Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TEiVIPORARY FRAME IDENTIFICATION FOR ARQ
IN A RESERVATION SLOITED-ALOHA TYPE OF PROTOCOL
BACKGROUND
Applicant's invention relates to mobile packet radio systems that use a
reservation-slotted-ALOHA type of protocol, and more particularly to automatic
repeat request (ARQ) in such systems.
In a mobile packet radio communication system, a base station (BS)
communicates with a number of mobile stations (MSs) via one or more shared
packet
radio channels. Such mobile packet radio systems are described in U.S. Patent
No. 4,887,265 to Felix and in K. Felix, "Packet Switching in Digital Cellular
Systems", Proc. 38th IEEE Vehicular Technology Conf. pp. 414-418 (June 1988).
Similar systems are described in U.S. Patent No. 4,916,691 to Goodman and in
"Cellular Digital Packet Data (CDPD) System Specification", Vol. 1, "System
Overview", Release 1.0 (July 19, 1993). Packet data communication in the
European
Global System for Mobile Telecommunication (GSM) is described in P. Decker,
"Packet Radio in GSM", Tech. Doc. SMG 4 58/93, European Telecommunications
Standards Institute (ETSI) (Feb. 12, 1993); in P. Decker et al., "A General
Packet
Radio Service Proposed for GSM", Aachen University of Technology (Oct. 13,
1993); in J. Hamalainen et al., "Packet Data over GSM Network", Tech. Doc.
SMG 1 238/93, ETSI (Sept. 28, 1993); and in European Patent Publication
No. 0 544 464 to Beeson et al.
Downlink (BS to MS) traffic is scheduled by the BS to avoid contention, but to
get access to the BS, MSs use random multiple access that inevitably leads to
contention for uplink (MS to BS) traffic. Also for both directions, it is
necessary that
each data packet include an identification that uniquely designates the MS
communicating with the BS (i.e., identifying the MS either as the recipient or
as the
sender of the data packet). It is desirable that such identifications
facilitate
implementation of an efficient radio protocol producing little overhead and
being
robust in various error situations.
The mobile packet radio communication system may have a single
communication channel optimized for packet data, which is to say that both
packet
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transfer and associated control signalling are conveyed across the same
channel. On
the other hand, the radio communication system may instead be a trunked multi-
channel system.
A trunked multi-channel mobile packet radio communication system is
described in U.S. Patent No. 5,590,133 "Apparatuses and Mobile Stations for
Providing Packet Data Communication in Digital TDMA Cellular Systems" and in
corresponding Swedish Patent Application No. 9304119 published December 10,
1993. A trunked multi-channel mobile packet radio communication system is also
described in "Tentative GPRS System Concepts", Tech. Doc. SMG GPRS 17/94,
ETSI (May 1994). The abbreviation "GPRS" stands for "General Packet Radio
Service".
In current cellular radio communication systems, radio channels are
implemented by frequency modulating radio carrier signals, which in' many
systems
have frequencies near 800 megahertz (MHz). In a TDMA cellular radiotelephone
IS system, each radio channel is divided into a series of time slots, each of
which
contains a burst of information from a data source, e.g., a digital computer.
During
each time slot in a GSM-type system for example, 114 bits are transmitted, of
which
the major portion is information to be transmitted, including bits due to
error
correction coding, and the remaining portion is used for guard times and
overhead
signalling for purposes such as synchronization. Other systems transmit other
numbers of bits during each slot (for example, 324 bits per slot in a system
according
to the digital advanced mobile phone service (D-AMPS) used in North America),
and
this should not be considered as limiting Applicant's invention.
The time slots are grouped into successive TDMA frames having a
predetermined duration. In a GSM-type system for example, a frame comprises
eight
time slots. The number of different users that can simultaneously share the
radio
channel is related to the number of time slots in each TDMA frame. In general,
the
maximum number of users is the number of slots in each frame, but it is
possible that
one user may be assigned more than one slot in each frame. The successive time
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slots assigned to the same user, which may or may not be consecutive time
slots on
the radio carrier, can be considered a logical channel assigned to the user.
For a better understanding of the structure and operation of Applicant's
invention, a communication system may be considered as having at least three
layers.
