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Tech Stuff - SS7 Protocols

ZYTRAX now offers SIGTRAN and SS7 training.

Contents

  1. SS7 Overview
  2. SS7 Signaling Points
  3. SS7 Stack
  4. ISUP - Call Setup Procedures
  5. SCCP Overview
  6. TCAP Overview
  7. Global Title Translation (GTT)
  8. SS7 & SIGTRAN Glossary

Common Channel Signaling System #7 (SS7)

Common Channel Signaling System #7 (SS7) separates the information required to set up and manage telephone calls in the Public Switched Telephone Network (PSTN) onto a separate packet switched network (The Signaling Network) rather than use the same circuit switched network that the telephone call is made on (The Voice Network). This technique is commonly called out-of-band signaling and contrasts with earlier in-band techniques (Channel Associated Signaling - CAS) which used MF tones. The SS7 network consists of Signaling Points.

ss7

Figure 1.0

Signaling Points
SSP = Service Switching Point
STP = Signal Transfer Point: Usually deployed in pairs in case of failure
SCP = Service Control Point: Usually deployed in pairs in case of failures.

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Signaling Points (SPs)

All nodes in the SS7 network are called Signaling Points (SPs). Each SP is identified by a unique address called a Point Code (PC). SPs have the ability to read a Point Code and determine if the message is for that node and the ability to route SS7 messages to another SP. The three types of Signaling Points are:

All SPs (signalling points) are connected using (typically) pairs of Links. Each Link type is identificed with a letter as defined below:

All links use the same physical connections (typically DS0A - 56K bit/s or DS1 (T1)). The letter designation allows differing congestion and recovery treatment.

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The SS7 Protocol Stack

The standard SS7 protocol has 4 levels (layers) as defined in the OSI 7 Layer Model. The levels 1 to 3 constitute the Message Transfer Part (MTP) and level 4 is the User Part (Transport Layer in OSI).

SS7 Stack

The SS7 Protocol Stack Model

MTP1 = Message Transfer Part 1
MTP2 = Message Transfer Part 2
MTP3 = Message Transfer Part 3
SCCP = Signaling Connection Control Part
TCAP = Transaction Capabilities Application Part
MAP = Mobile Application Part
INAP = Intelligent Network Application Part
ISUP = ISDN User Part
NOTE: TCAP, MAP and INAP are examples of services that use SCCP (list in SS7 glossary).

Blocks and Packets

SS7 uses three types of packets, called Signal Units (SUs) only one of which carries data. The Fill-in Signal Unit (FISU) is sent when the link is idle and allows the end-points to monitor signal quality and link integrity. The Link Status Signal Unit (LSSU) is sent when a link error occurs. Message Signal Units (MSU) carry protocol payloads such as ISUP, MAP etc.

Message Transfer Parts

The MTP acts as the carrier for all SS7 messages providing reliable transfer from one SP to another (node-to-node) with error detection and correction. MTP covers levels 1-3 in the OSI stack. [MTP ITU-T Specs: Q.701-Q.705 ANSI: T1.111.1 - T1.111.8]

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ISDN User Part: ISUP

ITU-T Specs: Q.760 - Q.769 ANSI: T1.113.1 - T1.113.4

ISUP defines one call control protocol used to set-up, manage and release circuits that carry voice and data calls in the PSTN. ISUP uses the MTP for routing messages from one SSP to another.

Notes

  1. ISUP is the most commonly used call control protocol in North America and is used to illustrate the general points. There are a number of other call control protocols and national variants, for example, TUP (Telephony User Part), IUP (Interconnect User Part - UK standard).
example

Example: ISUP signaling associated with a basic call

  1. When a call is placed to an out-of-switch number, the originating SSP1 transmits an ISUP Initial Address Message (IAM) to reserve a trunk circuit from the originating SSP1 to the destination SSP2. The IAM includes the originating and destination points codes, the circuit identification code and the global title digits. The IAM is routed via the local STP1 of the originating SSP1 to the destination SSP2.

  2. The destination SSP2 determines if it serves the called party. If so it generates a ringing tone at the called party's line and transmits an ISUP Address Complete Message (ACM) to the originating SSP1 via its local STP2. The ACM indicates that the remote end trunk circuit has been reserved. STP2 routes the ACM to SSP1 which generates a ringing tone to the calling party's line and connects it to the trunk circuit.

  3. When the called party picks up the phone, SSP2 terminates the ringing tone and transmits an ISUP ANswer Message (ANM) to SSP1 via STP2. STP2 routes the ANM to SSP1 which verifies that the calling party line is connected to the reserved trunk and if so starts billing.

  4. If the caller hangs up first SSP1 sends an ISUP RELease message (REL) to release the trunk circuit between the 2 switches. STP1 routes the REL to SSP2. Upon receiving the REL SSP2 disconnects the circuit from the called party's line and transmits an ISUP ReLease Complete message (RLC) to SSP1 to ack the release of the trunk circuit. When SSP1 receives the RLC it terminates billing.

  5. If the called person hangs up first, SSP2 sends a REL to SSP1 indicating the release cause. When REL is received, SSP1 disconnects the circuit from the caller's line and transmits an ISUP ReLease Complete message (RLC) to SSP2. When SSP2 receives the RLC it terminates and stops billing.

ISUP Messages
IAM = Initial Address Message
ACM = Address Complete Message
ANM = ANswer Message
REL = RElease Message
RLC = Release Complete Message

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Signaling Connection Control (User) Part: SCCP

ITU-T Specs: Q.711 - Q.719 ANSI: T1.112.1 - T1.112.5

SCCP provides connectionless and connection-oriented network services via MTP3 for the transfer of signaling messages between SSP's. While MTP3 provides point codes to allow messages to be addressed to specific signaling points, SCCP provides Subsystem Numbers (SSN) to let messages be addressed to specific applications at these signaling points. MTP transfers messages node-to-node while SCCP transfers messages end-to-end.

SCCP is used as the transport layer for TCAP based services like, freephone(800/888), local number portability and roaming. SCCP also provides the means by which an STP can provide Global Title translation. Because an STP provides global title translation, originating SSPs do no need to know the DPC or SSN of the associated service. only STPs need to maintain the tables of destination point codes or SSNs associated with specific services and possible destinations.

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Transaction Capabilities Application (User) Part: TCAP

ITU-T Specs: Q.770 - Q.779 ANSI: T1.114.1 - T1.114.5

TCAP messages are destined for application entities.

tcap

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Global Title Translation ( GTT )

In SS7, a dialed number is referred to as a global title. If the SSP does not know the destination of a global title, it sends a query in the form of TCAP messages to its local STP. When the query gets to the STP, the global title digits are given to the STP's SCCP. The STP will then translate the SCCP address fields and determine, through its own translation tables the address of the application (SCP). The Global Title Translation provides the SSN of the database and point code of the SCP that interfaces that database application.



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