Wireless Networking
Volume Number: 15
Issue Number: 12
Column Tag: Network Innovations
Wireless Networking and the Airport
by John C. Welch
Contributing Editor, Ilene Hoffman
Apple and Lucent redefine easy networking with the
AirPort, and 802.11
This article covers Apple's AirPort, the wireless networking system introduced in
July at MacWorld Expo, as well as the standards and technology behind the AirPort.
Along the way, we'll look at other wireless networking schemes and products, to see
how they stack up against the AirPort. We'll start with a basic introduction to the
AirPort, and its connection to the iBook,and to other members of the Macintosh family.
We will also go into a basic history of wireless networking, as it pertains to the
AirPort and the 802.11 standard. We'll also take a close look at the 802.11 standard
itself, and some of the work Lucent Technologies has done to get the speed and
capabilities that now exist out of that standard. Next we'll take a look at some of the
power management and other features that are integral to the Lucent work. Then we'll
look at the AirPort itself, and see how Apple has applied their philosophy of "It just
works" to wireless networking. Finally, we'll compare the AirPort and 802.11 to
some of the other wireless networking schemes.
Airport Introduction
The biggest news to come out of the New York MacWorld was the iBook, and its wireless
networking capabilities. Although I have no doubt the iBook will become as much a
success as the iMac, I think the really astounding part of this announcement was the
AirPort, Apple's release of an 802.11-compliant wireless networking system. The
AirPort is a one-stop wireless networking system that runs at common Ethernet
speeds. It is cheap enough to become an impulse buy for iBook, and other Mac
customers.
Now for the iBook, the AirPort is a great enhancement. The iBook's card, which, in
classic Macintosh fashion, is insanely easy to install, is about a hundred bucks; then
add about 300 bucks for the AirPort Pod, which is the base station for
AirPort-enabled Macs; a person or company can set up a fast, reliable, easy to use and
maintain wireless network in relatively short order. Even for other Macs, the AirPort
is as much of a blessing. With PC cards available, and more on the way from Apple and
other vendors, such as Farallon, and PCI cards for the tower and desktop Macs, even
non-iBooks can take full advantage of the AirPort. (Although not mentioned, I'll hazard
two predictions about the iMac. The first is that we will be quickly seeing a
USB-connected AirPort device, and the second is the iMac will eventually have AirPort
networking built-in.)
The extra plum in this bowl is the IEEE 802.11 wireless networking standard. Since
the AirPort is based on this standard, any computer with an 802.11-compliant
interface device can plug into the AirPort network with no more trouble than
connecting Macs and PCs or Unix workstations via conventional Ethernet. So much of
the AirPort techonology is based on the 802.11 standard, we shall go into the history
of 802.11 devices and the details of the standard itself.
The IEEE 802.11 Specification
Let's start with some background information on the foundation for the AirPort, the
IEEE 802.11 standard. This standard, defines the way that 802.11-compliant devices
communicate with each other. It operates at the physical, or hardware layer, and the
Media Access Control, or MAC layer. It is not concerned with TCP/IP or AppleTalk, but
rather the underpinnings. 802.11 defines wireless networking across a number of
bands, or frequencies. These frequencies, in the Industrial, Scientific, and Medical, or
ISM bands, range from just over 900MHz to approximately 5.8 GHz. The ISM bands
were chosen because of their worldwide availability. In practice, due to differences of
availability worldwide, 802.11 is primarily concerned with the 2.4 to 2.5 GHz bands.
The differences in band availability, power usage, and modulation rules in various
parts of the world are shown below in Table 1.