Layer 1(L1) is the physical layer, which defines the parameters of the
physical
communication channel, e.g., radio frequency spacing, carrier modulation
characteristics, etc. Layer 2 (L2) defines the techniques necessary for the
accurate
transmission of information within the constraints of the physical channel
(Li), e.g.,
error correction and detection, etc. Layer 3 (L3) defines the procedures for
transparent transfer of information over the L2 data link layer.
Since each TDMA time slot has a certain fixed information carrying capacity,
each burst typically carries only a portion of an L3 message. In the uplink
direction,
multiple mobile stations attempt to gain access to the channel resources on a
contention basis, while multiple mobile stations listen for L3 messages sent
from the
system in the downlink direction. In known systems, any given L3 message must
be
carried using as many TDMA channel bursts as required to send the entire L3
message.
Referring to Fig. 1, a data packet that is to be transmitted over a GSM-type
air-interface typically comprises a user data portion and a layer-3 header
portion L3H.
The packet is formatted, normally after encryption, into a frame comprising an
information field and a frame header FH. The frame is then segmented into as
many
blocks as are needed. Each block comprises a block header BH, an information
field,
and a BCS field, and each block is transmitted as four bursts in consecutive
TDMA
frames.
As illustrated in Fig. 2, an exemplary frame header FH might comprise forty-
eight bits that carry the following information: the identity of the MS
(thirty-two bits,
or four octets), the length of the frame in octets (ten bits, or one octet
plus two bits),
the type of the frame (three bits), a mobile/stationary flag (one bit), and a
frame
sequence number (two bits). It is possible to extend a frame header FH by
setting the
frame-type bits to predetermined values, e.g., 111. It will be appreciated
that Fig. 2
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shows only one possible example and that a great variety of other examples are
possible.
Control messages such as channel reservation messages and acknowledgment
messages occupy two time slots, while random access requests and reserved
access
acknowledgments are one-slot messages. These messages are discussed in more
detail
below.
It will be noted that every frame transferred across the air-interface
includes a
unique identification number of a MS, typically in the frame header (see Fig.
1). In
"ordinary" GSM, the unique global identification number of a MS is the
International
Mobile Subscriber Identity (IMSI). Some systems apply an identity-
confidentiality
service for IMSIs, in which case a Temporary Mobile Subscriber Identity (TMSI)
that
has significance only within a particular LA is used. Outside the LA, a TMSI
is
combined with a Location Area Identifier to maintain unambiguous
identification. See
"European Digital Cellular Telecommunications Systems (Phase 2); Mobile Radio
Interface Layer 3 Specification", GSM 04.08, Version 4.9.0, Section 4.3.1,
ETSI
TC-SMG (July 1994). Although another type of complete identity of a MS might
be
used in the frame header (e.g., a CDPD identity comparable to the TMSI), in
this
example it is assumed to be the TMSI.
Because the TMSI can be up to four octets in length and the frame itself is
fragmented into blocks, much overhead is created if every block must include
the
TMSI, i.e., carry the same amount of addressing information. This is also true
for
blocks that are retransmitted after errors. Therefore, only the first block in
the
compiete or partial data frame usually carries the complete identification of
a MS.
Thus in the case of retransmission, an additional primary block may have to be
added
to the partial data frame to be retransmitted, in order to accommodate the
MS's
complete identification. This itself is an increase in overhead that is
undesirable. =
SUMMARY
In accordance with one aspect of Applicant's invention, a method of
implementing ARQ in a mobile packet data communications system using a
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reservation slotted-ALOHA protocol comprises the step of assigning a temporary
frame identity (TFI) to each data frame transmitted to a mobile station. The
assigned
TFI is unique among concurrent frame transfer sequences in a cell, and may be
assigned based on information in a data frame sent to the mobile station or
based on
information in a channel reservation message that precedes the data frame sent
to the
mobile station.
In another aspect of Applicant's invention, a method of implementing ARQ in
a mobile packet data communications system using a reservation slotted-ALOHA
protocol comprises the step of assigning a TFI to each data frame transmitted
from a
mobile station. The TFI is assigned based on information in a channel
reservation
message that precedes a data frame sent from the mobile station, and the TFI
assigned
is unique among concurrent frame transfer sequences in a cell.
