Asterisk is a powerful open-source program of IPPBX, which is available at : http://www.asterisk.org. Its main pusher is Digium Corporation, whose PCI cards, including FXS card, FXO card, BRI card, PRI card, T1 card and E1 card, have become the first choice of most Enthusiasts. Plugged in these cards, a PC can connect the VOIP network and PSTN.

In fact, users can choose a gateway through the SIP ports of Asterisk. Compared to the manner of plug-in cards, using gateway has the features of flexible locations and better vocal quality. And our gateway products, through testing, can carry out all of the Asterisk functions.
The features of using geteway:
1. more flexible locations. You can manage the distribution without PCs.
2. better vocal quality. The gateways use the hardware of DSP, so can acquire a lot better vocal quality when there is substantial telephone traffic.

• voice process is a realtime task. PC operate system is not a realtime one. Voip gateway use its own dsp to do voice process while asterisk card use PC CPU to do this. Just like the DVD decode, on heavy task, voip gateway hardware will do better. So we sugguest you to use voip gateway on more than 4 phone line system.
• Asterisk PC+ voip gateway model, it is easy to expand to over 100 user. In this scale, you can not plug so many card into one PC.
• There are many analog voip gateway producer, the price is cheap, especially for Shang FXO/FXS Gateway fxs.

For FXS voip gateway, interoperate with Asterisk is easy. There are two requirements: one is sip interoperability. The other is DTMF transfer model.

If a voip gateway can make call with asterisk, that could to say sip interoperability is ok. If the auto attendant service is ok, that is mean DTMF transfer is ok.. The other things will be no problem.

Howerver there are some limits in Asterisk, especially on transcoding. If we use g.711, all is ok. but we are normal use g.723 or g.729. when we want to use conference service. Asterisk need to change codec to G.711.

The other things you must be careful. When a call setup successfully, there need call original voip gateway, and also the call termination voip gateway. The interoperation of such two type gateway is also important.

For FXO voip gateway, there is a lot of works. Such as the iver play feature, it need to special configed for asterisk.

Shang FXO/FXS Gateway fxo gateway provides more features to work with asterisk.
1.play asterisk ivr with no interuption.
when the Shang FXO/FXS Gateway received call from co line, it wouldn't conect instantly.instead, it start call to asterisk ivr first,when the ivr ready, it connect the co line. this feature make user feel friendly.

2. pbx voip/pstn inteleged route.
when you make pbx connect to voip/asterisk, how to make voip more stable. Shang FXO/FXS Gateway could detect the voip quanlity, when voip line failed, it change to pstn line automaticly.

3. pstn caller number transfer.
When pstn call in, the Shang FXO/FXS Gateway start voip call to the asterisk using pstn caller number instead of gateway number.

4. multy region pstn singal support.
By using MindSpeed technology, it integrated many region's pstn singal.

For voip base on the ip network, without a Qos assurence. How to make a voip service more like a telecom carrier grade service? There are many ways to do it.
1. PSTN life line.
When voip gateway is out of power, all phone that connected to that gateway are directed to the pstn life line. It seem to be better, but when the network is broken, or voip service is out of serivce, the gateway can't do any thing.
The Xia 304 has 4 lines to provide voip service and a lifeline for PSTN. When power is lost,  the lifeline connect the last port telephone with PSTN.

2. Use Route selector between voip gateway and phone.
Every route selector unit have 3 port, one for phone, one for voip gateway and one for PSTN line. When accident happend, you can change to the PSTN route manually, or easily dial phone with a prefix number such as 9. It is 1:1 PSTN backup and it is also a PSTN and VOIP integrated solution. Both VOIP and PSTN can work like normal way. Route selector work to distinguish them by phone number prefix.

3. MultiPath technology voip gateway .
Some foumous gateway like Quintum and Shang FXO/FXS Gateway use this technology to ensure the enterprise level voip service quality. Base on the Route selector, it can detect voip network quality automatically. When voip network is out of service, it change to PSTN automatically.

How Shang FXO/FXS Gateway backup voip with the PSTN line?
Shang FXO/FXS Gateway 4s4o owned 4 fxo and 4 fxs ports. When you work in backup mode, 4 fxs ports connect to pbx trunk line, 4 fxo ports connect to CO line. User start a voip call from fxs port, When a voip call failed, Shang FXO/FXS Gateway will automatically dial on the pstn line through fxo port. The end user use the gateway like it was on voip line. So the voip call quanlity is protected with pstn backup line.

 

There are many countries which are facing the voip block problem.

Generally, there are several means to block the voip business:
1. block the IPs and domain names of servers which have the main Voip stream.
2. shield the main Voip server ports, for  example the 5060 port for SIP protocol, or the 1720 port for H.232 protocol and so on.
3. monitor the Voip stream, if detected, send out the fake IP packages to disturb the Voip communication.
4. a lot of Middle-East countries like Dubai shut up most of the ports , only letting free limited ports such as 80 port.

There are 3 ways to resolve this problem:
1. use VPN. The flow picture is as follows: