The present invention relates to telecommunications networks and particularly, although not exclusively, to communicating packet data in Fixed Wireless Access (FWA) networks. 
In a fixed wireless access (FWA) telecommunications system, subscribers are connected to a backbone telecommunications network by means of radio links in place of traditional copper wires.  Each of a plurality of subscribers is provided with a subscriber radio terminal.  A plurality of subscriber radio terminals of a plurality of subscribers premises communicate with a base station which provides cellular coverage, typically in urban environments over a 5 km radius.  Each base station may be connected to a Public Switched Telecommunications Network (PSTN) switch via a conventional transmission link, known as a backhaul link.  A number of potential subscribers sites in a base stations"" area of transmission can be of the order of several thousands.  A single base station can serve up to several thousand subscribers, making the installation and maintenance cost of a fixed wireless access system lower than that of an equivalent copper wire access network. 
Referring to FIG.  1 herein, there is illustrated a prior art fixed wireless access system.  A plurality of subscriber radio terminals 100 each comprising a transceiver 101 and an antenna 102, part of a residential subscriber system (RSS) installed at a subscriber""s premises, communicate with a radio base station 103 having a base station antenna 104 and a base station transceiver apparatus 105.  A plurality of such radio base stations 103 each communicate with a central office switch 106 to gain access to a backbone telecommunications network 111, eg a Public Switched Telephone Network (PSTN) or an Integrated Services Digital Network (ISDN).  In a geographical area, each base station 103 is connected to a local exchange switch 106 via a backhaul transmission line 107 which may comprise for example a terrestrial line eg fiber optic cable or coaxial cable, or a microwave transmission link.  Communication between the subscriber radio terminal and the base station is via a wireless radio link 108.  Each local wireless link 108 between radio base station 103 and subscriber radio terminal 100 comprises an uplink beam from the subscriber transceiver and antenna to the radio base station, and a downlink beam transmitting from the radio base station antenna and transceiver to the subscriber antenna and transceiver.  Equal spectrum of frequency slots are allocated for uplink and downlink according to a frequency division duplex scheme, in which a first frequency of a frequency pair is allocated for uplink transmission and a second frequency of the frequency pair is allocated for downlink transmission. 
Conventional fixed wireless access systems are narrow band systems which are mainly designed for providing narrow band circuit switched telecommunications services such as telephony, fax or modem.  On the other hand, with increased penetration of personal computers into domestic residential markets, access to Internet services is increasingly demanded by subscribers.  Thus, it is desirable for subscribers to a FWA system to be able to connect their Personal Computers (PCs) 109 to their RSSs for accessing the Internet rather than require a conventional wire based communications network line to be installed at their premises.  The most popular Internet services include world wide web services and downloading of files.  In particular, subscribers are increasingly making use of Internet services downloaded from Internet service providers.  Internet access traffic is characterized as being highly asymmetric in the transfer of data as between a subscriber and an Internet service provider.  This results in a significant difference in data rate in one direction on a subscriber line compared with another, appropriate direction.  Internet access also has the characteristic of having a relatively long holding time.  For example, a World Wide Web (WWW) session may last for a few hours. 
For example, a dominant Internet access traffic type in the residential subscriber market is generated by web browsing.  Typical figures on an average download on a page of data by a subscriber is around 50 kBytes, and some estimates project an increase of this figure to around 150 kBytes by the year 2000.  Assuming subscribers will tolerate a 6 second waiting time during the page download, a peak data rate for download of web browsing data over a fixed wireless access link is of the order of 64 kbits/s increasing to around 192 kbits/s by the year 2000.  After page download, subscribers typically take time to study the information downloaded.  Estimates of an average time for such study is around 24 seconds, giving an average data rate on a fixed wireless access downlink of around 13 kbits/s, estimated to increase to around 39 kbits/s by the year 2000. 
However, on an uplink, data sent from a subscriber to an Internet service provider is typically very light compared to the data downloaded from the Internet service provider.  An approximate estimate of uplink loading requirement is that the uplink loading is typically around one tenth of the downlink loading for Internet services, ie one subscriber will produce a peak uplink rate of around 6.4 kbits/s (increasing to 19.2 kbits/s by the year 2000) and an average data rate of 1.3 kbits/s (increasing to 3.9 kbits/s by the year 2000). 
