Technical Principles of the network interface signaling protocol for ATM users (Q.2931)

1 Introduction
The world has entered the information age. On the one hand, the operation of the whole society, including science and technology, economy, environmental protection, and lifestyle, has been significantly affected by information. On the other hand, the amount of information in the whole society is growing at a rate of 0.2 million times that of population growth. It is precisely because so many people and so many aspects of people's lives are deeply influenced by information technology that communication technology is crucial to us. Today's new requirements for communication technology are to achieve digital, integrated, broadband, intelligent, and personalized communication on the basis of existing communication networks, it enables the voice, text, graphics, images, and other multimedia information that people need to freely transmit on the same Internet, and the cost is within the limits of people. It is in this context that B-ISDN came into being.
B-ISDNBroadhand Integrated Services Digital Network) Broadband Integrated Service Digital Network, which uses a variety of transmission technologies, such as FR Frame Relay and ATM asynchronous transfer mode. ATM was established as the final transfer mode of ITU-T at the Geneva Conference on January 1990 by CCITT as B-ISDN. The reason why ATM can be selected is that the ATM network is capable of transmitting media, voice, data, video and image through a single process, that is, Time Division multiplexing (TDM-Time Division Multiplex) compared with the group exchange network PSN, it also has advantages such as low cost, high bandwidth and high service quality assurance, and is a revolution in traditional exchange methods.
With the introduction of ATM technology, its internal specifications have also been introduced, the Q.2931 mentioned in this paper is the SG11 group of ITU-T proposed in February 7, 1995, and WTSCthe World Telecommunication Standardization Conference) through the 3rd specification for digital user signaling DSS2) User Network Interface UNI), it is based on N-ISDN user network connected UNI) DSS1 protocol Q.931.
2 Q.2931 and its Functions
2.1Q.2931 Protocol
It defines the process of establishing, maintaining, and releasing a network connection on a user's network interface and various messages used in the process. It is developed on the basis of N-ISDN user network interface DSS1 protocol Q.931, and according to the characteristics of B-ISDN business, it supplements and improves Q.931. It is worth noting that the Virtual Path Connection Identifier (VPCI) is introduced in Q.2931, which corresponds to VPIVirtual Path Identifier) the difference is that VPCI identifies a virtual channel connection, while VPI identifies a virtual channel link. It is introduced because in the B-ISDN, the user may be connected to the local exchange through the VP/VC Cross Connection Network, VP cross connection is controlled by the management plane, instead of using signaling, a signaling endpoint only knows its own VPI, but does not know the VPI of other signaling endpoints. Therefore, A unique VPCI must be introduced between the user and the local area switch.
2.2 architecture of the signaling protocol at the user's Network Interface
The architecture of the user network interface corresponds to the B-ISDN Protocol Reference Model. Both signaling and user information are transmitted in the frame structure of the physical layer in the cell format. Among them, SAALSignallingATM Adapter Layer is a signaling ATM adaptation Layer. It mainly adapts various signaling messages and processes them into the metadata format. The upper layer on SAAL is Q.293, which defines the process of establishing, maintaining, and releasing network connections on the user's network interfaces. Q.2931 communicates with peer-to-peer layers through messages, while it communicates with local upper and lower layers through primitives. Because Q.2931 only supports point-to-point signaling, Q.2971 protocol is used for point-to-point signaling.
The signaling mentioned here is transmitted between the control plane, while the ATM user information is transmitted between the user plane. In point-to-point signaling mode, the user side only has a unique signaling endpoint, And the signaling endpoint is connected through a permanent established signaling virtual path SVC), using this SVCVCI = 5) you can create a call and release the call.
2.3 Q.2931 Functions
Q.2931 layer, which defines the process of establishing, maintaining, and releasing network connections on UNI. Q.2931, also known as DSS2 ).
The basic functions of Q.2931 are as follows:
1) supports real-time SVC connections;
2) supports point-to-point SVC connections;
3) supports symmetric and asymmetric broadband connections;
4) supports single-connection calls;
5) support for basic signaling functions;
6) supports Class A, Class C, and Class X businesses;
Class A businesses: connection-oriented, constant bit rate and end-to-end timing requirements;
Class C businesses: connection-oriented, variable bit rate, but no end-to-end timing requirements;
Class X businesses: these services are intended for connection to ATM Transmission Services. The business types and timing requirements are customized by users.
7) supports VPI/VCI negotiation;
8) All signaling messages are transmitted on the specified out-of-band channel;
VCI = 5 in each VPCI is reserved for point-to-point signaling. Here, meta-signaling and broadcast-signaling virtual channels are not supported.
