U.S. patent application number 12/275462 was filed with the patent office on 2009-03-19 for method and apparatus for processing packets originating from local and neighboring basic service sets.
This patent application is currently assigned to INTERDIGITAL TECHNOLOGY CORPORATION. Invention is credited to Christopher R. Cave, Angelo A. Cuffaro, Diana Pani.
Application Number | 20090073948 12/275462 |
Document ID | / |
Family ID | 36815517 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090073948 |
Kind Code |
A1 |
Pani; Diana ; et
al. |
March 19, 2009 |
METHOD AND APPARATUS FOR PROCESSING PACKETS ORIGINATING FROM LOCAL
AND NEIGHBORING BASIC SERVICE SETS
Abstract
A method and apparatus for efficiently processing packets from
neighboring basic service sets (BSSs) to characterize the traffic
within a wireless local area network (WLAN), mesh network, or
similar wireless communication system is disclosed. The apparatus
may be an access point (AP) in a BSS which includes a measurement
period (MP) interval timer, a promiscuous measurement period (PMP)
timer and a receiver filter. An interval of the MP interval timer
is set while the receiver filter is enabled. During the interval,
the AP only processes packets which originated from the AP's own
BSS. When the MP interval timer expires, a value of the PMP timer
is set to a predetermined time duration and the receiver filter is
disabled. During the predetermined time duration, the AP processes
all received packets irrespective of which BSS the packets
originated from. The receiver filter is again enabled when the PMP
timer expires.
Inventors: |
Pani; Diana; (Montreal,
CA) ; Cave; Christopher R.; (Ile-des-soeurs,
(Verdun), CA) ; Cuffaro; Angelo A.; (Laval,
CA) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600, 30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
INTERDIGITAL TECHNOLOGY
CORPORATION
Wilmington
DE
|
Family ID: |
36815517 |
Appl. No.: |
12/275462 |
Filed: |
November 21, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11234804 |
Sep 23, 2005 |
7466660 |
|
|
12275462 |
|
|
|
|
60652413 |
Feb 11, 2005 |
|
|
|
Current U.S.
Class: |
370/338 |
Current CPC
Class: |
H04W 84/12 20130101;
H04W 92/16 20130101; H04W 40/246 20130101 |
Class at
Publication: |
370/338 |
International
Class: |
H04W 88/00 20090101
H04W088/00 |
Claims
1. An access point (AP) comprising: a processor; a measurement
period (MP) interval timer coupled to the processor; and a
transceiver coupled to the processor, the transceiver including a
receiver filter, wherein the processor sets an interval of the MP
interval timer while the receiver filter is enabled such that the
AP is restricted to processing only packets received on a given
channel which originate from the AP's own basic service set
(BSS).
2. The AP of claim 1 further comprising: a promiscuous measurement
period (PMP) timer coupled to the processor, wherein the processor
sets a value of the PMP timer to a predetermined time duration when
the MP interval timer expires and disables the receiver filter for
the predetermined time duration such that the AP processes packets
received on the given channel which originate from the AP's own
basic service set (BSS) and at least one neighboring BSS.
3. An access point (AP) comprising: a processor; a measurement
period (MP) interval timer coupled to the processor; a promiscuous
measurement period (PMP) timer coupled to the processor; and a
transceiver coupled to the processor, the transceiver including a
receiver filter, wherein the processor sets a value of the PMP
timer to a predetermined time duration when the MP interval timer
expires and disables the receiver filter for the predetermined time
duration such that the AP processes packets received on the given
channel which originate from the AP's own basic service set (BSS)
and at least one neighboring BSS.
4. The AP of claim 3 wherein the processor enables the receiver
filter when the PMP timer expires.
5. The AP of claim 4 further comprising: a silent measurement
period (SMP) timer, wherein the processor determines whether to
trigger an SMP or a PMP.
6. The AP of claim 5 wherein the transceiver switches from the
given channel to at least one other channel, and the AP processes
all packets received on the other channel.
7. The AP of claim 6 wherein the AP determines an activity level of
the other channel based on the packets received on the other
channel and the transceiver switches from the other channel to the
given channel when the SMP timer expires.
8. The AP of claim 3 wherein the interval of the MP interval timer
comprises a fixed component and a variable component.
