ATM WITHOUT STRINGS: An Overview of Wireless ATM A.S. Krishnakumar AT&T Bell Laboratories Nieuwegein, The Netherlands e-mail:
[email protected]
Abstract
of-Service(QoS) perspectives respectively. Interfacing the wired and wireless ATM networks is also considered in Section 4. Providing mobility management in the presence of ATM virtual circuits (VCs) and virtual Paths (VPs) is brie y discussed in Section 5. We conclude with some remarks in Section 6.
A majority of applications in the near-future are expected to be multimedia-capable. One of the enabling technologies for this to happen is Asynchronous Transfer Mode (ATM) networking. In addition, the exploding use of mobile terminals | laptop computers, personal digital assistants (PDAs) etc. | means there will be more demand to extend these multimedia-capable applications to the mobile terminal. This requires the extension of ATM technology to high-speed wireless communications (> 20 Mbps). This is an active area of research around the world. This talk will describe the technical challenges posed by wireless ATM networking and give an overview of wireless ATM e orts under way, focusing on speci c research projects in the USA and Europe.
2 ATM and the Wireless Environment
ATM(Asynchronous Transfer Mode) combines the highspeed switching of voice-oriented circuit-switched systems and the exibility of (primarily data-oriented) packetswitched systems in carrying di erent types of bit streams, e.g. data, voice, and video. While ATM was developed originally for wide area networks (WANs), its use in local area networks (LANs) is being de ned by the ATM-forum[1]. In an ATM network - consisting of high-speed switches and terminal nodes connected by point-to-point links - all data transfers occur using the basic unit of an ATM cell. Each ATM cell is 53 bytes long, with 5 bytes of header information and 48 bytes of payload. Before the exchange of cells, communicating terminals have to set up a connection (i.e. a VC). The link rate between the terminals and the switch is typically 155 Mbps while 25 Mbps is also encountered. Thus, an ATM network transfers xed-length cells at high speed across point-to-point links. It should be noted that these links are highly reliable with very low bit-error probabilities. The use of mobile terminals and wireless LANs is increasing rapidly. While they are useful in themselves, their utility is enhanced when they connect to other networks predominantly wired. Further, it is very desirable to have the increasing set of multimedia applications run on terminals connected to any network - wired or wireless. Since ATM is likely to be widely used in the wired networks and can carry multimedia trac eciently, it is advantageous to extend ATM networking to the wireless environment. There are many obstacles to achieving this extension. In contrast to the wired network, current wireless networks are slow (typically 2 Mbps) and unreliable (high bit-error rates). Further, the shared nature of the medium and the need to manage movement across cells serviced by di er-
1 Introduction
A majority of applications in the near-future are expected to be multimedia-capable. One of the enabling technologies for this to happen is Asynchronous Transfer Mode (ATM) networking. While these applications will be primarily intended for wired networks, the exploding use of mobile terminals | laptop computers, personal digital assistants (PDAs) etc. | means there will be more demand to extend these multimedia-capable applications to the mobile terminal. This requires the extension of ATM technology to wireless data communications. This motivates work in high-speed 1 wireless data communication techniques - both transmission and networking. This is an active area of research around the world. In this paper, we will describe the technical challenges posed by wireless ATM networking and some proposed solutions. We will also give an overview of wireless ATM e orts under way, focusing on speci c research projects in the USA and Europe. The rest of the paper is organized as follows: Section 2 provides a brief description of ATM networking and describes the challenges posed by the extension of ATM networking to the wireless environment. Section 3 and Section 4 consider in more detail these challenges and proposed solutions from transmission and MAC/Quality1 By high-speed we refer to raw transmission rates of at least 20 Mbps.
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ent base stations result in lowered performance compared to a wired equivalent. Another potential source of ineciency is that of the overhead needed for wireless operation. This takes the form of equalizer training, antenna adaptation, or channel estimation depending on the type of radio transceiver used2 . If single ATM cells are carried over the air, it is possible for the payload to be less than 50% in some cases. It is clear that link speed and reliability of wireless networks will continue to lag those of wired networks for the foreseeable future. Given this, the approach to provide ATM service in wireless networks should aim towards a) increasing the raw transmission rate over the wireless medium, and b) providing appropriate procedure at the wired/wireless network interface to ensure seamless internetworking. A typical wireless ATM scenario is show in Figure 1. We will consider these issues in more detail in the following sections.
