Various signals are transmitted in the network , some of which are what we need (such as voice calls, data packets for Internet access, etc.), and the other part is what we don't need (it can only be said that it is not directly needed ), it is used Special control circuit , this type of signal we call signaling (signaling, communication control command) , the transmission of signaling requires a signaling network.
Origin of name
The most traditional signaling is China's No. 1 signaling, which was used a lot by telephone in the past, but now it is basically No. 7 signaling (used for both telephone and network transmission) . The transmission of any actual application information between communication devices is always accompanied by the transmission of some control information . They work according to the established communication protocol to transmit the application information to the destination safely, reliably and efficiently. This information is called protocol control information in computer networks, and is called signaling in telecommunication networks . English materials often use the word "Signalling" (signalling process), but most Chinese technical materials only use the word "Signalling", that is, "Signaling" includes both "Signal" and "Signalling".
definition
Strictly speaking, signaling is a system that allows program-controlled switches, network databases, and other "intelligent" nodes in the network to exchange information about: call setup, supervision (Supervision), teardown (teardown), and distributed application processes information (challenge/response between processes or user-to-user data), network management information . In the wireless communication system, in addition to transmitting user information, signaling is used to ensure the control signals required for normal communication in order to make the entire network work sequentially.
effect
Signaling is different from user information. User information is directly transmitted from the sender to the receiver through the communication network, while signaling usually needs to be transmitted between different links of the communication network (base station, mobile station, mobile control switching center, etc.), each The link is analyzed and processed and a series of operations and controls are formed through interaction . Its function is to ensure the effective and reliable transmission of user information. Therefore, signaling can be regarded as the control system of the entire communication network, and its performance depends greatly To a certain extent, it determines the ability and quality of a communication network to provide services to users.
Classification
In the communication network, the signaling may adopt common channel signaling or channel associated signaling. The so-called co-channel signaling refers to a transmission channel dedicated to signaling transmission when signaling is transmitted; channel-associated signaling refers to the way signaling and signals are transmitted on the same channel.
According to the form of signaling signal, signaling can also be divided into two types : digital signaling and audio signaling . Because digital signaling has a series of obvious advantages such as fast speed, large capacity and high reliability, it has become the main form used in the current public mobile communication network.
According to its purpose, it is divided into two types : user signaling and inter-office signaling . User signaling acts between user terminal equipment (such as telephone sets) and the switchboard of the telephone office , and the latter acts between two switches connected by trunk lines .
The classification of inter-office signaling mainly includes channel-associated signaling and co-channel signaling . It is necessary to re-build a signaling network (mainly between the central office) , such as making a phone call: when we start to make a call, when we pick up the phone, the signal will be transmitted to the local telecommunications office . After a series of exchanges , the local office first sends a signaling on the network, and waits for the other end to receive a letter to respond to a signaling to agree to the call. At this time, the signaling transmission function on the network is completed, and the voice signal is started to be transmitted, and the call can be made. When the call ends, it is also necessary to control the circuit removal through signaling.
In short, signaling is actually a signal for control.
Figure 2 Signaling classification
SS7
1) Definition
Signaling No. 7: also known as public channel signaling. That is, the signaling method of transmitting a group of voice channel signaling on a high-speed data link in a time-division manner, usually used between offices .
The No. 7 signaling system used in China is called China No. 7 signaling system. The SS7 network is an out-of-band data communication network, which is superimposed on the operator's switching network and is an important part of the supporting network. Complete local, long-distance and international automatic and semi-automatic telephone connection between fixed-line telephone networks or ISDN network offices; provide local, long-distance and international telephone call services between switching offices in mobile networks, as well as related mobile services, such as SMS, etc. services; provide intelligent network services and other value-added services for fixed and mobile networks; provide transmission and collection of operation management and maintenance information.
2) Composition
The No. 7 signaling network is roughly composed of the following parts. The signaling point is the node that processes the control message in the SS7 signaling network. The signaling point that generates the message is the source signaling point of the message, and the signaling point that receives the message is the The destination signaling point of the message. There are three types of signaling points:
(1) ServiceSwitchingPoint (SSP) The service switching point is the generation or termination point of signaling messages, which is essentially the local switching system (or switching center CO), which initiates calls or receives incoming calls.
(2) SignalTransferPoint (STP) completes the function of the router, checks the messages sent by the SSP, and then exchanges each message to the appropriate place through the network. STP connects other signaling points and networks together to form a larger network.
(3) ServiceControlPoint (SCP) is a typical access database server. SCP is the control center of the intelligent network business. It is responsible for the execution of business logic, provides call processing functions, receives query information and query database sent by SSP, and sends it to SSP after verification. Call processing instructions, receive bills generated by SSP and perform corresponding processing.
In the No. 7 signaling network, the ISUP signaling (ISDN USER PART) message is used to establish a voice relay circuit between voice switches in the management and release central office, and provide information exchange required for voice and non-voice services. To support basic bearer services and supplementary services, for example: ISUP signaling messages can carry caller ID, caller's phone number, user name, etc. TCAP signaling (TransactionCapabilitiesApplicationPart) message is used to support telephone services, such as free calls, local number portability, card services, mobile roaming and authentication services. TCAP mainly includes Mobile Application Part (MAP) and Operation, Maintenance and Management Part (OMAP). MAP stipulates procedures such as roaming and channel over-office transfer in mobile services, and OMAP only provides formal test procedures for MTP routing and SCCP routing.
gateway
Telecommunications services will develop across networks. In the process of voice service and data service integration, the communication between the network based on circuit switching and the network based on packet switching will inevitably require interoperability of signaling. The two largest networks in the world will coexist for a long time and interoperate with signaling. The potential will be enormous. More importantly, the next-generation network is based on IP. Both the softswitch architecture and the third-generation mobile communication system use IP-based packet switching networks, so signaling gateways are essential. It is a signaling communication device between two different networks.
