Introduction to SS7 Protocoll

4. FSN (Forward Sequence Number)

The FSN contains the sequence number of the signal unit.

5. FIB (Forward Indicator Bit)

The FIB is used in error recovery like the BIB. When a signal unit is ready for transmission, the signaling point increments the FSN (forward sequence number) by one. The CRC (cyclic redundancy check) checksum value is calculated and appended to the forward message. Upon receiving the message, the remote signaling point checks the CRC and copies the value of the FSN into the BSN of the next available message scheduled for transmission back to the initiating signaling point.

6. SIO (Service Information Octet)

The SIO field in an MSU contains the 4-bit subservice field followed by the 4-bit service indicator. FISUs and LSSUs do not contain an SIO. The subservice field contains the network indicator (e.g., national or international) and the message priority (zero to three with three being the highest priority). Message priority is considered only under congestion conditions, not to control the order in which messages are transmitted.

7. SIF (Signaling Information Field)

The SIF in an MSU contains the routing label and signaling information (e.g., SCCP, TCAP and ISUP message data). LSSUs and FISUs contain neither a routing label nor an SIO as they are sent between two directly connected signaling points.

8. CRC (Cyclic Redundancy Check)

The CRC value is used to detect and correct data transmission errors.

3.c MTP Level 3

MTP Level 3 provides message routing between signaling points in the SS7 network. MTP Level 3 is equivalent in function to the OSI Network Layer. MTP Level 3 routes messages based on the routing label in the signaling information field (SIF) of message signal units. The routing label is comprised of the destination point code (DPC), originating point code (OPC) and signaling link selection (SLS) field. Point codes are numeric addresses that uniquely identify each signaling point in the SS7 network. When the destination point code in a message indicates the receiving signaling point, the message is distributed to the appropriate user part (e.g., ISUP or SCCP) indicated by the service indicator in the SIO. Messages destined for other signaling points are transferred provided that the receiving signaling point has message transfer capabilities (like an STP). The selection of outgoing link is based on information in the DPC and SLS.

3.d Signaling Link Selection (SLS)

The selection of outgoing link is based on information in the DPC and Signaling Link Selection field. The SLS is used to:

  • Ensure message sequencing. Any two messages sent with the same SLS will always arrive at the destination in the same order in which they were originally sent.
  • Allow equal load sharing of traffic among all available links. In theory, if a user part sends messages at regular intervals and assigns the SLS values in a round-robin fashion, the traffic level should be equal among all links (within the combined linkset) to that destination.

In ANSI networks, the size of the SLS field was originally five bits (32 values). In configurations with two links in each linkset of a combined linkset (totaling four links), eight SLS values were assigned to each link to allow an equal balance of traffic.

4. ISDN User Part

The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls. Calls that originate and terminate at the same switch do not use ISUP signaling.

4.a Basic ISUP Call Control

1. When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch (1a). The IAM includes the originating point code, destination point code, circuit identification code, dialed digits and, optionally, the calling party number and name. In the example below, the IAM is routed via the home STP of the originating switch to the destination switch (1b). Note that the same signaling link(s) are used for the duration of the call unless a link failure condition forces a switch to use an alternate signaling link.

2. The destination switch examines the dialed number, determines that it serves the called party and that the line is available for ringing. The destination switch rings the called party line and transmits an ISUP address complete message (ACM) to the originating switch (2a) (via its home STP) to indicate that the remote end of the trunk circuit has been reserved. The STP routes the ACM to the originating switch (2b), which rings the calling party’s line and connects it to the trunk to complete the voice circuit from the calling party to the called party. In the example shown above, the originating and destination switches are directly connected with trunks. If the originating and destination switches are not directly connected with trunks, the originating switch transmits an IAM to reserve a trunk circuit to an intermediate switch. The intermediate switch sends an ACM to acknowledge the circuit reservation request and then transmits an IAM to reserve a trunk circuit to another switch. This process continues until all trunks required to complete the voice circuit from the originating switch to the destination switch are reserved.

3. When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP (3a). The STP routes the ANM to the originating switch (3b), which verifies that the calling party’s line is connected to the reserved trunk and, if so, initiates billing.

4. If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches (4a). The STP routes the REL to the destination switch (4b). If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).

5. Upon receiving the REL, the destination switch disconnects the trunk from the called party’s line, sets the trunk state to idle and transmits an ISUP release complete message (RLC) to the originating switch (5a) to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC (5b), it terminates the billing cycle and sets the trunk state to idle in preparation for the next call.

Fig vii: ISUP Call Flow

4.b ISUP Message Format

ISUP information is carried in the Signaling Information Field (SIF) of an MSU. The SIF contains the routing label followed by a 14-bit (ANSI) or 12-bit (ITU) circuit identification code (CIC). The CIC indicates the trunk circuit reserved by the originating switch to carry the call. The CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC), which defines the contents of the remainder of the message. Each ISUP message contains a mandatory fixed part containing mandatory fixed-length parameters. Sometimes the mandatory fixed part is comprised only of the message type field. The mandatory fixed part may be followed by the mandatory variable part and/or the optional part. The mandatory variable part contains mandatory variable-length parameters. The optional part contains optional parameters, which are identified by a one-octet parameter code followed by a length indicator (“octets to follow”) field. Optional parameters may occur in any order. If optional parameters are included, the end of the optional parameters is indicated by an octet containing all zeros.

