Reducing MAC Layer Handoff Latency in IEEE

Reducing MAC Layer Handoff Latency in IEEE 802.11
Wireless LANs
Sangho Shin
Andrea G. Forte
Columbia University
Anshuman Singh Rawat
Henning Schulzrinne
asr245@nyu.edu
hgs@cs.columbia.edu
New York University
Columbia University
{ss2020,andreaf}@cs.columbia.edu
ABSTRACT
With the growth of IEEE 802.11-based wireless LANs, VoIP
and similar applications are now commonly used over wireless networks. Mobile station performs a handoff whenever
it moves out of the range of one access point (AP) and tries
to connect to a different one. This takes a few hundred
milliseconds, causing interruptions in VoIP sessions. We developed a new handoff procedure which reduces the MAC
layer handoff latency, in most cases, to a level where VoIP
communication becomes seamless. This new handoff procedure reduces the discovery phase using a selective scanning
algorithm and a caching mechanism.
Categories and Subject Descriptors
C.2.1 [Computer System Organization]: Computer- Communication Networks
General Terms
Measurement, Performance, Experimentation
Keywords
IEEE 802.11, Fast Handoff, Selective scanning
1.
INTRODUCTION
IEEE 802.11-based wireless LANs have seen a very fast
growth in the last few years and Voice over IP (VoIP) is
one of the most promising services to be used in mobile devices over wireless networks. One of the main problems in
VoIP communication is the handoff latency introduced when
moving from one Access Point (AP) to another. As we will
show below, the amount of time needed for the handoff in
the 802.11 environment is too large for seamless VoIP communications. We were able to reduce the handoff latency
using a modified handoff procedure, with modifications being limited to mobile devices and compatible with standard
802.11 behaviour.
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MobiWac’04, October 1, 2004, Philadelphia, Pennsylvania, USA.
Copyright 2004 ACM 1-58113-920-0/04/0010 ...$5.00.
Figure 1: Channel frequency distribution in IEEE
802.11b
Below, first we discuss in brief work done in this particular
area, followed by a brief description of how we tackled the
problem. Then, in section 3, we describe the IEEE 802.11
architecture focusing on the management frames and the
handoff process; then we will shortly describe the HostAP
driver and how we modified it in order to implement our
new algorithm. In section 5, we illustrate how we were able
to reduce the total handoff latency to an average value of
129 ms by using a selective scanning procedure and to an
average of 3 ms by using a caching mechanism. In section 7
and 8, we show the environment of the experiments and the
measurement results.
2. RELATED WORK
A lot of work has been done to reduce the handoff latency when roaming between different subnets and many
new schemes for mobile IP and route optimization have been
proposed.
In this paper, we focused on reducing handoff latency at
MAC layer. As we will describe in Section 4, MAC layer
handoff latency can be divided into three components: probe
delay, authentication delay and association delay.
Arunesh et. al. in [3] focused on reducing the reassociation delay. The reassociation delay is reduced by using a
caching mechanism on the AP side. This caching mechanism
is based on the IAPP protocol in order to exchange the client
context information between neighboring APs. The cache in
the AP is built using the information contained in an IAPP
Move-Notify message or in the reassociation request sent to
standard is backwards-compatible with the 802.11b standard while the 802.11a standard, because of the different
ISM band, is not compatible with the other two.
We will focus our attention on the IEEE 802.11b standard
even though most of the concepts and notions described here
are still valid for 802.11a and 802.11g. As we said earlier, the
802.11b operates in the 2.4 GHz ISM band. Its 14 channels
are distributed over the range from 2.402 GHz to 2.483 GHz
(see figure 1), each channel being 22 MHz wide. In US, only
the first 11 channels are used. Of these 11 channels, only
channels 1, 6 and 11 do not overlap. So, in a well configured
wireless network, all or most of the APs will operate on
channel 1, 6 and 11. Also, to avoid co-channel interference,
two adjacent APs should never be on the same channel.
Figure 2: IEEE 802.11 architecture
the AP by the client. By exchanging the client context information with the old AP, the new AP does not require the
client to send its context information in order to reassociate,
hence reducing the reassociation delay.