In addition, in the widely distributed access network, there is also a great market demand for using the ubiquitous IP network to transmit access signaling, such as DSS1 and V5.2.
The role of the signaling gateway is to complete the mutual conversion of information used for control between two different networks, so that the control information in one network can be continuously transmitted in the other network. A signaling gateway is an agent that receives and sends signaling at the border of two networks, and is a signaling gateway between the two networks, translating, relaying or terminating signaling messages. The signaling gateway can be set independently or integrated with other gateways to handle signaling related to access lines or trunk lines.
One of the two most widely used networks is the telephone switching network, which includes fixed telephone networks and mobile telephone networks, and the other is IP networks, including Internet, Intranet and Extranet.
In the telephone switching network, the commonly used signaling is shown in the figure on the right. In the figure, MAP, CAP, RANAP, BSSAP are the signaling used by the mobile communication system; ISUP, TUP, INAP are the signaling used by the fixed network; DSS1 and V5.2 are the signaling used by the access network.
At the end of the 1990s, IETF established the signaling transmission (SIGTRAN) working group to solve the transmission of packet-based telephone signaling on the IP network, determine the functional and performance requirements of the telephone signaling, and realize the transmission between IP network nodes such as SS7ISUP , signaling such as TUP and DSS1.
The SIGTRAN framework is to add a User Adaptation Layer (UAL) on the Stream Control Transmission Protocol (SCTP). The user adaptation layer is composed of multiple adaptation modules. They respectively provide the original primitive interface (such as management instruction, data operation and other primitives) for the existing upper-layer telephone users/applications, and package the upper-layer specific signaling protocol on SCTP for transmission. SCTP is based on standard Internet protocols (IPv4, IPv6). The structure of SIGTRAN is shown on the right:
The pink part in the figure is the protocol stack of SIGTRAN, above which is the corresponding telephone signaling protocol. SIGTRAN's protocol stack is divided into two layers: one layer is SCTP, which provides multiple streams, reliable data transmission, bundling and segmentation of user data, blocking and flow control, and prevents "denial of service" and "masquerading" attacks and other functions. The other layer is the user adaptation layer, which is composed of five adaptation modules SUA, M3UA, M2UA, M2PA, IUA and V5UA in the figure, which respectively meet the adaptation requirements of various telephone signaling protocols.
net
A signaling node in the telephone network cannot be directly connected to a signaling node in the IP network, and the mutual communication between them needs to rely on an intermediate device called a signaling gateway (Signaling Gateway, SG) . From the perspective of the IP network, this signaling gateway is sometimes called an application server communication device, because the above-mentioned IP signaling node is often called an application server (Application Server, AS) in UAL. The general structure of signaling intercommunication between the two networks through the signaling gateway is shown in FIG. 5 .
Generally speaking, the signaling gateway is always located between the telephone network and the IP network, completes the mutual conversion of messages between the traditional telephone signaling based on SS7 and the IP telephone signaling of SIGTRAN, and supports the high-level signaling protocol (SigProt) intercommunication . From the perspective of the telephone network, since the signaling gateway mainly completes the processing of SS7 signaling, it is usually called the No. 7 signaling gateway. When the signaling gateway is integrated with other network devices, the interface on one side will be the internal interface. For example, in the above figure, if the signaling gateway is integrated with the signaling endpoint (SEP) device on the left, the TTSP interface on the left will become the internal interface, and the IP phone signaling interface on the right will become the internal interface when it is integrated with the ISEP. .
develop
In order to speed up the research and standardization process of network signaling (NGN), the International Organization for Standardization is formulating NGN standards. ITU-T's research on NGN is mainly in FG and SG13; after the European standardization organization ETSI has successfully operated and dominated the global 3G standard, it is ambitious to carry out NGN's TISPAN (Telecommunications and Internet Converged Services And Protocols For Advanced Networking) telecommunications and Internet convergence advanced network services and protocols. NGN covers a wide range of content at the business layer, so it requires a lot of signaling and protocols. TISPAN does not specifically create a whole set of new signaling protocols, but uses the signaling and protocols defined by various standard organizations as much as possible. According to the requirements of NGN, add some signaling protocol content or formulate a new signaling protocol.
Architecture level
The frameworks proposed by different international standards research organizations are not the same, among which the European plan is advancing the fastest, and the corresponding standards are also implementable. So its standard will directly affect the NGN standard formulation of ITU-T. The functional structure of TISPANNGN includes the business layer and the transport layer based on IP.
Business layer composition
Includes the following parts:
(1) Resource and Admission Control Subsystem (RACS);
(2) Network attachment (attachment) subsystem (NASS);
(3) IP Multimedia Subsystem (IMS);
(4) PSTN/ISDN Emulation Subsystem (PES);
(5) Other multimedia subsystems (such as streaming media subsystems, broadcasting subsystems, etc.) and applications;
(6) Common components (that is, components shared by several subsystems), such as billing functions, user list management, and security management.
Reposted from: Signaling_Baidu Encyclopedia