The TFI is included in every block belonging to a particular frame, a block
being the unit of data on which ARQ is based. A partial data frame to be
retransmitted in case of a transmission error contains only the blocks
determined by
the ARQ protocol type (e.g., selective, and Go-back-N) used, whereby a primary
block need not be added to identify the mobile station. Blocks belonging to
frames
destined for different mobile entities can be multiplexed on the downlink
based on the
TFI. The mobile station may include, in its random access request, an
indication that
a TFI is already assigned.
With Applicant's invention, the first block of a data frame transmission may
be
erroneous and still the remaining blocks can be correctly received and
correctly
associated with a particular frame and a particular mobile station. In case of
an
uncompleted frame transfer caused by an erroneous acknowledgment message from
the mobile station, communication can be resumed if the base station sends a
message
with the TFI of the frame whose transfer was disrupted (e.g., by
retransmitting the
first block of the last transmission). In case of an uncompleted frame
transfer caused
by an erroneous acknowledgment message from the base station, communication
can
be resumed if the mobile station sends a random access request and, after
receiving a
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channel reservation, the TFI of the frame whose transfer was disrupted (e.g.,
by
retransmitting the first block of the last transmission).
BRIEF DESCRIP'I'ION OF THE DRAWINGS
Applicant's invention is described below in more detail with reference to
embodiments that are given only by way of example and that are illustrated in
the
accompanying drawings, in which:
Fig. 1 illustrates the block and frame structure of data packets;
Fig. 2 illustrates the structure of a frame header;
Figs. 3A, 3B illustrate structures of block headers;
Figs. 4A, 4B show message sequences across the air-interface for mobile-
originated (MO) packet transfers;
Figs. 4C, 4D, 4E show message sequences across the air-interface for mobile-
terminated (MT) packet transfers;
Fig. 5 shows the structure of a random access request message;
Fig. 6 shows the structure of a short acknowledgement message;
Fig. 7 illustrates a GSM-type communication system having packet data
functions;
Fig. 8 illustrates the protocol architecture of the system of Fig. 7; and
Fig. 9 illustrates another GSM-type communication system having packet data
functions.
DETAILED DESCRIPTION
The TDMA structure and the need to allow for some degree of timing
misalignment at first random access led to Applicant's selection of a
reservation-
slotted-ALOHA type of protocol for use in a GSM-type packet communication
system
as described in this application. Nevertheless, it will be understood that
Applicant's
invention can be embodied in other system platforms as well.
In accordance with Applicant's invention, a unique temporary frame identity
(TFI) is assigned to each frame to be transferred. The TFI is of local
character in the
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particular cell in which the transfer is to take place. In other words, the
particular
TFI assigned to a frame is dependent on the cell in which the transfer is to
take place
~ and is different from other TFIs used in concurrent packet transfers in that
cell.
Every block in a frame includes the same TFI, viz., the TFI uniquely assigned
to the
blocks' particular frame, and blocks that must be retransmitted include their
original
TFIs. It will be appreciated that the TFI substantially replaces the
combination of the
TMSI and the frame sequence number FN that is ordinarily included in blocks to
be
transferred.
Also in accordance with Applicant's invention, each block contains a
respective block sequence number for indicating the relative position of the
block in
the frame. Applicant's combination of the block sequence number and the TFI
unambiguously identifies an individual block as a specific block within a
particular
frame. In this way, Applicant's invention provides an ARQ protocol without
requiring inclusion of a primary block in each retransmitted frame. It will be
appreciated that although this description focusses on selective ARQ,
Applicant's
invention may be applied to other types of ARQ protocol, e.g., "Go-back-N
continuous" ARQ, which is described in the literature, including F. Halsall,
Data
Communications. Computer Networks and OSI, Addison-Wesley Publishing Co.
(1989).
