCAP query to an SCP database to get the real number before sending the IAM.

Importance of signaling

Signaling is the exchange of control information needed to set up, supervise and release a call and to manage the network.

• Call set-up: carries the request (off-hook), the dialled number and routing information between subscriber and exchanges.
• Supervision: shows whether lines and trunks are idle, busy, answered or cleared.
• Alerting and progress: ringing, dial tone, busy tone, ring-back and announcements.
• Charging: answer and release signals start and stop billing.
• Efficient use of trunks: fast signaling holds trunks for less time.
• Services: caller ID, call forwarding, roaming, toll-free numbers and number portability depend on signaling messages and database queries.
• Network management: congestion control, rerouting around faults, maintenance messages.

Without signaling, no automatic switched network can work.

Building blocks of the SS7 network

Signaling points (nodes)

• SSP (Service Switching Point): an exchange that originates or ends signaling for calls.
• STP (Signal Transfer Point): a packet router that forwards SS7 messages; deployed in mated pairs.
• SCP (Service Control Point): database for intelligent network services (800 numbers, HLR, prepaid).

Signaling links: 64 kbps (56 kbps in ANSI) links grouped in link sets: A (access), B (bridge), C (cross), D (diagonal), E (extended), F (fully associated).

   SSP ==A== STP ----C---- STP ==A== SSP
              |  \   B/D  /  |
              |   STP----STP |
              A              A
             SCP            SCP

SS7 protocol stack

  +------+------+------+
  | MAP  | INAP | OMAP |
  +------+------+------+
  |       TCAP         |  ISUP | TUP
  +--------------------+       |
  |       SCCP         |       |
  +--------------------+-------+
  |   MTP Level 3 (network)    |
  |   MTP Level 2 (link)       |
  |   MTP Level 1 (physical)   |
  +----------------------------+

MTP gives reliable transport; ISUP handles circuit-related calls; SCCP + TCAP handle non-circuit-related database queries.

In-channel signaling vs common channel signaling

In-channel signaling sends the signals of a call over the same channel (or a channel tied to it) as the speech, e.g. DC loop pulses, in-band tones (SF 2600 Hz, MF), or out-of-band/CAS bits in TS16 of E1. Common channel signaling (CCS) sends signals for many circuits as messages over a separate signaling link shared by all of them (SS7).

Types of signaling links in SS7

SS7 links connect signaling points (SSP, STP, SCP). They are named by their role:

     SSP1           SSP2
      | \  A      A / |
      |  STPa----STPb |   C = mated pair link
   F  |  | \ D  / |   |
      |  |  STPc  |   |   B/D = between pairs
     SSP3 -- E --> STPd

1. A-link (Access): connects an SSP or SCP to its home STP pair. Each SSP has A-links to both STPs of the pair for redundancy.
2. B-link (Bridge): connects two mated STP pairs at the same level (peer pairs); usually a quad of links.
3. C-link (Cross): connects the two STPs of a mated pair; used only when other links fail (for recovery).
4. D-link (Diagonal): connects STP pairs at different hierarchy levels (e.g. local STP to regional/gateway STP).
5. E-link (Extended): connects an SSP to a remote STP pair other than its home pair; an alternative route for reliability.
6. F-link (Fully associated): connects two SSPs directly, without STPs, when there is heavy traffic between them (associated mode).

Links between the same two points form a link set (up to 16 links); MTP level 3 shares load among links and reroutes traffic on failure.

Main signaling points in an SS7 network

A signaling point (SP) is any node that can send, receive or forward SS7 messages. Each has a unique point code (14 bits in ITU). There are three main types: SSP, STP and SCP.

   [SCP]         [SCP]       databases
     |A            |A
  [STP]=====C====[STP]       mated pair
    |  \          /  |
    A   \--B/D--/    A
    |                |
  [SSP] ======trunk===== [SSP]   exchanges
   |                       |
 phones                  phones

Roles in the signaling process:

• SSPs start and end call signaling (they are the "users").
• STPs relay messages (they are the "routers").
• SCPs answer queries (they are the "databases").

Signal Switching Point (SSP)

• A telephone exchange (local, tandem or mobile switch) with SS7 software.
• Converts subscriber events (off-hook, digits, answer, on-hook) into ISUP messages (IAM, ACM, ANM, REL, RLC) for trunk set-up and release.
• Detects calls needing special handling (e.g. 1660/800 numbers, prepaid) and sends TCAP queries to an SCP, then routes the call using the reply.
• Is the origin or end point of signaling messages; it does not normally relay messages for others.

Signal Transfer Point (STP)

• A high-capacity packet switch/router for SS7 messages; it does not switch speech.
• Receives messages on one link and forwards them on another using the destination point code (MTP level 3 routing).
• Performs global title translation (SCCP) to find which SCP should get a query (e.g. translate a toll-free number to the SCP point code).
• Provides screening (gateway security), measurement of traffic, and network management such as rerouting around failed links.
• Deployed in mated pairs joined by C-links so that one failure does not cut the network.

Signal Control Point (SCP)

• A database server connected to STPs by A-links.
• Answers TCAP queries from SSPs and gives instructions for the call.
• Supports Intelligent Network services: toll-free and premium-rate number translation, calling card and prepaid validation, number portability, virtual private networks.
• In mobile networks, the HLR/VLR databases play the SCP role through MAP.
• Usually duplicated (mated SCPs) for reliability.

Example: toll-free call

1. The caller dials a toll-free number; the SSP sends a TCAP query.
2. The STP performs global title translation and forwards it to the right SCP.
3. The SCP returns the real routing number.
4. The SSP sends an ISUP IAM through the STP to the far SSP to set up the trunk.

