ericsson review 1 1999

8/13/2019 Ericsson Review 1 1999

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Business customers are looking for ways tobecome more productive through conve-nient and cost-effective communication.Accordingly, the demand for in-building,wireless, voice-communication services isexpected to increase significantly over thenext few years. Anticipating this demand,Ericsson has developed and is marketing adigital wireless office system (DWOS) basedon the TDMA/136 standard. The commer-cial name of this system is Mobile AdvantageWireless Office.

The DWOS was developed especially forwireless, in-building communication.Being fully compliant with the TDMA/136

standard, the system operates on the licensed850 MHz cellular and 1900 MHz personalcommunications services (PCS) frequencybands. What is more, the indoor, wirelessservice provided by the DWOS is designedto minimize interference with the service of outdoor macro systems.• The operator can choose the most appro-

priate and convenient frequency band.• Frequency bands can be re-selected re-

motely and at any time.

Operator benefits

Operators of wireless networks make sig-nificant investments in the macro net-work—they purchase licensed frequenciesand build up the cellular/PCS infrastruc-ture. Since the wireless office system oper-ates on the same frequencies as those usedin the macro network, operators can employ

it to get a better return on investments inthe macro system and to generate addition-al revenue:• The wireless office system can be used to

attract business customers—who gener-ally use their phones more often than othersubscriber categories.

• Operators can charge businesses extra forusing licensed frequencies to operate thewireless office.

• Regular cellular/PCS charges apply tocalls made outside the office (off thepremises). Experience in the US hasshown that persons with wireless officeservices generate significantly more callsin the macro network, which amounts toincreased air-time usage in the macro sys-tem.

• US operators have also noted that user loy-alty is much improved, which results in adramatic reduction in churn.

• Operator administration and the costs of signing up new subscribers can be reducedthrough the signing of company-sponsored groups of subscribers. Opera-tors can therefore bundle services for their

customers; for example, long-distanceservice, outdoor/public wireless commu-nications, indoor/private wireless com-munications, and local access.

Customer benefits

Wireless office systems increase productiv-ity within an enterprise by making em-ployees much more accessible in and outsideof the office. As a result, fewer calls end upas voice mail (which when responded to,generate callback charges borne by the en-terprise). With a wireless office system, em-ployees can initiate calls from practicallyanywhere, which speeds up customer con-tacts and facilitates getting work donefaster. Improved accessibility and greatermobility increase customer and employeesatisfaction, which also contributes towardbetter business results. Moreover, wireless

20 Ericsson Review No. 1, 1999

Ericsson’s digital wireless office system was developed especially for in-

building communications operating on the licensed 850 MHz cellular and

1900 MHz PCS frequency bands. Because the system is fully compliant

with the TDMA/136 specification, any terminal that meets this specifica-

tion can be used with the system.

The authors describe the digital wireless office system concept, its

components and capabilities as well as the benefits it affords network

operators, businesses and end-users.

Mobile Advantage Wireless Office—A digitalwireless office system for TDMA/136 networks

Rune Johanson, Mikael Nilsson and Torbjörn Ward

Figure 1Commercial name and logo of Ericsson’sdigital wireless office system for TDMA/136networks.



Ericsson Review No. 1, 1999 21

office communication allows for more flex-ible office arrangements:• No need for extensive wiring/rewiring in

the building.• Wireless office systems that use standard

TDMA-compliant phones do not requirebusinesses to provide their employeeswith separate phones for in-building andout-of-building communication.

• Ultimately, wired desk phones can beeliminated, since the functionality offeredby the wireless office system will fully sat-isfy all voice-communication needs.

• Wireless office services enable employeesto telecommute from their homes or fromanother office location.

The ability to use standard TDMA termi-nals also implies that wireless operators andtheir customers are not bound to a particu-lar vendor of wireless office terminals. End-users can select any type of wireless devicethat suits their needs.

