Integrated cellular voice and digital packet data telecommunications systems and methods for their operation

An integrated voice and packet data telecommunications system has at least one dual mode channel. The system comprises a plurality of transceivers, at least one transceiver being operable to transmit and receive voice traffic on the dual mode channel, and at least one transceiver being operable to transmit and receive packet data traffic on the dual mode channel. The system further comprises a controller for controlling the transceivers so as to allocate the dual mode channel either to voice communications or to packet data communications. The controller responds to changing demand for voice channels and packet data channels by changing dynamically the allocation of the dual mode channel. The system is particularly useful for providing Cellular Digital Packet Data (CDPD) services.

FIELD OF INVENTION 
This invention relates to cellular voice and Cellular Digital Packet Data 
(CDPD) telecommunications systems, and to methods for their operation. 
BACKGROUND OF INVENTION 
In conventional cellular telephone networks, a base station is provided for 
each cell of the area served by the cellular network. Each base station 
comprises a plurality of radio transceivers which provide radio channels 
for voice communications between the base stations and mobile telephones 
in the cells served by the base stations. The base stations are connected 
to mobile switching centers which provide telecommunications switching 
between base stations. A gateway mobile switching center is connected 
between the mobile switching centers and a Public Switched Telephone 
Network (PSTN) so that mobile telephones served by the cellular telephone 
networks can be connected to telephones served by the PSTN. 
In addition to voice telephony services provided to mobile telephone users 
by cellular telephone networks, there is a demand for packet data services 
provided to mobile data terminals. In April 1992 an industry consortium 
was formed to develop standards for providing Cellular Digital Packet Data 
(CDPD) services. In July 1993 this consortium released version 1.0 of a 
CDPD Specification which defines standard interfaces and functionality for 
CDPD networks. Version 1.0 of the CDPD Specification is hereby 
incorporated by reference. 
A CDPD network may be implemented as an overlay on an existing cellular 
telephone network. The CDPD Specification calls for Mobile Data Base 
Stations (MDBSs) to serve mobile data terminals called Mobile End Stations 
(MESs). The MDBSs are connected to Mobile Data Intermediate Systems 
(MDISs) which are connected to external public or private Packet Data 
Networks (PDNs) so that the MESs can exchange packet data with Fixed End 
Stations (FESs) connected to the PDNs. 
The MDBSs use the same radio frequency channels to exchange packet data 
with the MESs as do voice base stations serving mobile telephones in the 
same serving area. To avoid radio interference bet-ween packet data 
transmissions and voice transmissions, the MDBSs must use radio frequency 
scanners to scan the voice channels to determine which voice channels are 
currently in use by the voice base stations serving the same area, and 
tune their transceivers to only those channels which are not currently in 
use for voice communications. Consequently, the MDBSs "hop" among the 
voice channels to avoid voice calls which are currently in progress. 
The frequency scanning and retuning operations of the MDBSs require 
considerable processing. Moreover, each frequency hop executed in order to 
"dodge" a voice call interrupts packet data transmission, reducing the 
data throughput of the CDPD network. Furthermore, because expensive MDBS 
hardware, MDIS hardware and transmission facilities linking the MDBS 
hardware to the MDIS hardware are needed to provide CDPD service, the cost 
of introducing CDPD service is higher than desired, particularly where the 
initial demand for CDPD service is limited. If the CDPD service providers 
price the service high enough to pay back their equipment investment 
quickly or limit deployment of CDPD service to high traffic areas, they 
risk limiting CDPD market growth. 
Moreover, the boundaries of cells served by MDBSs do not coincide exactly 
with the boundaries of cells served by voice base stations even when the 
MDBSs and the voice base stations are co-located. The cell boundaries do 
not coincide exactly because the intercell hand off criteria are different 
for voice and packet data transmission. The mismatch of cell boundaries 
can lead to excessive interference between channels used for voice 
communications and channels used for packet data communications. 
SUMMARY OF INVENTION 
An object of this invention is to reduce or avoid some or all of the 
disadvantages of CDPD networks as outlined above by integrating CDPD 
equipment with equipment providing voice services. 