Thus, in accordance with Applicant's invention and as shown in Figs. 3A, 3B,
a block header typically comprises the TFI (eight bits), the block sequence
number
(five bits), a type of the block (two bits), and a poll/final bit. The block
header of the
primary block in a frame (Fig. 3A) advantageously further comprises a number
of
random access tries (e.g., four bits), with the remaining bits of the
additional octet
being available for other uses. In the example illustrated, the length of a
block
header BH in a primary block is twenty-four bits (three octets), and the
length of a
block header BH in a following block (Fig. 3B) is sixteen bits (two octets).
Assignment and release of TFI values are administrative services provided by
the media access layer management on the network side. The minimum size of the
TFI field is determined by the number of possible concurrent packet transfers
in one
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cell. It is currently believed that eight bits is a good balance of
instantaneous capacity
against overhead, but other balances could be struck of course. The TFI values
can
be continually reused, thus distinguishing subsequent packet transfers from
earlier
ones.
For an MO packet transfer, the BS assigns the TFI as a part of the channel
reservation procedure and sends the TFI to the MS in a channel reservation
command
ChRe. For an MT packet transfer, the BS sends both the mobile's TMSI and the
TFI
assigned to the particular frame in the primary block of the frame. The TMSI
is sent
as the part of the frame header FH, and the TFI is sent as the part of the
block
headers BH. In this way, the MS is informed of the TFI for the current frame
transfer.
It is currently preferred that every block in a frame transmitted across the
air-
interface, whether on the uplink or the downlink, include the same TFI, which
unambiguously identifies that particular frame. It is not strictly necessary
for TFIs to
be included in blocks transmitted in a scheduled uplink transfer, but their
presence
adds to the robustness of the protocol.
An exemplary radio link protocol using Applicant's invention is a selective-
ARQ type of protocol. Selective ARQ means only erroneous blocks are
retransmitted. Basic scenarios are presented in Figs. 4A-4E that should be
sufficient
for those of ordinary skill in this art, and the following description is
based on the
patent applications cited above that have been incorporated here by reference.
In a GSM-type packet communication system using a reservation-slotted-
ALOHA type of protocol, a MS initiates a packet transfer by transmitting a
random
access request Ra on a packet data channel (PDCH) uplink when allowed to do
so.
This so-called random access sub-channel is determined by uplink state flags
(USFs)
marked as "free" (indicated as USF = f in Figs. 4A-4E) or "reserved"
(indicated as
USF = R in Figs. 4A-4E) on the PDCH downlink. As seen in Fig. 5, the random
access request Ra uses the same type of access burst as in ordinary GSM, and
comprises a five-bit random number for providing an initial identification of
the MS.
and other information that is described below.
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Figs. 4A-4E illustrate message sequences across the air-interface for mobile-
originated (MO) and mobile-terminated (MT) packet transfers. An index for time
slots numbered 1 through 51 is shown across the top of Fig. 4A.
As illustrated in Fig. 4A, packet transfer in the MO direction starts with a
mobile's transmitting a random access request Ra. The MS checks that the USF =
f,
and if it is, the MS transmits the random access request in the next time
slot. If the
USF = R, the MS typically would wait until the USF = f, randomly select one of
a
predetermined number of subsequent time slots, and transmit the random access
request in the selected slot if the USF is still free.
The normal response from the system to such a random access request is a
BS's transmitting a channel reservation command ChRe, reserving future slots
(USF = R) for uplink transfer of a variable length data packet. The channel
reservation command ChRe is sent on the PDCH downlink and usually includes a
request refered:ce and atim:ng, ad:'anC',P... The ~yulrpose~ of thc-, request
relGlenire
information is to address the particular MS by providing the random access
information used in the access request Ra and a frame number FN modulo 42432
for
the TDMA frame in which the access request was received (see "European Digital
Cellular Telecommunications Systems (Phase 2); Mobile Radio Interface Layer 3
Specification", GSM 04.08, Version 4.9.0, Section 10.5.2.30, ETSI TC-SMG (July
1994)). If the system does not respond to the mobile's random access request
Ra, the
MS makes a retry after a random backoff time.
