System and method relating to cordless communications

An arrangement for providing mobility in a communications system comprising a number of private branch exchanges to which a number of radio exchanges are connected over an interface and wherein a number of cordless extensions are connected to each private branch exchange over the radio exchanges. Between each private branch exchange and at least one radio exchange mobility serving means are arranged which in interworking with the private branch exchange emulate a radio exchange, thus providing cordless mobility between a number of radio exchanges.

BACKGROUND 
The present invention relates to an arrangement and a method for providing 
cordless mobility in a telecommunications system comprising a private 
network. 
The invention also relates to an arrangement for providing cordless access 
to the services provided by an exchange also when a cordless phone is not 
within coverage of a radio exchange adjunct to the exchange. Sure an 
exchange may e.g. relate to a private branch exchange, a centrex etc. The 
invention further relates to a telecommunications system comprising a 
private network wherein cordless mobility is provided and wherein cordless 
access to services offered by e.g. a private branch exchange is provided. 
In general a cordless portable telephone can access a private branch 
exchange and the services provided thereby in a way which is similar to 
that of a normal desk telephone using DTMF (Dual Tone Multiple 
Frequencies) signalling as long as the cordless phone is within the radio 
coverage of a radio exchange (radio switch) adjunct to the private branch 
exchange. If however the cordless telephone is not within radio coverage 
of a radio exchange adjunct to the private branch exchange it can not be 
used and the user instead has to use for example a PSTN (Public Switched 
Telephone Network) phone, a cellular phone or a desk phone in the private 
network. This means that the user has to change phones which is an 
apparent drawback since the user has to have at his disposal two different 
phones having two different directory numbers etc. 
In a system described in WO 94/16531 is illustrated an arrangement for 
establishing telephone links in a communications system by call 
transmission. A connection is provided between a number of radio exchanges 
and mobility is provided between them. Here a link thus is provided 
between the already existing private network and the links provided 
therein. This implies that in order to support those links, the private 
branch exchanges have to fulfil a number of requirements. Signalling 
between nodes independently of the private branch exchange will not be 
achieved and since existing private branch exchanges are used for 
providing the connections, the services offered will be limited and it 
will not be possible to get access to all the extension services existing 
in each of the nodes of the private network. 
SUMMARY 
It is an object of the present invention to provide an arrangement through 
which cordless phone access to a private network is provided regardless of 
whether the user is within radio coverage of a radio exchange interfacing 
an exchange or not (e.g. a private branch exchange). More particularly it 
is an object of the present invention to provide an arrangement through 
which a cordless portable user can access any of a number of e.g. private 
branch exchanges within a (private branch) exchange (PBX) network. 
Furthermore the invention is not limited to private branch exchanges, e.g. 
it also relates to centrex exchanges or e.g. more generally to Private 
Telecommunications Network Exchanges (PTNX). These are defined by ETSI 
(European Telecommunications Standard Institute) as: "A PTN nodal entity 
that provides automatic switching and call handling functions used for the 
provision of telecommunication services. A nodal entity consists of one or 
more nodes. The nodal entity, performing the functions outlined above, can 
be implemented by equipment located on the premises of the private network 
administrator or by equipment co-located with, or physically part of a 
public network." Two implementations of PTNX are: 
PBX; An implementation of a private telecommunications network exchange 
located on the premises of a private network administrator. 
Centrex: An implementation of a private telecommunications network exchange 
not located on the premises of a private network administrator. 
Consequently whenever referring to a PBX in the present application this 
is not for a limitative purpose but a PBX is here defined as also 
comprising other exchanges. 
PBX is in this document also defined as meaning Private Automatic Branch 
Exchange PABX. 
Still further it is an object of the present invention to interconnect a 
number of private branch exchanges whether or not there is an existing 
private branch exchange network so that cordless mobility is provided. 
Thus according to various aspects of the invention it is an object to 
provide mobility between cordless users within radio coverage of different 
radio exchanges (REX) interfacing one and the same private branch exchange 
(PBX) and/or between cordless users within radio coverage of radio 
exchanges (REX) adjunct to different private branch exchanges. Mobility or 
mobility management comprises two basic services, namely roaming and 
hand-over. 
