Note: Descriptions are shown in the official language in which they were submitted.
CA 02335751 2000-12-19
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A METHOD AND APPARATUS FOR ALLOCATING CHANNELS IN A MOBILE TELECOMMLT1ICATIONS
SYSTEM
SUPPORTING BOTH PACKET AND CIRCUTf SVIr~D TRp~C
The present invention relates to mobile telecommunications networks and in par-
ticular to the allocation of channels in such a network when both packet
switched
and circuit switched traffic is used in the network.
J
Description of Related Art
In many mobile telephony systems, for example the Global System for Mobile
communication (GSM), protocols exist both for circuit switched and packet
switched communication.
:io
In circuit switched speech systems one or more physical channels are allocated
to
each connection. Each channel is assigned to one connection only, until the
infor-
oration transfer on this connection is completed. In GSM, the High Speed
Circuit
Switched Data (HSCSD) protocol enables the allocation of several physical chan-
15 nets to one connection, and is mainly used for transfer of large amounts of
data.
In packet switched tragic data communication is often very intermittent and
the in-
formation to be transferred occurs in bursts of varying length. During a
burst, the
bandwidth demand may be very high, whereas between bursts it may be zero. To
20 utilize each channel optimally, a number of packet communication
connections of
ten share a number of physical channels.
The channel or channels assigned to a packet switched connection are assigned
to a
Particular connection only when information is being transmitted on the
connection.
25 The rest of the time, the channel or channels can be used for the other
connections
assigned to the same channel or channels. The effective transmission rate for
each
connection is of course reduced if many connections share the same physical
chan-
nels. Therefore, from a user's point of view it is beneficial not to share a
channel
with other users.
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The circuit switched connections often carry speech, whereas the packet
switched
connections are mostly used for data. Therefore, the circuit switched
connections
must be transmitted in real time, while this is not so critical for the packet
switched
connections. Therefore, circuit switched connections are usually given a
hisher pri-
ority 'than packet switched connections. This means that, if the total number
of
channels requested is higher than the total number of available channels,
packet
switched connections may be disconnected to enable a circuit switched
connection.
Depending on the traffic demand in the cell the number of channels required
for
each type varies with time. Therefore, the dynamic allocation of resources
between
circuit switched and packet switched traffic should be possible. Since the
total num-
ber of channels in a cell is fixed, the number of channels allocated to each
type of
tragic has to be balanced, especially when the total traffic demand in a cell
is high.
European Patent Specification EP 0 831 669 A2 describes a method for
controlling
the load of a Code Division Multiple Access (CDMA) system in which circuit
switched and packet switched traffic occurs. The number of channels that may
be
allocated to each type of tragic can be set dynamically in dependence of the
de-
mand for each type of circuit switched and packet switched traffic,
respectively. .
The solution described in EP 0 831 669 A2 relates to the overall distribution
of re-
sources between circuit switched and packet switched networks. It does not,
how-
ever, concern itself with optimuzing the utilization of the channels that may
be allo-
rated to each of the two types of traffic.
?5
Object of the Invention
It is an object of the present invention to enable the efficient traffic
handling in
TDMA systems using both circuit switched and packet switched traffic.
Summary of the Invention
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This object is achieved according to the invention by a method of allocating
char-
nels in a mobile telecommunications system in which packet switched and
circuit
switched tragic may be used, said method comprising the following steps:
specifying a maximum number of channels (TSB that may be allocated to
packet switched connections for each frequency;
specifying a maxzmum number of connections (NLJ~) that ~y be allocated to
one channel; .
- when a packet switched connection is requested, performing the following
steps:
- checking if the maximum number (TSB) of channels have been allocated
and, if not, allocating the available channels to the connection;
if more channels were requested, allocating other channels among the channels
that may be allocated to packet switched connections to the requested packet
switched connection.
