Sequence number ordering in a wireless communications system

Details Sequence number ordering in a wireless communications system Sequence number ordering in a wireless communications system

2001-01-10

2004-02-03

Pezzlo, John (Department: 2662)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S328000, C370S469000

Reexamination Certificate

active

06687248

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wireless communications system. More specifically, the present invention discloses a method and system that can properly determine sequence number ordering when performing an SDU discard procedure with a move receiving window (MRW) operation.
2. Description of the Prior Art
The surge in public demand for wireless communication devices has placed pressure upon industry to develop increasingly sophisticated communications standards. An example of such a standard is the 3
rd
Generation Partnership Project (3GPP™), Technical Specification Group Radio Access Network, RLC Protocol Specification. Such standards utilize a three layer approach to communications. Please refer to FIG.
1
.
FIG. 1
is a block diagram of the three layers in a communications protocol. In a typical wireless environment, a base station
10
is in wireless communications with one or more mobile units
20
. An application
13
on the base station
10
composes a message
11
and has it delivered to the mobile unit
20
by handing the message
11
to a layer
3
interface
12
. The layer
3
interface
12
delivers the message
11
to a layer
2
interface
16
in the form of layer
2
service data units (SDUs)
14
. The layer
2
SDUs
14
may be of any length. The layer
2
interface
16
composes the layer
2
SDUs
14
into one or more layer
2
protocol data units (PDUs)
18
. Each layer
2
PDU
18
is of a fixed length, and is delivered to a layer
1
interface
19
. The layer
1
interface
19
is the physical layer, transmitting data to the mobile unit
20
. The transmitted data is received by the mobile unit
20
by the layer
1
interface
29
and reconstructed into one or more layer
2
PDUs
28
, which are passed up to the layer
2
interface
26
. The layer
2
interface
26
receives the layer
2
PDUs
28
and builds up a layer
2
SDU
24
. The layer
2
SDU
24
is passed up to the layer
3
interface
22
. The layer
3
interface
22
in turn converts the layer
2
SDUs
24
(which may also be termed layer
3
PDUs) back into a message
21
, which should be identical to the original message
11
that was generated by the base station
10
application
13
. The message
21
is then passed to an application
23
on the mobile unit
20
.
Of particular interest is the layer
2
interface, which acts as a buffer between the relatively high-end data transmission and reception requests of the applications, and the low-level requirements of the physical transmission and reception process. Please refer to FIG.
2
.
FIG. 2
is a diagram of a transmission/reception process from a layer
2
perspective. The layer
2
interface
32
of a transmitter
30
, which may be either a base station or a mobile unit, receives a string of layer
2
SDUs
34
from layer
3
. The layer
2
SDUs
34
are sequentially ordered from 1 to 5, and are of an unequal length. The layer
2
interface
32
converts the string of layer
2
SDUs
34
into a string of layer
2
PDUs
36
. The layer
2
PDUs are sequentially ordered from 1 to 4, and are all of an equal length. The string of layer
2
PDUs is then sent off to the layer
1
interface for transmission. A reverse process occurs at the receiver end
40
, with the receiver layer
2
interface
42
converting a received string of layer
2
PDUs
46
into a received string of layer
2
SDUs
44
. Under certain transport modes, however, the multi-layered protocol insists that the receiver layer
2
interface
42
present the layer
2
SDUs to layer
3
in order. That is, the layer
2
interface
42
must present the SDUs
44
to layer
3
in the sequential order of the SDUs
44
, beginning with SDU
1
and ending with SDU
5
. The ordering of the SDUs
44
may not be scrambled, nor may a subsequent SDU be delivered to layer
3
until all of the prior SDUs have been delivered.
In line transmissions, such a requirement is relatively easy to fulfill. In the noisy environment of wireless transmissions, however, the receiver
40
, be it a base station or a mobile unit, often misses data. Some layer
2
PDUs in the received string of PDUs
46
will therefore be missing. Thus, ensuring that the layer
2
SDUs
44
are presented in order can pose a significant challenge. Please refer to FIG.
3
.
FIG. 3
is a block diagram of a data PDU
50
, as defined in the 3GPP™ TS 25.322 specification. In general, there are two types of PDUs: a control PDU or a data PDU. Control PDUs are used by layer
2
to control data transmission and reception protocols. Data PDUs are used to transmit acknowledged mode data, which is then reassembled and presented to layer
3
. The example PDU
50
is a data PDU, and is divided into several fields, as defined by the layer
2
protocol. The first field
51
is a single bit indicating that the PDU
50
is either a data or a control PDU. As the data/control bit
51
is set (i.e., equal to 1), the PDU
50
is marked as an acknowledged mode data PDU. The second field
52
is a sequence number (SN) field, and is twelve bits long. Successive PDUs have successively higher sequence numbers, and in this way a receiver can properly reassembled layer
2
PDUs to form layer
2
SDUs. That is, if a first PDU is transmitted with an SN equal to 536, the next PDU would be transmitted with an SN equal to 537, and so forth. A single polling bit
53
follows the SN field
52
, and when set indicates that the receiver should respond with an acknowledgment status PDU, which is one kind of control PDU, and which will be introduced later. Bit
54
is reserved and is set to zero. The next bit
55
a
is an extension bit, and when set indicates the presence of a following length indicator (LI). An LI may be either 7 bits long or 15 bits long, and is used to indicate the ending position of a layer
2
SDU within the layer
2
PDU. If a single SDU completely fills the data region
58
of the PDU
50
, then the bit
55
a
would be zero, thereby indicating that no LI is present. In the example PDU
50
, however, there are two layer
2
SDUs ending in the layer
2
PDU
50
: SDU
1
57
a
and SDU
2
57
b
. There must, therefore, be two LIs to indicate the respective ends of the SDU
1
57
a
and the SDU
2
57
b
. A PDU following the PDU
50
would hold the LI for SDU
3
57
c
. The first LI, LI
1
, is in field
56
a
following the extension bit field
55
a
, and marks the end of the SDU
1
57
a
. LI
1
56
a
has an extension bit
55
b
that is set, indicating the presence of another LI, LI
2
in field
56
b
. LI
2
56
b
indicates the ending position of the SDU
2
57
b
, and has an extension bit
55
c
that is cleared, signifying that there are no more LIs, and that the data region
58
is thus beginning.
The fact that the SN field
52
has a fixed bit length presents the peculiar fact that it is possible for layer
2
PDUs having higher SN values to be sequentially before layer
2
PDUs having lower SN values. To better understand this, please refer to FIG.
4
.
FIG. 4
is a phase diagram
60
of a sequence number transmission cycle. SN values behave very much like time values on a clock, due to overflow of the 12 bits in the SN field
52
. For example, an initial PDU may have an SN value of 0 (shown at position
66
), followed by a PDU with an SN=1 (at position
61
), and another with an SN=2, etc. The PDU SN values continue incrementing with each PDU, passing the 1024 mark at position
62
, the
2048
mark at position
63
, the 3072 mark at position
64
and finally reaching a maximum value of 4095 at position
65
. At 4095, the SN overflows when incremented, and returns to zero, just as 23:00 hours overflows to 0:00 hours at midnight. Thus, a layer
2
PDU with an SN=2 might be sequentially before a layer
2
PDU with an SN=1000, and yet sequentially after a layer
2
PDU with an SN=4092. If great care is not taken, this inequality of PDU sequentiality with SN numerical ascendancy can lead to confusion.
In the following discussion, receivers and transmitters may be eithe

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