All About GSM Wireless
FAQs on GSM Wireless Technology for Live Audio Broadcasts
In the US, T-Mobile and Cingular were offering this service, however recently they have been discontinuing the Circuit Switched Data service without warning. It has become increasingly difficult to reliably obtain CSD service or to be able to rely on this service for broadcasts and Comrex can no longer recommend this service in the US.
From the first time I saw mobile phones, I’ve dreamed about using them for remotes. Has Comrex made my dreams come true?
Comrex shared your dream, but for years the realities of available mobile networks and associated technologies would not support broadcast remotes with suffi cient quality and reliability. When we originally released the Matrix Portable Codec in 2000, it was designed to accept a high-quality wireless interface, but we would not release a product until we could provide the quality you expect from us.
Advances in GSM, as well as the ingenuity of our development team, enable us to now offer a fully-integrated Matrix GSM Module for the Matrix Portable for wireless remotes. The module includes a built-in GSM wireless phone, compatible with GSM carriers and capable of operation in most of the world.
Why is using mobile phones that much more diffi cult than using landlines? After all, my Comrex POTS Codecs sound great.
A little history is in order before we get to the technical issues.
Wireless telephony took different paths in the U.S. than it did in the rest of the world. In Europe, Groupe Speciale Mobile developed a digital standard in 1983. That standard, Global System for Mobile Communications, known by its acronym GSM, is now the predominant wireless telephone technology outside of the U.S. operating on the 900 MHz and 1800 MHz bands, depending on location.
Introduced in the early 1980s, the fi rst generation U.S. cellular telephone service was an analog system called Advanced Mobile Phone Service (AMPS), operating in the 800 MHz band. It was not until 1993 that a frequency range in the 1900 MHz band was auctioned to become the home for Personal Communications Service (PCS), a digital technology allowing phones to send and receive data, not just audio.
By the way, if you are interested in learning more, privateline.com has a wonderful history of wireless telecommunications and Cingular maintains a rather comprehensive glossary of terms, phrases, and acronyms related to wireless communications.
OK, still with us?
Yes. So what’s the status of PCS today?
Digital PCS encompasses three separate, incompatible technologies:
- Time Division Multiple Access (TDMA) was deployed by companies who had large, existing analog networks due to lower deployment costs. Currently, it is being phased out due to its lack of high-speed data capability. AT&T and Cingular are migrating from TDMA to GSM. Nextel uses a special form of TDMA with P2T, called “iDen”.
- Code Division Multiple Access (CDMA) is technically quite different from the other two protocols. CDMA is used by Sprint PCS and Verizon Wireless, and may also be found in some parts of Asia.
- The third technology is GSM. Worldwide, GSM is the most common standard for digital wireless phones. In the US, TDMA and CDMA have been more widely used. GSM is gaining traction and is available in most of the US. Carriers include T-Mobile and Cingular.
At this time, GSM is the only form of digital PCS that has a stable enough data spec to be suitable for audio codec use.
Why is that?
GSM offers Circuit-Switched Data (CSD) as part of the standard. This data service establishes a dedicated channel (or circuit), linking together all of the segments of the signal path to create a single unbroken line for each telephone call. This makes CSD appropriate for real-time transmission of audio data and is offered by most GSM and CDMA carriers.
In our testing, we looked at performance differences using CSD on both GSM and CDMA. We found that CDMA networks tend to be excessively “bursty” in nature – the data stream may become discontinuous, speeding up or slowing down, and even pausing at times. This burstiness causes time delays and stalls data throughput on CDMA networks, to the extent that real-time audio transfer is not feasible.
That leaves GSM, which offers CSD and low delay. But even GSM has its challenges.
OK, I am beginning to see the complexity of the matter. What are the other hurdles Comrex had to clear?
The data rate for GSM is limited to 9.6 kbps a challenge for transferring high-quality audio.
Also, the nature of the wireless telephone network makes sending real-time, high quality audio particularly diffi cult. Because the link from the wireless handset to the network is in a digital radio format, it is not possible to connect a normal modem to these phones. Yet it is desirable to have a normal modem, such as is found in our POTS Codecs, connected to the land-based telephone number you are calling. A conversion must therefore take place within the PCS network between protocols.
Let’s describe this another way. Think of a PCS data call as having three segments.
- The wireless connection between your handset and the PCS site nearest to you.
- The network between the local PCS site and a master central point, where a modem pool is located.
- An ordinary modem link between the PCS modem pool and your landline modem.
This is illustrated in figure below.
Keep in mind that these networks were not designed for realtime audio. In fact, the data connections between the PCS sites and the modem pools in CDMA use Internet-like protocols for data transfer, even on CSD. This means that portions of your data can be delayed signifi cantly.
In our tests, GSM CSD networks tend to be less “bursty” in nature, allowing real-time audio to pass easily.
This is where the new Comrex coding algorithm, error correction, and modem technology come in. We had to overcome both bandwidth and network limitations to achieve up to 7 kHz frequency response and reduced digital artifacts.
Does the future bode well for real-time audio data?
Wireless is moving toward the so-called 3G, the third generation of wireless technology, following analog (1G) and PCS (2G).
3G offers some promise. It will provide rates from 384 kbps to 2 Mbps in a stationary environment, supporting wide-bandwidth needs, such as full-motion video. While the bandwidth will remain Internet protocol-based and subject to congestion, design for multimedia and wide bandwidth will help reduce today’s limitations.
Early 3G networks have already begun to spring up. In Japan, NTT DoCoMo launched its 3G “FOMA” service in October 2001, and European networks are in development. Implementation in North America should be introduced in the upcoming years.
I have heard a lot about wireless Internet using GPRS. Is this compatible with Comrex’s GSM solutions?
