NASA Technical Reports Server (NTRS) 19940018281: Transparent data service with multiple wireless access

6 Pages·0.44 MB·English

by  NASA Technical Reports Server (NTRS)

N94- 754
Transparent Data Service with Multiple Wireless Access
Richard A. Dean Alien H. Levesque
Department of Defense GTE Government Systems
Ft. Meade, MD 20755 Waltham, MA 02254
[phone] [phone]
ABSTRACT ing and Data technologies. The integration of all
three disciplines into Multimedia Wireless Data
The rapid introduction of digital wireless net-
services combines the challenge of operating inthe
works is an important part of the emerging digital
harsh radio environment with the sophisticated
communications scene. The inU'oduction of Digital
protocols required for data, and with the seamless
Cellular, LEO and GEO Satellites, and Personal
services, switching, and transparency required of
Communications Services poses both a challenge and
modern networks.
an opportunity for the data user. On the one hand
wireless access will introduce significant new porta-
ble data services such as personal notebooks, paging,
E-mail, and fax that will put the information age in
the user's pocket. On the other hand the challenge of
creating a seamless and transparent environment for
the user in multiple access environments and across
multiple network connections is formidable.
This paper presents a summary of the issues
associated with developing techniques and standards
that can support transparent and seamless data ser-
vices. The introduction of data services into the radio
world represents a unique mix of RF channel prob-
lems, data protocol issues, and network issues. These
problems require that experts from each of these dis-
ciplines fuse the individual technologies to support
these services. Figure 1. Technologies and Applications
The fusion of these disciplines is particularly
INTRODUCTION
challenging in light of the variety of access mecha-
Multimedia wireless Data represents more than nisms that are possible, as with Cellular, Satellite,
just a combination of radio, network and data tech- and Wireless LAN, each with a separate set of
strategies, standards, and problems. The successful
nologies of which it is composed. These disciplines
integration of these technologies have typically
have evolved in largely independent communities
been viewed under the umbrella of Personal Com-
and their fusion is neither obvious nor direct. Each
munications Services and Networks (PCS &PCN).
has been spawned with separate technologies and in
While market forecasts for PCS are euphoric, the
separate markets. It is only recently that combina-
technical coordination and management problems
tions of these disciplines have been merged. Fig. 1 facing PCS are formidable. Essential to real
shows how the overlaps of these areas have com- progress in addressing these problems are first a
bined in new techniques and markets. clear strategy for accomplishing these technical
challenges and, second, much greater cooperation
Cellular telephones, for example, combine the
among the independent communities of interest.
Network switching and Radio technologies for voice
The potential for PCS applications appears un-
applications, while ISDN combines Network switch-
101
:mobilefaxterminalsforeachnet- bounded. It offers the possibility for creating a new defines differer
bettesrtrategayndmorecoordination infrastructure for the Information Age with im- work. Clearly a
proved services, convenience, and productivity. isrequired.
PROBLEM DEFINITION STRATEGY
¢FutureInteroperabWleirelessData Besides the historical differences that divide The hope fi
creatinagvirtualopensystemwithin these communities, Cellular Radio, Land Mobile Services lies in
redandwirelessnetworksA. strate- Radio, Mobile Satellite, and Wireless LANs are the emerging
pensystemsolutionliesinuseofa separated by the perception of a fractured market gy for such an_
suchasdefinedbyISOlayering which may lead in turn to a fractured solution. This layered strateg,.
hilethisstructureappliesingeneral, highly fractured approach persists today in spite of model (OSI).
cationtointeroperabwleirelessdata a high degree of commonality in the architectures, its specific app]
dstrategy. services, and issues. Figure 2 shows an architectur- leads to a natur
al view of these mobile services served by a com-
Media
Independent mon public network. They differ in the radio
Application channel and associated protocol but share common
Jk-
services, control, and network interworking fea-
_n i tures. 7. Applicati
/
tion 6. Present=
"%%
5. Session
t
._f %_,,.Satellite / @\ 14Tr_n_nn
3. Network
_'" _ %_ellula4[ I
i| 2. Link
..._tationl il
1. Physical
"Tansparent "% !
