Understanding Cellular

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Millions of people use cellular telephonic devices-primarily cell (or mobile) phones, which are essentially small, sophisticated two-way radios with a built-in antenna. These diminutive devices, however, do much more than a two-way radio. Depending on the device, they can store contact information, provide calendaring functions such as to-do lists, perform simple calculations using a built-in calculator, send and receive email messages, obtain Internet-based information, provide simple gaming capabilities, perform basic computer-related duties (if a supportive operating system is installed), and integrate with other devices (e.g. cameras, PDAs, laptops, MP3 players, and GPS receivers).

Here are the basics of how cellular works:

When you place a cellular telephone call, the phone converts your voice to modulated radio frequency energy. The radio waves travel through the air until they reach a receiver at a nearby radio tower (i.e. cell site antenna) and base transceiver station (i.e. transmitters and receivers in a box or cabinet which is connected to the radio tower by feeders). If there are sufficient resources available (i.e. an open channel), the cell tower will forward the signal to a centrally located device called a "switch," "mobile switching office" (MSO), or "mobile telephone switching office" (MTSO) via special leased phone lines (e.g. a T-1 line) that connect the switch to many cell sites. This way the switch can hand all of the network's regional phone connections to the Public Switched Telephone Network (PSTN) and also control all of the base stations in that region. The switch then patches the signal to a channel on the PSTN, where it will continue on its way until it reaches the person you are calling. If the other person is using POTS (Plain Old Telephone Service), it will be delivered over the PSTN lines. If that person is using a cell phone, the call will be handed off to another cell and make its way via radio waves to the recipient's phone.

When you receive a call on your cell phone, the reverse takes place-the message travels through the telephone network until it reaches a base transceiver station (more commonly referred to as a base station) close to the phone's location. Then the base station sends out radio waves that are detected by a receiver located within the phone, where the signals are changed back into the sound of a voice.

A cellular call occupies a wireless channel as well as a PSTN channel, with both being held open until the call is completed. This means that neither channel can be used for any other calls until the cell phone call is discontinued. (There are exceptions.)

All cellular telephonic devices have special codes associated with them. These codes identify the device, its owner and the cellular service provider. For example, when a cell phone is turned on, it listens on a designated frequency control channel for a System Identification Code (SID-a unique 5 digit number that is assigned to each carrier by the FCC). If no control channels are available, it displays a "no service" message. But if it obtains a channel and a SID, it matches the SID to the code programmed into the phone. If they match, the phone knows it is communicating with its home system. The phone also sends out its Mobile Identification Number (MIN-the phone number) and its Electronic Serial Number (ESN-a unique 32 number ID programmed into the communication device during manufacture) to the nearest cellular tower. This is then passed on to the MTSO, allowing the MTSO to track the phone's location. This way, when a call is received for that phone's MIN, the MTSO can forward the call to the right cell and pick the right frequency pair for the phone to use in that cell to take the call. Once that occurs, the MTSO communicates the frequency to the phone (using the control channel). After the phone and the tower switch to the right frequency pair, the call is connected.

Figure 13.1: How a call traverses a cellular network. Each hexagon represents a "cell" which is served by an antenna (radio/cell tower) and base transceiver station.

If the cellular telephonic device is mobile and moves away from the cell with which it is communicating, the base transceiver station will note the diminishing signal strength and the adjacent cell will note the increasing signal strength. (All base stations listen and measure signal strength on all frequencies.) The two base stations, using the MTSO as a go-between, will coordinate so that at the right moment the phone will get a signal on the control channel telling it to change frequencies and then it will be handed off to the new cell.

Roaming

Roaming refers to the ability to use a cell phone anywhere, anytime throughout the world where compatible technology is available. Initially this was a very challenging issue for the cellular industry. The difficulty arose in trying to get various systems to communicate and pass routing and billing information to each other. (The same problems also will have to be solved if Wi-Fi and cellular networks are to share a subscriber base.)