9. The AP of claim 5 wherein the result of the determination made
by the processor alternates between triggering the SMP and
triggering the PMP based on a periodicity constant.
10. The AP of claim 9 wherein if the periodicity constant is 2,
then every other MP corresponds to an SMP.
11. The AP of claim 9 wherein if the periodicity constant is 2,
then every other MP corresponds to a PMP.
12. The AP of claim 9 wherein the periodicity constant is set to 1
during a discovery phase of the AP such that every MP is provided
as an SMP.
13. The AP of claim 3 wherein the AP further comprises: a packet
information database, wherein the AP performs measurements and
collects packet information on out-of-BSS packets received on the
given channel, and the processor stores the packet information in
the packet information database.
14. The AP of claim 13 wherein the stored packet information
includes received signal strength indication (RSSI) measurements of
the out-of-BSS packets.
15. The AP of claim 14 wherein the RSSI measurements are used to
determine the proximity of neighboring APs and WTRUs.
16. The AP of claim 13 wherein the stored packet information
includes a number of packets received from individual nodes and
packet durations.
17. An access point (AP) comprising: a processor; a measurement
period (MP) interval timer coupled to the processor; a promiscuous
measurement period (PMP) timer coupled to the processor; and a
silent measurement period (SMP) timer, wherein the processor
determines whether to trigger an SMP or a PMP.
18. The AP of claim 17 further comprising: a transceiver coupled to
the processor, the transceiver including a receiver filter, wherein
the processor sets a value of the PMP timer to a predetermined time
duration when the MP interval timer expires and disables the
receiver filter for the predetermined time duration such that the
AP processes packets received on the given channel which originate
from the AP's own basic service set (BSS) and at least one
neighboring BSS.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/234,804 filed Sep. 23, 2005, which claims
the benefit of U.S. Provisional Patent Application No. 60/652,413
filed Feb. 11, 2005, which is incorporated by reference as if fully
set forth.
FIELD OF INVENTION
[0002] The present invention relates to wireless communication
systems. More particularly, the present invention relates to a
method and apparatus for setting a promiscuous measurement period
(PMP), or other measurement periods (MPs) that coexist with the
PMP, (e.g., a silent measurement period (SMP)), for processing
packets originating from local and neighboring basic service sets
(BSSs).
BACKGROUND
[0003] When a wireless local area network (WLAN) is operating in an
infrastructure mode or a mesh mode, each of the access points (APs)
operating within the BSSs of the WLAN communicate with one or more
wireless transmit/receive units (WTRUs), (i.e., wireless stations),
associated with the APs. The APs and the associated WTRUs
communicate with each other by sending and receiving packets over a
given channel. In typical deployments in the un-licensed spectrum,
there may be one or more APs operating on the same channel. In such
cases, it may be necessary to adjust both power and frequency
settings to improve the system performance by reducing interference
and the number of deferrals.
[0004] One metric used to aid in adjusting the power level and
frequency setting is the level of activity on the given channel. To
determine the level of activity on the given channel, it is
advantageous for the APs to process all of the packets received on
the same channel from the neighboring BSSs.
SUMMARY
[0005] The present invention is related to a method and apparatus
for efficiently processing packets which originate from neighboring
BSSs and estimating the characteristics of traffic between all
nodes within a WLAN, mesh network or similar wireless communication
system without impacting or disrupting the currently served traffic
of the measuring node.
[0006] Typically, each of the APs has a receiver filter used to
block unnecessary data and to pass only necessary data to a medium
access control (MAC) layer according to the requirements of the AP.
In order to limit the processing requirements, in normal operation,
the AP sets the receiver filter such that only the packets destined
to the AP are forwarded to the MAC layer of the AP and other
unnecessary packets are blocked by the receiver filter without
being further processed. However, to determine the level of
activity on the given channel, it is necessary to receive and
process all packets from neighboring BSSs, (out-of-BSS packets).
The out-of-BSS packet information includes header information and a
received signal strength indication (RSSI) as a primary measurement
required, or the like.
[0007] The receiver filter of the AP can be set such that all of
the received packets are forwarded to the MAC layer of the AP. This
mode of operation is called a promiscuous mode and allows the AP to
process all of the out-of-BSS packets and retrieve the necessary
information from the out-of-BSS packets transmitted from
neighboring BSSs.
