What is a Media Gateway?

Media Gateway (MGW: Media Gateway), a unit that connects different types of networks, performs conversion between disparate networks such as PSTN; data networks based on IP or ATM; 2.5G and 3G radio access networks or PBX Media gateways enable multimedia communications over next-generation networks through multiple transport protocols such as ATM, IP, and TDM. One of the main functions of MGW is the conversion and decoding technology between different transmissions. Media stream functions such as echo cancellation, DTMF, and voice senders are also located in the MGW. The media gateway is controlled by a media gateway controller (also called a call agent or soft switch), which provides call control and signaling functions. The communication between the media gateway and the call agent relies on some protocols such as MGCP or Megaco or H.248.

Chinese name: Media gateway
Foreign name: Media Gateway

Features: Call control and signaling functions
Agreement: Seven-layer protocol

Media Gateway Protocol

The media gateway's seven-layer protocol in the application network:

OSI is an open reference model for universal system interconnection. It is a well-defined protocol specification. The OSI model has a 7-layer structure, and each layer can have several sub-layers.

Let me briefly introduce the 7 layers and their functions.

OSI's 7 layers are from top to bottom

7 Application layer

6 presentation layer

5 Session layer

4 Transport layer

3 Network layer

2 Data link layer

1 Physical layer

The upper layers, namely layers 7, 6, 5, and 4, define the functions of the application program. The lower layers, namely layers 3, 2, and 1, are mainly oriented to the end-to-end data flow through the network.

At present, media gateways have firewalls on the application layer and network layer, and the role of VPN can also be seen on the third layer. The security gateway, such as the OfficeScan, acts on the second floor. According to the seven-level limitation, the principle of high-level protocols can govern the principles of low-level protocols, and the development of security gateways is moving towards the path of high-level protocols.

The difference between a gateway and a router:

The media gateway is the IP that accesses the router. Other computers must have an IP segment with the gateway to access the router. For example, the IP of the router is 192.168.0.1 (this is the gateway). 0.X (any number between 2 and 254) so that you can access the router, that is to say, you can access the Internet, and the gateway address on the computer must be 192.168.0.1

Media Gateway Features

2.1 Media gateway 2.1 intelligent access, perfect and reliable

The product supports ADSL, fiber optics, and other broadband access solutions, enabling flexible bandwidth expansion and cheap access. Through the functions of routing, NAT, multi-link multiplexing and detection, the company can solve the access solution of flexible bandwidth expansion and cheap access for enterprises.

2.2 Media Gateway 2.2 Healthy Network, Application Security

Through its own firewall, anti-virus, intrusion detection, user access active authentication and other functions, the product provides enterprises with a comprehensive LAN access security management solution. Through its own DHCP server, ARP firewall, DDNS and other functions to provide enterprises with a comprehensive LAN management solution.

2.3 Media Gateway 2.3 Mobile Office, Fast and Secure

The VPN functions such as SSLVPN, IPSEC, PPTP, and L2TP included in the product allow users to easily and quickly establish a low-cost WAN dedicated network through one-click operation, providing enterprises with a secure WAN transmission channel and convenient implementation. The connection between corporate headquarters and mobile workers, branches, partners, product suppliers, and customers improves the ability to conduct business with branches, customers, suppliers, and partners.

2.4 Media Gateway 2.4 suppresses bandwidth abuse and protects key services

Dynamic intelligent bandwidth management function, only need to set it up once, automatically suppress the users who occupy the bandwidth, and easily solve the problem of bandwidth occupation such as BT, P2P and video movie download.

Definition of media gateway

Figure 1 shows the functional entity diagram in the network structure. From the figure, you can see the location of the media gateway.

Figure 1 Network structure diagram

In related standards (such as H.248, MGCP, etc.), a media gateway is defined as a device that converts media in one network into a media format required by another network. The media gateway transfers various media (voice, data, fax, video, etc.) between circuit-switched networks (such as PSTN) and packet networks (such as IP networks). It provides mutual communication between related entities in different media networks. Two-way interface. The media gateway terminates the bearer control protocol. It contains the bearer end node and media operating equipment (such as a decoder, echo canceller, or voice transmitter). For example, the media gateway can convert between the bearer channel of the circuit-switched network and the media stream of the packet network. It can handle audio, video, or T.120. It also has the ability to handle any combination of the three, and can perform full duplex Media translation, which can demonstrate video / audio messages, implement other IVR functions, and also conduct media conferences.