As you now understand, when a cell phone is turned on, the cell phone listens for a SID on the control channel. If the SID does not match the SID programmed into the device, then it knows it is roaming. Since the phone also sends out its MIN (phone number) and ESN, the MTSO of the cell in which the phone is roaming contacts the MTSO of the home system (it does this based on the exchange, e.g. "305" if the phone number is "917-305-3336"). Once the home MTSO verifies that the SID is valid, the switch must then determine if roaming is possible, i.e. whether there is sufficient compatible equipment present. If roaming is possible, the local MTSO or "roaming switch" sets up a "Visitor Location Register" (VLR) registering the phone in its network and notifies the home switch of the phone's location so that it can route calls to the roaming switch as they come in. Outbound calls are handled through the roaming switch the same as they would be handled if the user were at home. Incoming calls are routed from the home switch to the roaming switch after sending a message to the roaming switch requesting a "Temporary Local Directory Number" (TLDN). This TLDN will be used to make a connection from the home switch to the roaming switch across the PSTN. (The process of registering the phone and notifying the home switch takes only a couple of seconds.)

Cellular's Evolutionary Path

Cellular technology evolved in three basic generations, with each successive generation being more reliable and flexible than the last. (See Fig. 13.2.)

Figure 13.2: GSM is the 2G technology of choice, but in North America it's more muddled with a mix of TDMA, CDMA and GSM being used for 2G cellular communications.

THE MARVEL OF THE CELL PHONE

The key to the cellular phone is the cell. By breaking a region into numerous cells and then spacing these cells fairly close to each other, cell phones can broadcast at very low power levels (typically 200mW - 1W). Because the cell phones can broadcast at such low power levels, they can be built with small transmitters and small batteries, enabling manufacturers to provide cellular devices that weigh less than three ounces.

Cell phones are remarkable. For an instrument with such a small "footprint" (i.e. it takes up very little space), it contains some amazing technology: antenna, liquid crystal display (LCD), keypad, microphone, speaker, battery and circuit board (the heart and brains of the phone). The typical cell phone's circuit board contains several computer chips, such as:

• The digital signal processor (DSP) performs signal manipulation calculations at high speed.

• The microprocessor manages all of the mundane tasks, such as controlling the keypad and LCD, handling the command and control signaling with the base station, and coordination of many of the functions on the circuit board.

• The analog-to-digital and digital-to-analog conversion chips translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog.

• The memory chips (ROM and Flash) provide storage for the phone's operating system and custom features, such as phone directory and calendar. Some phones also store the SID and MIN codes in Flash memory; others use external "Smart" cards for this purpose.

• The radio frequency (RF) and power sections manage the frequency channels and take care of power management and battery recharging.

• The RF amplifiers handle signals traveling to and from the antenna.

Cellular Networks and Data Transmission

Data transmission capabilities were added to most cellular technologies (GSM being the exception) much as an afterthought. For instance, Cellular Digital Packet Data (CDPD) is the technology used to add data capabilities to second generation TDMA (and FDMA) networks. This technology offers data transfer rates of up to 19.2 Kbps, quicker call set up, and good error correction.

CDMA networks, on the other hand, added data capabilities using one of two methods. A CDMA operator could use a circuit-switched approach (which involves setting up a data call in a manner similar to setting up a voice call except that digital data is transmitted rather than a voice signal). This allows data to be passed over the call circuit while the call is in operation. Or the operation could take a packet-switching approach, where the data is passed through the network without going through the typical call set up. This method requires that there be some form of arbitration to stop multiple users from sending data at the same time and interfering with each other. Because there is no need to set up a call, this method is sometimes referred to as "always-on." (Note: When the packet-switching approach is added to an existing 2G network, the resulting service is often referred to as "2.5G.")

2.5G

While the above data transmission techniques are still in use today in most 2G networks, efforts are underway to upgrade those 2G systems. These new hybrid "always on" networks (e.g. GSM/GPRS, EDGE, cdmaOne) are known as "2.5G." They can transfer data at speeds of up to 144 Kbps (faster than traditional 2G digital networks, but slower than true 3G networks). A phone with 2.5G services can alternate between using the Web, sending or receiving text messages, and making phone calls without losing its connection. Since it's these networks where Wi-Fi typically will be used to enhance their data capabilities, let's look a bit closer at some of their technologies.

