SS7 is structured in a multi-layered stack which corresponds closely to the layers of the standard OSI model, although some SS7 components span a number of layers, as illustrated in here.
The SS7 component parts are:
- Layer 1 (Physical): MTP-1 (Message Transfer Part-1)
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MTP-1 defines the physical means by which SS7 messages are transferred from one node to another. For E1 ot T1 networks, the physical layer is usually a timeslot of an E1 or T1 frame respectively.
The physical layer specifies only how a sequence of bits is conveyed from one SS7 node to another. It says nothing about the actual meaning of the bits or how they are grouped together to form a message.
- Layer 2 (Data Link): MTP-2
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MTP–2 defines how an MTP-1 bit transfer mechanism is used to reliably pass variable length messages from one SS7 node to another. MTP-2 uses a variant of the High level Data Link Control (HDLC) used in most modern data transfer protocols. This uses a delimiter to define the start and end of a data frame, prevents flags occurring in a frame (bit-stuffing) and protection for the entire frame (CRC at the end). It also defines how CRC errors are handled (by error response and retransmission).
MTP-2 says nothing about the actual content of a message. It simply defines a mechanism by which a message of any length can be sent 100% reliably between SS7 nodes and can be used by higher layers of the SS7 protocol.
MTP-2 knows nothing beyond the single point-to-point link it operates on.
- Layer 3 (Network Layer): MTP–3
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MTP-3 builds on top of the lower-level MTP layers to allow the creation of a network of telephony network nodes interconnected by SS7 links. Each node is assigned a unique address in the network (known as a Signaling Point Code or SPC). Messages can be sent at the MTP-3 level in one node to a topologically distant node (that is with one or more intermediate SS7 nodes) simply by specifying the Destination Point Code (DPC). MTP-3 entities on the SPC node, the DPC node, and all intermediate nodes coordinate the transfer of a higher-layer message through the network.
MTP-3 can use multiple parallel routes from SPC to DPC through the network to take account of link loading and availability (there should always be more than one way to get from any SPC to any DPC).
- Upper Layers: TUP (Telephone User Part)
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The Telephone User Part (TUP) is used to set up a telephone call between two SS7 nodes. It defines a set of messages and a protocol using these messages that allows a telephone call to be set up and torn down.
TUP messages flow only immediately before a call is established and then immediately before it is terminated. No TUP messages are exchanged when a call is in progress.
TUP was one of the first SS7 protocols and designed to support simple analog phones (with little function over and above call setup and tear-down).
- Upper Layers: ISUP (Integrated Services User Part)
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The ISUP performs the same function as the TUP (that is, it handles the setup and tear-down of telephone calls) but it is much more sophisticated providing function available with primary rate ISDN. This includes calling and called number notification (or suppression), the ability to control billing (charging) rates, advanced telephony functions such as transfer, and control over whether the voice channel is used for voice, fax, or data.
As with TUP, ISUP messages flow only during the setup and tear-down phases of a call.
- Upper layers: SCCP (Signaling Connection Control Part)
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The SCCP runs above the MTP layers and provides a set of facilities similar to those provided by the UDP and TCP layers of TCP/IP. Specifically, SCCP provides five classes of service such as connectionless (like UDP) and connection-oriented (like TCP) with options of error recovery and flow control. It also provides what is known in SS7 as Global Title Translation.
- Upper layers: TCAP (Transaction Capabilities Application Part)
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The TCAP is designed to implement functions in the SS7 network which are unrelated to the origination and termination of actual telephone calls. TCAP provides a means by which information can be transferred from an application at a switch location to another application in another network entity.
One example of TCAP usage is number translation and database transactions and lookup. Another example of the use of TCAP is the Message Waiting Indicator (MWI) on some telephones which indicates that a voice message is waiting for the subscriber. An SS7-connected voice mail system sends a TCAP message to the network to set the MWI flag in a subscriber's database.
Note that TCAP can be used by itself (on top of SCCP and the MTP layers), or it can be used as a transport layer for higher-level layers such as MAP and INAP (described in following sections).
- Upper layers: MAP (Mobile Application Part)
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Mobile Application Part (MAP) is the most complex SS7 component and is used in GSM mobile telephone systems to pass information between the components of the network.
- Upper layers: INAP (Intelligent Network Application Part)
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The Intelligent Network Application Part (INAP) is used to implement services within a network, which involve accesses to an SCP and might also involve the use of an Intelligent Peripheral (IP). INAP messages are sent between network entities using TCAP transactions.
- Upper layers: OMAP (Operations and Administration Application Part)
- The OMAP is typically used by a network administration facility to control an entire network from a central point. Facilities provided in OMAP include administration of system databases, maintenance access and performance monitoring.
SS7 Support for Blueworx Voice Response, which is discussed in the next section, supports the MTP layers and the ISUP. If you need other layers to be supported contact your Blueworx representative.