Media gateways are part of the softswitch architecture used in voice and data network integration. Service providers or telecommunications companies can replace the more expensive and bulky Category 5 voice telephone switches with less expensive soft switches, which support the interoperation between the Internet and the PSTN network.

The media gateway communicates with the media gateway controller through the standardized MGCP / Megaco protocol. MGCP / Megaco is a master-slave communication protocol. The media gateway controller (or Softswitch) controls and manages the media gateway through the MGCP / Megaco protocol.

The media gateway is located in the media access layer of the softswitch network. It can convert the media in one network into the media format required by another network. According to the location of the media gateway in the network and the media streams it processes, the media gateway can be divided into trunking gateway (TrunkingGateway), access gateway (AccessGateway), multimedia service access gateway (MultimediaServiceAccessGateway), and wireless access gateway ( WirelessAccessGateway) and so on.

3.1 Media Gateway 3.1 Relay Gateway

The relay gateway is located between the IP network and the PSTN network. It provides media mapping and transcoding functions between the IP network and the traditional PSTN network. It can group circuit-switched media streams and transmit them on the IP network. The transmitted media stream is converted into circuit-switched information. The gateway works with the signaling gateway to complete the interconnection between the circuit-switched network and the IP network. After the service stream enters or leaves the media gateway through the relay circuit of the circuit switch, the PSTN call can be transferred. Voice is transmitted through the IP network, so the main function of the relay gateway is to realize the transmission of media data streams in the IP network. For the relay gateways mainly for the traditional PSTN / ISDN, they are responsible for the tandem access of CTN / ISDN of PSTN / ISDN, connect them to the ATM / IP network, and realize VOATM / VOIP, so as to support the PSTN tandem relay. Features. By using a relay gateway instead of a traditional telephone tandem office, the next-generation network can provide PSTN with a virtual trunk gateway with local and long-distance tandem functions.

3.2 Media Gateway 3.2 Integrated Access Gateway

The integrated access gateway is an integrated access device in a softswitch system. It provides media mapping and transcoding functions, terminates TDM circuits, and groups media streams into packets and transmits them over packet networks. That is, the integrated access gateway terminates the voice relay of the circuit switch, and provides users with the ISDN user network interface (such as PRI) needed to access the Internet. The integrated access gateway is responsible for connecting various types of subscriber lines to a packet network (such as an IP network). Due to the different types of integrated access equipment, it can access various types of subscriber lines. Access, such as directly accessing PSTN / ISDN users, Ethernet users, xDSL users, or V5 users, and providing IP / ATM interfaces on the network side. This type of IAD is usually placed near the end office of the user, and it also has the function of modem data service offload. The purpose of the integrated access gateway is to provide a way for ordinary telephone users to access the Internet together with a network access server (NAS) or a remote access server (RAS). The MGCP / Magaco control protocol is used for communication between the integrated access gateway and the Softswitch.

3.3 Media gateway 3.3 resident gateway

The residential gateway (ResidentialGateway) provides a traditional analog subscriber line (RJ11) interface to the VoIP packet network. Examples of resident gateways are ordinary telephone CableModem, set-top boxes, xDSL equipment, and broadband and wireless access equipment.

The MGCP / Magaco control protocol is used for communication between the resident gateway and the Softswitch.

3.4 Media Gateway 3.4 Wireless Access Gateway

It supports the tandem relay function of the mobile network. Through the wireless access gateway, the next-generation network can provide a virtual relay with tandem function for 2G and 3G mobile communication networks.

Media gateway functional requirements

The media gateway terminates the media stream of the circuit-switched network and is responsible for the comprehensive access of various users or access networks. Its main functions are as follows.

