A gateway is an intermediary between two different networks, such as & nbsp; Public Switched Telephone Network (PSTN) and Intenet (see article "H.323 Architecture for IP Telephony Services Design" - TGVTA, January 2004, p. 75). Gateway can be built on a computer or hardware circuit that has multiple interfaces with different networks.
Connections are created by converting protocols to set up, maintain and release calls, convert signals, encrypt, encapsulate and transmit information between two different networks. This article introduces the Gateway models used in IP telephony (VoIP) networks that are popular today.
Voice Gateway
In order to be able to try voice, the voice gateway software must separate it into individual packets like data. Packets are pieces of information that are small enough to be sent and assembled in a suitable manner.
The process of calling a non-H.323 (PSTN) network to H.323 (LAN, Internet) occurs through phases: local PSTN calls to the nearest gateway server. Here the analog voice signal is digitized, compressed and encapsulated, and then through the Internet the IP packets are sent to the receiving gateway. Here the IP packets are allocated to the appropriate IP endpoint device (such as an IP phone or a computer with software installed ...). The reverse procedure applies similarly to the H.323 network call to the PSTN network.
Gateway building models
There are many different types of voice gateways available in the market. Gateway products are generally built into four basic configurations:
1. Gateway built on PC Server and sound card
This is Gateway with the simplest configuration with cheap price advantage, however it has the following limitations:
- These sound cards are not designed to work with real-time applications. IP telephony software must handle intermittent audio streams. These causes the latency of the conversation.
- The sound card does not have the function of compressing and decompressing the audio signal, so the PC will have to perform the entire task at the CPU, creating a delay in signal processing and reducing the Processor of the CPU.
This model is only suitable for small organizations or individual applications in the study study. The problem with this Gateway model is that it depends on the vendor because each vendor has its own standards.
2. Gateway built on PC Server using dedicated digital signal processing
Gateway is based on a dedicated digital signal processor (DSP) on a PC. Dedicated DSPs will replace CPUs that perform functions such as sampling, compression, and speech transmission, while the CPU performs real-time processing and call control. This solution produces better voice quality, reduces latency and allows the PC to serve multiple conversations.
Dedicated DSPs such as Dialogic, DM3 IP Link, NMS ... can support multiple voice communication ports on a single card for voice mail, call transfer, and voice response applications. organized with medium scale.
Gateway is associated with an internal network element
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This type of gateway is designed for local networks. Cisco is a leading manufacturer that adds Gateway H.323 functionality to its high-speed end-to-end routers. This solution enables both computer network and IP voice services in the same device.
Other manufacturers, such as Shoreline Teleworks and Touchwave Communications, have launched gateways associated with Ethernet hubs. Each device is connected to a standard RJ-14 telephone line and connected to an RJ-15 Ethernet network. In essence each Hub acts as a PBX.
This model is often used in representative offices of foreign companies to communicate between branches within the intranet. Figure 1 is a Cisco 3640 voice gateway model that establishes a local IP telephony network between two branches, connecting to the PSTN and the external Internet.
4. Independent gateway for Internet telephony
This is a gateway for the public network that connects directly to the telephone exchange and the Internet infrastructure, such as ADSL.
Figure 2 shows the solution for providing VoIP services through the Alcatel ASAM 7300 DSLAM model. This model can be deployed at the enterprise level to provide additional services on the Internet.
The ASAM 7300 is a multiplexer for Internet subscribers using the ADSL line (the technical features of the ASAM 7300 can be found at www.alcatel.com/dsl). The ADSL system has developed an ADSL call service (called VoDSL service), which means that the voice signal is packaged and transmitted over the ADSL network. This solution is very economical because of the cost per unit of data transmitted over the network, which we know that the volume of voice signal is very small. On the other hand, voice quality can be matched to traditional voice quality due to the high rate of ADSL transmission and the improved voice compression technology standards.
A few remarks
As already mentioned, there are many configurable Gateway options to suit your requirements. However, when designing a gateway used in IP telephony services, depending on the size of the corporate intranet or the deployment of a public service provider, they need to face many challenges. Requires voice service. These are the following criteria:
The voice quality must be comparable to that of the existing PSTN system and may vary depending on the network's QoS level, ie the level of control over the network's service bandwidth.
- IP networks must address common network problems. Minimize missed calls, network delays, lost packets. Especially when network congestion or too many people access the network at the same time.
- Call signaling should be handled so that users do not recognize which phone technology is performing call handling.
With the introduction of gateway models, we can see that the former depends on IP voice management software, such as Cisco's Cisco Call Manager software. Only the gateway model attached to the network element, or the independent gateway model integrated into the service provider network, can fully control the network quality.
The second standard in addition to being dependent on deployment technology is & nbsp; It depends on factors such as the network infrastructure used for the organization's internal network and the public network, the quality of network equipment such as IP phone terminals, switches, multiplexers ...
The last criterion for evaluation is for subscribers when deployed at the service provider level.
Thieu Quang Trung, Ho Chi Minh City Post Office
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