After the MS transmits a data frame in the reserved time slots, the BS
transmits a positive acknowledgment message Ack if the whole data frame was
correctly received. In the example illustrated in Fig. 4A, the data frame
transmitted
by the MS consists of four blocks, or sixteen bursts (time slots 16-31). If
the data
frame was received with an error, the BS transmits a two-burst negative
acknowledgment message Nack, in response to which the MS retransmits only the
erroneous blocks as a partial frame. This is illustrated by Fig. 4B: the third
block of
the frame (time slots 24-27) was received with an error, prompting
transmission of a
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Nack message by the system in slots 34, 35, a retransmission of the third
block by the
MS in slots 40-43, and transmission of an Ack message by the system in slots
46, 47.
Referring to Figs. 4C-4E, packet transfer in the MT direction starts with a
paging message transmitted by the system on the PDCH downlink. To economize on
radio spectrum, paging is limited to the smallest possible group of cells
based on
location area (LA), the mobile's recent cell location history, and MS submode.
A
paging message may include reservation of an access slot on the PDCH uplink
for the
MS's response to the paging message. After receiving a paging response from
the
MS, the system sends a data frame to the MS. Under certain conditions, e.g.,
when
the cell location of a MS is known with a high degree of probability, data
sent in the
MT direction is sent directly as "Immediate Data" without a preceding paging
message. This situation is illustrated in Figs. 4C-4E.
The MS replies to an "Immediate Data" transmission by sending an
acknowledgement message in a reserved access slot. The acknowledgment message
can be either a short (one burst) Ack message if all blocks were correctly
received
(see Fig. 4C) or a short (one burst) Nack message if all blocks were not
correctly
received (see Figs. 4D, 4E). As illustrated in Fig. 6, the short Nack message
includes a bitmap indicating the erroneous blocks, at least for smaller
packets (e.g.,
up to seven blocks), and an acknowledgment flag (ACK in Fig. 6). If the
acknowledgement flag is set to a predetermined value, e.g., 1, then all blocks
have
been received without errors. If a bit in the bitmap is set to a predetermined
value,
e.g., 1, an error in the corresponding block is indicated. In the situation
illustrated in
Fig. 4D, the frame consisted of four blocks, the third of which was
incorrectly
received; the ACK flag in the short Nack message sent by the MS was then set
to 0
and with the bitmap caused the system to retransmit the third block.
For larger packets (i. e. , more than seven blocks), the short Nack message
indicates to the system that a channel reservation is needed for a longer (two
burst),
complete negative acknowledgment message Nack. This can be indicated by
setting
the values of all bits in the bitmap to the value indicating no errors, e.g.
0, and at the
same time setting the value of the ACK flag bit to 0, indicating errors. In
Fig. 4E,
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blocks 8, 9, and 10 were incorrectly received, and the system responded to the
mobile's short Nack message (all zeroes) with the reservation message ChRe
indicating reserved time slots for a long (two-burst) Nack that includes a
complete
bitmap for all blocks in the frame.
The structure of the two-burst acknowledgement message comprises the TFI,
the Ack/Nack indicator, and a list (bitmap) of erroneously received blocks. On
the
downlink, the two-burst acknowledgement message also includes a channel
reservation
for retransmission. The MS transmitted the long (two-burst) Nack message in
the
assigned slots, and that message indicated to the system that retransmission
of
blocks 8, 9, and 10 was needed. For either short or long negative
acknowledgement
messages, partial frames that consist of not-acknowledged blocks are sent by
the
system until a positive acknowledgment message is received from the MS.
Thus in accordance with Applicant's invention, each transmission of a
complete or partial frame is followed by an acknowledgment message that
includes the
TFI of the frame to =which it refers and, if needed, a list of the blocks that
were
erroneous. Since a one-burst Ack/Nack message is sent only in a reserved slot,
the
sending mobile station is indirectly identified and does not need to include
the TFI in
the one-burst Ack/Nack message. Partial frames that consist of retransmitted
not-
acknowledged blocks are sent until a positive acknowledgment is received,
viz., until
reception of an acknowledgement message that does not include a list of
erroneous
blocks.