Sangheun et. al. in [11] and Park et. al. in [12] focused
on the IEEE 802.1x authentication process. This process is
perfomed after the STA has already associated to a new AP.
The IEEE 802.1x authentication delay is reduced by using
the Frequent Handoff Region (FHR) selection algorithm.
In [2], it is shown how the discovery phase (scanning time)
is the most time consuming part of the handoff process, taking over 90% of the total handoff delay, while (re)association
time contributes only a few milliseconds.
Our work follows a novel approach and reduces the total handoff latency by reducing the scanning time. This
is achieved by using a selective scanning algorithm and a
caching mechanism. This caching data structure is maintained at the client side and no changes are required in the
existing network infrastructure or the IEEE 802.11 standard
unlike in [3]. All the required changes are done on the client
side wireless card driver.
In [5], they also propose a selective scanning algorithm.
However, their proposition relies on the use of neighbor
graphs. This approach requires changes in the network infrastructure and use of IAPP. The scanning delay is defined
as “the duration taken from the first Probe Request message
to the last Probe Response message”. This definition does
not take into consideration the time needed by the client
to process the received probe responses. This processing
time represents a significant part of the scanning delay and
increases significantly with the number of probe responses
received. In our work and in [2], the time required for processing the probe responses received by the client is taken
into consideration.
3.
IEEE 802.11 STANDARDS
There are currently three IEEE 802.11 standards: 802.11a,
802.11b and 802.11g. The 802.11a standard operates in the
5 GHz ISM band, and it uses a total of 32 channels of which
only 8 do not overlap. The 802.11b and 802.11g standards
both operate in the 2.4 GHz ISM band and use 11 of the
14 possible channels. Of these 11 channels, only three do
not overlap. While 802.11b can operate up to a maximum
rate of 11 Mbit/sec, the 802.11g and 802.11a standards can
operate up to a maximum rate of 54 Mbit/sec. The 802.11g
3.1 The IEEE 802.11 Wireless LAN
architecture
The 802.11 architecture is comprised of several components and services that interact to provide station mobility
to the higher layers of the network stack. We outline the
following components as described in [8].
Wireless LAN station: The station (STA) is the most
basic component of the wireless network. A station is any
device that contains the functionality of the 802.11 protocol:
medium access control (MAC), physical layer (PHY), and
a connection to the wireless media. Typically, the 802.11
functions are implemented in the hardware and software of
a network interface card (NIC). A station could be a laptop
PC, handheld device, or an Access Point (AP). All stations
support the 802.11 station services of authentication, deauthentication, privacy, and data delivery.
Basic Service Set (BSS): Basic Service Set (BSS) is
the basic building block of an 802.11 wireless LAN. The
BSS consists of a group of any number of stations.
Service Set Identifier (SSID): A service set identifier
(SSID) is a unique label that distinguishes one WLAN from
another. So all APs and STAs attempting to become a part
of a specific WLAN must use the same SSID. Wireless STAs
use the SSID to establish and maintain connectivity with
APs.
3.2 IEEE 802.11 Management Frames
The IEEE 802.11 management frames enable stations to
establish and maintain communications. The following are
common IEEE 802.11 management frame subtypes, with the
description quoted from [6]:
“Authentication frame: The 802.11 authentication is a
process whereby the access point either accepts or rejects the
identity of a STA. The STA begins the process by sending
an authentication frame containing its identity to the access
point. With open system authentication (the default), the
STA sends only one authentication frame, and the access
point responds with an authentication frame as a response
indicating acceptance (or rejection).
Association request frame: 802.11 association enables the
access point to allocate resources for and synchronize with
a STA. A STA begins the association process by sending an
association request to an access point. This frame carries
information about the STA (e.g., supported data rates) and
the SSID of the network it wishes to associate with. After
receiving the association request, the access point considers
associating with the STA, and (if accepted) reserves memory
space and establishes an association ID for the STA.
Association response frame: An access point sends an association response frame containing an acceptance or rejection notice to the STA requesting association. If the access point accepts the STA, the frame includes information
regarding the association, such as association ID and supported data rates. If the outcome of the association is positive, the STA can utilize the access point to communicate
with other STAs on the network and systems on the distribution (i.e. Ethernet) side of the access point.
