IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 8, OCTOBER 1997 1391 Pigeon: A Wireless Two-Way Messaging System Thomas Y. C. Woo, Thomas F. La Porta, Member, IEEE, and Krishan K. Sabnani, Fellow, IEEE Abstract— Wireless messaging is an integral component of universal personal communication services (PCS’s). Its growth is likely to be further fueled by the availability of new data capabilities in the new PCS air interfaces. Our research focuses on high-level issues such as new mes- saging functionalities, high-layer protocols, and overall system design. Pigeon is our proposal of a wireless two-way messaging system. The novelty of our system lies in: 1) the techniques used in mitigating the wireless media and end device constraints, 2) the functionalities provided, and 3) its modular architecture. Examples of 1) include the use of asymmetric protocols and the introduction of user agents. Examples of 2) include group addressing, transaction support, and flexible messages. The mod- ularity of Pigeon allows its individual components to be adopted by specific systems. A prototype of Pigeon has been implemented, and is operational at Bell Laboratories. In this paper, we describe the motivation, design, and func- tionality of Pigeon. We also present, as an example, a mapping of Pigeon to a standard cellular/PCS messaging system. Index Terms—Cellular, GSM, IS-136, IS-95, messaging, PCS, SMS, two-way, wireless. I. INTRODUCTION A key distinction of personal communication services (PCS’s) is its integration of advanced data capabilities. These data capabilities are provided on both digital control and traffic channels, and include short messaging (SMS), circuit- switched data, and packet-switched data. For example, GSM (or equivalently, DCS-1900) [1] already supports SMS and circuit-switched data, and is currently developing multislot data (HSCSD) and packet data (GPRS). Similary, both IS-136 [2] and IS-95 [3] have implemented SMS, and are currently defining data standards. Separately in the United States, a new class of service called two-way paging has emerged in the narrow-band PCS spectrum, and new protocols such as ReFlex [4] and pACT [5] have been defined and implemented. The goal of PCS is to provide “anytime, anywhere” com- munication. This requires that the end device be truly portable, and that both real-time interactive communication (e.g., voice), and data services be available. A key component to PCS is the integration of messaging services, often derived from popular paging services, into service offerings. The messaging services may be used directly for end-to-end communication, or as a transport service for more generic data services such as e-mail and access to the World Wide Web. Messaging can also be Manuscript received September 12, 1996; revised April 30, 1997. A preliminary version of this paper was presented at the 7th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Taipei, Taiwan, October 1996. The authors are with the Wireless Networking Research Department, Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733 USA. Publisher Item Identifier S 0733-8716(97)06097-6. integrated into voice services, for example, by being used to deliver a directory entry for a voice call. Although a great deal of research has been done on the physical transmission issues (e.g., low-level access [6]–[8], protocol design to conserve end device power [9]) concerning wireless message transport, few efforts have addressed the protocol and system design aspects. This is not a problem for one-way systems as their functionalities tend to be simple. A two-way messaging system, however, has the same generality as a connectionless datagram service, and its design potentially entails similar considerations. Specifically, the availability of a return channel enables new features (e.g., acknowledgment, replies, registration, location tracking), which in turn impact the service that can be offered to end users, as well as the design of the system itself. By properly using a return channel, a two-way messaging system with maximum versatility, functionalities, and performance can be built. Pigeon is our proposal of such a system. It provides du- plex message communication among subscribers carrying self- contained portable wireless messaging devices, to be called two-way pagers 1 (or pagers for short) in the sequel. Unlike traditional pagers, two-way pagers are capable of originating as well as receiving messages. The “self-contained” requirement is especially important, as we will explain in Section II. Both Pigeon’s design and functionalities are heavily influenced by the constraints of the end device. Pigeon offers many functionalities, including some that are not even available in wireline messaging systems. The basic service offered by Pigeon is reliable end-to-end “flexible” messaging. We defer the precise definition of flexible mes- saging until Section III-A. For now, it suffices to say that it represents a compromise between traditional paging and electronic mail. It improves on traditional paging by adding message acknowledgment, pager-initiated messages, and direct reply capability; it is not as general as electronic mail in that it does not allow free-form messages. Apart from its functionalities, Pigeon is also interesting from a design perspective. First, Pigeon incorporates various techniques to mitigate the constraints of wireless communi- cation and end devices. Some of these techniques concern protocol design (e.g., the use of stateful protocols), while others concern system components (e.g., user agents). Sec- ond, Pigeon employs a distributed and modular architecture. Its control functions are distributed among a collection of intelligent servers, each specializing in a distinct function. 1 We note that the term two-way pager is used generically here. It does not refer only to devices resembling existing alphanumeric pagers. A two-way pager could be a cellular/PCS phone with messaging capabilities, or a PDA or laptop equipped with wireless communication. 0733–8716/97$10.00  1997 IEEE