Today’s cellular technology offers several different methods for data communications—an internal modem with built-in browser, an internal modem with an external port for connection (typically RS-232 or infrared (IR) to computer, or internal SMS messaging.

Modems fall into two categories: CDPD or traffic channel. CDPD offers true packet data communications at 19.2 Kbps whereas modems using the traffic channels are limited to the maximum rate for a traffic channel ( 14.4 Kbps, depending on the standard). Future 2.5G and 3G networks will differ in two distinct ways: Traffic channel rates will be higher, from  64 Kbps to 2.4 Mbps, and all data will be either packet-based or high speed circuit-switched.

CDPD MODEMS

Early  cellular  manufacturers  and  operators  recognized  the need for data communications, and the first modems were very similar  to  standard  modems  used  in  homes  and  offices.

However, cellular uses a valuable, shared commodity—spectrum. (Fixed telephone lines or wires may be shared, but they can always be increased in number if necessary.) CDPD was created as a digital packet data service over an analog cellular telephone: It uses the same analog channels as voice, but with different modulation applied to the air interface. Traffic channels not being used for voice calls may be used for CDPD calls. CDPD was the first digital data application to use packet data for cellular, and it is still very much in use today by carriers such as AT&T Wireless Services.

CDPD is fully compatible with analog cellular and is colocated with AMPS systems. Therefore, the analog infrastructure, such as frequency spectrum, cell sites, towers, and antennas, can be shared. The CDPD network elements overlay parallel to the analog infrastructure. Analog voice or analog data using an AMPS modem or digital data using a CDPD modem shares the same frequency spectrum. External modems are most common for CDPD communications, typically existing as PCMCIA cards for laptop computers, as accessories for PDA devices, or as external modems for connection to an analog phone. Some manufacturers actually include CDPD modems into their cellular telephone. This makes a 2G digital phone “Internet ready” because all TDMA and CDMA phones also include AMPS analog compatibility, and CDPD is carried on AMPS channels.

Two key design criteria were used to develop the CDPD protocol. From its inception, it was designed to use TCP/IP, the Internet protocol, making it transparent to Internet data. It was also designed to overlay an AMPS network, taking advantage of existing infrastructure. These design goals make CDPD very attractive to any carrier that manages an AMPS network. This becomes even more important when a carrier lacks true 2.5G or 3G capabilities and finds itself at a competitive disadvantage. CDPD can make any TDMA or CDMA phone “Internet ready” with 19.2 Kbps data rates by using TCP/IP in a packetdata network instead of on a circuit-switched connection.

As of the end of the February 2000, CDPD was available in 184 markets in the United States covering 56 percent of the United States population and 60 international markets. As 2.5G and 3G cellular networks launch, CDPD will begin to fade, but as long as there are 2G or analog phones, CDPD will retain its usefulness. And although it may compete with SMS for  short text  messages, it still  has  one advantage—CDPD works across networks with different physical layers. A CDPD modem in a laptop using CDMA can still send an email to a phone across the country, which is using TDMA.

TRAFFIC CHANNEL MODEMS

Many digital phones are advertised as Internet-ready, coming with a browser or a connectivity kit. Advertising for Wireless Internet modems or Internet-ready phones can be very deceiving, however. If the ad mentions a data rate of 19.2 Kbps, then it’s CDPD. The phone may be CDMA or TDMA but the data connection is through either an internal or external CDPD modem connected to an analog channel. If a TDMA or CDMA phone has a data connectivity kit, such as a cable to connect the phone and a laptop, and it does not mention the data rate, or if the phone has an internal browser, the modem is integrated into the phone and probably uses the rate of the traffic channel, 8 Kbps for TDMA and 9.6 or 14.4 Kbps for CDMA. GSM phones have long used data capabilities built into the phones so that they connect to a laptop by cable or include built-in modems to send data in the traffic channel at 9.6 Kbps. In all of these cases except CDPD, the connection is still circuit-switched for 2G networks.

A second type of modem is essentially a phone without voice capabilities on a PCMCIA card. There are modems of this type for every technology. They have an antenna integrated into them or are connected by a short cable to an antenna. Again, if the data rate is specified as 19.2 Kbps, it’s CDPD. If the rate is not specified, it’s probably using a traffic channel. Some PCMCIA modems offer a data rate of 56 Kbps and mention wireless in the same sentence. These actually combine two modems in one: a 56K landline modem and a wireless traffic channel modem.

With the launch of 2.5G and 3G networks, modems will become available having much higher data rates. They will fall into the same two categories: internal to a phone with a cable connection to a laptop or as a PCMCIA card. GPRS technology is just being launched in Europe but as with all new technologies, GPRS modems are still scarce. The United States will see next generation modems for CDMA and TDMA phones by 2002. The CDMA phones will use 1xRTT technology, and the TDMA phones will use GPRS. The CDMA standard 1xEV, with data rates up to 2.4 Mbps, will not be available until later. Products for W-CDMA will become available later in 2002 or 2003.

SHORT MESSAGE SERVICE (SMS)

If any application could be thought of as the “killer application,” messaging would certainly rank high on the list. First-generation digital cellular brought new data handling capabilities to the mobile community when a new service called SMS or Short Message Service was embedded into cellular protocols. All GSM phones support SMS but not all TDMA or CDMA phones fully support SMS yet. GSM was the first protocol to use SMS so the handsets have all caught up to the feature; the other protocols are working on enabling SMS in the network as more TDMA and CDMA handsets incorporate the ability to send an SMS. Remember—all digital phones can receive text messages.