One aspect of this invention provides an integrated voice and packet data 
telecommunications system having at least one dual mode channel. The 
system comprises a plurality of transceivers. At least one of the 
transceivers is operable to transmit and receive voice traffic on the dual 
mode channel, and at least one of the transceivers is operable to transmit 
and receive packet data traffic on the dual mode channel. The system 
further comprises a controller for controlling the plurality of 
transceivers so as to allocate the dual mode channel to one of voice 
communications and packet data communications. The controller is operable, 
in response to changing demand for voice channels and packet data 
channels, to change dynamically the allocation of the dual mode channel. 
The use of a common controller for voice and packet data services avoids 
the need to scan the voice channels to determine which voice channels are 
currently in use because that information is already available in the 
controller. This avoids the cost of radio frequency scanners and the 
processing resources needed to drive the radio frequency scanner. In 
addition, the common controller can be designed so as to assign channels 
to voice and packet data traffic in a more orderly manner to reduce the 
number of channel hops needed for packet data traffic, as will be 
explained in greater detail below. This increases the packet data 
throughput without increasing voice call blocking. 
In one embodiment suited to applications in which voice traffic is accorded 
priority over packet data traffic, the controller is responsive to a 
demand for a voice channel to allocate the dual mode channel to voice 
communications, and is responsive to no demand for a voice channel to 
allocate the dual mode channel to packet data communications. 
In most practical implementations, the system will have a plurality of dual 
mode channels, and the plurality of transceivers will be operable to 
transmit and receive voice traffic on any of the dual mode channels. The 
plurality of transceivers will also be operable to transmit and receive 
packet data traffic on any of the dual mode channels. The controller will 
respond to demands for voice channels by selecting dual mode channels for 
allocation to voice communications, and will allocate to packet data 
communications any dual mode channel not selected for allocation to voice 
communications. 
The controller may be operable to maintain a dual mode queue of dual mode 
channels not allocated to voice communications, and, in response to a 
demand for a voice channel, to select a dual mode channel according to its 
position in the dual mode queue. The controller may further be operable, 
in response to release of the dual mode channel allocated to voice 
communications, to return the dual mode channel to the dual mode queue and 
to reallocate the dual mode channel to packet data communications. The 
controller may operate the dual mode queue as a Last In, First Out (LIFO) 
queue so as to provide as many interruption-free packet data channels as 
the voice traffic conditions will permit. 
The system may further comprise a plurality of voice channels in addition 
to the plurality of dual mode channels, the voice channels being dedicated 
to voice communications. In this case, the controller may also be operable 
to maintain a voice queue of idle voice channels. The controller may be 
operable, in response to a request for a voice channel when at least one 
voice channel is present in the voice queue, to select a channel from the 
voice queue, and may be operable, in response to a request for a voice 
channel when no voice channel is present in the voice queue, to demand a 
channel from the dual mode queue. In this manner, the controller only 
allocates dual mode channels to voice calls when no voice channels are 
available, thereby minimizing interruptions to packet data transmission 
for maximum packet data throughput. 
Alternatively, because the number of interruptions to packet data 
transmission is reduced, the duration of the switching operations 
performed at each interruption has a smaller impact on the data 
throughput. Consequently, the design constraints on this switching 
duration may be relaxed, reducing the cost of the hardware and software 
implementation. 
To further improve packet data throughput, the controller may be operable, 
in response to release of a voice channel when at least one dual mode 
channel is allocated voice communications, to select a dual mode 
transceiver, to hand off a voice call served by the selected dual mode 
channel to the released voice channel, to return the selected dual mode 
channel to the dual mode queue, and to reallocate the selected dual mode 
channel to packet data communications. The hand off can be triggered only 
when the dual mode queue is empty to minimize voice call hand offs. 
Alternatively, the hand off can be triggered if any dual mode channels are 
allocated to voice communications to maximize packet data throughput. The 
controller can be made operator configurable with respect to these hand 
off options. 
The integrated voice and packet data telecommunications system may be a 
cellular system having a plurality of cells, a respective subset of the 
plurality of voice channels being assigned to each cell and a respective 
subset of the plurality of dual mode channels being assigned to each cell. 
In particular, the frequency plan for the dual mode channels may be 
distinct from the frequency plan for the voice channels. 
The use of distinct frequency plans for the dual mode and voice channels 
reduces the interference between voice transmissions and packet data 
transmissions that can result from the different intercell hand off 
algorithms used for voice and packet data communications. 
The system may also comprise one or more packet data channels which are 
dedicated to packet data operation to ensure a minimum level of packet 
data throughput regardless of the voice traffic. In this case the above 
hand off options may be disabled as long as one or more of the packet data 
channels is in operation. 