Why signaling is important

Signaling is the exchange of control information between subscriber and exchange, and between exchanges, needed to run a call. It is important because it:

• Requests, routes and sets up calls (off-hook, dial tone, dialled digits, route selection).
• Supervises lines and trunks (idle, busy, answered, cleared) and releases resources.
• Gives alerting and progress information (ringing, busy tone, ring-back).
• Starts and stops charging.
• Enables services such as caller ID, call forwarding, mobile roaming and toll-free numbers.
• Manages the network (congestion control, rerouting, maintenance).

ITU-T Signaling System No. 7 (SS7)

SS7 is the ITU-T standard (Q.700 series) common channel signaling system for digital networks. It sends signaling as labelled messages over separate 64 kbps signaling links, forming a packet network that runs alongside the speech network.

Network elements

• SSP: exchange that originates/ends signaling.
• STP: packet router for SS7 messages (mated pairs).
• SCP: database for intelligent network services.
• Links: A, B, C, D, E, F links joined into link sets.

Signaling modes

• Associated: signaling link runs directly between the two exchanges, parallel to the trunk group.
• Quasi-associated: messages go through STPs along a fixed predetermined route.
• Non-associated: messages go through STPs on routes that may change.

Protocol architecture

 OSI layer   SS7 part
  7      | MAP | INAP | OMAP |
  4-7    |       TCAP        |  ISUP  | TUP
  3      |       SCCP        |        |
  3      |  MTP Level 3: network        |
  2      |  MTP Level 2: link           |
  1      |  MTP Level 1: physical       |

• MTP-1: 64 kbps digital bearer (e.g. TS16 of E1).
• MTP-2: reliable link: flags, CRC-16, sequence numbers, retransmission, alignment; signal units FISU, LSSU, MSU.
• MTP-3: routing by point codes (DPC, OPC, SLS), message discrimination and distribution, and network management.
• SCCP: extra addressing (subsystem numbers, global title translation) and connectionless/connection-oriented services.
• TCAP: transaction (query/response) services for database access.
• ISUP: circuit-related call control: IAM, ACM, ANM, REL, RLC.
• TUP: older telephone user part, replaced by ISUP.
• MAP / INAP / OMAP: mobile, intelligent-network and maintenance applications.

Call example: IAM → ACM → ANM → conversation → REL → RLC.

Advantages: fast set-up, signaling during the call, large message capacity, fraud resistance, efficient trunk use, and support for ISDN, mobile and IN services.

CCS7 network architecture

Common Channel Signaling No. 7 (CCS7 / SS7) uses a separate packet-switched signaling network to carry call-control and database messages between exchanges. The speech network and signaling network are separate but linked through the exchanges.

       Signaling plane
  [SCP]                    [SCP]
    | A                      | A
  [STP]--------B/D--------[STP]
    | \ C                  / |
    |  [STP]--------[STP]    |
   A|                        |A
  [SSP]=======trunks=======[SSP]
       Speech (transport) plane

• Exchanges (SSPs) are connected to a mated pair of STPs by A-links.
• STP pairs are joined to each other by B- or D-links and internally by C-links.
• SCPs (databases) hang off STPs.
• Heavy routes may have direct F-links between SSPs.
• Everything is duplicated so that a single link or node failure does not stop signaling.

CCS7 terminologies

• Signaling Point (SP): any node that sends or receives SS7 messages; identified by a point code (14 bits in ITU).
• SSP (Service Switching Point): an exchange with SS7 capability; originates and terminates calls.
• STP (Signal Transfer Point): routes messages between links without being their origin or destination.
• SCP (Service Control Point): database answering queries (toll-free, prepaid, HLR).
• Signaling link: a 64 kbps bidirectional data link between two SPs.
• Link set: group of links between the same two SPs (up to 16).
• Route / route set: the sequence of link sets used to reach a destination.
• Link types: A (access), B (bridge), C (cross), D (diagonal), E (extended), F (fully associated).
• Signaling modes: associated (direct link alongside trunks), quasi-associated (via STPs on fixed route), non-associated (via STPs, route may vary).
• OPC / DPC / SLS: originating point code, destination point code and signaling link selection, forming the routing label.
• CIC (Circuit Identification Code): identifies the speech trunk to which an ISUP message refers.
• Signal units: MSU (message signal unit, carries user data), LSSU (link status), FISU (fill-in, keeps link alive).
• MTP (Message Transfer Part): levels 1–3 giving reliable transport and routing.
• User parts: ISUP (call control), TUP, SCCP (global title translation), TCAP (transactions), MAP, INAP.
• Global Title (GT): an address such as a dialled number that the STP translates into a point code and subsystem.

SS7

Signaling System No. 7 (SS7) is the ITU-T common channel signaling system. Call-control messages travel as data packets over separate 64 kbps signaling links between exchanges (SSPs), via STPs, while speech uses trunks. Call set-up and release use the ISDN User Part (ISUP) over the Message Transfer Part (MTP).

Call set-up and release

 Caller  SSP-A          STP          SSP-B   Called
  |off-hook|              |              |        |
  |<-dial--|              |              |        |
  |-digits>|              |              |        |
  |        |--- IAM ----->|---- IAM ---->|        |
  |        |              |              |--ring->|
  |        |<-- ACM ------|<--- ACM -----|        |
  |<-ringbk|              |              |        |
  |        |<-- ANM ------|<--- ANM -----|<-answer|
  |<=========== conversation on trunk ===========>|
  |on-hook>|              |              |        |
  |        |--- REL ----->|---- REL ---->|        |
  |        |<-- RLC ------|<--- RLC -----|        |

Set-up

1. Call origination: caller A goes off-hook; SSP-A gives dial tone and collects the digits.
2. Digit analysis and trunk selection: SSP-A finds that B is on SSP-B and reserves an idle trunk, identified by its CIC.
3. IAM (Initial Address Message): SSP-A sends IAM containing the called number, calling number, CIC and call type. MTP-3 routes it by the destination point code; the STP forwards it.
4. Continuity check (optional): the trunk may be tested (COT message).
5. Called side: SSP-B checks B is free, reserves the trunk and rings B.
6. ACM (Address Complete Message): SSP-B tells SSP-A that all digits are received and B is being alerted. Caller hears ring-back tone.
7. ANM (Answer Message): B answers; SSP-B sends ANM. The speech path is connected end to end and charging starts.
8. Conversation takes place on the trunk.