General Pack Radio Service (GPRS). GPRS is a packet-switched data service that can overlay a GSM (Groupe Speciale Mobile or Global System for Mobile Communications) network and can be ported to TDMA (Time Division Multiple Access) networks. Theoretically, GPRS over GSM can reach 172.4 Kbps, but in practice, transmission rates range between 25 and 50 Kbps in the downstream direction (i.e. sending data to the consumer's cellular device) and 10 and 25 Kbps in the upstream direction. GPRS introduces packet data and requires Mobile IP (a protocol that builds on the Internet Protocol by making mobility transparent to applications and higher level protocols like TCP) with a real-world data rate of approximately 86 Kbps.

cdmaOne. This refers to a digital service that provides data service that operates at a theoretical maximum data rate of 64 Kbps. It supports packet-switched data on CDMA (Code Division Multiple Access) networks, operates in the 800 MHz or 1.9 GHz radio frequency bands, and uses a 1.25 MHz-wide carrier. Basically what makes cdmaOne a 2.5G technology is that it provides "bandwidth on demand," i.e. all users share a "bandwidth pool," which can be dipped into by each mobile phone for whatever purpose-voice, data, fax, etc. When one mobile phone is using less bandwidth, more is available for others.

Personal Communications Services (PCS). This is an all-digital service with a top data rate of 144 Kbps. PCS was specifically designed for U.S. operations and is available mainly in large metropolitan areas. The term "Personal Communications Services" was coined by the Federal Communications Commission (FCC) to describe a digital, two-way, wireless telecommunications system licensed to operate between 1850-1990 MHz (although the FCC's rules describe "PCS" as a broad family of wireless services without reference to spectrum band or technology). But true PCS also provides features like paging, caller ID and email functions. One main advantage of PCS technology is that it can increase a cellular network's call capacity-a PCS network using TDMA has 200 kHz channel spacing and eight time slots (instead of the typical 30 kHz channel spacing and three time slots found in earlier digital cellular systems). PCS networks can use CDMA, TDMA or GSM technologies.

Third Generation

The original goal of 3G was to create a truly global mobile telephone system. After all, POTS is compatible all around the globe, so why shouldn't the same compatibility apply to cellular service? The International Telecommunications Union (ITU) initially took the lead. It adopted the same kind of approach it took with ISDN (Integrated Services Digital Network), which could be described as "3G for wired networks" since ISDN is designed to carry not just voice, but also a whole range of data services at what was at the time considered to be a high transmission rate.

Note 

The term, "third generation" or "3G" refers to pending improvements in cellular-based data and voice communications through any of a variety of proposed standards, with the immediate goal of providing data transmission speeds of 2 Mbps. However, any cellular network that supports packet-switching and promises data transmission speeds of at least 40 Kbps is oft-times mis-labeled "3G." But since technology to take cellular technology beyond 2G varies in its capabilities, perhaps a better vernacular for "beyond 2G" would be "next generation," "NextGen" or "nex-gen."

The first attempt at formulating a 3G system by the ITU was the Future Public Land Mobile Telecommunication Systems (FPLMTS), which would eventually evolve into the International Mobile Telecommunications 2000 (IMT 2000) specification.

The ITU was slow on the uptake. So, in the interim, the European Union decided to create its own version of IMT 2000, which it dubbed "UMTS" (Universal Mobile Telecommunications System). Toward that end, the EU reached an agreement between existing GSM equipment suppliers-mostly European and Japanese manufacturers-for a single, common technology. The basis of this technology (which was originally referred to as "3G/UMTS") is today's W-CDMA (Wideband CDMA).

