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Voice Feature Guide 01

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
H.248 Principles

H.248 Principles

Mechanism of the H.248 Protocol

Termination ID

A termination ID identifies a termination that is going to register or deregister a service. The termination ID of each termination is unique. During service configuration, the termination ID corresponding to each termination must be configured on the MG and the MGC. The root termination ID represents an entire MG. The ServiceChange command executed on the root termination ID is effective on an entire MG. The wildcarding principle is that the ALL wildcard (*) can be used but the CHOOSE wildcard ($) cannot be used.

Registration Mechanism of the H.248 Interface
The MG sends the ServiceChangeRequest command to inform the MGC that a user or a group of users are about to register or deregister service. After this command is executed successfully, the termination status is changed to InService or OutOfService. In addition, the MGC can unsolicitedly send the ServiceChangeRequest command to request the MG to register or deregister service for a user or a group of users.
NOTE:
Currently, the MG does not support the MGC to unsolicitedly send the ServiceChangeRequest command requesting the MG to register service for a user or a group of users.
Figure 1-92 shows the registration flow of the MG.
Figure 1-92 Registration flow of the MG

Description of the flow:

  1. The MG sends the ServiceChangeRequest command to the MGC. In the command, TerminationId is Root, Method is Restart, and ServiceChangeReason is 901 (cold boot, registering for the first time after power-on), 902 (warm boot, through command lines), or 900 (in other cases).
  2. The MGC sends the Reply message to the MG indicating the successful registration.
  3. The MGC sends the Modify command to the MG requesting the MG to detect the offhook of all users (al/of).
  4. The MG responds to the MGC with the Reply message.
Heartbeat Mechanism of the H.248 Interface

After the registration is successful, the MG and the MGC maintain communication by sending each other the heartbeat message Notify (it/ito). By default, the heartbeat message is sent every 60s. The sending interval can be set within the range of 5-655s.

After the MG sends the first heartbeat message to the MGC, if the MG does not receive the heartbeat response from the MGC before the preset interface heartbeat timer (for example, the length of three sending intervals) times out, the MG sets the interface status to "wait for response". Then, the MG keeps initiating a registration with the MGC. If dual-homing is configured, the MG initiates registration with the two MGCs alternatively. The registration is initiated once every 30s, every three trials of registration are one round, and every registration message is re-transmitted 7 times. Therefore, 24 registration messages in total are transmitted within 90s. Then, the MG starts the next round of registration with the other MGC.

Deregistration Mechanism of the H.248 Interface

Figure 1-93 shows the unsolicited deregistration flow of the MG.

Figure 1-93 Unsolicited deregistration flow of the MG

Description of the flow:

  1. The MG sends the ServiceChangeRequest command to the MGC. In the command, TerminationId is Root, Method is Forced, and ServiceChangeReason is 905 ("905" indicates that the termination is taken out of service because of maintenance operation, and now the MG uses "905" to initiate a deregistration request through command lines).
  2. The MGC sends the Reply message to the MG indicating a successful deregistration.

Figure 1-94 shows the flow of the MGC unsolicitedly deregistering the MG.

Figure 1-94 Unsolicited deregistration flow of the MGC

Description of the flow:

  1. The MGC sends the ServiceChangeRequest command to the MG. In the command, TerminationId is Root, Method is Forced, and ServiceChangeReason is 905.
  2. The MG responds to the MGC with the Reply message. The access device (MG) supports the registration and deregistration of not only an entire MG but also a single termination. The service status of a single user can be changed through the registration and deregistration of a single termination.
Authentication Mechanism of the H.248 Interface
Authentication is a security mechanism through which the MGC authenticates the legality of the MG user. The purpose of authentication is to prevent unauthorized entities from establishing illegal calls or interfering with legal calls through the H.248 or MGCP protocol. Authentication can be implemented only when it is also supported by the softswitch interconnected with the MG.
  • In H.248, the implementation of authentication complies with RFC2402.
  • MD5 is adopted as the encryption algorithm.