Whilst conventional fixed access wireless systems may be efficient for services, eg voice data, having a relatively balanced data rate in each direction, maintaining a circuit switched connection over a fixed wireless access link for services having an asymmetry of data rate as between different directions of a subscriber link is inefficient use of available wireless link bandwidth. 
FIG.  2 of the accompanying drawings schematically illustrates a prior art uplink transmission from RSS antenna 102 to radio base station 103.  The uplink transmission 201 comprises a sequence of timeslots, each lasting approximately 300 microseconds.  A first timeslot 202 of transmission 201 comprises an uplink transmission from subscriber to the base station.  Timeslot 203, immediately following timeslot 202, would usually represent a pause in the transmission by the the subscriber whilst the base station receives data transmitted in timeslot 202.  Timeslot 204, which immediately follows timeslot 203, usually represents a gap when the RSS is still unable to make a downlink transmission to the subscriber RSS.  Timeslot 205, which immediately follows timeslot 204, represents a second timeslot when the radio base station is able to make a downlink transmission to the subscriber RSS.  Thus, each downlink transmission timeslot is separated by two intermediate timeslots. 
Gap timeslot 204 represents a delay when the RSS may change the carrier frequency which it uses to transmit to the base station.  Other procedures may also be executed by the RSS during timeslot 204 and other subscriber terminals may communicate with the base station.  The gap may also exist in order to allow a transmit timeslot 205 (immediately following timeslot 204) to be time aligned with other transmissions, ie to attempt to ensure that all transmissions to the radio base station arrive at the same time. 
In addition to data transfer relating to internet services being asymmetric, ie, considerably more data being transmitted on the downlink than on the uplink as discussed herein above, the data transferred can be of a bursty nature, thus the data is transmitted intermittently, compared with, for example, voice data which tends to be continuous.  The continuous nature of circuit switch data, such as voice, means that it is undesirable for the RSS to change the carrier frequency which results in gaps, such as timeslots 204.  However, for package switch data, the constraints of maintaining continuous data transfer is not present, therefore, it is more acceptable for the RSS to change carrier frequency more frequently, because the bursty nature of the data transferred allows a training sequence to take place during the gaps in data transfer. 
In general, each subscriber RSS will experience different propagation characteristics over its uplink/downlink air interface.  The radio base station transmits on the downlink over a plurality of downlink frequencies.  Each downlink frequency is divided into a plurality of timeslots.  It may not be possible for each subscriber RSS to communicate on all of the downlink frequencies offered by the radio base station.  Individual subscriber RSSs may be restricted to a selection of a few of the downlink frequencies available.  As bearer timeslots on preferred downlink frequencies become occupied by communications, it is inefficient to place an existing circuit switched connection onto a different bearer timeslot on a different carrier frequency.  The reason for this inefficiency is that every time a circuit switched connection is moved from one to bearer timeslot to another bearer timeslot, there is a relatively long training sequence required for transfer of that circuit switched connection between different bearer timeslots.  Thus, consecutive bearer timeslots carrying a circuit switched connection must be at least two intermediate timeslots apart to allow transfer of a circuit switched connection from one bearer timeslot to another bearer timeslot.  For example, a circuit switched connection on a frequency division duplex pair may carry a first 500 xcexcs timeslot for transmit on the uplink, and then receive a second 500 xcexcs timeslot on the downlink, before proceeding to transmit a third 500 xcexcs timeslot on the uplink followed by a fourth 500 xcexcs timeslot on the downlink, and so on throughout the duration of a circuit switched connection.  For a change of carrier frequency, there must be an interruption in communication, since changing the circuit switch connection from one frequency to another takes longer than 1000 xcexcs, meaning that a transmit timeslot or a received timeslot must be omitted in the communication.  Since data carried on circuit switch connections, eg voice data is more susceptible to interruptions than packet switched data, to avoid degradation of grade of service, it is more efficient once the circuit switched connection is set up, to retain that circuit switched connection on a same bearer timeslot on a same carrier frequency.  However, packet switched data may tolerate higher levels of delay, due to the bursty nature of data transfer and also since the packet switched data may be completely retransmitted if necessary, it is not as delay sensitive as circuit switched traffic (eg voice data) in general. 
