Paging systems have traditionally consisted of infrastructure capable of simulcasting9.3 phone numbers to small portable receivers, notifying subscribers of the need to call whomever initiated the page. Today these systems are capable of two-way interaction, with subscribers initiating transactions from their portable devices as well as responding to messages by selecting one of a number of canned responses.
The traditional paging system provides the capability for a subscriber to receive simulcast messages from the network as long as they are in the coverage area of the paging service provider. Early incarnations of paging services limited the message to a 10-digit phone number and limited the coverage area to a specific market.
Both of these limitations have been extended with text messages (typically limited to about 80 characters in length) and simulcast coverage which is nationwide in scope (typically supported via satellite). With the proper equipment9.4 and provisioning, coverage areas can be extended internationally. Partly as a result of these extensions, at year-end 1995 there were some 34 million one-way paging subscribers in the United States.9.5
With increasing simulcast coverage comes increasing contention for a simulcast channel. It is significantly more resource-consumptive to use many transmission channels over a wide area for a single message than just one transmission channel over a single region. Prices for nationwide and international service are commensurately more expensive. This is the price paid for an essentially nonexistent mobility management scheme, in which the mobile is assumed to be everywhere.
A typical one-way paging network consists of a message center which receives messages to be simulcast to one or more mobile receivers. The message center consists of a combination of computers and operators to enter messages into the computers. Messages can originate from people phoning the operators or Email.
Messages are formatted per the protocol in use, then forwarded to one or more paging transmitters. The paging transmitters are RF towers, typically located in high places, which transmit messages over a wider area than cellular transmitters. Because one-way paging devices are provided with an excellent "sleep mode" capability by paging protocols, battery life is greatly extended (to approximately one month or more). As a result, these devices are very small and inexpensive. Battery life is further extended by the receive-only mode of operation of pagers, although to a lesser degree.9.6
One-way paging service is much less expensive than two-way service because there is no reverse path (i.e., mobiles never transmit) and they can thus be deployed with fewer larger cells. Fewer cells means lower fixed (infrastructure) costs for service providers. One-way paging systems also benefit from not having to perform mobility management functions-the network doesn't really care where the mobile is or if it is even powered on. The network simply simulcasts the message from all of the paging transmitters included in the service profile for the subscriber.
Each pager receives all messages transmitted on the channel it is tuned to and filters all messages not addressed to it. Pager addresses have traditionally been formatted as 10-digit phone numbers for user-friendliness reasons; all of these phone numbers would be received by the message center. Now a common number (often an 800-number) is used with the particular pager identified by a personal identification number (PIN), also often formatted as a 7-digit phone number.
Early one-way paging systems used proprietary protocols to convey the 10-digit phone numbers to pagers. From these proprietary systems evolved the Telocator Alphanumeric Protocol (TAP) and associated Telocator Network Paging Protocol (TNPP), with support for 7-bit ASCII text messages.
TAP is used today to access most text paging services, although it is inefficient in its inability to perform multicast transmissions. Instead it must generate a separate copy for each of the members of a group targetted to receive a common message. TAP also provides no support for binary file exchange (i.e., noncharacter-oriented messages), which limits its ability to support other data applications.
An improved standard called Telocator Data Protocol (TDP) has recently been developed by the Paging Communications Industry Association (PCIA), the primary paging industry trade group. TDP is aimed at addressing the shortcomings of TAP, which it will replace.
TDP supports 8-bit ASCII messages, which are capable of transporting binary files, facsimile and digitized voice as well as supporting longer messages, error correction and file compression. TDP provides for packetization of larger messages with reassembly of the messages required at the recipient device. TDP also provides the group multicast capability absent in TAP.
With TDP, subscribers now have the option of either receiving the message directly at the pager or having it forwarded to a mailbox for later retrieval with a spawned short notification message being sent to the pager in near real time. As shown in Figure 9.2, a message originator sends a message from their computer as a binary file to the message processor via the Telocator Message Entry (TME) protocol. The message processor packetizes the message and forwards either the message or its notification to the paging transmitter.
The paging transmitter broadcasts the packets (or notification) to the recipient pager via the Telocator Radio Transport (TRT) protocols. The recipient pager reassembles the packets into the original message (or notification). Finally, the paging receiver uses the Telocator Mobile Computer (TMC) protocol to relay the message (or a file) to an application on a computer.
The ability to use TDP is a powerful tool for distribution of messages or data (including software upgrades) to users of mobile computing equipment. It could even be used to update the control software in the pager itself, although complicated by the fact that one-way service-by definition-cannot provide the means for the pager to acknowledge the correct receipt of the message (data) or initiate procedures for error recovery (retransmission, etc.).
Despite these advances in capability and coverage, one-way paging remains a best-effort simulcast service. The embryonic two-way paging services are likely to obsolete one-way paging; in fact the 1994 narrowband PCS spectrum definition includes channels for incumbent one-way paging services to extend their capability to acknowledged paging. After all, who doesn't wonder whether or not their page has actually been received?
In response to the twin demands for two-way messaging capability and a reduced federal budget deficit, the FCC conducted auctions in mid-1994 for narrowband personal communication services (NPCS) licenses in the 900 MHz frequency bands.9.7 The auctions were a precursor to the subsequent broadband PCS auction (described in Chapter 2) and raised some $ 613 million for the U.S. Treasury.
