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					     COMMUNICATIONS



                                                                                                  and wireless networks. Figure 1 shows


     Issues in                                                                                    three possible architectures: using a mul-
                                                                                                  timode device, an overlay network, or a
                                                                                                  common access protocol.



     Emerging 4G                                                                                  Multimode devices
                                                                                                     One configuration uses a single physical
                                                                                                  terminal with multiple interfaces to access
                                                                                                  services on different wireless networks.


     Wireless Networks                                                                            Early examples of this architecture include
                                                                                                  the existing Advanced Mobile Phone
                                                                                                  System/Code Division Multiple Access
                                                                                                  dual-function cell phone, Iridium’s dual-
     Upkar Varshney and Radhika Jain, Georgia State University                                    function satellite-cell phone, and the
                                                                                                  emerging Global System for Mobile
                                                                                                  telecommunications/Digital Enhanced
                                                                                                  Cordless Terminal dual-mode cordless




     D
               eployment of International Mo-       ure or loss of one or more networks. 4G       phone.
               bile Telephony 2000 standards        networks will also feature IP interoper-         The multimode device architecture
               for third-generation wireless        ability for seamless mobile Internet access   may improve call completion and expand
               networks may begin this year,        and bit rates of 50 Mbps or more.             effective coverage area. It should also
               with NTT DoCoMo planning                Because deployment of 4G wireless          provide reliable wireless coverage in case
     to have a nationwide 3G network in             technology is not expected until 2006 or      of network, link, or switch failure. The
     Japan this year. These specifications give      even later, developers will hopefully have    user, device, or network can initiate
     existing 1G and 2G operators the flexi-         time to resolve issues involving multiple     handoff between networks. The device
     bility to evolve their networks, which         heterogeneous networks such as                itself incorporates most of the additional
     support low-bit-rate data and are pri-
     marily designed for voice, into 3G sys-
     tems. They also help satellite and terres-
     trial providers design new 3G systems.
                                                                                 4G wireless networks
        Third-generation networks offer mul-                                     support global roaming
     timedia transmission, global roaming
     across a cellular or other single type of                                   across multiple wireless
     wireless network, and bit rates ranging
                                                                                 and mobile networks.
     from 384 Kbps to several Mbps. Analysts
     expect worldwide migration to 3G to
     continue through 2005, depending on
     market needs, carrier and operator incen-        •   access,                                 complexity without requiring wireless
     tives, recovery on investments in existing       •   handoff,                                network modification or employing
     1G and 2G wireless systems, and per-             •   location coordination,                  interworking devices. Each network can
     ceived threats to monopolistic wireless          •   resource coordination to add new        deploy a database that keeps track of
     carriers in many countries.                          users,                                  user location, device capabilities, net-
                                                      •   support for multicasting,               work conditions, and user preferences.
     4G WIRELESS NETWORKS                             •   support for quality of service,         The handling of quality-of-service (QoS)
         Meanwhile, researchers and vendors           •   wireless security and authentication,   issues remains an open research question.
     are expressing a growing interest in 4G          •   network failure and backup, and
     wireless networks that support global            •   pricing and billing.                    Overlay network
     roaming across multiple wireless and                                                           In this architecture, a user accesses an
     mobile networks—for example, from a            Network architectures will play a key role    overlay network consisting of several
     cellular network to a satellite-based net-     in implementing the features required to      universal access points. These UAPs in
     work to a high-bandwidth wireless LAN.         address these issues.                         turn select a wireless network based on
        With this feature, users will have access                                                 availability, QoS specifications, and user-
     to different services, increased coverage,     POSSIBLE ARCHITECTURES                        defined choices. A UAP performs proto-
     the convenience of a single device, one bill     One of the most challenging problems        col and frequency translation, content
     with reduced total access cost, and more       facing deployment of 4G technology is         adaptation, and QoS negotiation-rene-
     reliable wireless access even with the fail-   how to access several different mobile        gotiation on behalf of users. The overlay

94                Computer
network, rather than the user or device,
performs handoffs as the user moves
from one UAP to another. A UAP stores             Fixed wireless network             Satellite network
user, network, and device information,
capabilities, and preferences. Because
UAPs can keep track of the various
resources a caller uses, this architecture
supports single billing and subscription.
                                                                                        Handoff

Common access protocol
   This protocol becomes viable if wireless
networks can support one or two standard
access protocols. One possible solution,
which will require interworking between
                                                  Wireless LAN                       Cellular network
different networks, uses wireless asyn-
chronous transfer mode. To implement
wireless ATM, every wireless network              (a)
must allow transmission of ATM cells with
additional headers or wireless ATM cells
requiring changes in the wireless networks.       Fixed wireless network             Satellite network

One or more types of satellite-based net-
works might use one protocol while one
or more terrestrial wireless networks use
another protocol.
                                                                                     Overlay network
QUALITY OF SERVICE
   Supporting QoS in 4G networks will
be a major challenge due to varying bit
rates, channel characteristics, bandwidth
allocation, fault-tolerance levels, and
handoff support among heterogeneous               Wireless LAN                       Cellular network
wireless networks. QoS support can
occur at the packet, transaction, circuit,        (b)
user, and network levels.