9) supports error recovery;
10) uniquely identifies an ATM endpoint using public UNI addressing;
11) support identification of compatibility parameters from terminal to Swiss;
In each connection, the compatibility test must be performed for the following specified client-to-kernel parameters:
I. AAL type, such as 1, 3/4, and 5 );
Ii. Methods and AAL parameters for Protocol use;
Iii. Protocols at the network layer.
12) Support interworking with N-ISDN signaling and provide N-ISDN business;
13) supports forward compatibility.
3 Q.2931 how to implement the call/connection process
3.1 Message format
Call Control on user network interfaces is implemented through message transmission. Each message definition in Q.2931 contains the following content:
A. The message is sent to the user's network, network to the user, or two-way );
B. Message Effective Range: local, access, dual or global );
C. constitute the Information Unit of the message.
Each message is composed of 9B public information units and variable length information units.
A. protocol Discriminator), used to distinguish user and network call control information from other messages in the scope of this suggestion; the recommended message is also used to distinguish it from the OSI network layer protocol units of other ITU standards and other standards.
B. Call reference: identifies the call on the local user's network interface;
C. Message type, which identifies the message being sent;
D. The message length, which identifies the length of the message content. It does not include the "protocol authentication", "Call reference", "slightly interest type", and the eight-bit Group of the message length;
E. variable-length information units. Common variable-length information units include AAL parameters, ATM service descriptor, broadband bearer performance, callee number, caller number, and connection identifier, client-to-Swiss transmission latency, service quality parameters, call status, and causes.
3.2 categories in Call Control messages and their functions
The signaling entity on both sides of UNI controls SVC through messages. The main call control information is shown in table 1.
Table 1 Call Control Messages for Q.2931
Range of process message names
Call SETUP Global starts the call establishment process, which reflects the service capabilities provided by the network to users.
Call proceeding Local instructs the peer that the CALL establishment request is being processed
CONNECT Global instructs the caller to agree to accept the call.
Connect acknowledge Local call established
Call clearing RELEASE Global indicates that the sender has removed the VC and will RELEASE the VC and call reference, requiring the recipient to remove the VC
Release com Local indicates that the sender has released VC and call Reference
Interworking with N-ISDN INFORMATION Global provides additional INFORMATION for call setup
PROGRESS Local is used to indicate the call PROGRESS during intercommunication.
Other STATUS Local reports error STATUS or response STATUS Request Message
Status enquiry Local request peer STATUS
RESTART Local is the specified virtual path returned to the idle state
Testart acknowledge Local indicates to the sender that the specified virtual path has returned to the idle state.
3.3 call/Connection Control Process
Call/connection control can be achieved through the communication of various signaling messages defined above. However, it is safer and more convenient to add the timer and call/connection control status.
Timer: to prevent one end from sending this message, it is set to wait for an endless loop. If this is not the case, the sender of the message will wait and the network resources will be occupied, which will increase the network load and cause system crash in severe cases. Therefore, measures should be taken to prevent such incidents. For example, if a timer is used, the sender starts the timer when sending a message. If a response is received before the timer times out, the timer is stopped. If the timer times out and still does not receive a response, you can resend the message or restart the timer or release network resources, and return to the null state.
Status: the status of the master, called, and network is set to different because of different messages. When a message is received, status transfer may be performed to promote the entire call/connection control process.
The UNI user side has the following statuses:
U0: zero state; U1: Call start; U3: Outbound call process; U4: Call submission;
U6: Call presentation; U7: Call reception; U8: Connection Request; U9: Incoming call process;
U10: running; U2: overlapping sending; U25: overlapping receiving; U11: release request;
U12: indicates the release.
U2 and U25 are used for intercommunication-related calls.
The status of the network side is similar to that of the secondary node.
3.3.1 call establishment process
If a caller needs to establish a connection for a call request, the user side of the initiator interface signaling entity sends a message SETUP request to establish a connection and starts the timer T303, at the same time, the status is migrated to the U1 state. If no network response is received before the timer times out, resend the SETUP message and restart the timer T303. If the call times out again, the caller releases the call request. At this time, the caller changes from U1 to U0.
The SETUP messages sent by the user side include the ATM service volume descriptor, broadband bearer capacity, high-and low-Layer Broadband information, QOS parameters, called user numbers, connection IDs, and other information units. After receiving the SETUP message, the network side checks the information unit to determine whether the network resources meet the requirements of users. If acceptable, the system forwards the SETUP message to the network interface of the called user, and sends a response call proceeding to the calling user to indicate that the CALL is being processed and the status is converted accordingly.
If the caller receives the call proceeding message, stop T303, start the T310 timer, wait for the message from the called party, And the status is changed from U1 to u3. If no message is received when T310 times out, the caller clears the call and migrates the call to U0.
