This tutorial is very rough and undergoing periodic updates as time permits --InetDaemon

Signalling System 7 is a digital communications standard for out-of-band management of voice trunk lines in the public switched telephone network. Signalling System 7 uses the International Telecommunications Union's Q.931 specification to manage call setup, teardown, routing and other special services including busy signals, transferring calls, accessing voicemail boxes, call waiting, *66 for redial, and much more. By performing all these functions out-of-band, bandwidth is conserved for handling transfer of actual voice data.

North American Signalling System Architecture

SSP (Service Switching Points)

  • Signalling Point where the PSTN connects to SS7. These are telephone switches in switching offices or tandems equipped with SS7 equipment/software. SSP's orginate, terminate or switch telephone calls.
    • Common Channel Signalling Switching Office (CCSSO)
      • End or Tandem offices w/SS7 Capacity
      • Can use SS7 to set up a Trunk
    • Signalling mode call
      • Handle call set up also, or stop calls (teardown)
      • Can also perform other functions based on softwar

Signal Transfer Point (STP)

Signal Transfer Points (STP) receive and route calls between service switching points in the network.

  • Makes Destination based Routing Decisions
  • Needs dedicated DS0's (Timeslots/Channels on T1)
  • Requires Redundancy so always Paired into Cross Links (C links)
  • Cross connect to other Signal Transfer Points (STP)
  • Hierarchy of Switches

Service Control Point (SCP)

The Service Control Point is a device with databases that provide the tables and information necessary to process calls and provide advanced services such as call waiting, call forwarding and more.

  • Database of Service Point Codes (addresses) is at SCP
  • Come in various varieties
  • Provide services to the network based on Software
  • SCP's identified by SPC's and a Sub-System Number (SSN)
  • SSN's are databases, or other software


Signalling Point Code (SEP)

This is a 24 bit address organized into three levels that indicate the identity of the signalling point within the North American Signalling System.

  • Level 1: Network ID (8 bits)
  • Level 2: Cluster ID (8 bits)
  • Level 3: Member ID (8bits)

Access links (A Link)

  • Link SEP's to Signal Transfer Point (Lines)
  • First link called LOCAL

Bridge Links (B Links)

  • Interconnect Signal Transfer Points (STP) - trunks

Diagonal (D links)

  • Connect levels of the hierarchy

Extended (E Links)

  • Connect SEP's to other STP's for additional redundancy

Fully Associated (F Links)

- SEP to SEP links
- Can transmit anything so long as
both sides understand format


Intelligent Peripheral (IP)

- Located off network
- Provide services to SCP's via the SSN #
- Husally more hardware than soft

Services Node (SN)

- Similar to an IP, but usually more
software than hardware

Mobile Switching Center (MSC)

(Mobile/Cellular services)
- Home Location Register (HLR)
- Authentication Centers (AUC)
- Short Message Centers (SMC)


Application OMA&P ASE ISDNUP
Presentation TCAP
Transport SCP

Other User's Parts
MTP L3    Telephone User's Parts
Data User's Parts

Data Link MTP L2
Physical MTP L1

** Other User Parts
Telephone Users Part
Data Users Part



Operations Administration Management & Provisioning


Application Service Element




Message Transport Part

  • L1 - Links, Clocking, physical considerations
  • L2 - ITU or ANSI signalling. Message assembly of signalling units,
    adds header flags (byte with zeroes at
    both ends and 1's in the middle).
    Bit stuffing to prevent accidentally
    creating fake flags. Link monitoring,
    Congestion monitoring/reporting
  • L3 - Congestion Control


  • Message Signal Units - Houses all SS7 msgs
  • Link Status Signal Unit - Information about the Link's status.
  • Fill In Signal Unit - No gaps in transmission, recognizeable data on the link


     NAME     BITS Description
   - SPARE	2
   - LI       6
   - FIB      1    Forward Indicator Bit
   - FSN      7    Forward Sequence Number
   - BIB      1    Backward Indicator Bit
   - BSN      7    Backward Sequence Number
   - FLAG     8    Fixed pattern: (01111110)

   Transmit Buffer
   Retransmit buffer
   FSN - Copies destroyed as ack'd as received at remote MTP

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