Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF HANDING OVER MOBILE STATIONS IN A GENERAL
PACKET RADIO SERVICE (GPRS) RADIO TELECOMMUNICATIONS
NETWORK
PRIORITY STATEMENT UNDER 35 U.S.C. ~ 119(e) & 37 C.F.R. ~ 1.78
This nonprovisional application claims priority based upon the prior U.S.
provisional patent application entitled, "Method of Handing Off Terminals in a
General Packet Radio System (GPRS) to Support Voice Services", application
number
60/140,346 filed June 21, 1999, in the names of Francis Lupien, Marlene Yared,
and
Jean-Francois Bertrand.
BACKGROUND OF THE INVENTION
Technical Field of the Invention
This invention relates to telecommunication systems and, more particularly,
to a system and method of handing over mobile stations in a General Packet
Radio
Service (GPRS) radio telecommunications network to support voice services.
Description of Related Art
In existing radio telecommunications networks, a cell change or handover
procedure in GPRS is executed when a Mobile Station (MS), which is
transmitting
and receiving user data payload while in GPRS mobility management (GMM) state
READY, roams into the coverage area of a neighboring cell, acquires service in
that
cell, and resumes data transmission. As the MS roams and changes cells while
in
READY state, the network tracks mobility information by updating the MS's
serving
routing area (RA) and cell. The process of updating cell information and
routing area
information is executed through standards defined by GMM procedures.
As defined in the GPRS specification, when a mobile station changes cells, it
must inform the network of its location. There are three types of cell change:
1 ) within one routing area (cell update);
2) between two routing areas but within one Serving GPRS Support Node
(SGSN) area (intra-SGSN routing area update); and
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3) between two SGSN areas (inter-SGSN routing area update).
Delay in the real time traffic flow is common to all types of cell change.
Interruptions arise due to the execution of various functions currently
prescribed in the
GPRS standard for radio access, radio resource allocation, location update
network
signaling, and inter-SGSN link handling.
Overview of the Current System
When a MS in LADY state roams to a new cell belonging to a different
location area (LA) or SGSN service area, the data stream is interrupted from
the time
the MS leaves the serving cell until the GPRS mobility management (GMM)
procedures terminate. When the MS changes cells, it must abort any temporary
block
flow (TBF) and cease transmission on the uplink and downlink. When the MS
switches to the new cell, it must read all system information messages before
making
a request for packet resources (new TBF). The typical time for the MS to
switch to the
new cell may be 40-60 ms.
In the intra-SGSN and inter-SGSN case, the MS must update its mobility
context in the Packet Switched (PS) network by sending a Routing Area Update
Request message. A new TBF resource must be used to send the Routing Area
Update
Request message. The estimated time to establish a new uplink TBF and send the
Routing Area Update Request Message is 100-150 ms. After the Routing Area
Update
Complete Message is sent, the mobile must then reestablish an uplink TBF (100-
150
ms) and a downlink TBF (60-100 ms) to resume data transfer.
In addition, the current mobility management procedures are designed to
minimize packet loss while a MS performs a cell change within and across SGSN
borders, and hence are biased towards loss-sensitive traffic types. Loss
minimization
is achieved by minimizing the loss of Logical Link Control (LLC) packets
during the
change of traffic flow connections between the old and new SGSN, and is
accomplished through a buffer transfer and associated synchronization of LLC
transmission states. The content of the old SGSN buffer is transferred to the
new
SGSN, including packets that were not acknowledged and packets received at the
old
SGSN before the transfer of control to the new SGSN was completed. In the
inter-
SGSN handover case, in addition to the above steps, the SGSN context and the
Packet
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Data Protocol (PDP) context must be updated in the packet-switched (PS)
domain.
This procedure includes the Home Location Register (HLR) location update
process,
the transfer of mobility context and buffers between the SGSNs, and the
establishment
of the new GPRS Tunneling Protocol (GTP) link. Currently, the packets are
tunneled
between the old SGSN and the new SGSN until the link is set up.
Additionally, if the subscriber is IMSI attached, the mobility management
context must be updated in the circuit-switched (CS) domain. Assuming that
each
network signaling procedure may have a delay budget of 100-200 ms, this yields
an
additional 1.1 to 2.2 seconds delay. Furthermore, approximately 50 ms is
needed for
the SGSN to process the RA Update Request and send the RA Update Accept.
While all three types of cell changes induce some measure of interruption in
real time traffic flow, the inter-SGSN cell change yields the longest
interruption time.
The total interruption time for an inter-SGSN routing area update is
approximately 3-4
seconds.
1 S SPECIFIC PROBLEM AREAS
While the preceding section broadly outlines the procedures associated with
handing off mobile terminals in GPRS to support voice services, it is
necessary to
explain more thoroughly specific problem areas that induce delay in the
existing
system. FIG. 1 is a simplified block diagram illustrating the existing GPRS
system.
A packet data network (PDN) 1 having terminal equipment (TE) 1 a is connected
to the
network through a gateway GPRS support node (GGSN) 2. A mobile station (MS) 12
is connected to the network through a serving (Old) base station system (BSS-
1) 10
and a serving GPRS support node (SGSN-1) 6. The BSS-1 also connects the MS to
a serving (Old) mobile switching center/visitor location register (MSC/VLR-1 )
8. The
SGSN-1 is also connected to a home location register (HLR) 5, an equipment
identity
register (EIR) 3, and to a target (New) SGSN (SGSN-2) 7. The SGSN-2 is, in
turn,
connected to a New BSS (BSS-2) 11 and a New MSC/VLR (MSC/VLR-2) 9. The
network may also be connected to other Public Land Mobile Networks (PLMN) 4.