Reassociation request frame: If a STA roams away from
the currently associated access point and finds another access point having a stronger beacon signal, the STA will send
a reassociation frame to the new access point. The new access point then coordinates the forwarding of data frames
that may still be in the buffer of the previous access point
waiting for transmission to the STA.
Reassociation response frame: An access point sends a reassociation response frame containing an acceptance or rejection notice to the STA requesting reassociation. Similar
to the association process, the frame includes information
regarding the association, such as association ID and supported data rates.
Disassociation frame: A station sends a disassociation
frame to another station if it wishes to terminate the association. For example, a STA that is shut down gracefully
can send a disassociation frame to alert the access point
that the STA is powering off. The access point can then relinquish memory allocations and remove the STA from the
association table.
Beacon frame: The access point periodically sends a beacon frame to announce its presence and relay information,
such as timestamp, SSID and other parameters regarding
the access point, to STAs that are within range.
Probe request frame: A station sends a probe request
frame when it needs to obtain information from another
station. For example, a STA would send a probe request
to determine which access points are within range.
Probe response frame: A station will respond with a probe
response frame, containing capability information, supported
data rates, etc., after it receives a probe request frame.”
4.
HANDOFF PROCEDURE WITH ACTIVE
SCANNING
Handoff is a procedure executed when a mobile node moves
from the coverage area of one AP to the coverage area of another AP. The handoff process involves a sequence of messages being exchanged between the mobile node and the
participating APs. This sequence of messages can be divided into three types: probe, authentication and association, which will be described later in detail. The transfer
from the old AP to the new AP results in state information
being transferred from the former to the latter, consisting of
authentication, authorization and accounting information.
This can be achieved by an Inter Access Point Protocol
(IAPP) that is currently under draft in IEEE 802.11f, or
by a proprietary protocol, specific to a vendor.
4.1 Steps during Handoff
The handoff process can be divided into two logical steps:
discovery and reauthentication [2].
Discovery: The discovery process involves the handoff
initiation phase and the scanning phase. When the STA
STA
Probe request (broadcast)
Probe responses
Probe delay
.
.
.
.
.
.
Probe request (broadcast)
Probe responses
Authentication request
Authentication delay
A
V
A
I
L
A
B
L
E
A
P
s
New AP
Authentication response
Reassociation request
Association delay
Reassociation response
Figure 3: Handoff process using active scanning [2]
is moving away from the AP it is currently associated with,
the signal strength and the signal-to-noise ratio of the signal
from the AP decrease. This causes the STA to initiate a
handoff. Now, the STA needs to find other APs that it
can connect to. This is done by the MAC layer scanning
function.
Scanning can be accomplished either in passive or active
mode. In passive scan mode, the STA listens to the wireless medium for beacon frames. Beacon frames provide a
combination of timing and advertising information to the
STAs. Using the information obtained from beacon frames
the STA can elect to join an AP. During this scanning mode
the STA listens to each channel of the physical medium to
try and locate an AP.
Active scanning involves transmission of probe request
frames by the STA in the wireless medium and processing of
the received probe responses from the APs. The basic procedure of the active scan mode includes the following steps
as explained in [7]:
1. Using the normal channel access procedure, Carrier
Sense Multiple Access with Collision Avoidance (CSMA/CA),
gain control of wireless medium.
2. Transmit a probe request frame which contains the
broadcast address as destination.
3. Start a probe timer.
4. Listen for probe responses.
5. If no response has been received by minChannelTime,
scan next channel.
6. If one or more responses are received by minChannelTime, stop accepting probe responses at maxChannelTime
and process all received responses.1
7. Move to next channel and repeat the above steps.
After all channels have been scanned, all information re1
minChannelTime and maxChannelTime values are device
dependent.
Selective Scanning
Do active scanning with
the channel mask
Detected any AP?
No
Flip the channel mask
Yes
Do active scanning with
the new channel mask
Figure 4: Handoff time in IEEE 802.11b
No
Detected any AP?
ceived from probe responses are processed so that the STA
can select which AP to join next.