End-user benefits

Most end-users of wireless office solutions willbe employees of an enterprise. The wireless-

office solution gives each employee onephone and one number for all business com-munication, regardless of his or her location.Other significant benefits include increasedaccessibility and improved communicationcapabilities. End-users can always access of-fice communications.

Also, as was pointed out earlier, employ-ees who remain accessible while afoot, andwho have the means of communicatingwhen needed, are more productive and de-rive greater job satisfaction.

System concept

Designers of the digital wireless office sys-tem approached their task with the aim of fulfilling certain specific market require-ments, namely:• cost effectiveness—the cost per user in

wireless office systems should not exceedthe cost of a line in the private branch ex-change (PBX). Furthermore, the marketwants a system that provides local, on-siteswitching;

• simple installation, operation and main-tenance—today, the infrastructure of most wireless communication systems iscostly and difficult to install. Installationprocedures for the DWOS should pre-clude costly and lengthy installations orthe need for specially trained installers.Ongoing operation and maintenance

(O&M) must be simple and easy to un-derstand;

• self-engineering radio-network plan-ning—several wireless-office systemsshould be able to operate in the same li-censed spectrum as public cellular/PCSsystems. Thus, frequency usage in the

AC/DC Alternating current-to-direct current AFA Adaptive frequency allocation

ANSI American National StandardsInstitute

API Application program interfaceBSC Base station controllerCRE Cellu lar radio exchange

DCCH Digital control channelDID Direct in-dialing (number)DWOS Digital wireless office systemE1 2 Mbit/s interface

EIA Electronics Industries AssociationGPRS General packet radio servicesGSM Global system for mobile communi-

cationHLR Home location register

IN Intelligent networkISDN Integrated services digital networkLAN Local area networkMAHO Mobile assisted handoffMSC Mobile switch ing center

O&M Operation and maintenancePBX Private branch exchange

PCS Personal communications servicesPLMN Public land mobile networkPRI Primary rate interfacePSID Private system identityPSTN Public switched telephone network

QSIG Protocol for PBX networkingRH Radio headSMS Short message serviceSNMP Simple network management protocol

SPU Signal-processing un itSS7 Signaling system no. 7T1 1.5 Mbit/s interface

TCP/IP Transmission control protocol/ Internet protocol

TDMA Time division multiple accessTIA Telecommunications Industry Asso-

ciationTRX Transceiver VLR Visitor location register

BOX A, ABBREVIATIONS



DWOS and macro systems must be coor-dinated. Where possible, for the planningof cell parameters and handoff in the net-work, indoor radio-network planningshould be automated. Capacity andcoverage-related expansion must be sim-ple and straightforward.

System overview

The DWOS equipment (Figures 2 and 3),consists of• radio heads (RH)—which provide indoor

radio coverage;• a cellular radio exchange (CRE);• a mobility server.The digital wireless office system is con-nected to the public land mobile network(PLMN) through a network protocol inter-face known as ANSI-41 (Box B). Ordinari-ly, the ANSI-41 protocol is carried over asignaling system no. 7 (SS7) network. How-ever, in the DWOS solution, the bearer of the ANSI-41 protocol has been replacedwith the transmission control protocol/In-ternet protocol (TCP/IP). This solution isless complex: instead of using the DWOS as

an SS7 terminating point, several hundredDWOS systems are connected to a gatewayunit (the terminating SS7 point) that trans-lates ANSI-41 messages from TCP/IP intoSS7 signals.

The DWOS is connected to the enterpriseoffice switch—a PBX—through an E1 orT1 ISDN primary rate interface (PRI). Aprotocol for PBX networking, called QSIG,can also be employed.

Where the PLMN is concerned, theDWOS behaves like a mobile switchingcenter/visitor location register (MSC/VLR).In this case, however, the MSC/VLR(DWOS) only serves the radio coverage areaof the business. Network signaling overANSI-41 is used for regular digital cellu-lar/PCS features and functions specified inthe standard, such as roaming, authentica-tion and short message service (SMS).