Roaming is according to ETSI/RES 3 (European Telecommunications Standard 
Institute, Technical Committee RES 3) described as "The process whereby a 
portable part can initiate and receive calls on more than one fixed part". 
A portable part here means a cordless phone and a fixed part means a base 
station and its associated radio exchange. 
This means that when a user moves from one location to another, the new 
location is automatically reported to the location register of the 
mobility network. The location register information is essential for both 
incoming and outgoing calls to/from the user. 
Hand-over is by ETSI/RES 3 described as "The process of switching a call in 
progress from one physical channel to another channel". There are 
different types of hand-over, namely intra-cell hand-over relating to a 
hand-over within a base station coverage and inter-cell handover meaning a 
hand-over between two different base stations which can be connected to 
the same or to different radio exchanges. 
Still further it is an object of the present invention to provide an 
arrangement through which the services provided by a private branch 
exchange can be accessed by a cordless user throughout a private network. 
Moreover it is an object of the present invention to provide an 
arrangement through which cordless coverage can be extended over a whole 
private network and through which given or specific phone services such as 
DTMF phone services of private branch exchanges can be maintained across 
the whole private network without any networking support from the private 
branch exchange or without imposing requirements on the private branch 
exchange itself. 
These as well as other objects are achieved through an arrangement and a 
method respectively wherein mobility serving means particularly referred 
to as a mobility server is arranged between a private branch exchange and 
a number of radio exchanges interfacing the private branch exchange using 
the existing interface from the private branch exchange wherein the 
mobility serving means emulates a radio exchange in interworking with the 
private branch exchange. 
Since the interface from the private branch exchange is unchanged no 
modification as to the software or the hardware of the private branch 
exchange is required by the mobility serving means, hereinafter referred 
to as the mobility server (MOBS). The mobility server can interface one or 
more radio switches depending on the circumstances, e.g. depending on the 
radio coverage that is wanted and on the number of portable phones to be 
supported etc. The mobility server thus provides mobility for cordless 
users between several radio exchanges (REX) without imposing any new 
requirements on the private branch exchange. Mobility for cordless users 
between a number of private branch exchanges is achieved through arranging 
mobility servers between a number of private branch exchanges and the 
radio exchanges (REX) interfacing the particular private branch exchanges 
and creating an overlay network of mobility servers. 
The copending U.S. patent application "System and method relating to 
cellular communications" filed by Applicant on Dec. 30, 1994, which is 
incorporated herein by reference relates to access by cellular telephones 
to a private branch exchange or a private network and to services provided 
thereby. 
It is an advantage of the present invention that the capacity relating to 
the number of cordless phones and to the radio coverage area is increased. 
Another advantage with the invention is that an extension can replace for 
example a desk phone with the cordless phone and still keep the directory 
number, i.e. the numbering gets flexible and the same number can be used 
whether using e.g. a desk phone or a cordless phone etc. 
A particular advantage with the invention is that the services provided by 
a private branch exchange are also available to a cordless user either 
within the site of a private branch exchange or within a network of sites 
of private branch exchanges.

DETAILED DESCRIPTION 
FIG. 1 illustrates mobility serving means, as referred to above also 
denoted mobility server, MOBS which is arranged between a private branch 
exchange PBX and two radio exchanges REX.sub.1, REX.sub.2. Of course it is 
just in this particular embodiment that the mobility server interfaces two 
radio exchanges; it can also interface one REX or more than two REXes. In 
known systems one or more REXes Interface a PBX. When a mobility server is 
arranged between a PBX and a number of REXes, the existing interface to 
the PBX is used and it does not have to be changed. Therefore no 
modification to the PBX software or the PBX hardware is required. In 
interworking with the PBX, the mobility server MOBS emulates a radio 
exchange. In the particular embodiment as shown in FIG. 1, the mobility 
server offers mobility for cordless users between REX.sub.1 and REX.sub.2, 
and since the interface to the PBX remains unchanged the PBX does not have 
to meet any further requirements. In the shown embodiment the mobility 
server interfaces the two radio exchanges REX.sub.1, REX.sub.2 on the ISDN 
S.sub.2 interface. 
The S reference point is adopted in PBX systems as the access interface for 
extension lines. The S.sub.2 indicates a 2 Mbps primary rate interface 
(30B+D). 