FIG. 2 is a message flow diagram illustrating the messages utilized in the
existing inter-SGSN handover process. As shown in FIG. 2, the delay problem
can be
broken down into the following areas:
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1 ) Cell re-selection procedures;
2) Establishment of the Uplink and Downlink TBF;
3) Establishment of the GTP link between the new SGSN land the
Gateway GPRS Support Node (GGSN) 2 (steps 20 - 27);
4) Updating location information in the HLR 5 for GPRS attached
mobiles (steps 28 - 33);
5) Performing GPRS security procedure (steps 23a and 23b);
6) Updating the mobility management context in the circuit switch
domain if the mobile station is IMSI attached (steps 34 - 4I);
7) Handling of delay-sensitive packets (versus loss-sensitive packets); and
8) Completion of the RA Update process (steps 42-44).
Delay inducing problem areas within GPRS can be broadly categorized into
two distinct types - MS Centric relating to mobile station related delays, and
Network
Centric relating to core network delays. Mobile station delays are related to
the air
link protocol and air interface delays such as those resulting from cell re-
selection
procedures and in the establishment of the Uplink and Downlink TBF. The air
link
includes delays from the physical RF link layer, the physical data link layer
(sub-
channeling (PBCCH, PACCH, PDCH, PCCCH, etc.)) and the Media Access
Control/Radio Link Control (MAC/RLC). Specific core network delays exist in
GTP
Link establishment procedures, policing functions, and admission control.
MS Centric Delavs
MS Cell Re-selection: The MS Cell Re-selection Procedure is described in the
GSM specifications 04.60 and 05.08. The MS Cell Re-selection Procedure is
controlled by the parameter Network Control Order. It is sent on the Broadcast
Control Channel (BCCH) as well as on the common control or Packet Associated
Control Channels (PACCH). Consequently, the cell re-selection mode can be set
for
all mobile stations but can be changed for a particular mobile station. The
available
settings for the parameter are:
NCO: MS control (autonomous cell re-selection);
NC 1: MS control with measurement reports sent to the network; and
NC2: Network control with measurement reports sent to the network.
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While in packet transfer mode, an MS constantly measures the signal strength
of its neighboring cells and of the serving cell. Once it has determined the
best
candidate target cell, based on signal strength measurements, the MS then
acquires the
target cell's system access information. The MS may continue its operation in
packet
idle or packet transfer mode in the old serving cell, while acquiring certain
system
information for the target cell. The MS may suspend its TBF in order to read
the
necessary information message on the BCCH and Packet Broadcast Control Channel
(PBCCH) of the target cell. When the MS performs this procedure, the network
is not
made aware of the MS's actions, and therefore the MS may miss downlink
packets.
It takes approximately 20 ms to read a system information message, and taking
retuning time into account, the maximum loss is 2 blocks or 40 ms. Assuming
one
PSI/SI message is read at a time, and the MS returns to the serving cell
between
reading attempts, the maximum time for loss of packets would be 40 ms for each
reading attempt. This operation could continue for up to 5 seconds. The MS may
move to the target cell without having completely read all of the necessary
system
information messages. This may be due to its multi-slot capabilities or that
(according
to GSM 05.08) the current serving cell radio conditions are no longer
fulfilled.
Currently, the minimum requirements for an MS to move to the new cell are:
~ The MS starts to receive information on the PBCCH of the target cell (if
the target cell contains a PBCCH);
~ The MS has received SI-13 on the BCCH of the target cell indicating that
the cell supports GPRS (if there is no PBCCH in the target cell); and
~ Required radio conditions (according to GSM 05.08) for the old cell are no
longer fulfilled.
If the target cell supports GPRS, then the MS may not perform a packet access.
The following paragraphs describe the information that the MS must decode
before
making a packet access request.
If the target cell contains BCCH only, the MS does not perform packet access
in the target cell or enter the packet transfer mode until it has performed a
"complete
acquisition" of the BCCH messages. In this case, the MS must acquire a PSI-1
message (20 ms) and make at least one attempt to receive other SI messages
that may
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be scheduled within one TC cycle on the BCCH. The time to complete the entire
transaction is almost 2 seconds. (See GSM 05.02 para 6.3.1.3: TC = (FN div 51
) mod
8, FN = Frame Number; Max 8 multi-frame lengths >> 8 X 235 ms = 1.9 seconds).
If PBCCH exists in the target cell, the MS delays the start of receiving
information on the PBCCH until the first occurrence of PSI-1 in block B0. If
reception of PS-1 and PS-2 fails, the MS can continue in the old cell until
the next
occurrence of PSI-1 in block B0. The mobile does not perform packet access in
the
target cell or enter the packet transfer mode until it has performed a
"complete
acquisition" of the PBCCH messages. In this case, the MS first acquires a PSI-
1
message (20 ms), then acquires a consistent set of PSI-2 messages (min 1 msg.,
max
8 msg. >> 20-160 ms), and finally attempts to receive the complete set of PSI
messages
on the PBCCH (min 4 msg. >> 80 ms).
The scheduling of the packet system information messages on the PBCCH is
completely network-determined. The network divides the messages into high
repetition rate and low repetition rate messages, and can use a repetition
cycle that is
anywhere from 1 to 16. In the worst case scenario, wherein the repetition
cycle is 16,
the maximum time it takes the MS to read all PSI messages is almost 4 seconds.
(16
X 235 ms = 3.7 seconds) (See GSM 05.02 para 6.3.2.4). However, even if the
cycle
is set to 8 (as in the BCCH case) and the messages are scheduled so that the
complete
set is read in one cycle, the time to read is still almost 2 seconds.
Set-up of Uplink and Downlink TBF: In the present GPRS system, a
temporary block flow (TBF) must be established before any user data can be
sent. The
TBF in the uplink and in the downlink are set up separately. It is estimated
to take
100-150 ms to set up an uplink TBF. It is estimated to take 60-100 ms to set
up a
downlink TBF.