Similarly, where the PBX is concerned,the DWOS behaves like a networked PBX.That is, the PBX is not aware that the newnetworked PBX (DWOS) provides wirelesscoverage.

Reaching end-users

The TDMA/136 standard permits thebroadcasting of private system identities(PSID). This allows TDMA terminals withan appropriate PSID to reselect and be ac-

cepted by the private system. For instance,as end-users commute to the office, theirwireless terminals connect to the publicTDMA network (the macro network). Asthey come within closer proximity of the of-fice, their terminals automatically reselectthe digital wireless office system. Once atthe office, the terminals register with theDWOS, which then notifies the home loca-tion register (HLR) in the PLMN that itssubscribers are “at home.”

Each DWOS user has one office phonenumber assigned to him or her in the PBX—a direct in-dialing (DID) number. Incom-ing calls to the DID line are forwarded bythe PBX to the “networked PBX” (DWOS).If the called terminal is in the office, the in-coming call is sent to both the wireless ter-minal and the PBX desk phone. The usercan therefore answer the call at either ter-minal.

Global mobility

As we have discussed, wireless-office ser-vices enable their subscribers to be mobilearound the office. Thanks to the ANSI-41networking protocol and the TDMA/136

air-interface standard, the DWOS also of-fers global mobility with a regular cellu-lar/PCS terminal. When subscribers leavethe building—for example, to eat lunch, towalk to another building of the office cam-pus, or to depart on an extended business


Figure 2DWOS components.

Radio Head

Cellular Radio Exchange.

MSC

PLMN

PBX

Voice mail

A NSI-41 T CP / IPI S D N P R I

HLR

Gateway

SS7

A N S I - 4

1 S S 7

E t h e

r n e t

Base

Station

Mobility Server

Internet

PSTN

Figure 3System integration into the telecommunications network.




trip—their terminals automatically reselectthe PLMN:• the macro system operated by the sub-

scribers’ cellular/PCS service provider;• an operator with which the regular service

provider has a roaming agreement.Each terminal registers with the macro sys-tem’s HLR, which notifies the DWOS sys-tem that the subscriber has left the officearea and is roaming in the macro network.After having been directed to the DWOS,incoming calls to the DID line result in aquery to the HLR—to find the roaming ter-minal. The DWOS then functions as a gate-way MSC, which controls service execution.If the terminal is turned off, the DWOS ex-ecutes the services set up in the user’s ser-vice profile, directing calls to voice mail orexecuting a search profile in a personal num-ber function. The DWOS uses the PBXvoice-mailbox system as default. Thus, usersneed only manage one voice mailbox.If the terminal is turned on while roaming,the HLR interrogates the MSC assigned tothe area in which the terminal resides at thetime of the call. The MSC returns a routing

number that the HLR forwards to theDWOS. This allows the DWOS to rerouteincoming calls through the PBX. The DIDline is charged for the leg of the call fromthe DWOS to the user.

Intelligent network (IN) features allowfour- and five-digit PBX numbering plansto be used even while roaming, thereby cre-ating a global, virtual wireless office.

Radio network concept

Several tactics can be used for providing in-door radio coverage for mobile communica-tions. Ultimately, the system equipmentshould offer excellent quality and be easy toadminister. Likewise, upgrading capacityand coverage should be easy.

One way of providing indoor coverage isthrough penetration from the outside macronetwork. This method implies that radiosignals penetrate building walls and struc-tures. Some building materials, however,significantly reduce signal strength, whichmakes it difficult and costly for operators toguarantee good signal coverage in all areasinside a building.

Capacity is another important issue. To

ensure good, ubiquitous, indoor coverage,the wireless office equipment should be lo-cated on the business premises. Good, effi-cient coverage can be provided in severalways. Two traditional alternatives are dis-tributed antenna/coax cable networks and

picocells. A third alternative—implement-ed in the DWOS—makes use of a virtual,single-cell solution.