An object of the present invention is to provide efficient packet switched data transfer for subscribers to a FWA network.  This may be achieved by taking advantage of the asymmetric and bursty nature of data transfer resulting from use of services such as internet access. 
A further object of the present invention is to provide a bandwidth on demand data packet transfer system over a FWA network. 
Preferably, a group of users in a cell transmitted to by a radio base station share a common logical channel on a frame by frame basis which can be yielded to circuit switched traffic when there are few or no more spare air side logical channels available for the circuit switched data.  Logical channel sharing is preferably provided in the form of closed user groups wherein each user of the user group is required to register and join the group before using the shared logical channel.  Preferably, a user group forms a virtual pipe over an air interface and data packet at both ends of the air interface may be dynamically routed or switched along a number of parallel user groups.  Preferably, the system can be added to an existing known air interface protocol.  The system may take advantage of a broadcast nature of downlink transmissions from a radio base station so that several subscribers belonging to a closed user group may listen to the same logical channel and, using a token mechanism, a user who may make an uplink transmission on a next frame may be decided. 
According to one aspect of the present invention there is provided a communications apparatus for transferring packet switched data said apparatus comprising:
a radio base station capable of transmitting downlink data packets each said data packet being transferred in a specified single timeslot replicating over a plurality of time frames; and
at least one network subscriber equipment capable of receiving said downlink data packets;
wherein said subscriber equipments are joined as members of at least one user group, each said user group configured to receive said downlink packets in said specified single repeating timeslot only. 
According to a second aspect of the present invention there is provided a method of communicating packet switched data comprising the steps of:
communicating a downlink data packet from a radio base station to at least one network subscriber equipment,
wherein said network subscriber equipment is registered as a member of a user group, said user group configured to receive said downlink packets in a specific single timeslot only. 
According to a third aspect of the present invention there is provided a method of communicating packet switched data over a wireless link, said method comprising the steps:
registering at least one subscriber lines for receipt of said packet switched data on a downlink channel of said wireless link; and
communicating said packet switched data on said downlink channel;
wherein said downlink packet switched data carries an address of a group of registered said subscriber lines, for which said data is intended. 
According to a fourth aspect of the present invention there is provided a method of communicating packet switched data over a wireless link, said method comprising the steps of:
registering at least one subscriber line for receipt of said packet switched data on a downlink channel of said wireless link;
allocating a single uplink channel of said wireless link for receipt of packet switched data from a group of said subscriber lines;
transmitting a token data on said downlink, said token data indicating which of said registered subscriber lines has authorization to transmit on said uplink channel. 
According to a fifth aspect of the present invention there is provided a method of communicating packet switched data over a wireless link, said method comprising the steps of:
receiving packet switched data on a downlink channel of said wireless link, said downlink channel allocated for receipt by a group of subscriber lines;
receiving a token data indicating authorization to transmit packet switched data on an allocated uplink channel; and
transmitting said packet switched data on said allocated uplink channel. 
According to a sixth aspect of the present invention there is provided a method of registering a plurality of users of a communications network for receipt of packet switched data services over a wireless link, said method comprising the steps of:
receiving a registration request from a said network user;
in response to said registration request, allocating a user identification data to said user;
allocating an identification data to said user; and
allocating at least one logical communications channel to said user. 
According to a seventh aspect of the present invention there is provided in a communications network operating a wireless link for communicating packet switched data to a network user of at least one channel of a plurality of channels, a method of re-synchronizing a user with a said channel to which said user has been previously assigned, said method comprising the steps of:
determining if said user is already assigned to a said logical channel;
if said user is already assigned to said logical channel, retrieving data identifying
a user group to which said user belongs;
a logical channel to which said group is assigned;
a physical channel to which said group is assigned; and
transmitting said data identifying said user group, said logical channel and said physical channel over said wireless link.