(1) The mapping function of the media stream. The architecture of the next generation network enables NGN to provide users with voice, high-speed data, and video information services. It can also provide users with video conferencing and teleconferencing services. , To provide users with this requirement with unified messaging, current affairs news and other services, and these functions can not be provided without the support of media gateways for various media formats. In the next generation network, any business data is abstracted into Media stream, which can be voice and video information, or comprehensive data information. Due to the inconsistency of the network transmission mechanism between user access and core media, it is necessary to map one media stream to another media stream format required by the network, and the media gateway can complete the mapping function of the media stream. However, due to the complexity of services and networks, media stream mapping is not a simple mapping. It involves media encoding formats, data compression algorithms, resource reservation and allocation, detection and processing of special resources, and confidentiality of media streams. Related content. In addition, there are special requirements for different service characteristics. For example, voice services have special requirements for echo suppression, mute compression, and comfortable noise insertion.

(2) The function of accessing the core media network. The media gateway is responsible for the comprehensive access of various users or various access networks, such as ordinary telephone users, ISDN users, ADSL access, Ethernet user access, or PSTN / ISDN network. Access, V5 access, 3G network access, etc., the media gateway accesses the core media network by means of broadband access. At present, access to the core media network is mainly through ATM or IP access. ATM is a connection-oriented layer 2 technology with reliable service quality assurance capabilities. IP is a widely used layer 3 technology. In short, the media gateway device is the "interface gateway" for the user or user network to access the core media layer.

(3) Management and statistics functions. As a member of the network, the media gateway is also under the unified management of the network management system. The media gateway also reports relevant statistical information to the softswitch or network management system.

(4) The function of receiving control. Softswitch controls the actions of the media gateway. Most of the operations of the media gateway, especially business-related operations, are performed under the control of the softswitch, such as encoding and compression algorithms. Selection, call establishment, release, interruption, resource allocation and release, special signal detection and processing, etc. The interaction between the media gateway and the softswitch is performed using a standard control protocol. MGCP and H.248 are the control protocols between the softswitch and the media gateway. MGCP is defined by the IETF and is relatively simple to implement. There are many early applications, but the current trend is to switch to the H.248 standard defined by the ITU-T or Megaco defined by the IETF.

Non-functional requirements for media gateways

5.1 Media Gateway 5.1 Quality

A major requirement for media gateways is to provide high-quality voice quality, because this has a direct impact on the user's perception of the service, but sometimes bandwidth is more important than quality, and compression becomes an important factor. Therefore, the media gateway should provide a series of codecs (G723.1, G.711, G.729, G.726, GSM) to meet different requirements, meeting both the voice quality requirements and the bandwidth requirements.

Moreover, media gateways should provide low packet loss and low latency, as both factors have an impact on the quality of the voice.

In addition, features such as echo cancellation and adjustable jitter buffers also help improve voice quality, so media gateways should also support it.

5.2 Media Gateway 5.2 Openness

Openness is another important requirement for media gateways. Media gateways must be able to use standardized protocols, such as MGCP, MEGACO / H.248, to interconnect with other network entities (such as softswitches). The use of standardized protocols allows operators to be less dependent on vendors and facilitates the updating of network entities.

5.3 Media gateway 5.3 good connectivity

Since the media gateway is located at the junction of two networks (circuit-switched network and packet-switched network), it should provide good connectivity to these networks. The media gateway should support PSTN connections such as E1, STM-1 or ISDNPRI and connections to packet-switched networks such as ATM or IP (Ethernet Connection).

5.4 Media Gateway 5.4 Security

Another important issue for media gateways is security. Only authorized users can use the media gateway, while non-authorized users cannot use the media gateway, so the media gateway should implement authentication protocols such as RADIUS, PAP, CHAP, or IPSec.

5.5 Media Gateway 5.5 Stability

Providing stability at the bearer level is a requirement for the stability of the media gateway. The stability of any network entity is important to the network operator. The media gateway can support redundant devices and support distributed deployment. Improve the stability of the media gateway.

5.6 Media Gateway 5.6 Scalability

Different operators have different requirements for the scale of network use. Operators should be allowed to expand and shrink the network as needed. Therefore, media gateways must support scalability. When considering media gateways, scalability is reflected in the support of distributed architecture (that is, stable In terms of performance) and the possibility of deploying a new gateway without affecting existing gateways.