Release 9. REL (Release): when A (or B) hangs up, that exchange sends REL with a cause value (e.g. normal clearing) and starts freeing the trunk. 10. RLC (Release Complete): the other exchange disconnects, makes the trunk idle and replies with RLC. Charging stops and the CIC can be reused.

Busy case: if B is busy, SSP-B replies to the IAM with REL (cause "user busy"); SSP-A gives busy tone, and the trunk is freed at once.

Since all messages travel at 64 kbps over the signaling network, set-up takes about a second, and trunks are held only for real conversations.

Channel associated signaling (CAS) sends the signals of each speech circuit on that circuit or on a channel permanently tied to it (e.g. the abcd bits of each channel in TS16 of an E1 multiframe, or in-band MF tones).

 Exchange A                         Exchange B
 +--------+  +------+  trunk 1  +------+  +--------+
 |        |--|Sig.  |===========|Sig.  |--|        |
 | Switch |  |unit 1| speech+sig|unit 1|  | Switch |
 |        |--|Sig.  |===========|Sig.  |--|        |
 |        |  |unit 2|  trunk 2  |unit 2|  |        |
 +--------+  +------+           +------+  +--------+
     ^  one signaling unit per trunk  ^
     +--------- control ---------------+

How CCS gives faster call set-up

• High-speed messages: in CCS (SS7) all digits and call data go in one IAM message over a 64 kbps link, taking a few milliseconds. In CAS, digits are sent one by one as MF tones or pulses, with handshakes at each exchange, taking several seconds.
• No register handshakes: CAS needs a register at each exchange to receive and resend digits; CCS passes messages directly between processors.
• Overlap of tasks: the next exchange can start processing as soon as the message arrives.
• Typical set-up time falls from about 10–20 s (CAS, multi-link calls) to 1–3 s (CCS).

How CCS gives greater trunking efficiency

• In CAS, the speech trunk is seized at the start of set-up and held while digits are sent and the called line is tested, even if B turns out to be busy or does not answer.
• In CCS, signaling is separate, so the trunk is used only for the conversation; for a busy called party, a REL comes back immediately and no trunk time is wasted.
• Shorter holding time per call means fewer trunks are needed for the same traffic (in erlangs) and grade of service.
• No per-trunk signaling units, and TS16 of E1 can carry speech (31 channels instead of 30).

The Message Transfer Part (MTP) is the lower three levels of SS7. Its purpose is to carry signaling messages from user parts (ISUP, SCCP, TCAP) reliably, in sequence and without loss or duplication between signaling points, and to keep the signaling network working when links or nodes fail.

MTP Level 1: signaling data link

• Physical, electrical and functional layer: a bidirectional 64 kbps digital channel (usually TS16 of a 2.048 Mbps E1; 56 kbps in ANSI networks).
• Transmits the bit stream; has no frame of its own.

MTP Level 2: signaling link

Functions: frame delimitation with flags, zero insertion, error detection (16-bit CRC), error correction by retransmission (basic or preventive cyclic), sequence numbering, link alignment, error-rate monitoring and flow control.

Signal unit format (bits, sent left to right):

• F: flag 01111110. BSN/BIB: backward sequence number and indicator bit (acknowledgement). FSN/FIB: forward sequence number and indicator bit. LI: length indicator. CK: CRC check bits.
• FISU (fill-in, LI = 0): no SIO/SIF; keeps the link alive.
• LSSU (link status, LI = 1 or 2): carries a status field for alignment.
• MSU (message, LI ≥ 3): carries SIO and SIF (up to 272 octets).

MTP Level 3: signaling network

Functions:

• Message handling: discrimination (is it for this node?), distribution to the right user part (from SIO), and routing to the next link using the routing label.
• Network management: traffic, link and route management (changeover, changeback, rerouting, congestion control).

Format used by level 3 (inside the MSU):

• SIO: service indicator (4 bits: which user part, e.g. ISUP) and subservice field (network indicator).
• Routing label (32 bits): destination point code, originating point code and signaling link selection (load sharing).

Signaling is the exchange of control information between the subscriber and the exchange, and between exchanges, needed to set up, supervise and release a call.

Why signaling is required

Without signaling an exchange cannot know that a subscriber wants a call, which number is wanted, whether the called line is free, when the called party answers (for billing) or when the call ends. Signaling therefore lets the network establish, monitor, charge and clear connections automatically, and also supports network management (busy circuits, faults, re-routing).

Signaling activities in a telephone system

1. Call request (seizure): calling party goes off-hook; exchange detects loop current and returns dial tone.
2. Address signaling: the dialled digits (pulse or DTMF) are sent to the exchange; between exchanges the digits are forwarded (MF or ISUP IAM).
3. Routing and trunk seizure: exchange analyses digits, selects a route and seizes a trunk to the next exchange.
4. Call progress / alerting: ringing current to the called party, ring-back tone to the caller, or busy tone / announcement if the call fails.
5. Answer (supervisory) signal: called party off-hook; ringing is stopped and charging starts.
6. Conversation supervision: the connection is held and monitored.
7. Clear (disconnect): on-hook from either side; clear-forward / clear-back signals release the trunks and charging stops.
8. Network management/maintenance: blocking, unblocking, circuit tests, congestion control.

These are grouped as supervisory, address and call-progress (information) signals.