Countries Frequency range Maximum RF power level Rules for
DSSS and FHSS
U.S.1, Canada, and Latin America (FCC Part 15,247) 902-928 MHz
2,400-2,483.5 MHz
5,750-5,850 MHz 1W (at ERP,2 and maximum 6 dBi antenna gain) DSSS:
Receiver processing gain >10 dB FHSS: 75 hops or more
Europe,3 (ETS,4 300 328) ‹2,400-2,483.5 MHz 100 mW (at
EIRP,5) DSSS: Power spectrum density maximum 10 mW/MHz FHSS: 20
hops or more
Japan (MPT,6 Ordinances 78 and 79) 2,471-2,497 MHz Not specified
DSSS/FHSS: Power spectrum density maximum 10 mW/MHz
Australia 2,400-2,450 MHz 500 mW
1. In Canada, not the 5,750-5,850-MHz band
2. ERP-Effectively radiated power
3. In France/Spain, only the 2,445-2,483.5/2,475-MHz band
4. ETS-European Telecommunication Standard
5. EIRP-Equivalent isotropically radiated power
6. MPT-Ministry of Posts and Telecommunications (in Japan)
Table 1. Frequency bands and power levels for wireless LANs.
As we can see, the 2.4 GHz band is the only one implemented worldwide. The 802.11
standard only is concerned with the MAC and physical layer parts of wireless
networking. For those of you unfamiliar with the layers of networking, in general, a
complete network stack, or system, is visualized as having up to seven layers. Each
layer sits on top of the other, and receives information from the layers above and
below, and passes information the same way. The MAC layer sits on top of one or more
physical layer systems, as shown below in Figure 1.
Figure 1. The physical layers handle the actual connections and
transmitting/receiving of electrical or optical signals that represent data.
802.11 Physical Layer
The physical layer of 802.11 is where the bits hit the wire. It is concerned with how
transmission and reception of data happens, and how data is encoded into the
corresponding RF signals by the transmitter, and decoded by the receiver. There are
three implementations of 802.11: Infrared (IR), Frequency Hopping Spread Spectrum
(FHSS), and Direct Sequence Spread Spectrum (DSSS). The IR implementation uses
infrared light to move data, much like a television remote, and the last two are radio
frequency, (RF) implementations.
Infrared - IR
The IR 802.11 implementation is based on diffuse IR. Rather than trying to line up the
transmitter and receiver, like a television and its remote, an IR-based 802.11 device
transmits a wideband, or diffuse signal at the ceiling, which reflects the data around
the area until it reaches its destination. Likewise, incoming data is bouncing off the
ceiling. While better than narrow-beam IR, this implementation can only be used
indoors, and requires the ceiling to be reflective to its wavelength, which is in the
850 to 950 nm range. In addition to the ceiling material requirements, 802.11 IR
only has a range of about 10 meters, which makes it suitable for a small room, say,
such as a work area with an IR-enabled printer.
802.11 IR supports 2 data rates, 1Mbps and 2 Mbps. At the 1Mbps rate, the data
stream is broken into quartets. Each quartet is then encoded into one of sixteen pulses
during modulation and transmitted. This modulation technique is called 16 Pulse
Position Modulation. At the 2Mbps rate, the modulation is somewhat different, with the
data stream being divided into data bit pairs, and each pair being modulated into one of
4 pulses.
Frequency Hopping Spread Spectrum - FHSS
FHSS systems break up the total bandwidth into narrower sub-bands, or channels, and
hop from channel to channel during transmissions. As the signal hops, it sends a packet
at one frequency, then hops to the next channel, and sends another packet, and so on.