The FCC definition of NPCS services is broad, including messaging, two-way paging, voice and mobile communications. Digitized voice paging services are also expected. Broadcast services are explicitly prohibited under the licensing provisions.
The NPCS licensing involves a complicated channel plan encompassing nationwide, regional and local coverage areas. Regional license areas are based on the 47 major trading areas (MTAs) defined by Rand McNally. Similarly, local license areas are based on Rand McNally's 487 basic trading areas (BTAs).
As displayed in Table 9.2, NPCS licensing includes twelve 50 kHz channels, each paired with a 12.5 kHz channel. These channel pairs are targeted at asymmetric services where messages and data are transmitted outbound to mobiles, which acknowledge receipt of the messages and data. Nine 50 kHz channel pairs are defined for symmetric data and message transmission. Eight unpaired 12.5 kHz channels are defined for use by existing paging services for two-way capability. Finally, five unpaired 50 kHz channels are defined. NCS channels may be aggregated up to symmetric 150 kHz channel pairs for services requiring greater bandwidth.
Although the media (RF frequencies) have been defined for NPCS, the standards and technology to be used are left to the discretion of license holders. From the NPCS community two basic standards have emerged-one based on a paging paradigm and the other based on a data networking model. These competing standards offer tradeoffs between system capacity and complexity versus subscriber battery life and simplicity. Other existing and intended paging equipment vendors have proposed alternative standards for use in the NPCS arena, but have since capitulated to the two dominant standards.
The paging-based standard for NPCS is the extended FLEXTM family of protocols by Motorola. The FLEX messaging protocol has now been augmented by the ReFLEXTM two-way messaging protocol and the InFLEXion voice nd data messaging protocol.
As listed in Table 9.3, new FLEX protocols are aimed primarily at asymmetric services, where greater capacity is needed in the forward direction. These protocols support a variety of data types, including binary files, and provide error detection and highly efficient sleep mode for mobiles a la one-way paging. These TDP-based protocols provide no security capabilities although they could be added later. As in any proprietary standard, licensing is required for any other manufacturer's products to be able to use these protocols.
The two-way extensions to FLEX include mobile registration to a particular local area, typically a city. Whenever outbound pages or data are destined for the mobile, all transmission facilities in that local area must participate. In a large city this could require simulcast transmission of the message from dozens or even hundreds of paging transmitters. This makes economic sense only if a few large paging cells can provide the service; otherwise fixed infrastructure costs would be excessive.
Although this simulcast bandwidth requirement is inefficient, it is mitigated somewhat by a much simpler mobility management scheme with support for limited tracking. This allows an efficient sleep mode capability, which in turn extends battery life. Unfortunately the need for supporting reverse channels for low power mobiles limits the size of cells, increasing the fixed (infrastructure) costs of the network.
Thus, an expensive bi-directional infrastructure is required to support the reverse channels but cannot be efficiently used (in terms of airlink capacity) because of simulcast forward transmission. However, the system retains the benefits of simple mobility management and long battery life, which also result from its paging history.
The second primary standard for NPCS is the personal Air Communicator Technology (pACT)9.8 standard developed by AT& T Wireless Services and others. This open CDPD-based messaging standard is oriented toward the data networking paradigm of the IP protocol suite. pACT is a variant of CDPD, with modifications to the airlink and radio resource management definitions.9.9
As in its CDPD heritage, pACT requires no licensing fees and is IP-based. Mobile units are IP-addressable network nodes. pACT preserves the security and mobility management capabilities of CDPD; a mobile device must notify the network whenever it moves to a new channel stream. This allows a pACT-based service provider to support much greater collective bandwidth because only a single transmitter is required to transmit to a mobile. The tradeoff is a somewhat reduced battery efficiency at the mobile device, because of the need for more continual reception of the forward channel to support mobility management.
The spectrum efficiency of pACT is offset somewhat by a low speed transmission capability. In support of its symmetric communication architecture, both forward and reverse channels support 8 kbps channel speed on a 50/50 kHz channel pair. This relatively low data rate includes error correction plus the capability for a cellular-type mode of frequency reuse. Each 50 kHz license is implemented as multiple sub-rate transmission channels.
pACT is designed for limited size messages and data. As such it includes the LSM and SNS protocols standardized in the CDPD Forum. Both symmetric (e.g., peer-to-peer Email) and asymmetric (e.g., acknowledged paging) applications are supported by pACT. Message and data store-and-forward capabilities are intrinsic to the system. But pACT is not intended to provide a general purpose mobile data service.
Although pACT mobiles are IP-addressable, the nature of the application-NPCS-suggests that the services are provided via a "closed" system. Therefore, there is no requirement that pACT-based service providers actually use globally-unique IP addresses. However, use of InterNIC-supplied IP address blocks will greatly facilitate the ease with which pACT-based applications can be developed to interact with the rest of the world.
Going forward, it seems likely that applications software developers will increasingly consider NPCS capability essential to the products they create. File compression and, possibly, encryption will likely become increasingly common. Of course, IP-based paging protocols like pACT also encourage the trend toward communications and computing commonality.