One or more of the plurality of transceivers may be a dual mode transceiver 
which is operable in a voice mode to transmit and receive voice traffic, 
and operable in a packet data mode to transmit and receive packet data 
traffic. In this case, the controller may be operable to switch the dual 
mode transceiver between the voice mode of operation and the packet data 
mode of operation. 
Thus, another aspect of the invention provides an integrated voice and 
packet data telecommunications system comprising at least one dual mode 
radio transceiver and a controller. The dual mode radio transceiver is 
operable in a voice mode to transmit and receive voice traffic and 
operable in a packet data mode to transmit and receive packet data 
traffic. The controller is operable to switch the dual mode transceiver 
between the voice mode of operation and the packet data mode of operation. 
Each dual mode transceiver may be implemented as a processor combined with 
at least one radio transmitter and at least one radio receiver. The 
processor may be operable in a voice mode for voice communications and in 
a packet data mode for packet date communications. 
Thus, another aspect of the invention provides a dual mode radio 
transceiver comprising at least one radio transmitter, at least one radio 
receiver and a processor for processing signals to be transmitted by the 
radio transmitter and for processing signals received by the radio 
receiver. The processor is configurable in a voice mode for processing 
voice traffic, and is configurable in a packet data mode for processing 
packet data traffic. 
Because the dual mode transceivers and the controller are shared between 
voice and packet data services, packet data services can added to voice 
services for the relatively low incremental cost of the software required 
to provide the packet data services. Moreover, packet data services can be 
added to existing voice services without coupling additional radio 
frequency equipment to existing cell site antennas, and without 
interruptions to existing voice services to install such equipment. 
Furthermore, because the transceivers required for voice and packet data 
services are located at a common base station site, voice signals and 
packet data signals can be multiplexed together for transmission to and 
from the base station site on a shared multiplexed transmission link to 
minimize transmission facility costs. 
Another aspect of this invention provides a method of operating an 
integrated voice and packet data telecommunications system, the system 
having a plurality of dual mode channels, each operable in a voice mode 
for voice communications and operable in a packet data mode for packet 
data communications. The method comprises maintaining a dual mode queue of 
dual mode channels allocated to packet data communications, selecting, in 
response to a demand for a voice channel, a dual mode channel according to 
its position in the dual mode queue, and allocating the selected channel 
to voice communications.

DETAILED DESCRIPTION 
FIG. 1 is a block schematic view of a CDPD system 200 overlaid on a 
cellular voice telephony system 100 according to the CDPD Specification. 
The cellular voice telephony system 100 comprises a plurality of voice base 
stations (VBSs) 110 interconnected by a plurality of mobile switching 
centers (MSCs) 120. Each VBS 110 comprises a plurality of voice radio 
transceivers (VTs) 112 which provide radio frequency channels for voice 
communications between the VBSs 110 and mobile voice terminals (for 
example, MVT 300) in cells served by the VBSs 110. 
The VBSs 110 are connected to the MSCs 120 via multiplexed transmission 
links, for example T1, E1 or other standard or proprietary format 
multiplexed transmission links. The MSCs 120 provide telecommunications 
switching between the VBSs 110. The MSCs 120 comprise a resource manager 
(RM) 122 which controls the allocation of radio channels to voice calls. 
A gateway MSC (GMSC) 130 is connected between the MSCs 120 and a Public 
Switched Telephone Network (PSTN) 400 so that MVTs 300 served by the 
cellular voice telephony system 100 can be connected to telephones 500 
served by the PSTN 400. 
The CDPD system 200 comprises a plurality of mobile data base stations 
(MDBSs) 210 interconnected by a plurality of mobile data intermediate 
systems (MDISs) 220. Each MDBS 210 comprises a plurality of packet data 
radio transceivers (PDT) 212 which provide packet data radio channels for 
packet data communications between the MDBSs 210 and mobile end systems 
(MESs) 600 in cells served by the MDBSs 210. The MDBSs 210 further 
comprise a scanning transceiver (ST) 214 which scans the radio frequency 
channels used by the VBSs 110 to determine which voice channels are 
currently in use. The PDTs 212 are tuned to radio frequency channels which 
are not currently in use by the VBSs 110 to provide packet data 
communications between the MDBSs 210 and the MESs 600. Consequently, the 
MDBSs 210 "hop" among the radio frequency channels to avoid voice calls 
which are currently in progress. (See part 405 of the CDPD System 
Specification, Release 1.1 issued by the CDPD-Forum on Jan. 19, 1995.) 