Reauthentication: The reauthentication process involves
authentication and reassociation to the new AP as well as
transfer of the STA’s credentials from the old AP to the new
AP. Authentication is a process by which the AP either accepts or rejects the identity of the STA. The STA begins the
process by sending the authentication frame, authentication
request, informing the AP of its identity; the AP responds
with an authentication response, indicating acceptance or
rejection. After successful authentication, the STA sends a
reassociation request to the new AP which will then send a
reassociation response, back to the STA, containing an acceptance or rejection notice. Figure 3, taken from [2], shows
the sequence of messages expected during the handoff. As
seen in Figure 3, the sequence of messages can be divided
into three types:
1. Probe messages: Once the STA decides to look for
other APs, the probing process starts. The STA starts sending out probe requests and then processes received probe
responses, based on the active scanning algorithm explained
above. The time involved in this probing process is called
probe delay.
2. Authentication messages: Once the STA decides to
join an AP, authentication messages are exchanged between
the STA and the selected AP. The time consumed by this
process is called authentication delay.
3. Reassociation messages: After a successful authentication, the STA sends a reassociation request and expects
a reassociation response back from the AP. These messages
are responsible for the reassociation delay.
5.
FAST HANDOFF ALGORITHM
As described in [2] and confirmed by our experiments,
the probe delay constitutes the biggest part (over 90%) of
the handoff latency (Figure 4); for this reason, we focus on
minimizing this delay.
In order to reduce the probe delay, we have focused our
attention on two different aspects of the problem. First, we
had to reduce the probe delay by improving the scanning
procedure, using a selective scanning algorithm; second, we
had to minimize the number of times the previous scanning
procedure was needed. This second point was achieved with
Do normal active
scanning(full
scanning)
Yes
Compute new
channel mask
Reassociate to
the best AP
Complete
Figure 5: Selective scanning procedure
the use of a caching mechanism.
We will now describe the two algorithms.
5.1 Selective Scanning
As we described in Section 3, among the 14 possible channels that can be used according to the IEEE 802.11b standard, only 11 are used in USA and, among these 11 channels,
only three do not overlap. These channels are 1, 6 and 11.
The selective scanning algorithm is based on this idea. In
the selective scanning, when a STA scans APs, a channel
mask is built. In the next handoff, during the scanning process, this channel mask will be used. In doing so, only a
well-selected subset of channels will be scanned, reducing
the probe delay. In the following, we describe in detail the
selective scanning algorithm.
1. When the driver is first loaded, it performs a full scan
(i.e., sends out a Probe Request on all the channels, and
listens to responses from APs).
2. A channel mask is set by turning on the bits for all the
channels in which a Probe Response was heard as a result
of step 1. In addition, bits for channel 1, 6 and 11 are also
set, as these channels are more likely to be used by APs.
3. Select the best AP, i.e. the one with the strongest
signal strength from among the scanned APs, and connect
to that AP.
4. The channel the STA connects to is removed from
the channel mask by re-setting the corresponding bit, as
the likelihood of an adjacent AP on the same channel of the
current AP is very small. So, the final formula for computing
Handoff process with cache
Check cache
Has a cache entry
for the current AP?
Yes
Try to associate to
the first AP in cache
Yes
Succeeded?
Complete
No
Try to associate to
the second AP in
cache
No
Yes
Succeeded?
No
Compute new channel mask
Do Selective scanning
Figure 6: Caching procedure
1
2
.̇.
10
Table 1: Cache structure
Key
Best AP
Second Best AP
MAC1 (Ch1) MAC2 (Ch2)
MAC3 (Ch3)
Our cache has a size of ten, meaning that it could store
up to ten keys and a width of two, meaning that for each
key, it can store up to two adjacent APs in the list. Below
we describe how the caching algorithm works:
1. When a STA associates to an AP, the AP is entered
in the cache as a key. At this point, the list of AP entries,
corresponding to this key, is empty.
2. When a handoff is needed, we first check the entries in
cache corresponding to the current key.
3. If no entry is found (cache miss), the STA performes
a scan using the selective scanning algorithm described in
Section 5.1. The best two results based on signal strength
are then entered in the cache with the old AP as the key.
4. If an entry is found (cache hit), we issue a command to
associate to this new AP. On success, the handoff procedure
is complete.
5. When the STA fails to connect to the first entry in
cache, the second entry is tried and if association with the
second entry fails as well, our selective scanning algorithm
is used.
