Jaringan Komputer Lanjut
Jaringan Nirkabel (Wireless LAN)
Rahmadian-Why have Wireless LANs Become
so Popular?
Business networks today are evolving to
support people who are on the move.
Employees and employers, students and
faculty, government agents and those they
serve, sports fans and shoppers, all are mobile
and many of them are "connected." Perhaps
you have a mobile phone that you route
instant messages to when you are away from
your computer.
This is the vision of mobility-an environment
Why Use Wireless?
Productivity is no longer
restricted to a fxed work location
or a defned time period.
People now expect to be
connected at any time and place.
Employees can check e-mail,
voice mail, and the status of
products on personal digital
Advantages
Flexibility
Reduced cost; on average, the IT
People Change The Way They
Live
The method of accessing the Internet
has quickly moved from temporary
modem dialup service to dedicated DSL
or cable service.
Home users are seeking many of the
same fexible wireless solutions as
ofce workers.
For the frst time, in 2005, more
Radio Frequency (RF)
RF does not have boundaries, such as the limits of a wire in a
sheath. The lack of such a boundary allows data frames traveling over the RF media to be available to anyone that can receive the RF signal.
RF is unprotected from outside signals, whereas cable is in an
insulating sheath. Radios operating independently in the same geographic area but using the same or a similar RF can interfere with each other.
RF transmission is subject to the same challenges inherent in any
wave-based technology, such as consumer radio. For example, as you get further away from the source, you may hear stations
playing over each other or hear static in the transmission.
Eventually you may lose the signal all together. Wired LANs have cables that are of an appropriate length to maintain signal
strength.
RF bands are regulated diferently in various countries. The use of
Other Things About WLAN
WLANs connect clients to the network through a wireless
access point (AP) instead of an Ethernet switch.
WLANs connect mobile devices that are often battery
powered, as opposed to plugged-in LAN devices. Wireless network interface cards (NICs) tend to reduce the battery life of a mobile device.
WLANs support hosts that contend for access on the RF
media (frequency bands). 802.11 prescribes
collision-avoidance instead of collision-detection for media access to proactively avoid collisions within the media.
WLANs use a diferent frame format than wired Ethernet
LANs. WLANs require additional information in the Layer 2 header of the frame.
WLANs raise more privacy issues because radio
History
When 802.11 was frst released, it
prescribed 1 - 2 Mb/s data rates in
the 2.4 GHz band. At that time, wired
LANs were operating at 10 Mb/s so
the new wireless technology was not
enthusiastically adopted.
Since then, wireless LAN standards
have continuously improved with the
release of IEEE 802.11a, IEEE
Wireless LAN Standards
Internationally, the three key organizations
infuencing WLAN standards are:
◦ ITU-R
Regulates the allocation of the RF spectrum and satellite orbits.
◦ IEEE
Developed and maintains the standards for local and metropolitan area networks with the IEEE 802 LAN/MAN family of standards. Specifed standards for RF
modulation devices.
◦ Wi-Fi Alliance
Association of vendors whose objective is to improve the interoperability of products that are based on the 802.11 standard by certifying vendors for conformance to
Wireless LAN Standards
The roles of these three
organizations can be summarized
as follows:
◦
ITU-R regulates allocation of RF
bands.
◦
IEEE specifes how RF is modulated
to carry information.
Wireless AP
An access point is a Layer 2 device that functions
like an 802.3 Ethernet hub. RF is a shared medium
and access points hear all radio trafc. Just as with
802.3 Ethernet, the devices that want to use the
medium contend for it. Unlike Ethernet NICs,
though, it is expensive to make wireless NICs that
can transmit and receive at the same time, so
radio devices do not detect collisions. Instead,
WLAN devices are designed to avoid them.
RF signals attenuate. That means that they lose
CSMA/CA
Access points oversee a distributed coordination
function (DCF) called Carrier Sense Multiple Access
with Collision Avoidance (CSMA/CA). This simply
means that devices on a WLAN must sense the
medium for energy (RF stimulation above a certain
threshold) and wait until the medium is free before
sending. Because all devices are required to do
this, the function of coordinating access to the
medium is distributed. If an access point receives
data from a client station, it sends an
Wireless Router
Wireless routers perform the role of access
point, Ethernet switch, and router. For
example, the Linksys WRT300N used is really
three devices in one box. First, there is the
wireless access point, which performs the
typical functions of an access point. A
built-in four-port, full-duplex switch provides
connectivity to wired devices. Finally, the
router function provides a gateway for
Wireless Operation
The wireless network mode refers to the WLAN
protocols: 802.11a, b, g, or n.
Because 802.11g is backward compatible with 802.11b,
access points support both standards. Remember that if all the clients connect to an access point with 802.11g, they all enjoy the better data rates provided. When
802.11b clients associate with the access point all the faster clients contending for the channel have to wait on 802.11b clients to clear the channel before transmitting.
When a Linksys access point is confgured to allow both
802.11b and 802.11g clients, it is operating in mixed mode.
For an access point to support 802.11a as well as
Wireless Operation
SSID
◦ A shared service set identifer (SSID) is a unique identifer that client devices use to distinguish between multiple wireless networks in the same vicinity. Can be any alphanumeric, case-sensitive entry from 2 to 32 characters long.
Channel
◦ The 2.4 GHz band is broken down into 11 channels for North America and 13 channels for Europe. These channels have a center frequency separation of only 5 MHz and an overall channel bandwidth (or frequency occupation) of 22 MHz. The 22 MHz channel bandwidth combined with the 5 MHz
separation between center frequencies means there is an overlap between successive channels.
WLAN Terms
Beacons - Frames used by the WLAN
network to advertise its presence.
Probes - Frames used by WLAN clients to
fnd their networks.
Authentication - A process which is an
artifact from the original 802.11
standard, but still required by the
standard.
Association - The process for
Threat to Wireless
Security
Unauthorized Access
There are three major categories
of threat that lead to
unauthorized access:
◦
War drivers
◦
Hackers (Crackers)
Threat to Wireless
Security
"War driving" originally referred
to using a scanning device to fnd
cellular phone numbers to
exploit. War driving now also
means driving around a
neighborhood with a laptop and
an 802.11b/g client card looking
for an unsecured 802.11b/g
Threat to Wireless
Security
The term hacker originally meant
someone who delved deeply into
computer systems to understand, and
perhaps exploit for creative reasons, the
structure and complexity of a system.
Today, the terms hacker and cracker have
come to mean malicious intruders who
enter systems as criminals and steal data
or deliberately harm systems.Hackers
Threat to Wireless
Security
A rogue access point is an access point placed on a
WLAN that is used to interfere with normal network
operation. If a rogue access point is confgured with the correct security settings, client data could be captured. A rogue access point also could be confgured to provide unauthorized users with information such as the MAC addresses of clients (both wireless and wired), or to capture and disguise data packets or, at worst, to gain access to servers and fles.
A simple and common version of a rogue access point is
one installed by employees without authorization.