The MDISs 220 are connected to public or private packet data networks (for 
example PDN 700) so that MESs 600 served by the CDPD system 200 can be 
connected to fixed end stations (for example FES 800) which are served by 
the PDNs 700. 
The frequency scanning and retuning operations of the MDBSs 210 combined 
with overhead data transfer operations needed to effect movement of packet 
data traffic from one channel to another channel amount to a considerable 
processing load on the MDBSs 210. Moreover, each frequency hop executed in 
order to "dodge" a voice call interrupts packet data transmission, 
reducing the data throughput of the CDPD system 200. Furthermore, because 
expensive MDBS hardware (including the ST 214), MDIS hardware and 
transmission facilities linking the MDBS hardware to the MDIS hardware are 
needed to provide CDPD service; the cost of introducing CDPD service is 
higher than desired, particularly where the initial demand for CDPD 
service is limited. The boundaries of cells served by the MDBSs 210 do not 
coincide exactly with the boundaries of cells served by VBSs 110 even when 
the MDBSs and the voice base stations are co-located because the intercell 
hand off criteria are different for voice and packet data transmission. 
The mismatch of cell boundaries can lead to excessive interference between 
channels used for voice communications and channels used for packet data 
communications. 
FIG. 2 is a block schematic diagram of an integrated CDPD and cellular 
voice telephony system 900 according to an embodiment of the invention. 
The integrated system 900 comprises a plurality of dual mode base stations 
(DBMSs) 910 interconnected by a plurality of Nortel MTX.TM. mobile 
switching centers (MTXs) 920. Each DMBS 910 comprises a plurality of voice 
radio transceivers (VTs) 912 which provide voice radio channels for voice 
communications between the DMBSs 910 and mobile voice terminals (for 
example, MVT 300) in cells served by the DMBSs 910. 
The DMBSs 910 are connected to the MTXs 920 via multiplexed transmission 
links, for example T1, E1 or other standard or proprietary format 
multiplexed transmission links. The MTXs 920 provide telecommunications 
switching between the DMBSs 910. The MTXs 920 comprise a resource manager 
(RM) 922 which controls the allocation of radio channels to voice calls. 
The MTXs 920 also perform the function of a gateway MSC, connecting the 
integrated system 900 to the Public Switched Telephone Network (PSTN) 400 
so that MVTs 300 served by the integrated system 900 can be connected to 
telephones 500 served by the PSTN 400. 
Each DMBS 910 further comprises a plurality of dual mode radio transceivers 
(DMTS) 914. FIG. 3 is a block schematic diagram showing a DMT 914 in more 
detail. The DMT 914 comprises a radio transmitter 10, and radio receiver 
20, and a signal processor 30 comprising a processing unit 32 and a memory 
34 for storing instructions to be executed by the processing unit and data 
required for execution of those instructions. The signal processor 30 
receives voice and packet data signals from the MTX 920 and processes 
those signals for transmission by the radio transmitter 10. The signal 
processor 30 also receives voice and packet data signals from the radio 
receiver 20 and processes those signals for transmission to the MTX 920. 
The signal processor 30 receives control signals from the RM 922 to switch 
the signal processor 30 between a voice mode in which it provides signal 
processing appropriate for voice signals and a packet data mode in which 
it provides signal processing appropriate for packet data signals. 
Consequently, the DMTs 914 are operable in a voice mode to exchange voice 
traffic with MVTs 300 in cells served by the DMBSs 910, and operable in a 
packet data mode to exchange packet data traffic with MESs 600 served by 
the DMBSs 910. The RMs 922 of the MTXs 920 operate as controllers for 
switching the DMTs 914 between the voice mode of operation and the packet 
data mode of operation as will be explained in greater detail below. 
Each DMBS 910 further comprises a packet data transceiver (PDT) 916 which 
operates only in the packet data mode and is dedicated to exchanging 
packet data signals with MESs 600 served by the DBMSs 910. 
The MTXs 920 perform MDIS functions for packet data transmissions and are 
connected to public or private packet data networks (for example PDN 700) 
so that MESs 600 served by the integrated system 900 can be connected to 
fixed end stations (for example FES 800) which are served by the PDNs 700. 