Similarly an S, interface indicates a 1.5 Mbps primary rate interface 
(23B+D) used mainly in the USA. The primary rate interface and its 
D-channel control singalling allows for traffic concentration, i.e. more 
than 30 or 23 users can be allocated to one interface (S.sub.2 or S, 
respectively). 
Through this arrangement a cordless user is provided with access to the PBX 
over an extension line. Therefore the cordless user will be represented in 
the PBX with a directory number, a class of service, a number of 
accessible features and a line status. Then both a desk telephone and a 
cordless telephone can use the same directory numbers from the same 
directory number series. Furthermore, the PBX will provide the same 
functions to a roaming cordless user as to a cordless user who is within 
radio coverage of a REX adjunct to the PBX in a manner known per se. In 
general it applies to all services provided by a PBX but here merely a few 
examples are given such as extension initiated services, for example 
enquiry and transfer, diversion services, busy line services for incoming 
calls such as call-back etc. 
Furthermore, operator services, call accounting services and private 
network services, for example least cost routing etc. can be provided to a 
roaming cordless user. The embodiment as described in FIG. 1 relates to 
providing mobility for cordless users who are within radio coverage of 
radio switches REX.sub.1, REX.sub.2 which are adjunct to one and the same 
PBX. In FIG. 2 a further embodiment will be illustrated wherein mobility 
for cordless users who are within radio coverage of radio exchanges 
adjunct to different private branch exchanges. 
In FIG. 2 a number of private branch exchanges PBX1, PBX2, PBX3 interface 
the public switched telephone network PSTN or the Integrated Services 
Digital Network ISDN. On a first site A a mobility server MOBS1 is 
arranged between the PBX1 and a number of radio exchanges REX1.sub.1, 
REX1.sub.2. Similarly, on site B a mobility server MOBS2 is arranged 
between a PBX2 and REX2.sub.1, REX2.sub.2 and on site C a mobility server 
MOBS3 is arranged between a PBX3 and REX3.sub.1 REX3.sub.2. The mobility 
servers MOBS.sub.1, MOBS.sub.2, MOBS.sub.3 interface each other to form an 
overlay network of mobility servers which provides cordless users with a 
network wide mobility. Thus mobility for cordless users is extended from 
just one site to a network of sites. The sites as defined herein (site A, 
site B, site C) may coincide or not with the "normal" sites of the PBXes. 
According to different embodiments there are different levels of 
integration between the existing PBXes and the ISDN network of adjunct 
mobility servers. 
In the following three different embodiments will be further discussed 
depending on whether the private branch exchanges are networked or not. 
In the embodiment illustrated in FIG. 3 the private branch exchanges are 
not so interconnected as to form a network. There are three separate sites 
A, B, C, each comprising a PBX1, PBX2, PBX3 and a mobility server MOBS1, 
MOBS2, MOBS3 which interfaces a number of radio exchanges REX1.sub.1, 
REX1.sub.2 ; REX2.sub.1, REX2.sub.2, REX2.sub.3 ; REX3.sub.1, REX3.sub.2. 
In order to provide cordless mobility, an overlay network is formed by the 
mobility servers through interconnecting them over an interface supporting 
ISDN signalling or digital connections. On each site the network is linked 
to the respective PBX. User information (B-channels) and control signals 
(D-channel) are carried in the overlay network formed by the mobility 
servers. 
In FIG. 4 one embodiment is shown wherein a PBX network already exists. 
However, in this particular embodiment the PBX network does not support 
ISDN signalling and it does also not support digital connections. Here 
like in the foregoing embodiment, user information as well as control 
signals are carried in the overlay network formed by the mobility servers. 
The same reference signs are used in this figure as in FIG. 3. If fixed 
modem connections are used for ISDN signalling, an analog PBX network can 
be used. In the embodiment as illustrated in FIG. 5 an ISON PBX network or 
a PBX network exists which supports digital connections. This means that 
this network, i.e. the PBX network, also can support signalling and 
traffic between the mobility servers. Also here the same reference signs 
denote the same features as in FIG. 3 and FIG. 4. The PBX network provides 
semi-permanent connections on which the signalling between the mobility 
servers can be carried. For example the "D-over-B concept" for ISDN 
signalling can be used. The D-over-B concept means that the D-channel, 
which carries control signals, in set up over a B-channel end-to-end 
between two nodes in a network. The B-channel is a bearer. The network 
supports digital transmission and can be ISDN or just a switched PCM 
network. It can be a private or a public network. 