Network Service
__Eb] _Base I il _ 1
twe 3. Layered Strategy Fi[
lodel is organized generally by the If the OSI
_r4-7) and the network (layer I-3) as application (la_
_ Sttat/oa_:a:_oI _,._ tatt/oan_a--/on'f,., _/,
3 then a general strategy for interop- shown in Figur,
organized as follows: erability can be
nmon sets of user applications that '%%° ,_ Develop cc
Ldependent. are media i
Figure 2 Strategic View of Wireless Networks
work services that are application in- Develop ne
These networks and services evolved from in- dependent.
enigent Network Services that in- dependent markets. Their integration with the Pub- Develop In
lic Network has been as ad hoc extensions rather
ary network interworking to support voke neces:
seamless connections. than as part of integrated services. This is true for transparent
voice services and is especially true for data servic-
ltegy simply organizes the applica- es such as G3 Fax. The myriad of approaches tak- The first st]
_geable set of standard protocols.The en, for example, by Inmarsat, GSM European tions into a mar
,for example, is made media inde- Cellular, and TIA North American Digital Cellular PSTN G3 FA_
102
pendenutnderTIA592byisolatingtheT4compres- Inherent in such a strategy is transparency to
sionprotocoal ndapieceoftheT30wireline the user and the application by provision of com-
controllerT.heUSGovernments'securetelephone mon physical, link and network layers or by auto-
(STU-III)haslikewisebeensegmenteindtoamedia matic interworking capability. This strategy
independepnrtotocoflormultiplemediaapplications. isolates the application from the variety of access
RF technologies that will evolve due to competi-
Thesecondstrategydeveloptsransparenatndin-
tion within markets and the differences among
teroperablneetworksserviceasndsignallingT.hisis
wireless channels.
accomplishebdyaccessin-dependenctallcontrol,
identityvalidationr,egistrationa,ndmobilitymanage- Figure 4 shows how layering might be accom-
ment.SuchanapproachisrecommendebdytheJoint plished with a G3 Fax connection between the
ExpertsMeetingofANSI TI and TIA TR45 in refer- PSTN and a Cellular network. A combination of
ence 1and summarized below in Table 1. Such an ap- common applications, common services, network
proach allows services to be independent of access interworking and intelligent networks create a vir-
technologies such as CDMA and TDMA on a given tual open system for this service.
network. It also allows services to be independent of
The Intelligent Network plays a potentially signifi-
the wireless access network such as satellite or cellu-
cant role in making operation transparent. The
lar.
Intelligent Network can support a transparency
of service to the user across multiple wireless
Table 1: Media Independence
networks by customizing the connection and
inserting the necessary interworking. Operation
Access Access
within the Intelligent Network can incorporate
Independent Dependent
the following phases and functions.
Application Protocol Multiplex Scheme Access Phase - Identify the User/Terminal
_all Control Radio Resource Mgmt <User Identity Established>
<Terminal Identity Established>
[dentity Validation Radio Link Protocol
<User Services Established>
Registration/Location <Terminal Services Established>
Flow Control
Connection Phase - Customize Network Service
Maintenance & Config <User Dialed Number>
<Examine User/Terminal Services>
7 -Application T4 i T4
Compression i Compression
6- Presentation
5- Session
| Digital PSTN
4 - Transport T30 _ _ } Cellular _, : T30
3 - Network _ Interworkin_ ___
q
Radio Link _ Function _ T30
2- Link
Protocol !
i
1 - Physical IS-54 _ _ V.21,V.27,V.29
Figure 4: Example of Interoperable Fax Connection - Data Plane
103
<Examine Destination Network Services>
which compatibility can be obtained by different
<Select Service Compatible Option> manufacturers. The TR45.3 architecture defines the
Transmission Phase - Support Transparent Service necessary entities for connection to the PSTN using a
<Establish Necessary Interworking> layered methodology as illustrated in Fig. 5. It repre-
<Clear To Send> sents the lower three layers of the Open Systems In-
terconnection model, the Physical, Link, and
<Data Transmission>
Network Layers. The reference model was adopted at
Earlier publications [4] have suggested use of IN
the outset to clearly define the protocols to be sup-
services for wireless network mobility subscriber mo-
ported by the different network elements. Where ap-
bility. The above sequence expands that role to sup- plicable common accepted data standards and
port service compatibility, service options, and
procedures were specified and adapted to the cellular
interworking.