From the above algorithm, we can see that scanning is
required only if a cache miss occurs; every time we have a
cache hit, no scanning is required.
Usually, using cache, it takes less than 5 ms to associate
to the new AP. But, when the STA fails to associate to the
new AP, the firmware waits for a long time, up to 15 ms2 . To
reduce this time-to-failure we are using a timer. The timer
expires after 6 ms, and the STA will then try to associate
to the next entry in cache. In selective scanning, when the
timer expires the STA performs a new selective scanning
using the new channel mask.
A cache miss does not significantly affect the handoff latency. As mentioned above, when a cache miss occurs, the
time-to-failure is only 6 ms. For example, if the first cache
entry misses and the second one hits, the additional handoff
delay is only 6 ms. When both cache entries miss, the total
handoff delay is 12 ms plus selective scanning time, all of
this still resulting in a significant improvement compared to
the original handoff time.
6. IMPLEMENTATION
the new channel mask is ’scanned channels (from step 2) +
1 + 6 + 11 - the current channel’.
5. If no APs are discovered with the current channel mask,
the channel mask is inverted and a new scan is done. If
still no APs are discovered, a full scan, on all channels, is
performed.
5.2 Caching
The selective scanning procedure described above reduced
the handoff latency in our experiments (Section 7) to a value
between 30% to 60% of the values obtained with the original
handoff (see Figure 9), bringing the average values for the
total handoff latency to 130 ms against an original handoff
latency of 343 ms (see Table 2). For seamless VoIP, it is
recommended that overall latency does not exceed 50 ms
[1]. This further improvement was achieved by using an
AP cache. The AP cache consists of a table which uses the
MAC address of the current AP as the key. Corresponding
to each key entry in the cache is a list of MAC addresses of
APs adjacent to current one which were discovered during
scanning. This list is automatically created while roaming.
To implement our new algorithm, we had to modify the
handoff procedure. Usually the handoff procedure is handled
by the firmware; using the HostAP driver [10], we were able
to emulate the whole handoff process in the driver.
6.1 The HostAP Driver
The HostAP driver is a Linux driver for wireless LAN
cards based on Intersil’s Prism2/2.5/3 802.11 chipset [10].
Wireless cards using these chipsets include the Linksys WPC11
PCMCIA card, the Linksys WMP11 PCI card, the ZoomAir
4105 PCMCIA card and the D-Link DWL-650 PCMCIA
card.
The driver supports a so-called Host AP mode, i.e., it
takes care of IEEE 802.11 management functions in the host
computer and acts as an access point. This does not require
any special firmware for the wireless LAN card. In addition
to this, it has support for normal station operations in BSS
and possible also in IBSS.3
2
Actual values measured using Prism2/2.5/3 chipset cards.
These values may vary from chipset to chipset.
3
IBSS, also known as ad-hoc network, comprises of a set of
The HostAP driver supports a command for scanning
APs, can handle the scanning results and supports a command for joining to a specific AP. It is also possible to disable the firmware handoff by switching to manual mode.
By enabling this mode, we were able to control the card
functionalities at the driver level and use our fast handoff
procedure.
MEASUREMENTS
This chapter describes the hardware and software used
for measuring the handoff latency and the environment in
which the measurements were taken.
7.1 Experimental Setup
For the measurements, we used three laptops and one
desktop. The laptops were a 1.2 GHz Intel Celeron with
256 MB of RAM running Red Hat Linux 8.0, a P-III with
256 MB of RAM running Red Hat 7.3, and another P-III
with 256 MB RAM running Red Hat Linux 8.0. Linksys
WPC11 version 3.0 PCMCIA wireless NICs were used in all
three laptops. The desktop was an AMD Athlon XP 1700+
with 512 MB RAM running WinXP. The 0.0.4 version of the
HostAP driver was used for all three wireless cards, with one
of them modified to load our improved driver, and the other
two cards were used for sniffing. Kismet 3.0.1 [9] was used
for capturing the 802.11 management frames, and Ethereal
0.9.16 [4] was used to view the dump generated by Kismet
and analyze the result.
Original Handofff
Selective Scanning
Cache
500
400
msec
7.