In addition, the GPRS specifications state that the current TBF must be
abandoned and a new TBF requested if there is a change in priority. Mobility
management messages, such as the Routing Area (RA) Update message, have a
higher
priority than user data. Therefore, before the RA Update sequence, a TBF must
be
established, and after the procedure is finished, a new TBF must be
established to
resume the data flow.
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Network Centric Delays
GTP Link Establishment: The current GPRS baseline text defines a procedure
for combined inter-SGSN RA/LA updates which is not appropriate to support real-
time service performance requirements. The data stream is interrupted from the
time
the MS leaves the serving cell until the RA Update terminates with the RA
Update
complete signal. Location update handling includes the following:
1) The SGSN Update Location process to the HLR, Insert Subscriber Data, and
location cancellation; and
2) The location update from the SGSN to the Mobil Switching Center/Visitor
Location Register (MSC/VLR), the MSC/VLR location update to the HLR
with associated insertion of subscriber data, and location cancellation.
The RA Update procedure assumes the following functions are performed
following
either an MS-controlled or network-controlled (i.e., Base Station System
(BSS)) cell
re-selection:
1) GTP Link Establishment
~ SGSN context transfer and GTP link set-up between the old and new
SGSN for inter-SGSN tunneling.
~ PDP context transfer for GTP link set-up between the Gateway GPRS
Support Node (GGSN) and the new SGSN.
For a cell change controlled by the MS, packet delays in mobile terminated
calls may range between 410 ms and 660 ms for packets arriving during the RA
Update procedure. Packets arnving later will suffer delays of 260 ms to 450 ms
due
to the priority transmission ofpackets accumulated during the RA Update
interruption
and the Upd PDP context procedure. In mobile originated calls, the total
packet delay
may be in the range of 550 ms to 910 ms, of which 100 ms to 200 ms is
attributable
to the GTP link establishment procedure.
For a cell change controlled by the network, packet delays in mobile
terminated
calls may range between 310 ms and 460 ms for packets arnving during the RA
Update procedure. Packets incoming to the new SGSN from the GGSN after the RA
Update procedure are buffered and await transmission due to the priority
transmission
of packets already accumulated. The delay may be quantified as 170 ms to 250
ms.
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In mobile originated calls, the total packet delay may be in the range of 450
ms to 660
ms. The delay is due to buffering at the old SGSN until the inter-SGSN
tunneling link
is established (200 ms to 400 ms) and buffering which may occur at the TE in
order
to minimize packet loss during preemption/interruption of packets transferred
at the
RA Update (250 ms to 360 ms).
2) Securityprocedures: authentication and potential fetching of security data
from
the Authentication Center (AuC) (security triplet). The security function
during RA Update may involve the following optional steps:
~ MS authentication request (the security triplets) may need to be
obtained from the HLR);
~ User identity confidentiality through reallocation of a new P-TMSI at
any time (included as part of the RA-update exchange);
~ Identity (IMEI) check procedures (the SGSN may decide to check the
IMEI against the EIR); and
~ Reset of Ciphering Key: The Kc[SN] (serial number encrypted using
Kc) and the ciphering algorithm is passed to the MS as the
authentication request.
It is to be noted that execution of the above security procedure during the RA
update with potential HLR query for security triplet(s), will further increase
the RA
update interruption time and thus data transfer delay.
3) MS location handling in the GPRS network
~ New SGSN Location update to HLR;
~ Subscriber data update from the HLR to the new SGSN; and
~ Location cancellation of the old SGSN location.
4) If the subscriber is IMSI attached,
~ New location update from the new SGSN to the associated MSCNLR;
~ New MSCNLR Location update to the HLR;
~ Subscriber data update in the new MSCNLR; and
~ Location cancellation of the old MSCNLR location.
S) TMSI reallocation.
As stated previously the total interruption time may total three to four
seconds.
The Swedish Patent office P C T / S E 0 0 / 0 1 2 7 7
PCT International Application
9
Policin~Functions: In systems that only support best efforts (loss sensitive)
services, the associated policing functions (if present at all) do not provide
the
characteristics needed for real-time payload services. Current policing
functions
are, at best, based on simple traffic shaping mechanisms for best effort
traffic only.
The shaping is meant to forge a "bursty" data stream into a desired traffic
profile.
Typically, for a data stream session, this is done through queuing of "bursty"
incoming packets to impose a constant delay within the specified traffic
profile.
The packets accumulate in a queue/buffer at a certain "bursty" rate, and are
then
removed from the queue at a constant rate, imposing a constant delay. The
policing
1o function consists of identifying packets that fall out of profile
parameters, typically
when the packet rate exceeds the profile rate limit.
Traffic shaping functions are typically implemented in edge nodes. If the
radio access network, the operator network, and the ISP network are considered
separate domains, then the SGSN, GGSN, and RNS/RNC can be edge nodes. In
the scope of GPRS Ph. 1, the SNDCP/LLC implements such functions.
Admission Control: Today's GPRS phase has no admission control besides
the QoS negotiation at PDP context activation.
A consequence of the various interruptions, whether MS Centric or
Network Centric, is the degradation of the delay performance. While it has
been
2o demonstrated that the user does not perceive any difference for
interruption times
less than 100 ms, a critical problem arises if the interruption lasts for more
than a
few tens of seconds since service interruption time in this range is not
acceptable
for low latency and delay-sensitive applications such as real-time speech.
The known prior art neither teaches nor suggests a solution to the
aforementioned deficiencies and shortcomings such as that disclosed herein.
European Patent Application EP 0898438A describes a system and method
for establishing an anchor Radio Network Controller (aRNC). In conjunction
with
the setup of a call, an aRNC is selected, and all user information flows
through the
aRNC for the entire duration of the call, even if the MS is handed over to a
3o neighboring RNC. In addition, in preparation for a handover, a list of
candidate
base stations is compiled in the neighboring RNC in case the neighboring RNC
is
made the active RNC. However, EP 0898438A does not disclose a method for
reducing delay time for a mobile station being handed over from an old SGSN to
a
new SGSN during a call handling a real-time payload. In particular, there is
no
teaching or suggestion of a method that reduces delay at both the air
interface level
and at the core network level, as disclosed herein.