[0008] However, when the AP continuously operates in the
promiscuous mode, the performance of the AP is degraded due to the
extra load that is imposed on the processor in the AP when
receiving and processing all of the packets. In addition, when the
AP operates in the promiscuous mode, new WTRUs are restricted from
associating with the AP if the load exceeds an established
threshold.
[0009] In order to minimize the degradation on the AP and to
characterize the traffic, operation in promiscuous mode is limited
to very finite and regular durations. The samples of collected
statistics are averaged over a longer interval to fully and
reliably characterize the traffic.
[0010] The apparatus may be an AP in a BSS which includes a
measurement period (MP) interval timer, a PMP timer and a receiver
filter. An interval of the MP interval timer is set while the
receiver filter is enabled. During the interval, the AP is
restricted to processing only packets which originated from the
AP's own BSS. When the MP interval timer expires, a value of the
PMP timer is set to a predetermined time duration and the receiver
filter is disabled. During the predetermined time duration, the AP
processes all received packets irrespective of which BSS the
packets originated from. The receiver filter is again enabled when
the PMP timer expires.
[0011] The PMP may be provided along with other types of
measurement period, such as an SMP. When the MP interval timer
expires, either a PMP or an SMP is provided. During the PMP, the AP
processes data packets on the same channel while, during the SMP,
the AP processes data packets on all of the channels in an active
channel set (ACS).
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A more detailed understanding of the invention may be had
from the following description of a preferred embodiment, given by
way of example and to be understood in conjunction with the
accompanying drawings wherein:
[0013] FIG. 1 shows a wireless communication system including a
plurality of BSSs having APs and WTRUs which operate in accordance
with the present invention;
[0014] FIG. 2 is an exemplary block diagram of an AP of a local BSS
in the wireless communication system of FIG. 1;
[0015] FIG. 3 is a flow diagram of a process for setting up a
periodic PMP in accordance with the present invention;
[0016] FIG. 4 is a flow diagram of a process for setting up a PMP
and an SMP in accordance with the present invention; and
[0017] FIG. 5 is a timing diagram for the PMP and the SMP of FIG.
4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Hereafter, the terminology "WTRU" includes but is not
limited to a wireless station (STA), a user equipment (UE), a
mobile station, a fixed or mobile subscriber unit, a pager, or any
other type of device capable of operating in a wireless
environment. When referred to hereafter, the terminology "AP"
includes but is not limited to a Node-B, a base station, a site
controller or any other type of interfacing device in a wireless
environment.
[0019] The features of the present invention may be incorporated
into an integrated circuit (IC) or be configured in a circuit
comprising a multitude of interconnecting components.
[0020] The invention provides a means to monitor and estimate the
characteristics of traffic between all nodes within a WLAN or MESH
or similar wireless system without impacting or disrupting the
currently served traffic of the measuring node
[0021] FIG. 1 shows a wireless communication system 100 including a
local BSS 106a and at least one neighboring BSS 106b which are
connected by a distribution system 108. The present invention is
applicable to an infrastructure network, as shown in FIG. 1, or a
mesh network (not shown). The local BSS 106a includes an AP 102a
and one or more WTRUs 104a, 104b, 104c. The at least one
neighboring BSS 106b includes an AP 102b and one or more WTRUs
104d-104n. The BSSs 106a, 106b and the distribution system 108 are
comprised by an extended service set (ESS), (not shown).
[0022] Each AP 102 has a receiver filter used to block unnecessary
data and to pass only necessary data to a medium access control
(MAC) layer according to the requirements of the AP 102. In order
to limit the processing requirements, in normal operation, the AP
102 sets the receiver filter such that only the packets destined to
the AP 102 are forwarded to the MAC layer of the AP 102 and other
unnecessary packets are blocked by the receiver filter without
being further processed.
[0023] However, to determine the level of activity on the given
channel, it is necessary to receive and process packets from
neighboring BSSs 106b, (i.e., out-of-BSS packets). The out-of-BSS
packet information includes header information and a received
signal strength indication (RSSI) as a primary measurement
required, or the like.