at 2 bits/symbol the signaling rate is 12.5 Msymbols/sec. At this rate, 200 ns translates to 2.5 symbols. The intersymbol interference gets worse with higher symbol rates. There are many di erent modulation schemes that can be used to achieve high data rates in a multipath, fading environment. They can be broadly classi ed as broadband and narrowband3. Narrowband systems can further be divided into single-carrier and multi-carrier systems. Spread spectrum modulation schemes are broadband while a single-carrier quaternary phase shift keying (QPSK) scheme is narrowband. An example of a multicarrier narrowband scheme is Orthogonal Frequency Division Multiplexing (OFDM)[3]. It is to be emphasized that with all these schemes good forward error-correction(FEC) coding is necessary to provide acceptable bit-error rate for ATM applications. We will not discuss FEC further in this paper. The di erent classes of systems are considered next.
3.1 Broadband Systems
Under this heading, we consider Spread Spectrum(SS) systems. These systems come in two major avors - direct sequence and frequency hopped. For an expanded discussion of spread spectrum techniques, see [4]. These techniques are robust against fading and interference. A RAKE receiver can be employed to deal with multipath propagation. A major limitation of these systems is the fact that the maximum link rate is limited to a small percentage of the rate achievable with a narrowband system using the same bandwidth. Thus, to achieve the high speeds contemplated for wireless ATM, impractically wide channels will be needed. For example, a 25 Mbps system with a modest chip rate of 64 will need approximately 1.5 GHz! Further, this complicates the front-end receiver processing that is done at the chip rate. However, this problem can be tackled to a certain Figure 1: A typical wireless ATM Network Scenario extent by the use of multiple spreading codes per user. More work is needed to determine the practical usefulness of this technique. Power control to eliminate near-far e ect and minimizing power consumption at the mobile transceiver The focus of e orts at the physical layer is to provide a are important issues to be considered. high link speed (> 20 Mbps) with a low bit-error probability ( 10?8). The radio channel is a fading environment and outage speci cations necessitate a fading margin. Usu- 3.2 Narrowband Systems ally, there is also a regulatory constraint on the maximum As mentioned earlier, narrowband systems can modulate transmitted power. Thus, the numbers of bits/symbol is information on a single carrier or on multiple carriers. Typtypically limited to about 2. At the frequency bands of in- ically, mPSK, QAM, or (G)MSK modulation schemes are terest - mostly around 5.2 GHz with 17 GHz and 60 GHz employed. If omnidirectional antennas are used, some form being future options - there is signi cant multipath propa- of equalization is unavoidable. This increases the complexgation in most operating environments. For example, typ- ity and power consumption of the receiver. It also adds ical oce environments show an rms delay spread in the to the overhead per transmitted packet as training symbols range of 100-200 nsec[2]. As we are limited in the number have to be sent with each packet. While this overhead is of bits conveyed per symbol, the symbol rate has to be quite not a big problem in systems with a large maximum packet high to achieve high link speeds. This leads to considerable length, it is of concern when short, xed-length ATM cells intersymbol interference. For example, to achieve 25 Mbps 3 ATM NETWORK
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3 The Physical Layer
Note that broadbandand narrowband refer to the bandwidth used We consider only radio systems in this paper. While infra-red relative to the data rate. Thus, a 1 Mbps system using 11 MHz transceivers have been used for wireless networks, most of the pro- bandwidth will be called broadband while a 25 Mbps system occupying 12.5 MHz is narrowband. posed wireless ATM networks are based on radio transceivers. 2
have to be transported. It is possible to avoid equalization by the use of directional antennas but some overhead for locking on to and tracking the signal is unavoidable. The trade-o s among these choices have to be carefully studied before a choice can be made[5]. The choice of the operating band plays a role too. At higher frequencies such as 60 GHz, the propagation loss is so high that re ected signals do not pose a signi cant problem. On the other hand, this implies that we have essentially line-of-sight operation and rather short range. At frequencies around 5.2 GHz, re ections are strong enough that measures such as equalization are needed but a larger range can be obtained without lineof-sight restriction. We consider examples of single carrier and multicarrier systems next.
3.2.1 Single-Carrier Systems In Europe, a high-speed system operating in the 5.2 GHz band is nearing standardization under the auspices of ETSI. This is the HiperLAN system with a link speed of approximately 20 Mbps. Some details on HiperLAN can be found in Kruys[6]. Though the system is not originally intended as a wireless ATM system, its physical layer is still of interest. While the standard is still in draft, the PHY working committee has settled upon equalized GMSK as the modulation technique for HiperLAN. This implies a signi cant equalization overhead that will have to be considered in ATM operation. Power consumption is another concern. On the positive side, GMSK is a constant envelope modulation scheme and does not require a linear ampli er. This leads to a highly ecient power ampli er operation. The use of more spectrally ecient =4-QPSK leads to lower eciency of the power ampli er due to a higher peak-toaverage power ratio. An extreme version of this problem is found in multicarrier systems and will be discussed in the next section. A research prototype of a 20 Mbps modem is discussed in [7]. This design uses =4-QPSK and an equalizer with programmable taps. Due to the exible nature of the design, this system can be used to implement the modulation scheme of HiperLAN also.
3.2.2 Multicarrier Systems Multicarrier systems are parallel transmission schemes. A prime example is Orthogonal Frequency Division Multiplexing (OFDM) where the available band is divided into a number of channels each of which is modulated using e.g. =4-QPSK but at a much lower symbol rate than a single-carrier system using the same bandwidth. By adding a small guard interval, intersymbol interference problems can be completely eliminated with this scheme. In addition, frequency-selective fades can be countered by avoiding channels that fall in the notch and using only the others or by coding across the channels. When a feedback channel is available to inform the transmitter of channel conditions, it
is possible to adapt the bits/symbol in each channel independently to optimize the data transfer. However, in order to preserve orthogonality at the receiver, it is necessary to correct for the channel distortion. This requires some overhead in training symbols but not quite as much as for an equalizer. An OFDM system uses less bandwidth than an equivalent multitone system as the subchannels are overlapped. In order to recover the information it is necessary to use orthogonal signals in the subchannels. An ecient way to generate and decode these signal using the Fast Fourier Transform (FFT) was presented in [3]. These schemes are
exible and can be adapted for di erent data rates and channel conditions. For example, to achieve higher data rates, one could simply increase the number of subchannels or increase the bits/symbol per channel as appropriate. Of course, this increases the Peak-to-Average Power(PAP) ratio and the size of the FFT to be performed. As with single-carrier, equalized systems, VLSI implementation complexity and power consumption is a concern. However, Daneshrad[7] estimates that the complexity of the two systems are comparable. A more serious concern is the very high PAP ratio of OFDM schemes and the consequent need for linear power ampli ers. With such high PAP ratios (e.g. 12dB for a 16-channel system and increasing by 3dB for each doubling of the number of subchannels) the ampli er eciency is very low leading to high power consumption. This is a serious concern for mobile terminals. The reduction of PAP ratio of multicarrier signals has received considerable attention [8, 9]. A technique of particular interest is the use of complementary sequences for PAP ratio reduction, proposed by Popovic[10]. While this work relates to radar signals, the use of the technique for data transmission was proposed by Wilkinson and Jones[11]. While complementary sequences achieve PAP ratio reduction, to be really useful in data transmission an error-correction scheme has to be developed to make use of the redundant information in these sequences. Any such code is likely to be non-linear and hence the traditional techniques of errorcorrection coding(linear) are not immediately applicable. Another example of a code for PAP ratio reduction may be found in Kamerman and Krishnakumar[12]. This remains an interesting area of research.
4 Internetworking
Although wireless networks can be used in stand-alone mode as replacements for wired LANs, their utility is greatly enhanced if they interoperate with other, mostly wired, networks. This is especially true in situations where multimedia applications are present. One is likely to engage in a videoconference when the other party or parties are in remote locations thus necessitating the use of a wide area network. To extend the multimedia applications to the mobile terminal, it becomes necessary to provide seamless internetworking between the wired and wireless networks. We will consider this from the perspective of ATM networks.
There are two types of ATM cell transport over the wireless link - encapsulated and native mode. In encapsulated ATM, the ATM cells are carried as the payload of a di erent wireless link protocol { typically a CSMA-variant. In the native mode, ATM cell is the basic transport unit (except for the radio link overhead). At rst glance, it would appear that the native mode avoids likely ineciencies due to protocol mismatch that may be present in the encapsulated mode. Indeed, encapsulated mode operation has many disadvantages - protocol conversion at every wireless/wired network interface, non-transparency to ATM protocols, lack of support for QoS guarantees, link turnaround times comparable to cell duration (even at 25 Mbps), to name a few. However, transporting single ATM cells in the native mode has its drawbacks too. The radio overhead can be so high that the payload is below 50%. Further, the need for resource sharing and mobility management capabilities (not part of traditional wired ATM protocols) dictates that native mode operation has to be designed carefully for the wireless environment. Block diagrams of systems operating in the encapsulated and native modes are shown in Figures 3 and 2 respectively. A MAC protocol(WASP-MAC) for native mode wireless ATM operation has been developed for the wireless Access System of the Platinum project [13]. In this system, the wireless Access Point(AP) services the mobile terminals in a sequence best suited to their negotiated trac and QoS needs (constant bit rate, variable bit rate, available bit rate, cell loss constraints etc.). Further, the AP allows the number of cells per packet to be larger than one for better eciency. The architecture for this system is shown in Figure 4. A similar MAC protocol, designed for ecient operation in the presence of diverse multimedia trac with di erent QoS requirements, is the DQ-RUMA protocol described by Karol et al. in [14]. This is used in the broadband wireless ATM LAN system BAHAMA, developed at AT&T Bell Laboratories, that is described in [15]. Other examples of wireless ATM systems, operating at link speeds less than or equal to 10 Mbps, are the RATM system developed at Olivetti Research Labs[16], the NEC wireless ATM system[17], and the SWAN system, developed at AT&T Bell Laboratories[18]. These systems transport single ATM cells over the air. Since they operate at relatively low link speeds, the overhead problem is not as stringent as it is at higher link speeds. It is also interesting to note that while the rst two systems aim at native mode ATM applications, SWAN considers tcp/ip over ATM also.
possible. The wireless environment introduces a new factor | mobility. During the lifetime of a VC, a terminal may move through the coverage area of several base stations | in fact, it may move from one cell to another between connection request and establishment. There is also the issue of locating the terminal when a connection request comes in for that terminal. Clearly, current ATM protocols need to be adapted for use in a heterogeneous environment. Researchers have started to address these issues but much work remains to be done. Most wireless LANs operate in a connectionless mode. In order to provide ATM service in such LANs, it is necessary to provide support for connection-oriented transport. This is further complicated by the fact that mobility has to be taken into account. Some approaches to provide connection-oriented transport services in a mobile environment have been published. Biswas and Hopper[19] introduce the concept of a software agent known as mobile representative to insulate xed network entities from the e ects of terminal mobility. A similar approach, but using home base stations, was described by Veeraraghavan et al. in [20]. Keeton et al.[21] describe several alternative schemes for maintaining VCs as the terminal moves through a number of cells. They also provide a simpli ed analytical comparison of the proposed schemes. Discussion of wireless ATM issues may also be found in [22]. A good summary of network management and control issues arising from the interfacing of wireless and wired ATM networks is given by Schwartz[23]. To summarize, the main issues that arise when considering ATM service in the presence of mobility are: mobile location for VC establishment, VC establishment in the presence of hando s, maintaining VCs through several hando s, network resources (link capacity, node bu ers etc.) required by alternative schemes, and impact of hando s (soft or hard) on QoS guarantees. Some approaches have been suggested in the literature cited earlier but this remains an active area of research.
6 Conclusions
In this paper, we have given a brief overview of wireless ATM system considerations. It can be seen that the wireless medium has fundamental di erences from the wired environment { signi cantly higher bit error rate, shared medium access, and mobility, to name a few { and that it is not sucient to simply transport ATM cells over the air. We have attempted to give a taste of the interesting interplay of design issues at di erent layers and their e ect on system design. It is impossible to give a thorough coverage of all the issues in this short paper and the reader is directed to the references for further information. Many ATM was developed in the wired network environment. In questions remain to be answered and should provide interthis environment, terminals are connected to switches by esting topics for research. a high-speed, low bit-error rate link. The terminals are stationary, at least for the duration of a VC's existence. All these implicit assumptions are re ected in the standards I would like to thank Geert Awater and Jan Kruys for allowdeveloped for ATM | indeed they are exploited wherever ing me to use material on ATM internetworking from their
5 Mobility
Acknowledgements
unpublished manuscript in Section 4 and Jo~ao Sobrinho for [13] G.Awater, R. van Nee, and J.F. Whitehead, Wireless discussions on mobility management. Access System Protocols Speci cations, Project Platinum Deliverable D1.3.2/01, December 1995. [14] M. J. Karol, Z. Liu, and K.Y. Eng, An ecient
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Figure 2: Native Mode Wireless ATM Operation
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Figure 3: Encapsulated Mode Wireless ATM Operation
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Figure 4: Wired/Wireless ATM internetworking architecture
