Data Link Control

Abstract: Describes data link control procedures for wireless ATM access channels based on a dynamic TDMA/TDD frame-work. The system provides integrated ATM services including available bit-rate (ABR) data and constant/variable bit-rate (CBR/VBR) voice or video through the addition of wireless-specific medium access control (MAC) and data link control (DLC) protocol layers between the physical and ATM network layers. The purpose of the DLC protocol layer is to insulate the ATM network layer from wireless channel impairments by selective retransmission of erroneous or lost cells before they are released to the ATM layer. The DLC methods described exploit the on-demand ABR burst transmission capability of the dynamic TDMA channel to retransmit unacknowledged cells in available slots not allocated to service data. Specific error recovery procedures are outlined for both (asynchronous) ABR and (isochronous) CBR services. For ABR, the DLC operation follows a group ACK/NACK based selective reject (SREJ) procedure on a burst-burst basis, without time limits for completion. For CBR, the retransmission procedure is constrained to complete within a specified time interval, so that isochronous delivery of cells to the ATM layer can be maintained. The proposed protocols have been validated using a software emulator which incorporates a choice of radio channel models, dynamic TDMA/TDD MAC, and ABR or CBR DLC. Numerical results show that the data link control protocols under consideration can significantly improve wireless ATM service quality over impaired radio channels for both packer data ABR and stream type CBR virtual circuits.

Abstract: A prototype microcellular wireless asynchronous transfer mode network (WATMnet) capable of providing integrated multimedia communication services to mobile terminals is described in this paper. The experimental system's hardware consists of laptop computers (NEC Versa-M) with WATMnet interface cards, multiple VME/i960 processor-based WATMnet base stations, and a mobility-enhanced local-area ATM switch. The prototype wireless network interface cards operate at peak bit-rates up to 8 Mb/s, using low-power 2.4 GHz industrial, scientific, and medical (ISM)-band modems. Wireless network protocols at the portable terminal and base station interfaces support available bit rate (ABR), variable bit rate (VBR), and constant bit rate (CBR) transport services compatible with ATM using a dynamic time-division multiple-access/time-division duplex (TDMA/TDD) MAC protocol for channel sharing and data link control (DLC) protocol for error recovery. A custom wireless control protocol is also implemented between the portable and base units for support of radio link related functions such as user registration and handoff. All network entities including the portable, base and switch use a mobility-enhanced version of ATM ("Q.2931+") signaling for switched virtual circuit (SVC) connection control functions, including handoff. In the first stage of the prototype, the application-level API is TCP/UP over ATM ABR service class using AAL5. Early experiments with the WATMnet prototype have been conducted to validate major protocol and software aspects, including DLC, wireless control, and mobility signaling for handoff, Selected network-based multimedia/video applications requiring moderate bit-rates (/spl sim/0.5-1 Mb/s) in the ABR mode have been successfully demonstrated on the laptop PC.

Abstract: Acoustic modems typically operate in half-duplex, which limits the choice of a data link control protocol to the Stop and Wait (S&W) type. Unfortunately, on channels with poor quality and long propagation delay-such as the majority of acoustic channels-S&W protocol has low throughput efficiency. The basic S&W can be improved by using a modification in which packets are transmitted in groups and acknowledged selectively. Throughput efficiency can now be maximized by selecting the optimal packet size, which is a function of range, rate, and error probability. Quantitative analysis for typical acoustic links shows that modified S&W protocols offer good performance, provided that packet size is chosen close to optimal. In addition, as the group size increases, sensitivity to packet size selection is reduced. To ensure best ARQ performance in mobile acoustic systems where link conditions vary with time, future generation of acoustic modems must focus on adaptive selection of protocol parameters.

Abstract: The concept of "wireless ATM", first proposed in 1992, is now being actively considered as a potential framework for next-generation wireless communication networks capable of supporting integrated, quality-of-service (QoS) based multimedia services. We outline the technological rationale for wireless ATM, present a system-level reference architecture, discuss key subsystem design issues, and summarize early prototyping results for a proof-of-concept system called "WATMnet". The reference architecture for wireless ATM consists of two major components: (a) a "radio access layer" for extension of ATM services over a wireless medium and (b) a "mobile ATM" infrastructure network capable of supporting terminal migration. Design considerations for both the radio access layer (e.g. physical layer, medium access control and data link control) and mobile ATM (e.g. handoff control, location management and routing/QoS control) are discussed, and key technical issues are identified in each case. An overview of experiences with the "WATMnet" system prototype developed at NEC USA's C&C Research Laboratories is given in conclusion.