  • Packet-level QoS applies to jitter,           Fixed wireless network             Satellite network
    throughput, and error rate. Net-
    work resources such as buffer space
    and access protocol are likely influ-
    ences.                                                                 Tunable
                                                                           adapter


Figure 1. Possible 4G wireless network archi-
tectures. (a) A multimode device lets the user,
device, or network initiate handoff between
networks without the need for network modifi-
cation or interworking devices. (b) An overlay    Wireless LAN                       Cellular network
network—consisting of several universal
access points (UAPs) that store user, net-        (c)
work, and device information—performs a                                     Database
handoff as the user moves from one UAP to                                   Interface
another. (c) A device capable of automatically                              Location management
switching between networks is possible if
                                                                            Universal access point
wireless networks can support a common
                                                                            Wireless link
protocol to access a satellite-based network
and another protocol for terrestrial networks.

                                                                                            June 2001    95
                                                                    Communications
     How to Reach
     Computer
     Writers
     We welcome submissions. For                                      • Transaction-level QoS describes both      networks instead of relying on each net-
     detailed information, write for a                                  the time it takes to complete a trans-    work’s QoS scheme requires study.
     Contributors’ Guide (computer@                                     action and the packet loss rate.
     computer.org) or visit our Web site:                               Certain transactions may be time-         Handoff delay
     http://computer.org/computer/.                                     sensitive, while others cannot toler-        Handoff delay poses another impor-
                                                                        ate any packet loss.                      tant QoS-related issue in 4G wireless
     News Ideas                                                       • Circuit-level QoS includes call block-    networks. Although likely to be smaller
     Contact Lee Garber at lgarber@                                     ing for new as well as existing calls.    in intranetwork handoffs, the delay can
     computer.org with ideas for news                                   It depends primarily on a network’s       be problematic in internetwork hand-
     features or news briefs.                                           ability to establish and maintain the     offs because of authentication proce-
                                                                        end-to-end circuit. Call routing and      dures that require message exchange,
     Products and Books                                                 location management are two impor-        multiple-database accesses, and negoti-
     Send product announcements to                                      tant circuit-level attributes.            ation-renegotiation due to a significant
     products@computer.org. Contact                                   • User-level QoS depends on user            difference between needed and available
     Stephanie Kawada at skawada@                                       mobility and application type. The        QoS.
     computer.org with book announce-                                   new location may not support the             During the handoff process, the user
     ments.                                                             minimum QoS needed, even with             may experience a significant drop in QoS
                                                                        adaptive applications.                    that will affect the performance of both
     Letters to the Editor                                                                                        upper-layer protocols and applications.
     Please provide an e-mail address or                                                                          Deploying a priority-based algorithm
     daytime phone number with your                                    Deploying a global QoS                     and using location-aware adaptive appli-
     letter. Send letters to Letters,                                                                             cations can reduce both handoff delay
     Computer, 10662 Los Vaqueros
                                                                      scheme may support the
                                                                                                                  and QoS variability.
     Cir., PO Box 3014, Los Alamitos,                                 diverse requirements of
                                                                                                                     When there is a potential for consid-
     CA 90720-1314; fax +1 714 821                                      users with different                      erable variation between senders’ and
     4010; computer@computer.org.                                        mobility patterns.                       receivers’ device capabilities, deploying
                                                                                                                  a receiver-specific filter in part of the net-
     On the Web                                                                                                   work close to the source can effectively
     Visit http://computer.org for infor-                             In a complete wireless solution, the        reduce the amount of traffic and pro-
     mation about joining and getting                               end-to-end communication between two          cessing, perhaps satisfying other users’
     involved with the Society and                                  users will likely involve multiple wireless   QoS needs.
     Computer.
                                                                    networks. Because QoS will vary across
                                                                    different networks, the QoS for such                lthough 4G wireless technology
     Magazine Change of Address
     Send change-of-address requests
     for magazine subscriptions to
     address.change@ieee.org. Make
                                                                    users will likely be the minimum level
                                                                    these networks support.                       A     offers higher bit rates and the abil-
                                                                                                                        ity to roam across multiple hetero-
                                                                                                                  geneous wireless networks, several issues
                                                                    End-to-End QoS                                require further research and development.
     sure to specify Computer.
                                                                       Developers need to do much more            It is not clear if existing 1G and 2G
                                                                    work to address end-to-end QoS. They          providers would upgrade to 3G or wait
     Missing or Damaged Copies                                      may need to modify many existing QoS          for it to evolve into 4G, completely
     If you are missing an issue or                                 schemes, including admission control,         bypassing 3G. The answer probably lies
     received a damaged copy, contact                               dynamic resource reservation, and QoS         in the perceived demand for 3G and the
     membership@computer.org.                                       renegotiation to support 4G users’ diverse    ongoing improvement in 2G networks to
                                                                    QoS requirements. The overhead of im-         meet user demands until 4G arrives. ✸
     Reprint Permission                                             plementing these QoS schemes at differ-
     To obtain permission to reprint                                ent levels requires careful evaluation.
     an article, contact William Hagen,                                A wireless network could make its cur-     Upkar Varshney is on the faculty of the
     IEEE Copyrights and Trademarks                                 rent QoS information available to all         Department of Computer Information
     Manager, at whagen@ieee.org.                                   other wireless networks in either a dis-      Systems at Georgia State University.
     To buy a reprint, send a query to
                                                                    tributed or centralized fashion so they can   Contact him at uvarshney@gsu.edu.
     computer@computer.org or a fax
     to +1 714 821 4010.                                            effectively use the available network
                                                                    resources. Additionally, deploying a
                                                                    global QoS scheme may support the             Radhika Jain is a doctoral student in the
                                                                    diverse requirements of users with differ-    Department of Computer Information
                 Innovative technology for computer professionals   ent mobility patterns. The effect of imple-   Systems at Georgia State University.
                                                                    menting a single QoS scheme across the        Contact her at rjain@cis.gsu.edu.

96             Computer

				
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