IS
The object is also achieved by a control node in a mobile telecommunications
net-
work comprising programs developed to perform said method
The method and control node according to the invention ensure that a certain
num-
ber of channels are always reserved for circuit switched traffic, and at the
same time
guarantees that a reasonable number of packet switched connections share each
of
the channels that may be used for packet switched connections
According to a first embodiment, if the maximum number (TS,,~;) of channels
have
been allocated, it is checked if the maximum number (NUB) of connections have
been assigned to each of the channels among the channels that may be allocated
to
packet switched connections, and, if not, the channels having the fewest
connec-
tions will be assigned to the requested connection.
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If the maximum number (NUB) of connections have been assigned to each of said
channels, another frequency is selected.
According to a second embodiment, if the maximum number (TSB) of channels
S , have been allocated, it is checked if the maximum number (NL1~ of
connections
have been assigned to each of the channels among the channels that may be allo-
cated to packet switched connections, and, if not, it is checked if the
requested
number of channels can be allocated to the connection and, if so, the channels
hav-
ing the fewest connections are assigned to the requested connection. If the
requested
number cannot be allocated to the connection, another frequency is selected.
~y~ty
In both embodiments, the steps are preferably performed for each frequency
accord-
ing to a priority order list, until a frequency is found on which the
connection re-
quest can be fulfilled, or until the Last frequency has been examined
If no frequency is found on which the connection request can be fulfilled, the
best
possible assignment of channels is made, the connection is denied or the
connection
is assigned to channels that may not normally be allocated to packet switched
con-
nections.
The maximum number of channels (TSB) that may be allocated to packet
switched connections and/or the number of connections (NUB) that may be as-
signed to each channel may be set individually for each frequency. This
enables the
establishment of a limited number of packet switched connections with a high
bandwidth without increasing the total number of channels that may be assigned
to
packet switched connections too much.
If a circuit switched connection is requested for which na channels are
available
among the channels that may not be used for packet switched connections and.
the
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necessary number of channels that may be used for packet switched connections
are
allocated to the circuit switched connection.
One or more packet switched connections may be disconnected to make idle chan-
nels for the circuit switched connection.
This ensures that the circuit switched connections are always given a higher
priority
than the packet switched connections, which do not require real=time communica-
tion.
Brief Description of the Drawings
14 The present invention will be described in more detail in the following, by
way of
preferred embodiments, and with particular reference to the drawings, in which
Figure 1 illustrates the communication between a mobile terminal and a base
station
terminal in a TDMA system;
Figure 2 is a flow chart of the inventive method;
15 Figures 3A-3E illustrate the use of the inventive method for allocating
channels to
one frequency.
Detailed Description of Embodiments
Figure I illustrates the communication between a mobile terminal 1 and a base
sta-
"~~'' ?~ tion 3 in a mobile telecommunications system according to TDMA. A
number of
channels, are transmitted in time slots on the same frequency. In the Figure,
eight
time slots, corresponding to eight physical channels, are transmitted on the
same
frequency. The base station 3 is controlled by a base station controller 5,
through
which it is connected to a telecommunications network 7. This description is,
how-
?~ ever, only concerned with the communication between the mobile terminal 1
and
the base station 3.
A circuit switched connection is assigned one or more physical channels, and
oc-
cupies this channel or these channels alone for the duration of the
connection. A
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packet switched connection is assigned one or more physical channels, which it
shares with other packet switched connections.
In the discussion below the following definitions are used:
' ~Max~ The maximum number of packet switched connections that can be ac-
commodated for each time slot.
TSB: The maximum number of time slots on one frequency that may be allo-
cated for~packet switched connections. If the TSB criterion is fi~iflied
for a frequency, this means that at least this number of time slots on this
frequency is allocated to packet switched connections.
Area The area criterion is fulfilled for a given frequency if there are at
least
NUB connections assigned to all the channels that may be allocated to
packet switched connections on that frequency.
Both NUB and TSB may be given different values for different frequencies.
. This may be used to make the system more flexible. For example, the value
for
TSB may be set relatively low for most frequencies, while a few frequencies
may
have a higher TSB to enable packet switched connections with higher bandwidth
on these few frequencies.
NU~"~,x and TSB are used together to determine when a new frequency should be
selected for new packet switched connections to be set up. They may be set by
the
operator and may be changed when the traffic situation changes.