In an alternative embodiment relating to the case wherein a PBX network 
supporting digital connections already exists, or to the case wherein PSTN 
supporting switched digital connections exists, so called "dynamic route 
allocation" can be used in order to adapt a number of reserved connections 
to the actual traffic situation in the overlay network. This will give a 
more cost efficient network. Dynamic route allocation is a service 
specified by ETSI (European Telecommunication Standards Institute) which 
provides a semipermanent route between two nodes over a switched network. 
The number of B-channels in the route can be dynamically controlled by the 
two nodes in order to match the actual traffic load and thus reduce 
transmission costs. 
In FIGS. 6 and 7 concepts are illustrated which are based on the 
application of transit nodes. In FIG. 6 a mobility server MOBS2 acts as a 
transit node between mobility server MOBS1 interfacing a PBX1 on site A 
and mobility server MOBS3 interfacing a PBX3 on a site C and thus there 
does not have to be interconnection between every mobility server. 
FIG. 7 is a very schematical illustration of a network with large sites 
wherein each site requires a number of networked mobility servers. In site 
A a PBX-A is interfaced by two mobility servers and MOBS A1, MOBS A2 which 
in turn interface with a transit node MOBS XA. The corresponding 
arrangement is relevant also to site B and the transit nodes formed by the 
mobility servers MOBS XA and MOBS XB are interfaced with each other. 
The concept of transit nodes can as referred to above be used when the 
sites are large but it can for example also be used in large networks 
comprising many nodes. Of course it can also be used under any other 
circumstances when it is found appropriate. 
Under reference to a particular embodiment as shown in FIG. 8 the 
interworking of a mobility server with a radio exchange and a private 
branch exchange respectively will be discussed as well as the signalling 
within an overlay network formed by a number of mobility servers. Of 
course the principles as discussed in relation to this embodiment are also 
valid to other embodiments, as for example those described in the 
foregoing examples. The interwork between a PBX or in the shown embodiment 
PBX1, with the mobility server and MOBS1 can take place using any type of 
extension interface. The extension interface may for example provide 
concentrated access e.g. via the ISDN S.sub.2 interface or may provide 
multiplexed access over the PCM/CAS interface but it may also be an 
analogue line. PCM (Pulse Code Modulation) is a common transmission 
technique worldwide. ITU (formerly CCITT) has issued a number of 
recommendations for this technique. CAS (Common Channel Signalling) 
indicates that all bearer channels share one signalling channel for line 
signalling information. The CCITT recommendations in series G.700-G.795 
describe general aspects of digital transmission systems. In particular 
the following CCITT recommendations apply to PCM and CAS as to be applied 
in the described application: G.703 "Physica/electrical characteristics of 
hierarchical digital interfaces", G.704 "Synchronous frame structures used 
at primary and secondary hierarchical levels", G.711 "Pulse code 
modulation of voice frequencies", G.731 "Primary PCM multiplex equipment 
for voice frequencies", G.732 "Characteristics of primary PCM multiplex 
equipment operating at 2.048 Mbps", G.733 "Characteristics of primary PCM 
multiplex equipment operating at 1.544 Mbps", Q.512 "Exchange interfaces 
for subscriber access". 
