STATION ID - 7047/3.12 9x Datakit Network FOR OFFICIAL USE ONLY This is a 9x system, restricted to authorized persons and for official 9x business only. Anyone using this system, network or data is subject to being monitored at any time for system administration and for identifying unauthorized users or system misuse. Anyone using this system expressly consents to such monitoring and is advised that any evidence of criminal activity revealed through -------------------------------------------------------------------------- The OSI Model and SS7 Protocol Stack by foneman -------------------------------------------------------------------------- Note: The following article explains the relationship between the OSI Model and the SS7 Protocol Stack, it does *not* explain the protocols within the stack itself. Those will be later explained in a future article. The OSI Model: History ====================== As you may or may not know, the Open Systems Interconnect (OSI) data communications standard was developed and published in 1982 by the International Standards Organization (ISO) mainly for use with mainframes. It wasn't until 1984 that it was actually adopted as a standard. OSI is a protocol which provides the methods necessary for mainframes to communicate with devices such as modems and terminals. Since SS7 was defined and being developed starting in 1981, the SS7 levels only map loosely to the OSI 7 layer model. The OSI Model: Layer Responsibilities ===================================== *Note: Each layer provides a service to the layer above and below it. Ex: Layer 1 provides a service to layer 2, and layer 2 provides a service to layer 3. OSI MODEL ____________ 7 |Application | | | ------------ 6 |Presentation| | | ------------ 5 | Session | | | ------------ 4 | Transport | | | ------------ 3 | Network | | | ------------ 2 | Data Link | | | ------------ 1 | Physical | |____________| Layer 1: The Physical Layer - The responsibility of this layer is to convert digital data into a bit stream to enable transmission over the network, such as conversion from electrical to audible and light. Layer 2: The Data Link Layer - The responsibility of this layer is to provide the services for reliable data communications between two devices by using some method of sequencing and error detection and correction, also called the reliability factor. This layer is *only* concerned with the transmission of data between the two devices and *not* the whole network. Layer 3: The Network Layer - The responsibility of this layer is to provide routing services for packets received from some other node. It is up to this layer to look at the destination address and find the link to be used to get there. Layer 4: The Transport Layer - The responsibility of this layer is to make sure the communications over the network are reliable and without error. The reliability factor, which was discussed in the Data Link section, can be built into the Transport layer should the Network layer become unreliable. Layer 5: The Session Layer - The responsibility of this layer is to establish a dialog with another entity as well as define what type of dialog to be established. It also provides flow control procedures and manages synchronization points. Layer 6: The Presentation Layer - The only responsibility of this layer is to compress and/or encrypt the data and to provide it in a syntax that can be sent and received over the network at a distant node and then decompressed and/or decrypted. Layer 7: The Application Layer - The Application layer is basically the interface between the application entity and the OSI model. This is the first stage in preparing the data to be sent over the network. The SS7 Protocol Stack: An Overview =================================== The SS7 protocol has proved to be an incredibly reliable and efficient packet-switching protocol that provides all of the services and functions required by telephone service providers. One thing that you might have noticed right off the bat is that while the OSI model is made up of 7 different layers, the SS7 standard only uses 4. This is because the functions carried out by the 4 SS7 levels correspond with the OSI model's 7 layers. Also, some of the OSI model's functions serve no purpose in the SS7 network. The fact that the SS7 stack doesn't perfectly align with the OSI model is due to the fact mentioned earlier in this text. The SS7 Layers: Level Definitions ================================= CCS7 LEVELS _ ______ __ __ __ | | TCAP | | || || | | |______| | || || | | ______ |T ||I ||B | | | ASP | |U ||S ||I | 4| |______| |P ||U ||S | | ______ | ||P ||U | | | SCCP | | || ||P | |_ |______| |__||__||__| ___________________ 3 | MTP Level 3 | | | ------------------- 2 | MTP Level 2 | | | ------------------- 1 | MTP Level 1 | |___________________| Level 1: The Message Transfer Part Level 1 - The MTP Level 1 is the SS7 equivalent to the OSI Physical Layer, except for the fact that while the OSI model doesn't specify which type of interface to be used, in SS7, we can specify that. Level 2: The Message Transfer Part Level 2 - The SS7 MTP Level 2 is the SS7 equivalent to the OSI Data Link Layer, except for the fact that the SS7 level does not provide the routing for SS7. Level 2 ensures reliable end-to-end data transfer over the network, implements flow control, message sequence validation, and error checking. Level 3: The Message Transfer Part Level 3 - The SS7 MTP Level 3 is the SS7 equivalent to the OSI Network Level. It provides the following functions: routing, message discrimination, and distribution. Message discrimination basically figures out who the message is addressed to. The distribution occurs when the discrimination determines that the address is a local address. In this case message distribution is responsible for identifying which user part the message is addressed to and routing the message to its internal user. Level 4: The User Parts Level - The SS7 User Parts Level is made up of multiple protocols called user parts and application parts. These protocols are responsible for functions from basic telephone call connection and disconnection, provided by the Telephone User Part (TUP) or the ISDN User Part (ISUP) protocols, to passing subscriber information from one cell network to another, provided by the somewhat new Mobile Application Part (MAP) protocol. Conclusion: What Comes Next? ============================ A few people have asked me if I think Signaling System 7 is going to become obsolete. SS7 is a digital and multi-layered signaling system. It is quite flexable and fully capable of adaptation. This has already been proved when application parts were added to SS7 when Public Land Mobile Networks were introduced. As stated at the beginning of the article, in the future I will be writing more in depth about the SS7 protocols within the stack. Greets: tprophet, lineman, fringe, elektron, c4, borodir, devolve, icbm, darkcube, subz, downtime, #darkcyde, radiofreq, zoro-a, mega elite. And all the people I haven't forgotten: baiac, panther, kool-aid, brain phreak, impy, bell phreak, scarface, channel surfer, sdphreak, doomd, hologram, chaos451, prodigy, chameleon, johnny yo yo, placid, sedition, water, william tell, vi, broken-, autopsy, theorem, the old sysfail crew, phriend, dizzy and the rest of the old #telephony cats.