[0024] The receiver filter of the AP 102a can be set such that all
of the received packets are forwarded to the MAC layer of the AP
102a. This mode of operation is called a promiscuous mode and
allows the AP 102a to process all of the out-of-BSS packets and
retrieve the necessary information from the out-of-BSS packets
transmitted from neighboring BSSs 106b.
[0025] When in a default operating mode whereby a receiver filter
in the AP 102a is enabled, the AP 102a processes only received data
packets which originated from its own BSS 106a. When in a
promiscuous mode whereby the receiver filter in the AP 102a is
disabled, the AP 102a processes all received data packets which
originated from both the BSS 106a and the at least one neighboring
BSS 106b.
[0026] The AP 102a sets the value of an MP. Between two consecutive
MPs is an interval, T.sub.MP. During the interval, T.sub.MP, the AP
102a is in the default operating mode whereby only the data packets
originating from its own BSS 106a, (i.e., in-BSS packets), are
processed. When the interval, T.sub.MP, expires, either a PMP or an
SMP is triggered during which the AP 102a is in the promiscuous
mode.
[0027] In a normal phase of the AP 102a, the MPs alternate between
the PMPs and the SMPs based on a periodicity constant, (P.sub.SMP).
Every P.sub.SMP-th MP corresponds to an SMP. For example, if the
P.sub.SMP is 2, then every other MP corresponds to an SMP.
Therefore, the first MP is a PMP, the second MP is an SMP, the
third MP is a PMP, the fourth MP is an SMP, and so on. During a
discovery phase of the AP 102a, the periodicity constant is set to
1. Therefore, every MP is provided as an SMP.
[0028] During the PMP, the AP 102a performs measurements and
collects packet information on out-of-BSS packets received on the
same channel as in-BSS packets. This allows the AP 102a to perform
measurements and collect packet information on out-of-BSS packets
received by the AP 102a on the same channel that in-BSS packets are
received.
[0029] The obtained measurements and information are averaged over
a longer period of time such that the information becomes
sufficient to understand the main characteristics of the channel.
When the PMP expires, the AP 102a returns to its default operating
mode; thereby reducing the processing load of the AP 102a.
[0030] The PMP is set to a short period of time. Even though the AP
102a is fully functional during the PMP, since the PMP will only
consume a small portion of the operating time, the performance of
the AP 102a remains substantially unaffected.
[0031] The SMP is an MP where the AP 102a halts traffic within it
own BSS 106a to gather information about other channels. Similar to
the PMP, the SMP is performed twice a second for a very short time
period (5 ms), during which the AP 102a stops transmitting on the
current channel, switches to at least one other channel contained
in the ACS, and listens to the at least one other channel while
processing all of the packets received on the other channel to
determine activity level of that channel. The AP 102a also performs
interference measurements on the other channel. Then the AP 102a
switches back to its normal channel and continues serving its WTRUs
104a, 104b, 104c.
[0032] In the PMP case, the AP 102a never switches channels.
Instead, the AP 102a disables its receiver filter such the AP 102a
can receive packets that are not destined for the AP 102a, (i.e.,
the AP 102a eavesdrops on the conversations of other APs and/or
WTRUs in at least one neighboring BSS 106b). During this time, the
AP 102a continues to serve the WTRUs 104a, 104b, 104c in the BSS
106a. Thus, there is no service interruption at all, unlike the SMP
where there is a service interruption during each time period it is
performed since the AP 102a stops transmitting when it switches
channels.
[0033] Since SMPs may cycle through several channels (e.g.,
channels 1, 6 and 11), relying on SMPs only may be insufficient to
determine information related to neighboring nodes that are
operating on the current channel. Some algorithms require a more
accurate determination of the load of neighboring nodes that are
operating on the same channel.
[0034] The operation of SMP is disclosed in detail in copending
U.S. patent application Ser. No. 11/009,821 entitled "wireless
communication method and apparatus for managing radio resources
using silent measurement periods" by Paul Marinier et al., which is
incorporated by reference as if fully set forth herein.
