How To Add a Cellular Trunk to Your VoIP System: Part 1
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.
| Carrier | Network Type |
|---|---|
| AT&T / Cingular | GSM |
| T-Mobile | GSM |
| Sprint | CDMA |
| Nextel | iDen (Motorola proprietary) |
| Verizon Wireless | CDMA |
| US Cellular | CDMA |
| Alltel | CDMA |
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.
Comments are closed.
Digium will be shipping a Skype connector module (commercial product) in Q1 of 2009, so you could connect any channel type that you can link into Asterisk through to your Skype account, bi-directionally.
I read through the docs on the MV-370, and it doesn’t seem like there is a way to receive SMS messages via any interface other than the built-in web server, even though there seems to be a way to send them via a telnet and “AT” command set. Have you found anything differently? Has anyone created any Perl routines to automated the transmission of SMS’s (not that it’s a big deal to do it oneself.) What happens when you log into the device’s AT command interface, set a debug mode, and send the unit an SMS? It seems that two-way SMS would be a pretty snappy feature even if it just relayed to a fixed SMTP address – XMPP would be asking too much. 🙂
JT
John,
I’ve not really pushed the SMS issue, as until recently I didn’t make much use of it even on my cell phone.The MV-370 unit that I have appears to have bidirectional SMS capabilities by way of AT commands over telnet. I know nothing of the command set.
This capability was implemented in firmware after I had the unit installed. I probably should upgrade, but it’s been doing what I need. The latest firmware with SMS is dated 10/13/2008.
The MT-350S specifically offers SMS service. It’s cheaper as its voice connectivity is by way of FXS/FXO.
I’m not much of code jockey so playing with AT commands is a little beyond my scope. However, there is a manual specifically on using the AT command set to send/receive SMS.
Michael
P.S. – the closing statement about Skype reflects the reality in early 2008 when this was originally written.
TellMe’s directory service (800-555-TELL) is actually pretty spartan. I’m not sure where they get their business directory from, but it’s pretty antiquated, and seems to be missing a LOT of businesses.
Free411 (800-free-411) has a MUCH better search capability, and includes municipal government numbers as well as business and residential, but at the expense of listening to an advert at the beginning of the call and before the call is connected.
GOOG-411 (800-GOOG-411 — Google’s Service) has a nice search feature that will rank top listings, and works for Canadian businesses as well, but it’s only business listings. It does not currently have adverts in it, but, being Google, one can assume those will appear eventually.
Incidentally, it’s rumoured that in the UK, this sort of cellular bridging is illegal to provide as a service. I’m not sure if it’s illegal in the SoHo arena. Again, this is just what I’ve heard. We’ve not done any real extensive research into that sort of issue, since we’re not in the UK. Any UKers out there care to comment on this?
Yes, I have heard that it’s illegal to provide such a gateway as a commercial service in the UK. Using it for private use is allowed AFAIK.
It is perfectly legal for any end user (business or consumer) to use GSM gateways, it is however illegal to use GSM gateways in order to offer a communications service.
Hmmm… I tried using the tags but it looks like it hasn’t worked. Please see below for the information I intended to post:
“Ofcom has recently clarified that it is entirely legal under UK law for end-users (whether businesses or ordinary consumers) to buy, install and use GSM gateways for their own use. However it is currently illegal under UK law for anyone to use GSM gateway equipment to provide a communications service by way of business to another person or organisation, irrespective of where the gateway equipment is located, or how many or few end-users are connected to each gateway. This prohibition on ‘commercial’ use applies equally to the mobile network operators (MNOs) as to other organisations, since the MNOs’ licences do not currently extend to the installation and use of GSM gateways.”
Taken from http://www.ofcom.org.uk/consult/condocs/gsm_gateways.
Thanks for delivering the details!
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I’m currently running two MV-370’s on a OnSip PBX from Junction Networks. One gateway is in US and the other is in Poland. This setup allows me to call toll free between my cell phones from US to Poland and back.I have one problem. I’m experiencing a delay (kind of an echo) pretty much every time a make a phone call between the gateways (sometimes more sometimes less). It’s usually better when one of the parties is on a regular sip phone and only one is talking via gateway. Have you expirienced such a delay? Do you know what can cause this?
Max
No, this has not been my experience. But then again, I only run one gateway. I would expect that the actual media would be negotiated to flow directly from one gateway to the other, not through OnSIP. That should be a very direct path.
You could try using another PBX to see if the situation changes. You could get a free account from IdeaSIP and give it a whirl. Or even set the gateways up to direct dial via IP, presuming that you have fixed IPs.