SmallNetBuilder

Cellular Networks and Gateways

There are two major types of cellular networks: CDMA and GSM. GSM is more common globally, CDMA is more common in the U.S. (Table 3) In the end, your choice of cellular carrier will decide the type of gateway device you require. Since T-Mobile is my cellular carrier I was going to need a GSM-based gateway.

Table 3: Network Types used by U.S. Cellular Carriers

Cellular gateways come in a range of sizes and capabilities. In searching, I found cellular gateways, sometimes called fixed cellular terminals, from one to dozens of ports. The larger gateways are carrier class devices that connect the cellular networks to large scale PBXs.

Portech, for example, makes a large gateway that provides an E-1 or T-1 interface at the PBX end, and can provide up to 30 simultaneous calls. Larger gateways, such as the Hypermedia HG-4000 shown in Figure 1, tend to be modular, so that an installation can be scaled to fit exact requirements.

Figure 1: Hypermedia HG-4000 GSM Gateway

Like most carrier class equipment, these devices are built to be extremely reliable, and they are priced accordingly. These are well beyond the scope of any home office.

For some applications, it may be desirable to have a handful of cellular trunks available. There are several mid-sized gateways that accommodate this, providing 2-8 ports. These usually target companies that would like to add wireless trunks to an existing commercial PBX. The cellular lines are exposed either as a number of analog line jacks or as SIP trunks access via IP.

While still costly, these smaller gateway devices are often priced so as to be practical for a business that would value the ability to keep their phones operating even while all land line and IP connectivity was lost.

My installation would be the smallest practical...just one trunk line...a single port. Even so, there are various approaches embodied in the various available products.

The least expensive models ($125-250), such as the Ericsson gateway shown in Figure 2, present a standard analog phone line jack as their means of connection to the PBX. This would require the addition of an analog FXO interface card to bring the line into in my Asterisk server.

Figure 2: Connecting GSM to Asterisk - Method 1

Somehow bringing the call from the digital domain, back to analog, then converting it to digital again in the cellular space seems less than ideal. Such transit would yield plenty of opportunity for call quality issues. The requirement for the analog line card also adds cost. As mentioned previously my experience with Asterisk and analog FXO interfaces was less than thrilling.

Within the Asterisk user community, two modules are available to allow the use of cell phones as the equivalent of analog trunks; chan_bluetooth and chan_mobile. These have the ability to bridge a call from the Asterisk server to a cell phone via Bluetooth.

From a hardware perspective, this is certainly a low cost approach. But it implies that I’d need to leave a Bluetooth-capable phone near the server at all times. It might be workable, but I would prefer a more elegant solution.

I eventually selected the Portech MV-370 SIP-GSM gateway (Figure 3). This gateway is a SIP device, so the connection between the PBX and the gateway would be via IP. By keeping the data path digital end-to-end, superior call quality is possible. Also, the configuration in Asterisk is simplified. The gateway just appears as a SIP peer, just like any of my SIP phones.

Figure 3: Portech MV-370 SIP-GSM gateway

In fact, I hunted around various Asterisk related web sites and found that configuration of the Portech GSM gateway was already well documented. This supported my decision to order the gateway directly from the manufacturer in Taiwan.

How About Skype?

There are a number of gateway devices available that are specifically designed to interface the cellular networks to the popular Skype IP communications service. However, while very popular, Skype is a completely proprietary system that doesn't interoperate with IP-PBX systems, except through dedicated gateways.

The inexpensive single-port versions of these devices often require a connection to a PC running Skype software. In this way, Skype allows third-party integration while maintaining a completely closed architecture. The gateway doesn't communicate with the Skype P2P network directly. It only acts as an audio device for the Skype client software. Communication is via Skype's published API, not by using the Skype protocol directly. Multi-port cellular-to-Skype gateways are available from a few sources.

So that's how I came to want to add a cellular-based alternative to my VoIP system and select the PORTech gateway. In Part 2, I'll describe how to install and configure the Portech GSM Gateway to operate with my Asterisk system.