Virtual, single-cell solution

The virtual, single-cell solution involvesseveral, small, radio base stations, calledradio heads. These contain the transmitterand receiver parts of a regular radio base sta-tion. The common control and signal-processing functions are centralized in a sep-arate, remote unit that can be pooled andshared by the entire system.

The radio heads are deployed inside abuilding to ensure that adequate coverage isprovided (Figure 4). All radio heads in thebuilding operate in unison. A single controlchannel is multicast throughout the entirebuilding. The radio-head transceiversmulticast the control channel.

Adaptive frequency allocation

Ericsson’s wireless office solution uses thesame licensed spectrum as the outdoormacro network without interfering with it.

Indeed, operators can deploy the systemwithout having to re-engineer the frequen-cy plan of the macro system. Conversely, op-erators can re-engineer the frequencies of themacro system without affecting installedwireless office systems.

Scanner

Radio head

Cellular radio exchange

• High capacity with minimum need for cell planning• No MAHO - Saved for CRE-CRE handoff• Most signal processing pooled in CRE

Algorithms• Adaptive frequency allocation• Adaptive public neighbor list• Dynamic channel selection• Adaptive radio head selection

• Synchronized radio heads• Multicasted DCCH • Uplink/downlink scanner• Macro diversity/soft handoff

Figure 4The virtual single cell. Radio heads are deployed inside the building to ensure that ade-quate coverage is provided.

ANSI-41 is composed of a series of recom-

mendations entitled “Cellular Radiotelecom-munications Intersystem Operations”

issued by the Telecommunications Industry Association/Electronics Industries Association(TIA/EIA). The recommendations describe pro-

cedures necessary to furnish cellular radio tele-phone subscribers with certain services requir-ing interaction between different cellularsystems.

BOX B, ANSI-41



The frequencies of the DWOS system areautomatically allocated to it through anadaptive frequency allocation (AFA)scheme. A dedicated scanner• scans the frequencies being used by the

macro system;• detects signal interference from the macro

system’s base stations (uplink) and frompublic mobile terminals (downlink).

The frequencies least affected by interfer-ence are automatically allocated for use inthe DWOS. If the scanner detects changesin the macro system, the DWOS immedi-ately yields to it, invoking a self-engineeredchange of frequencies.

Adaptive public neighbor lists

Information on surrounding macrocells isconveyed to DWOS subscribers (terminals)while at the office. The terminals use thisinformation to search for and detect theDWOS when they come into contact withnearby public macrocells; for example, whenen route to the office. Thus, terminals auto-matically locate a wireless office system inthe service area without burdening the pub-

lic radio network for information on the sys-tem’s identity and frequencies.

Adaptive radio-head selection and soft

handoff

During a call, a user might move away fromhis or her current position. Changes of thiskind are detected in the cellular radio ex-change, which monitors the signal strengthfrom all radio heads. If a stronger signal isreceived in an adjacent radio head, the CREorders that radio head to begin transmittingat the frequency and in the time slot allo-

cated to the call. It also connects a speechpath, so that speech information is sent toeach radio head concurrently. When the tar-get radio head indicates that it has estab-lished contact with the terminal, the “old”radio head is shut down. The terminal con-tinues to use the same frequency and timeslot as before, unaware that communicationhas been handed off. This procedure, called soft handoff , reduces the risk for dropped callsand minimizes speech muting when fre-quencies are retuned.

The virtual, single-cell solution and scan-ner combination enables operators to engi-neer the wireless office system with a mini-mum of cell planning.

System components

Mobility server

A mobility server, which functions as a localswitch, is fully capable of analyzing num-bers and routes, and provides common PBXservices, such as call forwarding, call trans-fer, inquiry, call diversion, call-back, call

waiting, and calling-line presentation. Italso provides access to the PBX and to thepublic switched telephone network (PSTN).Automatic roaming to the macro networkis facilitated through the mobility server’sANSI-41 connection.

