The form factor of an internal wireless radio network interface card (NIC)
defines a physical and electrical bus interface that enables the radio card
to communicate with a computing device. The barrage of form factors — a fancy
way of saying the physical size and shape of a device — demands that you be
careful when purchasing a card that matches your specific user device. Let’s
take a closer look at the various forms available for desktop PCs, laptops and
Personal Digital Assistants (PDAs).
Industry Standard Architecture (ISA)
The ISA bus is the most common bus interface in the desktop PC world. ISA has
been around since the early 80’s for use in the IBM PC/XT and PC/AT. Because
of this, the proliferation of the ISA has been significant.
Despite its lack of speed (2MBps), nearly all PC’s manufactured up until a
short time ago had at least one ISA bus. The ISA bus has failed, however, to
advance at the pace of the rest of the computer world, and other higher speed
alternatives are now available. ISA doesn’t impose too much of a performance
impact on 802.11b wireless LANs, but it’s not advisable to purchase new ISA
cards because of the possibility of them becoming obsolete.
Peripheral Component Interconnect (PCI)
The PCI bus is the most popular bus interface for PCs today and boasts a throughput
rate of 264Mbps. Intel originally developed and released PCI in 1993, and it
satisfies the needs of most recent generations of PCs for multimedia, graphics
and networking cards.
PCI cards were the first to popularize Plug and Play (PnP)<DEFINE: PnP>
technology. PCI circuitry can recognize compatible PCI cards and then works
with the computer’s operating system to set the resource allocations for each
card. This helps save time and prevents installation headaches.
Most wireless LAN vendors offer PCI-based radio cards instead of ISA… if
you must use an internal card, they’re the only way to go when equipping a desktop
PC with wireless LAN capability. In order to ensure good connectivity with an
access point, think about purchasing PCI cards having an external antenna
connector to allow the placement of the antenna on top of a desk. Permanently
attached antennas on PCI cards generally remain hidden behind PCs and low to
the floor where there are large obstructions, such as desks, that offer significant
signal attenuation.
PC Card
Developed in the early 90’s by the Personal Computer Memory Card International
Association (PCMCIA), the PC Card is a credit card sized peripheral device that
can provide extended memory, modems, connectivity to external devices, and,
of course, wireless LAN capabilities to laptops. They are the most widely available
WLAN cards available. In fact, they are much more popular than ISA or PCI cards
because of use in a growing number of laptops.
If you want to share a PC Card with your desktop PC, consider using an adaptor
that converts a PC Card into a PCI card. This allows purchasing one card for
use in both types of computers. Take the PC Card along with you a business trip
or home from work and utilize the same card when back in your office using a
PC.
Antennas are generally integrated into the wireless PC Card, with a stub that
extends outside the PC Card slot and provides omni-directional RF propagation
patterns. In most cases, these integrated antennas provide very good performance.
Some PC Cards, though, come with optional removable antennas in case you have
a need to utilize alternative, possibly more directive antennas.
In order to use a PC Card with a PDA, you’ll probably need a sled device that
accommodates the PC Card and mounts underneath the PDA. This is the only way
to add wireless LAN capability to some older PDAs. The combination of the sled,
PC Card and PDA, however, adds a lot of bulk and weight that depletes the usability.
You’ll find that many access points incorporate PC Card radio NICs and are
easily replaceable, accommodating newer technologies (e.g., 802.11a or 802.11g)
as they become available. In most cases, end users can swap the radio NICs along
with relatively simple firmware upgrades. This extends the life of the access
point hardware, which of course saves money in the long run.
You’ll sometimes see PC Card NICs referred to as "CardBus" — that’s
a 32-bit implementation of a PC Card. Which means, basically, it’s faster —
so it’s much more likely to be found for use with the faster 802.11a or g.
Mini-PCI
A Mini-PCI card is a small version of a standard desktop PCI card. It has all
the same features and functionality of a normal PCI card, but is about one quarter
the size. Mini-PCI cards are integrated within laptops as an option to buyers,
with antennas that are often integrated out of view within the monitor’s case
or even up next to the LCD screen. A strong advantage of this form of radio
NIC is that it frees up the PC Card slot for other devices. In addition, manufacturers
can provide Mini-PCI based wireless connectivity at lower costs.
The Mini-PCI card is not without disadvantages, however. If you want to replace
a Mini-PCI card yourself, you may have to disassemble most of the laptop. The
problem is that this could void the manufacturer’s warranty. Mini-PCI cards
may also lead to lower performance, because they require the computer to do
some, if not all of the processing. Despite these drawbacks, the Mini-PCI card
has revolutionized the wireless laptop world.
CompactFlash (CF)
SanDisk Corporation first introduced
CF in 1994, but wireless LAN radio cards were not available in CF form factors
until recently. A CF card is very small, weighing half an ounce and less than
half the thickness and one quarter the volume of PC Card radio card. The CF
cards also draw very little power, which enables batteries to last longer than
devices using PC Cards. Some PDAs come with direct CF interfaces, which results
in a very lightweight and compact wireless PDA.
A CF radio card is definitely the way to go, especially for compact computing
devices. Consider using a CF-to-PC Card adapter to operate the CF card in a
laptop via PC Card slot. This enables you to take advantage of the miniature-sized
radio card in the PDA and not have to purchase another card for your laptop.
This just covers the internal forms a wireless NIC can take — and doesn’t
cover the newest type of products, Secure Digital Input/Output or SD Cards that
will soon be the smallest Wi-Fi NICs in existence. There are also externals
wireless adapters that connect through the USB port and even the existing Ethernet
network port. We’ll cover them in the near future.
Jim Geier provides independent consulting services to companies developing
and deploying wireless network solutions. He is the author of the book, Wireless
LANs and offers computer-based
training (CBT) courses on wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
