Which Spread Spectrum Technology Delivers Better Performance, Direct
Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum
(FHSS)?
Technology Overview
Spread spectrum is a radio technique that continuously alters its data transmission pattern
either by constantly changing carrier frequencies or by constantly changing the data
pattern. In the world of wireless local area networks (WLAN), the majority of access
points (APs) and device radios use one of two technologies for signal transmission:
Direct Sequence Spread Spectrum (DSSS), which changes data patterns, or Frequency
Hopping Spread Spectrum (FHSS) which changes carrier frequencies. Each method has
its advocates. But, it is important to understand the differences in the technologies and the
performance they will provide in order to assess which approach is better suited for an
application. Legacy radio technology, such as UHF, 900 MHz and 2.4 GHz Open Air,
was primarily FHSS based. But, newer standards-based technologies have evolved into
DSSS with the use of Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a
modulation technique that distributes the data over a large number of carriers that are
spaced apart at precise frequencies to avoid signal interference of various types. It is
important to note that the Institute of Electrical and Electronic Engineers (IEEE)
standards setting body, has standardized on DSSS through its 802.11 Task Groups.
A commonly used analogy to understand spread spectrum is that of a series of trains
departing a station at the same time. The payload is distributed relatively equally among
the trains. Upon arrival at the destination, the payload is taken off each train and is
collated. Duplications of payload
are common to spread spectrum so
that when data arrives excessively
corrupted, or fails to arrive, the
redundancies inherent to this
technology provide the ability to
still interpret the message.
General Advantages
♦ The ability to eliminate or
alleviate the effects of
multipath interference
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♦
♦
♦
Can share the same frequency band with other users
Provides privacy due to unknown random codes
Involves low power spectral density since the radio signal is spread over a large
frequency band
Direct Sequence
DSSS is probably the most widely recognized form of spread spectrum. With DSSS, all
trains leave in an order beginning with Train 1 and ending with Train N. In DSSS
architecture, the trains
always leave in the same
order, although the
1
N
2
3
numbers of railroad tracks
can be in the hundreds or
even thousands.
In direct sequence spread spectrum, the stream of information to be transmitted is divided
into small pieces, each of which is allocated to a frequency channel across the spectrum.
A data signal at the point of transmission is combined with a higher data-rate bit
sequence (also known as a chipping code) that divides the data according to a spreading
ratio. The redundant chipping code helps the signal resist interference and also enables
the original data to be recovered if data bits are damaged during transmission. To an
unintended receiver, DSSS appears as low-power wide-band noise and is ignored by most
narrow-band receivers.
Pros
• Higher data rate per access point (AP) [11Mbps vs. 2Mbps]. Aggregate system
bandwidth accomplished with only 3 APs in one area (33 Mbps) vs. approx. 15
APs (30 Mbps). A serious cost issue to get needed bandwidth! [NOTE: 15 is the
standard number of “orthogonal”, or non-interfering channels (hop sequences) in
FH. In practice, with a lot of utilization, interference will increase and the channel
degrade noticeably. That is, 15 FH APs would not come close to the bandwidth of
the 3 DS APs]
♦ Faster AP signal acquisition (only 3 channels to probe vs. 79 separate frequencies
for FH) 1. If a client’s users roam a lot this is very useful property
♦ Beacon and hop rates do not need to be synchronized for DS (no hopping) which
allows easier AP setup, especially when balancing power management of clients
vs. response time (short beacon interval for better avg. response time, longer
interval for better battery life for idle clients)
♦ DS will withstand noise in channel with loss of Bandwidth, whereas FH will start
degrading
♦ Even if one or more bits in the chip are damaged during transmission, statistical
techniques embedded in the radio can recover the original data without the need
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There are 79 frequencies for FH (that 802.11 uses, 1MHz separation), and 11 for DS (but only 3 do not
overlap, thus the number 3 that we use). There are multiple hop sequences for FH which jump around the
frequencies. So one could say, in a very confusing way, that there are 15 FH channels (like the 3 DS
channels) that hop through 79 channels (freq).
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for retransmission. The longer the chip, the greater the probability that the original
data can be recovered (but this also requires more bandwidth)
♦ Higher speed connections can allow low power devices to get their data sent and
received more quickly and thus the client can go back to sleep. (i.e. the terminal
devices can have their radios on and back off quickly and save power)
♦ Many, many vendors of 802.11 DS chips, NICs and APs
♦ Current 802.11b radios will be compatible with 802.11g radios (both use DSSS)
Cons
♦
♦
Not very bandwidth efficient
In the face of noise in the channel and loss of bandwidth, DS may abruptly stop
working, whereas FH will continue to degrade gradually to no bandwidth
Frequency Hopping
As the name implies, FHSS hops from narrow band to narrow band within a spectrum of
frequencies. Specifically, FH radios send one or more data packets at one carrier
frequency, hop to another frequency, send more data packets and continue this hoptransmit sequence. The hopping pattern appears random but is actually a periodic
sequence tracked by sender and receiver. To an unintended receiver, FHSS appears to be
short-duration impulse noise.
With FHSS architecture, the
trains leave in a different
orderthat is, not
25
N sequentially from Train 1 to
9
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Train N. Trains that run into
interference are not sent out again until the interference ceases.
Pros
Proprietary systems may adjust channel selection so that highly interfered
channels are avoided, but frequency avoidance is not in the IEEE 802.11FH
standard
♦ In the face of noise in the channel and loss of bandwidth, FH will continue to
degrade gradually to no bandwidth. It can roam more quickly though when this is
detected
♦ Can typically re-transmit data that has been corrupted by interference during other
hops
♦
Cons
♦
♦
FH can be susceptible to noise during any one hop
Some claim that because a fixed frequency is not used, illegal monitoring of
spread spectrum signals is extremely difficult, if not downright impossible
depending on the particular method. But, if standard 802.11FH is used, then all
one needs to see is a beacon, then the hop sequence is known and it is a fairly
simple matter to sniff and track the data
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Because each carrier is treated independently of the others, a frequency guard
band must be placed around it, lowering bandwidth efficiency. In some systems
up to 50 percent of the available bandwidth is wasted
♦ Cost/efficiency ratio is high - In practice, 15 FH APs would not come close to the
bandwidth of the 3 DS APs, resulting in additional cost to users for same
bandwidth and throughput
♦ Few vendors of 802.11 FH chips, only one vendor of NICs and APs
♦ No new research is being done on FH and no new products are being developed.
Peripheral products are a good example; try finding a FH wireless printer!
♦
Conclusions
Frequency hopping can be a cost-effective wireless LAN to deploy if needs for network
bandwidth are 2 Mbps or less. Direct Sequence systems are more reliable and optimal for
bandwidth requirements higher than 2Mbps and/or user intensive applications.
Advantages include:
♦ Higher data rates per AP - Higher speed connections deliver more bandwidth,
faster throughput and saves battery life
♦ Faster AP signal acquisition keeps productivity high
♦ Greater tolerance of noise in the channel delivers better network efficiency
♦ Data delivery - If one or more DS chips in a data stream are disrupted the entire
data bit can still be recovered due to the redundancy of the transmission
♦ Easier AP setup and better power management
♦ Wider industry acceptance and radio chip availability, FH limits users to one
vendor
Although DS and FH networks may coexist, the devices are not interoperable. These
critical performance factors are part of the reason why the IEEE has adopted DSSS
technology as part of its 802.11 set of standards.
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