4.c Initial Address Message

An Initial Address Message (IAM) is sent in the “forward” direction by each switch needed to complete the circuit between the calling party and called party until the circuit connects to the destination switch. An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

4.d Address Complete Message

An Address Complete Message (ACM) is sent in the “backward” direction to indicate that the remote end of a trunk circuit has been reserved. The originating switch responds to an ACM message by connecting the calling party’s line to the trunk to complete the voice circuit from the calling party to the called party. The originating switch also sends a ringing tone to the calling party’s line.

4.e Release Message

A Release Message (REL) is sent in either direction indicating that the circuit is being released due to the cause indicator specified. An REL is sent when either the calling or called party “hangs up” the call (cause = 16). An REL is also sent in the backward direction if the called party line is busy (cause = 17).

4.f Release Complete Message

A Release Complete Message (RLC) is sent in the opposite direction of the REL to acknowledge the release of the remote end of a trunk circuit and end the billing cycle as appropriate.

5. Telephone User Part

In some parts of the world (e.g., China), the Telephone User Part (TUP) supports basic call processing. TUP handles analog circuits only – digital circuits and data transmission capabilities are provided by the Data User Part.

6. Signaling Connection Control Part

SCCP provides connectionless and connection-oriented network services above MTP Level 3. While MTP Level 3 provides point codes to allow messages to be addressed to specific signaling points, SCCP provides subsystem numbers to allow messages to be addressed to specific applications (called subsystems) at these signaling points. SCCP is used as the transport layer for TCAP-based services such as free phone (800/888), calling card, and local number portability, wireless roaming and personal communications services (PCS).

6.a Global Title Translation

SCCP also provides the means by which an STP can perform global title translation (GTT), a procedure by which the destination signaling point and subsystem number (SSN) is determined from digits (i.e., the global title) present in the signaling message. The global title digits may be any sequence of digits (e.g., the dialed 800/888 number, calling card number or mobile subscriber identification number) pertinent to the service requested. Because an STP provides global title translation, originating signaling points do not need to know the destination point code or subsystem number of the associated service. Only the STPs need to maintain a database of destination point codes and subsystem numbers associated with specific services and possible destinations.

6.b SCCP Message Format

The Service Indicator of the Service Information Octet (SIO) is coded three (binary 0011) for SCCP. SCCP messages are contained within the Signaling Information Field (SIF) of an MSU. The SIF contains the routing label followed by the SCCP message contents. The SCCP message is comprised of a one octet message type field that defines the contents of the remainder of the message

Each SCCP message contains a mandatory fixed part (mandatory fixed-length parameters), mandatory variable part (mandatory variable-length parameters) and an optional part that may contain fixed-length and variable-length fields. Each optional part parameter is identified by a one-octet parameter code followed by a length indicator (“octets to follow”) field. Optional parameters may occur in any order. If optional parameters are included, the end of the optional parameters is indicated by an octet containing all zeros.

7. Transaction Capabilities Application Part

TCAP enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signaling points using the SCCP connectionless service. The SCP uses TCAP to return a response containing the routing number(s) (or an error or reject component) back to the SSP. Calling card calls are also validated using TCAP query and response messages. When a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register requests service profile information from the subscriber’s home location register (HLR) using mobile application part (MAP) information carried within TCAP messages. TCAP messages are contained within the SCCP portion of an MSU. A TCAP message is comprised of a transaction portion and a component portion.

7.a Transaction Portion

The transaction portion contains the package type identifier. There are seven package types:

  • Unidirectional: Transfers component(s) in one direction only (no reply expected).
  • Query with Permission: Initiates a TCAP transaction. The destination node may end
    the transaction.
  • Query without Permission: Initiates a TCAP transaction. The destination node may not
    end the transaction.
  • Response: Ends the TCAP transaction. A response to a 1-800 query with permission may
    contain the routing number(s) associated with the 800 number.
  • Conversation with Permission: Continues a TCAP transaction. The destination node
    may end the transaction.
  • Conversation without Permission: Continues a TCAP transaction. The destination
    node may not end the transaction.
  • Abort: Terminates a transaction due to an abnormal situation.

The transaction portion also contains the Originating Transaction ID and Responding
Transaction ID fields, which associate the TCAP transaction with a specific application at
the originating and destination signaling points respectively.

7.b Component Portion

The component portion contains components. There are six kinds of components:

  • Invoke (Last): Invokes an operation. For example, a Query with Permission transaction
    may include an Invoke (Last) component to request SCP translation of a dialed 800
    number. The component is the “last” component in the query.
  • Invoke (Not Last): Similar to the Invoke (Last) component except that the component
    is followed by one or more components.
  • Return Result (Last): Returns the result of an invoked operation. The component is the
    “last” component in the response.
  • Return Result (Not Last): Similar to the Return Result (Last) component except that
    the component is followed by one or more components.
  • Return Error: Reports the unsuccessful completion of an invoked operation.
  • Reject: Indicates that an incorrect package type or component was received.

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