[0035] FIG. 2 is an exemplary block diagram of the AP 102a of the
local BSS 106a of the wireless communication system of FIG. 1. The
AP 102a includes a transceiver 202 with a receiver filter 204, a
MAC entity 206, a processor 208, a packet information database 210
and a timer 112. The transceiver 202 may be configured as a
physical layer entity for receiving and transmitting packets. The
received packets are forwarded to the MAC entity 206 for MAC layer
processing. The processor 208 controls the transceiver 202 such
that either the AP 102a enters a default operating mode whereby the
receiver filter 204 is enabled, (i.e., turned on or inserted into a
path through which packets received by the AP 102a are routed),
such that only the packets originating from the BSS 106a of the AP
102a are forwarded to the MAC entity 206, or the AP 102a enters a
promiscuous mode whereby the receiver filter is disabled, (i.e.,
turned off or bypassed), such that all of the packets received on
the channel from the BSS 106a and the at least one neighboring BSS
106b are forwarded to the MAC entity 206.
[0036] The processor 208 selects between the default operating mode
and the promiscuous mode in accordance with the timer 212. The
timer 212 includes a PMP timer 214, an MP interval timer 216 and an
SMP timer 218. The processor 208 computes and sets up an interval,
T.sub.MP, for the MP interval timer 216 based on a fixed component,
(T.sub.MP.sub.--.sub.fix), and a variable component,
(T.sub.MP.sub.--.sub.var).
[0037] For example, the fixed component may be set to 250 ms during
normal operation and 1,000 ms during a discovery phase, and the
variable component may be set to .+-.20 ms during normal operation
and 0 ms during discovery phase.
[0038] The fixed component is used as the main periodic timing
mechanism for triggering a PMP or SMP. Thus, the fixed component
determines the periodicity at which MPs occur.
[0039] The variable component is used to make sure that all of the
APs 102a, 102b are not synchronized when performing the SMPs or
PMPs, as it is not desired to measure activity on a given channel
at the same time that another AP 102 under study is also performing
an SMP on another channel, (i.e., an empty channel is being
listened to where there is normally traffic on that channel). By
introducing this variability, AP synchronization is avoided.
[0040] When the MP interval timer 216 expires, the processor 208
sets the value of the PMP timer 214 to a predetermined time
duration and sends a control signal to the transceiver 202 via a
path 207 to disable the receiver filter 204, (i.e., place the AP
102a in the promiscuous mode), such that all of the packets
received on a given channel which originated from both the BSS 106a
of the AP 102a and the at least one neighboring BSS 106b are
forwarded to the MAC entity 206.
[0041] When the PMP timer 214 expires, the processor 208 sends a
control signal to the transceiver 202 via the path 207 to enable
the receiver filter 204, (i.e., place the AP 102a in the default
operating mode), such that only the packets received on the given
channel which originated from the BSS 106a are forwarded to the MAC
entity 206. The processor 208 then determines whether the next MP
should be a PMP or an SMP based on a periodicity constant, as
discussed in detail above. Thus, the processor 206 alternates
between an SMP and PMP every fixed component period.
[0042] If the processor 208 determines that the next MP should be a
PMP, the processor 208 sets the value of the PMP timer 214 to a
predetermined time duration and sends a control signal to the
transceiver 202 via the path 207 to disable the receiver filter
204, (i.e., place the AP 102a in the promiscuous mode), such that
all of the packets received on a given channel which originated
from both the BSS 106a of the AP 102a and the at least one
neighboring BSS 106b are forwarded to the MAC entity 206. When the
PMP timer 214 expires, the processor 208 sends a control signal to
the transceiver 202 via the path 207 to enable the receiver filter
204, (i.e., place the AP 102a in the default operating mode), such
that only the packets received on the given channel which
originated from the BSS 106a are forwarded to the MAC entity
206.
[0043] If the processor 208 determines that the next MP should be
an SMP, the processor 208 sets a value of the SMP timer 218 to a
predetermined time period, changes to the next appropriate channel
contained in the ACS, and sends a control signal to the transceiver
202 via the path 207 to disable the receiver filter 204, (i.e.,
place the AP 102a in the promiscuous mode), such that all of the
packets received on at least one other channel that the AP 102a
switches to which originated from at least one neighboring BSS 106b
are forwarded to the MAC entity 206. When the SMP timer 218
expires, the channel is changed to the previous operational channel
if required, the processor 208 sends a control signal to the
transceiver 202 via the path 207 to enable the receiver filter 204,
(i.e., place the AP 102a in the default operating mode), such that
only the packets received on the given channel which originated
from the BSS 106a are forwarded to the MAC entity 206.