Carriers in Europe report SMS revenues as up to 15 percent of revenues and an even greater percent of profits. Global SMS traffic is estimated at 15 billion messages by December 2001.* While SMS is immensely popular in Europe, it will be the first  step  in  most carrier messaging strategies and will therefore be the first non-voice application that the majority of American consumers will experience. Although mainstream launches and promotions of SMS have taken place, widespread adoption is far from complete.

SMS teaches consumers to use wireless devices for nonvoice services, and it will be the bearer for the next stage of messaging that incorporates elements other than simple text— graphics, sound, and specific formatting. Before United States customers can enjoy the same widespread usage as GSM users, however, several problems must be solved.

Not all SMS messaging is created equal. The number of characters that can be sent using SMS varies by protocol and carrier. Typically GSM sends 160 characters, TDMA sends 150 characters, and CDMA can handle up to about 200. Some phones only receive and cannot send SMS. Addressing and interconnectivity for SMS is a major challenge for substantial growth of SMS traffic. Users of CDMA cannot send SMS to TDMA users.

A second problem area is billing. Billing impacts the adoption of SMS because most consumers will be unsure of the need for SMS and will not have any reference point for usage. Billing can be in “buckets of messages,” “per SMS,” or free. Most carriers will launch SMS with a period of free SMS messages before moving to the primary offer of a bucket of SMS messages (200-800) for $4 to 8 per month.

Speed and latency offer another potential problem area in the United States. This was a problem in Europe six years ago, but because SMS is a mature technology in Europe, latency has been “fine-tuned” out. A typical SMS message is very fast— less  than  5 seconds from send to receive. At times in the United States, however, the SMS traffic is so heavy (holidays, etc.) that the delay is measured in hours not seconds. SMS quickly loses value as latency increases. Carriers control latency by adding processing power to the Short Message Service Center (SMSC).

Every technology-based  service  such  as  cellular  or  the Internet constantly evolves into something different and, hopefully, better. SMS is no different; it will migrate to newer versions  such  as  Smart  Messaging,  Instant  Messaging, Multimedia Messaging, and Enhanced SMS (EMS or E-SMS). SMS is characterized by out-of-band packet delivery and low-bandwidth message transfer, which results in a highly efficient means for transmitting short bursts of data. Initial applications of SMS focused on eliminating alphanumeric pagers by permitting two-way general-purpose messaging and notification services, primarily for voice mail. As technology and networks evolved,  a  variety  of  services  has  been  introduced, including email, fax, and paging integration, interactive banking, information services such as stock quotes, and integration with Internet-based applications. Wireless data applications include  downloading  of  subscriber  identity  module (SIM) cards for activation, debit, profile-editing purposes, wireless points of sale (POS), and other field-service applications such as automatic meter reading, remote sensing, and locationbased  services.  Additionally,  integration  with  the  Internet spurred the development of Web-based messaging and other interactive applications such as instant messaging, gaming, and chatting.

Clearly, mobile messaging is a valuable application that is gaining popularity in both the business and consumer sectors. Mobile messaging services will continue into the next-generation networks, and multimedia messaging will emerge as more bandwidth becomes available.

GENERAL PACKET RADIO SERVICE (GPRS)

General Packet Radio Service (GPRS) is a GSM Phase 2+ bearer service. It represents the first true advance in packet data service since CDPD and is the first packet data service on wireless digital  networks. It is  currently being launched in Europe on the GSM networks, but a common start-up problem has hampered its growth—lack of equipment! GPRS handsets are still in short supply. This is a recurring nightmare for operators of all  new technologies: When WAP was introduced, there was a lack of handsets and content.

This results from the classic “chicken-and-egg” syndrome. Because GPRS handsets cost more to make in small quantities, prices to consumers are higher. With low sales figures, manufacturers produce small quantities of product. The ramp-up to higher production volumes will take time, but it will happen, of that you can be sure. GPRS will be the backbone of GSM and TDMA networks for wireless packet data communications. Radio resources are shared by all mobile stations, and GPRS parses out those resources as needed to each user because Internet browsing usually results in data communication that is transmitted in bursts rather than steady streams. This creates greater efficiency in network capacity management: Data rates as high as 115 Kbps can be achieved.

Unlike SMS messaging, GPRS was not originally a part of the GSM (or TDMA) network. For this reason, some new network elements must be introduced to the GSM architecture, and some mobility management functions must be modified. Unlike CDPD, however, GPRS provides a data overlay within the standard GSM infrastructure by adding these additional elements. Packet data through a GPRS network does not use any circuit-switched network resources. One of these additional network elements is called the Gateway GPRS Support Node (GGSN). Essentially, this is a packet router with some mobility management functions. It connects to the GSM network and the external packet node network through standard interfaces.

The second new element is very similar in function except that it connects directly to the Base Station Controller (BSC). The Serving GPRS Support Node (SGSN) is responsible for handling packet data to and from the mobile unit.
BT Cellnet began offering GPRS network access to mobile phone users in Europe in 2001. Recent tests of those services, however, have not impressed many customers. Actual data rates have not matched expectations, but it is a new service and there will be a great deal of “fine tuning” to the system over the next few months.

When GPRS fulfills its promise of higher data rates, many new applications will be possible over GSM and TDMA networks. GPRS will fully enable mobile Internet applications similar to Web browsing on a desktop computer. Applications will include file transfer, Web surfing, and of course, email with attachments.

As with any new technology, GPRS does have some negative impact on a network. Not only are data resources shared, they are shared with voice resources—for any given cell site, channels must be divided between voice users and GPRS users. If all voice channels are in use and file transfers are taking place on all packet-data channels, there is no more capacity for that particular cell site until someone stops using some of the resources. Dynamic allocation of resources can only do so much.