network environment. Figure 5 illustrates the Data
EXAMPLE: TR45.3 DIGITAL CELLULAR Plane protocols used for G3 interoperable Fax ser-
vice from a Cellular Mobile Terminal Equipment
This is not the first paper to suggest strategies for
(TE) through the Base Station (BS)/Mobile
future wireless networks. Most network designers be-
Switch(MSC) and Interworldng Function (IWF).
gin with such goals. For this reason it isperhaps more
Note how the layering allows clear visibility of the
appropriate to search out good example to build upon.
protocols and interfaces necessary at each element in
The European GSM and North American Digital Cel-
the connection. Note also that the only variation one
lular TR45.3 are two such examples where this prob-
might expect between a cellular connection and a sat-
lem has been addressed [2]. Several significant
ellite connection would be in the Radio Link protocol
approaches have been demonstrated in TR45.3 in ref-
(RLP) unique to that network.
erence [3] and are summarized below.
Standard Services
Layered Approach
Standard sets of bearer services are necessary
A layered network model has been developed in
across the variety of wireless access networks if
TR45.3 to represent each networkelement needed to
transparent teleservices are to be provided. Bearer
support a specific service. The notion of segmenting
services would typically be synchronous, asynchro-
the network features into layers separates out the net-
nous or packet services necessary to support a broad
work's unique protocols and identifies a means by
base of teleservices across multiple networks. Rates
lStation[ [ IWF I I-
\reiay/ \re ay/ _-:N, re. ay/
Network
T30
r3o i!
Link mr T3(3 T30
Y_II21
"_}_
: V
EIA- EIA. :SS IS5_ V.2_ 2W 2W v.27Eia EIA-
Physical 232 232 I 232
i 12321
: .... i
Rm Um Ai W Rv
Standardized Interfaces
Figure 5. Layered Standards and Interfaces - Data Plane
104
from2.4upto28.8kbpsfortraditionawl irelinedata, hardwares,erviceasndinteroperabilit_y.lhisisper-
andupto64kbpsforISDNservicearebeingconsid- hapsthebiggesctoncernusershaveabouttheexplo-
ered.CustomservicessuchastheG3FaxandSTU- sionofwirelestsechnologieTs.R45.3hasmadea
III arealsoaccommodateAd.minimumsubseotf judiciousselectionofcommonlyusedstandardfsor
thesehowevecranclearlybeaccommodateadcross interfaceasndsignallingasshowninFig.5andsum-
allmedia. marizedinTable2.
Networklnterworking
Table 2:TR45.3 Interfaces & Protocols
Consistenwtithsupportintgeleserviceidsentified
Function Standard
aboveistheincorporatioonfnecessarnyetworkin-
terworkingT.ransparenintterworkingisthemostim-
Mobile Station Rm Sm (ISDN)
portanta,ndperhaptshemostdifficultpartofthe
solutionI.t isatthispointthatallpossibleservices
_ignaling EIA/TIA 602 Q.931. V. 120
connecwt ithallpossiblenetworksI.ngeneratlhere
areacommonsetofservicesandconnectiontshat Rate .3,2.4,4.8,..38.4 64kbps B Chan
simplifythisproblemF.igure5showsaTR45.3ex-
[nterface EIA/TIA 232E 1.430
ampleofInterworkingforG3FaxwiththePSTN.
TheIWFsupportsatranslationofdataontheradio
IWF Modems V.21,V.22,V.22bis, V.32,V.32bis
linkprotocoilntoaV.27wirelinemodemwiththe
V.42, V.fast,Bell 103, Bell 212a
necessarTy30controlsignallingtogeneratGe3com-
patibleserviceonthePSTN.Otherexampleosfin-
terworkingwouldbeuseofcommonlyavailableV Radio Access Independence
seriesmodemfsorsynchronousserviceoraHayes
The TR45.3 Data Services have accomplished a
compatiblecontrolschemfeorasynchronoudsata.
large degree of access independence in their designs
ConnectiontsoX.25Packentetworkswouldrequire
to date. This is largely a result of a layered approach
packetassembly-disassem(PblAyD)functions.
and represents a major advantage for future applica-
ISDNconnectionwsouldrequireV.110orV.120rate
tions. Figure 5 shows how the Radio Link Protocol
adaptionT.heGovernmentS'sTU-IIIcanbeaccom-
and the IS54 TDMA physical layer are isolated from
modatedwitha4.8kbpssynchronoubsearesrervice
the other components in the network. This isolation
andanIWFthatincludesV.21,V.26andV.32echo
will enable support of the same architecture, protocol
cancellingmodems. and functional elements in the TR45 CDMA solution.
