A New Layered Protocol Integrating 5-kHz and 25-kHz DAMA Operations: A Proposed Improvement to the UHF DAMA Standards

Transcription

1 1 of 5 A New Layered Protocol Integrating 5-kHz and 25-kHz DAMA Operations: A Proposed Improvement to the UHF DAMA Standards Gary R. Huckell, Frank M. Tirpak SPAWAR Systems Center San Diego, California Edward W. Chandler Milwaukee School of Engineering (MSOE) Milwaukee, Wisconsin Abstract-The Joint UHF MILSATCOM Technical Working Group (TWG) is developing a single integrated waveform that will provide a significant improvement over the two current Demand Assigned Multiple Access (DAMA) waveforms now required for operation over 5-kHz and 25-kHz UHF military satellite channels. The integrated waveform will be defined in revisions to the two existing standards. Each revised standard will cover a different protocol layer for the integrated waveform. The revised standard for the lower layer will define the interoperable modulation, error-correction coding and channel. The revised standard for the higher layer will define demand-assignment protocols and requirements for voice and data interoperability. This paper describes the proposed waveform and protocols. I. INTRODUCTION The Joint UHF MILSATCOM Technical Working Group (TWG) is developing a single integrated waveform that will provide a significant improvement over the two current Demand Assigned Multiple Access (DAMA) waveforms [1,2] now required for operation over 5-kHz and 25-kHz UHF military satellite channels. While the two current waveforms may have been sufficient for their intended purpose, they are not interoperable with each other and are not capable of satisfying all current and developing user requirements. Poor user acceptance, particularly of the voice service provided by the 5-kHz DAMA waveform, has greatly slowed the transition of UHF MILSATCOM users from dedicated access to DAMA operation. The TWG is presently developing a new layered protocol to be incorporated in revisions to the existing standards. The integrated waveform will be defined in two standards each covering a different protocol layer. The revised standard for the lower layer, MIL-STD B, will define the interoperable modulation, error-correction coding and channel. The revised standard for the higher layer, MIL-STD B, will define demand-assignment protocols and requirements for voice and data interoperability. MILSATCOM user systems not requiring interoperable voice or data would be required to implement only the protocols of the lower-layer standard. The integrated waveform standards will accommodate backward compatibility with the present standards. This paper describes the proposed waveform and protocols. II. UHF MILSATCOM BACKGROUND A pair of UHF Follow-On (UFO) satellites in geostationary orbits operates over each of four overlapping satellite coverage areas providing around-the-world coverage. Each satellite contains a mixture of 5-kHz and 25-kHz bandwidth channels, each using an independent transponder. A satellite pair operating together provides a total of 78 channels for use within each coverage area. Of these 78 channels, there are 42 5-kHz channels, kHz channels, and two 25-kHz fleet broadcast downlinks that are fed by jam-resistant SHF and EHF uplinks. Channel transponders are unprocessed (they do not demodulate the data). They simply hard-limit, filter, frequency-translate and amplify the received signal. The use of unprocessed transponders has allowed UHF MILSATCOM users to take advantage of improved modulation techniques that have been developed since the original UHF satellites were launched. While twenty years ago a 25-kHz bandwidth transponder was often used at only 2400 bps, today's standards [3] allow operation at rates as high as 56,000 bps. The transponders hard-limit received signals, providing maximum gain for weak signals, but also preventing the use of bandwidth-efficient modulation techniques that depend on amplitude modulation. Hard limiting makes it difficult for simultaneous signals to share a channel. UHF DAMA waveforms use Time-Division Multiple Access (TDMA) to share channels since this technique doesn t require simultaneous access. The main problem with UHF MILSATCOM is that there just isn t enough capacity. There are many more potential users of UHF MILSATCOM than there are available channels, and channel capacity is primarily limited by the UHF spectrum allocated for military use, not by an inability to build and launch satellites. The combination of earth coverage and non-processed transponders results in no antijam capability, and UHF is subject to ionospheric scintillation, multipath, and unintentional interference. By international agreements, military satellite communication is the secondary user for the allocated UHF spectrum. Interference from a variety of sources is preventing the use of many of the satellite channels and the military users have no legal recourse when interference is caused by other legitimate uses of these frequencies. A. Evolution of UHF DAMA The first and still most prevalent use of UHF MILSATCOM channels is in the single-access mode (also called dedicated access mode) where the entire channel bandwidth is dedicated to a single communications requirement, regardless of the actual bandwidth required. Each such communications requirement is also referred to as a user service, or as a network, because it typically involves a network of users that exchange data with one another either in a push-to-talk mode or by using higher-layer protocols. With

