Why are
multiple accesses required in LAN technologies? Compare different multiple
accesses techniques.
Ans
multiple
access protocols - by which nodes regulate their transmission onto the shared
broadcast channel. multiple access protocols are needed in a wide variety
of network settings, including both wired and wireless local area networks, and
satellite networks. Although technically each node accesses the broadcast channel
through its adapter, in this section we will refer to the node as
the sending and receiving device. In practice, hundreds or even thousands of
nodes can directly communicate over a broadcast channel.
In
order to ensure that the broadcast channel performs useful work when multiple
nodes are active, it is necessary to somehow coordinate the transmissions of
the active nodes. This coordination job is the responsibility of the multiple
access protocol. Over the past thirty years, thousands of papers and
hundreds of Ph.D. dissertations have been written on multiple access protocols;
a comprehensive survey of this body of work is [Rom 1980] Furthermore, dozens
of different protocols have been implemented in a variety of link-layer
technologies. Nevertheless, we can classify just about any multiple access
protocol as belonging to one of three categories: channel partitioning
protocols, random access protocols, and taking-turns
protocols. We'll cover these categories of multiple access protocols
in the following three subsections. Let us conclude this overview by
noting that ideally, a multiple access protocol for a broadcast channel of
rate R bits per second should have the following desirable
characteristics:
1. When only one node has data to send, that node has a
throughput of R bps.
2. When M nodes have data to send, each
of these nodes has a throughput of R/M bps. This need
not necessarily imply that each of the M nodes always have an
instantaneous rate of R/M , but rather that each node
should have an average transmission rate of R/M over some
suitably-defined interval of time.
3. The protocol is decentralized, i.e., there are no
master nodes that can fail and bring down the entire system.
4. The protocol is simple, so that it is inexpensive to
implement.
Cellular
systems divide a geographic region into cells where a mobile unit in each cell
communicates with a base station. The goal in the design of cellular systems is
to be able to handle as many calls as possible (this is called capacity in
cellular terminology) in a given bandwidth with some reliability. There are
several different ways to allow access to the channel. These include the
following.
·
frequency
division multiple-access (FDMA)
·
time division
multiple-access (TDMA)
·
time/frequency
multiple-access
·
random access
·
code division
multiple-access (CDMA)
o frequency-hop CDMA
o direct-sequence CDMA
o multi-carrier CDMA (FH or DS)
As
mentioned earlier, FDMA was the initial multiple-access technique for cellular
systems. In this technique a user is assigned a pair of frequencies when
placing or receiving a call. One frequency is used for downlink (base station
to mobile) and one pair for uplink (mobile to base). This is called frequency
division duplexing. That frequency pair is not used in the same cell or
adjacent cells during the call. Even though the user may not be talking, the
spectrum cannot be reassigned as long as a call is in place. Two second
generation cellular systems (IS-54, GSM) use time/frequency multiple-access
whereby the available spectrum is divided into frequency slots (e.g., 30 kHz
bands) but then each frequency slot is divided into time slots. Each user is
then given a pair of frequencies (uplink and downlink) and a time slot during a
frame. Different users can use the same frequency in the same cell except that
they must transmit at different times. This technique is also being used in
third generation wireless systems (e.g., EDGE).
Code
division multiple-access techniques allow many users to simultaneously access a
given frequency allocation. User separation at the receiver is possible because
each user spreads the modulated waveform over a wide bandwidth using unique
spreading codes. There are two basic types of CDMA. Direct-sequence CDMA
(DS-CDMA) spreads the signal directly by multiplying the data waveform with a
user-unique high bandwidth pseudo-noise binary sequence. The resulting signal
is then mixed up to a carrier frequency and transmitted. The receiver mixes
down to baseband and then re-multiplies with the binary {± 1} pseudo-noise
sequence. This effectively (assuming perfect synchronization) removes the
pseudo-noise signal and what remains (of the desired signal) is just the
transmitted data waveform. After removing the pseudo-noise signal, a filter
with bandwidth proportional to the data rate is applied to the signal. Because
other users do not use completely orthogonal spreading codes, there is residual
multiple-access interference present at the filter output.
This
multiple-access interference can present a significant problem if the power level
of the desired signal is significantly lower (due to distance) than the power
level of the interfering user. This is called the near-far problem. Over the
last 15 years there has been considerable theoretical research on solutions to
the near-far problem beginning with the derivation of the optimal multiuser
receiver and now with many companies (e.g., Fujitsu, NTT DoCoMo, NEC) building
suboptimal reduced complexity multiuser receivers. The approach being
considered by companies is either successive interference cancellation or
parallel interference cancellation. One advantage of these techniques is that
they generally do not require spreading codes with period equal to the bit
duration. Another advantage is that they do not require significant complexity
(compared to a minimum mean square error-MMSE-detector or a decorrelating
detector). These interference cancellation detectors can also easily be
improved by cascading several stages together.
As a typical example, Fujitsu
has a multistage parallel interference canceler with full parallel structure
that allows for short processing delay. Accurate channel estimation is possible
using pilot and data symbols. Soft decision information is passed between
stages, which improves the performance. Fujitsu's system uses 1-2 stages giving
fairly low complexity. Fujitsu claims that the number of users per cell
increases by about a factor of 2 (100%) compared to conventional receivers and
1.3 times if intercell interference is considered.
Very costly
·
Hard to install
The data transmission
capabilities of various Medias vary differently depending upon the various
factors. These factors are:
1. Bandwidth.
It refers to the data carrying capacity of a channel or medium. Higher
bandwidth communication channels support higher data rates.
2. Radiation.
It refers to the leakage of signal from the medium due to undesirable
electrical characteristics of the medium.
3. Noise Absorption.
It refers to the susceptibility of the media to external electrical noise that
can cause distortion of data signal.
4. Attenuation.
It refers to loss of energy as signal propagates outwards. The amount of energy
lost depends on frequency. Radiations and physical characteristics of media
contribute to attenuation
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