The mobility server interacts with theHLR in the macro network. It also includesa TCP/IP interface for connecting to a localarea network (LAN) and the Internet.DWOS administration and management isperformed over the Internet/intranet via aWeb browser.

In the initial release, the DWOS is con-nected to the PSTN through a PBX trunkconnection. In subsequent releases it will bepossible to connect the DWOS directly tothe PSTN. Thus, the DWOS might be em-ployed as the sole system for office commu-nication.

Cellular radio exchange

The cellular radio exchange, whose functionis similar to that of a base station controller(BSC) in a GSM system, is the central pointin the radio infrastructure (Figure 5). It con-trols all radio heads and communicates with

mobile terminals over the TDMA air inter-face by adapting and translating ISDN callcontrol to TDMA call control. The CRE alsoincludes ISDN transmission to the PBX aswell as transmission to radio heads and scan-ners, which includes capability for power-


Figure 5The cellular radio exchange (CRE) is thecentral point in the radio infrastructure.




ing –48 V over the transmission cable. Thevirtual, single-cell logic for managing andcontrolling the radio network is also imple-mented in the CRE.

Control and signal-processing functions(speech coding, echo cancelling, modula-tion, error detection and correction process-ing) for TDMA air-interface communica-tion are centralized in a pool in the CRE.This configuration provides improved, cost-effective use of resources compared with dis-tributing the functions to each radio head.

Operation and maintenance is handledusing the simple network management pro-tocol (SNMP) over an Ethernet connectionto the mobility server. Performance-monitoring data and event recordings arestored temporarily in the CRE before theyare transferred to the mobility server for per-manent storage on disk. CRE administra-tion and management is integrated into theWeb-based management system providedby the mobility server.

From the outset, the system was designedfor simple operation and maintenance andeasy installation. For example, the design

supports the swapping of boards while thesystem is running (hot-swapping): when ca-pacity is to be expanded, a board compris-ing control and signal-processing capabili-ties (the signal-processing unit, SPU) can beinserted into any of the available slots in thecabinet. Once inserted, its presence is de-tected and its capabilities are evaluated. Theboard is then assigned an identity, loadedwith software, and taken into service.

Radio head

The radio head is a dual-band radio unit thatcan operate on any 850 MHz cellular band(A and B) or any 1900 MHz PCS band (A,B, C, D, E or F). Output power and the fre-quency band of operation can be configuredremotely. Moreover, the output power canbe set in variable steps from 0.5 mW to 25mW.

The radio head is a small, four-liter unitdesigned for indoor environments (Figure 6).It contains• five transceivers (TRX);• one scanning receiver;• circuits for an E1 interface to the CRE.Given the three time slots per radio carrier

in the TDMA air interface, each radio headhas a capacity of 14 digital traffic channelsand one digital control channel.

The radio head can be powered remotelyover the twisted-pair transmission cable(Category 5) from the CRE, which elimi-

nates the need for battery backup or a powerconnection at the radio head. When pow-ered remotely, the radio head can be locat-ed up to 800 m from the CRE. Otherwise,using a regular AC/DC converter to providelocal power, it can be mounted up to

1,600 m from the CRE.Radio heads have been designed for easy

installation using wall brackets and a cableconnection to the CRE. As soon as the CREdetects the presence of a new radio head, itautomatically configures the radio head, up-loads the correct software, and takes thetransceivers into service. Without furtherplanning, the CRE can immediately beginreceiving calls from, or handoff calls into,the newly added coverage area.

Scanner

The scanner scans for the adaptive frequency-allocation function, providing input forgenerating the adaptive public neighborlist. The scanner uses the same control-logichardware and is based on the same mechan-ical platform as the radio head. The radiocircuitry includes three receivers:• An uplink receiver that scans for inter-

ference on frequencies used by public mo-bile terminals in the area.