It can also be emulated for satellite and PCS networks
StandardSignalingInterfacesandProtocols
to support uniformity of products and user services.
Uniformityofsignalingi,nterfaceasndprotocols
acrosswirelessdatanetworkssupportcsommon
BS/MSC
Interwork
Mobile I Function PSTN -- M
®
[.../1 IV serie_
BS/MSC
(_) Source
0 Recovered
Fig.6 Cellular/PSTN Data Configuration
105
Timing and Synchronization MSC clock. Hence there is no real issue with bit syn-
Data service for the wireless user connected chronization in this direction. The bit sync problem
as presented here will be common to all mobile net-
across the Public Switched Telephone network pre-
works independent of access scheme.
sents several timing issues that must be resolved. The
two significant issues to be addressed are Frame Sync
Frame Sync problems occur in Digital Cellular
and Bit Sync. Frame sync addresses the need to
Networks primarily during handoffs between cell
accommodate timing operation across a handoff. Bit
sites that have independent timing references. In gen-
sync addresses the need to accommodate clock mis-
eral a gap in transmission can occur and the signal
matches across the network connection. Figure 6
will reappear in some random phase alignment. Solu-
highlights the different clocks in the system which
tions to the frame sync problem are limited for the
leads to the sync problem in the internetwork connec-
tion. The PSTN V series modem transmit clock is TDMA system as the overall timing of the Digital
shown as Tx and is the data rate of the PSTN source Cellular system isperformed independently by each
based on that PSTN modem's internal clock. This Base Station. The remaining practical solution under
consideration in TR45.3 is to maintain frame sync
timing source is tracked by the Interworking Func-
across the handoff by inserting a frame counter in
tion modem which develops an estimate of this clock
each frame so that added and deleted frame can be
using a phase lock loop. This clock is further promul-
accounted for in the alignment of the data stream at
gated across the Cellular channel and appears again
the BS/MSC or the Interworking function. A modulo
in the mobile as a reconstituted Tx. Figure 6 also
2ncounter with n ranging from 4 to 8 can accomplish
shows the Digital Cellular synthesized clock Mx.
this task. If this scheme is accomplished, the net
This clock is generated at the MSC and is recon-
effect of handoff would be the insertion of burst
structed at the mobile station. In general for an
errors in that data stream when frames are lost but bit
agreed upon data rate Mx and Tx are equal to the
sync is maintained.
nominal rate of the data service (E.G. 4.8 kHz). In
practice however the data rates of both clocks will
Conclusions
differ from the nominal rate and will have some drift.
Furthermore, the Mx clock cannot phase lock with The standardization of wireless data services rep-
the PSTN modem clock as itis locked to the rest of resents significant challenges but the potential exists
the cellular TDMA systeml For PSTN modems the for a smooth path for a broad range of services and
worst case clock accuracy is lxl0 -4while the Digital markets. Technical solutions lie in a fusion of exist-
Cellular clock is assumed to be lxl0 6 or better. The
ing network, radio, and data technologies. A strategy
differences in these clocks results in different trans- for such a fusion across the multiple wireless net-
mission rates across the network. This difference can works will require a clear technical strategy as out-
result in a slip in bit sync when there is either too lined in this paper and, perhaps more importantly,
much or too little data at the interface. These differ- cooperation among the currently insular cellular, sat-
ences must be accommodated by the network if the ellite, network, and data communities.
service is to be usable. TR45.3 is currently consider-
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2'lnteroperable Wireless Data, D Weissman, A.
The case shown reflects the situation with the
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3. Stage 2 Service Description - Circuit Mode Data
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106