2 2 of 5 channels operating in the single-access mode, there is a simple one-to-one correspondence between the number of UHF MILSATCOM channels available and the number of communications requirements or networks that can be supported at any given time. DAMA standards were introduced in an attempt to make more efficient use of the limited UHF MILSATCOM resources. DAMA provides multiple access to a UHF channel through the use of TDMA, and allows such accesses to be demand-assigned. The two current DAMA standards that define operation over UHF satellite channels are MIL-STD A [1] for operation on 5-kHz channels and MIL-STD A [2] for operation on both 5-kHz and 25-kHz channels. MIL-STD protocols were originally developed to support an Air Force requirement for around-the-world messaging among Military Airlift Command (MAC) aircraft. MIL-STD can support circuits (i.e., stream traffic) at user data rates as high as 2400 bps and provides a messaging capability. MIL-STD protocols were originally developed to allow the Navy to use a single suite of radio equipment for simultaneous communication over multiple circuits. MIL-STD can support user data rates as high as 16-kbps, but generally is used in a mode where the maximum data rate available for each multiplexed communications requirement is 2400 bps. The demand for UHF MILSATCOM resources to support the communications requirements of all the services has continued to increase at a rate that far outpaces the availability of these resources. The problem of over-demand for UHF resources became so great that the Joint Chiefs of Staff (JCS) mandated a requirement to transition all UHF MILSATCOM users to DAMA operation. JCS made the decision to require all terminals using UHF MILSATCOM to be certified to be interoperable with both MIL-STD and MIL-STD B. Problems with UHF DAMA Although the two current UHF standards [1,2] may have been sufficient for their intended purpose, they are not interoperable with each other and are not capable of satisfying all current and developing user requirements. This has contributed to the low user acceptance of DAMA, greatly slowing the transition of UHF MILSATCOM users from dedicated access to DAMA operation. While TDMA and demand assignment can greatly expand the number of user services that UHF MILSATCOM can support, some performance characteristics of the user services can be adversely affected. When a 25-kHz channel is used for a single nondama voice network, the effective information data rate over the satellite channel is equal to the baseband rate of the voice service. When TDMA is used, the information data rate over the satellite channel has to exceed the sum of the data rates of all users. For example, when five users all operating at 2400-bps share a 25-kHz channel, each user is allocated 1/5 of the channel time and must operate at five times the normal modulation rate. The increased modulation rate, along with the requirement to break the signal into small bursts, requires a higher quality signal than required for single-access operation. A channel that can provide good service for a single communications requirement may operate poorly when TDMA is used. UHF terminals operating in the current DAMA modes do not provide the ease of use normally associated with a commercial cellular phone system. While the DAMA standards fully define how terminals must communicate with each other through the satellite, the operator interface between the terminal and the user is not prescribed by the standards, and can be significantly more complex for DAMA operation than for fixed-assignment operation. The requirement that the DAMA control orderwire be encrypted adds another level of complexity to overall UHF system operation. In one of the modes within MIL-STD A [2], called the distributed control (DC) mode, the assignment of each time slot is fixed and the orderwire is not encrypted. A voice network is given a Circuit Identification Number (CIN) to identify its allocated time slot and the terminal operator only needs to command acquisition of the orderwire and enter the CIN to join the network. In the automatic control (AC) mode within MIL-STD A [2], which operates with demand assignments, joining a network is more involved. After acquiring the encrypted orderwire, the terminal must successfully transmit a return orderwire message to log in. The terminal operator must then request to join a network via the transmission of another return orderwire, and then wait for an orderwire message to direct the terminal to connect to the network. If the operator improperly sets the terminal s address, port configuration code, or network address, the connection will be refused via the orderwire and the terminal will display an error message. The increased complexity of AC-mode operation, as well as limited user knowledge of the benefits and tradeoffs inherent in both 5-kHz and 25-kHz DAMA operation, are two factors contributing to the hesitancy to move to DAMA. Currently twenty 25-kHz channels operate in the AC mode, however there is little or no demand assignment being used. Although DAMA is possible, time slots are still being preassigned to each network or communications requirement. Many time slots are assigned to Navy messaging systems that require communications availability 24 hour per day, 7 days per week. Other time slots need to be preassigned because the baseband equipments being used are not capable of generating a DAMA request or responding to an incoming call. For example, typical voice-network baseband equipment can alert an operator only via a speaker. Hearing a voice from the speaker is the only indication that a "call" is coming in. There are many baseband systems that operate over UHF MILSATCOM using protocols that are incompatible with DAMA operation. The Navy has redesigned some of its baseband systems to allow operation over DAMA, but many systems have requirements or funding problems that make it impossible to move them to DAMA operation. A major limitation to the use of TDMA is the Navy's use of single radios to operate in multiple networks. A single TDMA radio is used to simultaneously connect to as many as eight separate networks. To operate on eight networks with a single radio requires that no two time slots overlap in time. Since each such radio must, in general, operate on a different set of networks, careful planning is required when assigning networks to time slots. A problem that has kept many voice networks from transitioning to DAMA is user reluctance to accept the lower

