ISUP message types

-       IAM

o   Initial Address Message (IAM) is sent in the forward direction by each switch in the circuit between the calling party and the destination switch of the called party.

o   Contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

-       SAM

o   Subsequent Address Message. In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.

-       ACM

o   Address Complete Message is sent in the backward direction to indicate that the remote end of a trunk circuit has been reserved.

o   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.

-       ANM

o   Answer Message sent when the subscriber picks up the phone

o   Normally charging starts at this moment

-       CPG

o   Call Progress (CPG) can be sent by the terminating SSP after it has sent an ACM. For example, this could be used to signal that a call has been forwarded as a result of a busy line.

-       REL

o   Release Message is sent in either direction indicating that the circuit is being released due to a specific cause indicator

o   Is sent when either calling or called party hangs up the call (cause =16)

o   Is sent back to the calling party if the called party is busy (cause 17).

-       RLC

o   Release Complete Message is sent in the opposite direction of a REL to acknowledge the release of the remote end of a trunk circuit and to end the billing cycle.

ISUP Overview

ISDN User Part (ISUP) provides signaling support for all voice and non-voice services identified as part of ISDN. ISUP includes all of the flexibility found in the message-oriented signaling of ISDN’s Q.931 message format. It also extends beyond simple call establishment and disestablishment to include network management.

Then ISUP is part of the SS7 which is used to set up telephone calls in Public Switched Telephone Networks. It is specified by the ITU-T as part of the Q.76x series.

ISUP Message Format

-       ISUP information is carried in the Service Information Field (SIF) of an MSU.

-       SIF contains the routing label followed by a 12-bit Circuit Identification Code (CIC).

-       CIC indicated the trunk circuit reserved by the originating switch to carry the call

-       CIC is followed by the message type field

o   IAM, SAM, ACM, ANM, CPG, REL, RLC

SS7 Signal Units

-       Signaling information is passed over the signaling links which are called signal units. Signal Units are continuously transmitted in both directions on any link that is in service.

-       SS7 uses 3 different types of signal units:

o   Message Signal Units (MSU) :

§   All signaling associated with call setup and teardown, database query and response, and SS7 management requires the use of MSUs

o   Links Status Signal Units (LSSU):

§  communicate information about the signaling link between the nodes on either end of the link. This information is contained in the status field of the signal unit. They signal the initiation of link alignment, quality of received traffic, and status of processors at either end of the link.

o   Fill In Signal Units (FISU):

§  do not carry any information; they simply occupy the link when there are no LSSUs or MSUs. FISUs support the monitoring of link traffic because they undergo error checking. They can also be used to acknowledge the receipt of messages using backward sequence number (BSN)

SS7 Links

-          Is the physical transmission line that connects the individual nodes in an SS7 network

A-Links

-          Interconnect an STP and either an SSP or an SCP

-          their sole purpose is to deliver signaling to and from end points

B and D Links

-          bridge links (B-Links) are the quad of links interconnecting peer pairs of STPs

-          Diagonal Links (D-Links) are the quad of links interconnecting mated pairs of STPs at different hierarchical levels

C-Links

-          Cross Links (C-Links) interconnect mated STPs and are used to enhance the reliability of the signaling network

E-Links

-          Extended links (E-Links) connect an SSP to an alternate STP to provide backup connectivity to the network if the SSP’s “home” STP cannot be reached on it’s a-Link.

Linksets

-          Links are put into groups called linksets

-          Up to 16 links can be assigned to a linkset

-          All links in a linkset must have the same adjacent node

Routes

-          Route is a collection of linksets to reach a particular destination

-          A linkset can belong to more than one route

Routesets

-          A collection of routes that are assigned to destinations and also provide alternate routes

Destination

-          An address entered into routing table of a remote signaling point. A destination need not to be adjacent to the signaling point but must be a point code tha can be reached by the signaling point.

Point Codes

-          In SS7, addresses are assigned using a 3 level hierarchy:

o   Member: a signaling point within a cluster

o   Cluster: a collection of signaling points

o   Network: each cluster is defined as being part of a network

SLS

-          Signaling link selection is used as a label to route MTP level 3 messages in the SS7 network

-          Can be used in reference to the algorithm that an SS7 node uses for selection of the appropriate signaling link to use for communication with an adjacent SS7 node.

SS7 Components

The SS7 signaling architecture consists of three essential components, interconnected via signaling links. 

Signal Switching Point (SSP)

-are switches that have SS7 software and terminating signaling links

-can originate, terminate, and switch calls

-use the calling number party info to determine how to route the call

-looks up the dialed digits in the SSP routing table to find the corresponding trunk circuit and terminating exchange

-then sends an SS7 message out to the adjacent exchange requesting a circuit connection on the trunk which was specified in the routing table

-manages all of those connections until the destination is reached

Signal Transfer Point (STP)

-are packet switches and act like routers in SS7 networks

-messages are not usually originated by an STP

-three levels of STP:

            -National: exists within the national network. It can transfer messages that use the
             same national standard of protocol.

-International: functions within an international network. It provides for SS7 interconnection of all countries, using the ITU-TS standard protocol.

-Gateway: converts signaling data from one protocol to another. Gateway STPs are often used as an access point to the international network.

Signal Control Point (SCP)

-is usually a computer used as front end to a database syatem

-is an interface to telco database

SS7 Overview

Common Channel Signaling

is the transmission of signaling information on a separate channel from the data and more specifically where that signaling channel controls multiple data channels.

Signaling Modes

• Associated Signaling

•  Non-associated Signaling

• Quasi-associated Signaling