Figure 1-95 shows the authentication flow.

Figure 1-95 Authentication flow

The basic flow is as follows:

  1. The MG sends the ServiceChange command to register with the MGC. The command contains the digital signature of the MG.
  2. After receiving the ServiceChange command, the softswitch verifies the MG and sends a reply.
  3. The softswitch sends the Modify message to the MG. The message contains the required algorithm ID and random number.
  4. The MG verifies the message sent by the softswitch and sends a reply.
  5. The softswitch authenticates the MG periodically.
  6. The MG sends replies to the softswitch.

H.248-Based VoIP

Figure 1-96 shows the principles of call establishment and release in the H.248-based VoIP service.

Figure 1-96 Principles of the H.248-based VoIP service

Figure 1-97 shows the flowchart of call establishment and release in the H.248-based VoIP service.
Figure 1-97 Flowchart of call establishment and release in the H.248-based VoIP service
  1. MG-0 detects the offhook of user A0, and notifies the MGC of the offhook event through the Notify command.
  2. After receiving the offhook event, the MGC sends a digitmap to MG-0, requests MG-0 to play the dial tone to user A0, and at the same time checks for the digit collection event.
  3. User A0 dials a telephone number, and MG-0 collects the digits according to the digitmap issued by the MGC. Then, MG-0 reports the result of digit collection to the MGC.
  4. The MGC sends the Add command to MG-0 for creating a context and adding the termination and RTP termination of user A0 into the context.
  5. After creating the context, MG-0 responds to the MGC. The response contains the session description that provides the necessary information for the peer end to send the packet to MG-0, such as the IP address and UDP port number.
  6. The MGC sends the Add command to MG-1 for creating a context and adding the termination and RTP termination of user A1 into the context, and then issues the IP address/UDP port ID of user A0 to user A1.
  7. After creating the context, MG-1 responds to the MGC. The response contains the session description that provides the necessary information for the peer end to send the packet to MG-1, such as the IP address and UDP port ID.
  8. MG-1 detects the offhook of user A1, and then reports the offhook event to the MGC. The softswitch (MGC) sends the Modify command to stop the ring back tone of user A0 and the ringing of user A1.
  9. The MGC sends the session description of MG-1 to user A0 through the Modify command. Then, the conversation is set up between users A0 and A1.
  10. MG-0 detects the onhook of user A0, and notifies the MGC of the onhook event through the Notify command.
  11. The MGC sends the Modify command to MG-0 and MG-1 respectively to modify the RTP mode to receive-only.
  12. The MGC sends the Modify command to MG-1 requesting MG-1 to play the busy tone to user A1, and at the same time checks for the onhook event.
  13. The MGC sends the Subtract command to MG-0, requesting MG-0 to release the resources that are occupied by the call of user A0.
  14. MG-1 detects the onhook of user A1, and notifies the MGC of the onhook event through the Notify command.
  15. The MGC sends the Subtract command to MG-1, requesting MG-1 to release the resources that are occupied by the call of user A1.
  16. The call between users A0 and A1 is terminated, and all the resources occupied by the call are released.

H.248-Based MoIP

This topic describes the principle of the H.248-based modem over Internet protocol (MoIP) service.

MoIP refers to the modem service provided on the IP network or between the IP network and the traditional PSTN network. According to different control devices, MoIP can be classified as softswitch-controlled MoIP and self-switching MoIP.