3 3 of 5 voice quality provided by 2400-bps LPC-10 narrowband vocoders. Some users currently use CVSD vocoders operating at 16-kbps on dedicated 25-kHz channels. This data rate is too high to be used efficiently with TDMA on 25-kHz UHF channels. The higher quality of 16-kbps CVSD allows voice recognition and works well in the high background-noise environment found in a helicopter and in the battlefield. The 2400-bps LPC-10 works very poorly in a high background-noise environment. Mixed Excitation Linear Prediction (MELP), a newly developed 2400-bps vocoder algorithm, solves both of these deficiencies but will not be generally available for several more years. In addition, NATO is currently testing two other 2400-bps vocoder algorithms and it is possible that MELP may not become the NATO standard. Users accustomed to operating over a dedicated channel are hesitant to accept the extra setup time required for operation over a DAMA channel. Users want guaranteed and immediate service when operating under battlefield conditions. C. Future UHF Satellites The current UHF constellation will be complete after the launch of UFO-10 scheduled for late To prepare for the possible loss of one or more satellites during the planned lifetime of the UFO constellation, the Navy is in the process of procuring a single gapfiller satellite, UFO-11. The Mobile User Objective System (MUOS) is intended to provide the next generation of satellites. Major MUOS goals are greatly increased circuit capacity, increased maximum data rate, and a greatly improved ability to communicate with small terminals. The most promising approach to maintain compatibility with existing terminals, increase circuit capacity, and allow small terminal operation is through the use of a mixture of earth coverage and spot beams. The Joint UHF MILSATCOM TWG is working closely with the MUOS development team to insure that the new integrated waveform can take advantage of any new capabilities provided by the MUOS. III. INTEGRATED WAVEFORM The Joint UHF MILSATCOM TWG is developing a single integrated waveform having a new layered protocol to be incorporated in revisions to the existing standards. Interoperability requirements for participation in user services that use fixed-assignment time slots and channels will be specified in the lower-layer standard, MIL-STD B. This standard will define lower-layer protocols including interoperable modulation, error-correction coding, and channels. Interoperability requirements for participation in user services in DAMA modes, including required interoperability with legacy terminals in the DAMA modes, will be specified in the higher-layer standard, MIL-STD B. This standard will define demandassignment protocols and requirements for full voice and data interoperability. MILSATCOM user systems not requiring interoperable voice and data will be required to implement only the lower-layer protocols of MIL-STD B. The integrated waveform standards will accommodate backward compatibility with the present standards. The lower-layer multiple-access protocol will operate much like the 25-kHz TDMA DC mode, broadcasting relatively fixed (i.e., not demand-assigned) channel and time-slot assignments for all UHF MILSATCOM users. A Network Management System (NMS) will perform communications planning and management of these assignments. At this protocol layer terminals will have no ability to directly affect time-slot assignments. An example of a service that could operate in a fixed-assignment time slot or channel is a data messaging system that requires communications availability 24 hours per day, 7 days per week. Likewise, voice networks that cannot tolerate the setup delays imposed by the higherlayer demand-assignment protocols could operate in fixedassignment time slots or channels. It is anticipated that many systems that cannot use DAMA today will be able to operate using the new lower-layer multiple-access protocol. The higher-layer protocols for demand-assigned voice and data interoperability will be executed on top of the multipleaccess protocol layer, providing voice and data services similar to what is provided by commercial telephone and systems. The TWG has produced a limited-distribution working draft of MIL-STD B. As of this writing, the TWG has not come to a consensus on either the scope or contents of MIL-STD B. This paper describes the protocols defined in the draft MIL-STD B and contains general descriptions of the interoperable voice and data protocols that are under consideration for inclusion in MIL-STD B. The contents of both standards will be the subject of intense review by the TWG and representatives from industry. It is possible that the published standards will be different from what is presented here. A. MIL-STD B The lower-layer standard, MIL-STD B, will define the protocols and waveform parameters necessary to receive and interpret forward orderwire (FOW) messages from the NMS controller and to participate in fixed-assignment communications (i.e., networks) on both single-access and TDMA UHF channels. The lower-layer standard will also define orderwire decryption protocols and specify methods for both passive and active terminal ranging. The integrated waveform will use the same second frame duration being used by the existing MIL-STD A waveform. Frame timing for both the legacy MIL-STD A waveform and the integrated waveform will be synchronized to a global positioning system (GPS) time reference. The NMS controller will repetitively transmit FOW messages on a 25-kHz channel called the master channel. The NMS FOW will provide userservice announcements for all types of user services supported by UHF MILSATCOM. The NMS FOW will also provide a GPS frame timing reference and satellite ephemeris data to support terminal passive ranging, and announcements of the locations of time slots available to support active ranging. User-service announcements within the NMS FOW will identify for each UHF MILSATCOM user service, the home channel (i.e., UHF transmit and receive frequencies) and, if applicable, the TDMA time slot, where terminals can access that particular user service. The user-service announcements will also identify for each user service the particular satellite