A TFI remains valid even if the communication is disrupted for some short
period of time. For example, when an erroneous acknowledgment message is sent
by
the BS, the MS can reestablish the communication by sending a random access
request message Ra that notifies the BS that a TFI is already assigned. As
illustrated
~ in Fig. 5, the random access request message Ra advantageously comprises a
single
octet in which some bits (e.g., five) convey the pseudo-random number
identifying the
MS, one bit (RETR in Fig. 5) indicates an initial/retransmission request, one
bit
(SING in Fig. 5) indicates whether the MS intends to transmit only a single
block on
the uplink, and another bit (PRIO in Fig. 5) indicates a priority. The
pseudorandom
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number is used to distinguish access requests from different mobiles. The
initial/retransmission request indicates that a TFI is already assigned.
Upon receiving the channel reservation message ChRe sent by the BS in
response to the random access message Ra, the MS can identify and resume the
uncompleted frame transfer by restating its TFI (e.g., by retransmitting the
primary
block of the incomplete transfer), and the BS can then continue the frame
transfer by
sending the same acknowledgment message that was erroneous in the previous
transmission. Operation in this way has significant advantages. When
reestablishing
communication with a BS in case of an uncompleted packet transfer (e.g., a
missing
acknowledgment message from the BS), it is sufficient to state the TFI of the
frame
whose transfer was interrupted (e.g., by retransmitting the first block of the
last
transmission).
After a frame is successfully transferred across the air-interface, viz.,
after a
positive acknowledgment has been received, the network-side layer management
entity
can release the TFI value and make it available for future use. Release of the
TFI
value can also occur in the case of persistent loss of response from an MS or
when a
MS roams to another cell.
It should be appreciated that Applicant's invention provides a communication
system having many advantages over other systems. The TFI is a short identity
that
replaces the combination of the TMSI and frame sequence number FN during frame
transfers across the air-interface. Also, the combination of the TFI and the
block
sequence number unambiguously identifies a block in a particular frame sent
to/from a
particular MS. Moreover by using a TFI as a part of every block's header,
there is
no need for the additional primary block that is necessary for each
retransmitted
partial data frame in order to accommodate the complete identification of the
MS.
Furthermore by including a TFI in every block of a frame, the first block of
the
frame can be incorrectly received and still the remaining blocks can be
correctly
received and correctly associated with a particular frame and a particular MS.
As a result of Applicant's invention, the scheduling of downlink traffic is
made
simpler, more flexible, and more spectrum efficient for the following reasons:
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(1) blocks belonging to frames destined for different MSs can be multiplexed
on the
same downlink channel (e.g., while waiting for an acknowledgment message from
a
first MS, a few blocks can be sent to a second MS, thereby fully utilizing the
downlink spectrum); (2) the downlink transmission of a data frame comprising a
plurality of blocks can be interrupted, for example by a control message to
some other
MS, and then resumed; and (3) if more than one data channel is available for
the
downlink traffic, blocks belonging to the same frame can be transmitted on
different
channels and the intended MS can still properly receive them, provided the MS
is
capable of monitoring multiple channels in parallel.
As described above, Applicant's invention may be applied in a digital TDMA
cellular radiotelephone system having a GSM-type architecture. In one such
system
that is described first below, packet data services are added to a GSM-type
system in
a closely integrated way, using current GSM infrastructure to the maximum
extent
possible. The second such system described below uses primarily the BS portion
of a
GSM network and adds a separate mobile packet data infrastructure for the
other
network parts.
Fig. 7 illustrates a GSM system enhanced with packet data (PD) functions, the
major PD function blocks being indicated by bold contour lines. A plurality of
base
transceiver stations (BTSs), each providing radio communication service to
multiple
MSs in a respective cell, together provide complete coverage of the GSM public
land
mobile network (PLMN) service area. Only one BTS and one MS, which comprises
a mobile termination (MT) part and a terminal equipment (TE) part, are shown
schematically in Fig. 7. The functional units of the BTS carry out the above-
described steps of assigning, to each data frame transmitted to an MS, a TFI
that is
unique among other TFIs assigned to data frames concurrently transmitted to
other
mobile stations.
A group of BTSs is controlled by a base station controller (BSC), and these
together form a base station system (BSS). In the preceding description, a BS
may be
considered as a combination of a BTS and its BSC. One or more BSSs are served
by
a mobile services switching center (MSC) and an associated visitor location
register
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(VLR). The MSC controls calls to and from other networks, such as the public
switched telephone network (PSTN), an integrated services digital network
(ISDN),
and other PLMNs. An MSC equipped for routing incoming caIls is referred to as
a -
gateway MSC (GMSC). One or more MSC service areas together constitute the
PLMN service area.