Softswitch-Controlled MoIP
The basic flow of the softswitch-controlled MoIP service is as follows:
  1. Establish a call. If the MoIP service is configured on the softswitch, the softswitch sends a command to the MG instructing the MG to detect the modem event.
  2. The calling party and called party start communicating with each other.
  3. During the call, when the MG detects the ANS or ANSAM modem start event (low-speed modem signal), or detects the ANSBAR or ANSAMBAR modem start event (high-speed modem signal), the MG sends the event to the softswitch.
  4. According to the event, the softswitch sends a command instructing the MG to switch the DSP channel of the calling and called parties to the low-speed or high-speed modem mode.
  5. According to the command sent by the softswitch, the MG switches the DSP channel to the corresponding modem mode. At this stage, the MG adopts the encoding format and port number specified by the softswitch.
  6. The settings of echo cancellation (EC), voice activity detection (VAD), and DSP working mode are as follows:
    1. Low-speed modem: EC-ON, VAD-OFF, DSP working mode-modem mode
    2. High-speed modem: EC-OFF, VAD-OFF, DSP working mode-modem mode
  7. After the modem data is transmitted, if the conversation proceeds, the DSP working mode does not automatically switch from the modem mode to the voice mode, because the modem end event is not issued.
Self-switching MoIP
The basic flow of the self-switching MoIP service is as follows:
  1. Set up a conversation.
  2. The MGs at both ends check for the modem event on the IP side and the TDM side. When the modem event is detected, if the modem transmission mode is configured as self-switching, the coding mode is switched to G.711 (the A/μ law is configurable), and the DSP parameters are modified according to the modem mode (high-speed/low-speed) detected.
  3. When the modem service is terminated, the call is released.

H.248-Based FoIP

Fax over IP (FoIP) is used to provide fax services on an IP network or between an IP network and a traditional public switched telephone network (PSTN) network. A fax machine can be regarded as a special modem. In FoIP negotiation, modem negotiation is performed before fax negotiation.

According to different transmission protocols, two modes are available for transmitting fax services on the IP network: T.30 transparent transmission mode and T.38 transmission mode. FoIP are classified into softswitch-controlled FoIP and self-switching FoIP according to different control devices.

Common Concepts for Fax Services
  • Fax parameter negotiation mode
    • Parameters involved in negotiation: codec mode, fax mode, IP port ID, voice activity detection (VAD), packetization interval, and echo cancellation (EC).
    • Self-switch:

      After a media gateway (MG) detects a fax signal tone, it automatically uses a transmission mode according to system configurations and does not need to send a signaling to the peer end. The transmission mode can be transparent transmission or T.38-based transmission.

    • Negotiation:

      After an MG detects a fax signal tone, it negotiates the fax parameters with the peer end.

  • Negotiation process
    • V2 negotiation process:

      The process focuses on the MG capability. The MG determines the transmission mode to be used (transparent transmission or T.38-based transmission), and determines whether the fax port ID is increased by 2. No signaling negotiation is performed when the port ID is increased by 2. Therefore, when the V2 T.38 is configured, the IDs of the local and peer ports must be configured consistently. That is, the port IDs must be increased by 2 or not increased by 2 at the same time.

    • V3 negotiation process:

      The process focuses on the softswitch capability. If the softswitch requires the T.38-based transmission, this transmission mode is used; otherwise, the transparent transmission mode is used. The softswitch uses the signaling negotiation to determine whether the fax port ID is increased by 2.

    NOTE:

    V2 and V3 indicate the negotiation process versions and are both defined by China Telecom.

Softswitch-Controlled FoIP

A fax service can be classified into high-speed fax and low-speed fax. The softswitch-controlled low-speed fax service supports the T.30 transparent transmission mode or the T.38 transmission mode. The basic service flow is as follows:

  1. A user configures the fax service and fax flow on MGs and a softswitch.
  2. After a voice channel is set up, the softswitch instructs an MG to detect a fax or modem event.
  3. When detecting a fax event, the MG reports the event to the softswitch. The event can be a low-speed modem event (ANS or ANSAM) or a low-speed fax event (V.21Flag).
  4. According to the preset fax flow, the softswitch instructs the MGs at both ends to change the digital signal processing (DSP) channel working mode to the T.30 transparent transmission mode or T.38 transmission mode.
  5. The fax service starts.
  6. After the fax service is complete, the MG reports the event to the softswitch if the MG detects a fax end event.
  7. The softswitch instructs the MGs at both ends to change the DSP channel working mode back to the voice mode.
  8. The voice service proceeds.

