Gateway model for IP

Gateway is an element that connects intermediaries between two different networks, such as the public switched telephone network PSTN and Intenet network (refer to the article "H.323 architecture IP voice service design platform" - TGVTA number of months) January 2004, p.75). Gateway can be built on computers or hardware circuits that have multiple interfaces with different networks.

Connections are created by converting protocols to establish, maintain and release calls, convert signals, encode, encapsulate and transfer information between two different networks.The article introduces Gateway models used in the popular IP voice service (VoIP) network today.

Voice Gateway

To be able to test the voice, the software of the voice gateway must split it into separate packages like data.Data packets are pieces of information that are small, appropriately sized to be sent and assembled together in a suitable way.

The cycle of a call from the non-H.323 network (PSTN network) to the H.323 network (LAN, Internet) takes place through phases: local PSTN calls to the nearest Gateway server.Here analog voice signals are digitized, compressed and encapsulated IP, then via Internet the IP packets are transferred to the receiving Gateway.Here IP packets are allocated to the appropriate terminal IP device (such as an IP phone or a computer with software installed .).The reverse cycle will apply similarly to the call from H.323 network to PSTN.

Gateway construction models

On the market today there are many types of Voice Gateways of many different companies, generally Gateway products are usually built in the following 4 basic configurations:

1. Gateway built on PC Server and sound card

This is the Gateway with the simplest configuration with the advantage of low cost, however it has the following limitations:

- These sound cards are not designed to work with real-time applications.IP phone software must handle intermittent audio signal flow.These causes the conversation delay.

- Sound cards do not have the function of compressing and decompressing audio signal streams, so the PC will have to perform this task at the CPU, creating latency during signal processing and reducing processing capability. of the CPU.

This model is only suitable for small organizations or individuals applying for academic research.The disadvantage of this Gateway model is that it depends on the equipment provider because each manufacturer has its own standard.

2. Gateway built on PC Server using dedicated digital signal processing card

Gateway is built on a digital signal processor (DSP) dedicated to the PC platform.Dedicated DSP cards will replace the CPU to perform functions such as sampling, compression and voice transmission, while the CPU performs the function of real-time processing and call control.This solution creates better voice quality, reduces latency and allows the PC to serve multiple conversations.

Dedicated DSP cards like Dialogic, DM3 IP Link, NMS . can support multiple voice communication ports on one card suitable for voice mail applications, call receiving system, automatic voice answering in organization with medium scale.

3. Gateway is attached to an element of the intranet

This type of Gateway is designed to serve local networks.Cisco is a leading manufacturer that has added the functionality of Gateway H.323 to its high-speed terminal routers.This solution allows to perform both computer network functions and IP voice services in the same device.

A number of other manufacturers, such as Shoreline Teleworks and Touchwave Communications, have introduced the Gateway associated with Ethernet Hubs.Each device connects to the phone line with a RJ14 standard cable and connects to an Ethernet network with a RJ15 standard cable.In fact, each Hub acts as a PBX.

It can be seen that this model is often used in foreign companies' representative offices to communicate between branches in the intranet.Figure 1 is a model that uses voice gateway Cisco router 3640 to establish an internal IP voice network between two branches, connecting to PSTN and external Internet.

4. Independent gateway for Internet telephone network

This type of Gateway serves the public network, it connects directly to the telephone switchboard and the Internet infrastructure, such as ADSL network.

Figure 2 shows the solution for providing VoIP services through Alcatel's DSLAM model ASAM 7300.This model can be deployed at the enterprise level providing additional services on the Internet.

ASAM 7300 is a centralized multiplexer of Internet subscribers using ADSL lines (technical features of ASAM 7300 can be found at www.alcatel.com/dsl).The ADSL system was developed and developed a telephone service via ADSL (called VoDSL service), ie the voice signal will be packaged and transmitted on ADSL network.This solution brings very high economic efficiency because of the cost of data transmission on the network, but we know that the capacity of the voice signal is very small.On the one hand, cost-saving, on the other hand, voice quality can be equivalent to traditional voice quality thanks to the high transmission speed of ADSL lines and improved voice compression technology standards.

A few comments

As introduced, there are many Gateway configuration options that are suitable for use.However, when Gateway design is used in IP telephony service, depending on the size of the corporate network or deployed by the public service provider, they need to face many challenges and required from users of voice services.These are the following criteria:

- Voice quality must be comparable to the current PSTN system and can be changed depending on the QoS level of the network, ie the level of network bandwidth control of the service provider.

- IP networks must resolve network problems that often occur.Minimize the number of unsuccessful calls, network delay and packet loss.Especially when network congestion or too many people access the network at the same time.

- How the call signal must be handled so that the user does not identify which phone technology is performing the call processing.

Through the recently introduced gateway models, we can see that the first standard depends on the software for managing IP voice networks, such as Cisco's Cisco Call Manager software.Only the Gateway model attached to the network element, or the independent Gateway model integrated in the new service provider network, is capable of fully controlling network quality.

The second standard, in addition to depending on technology, the deployment solution also depends on factors such as private network infrastructure (organization's intranet) and shared network (public network like Internet . .), quality of network devices such as IP phone terminals, switches, multiplexers .

The final criterion for evaluation is for subscribers when deploying at the service provider level.

Thieu Quang Trung, Ho Chi Minh City Post Office