SS7 protocol architecture

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

• MTP Level 1 (Signaling data link): bidirectional 64 kb/s digital path (usually a timeslot, TS16, of an E1).
• MTP Level 2 (Signaling link): flags, error detection by CRC-16, retransmission, sequence numbers (FSN/BSN), alignment and link error monitoring. Carries MSU, LSSU and FISU.
• MTP Level 3 (Signaling network): message routing using point codes (DPC/OPC), discrimination and distribution, and signaling network management (re-routing on link failure).
• SCCP: adds global-title translation and connectionless / connection-oriented services; together with MTP it forms the Network Service Part (NSP), equal to OSI layer 3.
• TCAP: transaction capabilities for non-circuit queries to databases (e.g. free-phone, roaming).
• ISUP / TUP: set up and release of voice/ISDN circuits (IAM, ACM, ANM, REL, RLC).
• MAP, OMAP, INAP: mobile application part, operation and maintenance part, intelligent network application part on top of TCAP.

In SS7 a call is set up and released by ISUP messages exchanged between the originating and terminating Service Switching Points (SSPs) through Signal Transfer Points (STPs), while the voice trunk itself carries only speech.

Network arrangement

 Caller--SSP-A ----- STP ----- SSP-B--Called
           |   (signaling link)  |
           +======= voice trunk ==+

Sequence of steps for a successful call

1. Off-hook and dial tone: calling subscriber lifts the handset; SSP-A detects it and gives dial tone.
2. Digit collection and analysis: SSP-A receives the dialled number, finds that the called party is on SSP-B, and selects an idle trunk (identified by its CIC, circuit identification code).
3. IAM (Initial Address Message): SSP-A sends IAM via the STP to SSP-B. It contains the called and calling numbers, the CIC and the nature of connection. The trunk is reserved.
4. Called line check: SSP-B checks that the called line is free, applies ringing to it and connects ring-back towards the trunk.
5. ACM (Address Complete Message): SSP-B returns ACM to SSP-A through the STP, telling that all digits are received and the called party is being alerted. Caller hears ring-back tone over the trunk.
6. ANM (Answer Message): when the called party lifts the handset, SSP-B sends ANM; the speech path is through-connected in both directions and charging starts.
7. Conversation on the voice trunk; no signaling traffic on it.
8. REL (Release): when either party hangs up, its switch sends REL with a cause value (e.g. normal clearing).
9. RLC (Release Complete): the other switch frees the trunk and replies RLC; the CIC becomes idle, charging stops.

Message flow

 SSP-A            STP            SSP-B
   |---IAM-------->|---IAM-------->|  ringing
   |<--ACM---------|<--ACM---------|
   |  (ring-back)                  |
   |<--ANM---------|<--ANM---------|  answer
   |====== conversation (trunk) ===|
   |---REL-------->|---REL-------->|  clear
   |<--RLC---------|<--RLC---------|

Unsuccessful call

If the called line is busy, SSP-B sends REL with cause "user busy" instead of ACM; SSP-A gives busy tone to the caller and returns RLC. If the trunk or route is congested, SSP-A itself gives a congestion tone.

Points to note

• Signaling and speech use separate paths, so the trunk is held only after IAM and is released immediately on failure.
• Because the trunk is not seized for ringing on a busy line, call set-up is fast (under about 1 s) and trunk usage is efficient.
• Database-type steps (free-phone 1660 numbers, number portability, mobile location) use TCAP queries to an SCP before the IAM is sent.

In-channel multi-frequency (MF) signaling sends address digits as combinations of voice-band tones over the same channel that later carries speech. Its design must make the signals fast and reliable while never being confused with speech.

Design considerations

1. Choice of frequencies: tones must lie inside the 300–3400 Hz voice band but be spread so filters can separate them. Example: CCITT R1/No. 5 uses 700, 900, 1100, 1300, 1500, 1700 Hz (200 Hz spacing); R2 uses 1380–1980 Hz forward and 1140–540 Hz backward (120 Hz spacing).
2. Code structure (2-out-of-N): each digit is two tones out of six, giving $\binom{6}{2} = 15$ codes. Exactly two tones must be present, so any missing or extra tone is detected (self-checking).
3. Protection against speech imitation (talk-off): speech rarely contains two pure tones of exactly the right level and duration, so detectors require precise frequency, level and minimum duration; MF receivers are usually connected only during set-up.
4. Signal level and twist: tones must be loud enough above noise but below overload; the level difference between the two tones (twist) must be limited.
5. Frequency tolerance: allow for carrier-system frequency shift (a few Hz) while still rejecting adjacent tones.
6. Timing: minimum tone duration and inter-digit pause (e.g. about 68 ms tone in R1) set the signaling speed.
7. Compelled or non-compelled operation: compelled (R2) — each forward signal stays on until a backward acknowledgement arrives, giving reliability on unknown links; non-compelled (R1) is faster but needs good links.
8. Start/stop signals: KP (start-of-pulsing) and ST (end-of-pulsing) codes mark the digit string.
9. Separation from line signaling: supervisory signals (seize, answer, clear) are handled by a different method (e.g. 2600 Hz or out-band), so the MF set is reserved for addresses.

SS7 vs OSI layers

ISUP and TUP sit directly on MTP-3 (they skip SCCP), so SS7 is a 4-level stack rather than a strict 7-layer one.

Purpose of MTP Level 2

MTP-2 makes a single signaling link reliable: it delimits units with flags, detects errors with a 16-bit CRC, keeps the order with sequence numbers, retransmits lost or errored units, aligns a new link, and monitors the error rate so a bad link is taken out of service. Three kinds of signal units (SU) are used.

1. Message Signal Unit (MSU)

Carries real signaling messages from MTP-3 users (ISUP, SCCP, network management).

| F | BSN|BIB| FSN|FIB| LI |Sp| SIO |   SIF    | CK | F |
| 8 |  7 | 1 |  7 | 1 | 6  |2 |  8  | 8n, n>=2 | 16 | 8 |

2. Link Status Signal Unit (LSSU)

Carries link status (alignment, out of service, processor outage, busy) in a Status Field (SF) of 8 or 16 bits in place of SIO+SIF.