[0044] FIG. 3 is a flow diagram of a process 300 including method
steps for setting up a PMP for the AP 102a in accordance with the
present invention. In step 302, a PMP setup process for the AP 102a
is triggered. In step 304, the AP 102a sets the value of the PMP
timer 214 to a predetermined time duration,
T.sub.PMP.sub.--.sub.dur. The predetermined time duration,
T.sub.PMP.sub.--.sub.dur, may be set to any value, (e.g., 10 ms).
However, the predetermined time duration, T.sub.PMP.sub.--.sub.dur,
should not be too long to substantially disrupt the normal
operation of the AP 102a, but should be long enough to allow the
reception of a full packet from its neighboring BSS 106b.
[0045] As shown in step 306 of FIG. 3, the AP 102a disables a
receiver filter in the AP 102a, (i.e., places the AP 102a in a
promiscuous mode), such that all packets, (in-BSS and out-of-BSS
packets), that are received by the AP 102a on the same channel are
processed. In step 308, the AP 102a performs measurements and
collects packet information on the received out-of-BSS packets and
stores the packet information in the packet information database
210 for post-processing.
[0046] The packet information may include the RSSI of packets, the
duration of packets and various MAC header fields, such as source
address, (MAC address of transmitting node), as well as destination
address and BSSID, which specify the BSS to which the AP 102a
belongs. The information of packets received during a PMP is stored
in the packet information database 210 such that the processor 208
can average measurements collected over a substantial period of
time. This average is then used to understand the main
characteristics of the channel for post-processing.
[0047] For example, the RSSI measurements of received packets,
observed over a certain period of time, may be used to determine
the proximity of neighboring WLAN nodes, (e.g., determine the radio
frequency (RF) path loss to neighboring APs and neighboring
WTRUs).
[0048] In another example, accumulating the number of packets from
individual nodes while considering packet durations may be used to
estimate the medium usage from individual nodes and/or from
neighboring BSSs that are operating on the same channel.
[0049] The processor 208 in the AP 102a continuously monitors the
PMP timer 214 while performing the measurements to determine when
it expires (step 310). When the PMP timer 214 is determined to have
expired in step 310, the AP 102a enables the receiver filter 204,
(i.e., places the AP 102a in a default operating mode), such that
only in-BSS packets are processed.
[0050] Each MP corresponds to either a PMP or an SMP. When in the
default operating mode, the AP 102a periodically enters either a
PMP or an SMP and receives and processes out-of-BSS packets
destined to other APs. During the PMP, the AP 102a listens to only
its own channel, but during the SMP, the AP 102a not only listens
to its own channel but the AP 102a also listens to all of the
available channels in an ACS.
[0051] FIG. 4 is a flow diagram of a process 400 including method
steps for setting up a PMP and an SMP in accordance with the
present invention. Upon initiation of the process, the AP 102a
computes and sets the value T.sub.MP of an MP interval timer (step
402). The AP continuously monitors the MP interval timer 216 to
determine when it expires (step 404). When the MP interval timer
216 expires, the AP 102a determines whether an SMP or a PMP should
be triggered in the next MP (step 406).
[0052] If the next MP is determined in step 406 to be the
P.sub.SMP-th MP, (i.e., a multiple of P.sub.SMP), an SMP is
triggered in the next MP (step 408). When entering the SMP, the SMP
timer 218 is set, the channel is changed to next appropriate
channel contained in the ACS, and the AP 102a enters the
promiscuous mode such that it processes all of the packets received
from at least one neighboring BSSs 106b. When the SMP timer 318
expires, the channel is changed back to the previous operational
channel, and the process 400 returns to step 302 to set the MP
interval timer 216 and the AP 102a enters the default operating
mode.
[0053] If the next MP is not the P.sub.SMP-th MP, a PMP is
triggered in the next MP (step 410) and the process 300 of FIG. 3
is initiated. Upon expiration of the PMP, the process 400 returns
to step 402 to set the MP interval timer 216 and the AP 102a
returns to its default operating mode.
[0054] FIG. 5 shows a timing relationship between the PMP and the
SMP in the case of P.sub.SMP=2.
[0055] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention.
* * * * *