The FHSS signal dwells on each band a predetermined amount of time. In the case of
802.11, the time is up to 300msec. The hopping sequence is pseudo-random,
(computers can't generate true random sequences, but they can come very close, hence
the term pseudo random.) The sequence and pattern of the frequency hops are partially
determined by the geographical location. For example: Japan specifies three sequences
with four patterns, Spain specifies three sequences with nine patterns, France
specifies 3 sequences with eleven patterns, and the U.S. and the rest of Europe use
three sequences with 26 patterns. This frequency-hopping also has the serendipitous
side-effect of assisting with the collision avoidance process. Since the signal is
transmitted on any given channel for a fairly short period of time, collisions happen
less often. Within the overall bandwidth are a number of 1MHz wide channels, the
number of which depends on the locality of the system. In Japan, there are 23 of these
channels between 2.473GHz and 2.495GHz, whereas in the U.S. there are 79 channels
between 2.402GHz and 2.48GHz. Another important item in the FHSS algorithm is all
available channels must be used before a repeat use of a channel. The FHSS transmitter
converts the bitstream from the transmitting device to a symbol stream, where each
symbol represents one or more bits. The signal is modulated via a Frequency Shift
Keying (FSK) method, with the specific type of FSK depending on the number of
modulating frequencies desired. If two modulating frequencies are used, then binary
FSK is used, and if four frequencies are used, then quaternary FSK is used. This
FSK-modulated signal is what hops frequencies during the data transmissions and
receptions. 802.11 FHSS uses a third type of FSK modulation, Gaussian FSK. Finally,
although the Gaussian FSK used by 802.11 FHSS gives higher bitrates in its channels,
it has more sensitivity to noise and other poor conditions. (Interesting historical
tidbit: Spread Spectrum using frequency hopping was invented in 1940 by actress
Hedy Lamarr when she was 26.)
Direct Sequence Spread Spectrum - DSSS
The final 802.11 physical implementation is DSSS. This is the implementation used by
Apple, Lucent, Farallon and others to create 802.11 wireless networks. DSSS differs
from FHSS - instead of subdividing the bandwidth into channels and switching between
them, DSSS spreads the signal across the entire bandwidth, thereby increasing
bandwidthutilization. As in FHSS, bitstreams are converted into symbol streams, with
each symbol containing one or more bits. The number of bits is determined by the
modulation technique used, however unlike FHSS, DSSS bases its modulation on Phase
Shift Keying, or PSK. The PSK-modulated symbol stream is converted to a
complex-valued signal, which is then fed into a spreader chip. The spreader chip
multiplies this signal with a pseudo-noise, PN signal, called a chip sequence. 802.11
DSSS bases its chip sequence on the eleven-chip Barker sequence. The Barker sequence
is just a series of positive and negative values that is used to force transitions in the
signal. For example, you have a pulse that looks like Figure 2.
Figure 2. Basic Pulse.
If you modulate that signal with the following sequence:
+1-1+1+1-1+1+1+1-1-1-1
and you invert the pulse on every transition, (going high if it was low, going low if it
was high), and keeping state if no transitions are called for, you get a modulated pulse
that looks like Figure 3.
Figure 3. Sample Modulated Pulse.
Although the pulse seems to be out of sequence for the last half of the pulse, the
sequence starts over once the 11th key is used, so although the last key is a -1, and the
first key is a +1, this is a restart/reset, not a transition. No phase shift occurs until
the second key, which is a -1, and a transition. Spreading the signal on this sequence
makes the total occupied bandwidth larger, and brings the effective bandwidth up to 11
MHz from 1 MHz, while still allowing fallback to 5.5, 2, or 1Mbps if needed.
Spreading the signal also makes it less susceptible to interference, as to completely
block the signal, the interference must occur across the entire band. However,
spreading also reduces overall transmitted signal power, as the output power is
applied over a wider bandwidth. Both effects are shown in the diagram below, with
signal strength as the y-axis, signal bandwidth as the x-axis, data in blue, and noise in
pink.
Figure 4.
The outputs of the spreader are then fed into a quadrature modulator, and then into the
transmitter front end. 802.11 DSSS specifies 2 bitrates: 1Mbps using Binary PSK,
BPSK, and 2Mbps using Quadrature PSK, QPSK.