The introduction of W-CDMA put a monkey wrench in the ITU's plans for a "global cellular network," forcing the organization into a series of compromises. Ultimately, UMTS, as specified by the Third Generation Partnership Project (3GPP), was formally adopted by the ITU as a member of its family of IMT-2000 Third Generation Mobile Communication standards. The outcome is set forth in the latest version of the ITU document entitled "International Mobile Telecommunications-2000" (IMT 2000). UMTS is now therefore a part of the ITU's IMT-2000 "vision" of a global family of 3G mobile communications systems. However, because of the popularity of the term UMTS, the final 3G system goal is sometimes referred to as "IMT 2000/UMTS third generation personal and mobile services."

The IMT 2000 defines an "anywhere, anytime" standard for the future of cellular communications with a goal of making available all kinds of service-wide-area paging, voice, high-speed data, audio/visual, etc.-within a common system. The framers of the IMT 2000 also envisioned seamless service across the globe that merged (or at least made compatible) PSTN service, the various approaches to wireless (cellular, Wi-Fi and satellite), the Internet, and even cordless phone technologies. Moreover, they would like to see multi-function mobile terminals, worldwide roaming, and even a personal phone number or a Universal Personal Telecommunications (UPT) number to allow a user to receive any kind of communication, on any terminal, anywhere. Thus, IMT 2000 promises:

THE MIGRATION PATH

The cellular operators that go the 3G route will typically take one of two paths. The umbrella names for these two paths are: IMT-2000 MC for multicarrier CDMA (which includes 1X, 1xEV, and 3x) and IMT-2000 DS for direct spread W-CDMA. Let's take a closer look at each:

The IMT-2000 MC Path: Since CDMA already uses a wide (1.25MHz) carrier, the task becomes one of making the most efficient use out of what's already there. Providers currently using cdmaOne (the technology that provides services that are somewhat similar to W-CDMA) can rather easily migrate to one of the cdma2000 family of 3G technologies. This path is favored by cellular operators in the Americas and the Pacific Rim. It includes:

cdma2000 1X can double the voice capacity of cdmaOne networks and delivers peak packet data speeds of 307 Kbps in mobile environments.

cdma2000 1xEV includes cdma2000 1xEV-DO (Data Only), which delivers peak data speeds of 2.4 Mbps (although real-world speeds are more in the 300 Mbps range) and supports applications such as MP3 transfers and video conferencing.

cdma2000 1xEV-DV (Data Voice) can provide integrated voice and simultaneous highspeed packet data multimedia services at speeds that have tested as high as 3.09 Mbps.

1xEV-DO and 1xEV-DV are both backward compatible with cdma2000 1X and cdmaOne. Today, there are 26 cdma2000 1X and three 1xEV-DO commercial networks across the globe. Moreover, 22 1X and three 1xEV-DO networks are scheduled for deployment during 2003. For the current status of cdma2000 deployments, the reader can visit the CDG website: www.cdg.org/worldwide/index.asp and then click on the "View Entire Worldwide Database" link.

The IMT-2000 DS Path: GSM's evolution is from 9.6 Kbps circuit-switched (non-packet) data to the interim GPRS solution, then to Enhanced Data rates for Global Evolution (EDGE). EDGE is a further improvement to GSM/TDMA and it optimizes for GPRS. In fact, EDGE was developed specifically to meet the bandwidth needs of 3G-like services. Since EDGE works with current GSM networks and offers a top speed of 384 Kbps, it is the technology du jour in the GSM community. It's noted, however, that both GPRS and EDGE continue to use GSM's narrow 200 KHz carrier and TDMA technology.

GSM's final evolutionary step to 3G is to switch over to W-CDMA, which is a huge leap-moving from its TDMA roots over to CDMA, and from a 200 KHz carrier to a 5 MHz carrier. New frequency allocations, infrastructure, and handsets will be required for this last step.

W-CDMA (Wideband CDMA), also known as CDMA Direct Spread or 3G/UMTS, is the 3G radio transmission technology that is expected to be favored by any European operator that build-outs a 3G network. W-CDMA can be built upon existing GSM networks and represents the obvious next step for current system operators. This 3G technology sends data in a digital format over a range of frequencies, which makes the data move faster, but it also uses more bandwidth than digital voice services. (W-CDMA has also found favor in the U.S. AT&T Wireless and a few other U.S telecos are slated to roll out W-CDMA networks in a few large U.S. metropolitan areas sometime prior to 2005.)