In order to overcome the disadvantages of existing solutions, it would be
advantageous to have a method of introducing new capabilities within the GPRS
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9a
system to better handle the handover procedure for delay-sensitive type
traffic
thereby reducing the handover interruption time while keeping the delays to
affordable values for the real-time payload services in a PS GPRS network. The
present invention provides such a method.
Amended Sheet
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SUMMARY OF THE INVENTION
The present invention is a method of improving cell change procedures within
the GPRS system to reduce the handover interruption time while keeping the
delays
to affordable values for real-time payload services in a PS GPRS network. The
S method consists of procedural mechanisms which reduce the data stream
interruption
at the air interface RLC/MAC level and shorten the inter-SGSN Routing Area
Update
interruption interval at the core network level.
Improvements at the air interface are achieved by reducing the system
information retrieval time and pre-allocating the TBF prior to the MS
accessing the
new cell/BSS. The system information retrieval time is reduced by sending
system
information messages about the target cell for handover on a control channel
in the
MS's current serving cell. A Packet Associated Control Channel (PACCH) may be
utilized as the control channel for sending such system information messages.
Additionally, silent periods, when no speech is in progress, are utilized for
sending
system information messages. Where the MS is capable of supporting multiple
time
slots, system information messages are sent on a different time slot than the
time slot
carrying the speech. Pre-allocation of the TBF is accomplished by reserving a
TBF in
the target cell for the MS prior requesting a handover. Reserving a TBF in the
target
cell for the MS prior to requesting handover may also include the following:
reserving
resources in the target cell for the MS based on the MS's quality of service
profile;
modifying a Packet Cell Change Order message to assign the TBF in the target
cell to
the MS; and/or modifying a Packet Time Slot Reconfigure message to assign the
TBF
in the target cell to the MS.
Improvements within the core network are achieved by utilizing low latency
delay-sensitive requirements and shaping packet traffic for premium traffic
(e.g.,
speech). Implementing low latency delay-sensitive requirements, instead of
loss-
sensitive requirements, provides a more efficient approach to packet dropping,
redirection, and buffering. A combination of procedural mechanisms is utilized
to
improve traffic shaping. The GTP link establishment procedure is improved by
modifying the GTP link handling logic so that mobility management procedures
are
delayed until after handover. Location update procedures and security
functions are
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postponed until after the handover is completed. The current inter-SGSN
tunneling
mechanism may also be removed. In addition, packet multicasting or "soft
handover"-
like mechanisms, between the GGSN and the serving and target SGSN are utilized
to
improve handover performance.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood and its numerous objects and
advantages will become more apparent to those skilled in the art by reference
to the
following drawings, in conjunction with the accompanying specification, in
which:
FIG. 1 (Prior Art) is a simplified block diagram illustrating the existing
GPRS
system;
FIG. 2 (Prior Art) is a message flow diagram illustrating the messages
utilized
in the existing inter-SGSN handover process;
FIG. 3 is a simplified functional block diagram illustrating the traffic
shaping
function for delay-sensitive and loss-sensitive traffic requirements;
FIG. 4 is a message flow diagram illustrating the messages utilized in an
improved method of MS-controlled cell change;
FIG. 5 is a message flow diagram illustrating the messages utilized in a first
embodiment of an improved method of Network-controlled cell change;
FIG. 6 is a message flow diagram illustrating the messages utilized in a
second
embodiment of an improved method of Network-controlled cell change; and
FIG. 7 is a message flow diagram illustrating the messages utilized in a third
embodiment of an improved method of Network-controlled cell change.
DETAILED DESCRIPTION OF EMBODIMENTS
The preferred embodiment of the present invention is derived from the
implementation of a plurality of optimization procedures to reduce the
handover
interruption time. It is, therefore, necessary to discuss the possible
optimization
solutions to delay-inducing problem areas within GPRS prior to describing the
preferred embodiment of the present invention.
MS Cell Re-Selection: The present invention improves the MS Cell Re-
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selection Procedure by sending PSI-1 and PSI-2 information, or SI-13 and any
other
system information messages about the target cell, on the old serving cell's
common
control channel or Packet Associated Control Channel (PACCH). This procedure
reduces the cell re-selection time and the loss of downlink radio blocks on
the serving
cell.
The maximum amount of system information messages sent on the BCCH is
ten ( 10). Each message takes approximately 20 ms to transmit. Therefore, it
requires
a utilization of the PACCH for a maximum total of 200 ms to transfer all of
the system
information messages. These do not have to be sent all at once, but can be
multiplexed within a 5-second period (the cell re-selection period).
Likewise, the amount of packet system information messages sent on the
PBCCH ranges from 4 to 28. Each message takes approximately 20 ms to transmit.
Therefore, it requires a utilization of the PACCH for a total of 80-560 ms to
transfer
all of the system information messages. These do not have to be sent all at
once, but
can be multiplexed within the S-second cell re-selection period. The present
invention may also send the messages in such a way as to minimize the
disruption to
the speech or other real-time payload. For example, messages may be sent
during
silent periods or on another time slot if the mobile station's mufti-slot
capabilities
permit.
The effect of sending PSI-1 and PSI-2 information, or SI-13 and any other
system information messages about the target cell, on the old serving cell's
common
control channel or PACCH is that the system information retrieval time on the
target
cell is reduced to zero if a change in the broadcast channel information has
not
occurred from the time the terminal received the SI messages on the old
serving cell
until it resynchronizes on the new cell. The time for the cell change
therefore is
reduced to only the resynchronization time.