3. Fill-In Signal Unit (FISU)

Sent when there is no traffic, to keep the link synchronised and to carry acknowledgements; it has no SIO/SIF (LI = 0).

Fields

• F (Flag): 01111110, opens and closes each unit.
• BSN / BIB: backward sequence number and indicator bit, used to acknowledge or ask for retransmission (BIB inverted = negative ack).
• FSN / FIB: forward sequence number of this unit and its indicator bit.
• LI (Length Indicator): number of octets after LI: 0 = FISU, 1 or 2 = LSSU, 3 or more = MSU (63 if above 62).
• SIO (Service Information Octet): service indicator (which user: SNM, SCCP, TUP, ISUP) and network indicator (national/international).
• SIF (Signaling Information Field): routing label (DPC, OPC, SLS) and user data, up to 272 octets.
• CK: 16-bit CRC check bits.

In-channel vs common channel signaling

SS7 network architecture

SS7 is a separate packet network that carries signaling for the voice network.

        mated pair        mated pair
 SSP-A --A-- STP1 ---B--- STP3 --A-- SSP-B
   \          |   \     /   |          /
    \         C    B   B    C         /
     \--A--   |   /     \   |   --A--/
           \ STP2 ---B--- STP4 /
               |
              SCP  (A link to STP pair)
 SSP-A ==========F (direct)========= SSP-B

• SSP (Service Switching Point): telephone exchange that originates/terminates SS7 messages.
• STP (Signal Transfer Point): packet switch that routes SS7 messages; deployed in mated pairs for reliability.
• SCP (Service Control Point): database for 800/free-phone, number portability, HLR, prepaid.
• Links: A (SSP–STP), B (STP pair to STP pair), C (between mated STPs), D (local to regional STP), E (SSP to alternate STP), F (direct SSP–SSP).

SS7 protocol stack

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

1. MTP-1: 64 kb/s physical signaling data link.
2. MTP-2: reliable link — flags, CRC, sequence numbers, retransmission; units MSU, LSSU, FISU.
3. MTP-3: message routing by point codes and signaling network management.
4. SCCP: global title translation, connectionless and connection-oriented classes.
5. TCAP: dialogues with databases (query/response).
6. ISUP/TUP: circuit-related call control (IAM, ACM, ANM, REL, RLC).
7. MAP/INAP/OMAP: mobile, intelligent network and maintenance applications.

Signaling system

A signaling system is the set of signals, codes and procedures used to exchange control information between subscriber and exchange and between exchanges, so that calls can be set up, supervised, charged and released. Signals are of three kinds: supervisory (seize, answer, clear), address (dialled digits) and call-progress (dial tone, ringing, busy tone).

Reasons behind the development of SS7

Older systems (DC loop, single-frequency 2600 Hz, MF R1/R2 and CAS on E1 TS16) sent signals inside or alongside each voice channel. As networks became digital and larger, these methods had serious limits, which SS7 (ITU-T Q.700 series) was designed to remove.

1. Slow call set-up: MF digits are sent one by one with acknowledgements, taking several seconds per link. SS7 sends the whole address in one IAM message at 64 kb/s; set-up falls to well under a second.
2. Wasted trunk capacity: with in-channel signaling the speech trunk is seized for the whole set-up even if the called line is busy. SS7 checks first and seizes the trunk only when needed.
3. Limited signal repertoire: tone systems have only about 15 codes. SS7 messages are flexible and new messages and parameters can be added (ISDN, calling line identification, call forwarding).
4. Fraud: in-band tones could be imitated by users ("blue box" with 2600 Hz). SS7 links are out of reach of subscribers.
5. No database access: modern services (toll-free numbers, number portability, credit-card calling, mobile roaming via HLR/VLR, SMS) need queries to remote databases. SS7 with SCCP/TCAP supports these non-circuit transactions.
6. Signaling during the call: CAS cannot easily exchange information once speech starts; SS7 can at any time (supplementary services).
7. Efficiency: one 64 kb/s SS7 link can serve thousands of trunks, replacing per-trunk signaling equipment.
8. Reliability and management: SS7 has CRC error control, retransmission, redundant STP pairs and automatic re-routing.
9. Digital and ISDN compatibility: a message-based system suits stored-program-control digital exchanges and ISDN/mobile (GSM) networks.
10. International standard: one common system makes interworking between operators and countries simple.

Result

SS7 became the "nervous system" of the PSTN and GSM networks: an out-of-band packet network of SSPs, STPs and SCPs that is fast, secure, flexible and supports intelligent network services.

Common Channel Signaling System No. 7 (SS7) is an ITU-T standard in which signaling for many trunks is carried as packets on a separate 64 kb/s link. Its protocol is layered into a Message Transfer Part (MTP) and several user parts.

Protocol structure

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

1. MTP Level 1 – Signaling data link

• Physical, bidirectional digital path, normally 64 kb/s (a timeslot of E1/T1); 56 kb/s in some ANSI networks.
• Equivalent to the OSI physical layer.

2. MTP Level 2 – Signaling link

• Gives reliable, in-sequence, error-free transfer on one link.
• Flag delimitation, 16-bit CRC, forward/backward sequence numbers, retransmission (basic or preventive cyclic).
• Initial alignment and signal unit error-rate monitoring.
• Signal units: MSU (messages), LSSU (link status), FISU (fill-in).

3. MTP Level 3 – Signaling network

• Message handling: discrimination (is it for this node?), distribution to the right user part (by service indicator), and routing using the routing label (DPC, OPC, SLS).
• Network management: traffic, link and route management (changeover, changeback, rerouting on failure or congestion).

