Subject: [NEXTEL-1] W-LAN performance & testing issues & challenges

From: Marcel <[email-masked]>
Date: Sun, 15 Dec 2002 13:46:26 -0500

WLAN
Performance Analysis
by Richard O'Dea
Product Manager, Spirent Communications

The growth of wireless networks is being driven by the need to
lower costs associated with network infrastructures and to support
the mobile networking applications that offer gains in process
efficiency, accuracy, and lower business costs. Worldwide wireless
LAN (WLAN) hardware revenue exceeded $390 million in the second
quarter of 2002 with WLANs showing strong penetration into
enterprise and consumer sectors. But the really big news could be
in the service provider sector. As the popularity of public access
Internet hot spots grows, the debate as to whether WLAN competes
with or compliments 3G deployment is still being hotly contested.

This being said, the advent of WLAN technology brings new
challenges to the forefront for semiconductor chip manufacturers,
network equipment manufacturers (NEM), service providers and
network administrators. The risk of failed networks due to
untested or unproven devices or equipment performance is high.
Before shipping new WLAN products or deploying live WLAN networks,
certain questions need to be answered. For example, how
effectively is interference handled in the 2.4 GHz RF range? What
sort of roaming hand-off delay will there be as mobile users move
from access point cell to access point cell? How will the
implementation of various MAC and upper layer security mechanisms
impact product and network performance? How effectively does the
WLAN station and access point handle power management? All of
these issues, along with traditional end-to-end performance,
latency and packet loss, can be addressed through comprehensive
testing.

The focus of this article is on addressing the importance and
benefits of WLAN performance testing and will attempt to
specifically address the value and benefits of testing some of the
key areas of concern relative to WLAN device development and
network deployment.

Traffic Generation and Impairment Emulation for an AP-to-AP
Testing Environment

Standardized, real-life RF propagation modeling
The biggest challenge facing WLAN chip and equipment manufacturers
will be the development of robust, high performance, physical
layer implementations using 2.4 GHz and 5 GHz radio frequency
signaling. Although 802.11b and 802.11a are rated at 11 Mbps and
54 Mbps of potential throughput respectively, actual throughput
will vary greatly depending on how well the physical layer
interface deals with RF channel characteristics such as multi-path
fading, path loss, delay spread and log normal shadowing.
Laboratory recreation of real world radio frequency impairments
will be required by developers and service providers so that true
product performance can be determined. This can be a significant
challenge as the environmental variables that can impact radio
frequency transmissions are far more difficult to recreate than a
simple twisted pair, copper wire connection. Take, for example,
the issue of multi-path interference. Radio waves can reflect off
of solid objects and show up at the WLAN AP or W-NIC receiver
slightly delayed from the original signal causing inter-symbol
interference and degrading the signal quality. Typical reflected
signal delay in an office environment is 50 nsec. and 300 nsec on
a manufacturing floor. Inter-symbol spacing can be used to counter
the effect of multi-path interference but at a cost to throughput.
Additionally, flutter may occur as the radio tries to lock on to
the reflected signals as well as the original.

Multi-path fading emulators can reproduce these types of RF
characteristics, recreating real-world impairments against which
the System Under Test (SUT) can be evaluated in the lab. The
connection to the RF channel emulator and interference emulator
requires either a RF isolation chamber or direct connection to the
RF impairment emulator equipment through a coaxial cable. RF
circulators may also be required to isolate the transmit and
receive signal for connection to the emulator. The SUTs
performance should first be calibrated by testing it in a
impairment free environment. Once this benchmark is established,
progressive physical layer impairment testing can be performed by
introducing various channel models such as JTC, or an
exponentially decaying Rayleigh model, for example. One advantage
of the JTC channel model is that it was agreed upon by the Joint
Standards Committee as an acceptable model for wireless indoor
communications and is therefore appropriate as a way to compare
various WLAN systems' performance.

[JJ1] The graph illustrates: average transfer delay, packet rate
and bandwidth percentage test results with an alternating 90
degree phase shift impairment

Performance when interference is present in the 2.4 GHz RF range
RF interference involves the presence of unwanted, disruptive RF
signals that disturb normal system operations. An interfering RF
signal of sufficient amplitude and within the receiver's frequency
spectrum can appear as a bogus 802.11 station transmitting a
packet. This can cause legitimate 802.11 stations to wait for
indefinite periods of time until the interfering signal goes away.

This is of particular concern in the crowded 2.4 GHz range where
microwave ovens, radiophones and Bluetooth devices and other
non-802.11 devices share the spectrum with 802.11b devices. Noise
and interference emulators can simulate these types of impairments
in a lab and should be include as a component of any fully
equipped WLAN test lab. Standardized WLAN interference models are
being developed for the WLAN market but non have been released at
the time this article was being written.

Measuring roaming hand-off delays and RF rate adaptation
performance
When a WLAN user is moving about the AP will try to maintain the
connection as the user moves out of range by reducing the speed of
the connection first. Then, if the signal continues to degrade,
the client has the option of looking for another AP with a
stronger signal. Intuitively, one might think that roaming
hand-off delay would not be a significant issue for WLAN users
since they are not typically on-line while walking around with
their laptops. But there are a number of scenarios that must be
considered where poor performance of the rate adaption function
and roaming hand-off to another AP could impact the effectiveness
of the wireless application.