Link Layer Handoff Time in IEEE 802.11b
600
300
200
100
0
1
2
3
4
5
6
Experiments
7
8
9
Figure 7: Link layer handoff time in 802.11b
7.2 The Environment
The experiments were conducted in the 802.11b wireless
environment in the CEPSR building of Columbia University,
on the 7th and the 8th floor. With only two laptops running
the sniffer (Kismet), many initial runs were first conducted
to get to know the wireless environment, specifically the
channel numbers the APs were operating on, the handoff
starting points and the next AP and channel the STA would
connect to.
The environment in which the packet loss measurements
were taken differed from the one above. The measurements
were still taken in the CEPSR building of Columbia University, on the 7th and 8th floor, but some rogue APs were removed. This change in the environment caused a reduction
of the original handoff time and consequentially a drastic
reduction of the packet loss. This will be shown in section
8.2.
7.3 Experiments
After gathering sufficient information about the environment, we started taking the actual handoff measurements.
One sniffer was set to always sniff on channel 1 (as the first
Probe Request is always sent out on channel 1 in normal active scanning), and the other sniffer on the next channel the
STA was expected to associate to. For the measurement, the
system clock of the three laptops was synchronized using the
Network Time Protocol (NTP). Also, to avoid multi-path
delays, the wireless cards were kept as close as physically
possible during the measurements.
For measuring the packet loss, in addition to the three
laptops, the desktop was used as a sender and receiver. A
stations which can communicate directly with each other,
via the wireless medium, in a peer-to-peer fashion.
Figure 8: Packet loss and packet delay
UDP packet generator was used to send and receive data
packets. Each UDP packet contained in the data field the
packet sequence number to improve accuracy by linking the
packet sequence number to its timestamp on all three laptops.
8. MEASUREMENT RESULTS
In the following sections we present the results for the
total handoff time and packet loss.
8.1 Handoff Time
Figure 7 and Table 2 present the results we obtained.
As can be seen, with selective scanning the handoff latency improved considerably, with an average reduction of
40%. But even this reduced time is not good enough for
seamless VoIP. However, using the cache, the handoff latency time drops to a few ms, making it possible to have
seamless VoIP. This huge reduction was possible because
scanning, which took more than 90% of the total handoff
time, was eliminated by using the cache.
8.2 Packet Loss
Table 3 present the results we obtained when the STA
performing the handoff was the receiver. Table 4 present the
results we obtained when the STA performing the handoff
was the sender.
For measuring packet loss, UDP packets were transmitted to and from the STA, to simulate voice traffic during
10
Table 2: Handoff delay (ms) of 802.11b in link layer
Experiment
1
2
3
4
5
6
7
8
9
Original handoff
457 236 434 317 566 321 241 364 216
Selective scanning 140 101 141 141 141 139 143
94 142
Caching
2
2
4
3
4
2
2
2
2
Table 3: Packet loss during handoff in
Experiment
1
2
Original Handoff
36 55
Selective Scanning 88 24
Cache
16 15
mobile
3
4
32 79
26 19
14 14
Bridging Delay
150
145
140
135
ms
130
125
120
115
110
105
100
1
2
3
4
5
6
Experiments
7
8
Figure 9: Bridging delay
9
10
receiver (number of
5
6
7
8
37 122 134 32
31
28
46 26
16
15
23 21
10
274
101
2
avg
343
129
3
packets; 20 ms interval)
9 10 avg
69 36
63
64 18
37
15 14
16
the handoff. Transmitting data packets adds to the handoff
delay. This delay is caused by the fact that data packets
are transmitted during the handoff process, in particular
between the last probe response and the authentication request. This behaviour is only noticed when the STA performing the handoff is the sender (see Table 5). When the
STA performing the handoff is the receiver, no such delay is
introduced. However, a new delay is introduced. This new
delay, bridging delay [2], is caused by the time needed for updating the MAC addresses to the ethernet switches forming
the distribution system. In particular, when handoff happens and the STA associates to the new AP, the switch continues to send the packets, addressed to the STA, through
the old AP which after many retries, discards them. This behaviour persists for about 140 ms4 (see Figure 9) after which
the MAC addresses have been updated and the switch starts
forwarding the data packets, addressed to the STA, through
the new AP. This results in an additional packet loss.