4. SCCP – Signaling Connection Control Part

• Adds to MTP-3 to give full OSI layer-3 service.
• Global Title Translation (e.g. a dialled number to point code + subsystem number).
• Four protocol classes: 0, 1 (connectionless), 2, 3 (connection-oriented).

5. TCAP – Transaction Capabilities Application Part

• Supports non-circuit dialogues such as database queries (free-phone, number portability, roaming).
• Consists of a component sub-layer (invoke, result, error) and a transaction sub-layer.

6. User parts

• TUP (Telephone User Part): older, basic telephone call control.
• ISUP (ISDN User Part): circuit set-up and release for voice and ISDN: IAM, ACM, ANM, REL, RLC.
• MAP (Mobile Application Part): GSM location update, authentication, SMS.
• INAP: intelligent network services; OMAP: operation and maintenance.

Summary

MTP 1–3 plus SCCP form the Network Service Part (NSP), giving the transport. ISUP/TUP use MTP directly for circuit-related signaling, while TCAP-based applications use SCCP for non-circuit transactions.

Signaling Connection Control Part (SCCP)

SCCP is the SS7 layer above MTP Level 3. MTP-3 can only route to a node by point code and deliver to a user part. SCCP adds addressing of subsystems inside a node (by Subsystem Number, SSN) and Global Title Translation (GTT), so a message addressed by a dialled number or IMSI can be routed. MTP + SCCP form the Network Service Part, equal to OSI layer 3. SCCP is used by TCAP, MAP and INAP.

It offers four protocol classes:

SCCP message structure

An SCCP message is carried in the SIF of an MTP MSU (Service Indicator = 3).

+---------------------------------------+
| Routing label (DPC, OPC, SLS)         |
+---------------------------------------+
| Message type code (1 octet)           |
+---------------------------------------+
| Mandatory fixed part                  |
+---------------------------------------+
| Mandatory variable part               |
|  (pointers + length + value)          |
+---------------------------------------+
| Optional part (name, length, value)   |
+---------------------------------------+

1. Routing label: DPC (14 bits), OPC (14 bits), SLS (4 bits) in ITU format; used by MTP-3.
2. Message type code: one octet giving the message, e.g. CR (connection request), CC (connection confirm), CREF (refused), DT1/DT2 (data form 1/2), RLSD (released), RLC (release complete), UDT (unitdata), UDTS (unitdata service), XUDT (extended unitdata).
3. Mandatory fixed part: parameters of fixed length and order for that message type, so no name or length is needed, e.g. protocol class, source/destination local reference.
4. Mandatory variable part: variable-length parameters such as called party address, calling party address and data. Pointers at the start give the offset of each parameter; each parameter starts with its length.
5. Optional part: parameters that may or may not appear; each coded as name + length + value, ending with an end-of-optional-parameters octet.

SCCP address (called/calling party)

| Address indicator | SPC | SSN | Global title |

• Address indicator: shows whether PC, SSN and GT are present and whether routing is on GT or on PC+SSN.
• SPC: signaling point code; SSN: subsystem (e.g. HLR = 6, VLR = 7, MSC = 8).
• Global title: digits (e.g. E.164 or E.212 number) with translation type and numbering plan.

Example

A UDT carrying a MAP "send routing info" query from an MSC to an HLR uses connectionless class 0; the called party address holds the subscriber's MSISDN as global title, which the STP translates to the HLR's point code and SSN 6.

CCS7 protocol stack

CCS7 (SS7) is a layered, message-based signaling protocol carried on dedicated 64 kb/s links separate from speech.

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

1. MTP Level 1 (Signaling data link): 64 kb/s bidirectional channel, e.g. TS16 of an E1. Same as OSI layer 1.
2. MTP Level 2 (Signaling link): flag delimiting, CRC-16 error detection, sequence numbers (FSN, BSN), retransmission, alignment and error monitoring. Signal units: MSU, LSSU, FISU.
3. MTP Level 3 (Signaling network): routes messages by the routing label (DPC, OPC, SLS); discriminates and distributes to user parts; manages links and routes during failures.
4. SCCP: global-title translation and subsystem addressing; connectionless (classes 0, 1) and connection-oriented (classes 2, 3) service. MTP + SCCP = Network Service Part (OSI layer 3).
5. TCAP: handles query/response transactions with remote databases (SCP, HLR).
6. ISUP: ISDN user part for call set-up/release (IAM, ACM, ANM, REL, RLC); TUP is the older telephone-only version.
7. MAP / INAP / OMAP: mobile, intelligent-network and maintenance applications over TCAP.

Why common channel is preferred over in-channel signaling

• Faster set-up: whole address in one message at 64 kb/s.
• Better trunk use: trunks are seized only after the called line is found free; one link serves thousands of trunks.
• Rich, extendable signal set: supports ISDN, caller ID, call forwarding, mobile and IN services.
• Signaling at any time, even during conversation.
• Secure: users cannot inject tones (no blue-box fraud).
• Database access through SCCP/TCAP (toll-free, number portability, roaming).
• Reliable: error correction and redundant STPs.

Importance of signaling

Signaling is the control information that lets a telephone network work automatically. It is important because it:

• detects a call request and returns dial tone;
• carries the called number so the exchange can route the call;
• tells whether the called party is free, alerts it (ringing) and informs the caller (ring-back or busy tone);
• detects answer and clear, so the connection is made, charged and released correctly;
• supports network management (blocking circuits, re-routing on faults) and advanced services such as caller ID, free-phone and mobile roaming.