Furthermore, to support the concept of network-assisted system information
retrieval, the serving BSS/RNC has to obtain the information from the target
cell's
BSS/RNC either directly or via their respective SGSNs. In a network-controlled
cell
re-selection mode (NC2), the network selects the target cell based on the
information
sent by the MS in the measurement reports. In a MS-controlled cell re-
selection mode
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(NCO), the MS determines the target cell. However, the MS can then request the
target
cell's system information from the network.
Set-up of Uplink and Downlink TBF: To reduce the TBF set up time for a
real-time application handover, the network may pre-reserve the TBF in the
target cell
so that the serving cell can forward the necessary information to the mobile
station.
In the present invention, two RLC/MAC control messages are modified and used
by
the network in a handover situation to assign a TBF to a mobile station. For
example,
the existing MAC control messages (i.e., Packet Cell Change Order and Packet
Time
Slot Reconfigure) can be modified and used for a handover order.
Alternatively, a new
message can be created.
The Packet Cell Change Order message is used to order the MS to another cell.
It contains information elements necessary to identify the BCCH of the target
cell
(i. e., BSIC + BCCH frequency). It does not contain the information elements
required
for frequency hopping, and it does not contain any reference to a specific
packet data
1 S traffic channel in the target cell.
The Packet Time Slot Reconfigure message is used to reassign the mobile to
another packet data channel within the same cell. It contains frequency and
time slot
information for the target packet data channel as well as the TBI to be used
in the
uplink and/or downlink. The message is designed to offload the mobile station
to
another packet resource within the same cell and therefore does not include
any
Broadcast channel information.
The present invention creates a combination of the above two messages. Either
one may be modified to include the required information elements or a
completely new
message may be created. The time for this transaction is estimated as:
~ receive "Cell Change / TS Reconfigure" order - 20 ms
~ retune a new cell - 5 ms
~ send shortened access on new cell - 5 ms
~ receive Time Alignment information from system - 20 ms
In summary, this procedure reduces the TBF set up time from the current 60-
150 ms down to 50 ms.
To support the concept of pre-assignment of resources, a form of resource
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reservation must be used. This has the added benefit of guaranteeing resources
in the
target cell. Resource reservation based on the mobile station's Quality of
Service
(QoS) profile can also be used. Naturally inter-BSS/RNC communications are
necessary for this implementation. The communications can be between the
BSS/RNCs directly or via their respective SGSNs. This concept is easily
supported
in a network-controlled cell re-selection mode since the target cell is
selected by the
network based on the measurement reports sent in by the mobile station. The
concept
can also be supported in a MS-controlled cell re-selection mode, but it is a
little more
complex. In this case, the mobile station selects a target cell and requests
the network
to reserve resources for it.
According to current specifications, any mobility management or signaling
procedure causes the release of the TBF used for real-time payloads (e.g.,
speech).
Possible modifications to the specifications could include assigning the same
or higher
priority to signaling traffic that is used for real-time traffic.
Alternatively, the
performance of mobility management procedures may be delayed until after the
handover process is completed so as to reduce the interruption time. The start
of the
mobility management procedure may be controlled by a timer which specifies the
time
interval between the two events. The procedure for of delaying the mobility
management procedure until after the call completion is discussed later.
Optimizi~~Routing Area Information Flows: The present invention includes
two methods for optimizing Routing Area (RA) information flows, both of which
involve delaying certain functions until after the handover process is
completed.
a. Delaying the GPRS-GSM/ANSI-41 Location Update Procedures; and
b. Delaying the Security Functions.
If the Location Update Procedures are delayed, the location update handling
in the GPRS-GSM/ANSI-41 network is completed as soon as the relevant
subscriber
SGSN context has been updated to the new SGSN, and the relevant PDP contexts
and
GTP have been updated in the new SGSN and GGSN. Inherent in this method are
some foreseeable problems.
The first problem is that an additional Location Area Update Accept message
is returned to the MS to transfer the result of the Location Update response
received
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at the new serving SGSN. This introduces an extra interruption of the real-
time
payload, but the delay can be minimized by avoiding the reallocation of a TBF.
Although this is a disadvantage, it may be outweighed by the advantage of
minimizing
data-transfer delay in the combined RA/LA update situation.
The second problem is that the response from the network is also delayed with
respect to the point in time when the real-time payload resumes. Normally, the
mobility management procedure triggered by the RA Update is not completed
until the
location update process has terminated. In the present invention, the scope of
GMM
procedures associated with GMM state transitions are modified. In case of
failure of
either of the location update processes, there is a potential dropped call
situation (e.g.,
a rejected location update) or, the MS may be required to make a new GPRS
attach or
new PDP context activation, as indicated by the SGSN in the SGSN-initiated
GPRS
Detach procedure. In any case, the reason for the failure justifies that the
call be
aborted. There is a drawback due to the misuse of handover resources in such a
case.
A fallback procedure must then be implemented to send the MS back to the old
SGSN.
However, this scenario has a low likelihood of occurrence.
The third problem is that there is an increased likelihood that incoming
circuit-
switched calls maybe delivered to the wrong MSC/VLR due to delays in updating
the
MSC/VLR location and the SGSN-MSC/VLR association. These calls could not be
completed as specified by the subscriber profile (e.g., CAW, TRN, TRB, etc.).
It
should be noted that this situation does not apply in a circuit-switched
network since
the Mobility State is not changed at inter-MSC handover.
If, instead of delaying the Location Update Procedures, the Security Functions
may be delayed in order to reduce the interruption time for the RA updates.
This is
achieved by postponing optional security procedures until after data transfer
resumes.
During handover, the SGSN may choose not to perform any of the security
procedures.