As can be seen in Figure 8, when the receiver is performing the handoff, the packet loss drops to about 60% and 40%
of the value obtained with the original handoff using selective scanning and cache, respectively. When using cache,
the effect of the bridging delay is particularly prominent.
Table 3 shows how, even though the handoff time is only a
few milliseconds, when using a cache, the packet loss is still
considerable.
Figure 8 also shows the average packet delay introduced
by the handoff procedure when the sender is performing the
handoff. For VoIP sessions, packets exceeding a delay of
100 ms can be considered lost. Table 4 shows the packet delay when using the original handoff scheme, selective scanning and cache. Even though selective scanning reduces
such a delay by about 25%, in order to achieve seamless
VoIP communication, the caching mechanism is necessary.
As a note, the behaviour of the selective scanning algorithm is not dependent on the environment, while the original handoff performance is very much affected by it. Table
2 and Figure 7 show an environment in which rogue APs are
present. Table 5 refers to a clean environment, without the
presence of rogue APs. As we can see, the selective scanning behaviour is very consistent, while the original handoff
performance deteriorates with the environment.
9. CONCLUSIONS AND FUTURE WORK
Figure 10: Handoff time in IEEE 802.11a
In this paper we described the handoff procedure and
demonstrated how the handoff latency can be substantially
4
Actual values may vary according to the environment.
Table 4: Delay during handoff in mobile sender (ms)
Experiment
1
2
3
4
5
6
7
8
9
Original Handoff
281 229 230 210 209 227 185 174 189
Selective Scanning 185 132 147 131 204 182 164 133 151
Cache
0
0
0
0
0
0
0
0
0
Original Handoff
Selective Scanning
Cache
Table 5: Summary of result
Handoff time in
Packet loss in
Handoff time in
mobile receiver (ms)
mobile receiver mobile sender (ms)
(num of packets)
182.5
63.2
201.5
102.1
37.0
141.1
4.5
16.3
3.9
reduced with the use of a caching mechanism combined to
a selective scanning algorithm. We demonstrated how, in
the best case, we were able to reduce the handoff latency to
an average value of 129 ms by only using the selective scanning algorithm and to an average value of 3.0 ms by using
the caching mechanism (Table 2). This reduction in handoff
latency also considerably decreased packet loss and packet
delay (Table 5).
By using a dynamic channel mask (refer to selective scanning algorithm 4 and 5 in section 5.1), scanning a subset of
channels can be used as a generic solution.
Another important result of our work is that by using selective scanning and caching, the probing process, the most
power consuming phase in active scanning, is reduced to the
minimum. This makes it possible to use the active scanning
procedure also in those devices such as PDAs where power
consumption is a critical issue.
With the new IEEE 802.11g standard out, we will be testing our algorithm to the new standard. The extension of our
algorithm to the new 802.11g standard will only require minor adjustments, if any.
Figure 10 shows the original handoff time in IEEE 802.11a
networks. As can be seen, the discovery phase is still the
most time consuming phase of the handoff process. Future
testing will be done in an IEEE 802.11a environment. The
extension of our algorithm to this standard will also require
minor adjustments such as modification of the channel mask
(selective scanning), improved cache dimensioning and management.
The channel with the best signal is not necessarily the
best channel to connect to because it could be much more
congested than a channel with a lower signal strength. Because of this, a heuristic which considers bit rate information
together with signal strength can achieve optimal performance.
A procedure in which an AP somehow knew its neighboring APs [3] and could provide that information to the
STA could be used to fill the cache. This could also be
combined with a positioning algorithm such as GPS or any
other WiFi positioning algorithm allowing a real-time filling
and refreshing of the cache, according to the STA actual
position, always resulting in a cache hit.
Very critical issues are the cache size and cache management policy. A good cache policy, together with the use of
AP neighboring and other heuristics, can achieve seamless
VoIP sessions.
10
168
184
0
avg
210
161
0
Packet delay in
mobile sender
(num of packets)
210.7
161.7
0
10. ACKNOWLEDGMENTS
This work was supported by grant of SIPquest Inc. Equipment was partially funded through grant CISE 02-02063 of
the National Science Foundation.
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