4 4 of 5 over which that user service operates, and the modulation and coding parameters for that user service. The user-service announcement segment of the NMS FOW message can be lengthy, possibly requiring it to be split up over several frames. A list of recent changes to user services, including moved or terminated user services, will be broadcast at the beginning of every NMS FOW message. Terminals will be required to periodically monitor this update portion of NMS FOW messages. The update portion of the NMS FOW message may be broadcast in one or more additional time slots to make the information available for those terminals participating in user-service time slots that prevent reception of the main NMS FOW message (see Fig. 1). The NMS FOW burst is expected to have the format and fields identified in Table I. In order to achieve simple UHF terminal operation, the standard will require a terminal to search over a set of userspecified UHF receive channels for the NMS FOW message. The terminal's operator interface will allow advance entry (i.e., prior to UHF SATCOM operation) of the ordered list of these UHF frequencies. Since some airborne and handheld terminals will not be capable of receiving the standard NMS FOW message, it is expected that a lower burst-rate version of the NMS FOW message will also be broadcast. When a terminal is not carrying the current version of the key required for FOW decryption, the terminal will automatically request the current key using a public key protocol. When a terminal has been properly set up, connecting to a user service or network should require no operator actions other than pointing of the antenna and selection of the satellite mode of operation (i.e., selection of the user service). B. MIL-STD B The higher-layer standard, MIL-STD B, will define the protocols and waveform parameters necessary to provide for interoperable voice and data communication between users of UHF MILSATCOM requiring that capability. Placing the terminal into the interoperable voice and data mode will cause the terminal to automatically acquire orderwires on the interoperable voice and data control channel. The interoperable voice and data control channel can be located using the user-service announcement within the lower-layer NMS FOW message. An entity called a Service Controller (SC) will control communications within a demand-assigned user service such as the interoperable voice and data service. Protocols for voice operation will provide for both point-to-point and group call setups along with the ability to join fixed-assignment services (networks). The TWG's selection of data service types to include within the standard will be difficult. There is an almost infinite variety of data protocols from which to choose. Even a capability as common as circuit service has many possible variations. Legacy CCOW Begin frame n Begin frame n+1 NMS FOW Burst NMS Update Legacy CCOW Fig. 1. Typical frame format on master channel. NMS FOW Burst NMS Update TABLE I. NMS FOW BURST FORMAT AND FIELDS FIELD BITS DESCRIPTION Preamble 250 Redundant synchronization preamble Burst Type 10 Burst type indicator Frame Count 21 Frame count of the current frame NMS Controller ID 32 Unique address for NMS controller KG Net Number 5 KG Day 3 Controller Standard Version of NMS Controller Version 3 0 = MIL-STD B 1-7 = reserved Satellite ID 6 ID uniquely identifying satellite being used for this NMS FOW Cycle Length 3 Number of frames per cycle Cycle Value 3 Cycle value for this burst Range Update 1 Indicates change in range slot locations (value=1 only if previously designated range slot(s) are no longer valid for Ranging) Update Length 8 Number of services with changes Update Data 16-bit service numbers with changes Update CRC bit CRC for error checking GPS Alignment 7 Indicates NMS FOW Alignment with GPS time Number of Satellites 4 Number of satellites for which ephemeris data is provided Satellite Ephemeris Data Satellite ephemeris data RBN 4 Ranging Backoff Number No. of Range Channels 8 Number of UHF channels with range slots Range Slots Range-slot announcements for each channel Range CRC bit CRC for error checking Service Size 8 Number of UHF channels with user services Services Data Service announcements for each channel Service CRC bit CRC for error checking For interoperable voice operation the standard will fully define all the usual signaling and supervision tones and messages that a user is accustomed to when placing a commercial telephone call. Operation will be modeled after a commercial cellular phone system, providing a familiar, easyto-use interface. Additional signaling and supervision tones and messages will be required to provide for priority and preemption capabilities not normally provided by commercial systems. As in the case of commercial phone systems, the SC will be responsible for most of the signaling and supervision protocols. The terminal will be responsible for generating required signaling, usually in direct response to an operator action, and for producing the supervision tones and messages as directed by the SC. Complex functions such as Call Waiting will require no special logic within a terminal. The signaling and supervision protocol will be operable inband (communicated over the same time slot used for voice data) making it possible to interface to other secure voice systems.