• A downlink receiver that scans for inter-ference frequencies used by neighboringpublic base stations.

• A downlink receiver that detects and de-

codes cell identities from neighboringpublic base stations.

Terminals

Ericsson’s digital wireless office system op-erates with any standard TDMA wireless

Figure 6The dual-band radio head is a small, four-liter unit designed for mounting on walls inindoor environments.



mobile terminal. That is, any terminal canbe used as long as it meets the TDMA/136

specification.

Capabilities

Operation

The DWOS provides local and global mo-bility in a private office environment. Glob-al mobility enables any caller to accessDWOS subscribers wherever they happento be. If they want to, however, subscriberscan turn the global-mobility feature off.

The digital wireless office system is a dig-

ital system that provides wireless commu-nication according to the TDMA/136 stan-dard on 850 MHz cellular and 1,900 MHzPCS frequencies. It includes features for au-thentication; supports automatic roamingfrom the in-building system to the publicsystem via the ANSI-41 connection; and isbased on the principles of adaptive channelallocation, which means it yields to the out-door macro system—whenever the systemdiscovers that its frequency selection is inconflict with the macro system, it switchesto an idle frequency.

A PBX numbering plan can be used in-side an area supported by DWOS—sub-scribers are accessed through the PBX num-ber or through their mobile subscriber num-ber. The DWOS system reroutes incomingcalls to reach users wherever they are,whether inside or outside of the DWOS area.Finally, DWOS operation, administrationand maintenance is simple—all necessaryadministration can be performed via Inter-net connections.

Capacity and coverage

The first version of the digital wireless of-fice system targets systems of 25 to 500+users. For wireless-based calls, the systemcan carry 45 Erlang at 0.5% blocking. Thisallows up to 450 users, assuming the fol-lowing call loads:• 100 mE wireless call load per user• 100 mE load on the desk phoneIn the initial system release, up to 32 radioheads can be connected to the CRE. In a typ-ical office building, this would provide cov-erage to approximately 500,000 square me-ters (5,381,958 sq. ft). Note: Actual cover-


Figure 7

DWOS uses a standard TDMA/136 wirelessmobile terminal—any terminal can be usedas long as it meets the TDMA/136 specifica-tion.




age by in-building wireless systems varieswith building structure.

System evolution

Market and customer responses to the ini-tial product release will influence further de-velopment of the digital wireless office sys-tem. Some of the capabilities being consid-ered for future releases include• stand-alone operation—that is, as a PBX

replacement;• greater system capacity;• multi-site networking;• radio heads for outdoor use;• packet data services—GPRS (TDMA en-

hancement);• voice over IP (based on the H.323 stan-

dard);• semi-private systems that would allow

–handoff to an outdoor system;–charging of external users;

• using DWOS as an indoor extension to anoutdoor public cellular/PCS system.

Conclusion

The digital wireless office system is imple-mented as a private network usingTDMA/136 technology. It offers fullMSC/VLR functionality and is fully inte-grated into the core PLMN through theANSI-41 network protocol.

The DWOS embodies a highly flexibleradio-network architecture based on a vir-tual, single-cell concept that allows inde-pendent dimensioning and expansion of ca-pacity and coverage. The virtual, single-cellsolution and the scanner are instrumental in

providing a wireless office system that• is simple to engineer;

• requires minimum cell planning;• is easy to install and support.Efforts to facilitate installation and supportinclude the implementation of plug-and-play capabilities, a Web browser-basedO&M application, and remote powering viastandard twisted-pair cables between theCRE and radio heads.

The DWOS offers a wide range of featuresand services, including open applicationprogram interfaces (API) for third-partyvendor applications for interfacing with andusing unified messaging systems.

Figure 8 A docking station will be available for the Ericsson terminal to improve its use as an officephone. The docking station includes a microphone, a loudspeaker and a battery charger.

ericsson review 1 1999

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