All of the security parameters are maintained as they were before handover,
and the
authentication triplets have already been downloaded into the new SGSN as a
part of
the SGSN-context transfer. The SGSN and the MS continue to use the same
ciphering
algorithm as before the handover. Since TLLI/P-TMSI reallocation is already a
part
of the RA Update procedure, there is no need to duplicate this procedure by
having a
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separate P-TMSI reallocation procedure. The only security procedure left is
the
identity check, which does not have to be performed during handover. If a
signaling
exchange with the EIR occurs, handover delay increases. While postponing these
security procedures may further increase the likelihood of delayed failure
discovery,
and introduce additional interruption of the real-time payload, this can be
minimized
by avoiding the reallocation of a TBF.
Thus, in the present invention, the GMM mobility context handling (04.08) is
updated to extend the mobility procedure or change the Mobility Context State
handling after the RA Update is completed. It is unlikely that the failure of
the HLR
location update procedures can be avoided; therefore, the MS may have to be
redirected to the old SGSN. The security procedures can be postponed till
after the
RA update procedure terminates and data transfer resumes. The effect of the
above
procedures is to avoid the reallocation of TBF and thus keep the interruption
time to
a minimum.
In order to handle the interaction of CS services with the RT voice service in
the packet-switched domain, one of the following approaches can be taken:
1. Provide for extra functionality between the CS and PS networks:
~ Mark subscribers "Busy" at the MSC/VLR when a RT voice call is on-
going;
~ Allow for rerouting of calls from the PLMN to the VoIP
gateway/network; and
~ Allow for the execution of other services in the PLMN through
interwork with the VoIP Network.
2. Provide for service handling through a service convergence layer:
~ The IP paradigm allows for handling of new applications and services
outside of the traditional infrastructures. Tracking of the subscriber's
location in the TDMA circuit switched or RT packet-switched network
can be handled by a service layer VAS (value added service) function.
This allows, for instance, the delivery of incoming ANSI-41
PSTN/PLMN voice calls to the RT IS-136HS VoIP infrastructures
where the subscriber is active (e.g., a mobility gateway can handle
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ANSI-41 call delivery and other service interaction when the MS is
located in the RT PS network).
~ All RT services are provided from the service layer, so they appear the
same on all access networks which are a part of the IP network.
S ~ ANSI-41 services such as basic, supplementary, and WIN services may
be implemented/replicated (either fully or in part) in the service layer,
as value added service to existing IP services (VAS voice services).
Thus, the HLR, SCP, and Special Resource Functions (SRF) may be
functionally mapped to IP-specific associated functionality, although
the mechanics of implementation of the services may be different from
that implemented over ANSI-41.
Packet Dropping Policy: FIG. 3 is a simplified functional block diagram
illustrating the traffic shaping function for delay-sensitive and loss-
sensitive traffic
requirements. Profile policing is performed at 45 for loss-sensitive traffic
such as Best
Effort traffic which is then buffered at 48 and 49. For delay-sensitive
traffic such as
speech and RT streaming, the packets that do not adhere to the profile are
dropped at
46. The others are buffered at 47 and 49. Important considerations for
determining
the criteria for dropping packets include the following:
a. Shaping for real-time service requires that the ordering be preserved,
especially when dropping a packet;
b. Compression/decompression works synchronously and needs to
resynchronize when packets are dropped; and
c. Encryption, when state driven, may also need to be resynchronized.
As discussed above, the handover interruption has the effect of a faster
increase
in the number of queued packets per session in the associated buffer.
Typically, a
queuing approach based on the "leaky bucket" is used at 46. A variation of the
WFQ
algorithm can be used or, as an alternative, a less complex method can be used
which
consists of time-stamping the packets in order to calculate the rate of packet
queuing
in comparison with the profile/service specification.
Admission Control Improvements: The present invention includes two
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procedures for improving Admission Control. First, through the use of a soft
reservation after call setup, if a neighboring cell is found to be part of a
different
service area (different LAI), inter-BSS or inter-SGSN, the MS issues a request
to the
network to perform a pre-allocation of resources to the MS. Resources may be
requested in the targeted BSS and/or SGSN through inter-BSS communications or
inter-system (SGSN) communications. The allocation of resources may be valid
for
a certain amount of time as determined by a timer in the MS or the network.
The second procedure consists of a QoS negotiation after handover. At
handover, it may be impossible to allocate the QoS level negotiated/allocated
to the
old SGSN. A temporary QoS fall back in the GPRS core and radio access networks
may be effected at handover. It takes too long to renegotiate the QoS through
all
protocol layers up to the application level and with the peers residing
somewhere in
the network. Subsequent QoS fall forward must be handled with a QoS
renegotiation
procedure between the MS, radio access network, and core network only, based
on the
1 S changed load situation in both the radio network and the core network.
In order to support the TBF pre-assignment, the network can reserve the radio
resources in the target cell prior to the handover. In this embodiment of the
present
invention, the resource reservation is also used to relay the offered quality
of service
of the target cell to the MS. This gives the MS the ability to renegotiate the
QoS in the
event that the original service QoS cannot be satisfied.
Embodiments of GPRS Network Logic Improvements
As discussed in the previous sections, all cell change procedures require
mechanisms which reduce the data stream interruption at the air interface
RLC/MAC
level. This can be accomplished by:
~ Reducing the system information retrieval time by providing the target
cell's system information in the serving cell; and
~ Pre-allocating the TBF prior to the MS accessing the new cell/BSS.
The core network implements low latency delay-sensitive requirements instead
of loss-sensitive requirements which provide:
~ an efficient packet dropping approach; and
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~ an efficient packet redirection and buffering approach.
The core network should also implement traffic shaping for premium traffic.
Specific improvements include:
~ Removing all or part of the tunneling mechanism;
~ Delaying the security functions;
~ Defining a new GTP link handling logic;
~ Utilizing packet multicasting or "soft handover" mechanisms between the
GGSN, the serving SGSN, and the target SGSN;
~ Delaying the circuit-switched domain mobility management update until
after the handover or the call is completed; or
~ A combination of all of the above.
