Signaling System 7 / Edition 2 available in Hardcover
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The only complete guide to SS7-revised and updated. Don't rely on ANSI publications or Bellcore standards to learn SS7 from the inside out. Turn instead to this bestselling guide,now fully updated to reflect the latest changes in the SS7 signaling network and its cellular,broadband,and international applications. Inside you'll discover: a complete "short course" on SS7,its architecture,and protocols; specific technical data for design and specifications; ramifications of SS7 changes following the Telecommunications Act of 1996; practical analysis of Bellcore GR-246-CORE and other key specs; full coverage of the latest ITU standards for global SS7 use; insights into revised broadband ISUP protocols and procedures; an overview and explanation of local number portability (LNP) based on the SS7 protocols,which has been mandated by the FCC for deployment by telcos and wireless providers by 1999. Packed with real-world examples and applications,this hands-on guide is for everyone who works with SS7. Novices will find it readable and informative. . . engineers designing new products will embrace it as a useful reference and a refreshing break from the monotonous specifications used throughout the industry. . . and professionals in the field will find it such an invaluable working tool that they may even want to offer it to their customers
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Chapter 3: Overview of a ProtocolBefore looking at the OSI model, let us examine the functions of a protocol. A protocol is a set of rules governing the way data will be transmitted and received over data communications networks. Protocols must provide reliable, error-free transmission of user data, as well as network management functions. Protocols packetize the user data into data envelopes, some being of a fixed length while others can be variable lengths, depending on the protocol used.
Protocols are used whenever a serial bit stream is used. The protocol defines the order in which the bits will be sent and also appends information for use by the network in routing and management of the network. This appended information is used only by the protocol and is transparent to the user.
Some protocols, such as SS7, actually send predefined messages to the other nodes in the network. Messages can be used at any layer above layer one and are commonly found at layers two and three. A typical example of a protocol message is the initial address message (IAM) sent by the SS7 protocol to establish a connection on a voice circuit between two end offices. Other messages exist for SS7 and will be discussed in greater detail in the other chapters. Predefined messages are an excellent way to send network management functions and handle data error procedures.
Other functions of a protocol include the segmentation of blocks of data for easier transmission over the network and reassembly at the receiving node. When sending multiple blocks of associated data, procedures must be provided that allow the blocks to be identified in the order they were sent and reassembled as such. In large networks, these data blocks can be sent in order but received out of order.
There are three basic modes of operation for a protocol, depending on the type of network. A circuit-switched network protocol establishes a connection on a specific circuit, and then sends the data on that circuit. The circuit used depends on the destination of the data. A good example of a typical circuit-switched network is the Public Switched Telephone Network (PSTN), which uses various circuits for the transmission of voice from one exchange to another.
Once the transmission has been completed, the circuit is released and is ready to carry another transmission. The protocol must manage the connection and release when transmission has been completed, and must also maintain the connection during the data transmission.
Another type of network is a local area network (LAN). LANs use different types of protocols, but the method of transmission is usually very similar. The topology of a LAN is usually a bus topology or a ring topology. In both topologies, the data is transmitted out on the LAN, with an address attached in a protocol header.
When a data terminal recognizes its address, it reads the data. Some mechanism must be used within the protocol to remove the data from the LAN once it has been read. This differs from one protocol to the next. These types of networks only allow one message at a time to be transmitted across the LAN.
Packet-switching networks provide multiple paths to the same destination. Each message has both an originating address and a destination address. The addresses are used to route the message through the network. Unlike LANs, a packet-switched network allows many messages to be transmitted simultaneously across the network.
The circuits used for this type of network are always connected, and transmission is taking place continuously. The direction the message takes from one node to the next depends on the packet address. Each packet provides enough information regarding the data to allow the packet to reach its destination without establishing a connection between the two devices. The X.25 and SS7 networks are both packetswitched networks.
In any protocol stack, there are several layers of addressing used. Typically, at least three layers of addressing can be found. Each device on the network must have its own unique physical address. The node address identifies the particular device within its own network. The layer two protocols are users of this address, since they are responsible for the routing from one device to the next adjacent device.
The next address layer is that of the network itself. This address is used when sending messages between two networks. This address can usually be found in layer three of most protocols. The network address is used by those devices which interconnect two or more networks (such as a router).
Once a message reaches its final destination, the logical address within the destination node must be provided to identify which operation or application entity within the node is to receive the data. An application entity is a function within a network node, such as file transfer or electronic mail. Application does not imply something like word processing (in the network sense).
In the SS7 network, application entities are objects such as IS-41, which allows Mobile Switching Centers in the cellular network to exchange data from one to the other, using the services of SS7 protocols.
As the information is handed from one layer to the next, the protocol appends control information. This control information is used to ensure that the data is received in the same order it was sent, and allows the protocol to monitor the status of every connection and automatically correct problems that may occur.
Control information includes sequence numbering and flow control. This function is usually found at layers two through four, but can also be found in the higher layers. In the SS7 protocols, levels two through four provide varying levels of control.
As mentioned earlier, segmentation and reassembly are also tasks of the protocol. This is necessary when large blocks of data must be transmitted across the network. Large blocks of data can be time consuming and, if an error occurs during transmission, can cause congestion on the network while retransmitting.
For this reason, blocks are broken down into smaller chunks, which make it faster and easier to control and transmit through the network. When a retransmission becomes necessary, only a small portion of the original data must be retransmitted, saving valuable network resources.
Encapsulation is the process of appending the original data with additional control information and protocol headers. This information is stripped off the message by the receiving node at the same layer it was appended. This information is transparent to the user.
Connection control is one of the most important tasks of a protocol. Connections must usually be established not only between two devices, but between two application entities as well. These logical connections must be maintained throughout the data transmission. The establishment of a logical connection ensures reliable data transfer, with the use of positive and negative acknowledgments to advise the adjacent node of transmission status.
Sequence numbering is also used in these types of services to ensure that the data is received in the same order it was transmitted. This type of protocol service is referred to as connection-oriented. Each node may have multiple logical connections established at one time.
When the data transmission is complete, the logical connection must be released to allow another application entity to establish a connection and transmit data. Protocol messages (such as connect requests and disconnects) are used to manage these logical connections....
Table of ContentsSignaling System #7.
Overview of a Protocol.
Overview of Signal Units.
Message Transfer Part (MTP).
Message Transfer Part Level Three.
General Description of SCCP Functions.
Overview of TCAP.
Overview of ISUP.
Local Number Portability.
A: ITV-TSS 227 Publications.
B: American National Standards Institute (ASA).