The softswitch-controlled high-speed fax service supports the T.30 transparent transmission mode. The basic service flow is as follows:

  1. A user configures the fax service and fax flow on MGs and a softswitch.
  2. After a voice channel is set up, the softswitch instructs an MG to detect a fax or modem event.
  3. When detecting a fax event, the MG reports the event to the softswitch. The event can be a high-speed modem event (ANSBAR or ANSAMBAR) or a high-speed fax event (V.21Flag; if the peer end is a low-speed fax machine or network quality is poor, the fax speed is automatically decreased and this event is reported).
  4. According to the preset fax flow, the softswitch instructs the MGs at both ends to change the DSP channel working mode to T.30 transparent transmission mode.
  5. The fax service starts.
  6. After the fax service is complete, the MG reports the event to the softswitch if the MG detects a fax end event.
  7. The softswitch instructs the MGs at both ends to change the DSP channel working mode back to the voice mode.
  8. The voice service proceeds.
Self-switching FoIP

The self-switching fax service supports the T.30 transparent transmission mode and the T.38 transmission mode. The basic service flow is as follows:

  1. A user configures the self-switching fax service on MGs at both ends.
  2. After a call connection is set up, the voice service starts.
  3. The MG checks for a fax event on the IP side and the TDM side. When detecting a fax event, the MG changes the DSP channel working mode to the T.30 transparent transmission mode or the T.38 transmission mode.
  4. After the fax service is complete, the MG changes the DSP channel working mode to the voice mode if the MG detects a fax end event.
  5. The voice service proceeds.
Transparent Transmission Mode (T.30-Based Transmission Mode)

ITU-T Recommendation T.30 (T.30) is a PSTN-based fax protocol. The protocol defines the transmission flow, data modulation mode, data transmission format, and physical standards for fax signals on the PSTN. Transparent transmission enables T.30 fax messages and data to be transmitted transparently inside MGs or between MGs.

Transparent transmission has the following advantages and disadvantages:

  • Advantages: Less DSP resource is consumed and there is less dependence on the softswitch.
  • Disadvantages: Anti-interference capability is weak and fax quality is poor.

Figure 1-98 shows the transparent transmission mode.

Figure 1-98 Transparent transmission mode

T.38-Based Transmission Mode

ITU-T Recommendation T.38 (T.38) is an IP-based real-time fax mode. In this mode, an MG terminates T.30 signals sent from a fax machine, and transmits the data to a peer MG in T.38 mode. The peer MG receives T.38 packets and converts them to T.30 signals.

The transmission mode has the following advantages and disadvantages:

  • Advantages: A redundancy processing mechanism is provided and there is no strict requirements on network quality (the fax service can be successful even when a 20% packet loss occurs on the network).
  • Disadvantages: The DSP on the MG needs to participate in parsing the T.30 signals. Compatibility problems may occur due to various terminal types.

Figure 1-99 shows the T.38-based transmission mode.

Figure 1-99 Principles of the T.38-based transmission mode

Comparison Between the High-Speed and Low-Speed Faxes

The high-speed fax differs from the low-speed fax in the following aspects:

  • Standard: The high-speed fax uses V.8 data transmission process. The low-speed fax uses the transmission process defined by T.30. Some low-speed fax terminals may use earlier standards.
  • Rate range: The rate range is 2400-33600 bit/s for the high-speed fax and 2400-14400 bit/s for the low-speed fax.
  • Upstream transmission mode: The high-speed fax uses only the transparent transmission mode. The mode is high-speed modem transmission for the MG. The low-speed fax uses the transparent transmission mode or the T.38 transmission mode based on configurations.
  • Requirement for the error correction mode: The error correction mode is mandatory for the high-speed fax and is optional for the low-speed fax.
  • EC requirement: The high-speed fax provides an EC mechanism by itself and therefore the EC must be disabled. The low-speed fax requires that the EC be enabled.
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Updated: 2019-02-22

Document ID: EDOC1100067358

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