GPRS (General Packet Radio Service) is one of several interim solutions that use the public Internet for data transfer. GPRS is a good way to provide web browsing, e-mail and portable multimedia. But packet-based data, sent through the Internet, is in danger of network congestion and delays, which is clearly not good for real-time broadcast.
We are not supporting codecs on these so called “2.5G” services for several reasons:
- Data throughput is typically asymmetrical on these services. They transmit high speed from the network to the wireless phone, but allowing only low speed transmission from the wireless phone to the network. The path from the phone is the direction we need the greatest throughput.
- Rather than dial-up services, these are connections via the public Internet. Similar to the services from your ISP, they are subject to the congestion and unpredictably associated with Internet delivered data.
- These services are “best effort,” meaning that if someone makes a voice call within your cell site, your data rate can be throttled down.
Continuing on the subject of other available services, some European wireless providers have implemented High Speed Circuit Switched Data (HSCSD), which uses two telephone channels for up to a 28.8 kbps data rate over GSM with audio frequency response as high as 10-12 kHz. The problem with HSCSD is its “best effort” implementation, which means that, when demand on the GSM system is increased, the carrier can remove the high-speed data by dropping one of the two channels whenever it chooses to and without notifying you. This results in the interruption of audio. The uncertainty of the HSCSD connections in real world situations makes it diffi cult, if not impossible, to use for live broadcast.
An “enhanced” GSM data service delivering a data rate of up to 14.4 kbps on a single wireless channel is also available in the UK and other places in Europe. The Matrix Portable is being used successfully on this service, delivering higher quality audio than can be achieved on the more limited 9.6 kbps GSM available to most users. This 14.4 kbps service tends to be far more robust that the HSCSD dual channel implementation.
Comrex’s research and development for our GSM technology has a decidedly international fl avor. Over the years, we have made countless GSM trials in every part of the world. This global effort continues as GSM services evolve.
About the Matrix GSM Module
Is my current Matrix compatible with the Matrix GSM Module?
Yes, but your Matrix Portable units may require a free upgrade. Contact Comrex Tech Support for more information.
What phone does the Matrix GSM Module use?
No external phone is necessary, because the phone is actually embedded into the module. Simply insert a GSM SIM card to activate the phone.
The SIM (subscriber identity module) contains your subscriber-related data, including your phone number and carrier information. It can be moved from wireless phone to wireless phone.
The Matrix GSM Module is available in two versions. The 850/1900 MHz version is suitable for locations in the United States and the 900/1800 MHz version will cover most of the rest of the world. Your SIM card should work worldwide, but you may pay less for airtime using a locally acquired SIM.
What do I need on the studio side? Someone told me I need another mobile phone.
Comrex’s mobile wireless solution requires only a SIM card in the Matrix GSM Module in the field and a compatible Comrex POTS Codec at the studio. Because our solution is fully integrated into Comrex products, we have more control of the technology and you benefi t with more predictable results.
POTS Codecs that will function on the studio side of wireless remotes include the Matrix Rack, the Matrix Portable, and the BlueBox. Some of these units may need to be upgraded for compatibility, but the upgrade is free. Contact Comrex Tech Support, providing the serial number of your unit to determine if an upgrade is needed.
You mean that you will upgrade both of my Comrex POTS for compatibility with the Matrix GSM Module Codecs at no charge?
You heard it right! Comrex does everything we can to put the latest technology in the hands of our customers. We even pack an adapter to power the Matrix Portable from a car cigarette light jack with the Matrix GSM Module.
How’s the wireless sound quality?
We’ve made very effort to have it as good as possible. A new Comrex coding algorithm results in frequency response of up to 7 kHz with reduced digital artifacts. New error correction and modem technology mean better stability for your connection.
However, you have a mobile telephone call and coded audio. Both are subject to situational variables. The reviews from the field range from “satisfactory” to “ I can’t believe it sounds so good.”
Quality is subjective, so you should listen to it for yourself before deciding. To hear clips, visit our website or contact Comrex Tech Support on techies@comrex.com for a free demo CD.
How do I dial the phone when using the Matrix GSM module?
The phone is dialed from the Matrix Portable keypad for either enhanced (data) or voice calls. Just follow the menus.
How do I know my signal strength?
The Matrix Portable displays the wireless signal strength, on a scale ranging from 0 to 32. We recommend a reading of at least 20.
Can the Matrix GSM Module improve my reception?
Yes. The module kit includes a three-inch external magnet-mount antenna for optimal reception. Also, since the phone is not handheld, it is not limited by radiation concerns and can safely transmit more power than traditional handheld phones.
Where should I place the antenna?
As high as possible. If you have trouble, try reorienting the antenna. Sometimes placement in the immediate area will change reception dramatically. Whenever possible, attach it to a steel surface, like a car body or even a pie pan, for a ground plane.
Do the other Comrex POTS Codecs function wirelessly using the mobile phone network?
The Matrix Portable, BlueBox, and Vector can be connected to the hands-free port of many mobile phones. Although audio quality is not much better than a standard mobile phone call, it provides a useful backup tool.
On the other hand, the optional GSM wireless module for the Matrix Portable is an integrated solution and is the only solution that will provide a frequency response of up to 7 kHz with reduced artifacts.
You seem to suggest that, even if the quality is not as good, I can do a wireless remote using my current Comrex POTS Codec. Something is better than nothing, eh?
Microphones and headphones may be interfaced with the “hands-free port” on a variety of cell phones by using the optional TelCell module on the Matrix Portable or the “Cell Phone Jack” on the BlueBox. While this method is limited to the phone’s frequency response, professional microphones will typically lead to an audible improvement over the cheap ones found in most mobile phones. For detailed information, see Comrex Tech Note 229.
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