SS7 protocol stack

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

1. MTP Level 1 – Signaling data link: 64 kb/s digital bidirectional path; corresponds to the OSI physical layer.
2. MTP Level 2 – Signaling link: reliable transfer over one link: flags, 16-bit CRC, FSN/BSN sequence numbers, retransmission, alignment and error-rate monitoring. Uses MSU (messages), LSSU (link status) and FISU (fill-in) units.
3. MTP Level 3 – Signaling network: message discrimination, distribution and routing using the routing label (DPC, OPC, SLS); signaling network management for link failures and congestion.
4. SCCP – Signaling Connection Control Part: extends MTP-3 with global title translation and subsystem numbers; offers connectionless and connection-oriented classes. MTP + SCCP give OSI layer-3 service (Network Service Part).
5. TCAP – Transaction Capabilities Application Part: carries query/response dialogues not tied to a voice circuit (e.g. 800-number lookup, HLR query).
6. ISUP – ISDN User Part: controls trunk circuits: IAM, ACM, ANM, REL, RLC. TUP is the earlier telephone user part.
7. Applications: MAP (GSM mobility, SMS), INAP (intelligent network), OMAP (operation and maintenance).

Message Transfer Part (MTP)

The MTP is the lower part of the SS7 stack that transfers signaling messages reliably from one signaling point to another for all user parts (ISUP, TUP, SCCP). It has three levels.

MTP Level 1 – Signaling data link

• A bidirectional transmission path at 64 kb/s (56 kb/s in ANSI), usually a timeslot of E1/T1.
• Defines electrical and physical characteristics (OSI layer 1).

MTP Level 2 – Signaling link functions

• Delimitation: each signal unit starts and ends with flag 01111110 (bit stuffing used).
• Error detection: 16-bit CRC check bits.
• Error correction: basic method (go-back-N using FSN/BSN and FIB/BIB) or preventive cyclic retransmission on long-delay links.
• Initial alignment of a new link and error-rate monitoring (SUERM).
• Flow control using LSSU "busy".
• Signal units: MSU, LSSU, FISU.

MTP Level 3 – Signaling network functions

• Message handling: discrimination (is the DPC this node?), distribution (by service indicator to SCCP, ISUP, TUP...) and routing (choice of outgoing link set; load sharing with SLS).
• Network management: traffic management (changeover, changeback, forced rerouting), link management (activate, restore links), route management (transfer-prohibited/allowed messages between STPs).

Addressing of signal units

Addressing lives in the MSU, in the SIO and the routing label at the start of the SIF.

MSU: |F|BSN|BIB|FSN|FIB|LI|SIO|  SIF  |CK|F|
                            |
          +-----------------+------------+
          | Routing label   | User data  |
          | DPC | OPC | SLS |            |

Service Information Octet (SIO), 8 bits

Routing label (ITU-T), 32 bits

In ANSI networks point codes are 24 bits (network–cluster–member, 8 bits each) and SLS is 5 or 8 bits, giving a 56-bit label. For ISUP messages a CIC (circuit identification code, 12 bits in ITU) follows the label and identifies the speech trunk.

LSSU and FISU are link-local, so they carry no routing label: they are "addressed" only by the link on which they travel.

Signaling

Signaling is the exchange of control information needed to set up, supervise, charge and release a call: between the subscriber and the local exchange (subscriber loop signaling) and between exchanges (inter-exchange or trunk signaling). It includes supervisory, address and call-progress signals.

CAS vs CCS

In Channel Associated Signaling (CAS) the signals for each speech circuit travel on that circuit itself or on a channel permanently tied to it (in-band tones, or the TS16 bits of an E1 frame assigned to each channel). In Common Channel Signaling (CCS) the signals for many circuits are sent as labelled messages over a separate shared data link (e.g. SS7).

CAS:  [speech+signal ch1] [speech+signal ch2] ...
CCS:  [speech ch1] [speech ch2] ... [speech chN]
      [====== one common signaling link ======]

CAS is simple and cheap for small networks, whereas CCS is used in modern digital and mobile networks because of its speed, flexibility and support for advanced services.

Signaling in a telecommunication network is the transfer of control information that establishes, supervises, charges and releases connections, and manages the network. It is the "language" between telephones and exchanges and between exchanges.

Forms (functions) of signals

1. Supervisory signals – show the state of a line or trunk: idle/busy, seize, off-hook, answer, clear-forward, clear-back. Example: loop closed by lifting the handset.
2. Address signals – carry the called number and other routing information: rotary dial pulses, DTMF tones, MF digits, or the called-party number in an SS7 IAM.
3. Call-progress (information) signals – inform the user: dial tone, ringing current, ring-back tone, busy tone, congestion tone, recorded announcements.
4. Network management / maintenance signals – blocking/unblocking of circuits, alarm, test, re-routing information.

Types of signaling by location

• Subscriber (access) signaling: between telephone and local exchange — DC loop (off-hook/on-hook), dial pulses at 10 pps or DTMF (697–941 Hz row, 1209–1633 Hz column tones), 75 V/20–25 Hz ringing. In ISDN, DSS1 on the D-channel.
• Inter-exchange (trunk) signaling: between exchanges over trunks — line signals and register signals.

Types by frequency relation

 0   300                 3400 3825      Hz
 |----|=====voice band=====|----|-----
   DC   in-band (e.g.2600)   out-band

• DC signaling: loop or polarity changes; used on short metallic lines.
• In-band signaling: tones within 300–3400 Hz, e.g. 2600 Hz SF, MF R1/R2. Passes through any voice channel but can be imitated by speech; signals cannot be sent during speech.
• Out-of-band signaling: a tone outside the speech band but inside the 4 kHz channel, e.g. 3825 Hz. Can signal during conversation; needs filters.

Types by relation to speech channel

• Channel Associated Signaling (CAS): signals tied to each speech channel, e.g. ABCD bits in TS16 of E1 for each of 30 channels, MF R2.
• Common Channel Signaling (CCS): one separate data link carries labelled messages for many channels, e.g. CCITT No. 6 and SS7 (MTP, SCCP, ISUP, TCAP).