5 5 of 5 Interoperable voice will use its own addressing plan, modeled after that used in commercial cellular phone systems, rather than the addressing plans in the existing UHF DAMA standards. A telephone number will be associated with each voice port that a terminal provides. Multi-port terminals may have multiple phone numbers and it is the terminal s responsibility to maintain the mapping between each voice port and the assigned telephone number. Telephone numbers will be associated with the user, not the physical terminal, and will be entered into the terminal as part of the initial terminal setup. The numbering plan will include the ability to establish group calls and join networks operating within fixed-assignment time slots. A telephone number can contain as many as seven digits and will be reported to the SC as part of the log-in protocol. Use of a standard numbering system will simplify interfacing with other secure voice systems. For interoperable data operation, the standard will define both circuit (i.e., stream traffic) and message (i.e., datagram) services. It is expected that a connectionless message service will be defined, using commercial Internet protocols, allowing datagrams to be sent to users at UHF terminals that are not necessarily operating (or logged in) at the time the messages are sent. It is expected that a separate message delivery protocol will be specified capable of multicast message delivery to a group of UHF terminals operating in a network. Connection-oriented data services will be established using standard voice-service call-setup protocols, and users will be able to switch between voice and data without reestablishing a call. To improve interoperability with external systems, the standard will provide optional requirements for more fully defining baseband serial port characteristics. Terminals implementing this option will be required to have a specified set of port clocking options, have the ability to operate a port in both full- and half-duplex baseband modes, and provide discrete control signals to indicate the presence of data and indicate the start of the transmit time slot. A capability to access data using standard Web browsing protocols will be provided to support data pull. Since newer data protocols are generally designed to move data asynchronously within Internet Protocol (IP) datagrams, protocols that support the efficient handling of IP datagrams will be developed to operate over the lower-layer multipleaccess protocols. A paper [4] presented at MILCOM'98 describes a proposed packet transfer mechanism, called the Variable Rate Data Packet (VRDP) transfer protocol, which could be used for this purpose. A revised version of MIL-STD [5] will be used to provide error-free transfers of the contents of the IP datagrams. IV. CONCLUSION It is anticipated that the increased circuit capacity, higher data rates, and improved quality of service that will become available with the launch of the next generation of satellites, along with the introduction of the integrated DAMA waveform, will improve user acceptance of UHF DAMA. The layered protocol design of the integrated DAMA waveform will allow the use of commercial communication protocols over UHF satellite channels, and will provide simplified terminal operation and improved communication services similar to those provided by cellular phone systems and commercial systems. REFERENCES [1] MIL-STD A, Department of Defense Interface Standard, "Interoperability Standard for 5-kHz UHF DAMA Terminal Waveform" 31 March 1997, U.S. DoD. [2] MIL-STD A, Department of Defense Interface Standard, "Interoperability Standard for 25-kHz TDMA/DAMA Terminal Waveform (Including 5-kHz and 25-kHz Slave Channels)," 20 March 1998, U.S. DoD. [3] MIL-STD B, Department of Defense Interface Standard, "Interoperability Standard for Single-Access 5-kHz and 25-kHz UHF Satellite Communications Channels," 20 March 1999, U.S. DoD. [4] G. Huckell, G. Clinesmith, S. Graser, E. Chandler, "Integration of IP-packet data transfers within UHF DAMA (unclassified), MILCOM'98 Proceedings, Classified Vol., pp , Nov [5] MIL-STD , Department of Defense Interface Standard, "Interoperability and Performance Standard for the Data Control Waveform," 20 Aug 1993, U.S. DoD.