The only W-CDMA network that is fully operational at this writing is the aforementioned NTT DoCoMo network in Japan. The reason cdma2000 deployments are so far ahead of W-CDMA rollouts is because W-CDMA requires new spectrum allocations and cdma2000 operators can build their systems atop their existing CDMA infrastructure, allowing them to reuse much of the existing gear. Thus, cellular providers implementing W-CDMA networks must spend more time and money on their network build out, versus cdma2000 providers. For now, W-CDMA will do well only in areas in which regulators favor W-CDMA over cdma2000, such as Western Europe.

U.S. cellular providers AT&T Wireless, Cingular and T-Mobile are taking a torturous route to W-CDMA. AT&T Wireless's path toward 3G began with the installation of voice-centric GSM network on top of its existing analog and TDMA networks. It then installed a data-only GPRS system (which reduced the network's voice capacity). Next came an all-new radio access network (RAN) to provide higher-speed data using EDGE. Its most recent step is to install yet another all-new RAN (and potentially a new core network) for W-CDMA, slated for completion 2005.

Cingular, which initially straddled the fence about its technology choice, committed to the same 3G migration path as AT&T. T-Mobile (VoiceStream), already a GSM carrier, also committed to this same three-stage implementation path. But elsewhere in the world, the news isn't particularly good for cellular providers choosing this technology route-a host of W-CDMA carriers have announced delays in deployment.

• Increased bandwidth, up to 384 Kbps when a device is stationary or moving at pedestrian speed, 128 Kbps in a vehicle such as a car, bus, train or plane, and 2 Mbps in fixed applications.

• Circuit-switched and packet-switched services, such as Internet Protocol (IP) traffic and real-time video.

• Voice quality that's comparable to POTS.

• Greater capacity and improved spectrum efficiency than currently available in 2G networks.

• Ability to provide several simultaneous services to end-users and terminals, for multimedia services.

• The seamless incorporation of 2G cellular systems, in order not to have any discontinuity between 2G and 3G systems.

• Global, i.e. international, roaming between different IMT-2000 operational environments.

• Economies of scale and an open global standard that meet the needs of the mass market.

• Common billing/user profiles, e.g. sharing of usage/rate information between service providers, standardized call detail recording, and standardized user profiles.

• Enhanced 911, which refers to the network's ability to determine geographic position of cellular devices and report it to both the network and the mobile terminal.

• Support of multimedia services/capabilities, e.g. fixed and variable rate bandwidth on demand, asymmetric data rates in the forward and reverse links, multimedia mail store and forward, broadband access up to 2 Mbps.

The ITU accepted five different standards as suitable for IMT-2000 (i.e. 3G) environments: Direct Spread, Multicarrier, Time Code, Single Carrier, and Frequency Time. Actually, only the first three standards are being adopted; the last two standards appear to be attracting very little interest. For further explanation of these five standards, see Fig. 13.3.

Figure 13.3: Official IMT 2000 recognized 3G transmission standards.

Adopting any of the IMT 2000 standards requires a lot for a new technology and new networking environments. The cost of building such a network will be exorbitant. Furthermore, it is highly unlikely that early 3G networks will be able to provide for all of the above requirements. This may cause many cellular providers to look at Wi-Fi in a different light.

Consider some of the limitations of 3G networks:

• Unlikely to provide end-users with a 2 Mbps data transfer speed, since to attain such speed would require that no other users be registered on a base station. Because spread spectrum systems (which all 3G standards are) transmit the same signal to multiple base stations simultaneously, finding a base station with no other users is highly unlikely.

• Cannot support both circuit- and packet-switched communications concurrently because of the complexity of implementation.

• The technology to seamlessly incorporate existing 2G networks does not exist. The only fully operational 3G network (at this writing) is Japan's NTT DoCoMo network and it doesn't support the Japanese 2G standard.

• Cannot provide international roaming between different IMT-2000 environments. But this could be remedied in the future-it was some time before GSM roaming became available.