The VTs 912, DMTs 914 and PDT 916 are connected to the RMs 922 of the MTXs 
920 via one or more shared multiplexed transmission links. 
The RM 922 of each MTX 920 maintains a VT queue and a DMT queue for each 
DMBS 910 served by the MTX 920. The VT queue contains identifiers of idle 
VTs 912. The DMT queue contains identifiers of DMTS 914 which are 
currently operating in packet data mode. 
FIG. 4 is a flowchart illustrating a first part of a channel allocation 
algorithm used by the RMs 922 to allocate radio channels in the integrated 
system 900. When a request for a voice channel is received by the RM 922 
(either because a MVT 300 is attempting to initiate a voice call or 
because another terminal is attempting to initiate a call to a MVT 300), 
the RM 922 first examines the voice queue to determine whether any VTs 912 
are idle. If idle VTs 912 are found in the voice queue, the RM allocates 
an idle VT 912 to the voice call and updates the voice queue. 
If the voice queue is empty, the RM 922 examines the dual mode queue to 
determine whether any DMTs 914 are operating in packet data mode. (The RM 
922 automatically configures any DMTs 914 that are not allocated to a 
voice call in packet data mode for transmission of packet data on demand.) 
If DMTs 914 are found in the dual mode queue, the RM 922 allocates a DMT 
914 from the dual mode queue on a last in, first out (LIFO) basis, and 
updates the dual mode queue. 
If the voice queue and the dual mode queue are both empty, the RM 922 
initiates refusal of the voice channel request. 
The channel allocation algorithm described above ensures maximum use of all 
voice channels before packet data transmission are interrupted to provide 
voice communications. Moreover, as many of the DMTs 914 as possible are 
used for uninterrupted packet data transmissions. In addition, the PDT 916 
is always used for uninterrupted packet data communications. 
FIG. 5A is a flowchart illustrating a second part of the channel allocation 
algorithm when the RM 922 is configured in a full dual mode queue hand off 
configuration. Voice channels are released when a voice call served by a 
DMBS 910 is handed off to another DMBS 910, when a MVT 300 disconnects or 
when a release order is received indicating that the network or another 
terminal has disconnected. When a voice channel is released, the RM 922 
determines whether the released voice channel was provided by a VT 912 or 
a DMT 914. If the released voice channel was provided by a DMT 914, the 
DMT 914 is returned to the dual mode queue and switched to packet data 
mode. 
If the released voice channel was provided by a VT 912, the transceiver 
which provided by the released voice channel is now available for use by 
any other voice call. In particular, the VT 912 which provided the 
released voice channel could now provide a voice channel for any voice 
call which is currently being handled by a DMT 914 operating in voice 
mode. This would enable the DMT 914 to switch to packet data mode to 
provide higher packet data throughput. 
Consequently, if the released voice channel was provided by a VT 912, the 
RM 922 selects a DMT 914 which is currently operating in voice mode, hands 
off a voice call from the selected DMT 914 to the VT 912 which previously 
provided the released voice channel, and returns the selected DMT 914 to 
the dual mode queue, switching the selected DMT 914 to the packet data 
mode. 
Advantageously, the RM 922 may select the DMT 914 which was last allocated 
from the dual mode queue for the voice call hand off. The RM 922 may 
maintain a DMT voice queue for this purpose, entering the DMTs 914 into 
the DMT voice queue when they are switched from the packet data mode to 
the voice mode. The RM 922 may then select DMTs 914 from this queue on a 
LIFO basis when VTs 912 able to provide voice channels become available. 
The full dual mode hand off arrangement described above may result in a 
large number of voice call hand offs in heavy voice traffic and this may 
be deemed unacceptable for some applications. To increase packet data 
throughput while reducing voice call handoff activity, the steps of FIG. 
5A may be replaced by the steps of FIG. 5B which implement a partial dual 
mode handoff strategy. According to the partial dual mode hand 
off-strategy, voice calls are handed off from DMTs 914 to released VTS 912 
only when the dual mode queue is empty (i.e. when all DMTs 914 are 
operating in voice mode, so that none of the DMTs 914 are contributing to 
packet data throughput). 
Full dual mode handoff and partial dual mode hand off as described above 
may be implemented in the RM 922 as options which are configurable by the 
service provider who operates the integrated network 900. The hand off 
options may be automatically disabled when operational measurements 
indicate that the integrated system 900 is in a voice overload condition. 