The interworking between REX1 and MOBS1 (and REX2 anad MOBS2) is effected 
via the ISDN S.sub.2 interface supporting intelligent D-channel signalling 
e.g. for mobility management. The overlay network formed by a number of 
mobility servers, in this case MOBS1 and MOBS2, acts as an access network 
between REXes and PBXes (here REX1 and REX2). Now a new concept is 
introduced, namely the application of home-PBXes. This implies that each 
cordless user is an extension In a specific PBX, the home-PBX, as any 
conventional extension, also non-mobile extensions, with a directory 
number, a class-of-service, a number of accessible features and a line 
status. Connections to and from a cordless user such as incoming and 
outgoing calls are always routed through the home-PBX. In the illustrated 
example PBX1 forms a home-PBX for the portable 2525. The overlay network 
of mobility servers MOBS1, MOBS2 uses in this embodiment ISDN Q-SIG 
private network signalling for mobility management. The signalling 
provides transparent access signalling between portables and the private 
branch exchange. Q-SIG is an ISDN network protocol set for private 
networks specified by ETSI. The following ETSI standards apply to basic 
call and as a basis for supplementary services: ETS 300 170, "Data link 
layer protocol at the Q reference point for the signalling channel between 
two private telecommunication network exchanges", ETS 300 172, "Layer 3 
protocol for signalling between exchanges of private telecommunication 
networks for control of circuit switched calls", ETS 300 239, "Generic 
fuctional protocol for the support of supplementary services". The 
following draft ETSI standards apply to cordless mobility in private 
networks: QSIG-CTAU, "Cordless terminal authentication", QSIG-CTLR, 
"Cordless terminal location registration" QSIG-CTMI, "Incoming CTM call 
handling", QSIG-CTMO, "outgoing CTM call handling". 
Mobility management signalling between the mobility servers MOBS1 and MOBS2 
is in an advantageous embodiment carried in ISDN Q-SIG facility 
information elements in agreement with standards proposed by ETSI/ECMA for 
private network cordless mobility. (ECMA relates to a standard body 
(formerly European Computer manufactures Association) standardizing 
information and communication systems, e.g. private telecommunications 
networks.) In order to provide for authentication the DECT (Digital 
European Cordless Telecommunications Standard; the standard e.g. specifies 
the radio interface between a cordless portable and a base station. The 
standard is specified by the technical committee RES3 of ETSI.) standard 
may be applied. This is effected from the radio exchange REX1 via the 
adjunct mobility server MOBS1 across the network to an authentication 
database (not illustrated in the figures) In a manner known per se. Also 
for the identification function through which a portable automatically is 
identified by the network the DECT standard may be used, according to an 
advantageous embodiment in combination with the PTN standard. For the 
roaming function, updating of location register, a common database is used 
according to an advantageous embodiment. This common database can be used 
for the entire overlay network and it may for example be in the form of an 
internal central database but it may also take the form of an internal 
distributed database. In another embodiment the common database comprises 
a central external data base. The implementation of the data base can also 
be done in other ways and which implementation that is used for example 
depends on the size of the overlay network and the size of the extension 
profile etc. 
For ciphering the DECT standard may be used. 
What has been described in relation to FIG. 8 in general relates to 
mobility within a network (in the FIG. merely 2 are shown) of mobility 
servers. The invention however also relates to providing mobility within 
one site but between different radio exchanges either as a standalone 
function but of course also within a network. Handover between radio 
exchanges REX interfacing one and the same mobility server is supported by 
this mobility server. As referred to above the mobility server provides 
for transparent signalling between a radio exchange REX and a private 
branch exchange PBX wherein transparent signalling means that it appears 
that the REX and the PBX are directly interconnected without the 
intermediate mobility serving means. This transparent link is set up both 
for intra-site calls and for inter-site calls. For inter-site calls, i.e. 
calls between different sites, a roaming cordless portable telephone gets 
a direct link to its home-PBX via the overlay network. The mobility server 
emulates a REX in its interwork with the PBX. Through this arrangement all 
the extension features and services which are applicable to a DTMF 
extension will also be applicable to a cordless portable. These extension 
features may for example relate to user procedures comprising digits 0-9, 
* and #, as well as the R-key. The services are available both on one site 
and throughout the overlay network. The key-pad procedures are carried in 
ISDN Q-SIG facility Information elements. ECMA provides the unique object 
identifier. This makes the proprietary signalling information being 
handled as transparent information by any foreign network node supporting 
the generic procedures which are defined by ETSI and ECMA. Thereby a mixed 
network of switches can be allowed. 
The invention as described in relation to the particular embodiments is of 
course not limited to the shown number of exchanges but a network may 
comprise any number thereof and take any form. The invention is also not 
limited to the mentioned interfaces; other interfaces can also be used as 
well as the invention is not limited to the mentioned standards but it is 
applicable using other standards as well. The examples of services 
provided is of course also not exclusive; a number of other services can 
also be provided and it is only a question of which services are provided 
by the private network or the private exchange.