The SRNS relocation procedure is similar to the GPRS Network-controlled cell
change procedure. Therefore, the proposed solution may also be applicable for
the
SRSN relocation problem.
The following embodiments of the present invention assume that traffic
shaping is implemented in the SGSN and that the security and mobility
management
context updates are delayed until after the handover. Modified GTP link
reestablishment procedures are presented.
Improvements Specific to MS-Controlled Cell Change: The initiation of the
inter-SGSN tunneling and security functions are removed from the RA update
procedure, thereby reducing the delay overhead of the RA update procedure.
Speech
or other real-time payload packets may be lost with the inter-SGSN tunneling
being
removed. Packets arnving after the RA Update will not be delayed. The
complexity
of Inter-SGSN tunneling and buffer synchronization is thus avoided at the
expense of
real-time packet loss. However, for low latency and delay-sensitive traffic
types,
dropping of packets may be an unavoidable reality. Real-time payload packets
with
an end-to-end delay on the order of the interruption time may be considered
being
outside of the traffic profile requirements, and may be ignored totally or in
part. With
this improvement, the interruption time is reduced, realizing that some of the
oldest
packets will be dropped, in addition to those not delivered by the old SGSN
after the
MS has left the old serving cell. For MS terminated packets, the maximum
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interruption is now on the order of 300 ms instead of up to 600 ms in the
original
scenario. This may represent up to an extra 200 ms of loss for a real-time
payload.
The execution of postponed security procedures causes an interruption since
the required signaling is still assumed to have a higher priority than traffic
(GPRS/GSM 4.08), which results in suspending traffic during the procedure.
This
interruption should be shorter than an inter/intra-SGSN RA Update. It is also
assumed
that the TBF used for the RA Update can be used for the Security functions.
FIG. 4 is a message flow diagram illustrating the messages utilized in an
improved method of MS-controlled cell change. The signaling takes place in a
GPRS
network in which the nodes have been modified to recognize certain modified
messages. Alternatively, the order of standard procedures may be altered in
order to
move some procedures that are not time-sensitive to a later time after data
transfer is
resumed.
At 50, Mobile Station (MS) 112 sends an RA Update request to a New SGSN
107. The New SGSN sends an SGSN context request 51 to an Old SGSN 106. The
Old SGSN responds with an SGSN context response 52. The New SGSN then sends
an RA Update 53 to the MS. The transfer of data is then resumed at 54. At 55,
a PDP
context update request is sent from the New SGSN to a GGSN 102, and the GGSN
returns a PDP context update response at 56. Security functions are then
performed
between the MS and the New SGSN at 57. At 58, after traffic resumption, an HLR
update is performed in accordance with the standard.
Two alternative embodiments are proposed to improve the MS-controlled cell
change procedure:
a. The first alternative involves no change in the flow of signaling as
shown in FIG 4. The only required modification is to keep track of the lost
LLC
frames at the Old SGSN 106 during RA Updates since these can be later tunneled
to
the New SGSN 107 (i.e., an Acknowledge mode without retransmission). There is
no
need to maintain the whole buffer synchronization procedure.
b. The second alternative is to remove inter-SGSN tunneling and buffer
synchronization, but keep the transfer of context information as shown FIG. 4.
No
TBF reassignment is used in this scenario.
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Improvements Specific to Network-Controlled Cell Change: Three alternative
embodiments are proposed to improve the Network controlled cell change
procedure.
FIG. 5 illustrates the most efficient embodiment. FIG. 6 illustrates the least
efficient
embodiment, although it is still an improvement over the prior art. FIG. 7
illustrates
an embodiment whose efficiency lies between that of FIG. 5 and FIG. 6.
The embodiment of FIG. 6 improves on the RA Update interruption because
of the pre-allocation of TBF and QoS, but a good fraction of the packets
tunneled to
the old SGSN may not be delivered to the MS after the cell change is ordered.
The
embodiment of FIG. 7 improves upon FIG. 6 because the removal of the tunneling
avoids the setup time for the inter-SGSN tunnel. Therefore, the MS cell change
is
faster (the MS receives the cell change order earlier) while MS-destined
packets are
buffered at the new SGSN. However, FIG. 7 still offers a two-step
interruption: (1)
the interruption for setting up links between the GGSN and the new SGSN (with
its
associated buffering), and (2) the RA Update. Finally, FIG. 5 improves upon
the
process illustrated in FIG. 7 because the MS cell change is faster. The link
between
the GGSN and the new SGSN is actually changed at step 65, just prior to the
cell
change order, thus avoiding the execution of one primitive (SGSN context rsp
at the
new SGSN) with its associated buffering.
a. FIG. 5 is a message flow diagram illustrating the messages utilized in
a first embodiment of an improved method of Network-controlled cell change. As
shown in FIG. 5, the network initially treats the cell change request as a PDP
update
request. After the Old BSS 110 indicates it wants to perform a cell change at
59, the
Old (serving) SGSN 106 sends a PDP context update request 60 to the GGSN 102
indicating that the currently used context needs to be configured at the new
SGSN 107.
The target cell is indicated in the request. The MS-terminated and originated
RT
speech packet transmission between the old SGSN 106 and GGSN 102 continues
uninterrupted. The GGSN then initiates the GTP link establishment procedures
61
towards the New SGSN 107, including the MS context (with the QoS, etc.).
The New SGSN 107 then issues a Radio Resources (RR) reservation request
62 to the New (target) BSS 111 requesting an allocation ofresources, for both
mobility
and user data payload, in the specified cells and for the specified QoS level
and RAB.
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The target BSS responds at 63 with the identifier of the allocated resources
and
associated QoS level. Once the GTP link is set, the MS context, and radio
resources
are set-up, the new SGSN 107, replies to the GGSN 102 with an Update PDP
context
response 64. The GGSN then directs the traffic from the Old SGSN 106 to the
new
S SGSN, where the packets are buffered. The GGSN replies to the Old SGSN with
an
Update PDP context response 65.