Roaming can create a scalability issue for APs in certain
applications. Consider how roaming typically happens in a campus
or large enterprise environment. In both of these cases the AP may
be required to deal with huge spikes in roaming activity.

In the IEEE 802.11 specification, there are three classes of
client mobility:

. 1. No-transition: movement may or may not occur and the client
never changes its AP association.

. 2. BSS-transition: A wireless client moves between multiple AP's
that are a part of the same wireless infrastructure.

. 3. ESS-transition: A wireless client moves between multiple AP's
that are not a part of the same wireless infrastructure.

When the wireless client changes its AP association, any traffic
flows destined for the wireless client must change their physical
path. Such an exchange is unlikely to be instantaneous and is
likely to trigger various errors, including lost, misinserted,
duplicate, and out-of-sequence packets as well as increased
latency.

A wireless client that is moving out of the range of an AP can
constantly rate adapt between a lower speed that is passing data
and a high speed rate that is not. Additionally, if it is in range
of two APs it may constantly switch its association between the
two APs. Depending on the performance impact of re-association and
the rate of association changes, this ping-pong scenario can have
a dramatic impact on performance.

The 802.11 specification also states that service interruptions
are likely during ESS-transitions, but implies that
BSS-transitions can be completed without interruption. However,
the documentation provides no procedure for implementing either
the client re-association or AP hand-off functions. Since there is
no standardized method for performing this function, each vendor
implementation provides a unique testing opportunity.

Scalability of various security solutions; WEP, 802.11i, 802.1x,
IPSec, SSL, Firewalls
Every standard has its "gotchas" - X.25 was too slow, IP didn't
have enough addresses, and 802.11's "gotcha" is its security
definition, Wired Equivalency Protocol (WEP). WEP fails to be
specific about how Initialization Vectors (IVs) for the RC4 stream
cipher are to be implemented. Some implementations reset IVs to
zero every time they are restarted, and then increment them by one
for every use. This results in the likelihood that key-streams
will be reused. Combine this with the fact that the encryption key
and IV were not hash coded using MD5 or SHA-1 and WEP is open to
simple crypto-analytic attacks against the cipher and decryption
of message traffic.

The IEEE standards group is working on correcting the short
comings of WEP with its 802.11i working group, but equipment
vendors and service providers have already turned to other
security solutions like 802.1x, IPSec, SSL, integral firewalls and
other proprietary solutions. This creates a playing field in which
WLAN performance can vary significantly from solution to solution
depending upon the security mechanism chosen and the efficiency of
its implementation. To understand the performance issues
associated with a WLAN security solution, realistic testing of the
control plane with IP Virtual Private Network (IP-VPN) protocols,
as well as the data plane with connection-oriented (HTTP) and
connectionless (UDP) traffic is key.

It is important to determine IP-VPN tunnel creation capacity using
IPSec protocols and also generate UDP or HTTP traffic over each
tunnel and measure data performance characteristics such as packet
loss, latency, and response time. Results should be displayed in
an efficient layout for quick performance benchmark comparisons
and should function as an aid to troubleshoot interoperability
issues.

Firewall testing should determine the maximum application
transaction capacity and measure application throughput with TCP
acting as the transport agent. It should gauge the performance of
firewalls executing Network Address Translation (NAT) as well as
the impact of multiple packet filtering rules being set up.
Firewall test equipment should also evaluate a firewall's ability
to deal with Denial of Service (DoS) attacks by allowing different
ratios of attack traffic to be generated in parallel with other
application traffic. This permits bench marking of legal traffic
performance while attack traffic is being directed at the device
under test-consuming its resources.

Qualifying firewall performance requires exposure to a high volume
of Web transactions sufficient to model peak Internet conditions
and the ability to simulate an Internet mix of traffic as stateful
firewalls require real application traffic generation in order to
forward traffic properly. Scalability is a key component in
choosing the right test equipment for firewall testing as
thousands of HTTP transactions per second and millions of
concurrent TCP connections may be needed to effectively test a
firewall's performance.

Testing will accelerate deployment of WLAN technology
Because WLAN technology brings real benefits in cost savings,
mobility and increased productivity to the work place, campus,
home and hot spots in between, it is being accepted and deployed
at an increasingly rapid rate. Triple digit growth rates are
projected out through 2005, for 802.11a, 802.11a/b dual mode and
802.11g devices. With rapid acceptance and deployment, though,
comes the specter of unrealized performance and less than
acceptable quality of service (QoS). The early buyer will tolerate
some shortcomings in these areas just to get the latest technology
on the block, but as the solution moves into the mainstream the
buying public will become less tolerant of performance and QoS
issues. Users aren't happy to be able to just roam, for example,
they want to move about freely without interruptions in service as
they dis-associate and re-associate with APs. Realizing this, chip
and equipment manufacturers and service providers are beefing up
their test labs with test equipment that will allow them to
emulate real world RF network conditions and to test the
robustness and scalability of their products and network
solutions. WLAN technology has enormous revenue generating
potential for those who can deliver a solution that meets the
performance demands of the mainstream buyer.

SPIRENT Communications, 541 Industrial Way West, Eatontown, NJ
07724.

WirelessDesignMag.com

http://www.wirelessdesignmag.com/scripts/ShowPR.asp?PUBCODE=055&ACCT=0007220&ISSUE=0212&RELTYPE=pr&PRODCODE=00000&PRODLETT=C

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