The 3G Hyperbole

When the cellular industry realized the costly and technological challenges they faced in upgrading their current 2G infrastructure to 3G, some opted to use 2.5G technology as an interim step. But download speeds that 2.5G networks offer are, to put it bluntly, paltry when compared to Wi-Fi's speeds.

Compare the industry's marketing campaigns to reality. Many ads tout that these new 3G systems (actually 2.5G role-playing as 3G) are fast enough to blow the typical dial-up experience via ISPs, like America Online, out of the water. But the current crop of next-gen networks just barely manages to equal a typical America Online dial-up session-a fact that is documented by figures provided by the cellular carriers themselves.

Providing an explanation for the discrepancies, Keith Nowak, a representative for handset marker Nokia states, "We tried real hard not to hype that peak number [144 Kbps], but some other parties might have been more likely to hype it. In the long term, it's better to use the more realistic speeds."

Customer disappointment over a cellular network's data delivery doesn't bode well for an industry hoping to recoup the high cost of building these new networks by selling downloadable games or business applications needing speed to succeed. And, this gap between hype and reality has caused at least one analyst to state that the actual performance of the networks raises the question of why they were built at all. IDC analyst Keith Waryas said when commenting on the low end of a typical web session on new next-gen networks from AT&T Wireless, Cingular Wireless and T-Mobile, "You build this big network, and all you can offer is a 20 Kbps download? That's not much of an improvement over what the carriers already had."

In defense, Jim Gerace, a Verizon Wireless representative, said that his company has been insisting all along that it's more likely that a subscriber to its Express Network, which peaks at 144 Kbps, will experience data transfer speeds of between 40 Kbps and 60 Kbps on average. "Some will experience even better than that," Gerace said. "But 40 to 60 is the number we're sticking with."

AT&T Wireless's new mMode service (which uses the GPRS standard) replaces the carrier's PocketNet wireless web service (an 8-year-old effort that uses CDPD and operates at 19.2 Kbps with an average user experience of between 5 Kbps and 9 Kbps). A company spokesperson says that subscribers to its new mMode service can expect on crowded days to experience data transfer speeds of around 20 Kbps to 30 Kbps and the news gets even worse: speed can drop to between 10 Kbps and 20 Kbps if someone's trying to download, say, video using a heavily trafficked network. Those startling numbers come straight from AT&T Wireless's own data.

As justification for the new mMode service, AT&T Wireless Chief Technology Officer Rod Nelson says that its mMode technology enables "streaming media, which wouldn't even work at all over CDPD [the technology used by PocketNet]."

A T-Mobile representative reports that its average user experience on its highly promoted next-gen service is about 40 Kbps. T-Mobile launched its nationwide GPRS service as an extension of its GSM voice network in late 2001. The carrier is pushing its data network to the limit. Its "you had to be there" ad campaign introduces customers to the benefits of visual, wireless communications via T-Mobile's next-gen cellular network.

Sprint PCS's new telephone network uses the same technology as that of Verizon Wireless, and while its data transmission rates peaks at 144 Kbps, Sprint describes an average user's experience at between 40 Kbps and 60 Kbps (the same as Verizon).

Is it any wonder that some cellular providers are turning to Wi-Fi technology? These cellular operators need some way to meet customer's high expectations brought about by over-optimistic marketing campaigns. Wi-Fi can help these hapless operators to match the promotional campaigns that promise a nationwide network capable of speeds that make it easy to download (and send) music and video files via a cell phone. With Wi-Fi as part of their service package, cellular providers and their customers can use Wi-Fi technology to do the "heavy lifting." In other words, with Wi-Fi to augment the cellular operator's current network, a cell phone with a dual-mode Wi-Fi/cellular chip could, for example, use Wi-Fi to download a huge document directly to a PDA or laptop.

Also, since Wi-Fi networks can also be optimized to carry voice calls, these networks could be used in the future to improve cell phone reception in buildings, where cellular coverage is traditionally poor. According to IDC analyist Waryas, "there is some very real potential to offloading some of the voice calls onto Wi-Fi."

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