Upon receiving the response 65, the Old SGSN 106 forwards a Modified PDP
context request 66 to the MS 112 indicating that a change of cell is
requested. The MS
in turn accesses the new channel using the pre-allocated TBF and sends a
Routing
Area Update Request 67 to the New SGSN 107. Security functions are then
performed
at 68. Since the New SGSN previously acquired the context from the old SGSN
and
the GGSN GTP link set-up, the New SGSN immediately sends a Routing Area Update
Accept 69 to the MS. Data transfer then resumes at 70 and packets are sent
directly
to the MS with minimum interruption. At 58, after traffic resumption, an HLR
update
is performed in accordance with the standard.
Thus, in this embodiment, the GGSN 102 controls the link re-establishment
process between the Old SGSN 106 and New SGSN 107. The GGSN serves as the
anchor point in the link re-establishment procedure between SGSNs in the same
fashion that the SGSN anchors the link re-establishment procedure between own-
controlled BSSs, and the Base Station Controller (BSC) anchors the link re-
establishment between own-controlled Base Stations (BS). The anchors are,
therefore,
in control of the various links. This concept is not applied in the current
GPRS
procedures.
A network utilizing the present invention is thereby more proactive than in
the
current network-controlled cell change scenario. While the cell change is
being
requested, the network prepares for the new links to be configured and for QoS
resource reservation and admission control. The New SGSN 107 is configured
with
the MS context (including QoS). The target BSS 111 is requested to allocate
resources
for the session's QoS. Concurrently, a new GTP link is set up between the GGSN
102
and New SGSN. The data stream transfer between the MS 112, serving (Old) SGSN
and GGSN is not interrupted during execution of these procedures.
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Once the configuration and link set-up procedures are completed, the MS 112
is ordered to make the cell change. Since the radio resources have already
been
allocated to the MS, the interruption time is reduced substantially. The
Mobility
management signaling procedures (such as the RA Update) are delayed until
after the
data transfer resumes.
The main advantage of this method is the pre-allocation of QoS resources at
the New BSS 111 coupled with the ability of the MS 112 to start renegotiating
the QoS
profile in the event that the original service QoS is not satisfied.
The use of a modified Update PDP context request 60-61 and the RR
reservation request 62 allows the MS 112 to be informed of the allocated QoS
and to
readily start to renegotiate and/or set the QoS profile with the new SGSN 107
and BSS
111. This is not feasible under the current GPRS procedures, where the new BSS
and
MS have no way of knowing the allocated QoS profile in the new SGSN. As shown
in FIG. 2, the MS must currently wait until after it receives the necessary
information
in the RA Update accept message 42.
b. FIG. 6 is a message flow diagram illustrating the messages utilized in
a second embodiment of an improved method of Network-controlled cell change.
After the Old BSS 110 indicates it wants to perform a cell change at 71, the
Old SGSN
106 sends an SGSN context indication 71 to the New SGSN 107. The New SGSN
sends a PDP context update request 73 to the GGSN 102 indicating that the
currently
used context needs to be configured at the new SGSN. A Radio Resources (RR)
reservation request 74 is sent to the New BSS 111 to pre-allocate radio
bearers prior
to the MS 112 change of cell. The New BSS responds at 75 with the identifier
of the
allocated resources and associated QoS level. The GGSN then sends a PDP
context
update response 76 to the New SGSN which sends an SGSN context indication
acknowledgment 77 to the Old SGSN. Tunneling of packets then occurs between
the
New SGSN and the Old SGSN at 78.
At 79, the Old SGSN 106 issues a change cell order to the Old BSS 110 which
is forwarded to the MS at 80. Tunneling is then stopped at 81. The MS sends an
RA
Update request 82 to the New SGSN which returns an RA Update accept message to
the MS at 83. Data transfer is then resumed at 84, and security functions are
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performed at 85. At 58, after traffic resumption, an HLR update is performed
in
accordance with the standard.
c. FIG. 7 is a message flow diagram illustrating the messages utilized in
a third embodiment of an improved method of Network-controlled cell change.
This
embodiment is a variation of the embodiment shown in FIG. 6. Because tunneling
is
inefficient and packet loss is reduced substantially when the MS 112 accesses
the new
cell, the inter-SGSN tunneling 78 is removed in this embodiment to reduce
complexity. After the Old BSS 110 indicates it wants to perform a cell change
at
86, the Old SGSN 106 sends an SGSN context indication 87 to the New SGSN 107.
The New SGSN sends a PDP context update request 88 to the GGSN 102 indicating
that the currently used context needs to be configured at the new SGSN. A
Radio
Resources (RR) reservation request 89 is sent to the New BSS 111 to pre-
allocate
radio bearers prior to the MS 112 change of cell. The New BSS responds at 90
with
the identifier of the allocated resources and associated QoS level. The GGSN
then
sends a PDP context update response 91 to the New SGSN which sends an SGSN
context indication acknowledgment 92 to the Old SGSN. At 93, the Old SGSN
issues
a change cell order to the Old BSS 110 which is forwarded to the MS at 94. The
MS
sends an RA Update request 95 to the New SGSN which returns an RA Update
accept
message to the MS at 96. Data transfer is then resumed at 97. Security
functions are
performed at 98. At 58, after traffic resumption, an HLR update is performed
in
accordance with the standard. In this embodiment, the new SGSN 107 starts
buffering
packets as soon as packets are directed to it when the new GTP link is
established.
It is thus believed that the operation and construction of the present
invention
will be apparent from the foregoing description. While the method shown and
described has been characterized as being preferred, it will be readily
apparent that
various changes and modifications could be made therein without departing from
the
scope of the invention as defined in the following claims.