The hand off options may also be disabled so long as at least one 
dedicated packet data channel (provided by the PDT 916) remains in 
service. 
FIG. 6 illustrates distinct dual mode, packet data and voice frequency 
plans for the integrated system of FIG. 2. The frequency plans are based 
on seven groups of channels dedicated to voice communications (Va, Vb, Vc, 
Vd, Ve, Vf, and Vg), seven groups of channels dedicated to packet data 
communications (PDa, PDb, PDc, PDd, PDe, PDf and PDg), seven groups of 
channels that can be used for either voice or packet data communications 
(DMa, DMb, DMc, DMd, DMe, DMf and DMg), and assume two VTs 912, two DMTs 
914 and one PDT 916 per cell. The use of distinct frequency plans for the 
dual mode channels, packet data channels and the voice channels reduces 
the interference between voice transmissions and packet data transmissions 
that can result from the different intercell hand off algorithms used for 
voice and packet data communications. 
In the integrated system 900, the RM 922 and the DMTs 914 are shared 
between voice and packet data services, so that packet data services can 
be added to voice services for the relatively low incremental cost of the 
software required to provide the packet data services. Moreover, the use 
of a common RM 922 for voice and packet data services avoids the need to 
scan the voice channels to determine which voice channels are currently in 
use because that information is already available in the controller. This 
avoids the cost of radio frequency scanners 214 of the CDPD network 200 
and the processing resources needed to drive the radio frequency scanners 
214. In addition, the common RM 922 assigns channels to voice and packet 
data traffic in a more orderly manner to reduce the number of channel hops 
needed for packet data traffic. This increases the packet data throughput 
without increasing voice call blocking. 
Alternatively, because the number of channel hops is reduced, the duration 
of the switching operations performed at each channel hop has a smaller 
impact on the data throughput. Consequently, the design constraints on 
this switching duration may be relaxed, reducing the cost of the hardware 
and software implementation. 
The embodiments described above may be modified without departing from the 
principles of the invention, the scope of which is defined by the claims 
below. 
For example, the integrated system 900 could have more or fewer DMBSs 910 
or more or fewer MTXs 920 than illustrated. Some or all of the MTXs 920 
could serve multiple DMBSs 910. 
Each DMBS 910 could have a different number of the various transceiver 
types. For example, some or all of the DMBSs 910 could have no VTs 912 so 
long as enough DMTs 914 are provided to meet the demands of the voice 
traffic. 
The VTS 912 may be DMTS 914 that are operator configured to operate only in 
voice mode. Similarly, the PDTs 918 may be DMTS 914 that are operator 
configured to operate only in packet data mode. 
The VTS 912 could be AMPS, TDMA or dual mode AMPS/TDMA transceivers and the 
DMTS 914 could be operate in AMPS mode or TDMA mode when in voice mode. 
The DMTs 914 could even be "triple mode transceivers" selectively operable 
in AMPs mode, TDMA mode and packet data mode. If transceivers selectively 
operable in both AMPS and TDMA voice modes are used, the channel 
allocation algorithms described above will need to be extended 
accordingly. 
Where the demand for mobile packet data services is relatively light, no 
PDTs 916 may be provided, all packet data services being provided by the 
DMTs 914. In this case, the partial or full dual mode hand off procedures 
are particularly advantageous as means for increasing packet data 
throughput. 
The invention could also be implemented on a network architecture having 
separate voice transceivers 112 and packet data transceivers 212 that can 
operate on the same radio frequency channels as illustrated in FIG. 1, 
provided that the radio frequency channels that can be used for both voice 
and packet data communications are allocated from a common queue. This 
could be implemented, for example, by connecting the VTs 112 and PDTs 212 
in FIG. 1 to a common controller which manages the queue. 
In the embodiment described above, a separate processor 30 is provided for 
each DMT 914. Alternatively, a processor 30 could be shared by multiple 
DMTS 914, or separate processing units 32 could be provided for each DMT 
914 while a single memory 34 could be shared by multiple processors 32. 
The embodiment described in detail above is particularly suited to 
applications in which voice traffic is given priority over packet data 
traffic. Some applications may place other relative priorities on voice 
traffic and packet data traffic, and the control algorithm may be modified 
to suit the modified priorities. Similarly, some applications may favour 
queue management schemes other than LIFO, for example FIFO or 
activity-based queuing schemes. 
These and other modifications of the embodiment described in detail above 
are within the scope of the invention as defined by the claims below.