After checking whether resources are available, the network sends a SETUP message indicating that the call has reached the user's network interface, starts the timer T303, and enters the corresponding status. If the network does not receive a response when the timer times out for the first time, resend the SETUP message and restart the timer T303. If the call times out twice, the network needs to clear the call.
After receiving the SETUP message, the called user checks the address and compatibility. If not, the called user releases the call and sends the RELEASE message to the caller. If yes, send a call proceeding message to the network, and its status is migrated from U0 to u6.
When a user at a called address has started the user's ringing instructions, the network should send the ALERTING message back to the user network through the caller address, and enter the corresponding status, when the user receives the ALERTING message, the user starts internal ringing instructions, stops T310, enters the U4 state, and starts the T301 timer. When receiving an accepted call instruction, the network should send the CONNECT message to the caller through UNI and enter the U10 state.
The CONNECT message instructs the caller that a connection has been established through the network and that a possible local ringing indicator has been stopped. When receiving the CONNECT message, the caller should stop the timer and stop any user from generating the ringing indicator, CONNECT to the user plane virtual path, send the connect acknowledge message to the network that has responded to the CONNECT message, and enter the U10 state at the same time.
During this process, parameter negotiation may be performed between the caller and the called party, such as the user peak rate and transmission latency. If these parameters cannot be consistent, the call will be cleared; if the format of the message is sent. The call is cleared and the connection is released when an error occurs in the length and information unit or a inconsistency is not met.
3.3.2 call release process
Under normal circumstances, after the communication between the master and the called party ends, the user or the network sends a RELEASE message to start the call clearing. The exception is that the user or network can reject the call/connection by sending the RELEASE COM-PLETE message as the first response message in response to the SETUP message, releasing the call reference and entering the zero state.
A. Cleanup started by the user
Normally, the user sends the RELEASE message, starts the timer T308, removes the virtual path, and enters the RELEASE Request status.
When a RELEASE message is received, the network enters the RELEASE Request status. This message prompts the network to remove the virtual path and start the program to clear the network connection to the remote user. Once the virtual path used for this call is removed, the network should send the release complete message to the user, RELEASE the call reference and virtual path, and enter the zero state.
When the timer T308 times out for the first time, the user should resend the RE-LEASE message to the network, and restart the timer T308 using the cause number that contains the first RELEASE message. If the timer T308 times out for the second time, you should maintain the virtual path, release the call reference, and return the zero state.
When the release complete message is received, the user should stop the timer T308, RELEASE the virtual path and call reference, and enter the zero state.
B. cleanup by network startup
Under normal circumstances, the network sends a RELEASE message, starts the timer T308, removes the virtual path, and enters the RELEASE Request status.
When a RELEASE message is received, the user enters the RELEASE status. Once the virtual path used for this call is removed, the user should send the RE-lease complete message to the network, release the call reference and virtual path, and return the zero status.
When the release complete message is received, the network should stop the loose T308, RELEASE the call reference and virtual path at the same time, and enter the zero state.
If the timer T308 times out, the troubleshooting method is the same as that started by the user.
3.3.3 restart
The restart program is used to return one or all virtual channels in the virtual channel, or all virtual channels controlled by the signaling virtual channel, to the idle state. When one end of the interface does not respond to the call control message on the other side or a fault occurs, this program is usually called and may also be used to call the program due to local faults, maintenance operations, or misoperations.
If you want to know the STATUS of the other party during the call/connection process, you can send the STA-TUS ENQUIRY message to request, the other party will return the STATUS message after receiving the message, it carries its status.
3.3.4 normal call/Connection Control Process
The user network interface signaling establishes the corresponding SVC connection for a call request.
The caller first sends a set-up message to the network. After receiving the message, the network determines whether the request can be met. If yes, the caller sends the set-up message to the called party and sends the call back message to the user; after receiving the established message, the called party performs consistency check and processing. After confirming that the message is accepted, the caller sends a bell message to the network. After receiving the message, the network forwards the ring message to the caller.
After the network receives a connection message from the called party, it establishes the connection message used by the call in the network. After receiving the connection message, the caller establishes the corresponding VC connection, then send the connection back to the network to confirm the message. In this way, the connection between the master and the called is established. Once the communication ends, the caller sends a release message to the network and removes the virtual path. After receiving the release message, the network immediately removes the virtual path and sends the release message to the user, call reference and virtual path are released. After the called user receives the release message sent from the network, the virtual path and call reference are released.
The entire communication process ends.
4 Conclusion
Through the above process, Q.2931 will complete the entire process of establishing an ultra-holding and releasing connection between the two parties. It is worth noting that Q.2931 only supports point-to-point connection control. For point-to-point multi-point and broadcast methods, please refer to the recommended Q.2971 books. For point-to-point signaling mode and point-to-point signaling mode access configuration problems, please refer to the Q.2010 ITU-T recommendations.