The MSC/VLR(s) communicate with a home location register (HLR) via a
common channel signalling (CCS) system No. 7 network, which is standardized by
the International Telegraph & Telephone Consultative Committee (CCITT), now
the
International Telecommunications Union (ITU). The HLR is a data base
comprising
information on all subscribers, including location information identifying the
MSC/VLR where a subscriber is currently (or was last) registered. Connected to
the
HLR is an authentication center (AUC) that provides the HLR with
authentication
parameters. To allow identification of subscriber equipment, an equipment
identity
register (EIR) is also connected to the MSC(s). Finally, an operations and
maintenance center (OMC) may be included for providing overall network
support.
The packet data functionality in the BTS includes capability to provide one or
more shared PDCHs, depending on demand. In a cell only occasionally visited by
a
packet data user, a PDCH may be allocated temporarily on user demand. In a
cell
having continuous packet data traffic demand, one or more PDCHs may be
allocated
either semi-permanently or dynamically, adapted to the current load situation.
The
allocation of PDCHs is controlled from the BSC. Information defining the
support
level and any PDCH allocated for initiating packet transfer is broadcast on a
conventional GSM broadcast control channel (BCCH).
The packet data radio link protocol over the PDCH(s) allocated in a cell is
handled by a PD transfer controller in the BTS. In a BTS having at least one
PDCH
allocated, the PD transfer controller has a physical connection for packet
transfer to
and from the MSC. The physical connection is typically unique and uses
ordinary
internode trunks.
The MSC/VLR includes a PD router for routing packets to and from the MSC
service area and a PD signalling controller for handling signalling exchange
with a
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circuit mode MSC. The PD signalling controller also handles control,
monitoring,
and parameter storage functions related to packet data MSs. The PD controller
comprises a processor, memory, signalling interface functions, and software.
Although the PD router and PD signalling controller are shown as parts of the
MSC/VLR, it will be understood that either or both, in whole or in part, could
be
physically realized as external equipment attached to the MSC.
MSCs (PD routers) are interconnected via a backbone network, to which one
or more interworking functions (IWFS) are also connected. IWFs enable
internetworking with external network(s), such as the Internet (i.e., IP
network)
and/or a packet switched public data network (PSPDN) (i.e., an X.25 network),
thus
interconnecting fixed stations (FSs) with the MSs. An IWF may perform protocol
conversion and address translation, as required, and an IWF may also route
packet
data traffic between cooperating PLMNs. Packet data traffic between MSs in
different MSC service areas in the same PLMN is normally routed directly
between
the respective MSCs, across the backbone network. For routing purposes, the
HLR
may be interrogated from entities on the backbone network through an HLR
interrogation server, which provides the functions necessary for such
interrogation
from the packet data network. The AUC, EIR, OMC, and HLR interrogation server
may also be enhanced from time to time to support new types of subscriptions,
services, and equipment.
The basic packet data network service provided by a cellular packet data
PLMN such as that illustrated in Fig. 7 is a standard connectionless network
(datagram) service based on a standard connectionless IP protocol. The term
"IP
protocol" should be understood to denote either the Internet Protocol (the de
facto
standard IP protocol used in the TCP/IP protocol suite) or the International
Standards
Organization (ISO) Internetwork protocol ISO 8473. Value-added services,
including
multicast, broadcast, and electronic mail services, may be provided by Network
Application Server(s) (NAS(s)), attached to the backbone network and accessed
by
using higher layer protocols on top of the IP. Thus, from a packet data
communication point of view, the PLMN basically appears as an IP network.
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The protocol architecture is illustrated by Fig. 8, which shows an example of
communication between an MS and an FS, e.g., a host computer, attached to an
external IP network. Thus, the IWF and MSC both play the role of IP (layer 3)
routers, and the MS and FS can communicate end-to-end using a transmission
control
protocol (TCP), or transport (layer 4) protocol. For the architecture
illustrated in
Fig. 8, the MT and TE parts of the MS would be integrated in one unit.