'S Two fundamental assumptions are made, which are true for most TDMA systems:
1 ) A priority order has been defined for the allocation frequencies to
circuit
switched and packet switched traffic, respectively.
2) In the case of congestion, circuit switched traffic has a higher priority
than packet
switched traffic. This means that, if more than the available number of
channels
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are requested a channel used for packet switched connections may be pre-empted
to make room for a circuit switched connection.
If TSB has different values for different frequencies, different priority
lists may be
, used. For example, the first frequency examined may always be the frequency
for
which TSB is equal to the number of channels requested.
Figure 2 is a flow chart of the method according to the invention. In the
initial
situation all channels are either idle or used for circuit switched
connections.
Step S I: A packet switched connection utilizing a number N of channels is re-
quested
Step S2: Select the first frequency in the above mentioned priority list.
Step S3: Is the TSB criterion fulfilled for this frequency? If no, go to step
S4;
if yes, go to step S5.
Step S4: Allocate the channels that may be allocated to packet switched connec-
tions and that are not currently allocated to any connection on this fre-
quency to the connection. If more channels are requested, go to step S6,
otherwise, end of procedure.
Step S5: Is the Area criterion fulfilled for this frequency? If no, go to step
S6;~if
~.~~-~ 20 yes, go to step S7.
Step S6: Assign the requested number of channels on this frequency to the
. ~ packet switched connection, or, if the number of channels requested is
higher than TSB, assign TSB channels. The channels are selected as
the ones that have the fewest connections allocated to them among the
ones already used for packet switched connections. End of procedure.
Step S7: Is this the last frequency in the priority list? If yes, go to step
S8, if no,
go to step S9.
Step S8: Make the best possible assignment of channels.
Step S9: Select the next frequency in the priority list as the current
frequency.
'0 Go to step S3.
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In step S8, that is, if the TSB and the Area criteria are fulfilled for all
frequencies,
alternative actions are possible. In the flow chart it has been assumed that
the con-
nection is assigned to channels already having more than NUB co~ections as-
s , signed to them. It would also be possible to deny the connection
altogether, to guar-
antee a minimum bandwitdh to eac):~ux~ection actually set up. A third
possibility
would be to assign channels that may not normally be used for packet switched
connections to the connection.
Figures 3A-3E illustrate the allocation of channels according to the procedure
de-
scribed above. In these figures TSB is set to four and NUB is set to three.
This ~:;:
means that, of the eight channels available on the frequency shown, four may
be
allocated to packet switched connections. Three connections may share one
physical
channel before it will be considered to allocate a newly requested connection
to a
IS new frequency.
Figure 3A shows the situation when there is only one connection on the
frequency.
Two channels have been allocated to the connection, as indicated by the two
boxes
with diagonal shading.
Figure 3B shows the situation after a second connection has been established.
Three
channels have been assigned to this connection, as indicated by three boxes
with
vertical shading. Since fewer than TSB (that is, four) channels were allocated
to
packet switched data before this connection was set up, the connection uses
the two
channels that were not previously used and shares one channel with the first
con-
nection.
Figure 3C shows the situation after the establishment of a third connection
using
four time slots. Since all the four channels allocated to packet switched
connections
0 already had been assigned, a check was made to see if the Area criterion was
ful-
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0
filled, that is, if the maximum number of connections had already been
assigned to
each channel. Since this was not the case, all the four channels of the
frequency that
may be allocated to packet switched connections have been allocated to the
connec-
tion, as indicated by the dotted shadow squares.
Figure 3D shows the situation after two more one-channel connections have been
allocated. The time slots used are shown as black squares. Now all the
channels ex-
cept one have the maximum number of connections assigned to them.
If a connection request for two or more channels is made at this stage, there
are two
alternatives:
w
~;"'1) The connection may be set up on this frequency, which means that on two
chan-
nets there will be one more than NUB (that is, three) connections. This is the
situation shown in Figure 3E.
2) Another frequency may be selected for the new connection, since it cannot
be set
up on this frequency without violating the Area criterion.
The functions described above may be implemented as software packages included
in a node in the network. A separate node may be included, or the functions
may be
implemented in an existing node, for example, the base station controller 5
shown in
Figure 1.