5.7OAM Media Gateway 5.7OAM

The management and monitoring of the media gateway should be as easy as possible, and generally there should be a graphical user interface. These operations should also be done remotely, which can make the management of multiple distributed media gateways easier. The media gateway should support standard management protocols (SNMP, CORBA) to be compatible with widely used management tools and integrate with old OSS, so it should support SNMP-compatible MIB, and it should be possible to view the main performance parameters and generated warnings on the management platform (there Different alarm levels), you should see the network topology (such as how the MG is connected to the softswitch) to configure the network.

Media gateway media gateway implementation technology

The architecture of the media gateway should be highly configurable and programmable, with good scalability and high flexibility, all of which depend on a good architecture-hardware and software structure, the software structure mainly involves equipment The implementation of drivers, network protocols, and media gateway control protocols (MGCP, Megaco / H.248, etc.). The hardware structure must be scalable, requiring integrated voice packetization, multiple QoS transmission mechanisms, and adaptive QoS guarantee mechanisms at the application layer. And transport layer QoS guarantee mechanism, and other key technical mechanisms. Voice packetization is a computationally intensive task and requires the use of high-performance computing engines. At the same time, the realization of transport layer QoS guarantee also mainly depends on hardware. The following are supported.

(1) Support multiple speech coding algorithms

In order to ensure that the existing network can smoothly access NGN and can be interconnected, because different networks use different voice coding algorithms, the media gateway needs to support multiple voice coding algorithms, and it must support multiple voice coding algorithms used in existing communication systems. , Such as G.711 (PSTN), G.723.1 (IP phone), G.729A (IP phone), EFR (GSM), AMR (3GPP), etc., the media gateway can achieve self-adaptation by using a variety of voice coding algorithms The QoS guarantee mechanism, for example, according to the different or changes in the network topology, dynamic load, and link status, the system dynamically adjusts the voice coding algorithm used to ensure the optimal QoS of the entire network.

(2) Support multiple transport layer QoS guarantee mechanisms

The next-generation network is an IP-centric full-service network that supports both voice, data, and multimedia services. However, the existing IP protocol cannot completely solve the QoS problems faced by the next-generation network. This requires the media gateway to support multiple This transport layer QoS guarantee mechanism can not only support the existing transport layer QoS mechanism, but also quickly support the emerging QoS mechanism through limited modification. This requires the media gateway to have sufficient flexibility in the architecture.

(3) Voice packetization

Voice packetization (speech coding algorithm) is the prerequisite and foundation for multimedia voice communication in next-generation networks. Voice packetization has strict real-time requirements. A frame of speech must obtain parameters within a specified time, and there is a certain correlation between the front and back speech frames. In the case of short-term transmission interruptions or errors, voice can be predicted using the characteristic parameters of the previous frames, so that users do not feel a sudden drop in service quality, but long-term interruptions will inevitably interrupt the call, and rely only on the voice encoding algorithm QoS cannot be guaranteed and must be combined with other mechanisms.

(4) Adaptive QoS guarantee mechanism

Media gateways rely on the transport-layer QoS mechanism to achieve user-oriented QoS, which is far from sufficient, because media gateways must support not only PSTN, but also GSM, CDMA, and future 3G mobile networks. When the mobile communication channel deteriorates or the user capacity increases, only adaptively adjusting the coding algorithm used by the user can ensure QoS in different network environments. Only by making full use of wireless spectrum resources can the optimal network QoS be achieved. Based on the current network conditions, a suitable encoding algorithm is selected from a variety of adaptive multi-rate speech codecs used by 3G.

(5) Highly reliable, easy to control and maintain

Because there will be many nodes with powerful computing capabilities in the next-generation network, the next-generation network will be a huge distributed system. In such a computing environment, the media gateway must have high reliability. To accept the control of a softswitch device, the media gateway needs to support a variety of different control protocols, and it should be easy to implement functions such as maintenance, monitoring, and management of the media gateway.