Between the MSC and the MS, the BTS acts as a link layer (layer 2) relay
between the radio link protocol (denoted RL2 in Fig. 8) and the link protocol
(denoted
L2) used across the trunk connection. The radio protocol handled by the BTS is
Applicant's ARQ type of protocol described above. In contrast to ordinary GSM,
encryption/decryption is performed between the MT part of the MS and the MSC.
An MS is identified on layer 3 with an IP address and on layer 2 with standard
GSM
identities: the IMSI, or more typically, the TMSI.
Another GSM-type system that can benefit from Applicant's invention is
schematically illustrated in Fig. 9, in which the major PD function blocks are
indicated by bold outlines. In Fig. 9, only the BSS portion of the GSM
infrastructure
is utilized for packet data. The PD functions in the BTS are almost the same
as in the
system illustrated in Fig. 7, as are the PDCH-allocation functions in the BSC.
Again,
the functional units of the BTS carry out the above-described steps of
assigning, to
each data frame transmitted to an MS, a TFI that is unique among other TFIs
assigned to data frames concurrently transmitted to other mobile stations.
As illustrated in Fig. 9, the packet data transfer connection of a BTS is
coupled to a separate Mobile Packet Data Infrastructure (MPDI) instead of a PD
router in the MSC/VLR. The MPDI provides the necessary packet routing,
mobility
management, authentication, and network management functions. Together, the
MPDI and the portions of BSS(s) utilized for packet data constitute a mobile
packet
data system. With respect to GSM, the system may be regarded as a separate
system,
and a GSM operator may choose to lease radio channel capacity to a separate
packet
data system operator. An MS requiring both packet data and regular GSM
services
may then need a separate subscription in each system. The packet data services
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provided by the system may (depending on the functionality of the MPDI) be the
same as those provided by the system illustrated in Fig. 7.
In the system shown in Fig. 9, the only radio channels available for MSs are
PDCHs and regular GSM broadcast channels. Thus, registration, location
updating
(or cell location reporting), authentication, and similar signalling are
performed via
PDCHs, and thus allocation of a first master packet data channel (1ViPDCH) on
user
demand, using ordinary GSM signalling as in Fig. 7, is not possible. With this
exception, the functions for providing PDCHs are the same as described for
Fig. 7.
The MPDCH is the first PDCH allocated in a cell on which packet transfers
are initiated and is normally allocated by system configuration, although the
method
of using a PDCH of an adjacent cell for requesting allocation of an MPDCH in a
"PDCH on demand" cell, prior to moving into that cell, is also feasible. In
that case,
the allocation request would be transferred to a system entity in the MPDI.
This
system entity would then send an allocation request to the BTS of the "PDCH on
demand" cell in question, and that cell would, in turn, convey the request to
the
PDCH allocation controller in the BSC.
Regular GSM broadcast control channels are used in the same way as in the
system illustrated in Fig. 7 for defining PDCH support level and MPDCH
allocated in
the cell (via information on BCCH) and for performing cell selection with two
alternative criteria for cell selection. Listening to cell broadcast short
messages is
also possible in a way similar to that of the system in Fig. 7.
Packet transfer across PDCHs may be performed according to the principles
described for Fig. 7. The PD transfer controller and associated interface
functions in
the BTS are also adapted to the interconnection requirements of the MPDI,
e.g., to
allow interconnection via a routing network. The functions of the MS in the
system
of Fig. 9 are basically the same as the MS functions in the system of Fig. 7,
except
for functions related to regular GSM signalling and PD mode, which are not
applicable in the system of Fig. 9.
While specific embodiments of the present invention applied to a GSM type of
cellular system have been described, it should be understood that the present
invention
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may be applied also to other TDMA cellular systems including D-AMPS and PDC
systems. Although in these systems, BSC is not provided as a separate
functional
entity, corresponding base station controller functions and associated new PD
functions are instead divided between MSC and base stations.
It wiIl be understood that Applicant's invention is not limited to the
particular
embodiments that have been described and illustrated. This application
contemplates
any and all modifications that fall within the spirit and scope of Applicant's
invention
as defined by the following claims.