Other classifications

• Line vs register signaling: line signals supervise the trunk (seize, answer, clear) throughout the call; register signals carry digits during set-up only.
• Compelled vs non-compelled: in compelled signaling (R2) each signal stays until acknowledged; non-compelled signals have fixed duration.
• Link-by-link vs end-to-end: link-by-link signals are regenerated at each exchange; end-to-end signals pass from origin to destination exchange directly.

Comparison of main approaches

Modern networks use DTMF/DSS1 on the access side and SS7 (or SIP in IP networks) between exchanges.

Major building blocks of an SS7 network

SS7 is a packet-switched signaling network separate from the voice network. Its nodes are called signaling points, each identified by a point code.

1. Service Switching Point (SSP): a telephone exchange (local, tandem or MSC) with SS7 software. It originates and terminates ISUP call-control messages, and sends TCAP queries to SCPs when a call needs database information (e.g. toll-free number).
2. Signal Transfer Point (STP): a packet switch that receives SS7 messages and routes them to the correct destination by point code or global title translation. STPs are installed in mated pairs so that the failure of one does not stop signaling. They may also screen messages (gateway STP between operators).
3. Service Control Point (SCP): a database server that answers TCAP queries: 800/free-phone translation, local number portability, calling-card validation, prepaid, HLR in mobile networks. Also usually deployed in pairs.
4. Signaling links and link sets: 64 kb/s bidirectional channels connecting the nodes; parallel links between two nodes form a link set (up to 16 links) and share load.

Links used in SS7

        A                      A
  SSP1 ---> STP-a <==C==> STP-b <--- SSP1
              ||  \      /  ||
           B/D||   B    B   ||B/D
              ||  /      \  ||
            STP-c <==C==> STP-d ---A--- SSP2
  SSP1 -------------F------------------ SSP2
  SSP1 ----E---- STP-c  (alternate pair)
  SCP  ----A---- STP-a / STP-b

The "quad" arrangement of B/D links between two mated pairs gives each message several alternative paths, giving very high availability.

Advantages of SS7

• Fast call set-up (whole number sent in one message at 64 kb/s).
• Trunks are seized only after the called party is free, so trunk efficiency is higher.
• One signaling link serves thousands of trunks; less per-circuit equipment.
• Large, extendable message set: ISDN, caller ID, call forwarding, mobile (MAP), IN services.
• Signaling possible during conversation; no fraud by tone imitation.
• Database access (toll-free, number portability, HLR) through SCCP/TCAP; high reliability with CRC and redundant STPs.

Working principle

SS7 sends all control information as digital messages on a separate signaling network, while the voice trunks carry only speech.

  Tel-A                                   Tel-B
   |                                        |
  SSP-A ---A link--- STP pair ---A link--- SSP-B
   |                    |                   |
   +====== voice trunks (CIC) ==============+
                        |
                       SCP (database)

1. Message creation: the originating SSP's user part (ISUP) builds a message (e.g. IAM) containing called/calling number and the CIC of the chosen trunk.
2. Routing label: MTP-3 adds DPC (destination), OPC (origin) and SLS. MTP-2 puts it in an MSU with sequence numbers and CRC; MTP-1 sends it on the 64 kb/s link.
3. Transfer: the STP reads the DPC and forwards the message on a link toward SSP-B; MTP-2 at each hop checks the CRC and acknowledges or asks retransmission.
4. Delivery: SSP-B's MTP-3 sees its own point code and passes the message (by the service indicator) to ISUP.
5. Call set-up sequence: IAM → ACM (called party ringing) → ANM (answer; charging starts) → conversation on the trunk → REL → RLC (trunk freed).
6. Non-circuit services: for an 800 number, SSP-A first sends a TCAP query through SCCP/STP to the SCP, which returns the real routing number; then the normal ISUP sequence follows.
7. Network management: if a link fails, MTP-3 changes traffic over to another link or STP automatically.

Common Channel Signaling

Common Channel Signaling (CCS) is a method in which signaling information for a large group of speech circuits is sent as labelled data messages over a separate, shared signaling link, instead of on each speech channel. Each message carries a label (e.g. circuit identification code) showing which circuit it refers to. SS7 (ITU-T Q.700) is the standard CCS system.

Working principle of SS7

  Tel-A                                   Tel-B
   |                                        |
  SSP-A ---A link--- STP pair ---A link--- SSP-B
   |                    |                   |
   +====== voice trunks (CIC) ==============+
                        |
                       SCP (database)

Network elements: SSPs (exchanges that originate/terminate messages), STPs (packet switches that route messages, used in mated pairs) and SCPs (databases).

Protocol layers used:

• MTP-1: 64 kb/s link; MTP-2: framing, CRC, sequence numbers and retransmission; MTP-3: routing by DPC/OPC/SLS and network management.
• SCCP + TCAP: database queries; ISUP: circuit set-up and release.

Steps of a call

1. Subscriber A lifts the handset and dials. SSP-A analyses the digits and picks a free trunk to SSP-B (CIC).
2. SSP-A's ISUP forms an IAM with called number, calling number and CIC. MTP-3 adds the routing label; MTP-2 frames it as an MSU with CRC; it is sent on the A-link to the STP.
3. The STP reads the DPC and forwards the MSU to SSP-B.
4. SSP-B checks B's line. If free, it rings B and returns ACM; A hears ring-back.
5. When B answers, SSP-B sends ANM; the trunk is through-connected and billing starts.
6. On hang-up, REL is sent and the other side answers RLC; the trunk becomes idle.
7. If B is busy, SSP-B sends REL (cause: user busy) at once, so no trunk is held uselessly.
8. For services such as toll-free numbers or mobile roaming, the SSP first sends a TCAP query through SCCP to an SCP or HLR and uses the reply to route the call.

Reliability: errors on a link are corrected by retransmission at MTP-2; failed links or STPs are bypassed by MTP-3 network management using the redundant STP pairs.