FHSS v. DSSS
In comparing FHSS and DSSS, we notice that DSSS has some immediate advantages over
FHSS. The first is more robust modulation, and greater range, even when operating at
half the signal strength of a comparable FHSS system. While the channel-hopping
behavior of FHSS gives it more overall frequencies, interference between adjacent
channels limits the total number of collocated FHSS systems. However, FHSS does have
an advantage over DSSS because it degrades more gracefully than DSSS, and can work
better under worse conditions. Much of this advantage is due to FHSS not being spread
out like DSSS. Since the FHSS signal is concentrated across a much narrower
bandwidth, its amplitude is greater, and FHSS can therefore 'punch through'
interference better. Also, the hopping aspect of FHSS assists with frame collision
avoidance. These advantages are minimized by the fact that DSSS works reliably at
much greater distances than FHSS, as shown in Figure 5 below.
Figure 5.
Another advantage to DSSS is efficiency. DSSS is able to give better performance with
fewer access points than FHSS., Plus, FHSS reaches a point of diminishing returns
much faster than DSSS, as shown in Figure 6.
Figure 6.
In addition, DSSS can use a higher number of access points to get an overall higher
aggregated bandwidth than FHSS.
Figure 7.
Also, in collocated networks, DSSS gives higher speeds with fewer access points than
FHSS.
802.11 MAC Layer
Now that we have taken a look at the physical layer of 802.11, let's move on to the
next part of the standard, the Medium Access Control or MAC layer. As a wireless
network standard, the MAC for 802.11 is different from the MAC for a wired network
such as Ethernet. One example of this is the expectation that 802.11 Access Points
(AP) are always acting as bridges between the wired network, and the wireless
network, which is not an assumption in wired networks. Plus 802.11 frames have
some unique features that assist in wireless data transmission and reception. Each
frame has sequence control and retry fields that help to minimize interference
between stations. Since RF is omnidirectional, regardless of which AP a particular end
node is connecting to, its frames are received by every AP in range, so the sequence
control fields help deal with this. In conjunction with the sequence control fields, you
have the type/subtype and duration fields, that help ensure reliable communications
with 'hidden' stations. The sequence control fields also work with the fragmentation
fields, which allow each frame to be further subdivided into smaller fragments if
conditions are bad. There are also ToDS andFrom DS fields that assist in sett up and use
of single-channel wireless backbones.
Carrier Sense Multiple Access/ Collison Avoidance - CSMA/CA
802.11 uses a MAC scheme that is similar to Ethernet's CSMA/CD, called CSMA/CA.
The main difference is that 802.11 practices collision avoidance (CA), as opposed to
Ethernet's collision detection (CD). The reason is that in a distributed wireless
network, it is highly impractical to attempt to detect collisions, because a weak
incoming signal could either be a frame or noise. The CA in 802.11 is designed to avoid
collisions entirely. It reduces the chances for a collision during the period of time that
has the highest probability of a collision, which is the time just after a station stops
transmitting. At that point, many other stations are waiting for access, and will
attempt to transmit their data. To avoid collisions, 802.11 uses a random back-off
arrangement.
Figure 8.
As shown in Figure 8, after the busy medium period, there is an Interframe spacing
period (IFS), which for 802.11 is 50µsec. All devices on the segment must wait for
that IFS period. Following the IFS, devices wait an additional random number of
20µsec slots, the number of which is determined by a binary exponential backoff
algorithm. If after this time has passed, the medium is still free, (no other stations
transmitting), then the stations can attempt to transmit. Each station uses its own
random, (actually pseudorandom) amount of wait time, the chance of a collision is
reduced. If a collision is detected, the devices go back into the slot time wait mode,
until the medium is free. Another difference is in the frame acknowledgement.
Although most LAN systems require some form of frame reception acknowledgement,
the wireless nature of 802.11 forces some unique requirements in this area. As with
other LANs, 802.11 does all its frame acknowledgement at the receiving end. However,
unlike most LANs, 802.11 handles this at the MAC layer, whereas other LANs handle
this at higher layers. The reason is the timing requirements imposed by 802.11. With
the IFS lasting for only 50µsec, the receiver is required to send an acknowledgement
within 10µsec, only after verifying the CRC for the frame. By performing all of these
functions within 10µsec of receiving the frame, the receiver can immediately send the