2. In a PSTN network, a separate physical wire
is used to connect the subscriber’s telephone
with the switch.
Multiple users can have speech
communication at the same time without
causing any interference to each other.
The scene is different in the case of wireless
communication.
Radio channel used in a wireless network, is
shared by multiple subscribers.
Unless we control simultaneous access of
the radio channel, collisions can occur.
In a connection oriented communication, a
collision is undesirable.
This is achieved by using different
multiplexing techniques.
3. In mobile computing we have four types of
multiple access procedures
1. FDMA (Frequency Division Multiple
Access)
2. TDMA (Time Division Multiple Access)
3. CDMA (Code Division Multiple Access)
4. SDMA (Space Division Multiple Access)
4.
5. FDMA (Frequency Division Multiple
Access)
FDMA is one of the most common
multiplexing procedures.
The available frequency and is divided
into channels of equal bandwidth so that
each communication is carried on a
different frequency.
This multiplexing technique is used in all
the first generation analog mobile
networks like Advanced Mobile Phone
System in the US and Total Access
Communication System in the UK.
6. TDMA (Time Division Multiple
Access)
TDMA is a multiplexing technique where
multiple channels are multiplexed over
time.
In TDMA several users share the same
frequency channel of higher bandwidth
by dividing the signal into different time
slots.
Users transmit their data using their own
respective time slots in rapid succession
The transmitter and receiver need global
clock.
7. In TDMA system divides its
transmission medium into frames.
Each TDMA frame is then divided into
time slots of same temporal width that
are allotted to individual users.
TDMA is a very common multiplexing
technique and used in many digital
transmissions.
8.
9. In TDMA we have the following
categories.
1. Fixed TDMA
2. Dynamic TDMA
3. Packet based TDMA
10. Fixed TDMA
In Fixed TDMA, connections between
time slots in each frame an data streams
assigned to a user remain static and
switched only when large variations in
traffic are required.
In this variant of TDMA, the slot sizes
are fixed at T/N.
If a station does not transmit during its
assigned slot, then the corresponding
bandwidth is wasted.
This variant is simple to implement but
performs poorly since the entire
bandwidth is not used.
11. Dynamic TDMA
In Dynamic TDMA or Dynamic
Reservation TDMA, a scheduling
algorithm is used to dynamically reserve
a variable number of time slots in each
frame to variable bit-rate data streams.
This reservation algorithm is based on
the traffic demand of each data stream.
The fixed length DR TDMA frame is time-
duplexed into an uplink and downlink
channel and the boundary between these
two parts is dynamically adjusted as a
function of the traffic load.
12. Packet based TDMA
PRMA(Packet Reservation Multiple
Access) is a packet based TDMA where
the users contend for the time slots.
In PRMA the user can reserve a time slot
in advance for future use and optimize
the bandwidth in radio transmission.
The two prominent variants of PRMA are
Dynamic PRMA and PRMA Hindering
States.
In DPRMA, each mobile station is
responsible for making a reasonable
estimate of its bandwidth requirements
an then request for resource allocation.
13. CDMA (Code Division Multiple
Access)
DDMA is a broadband system. CDMA
uses spread spectrum technique where
each subscriber used the whole system
bandwidth.
Unlike the FDMA or TDMA where a
frequency or time slot is assigned
exclusively to a subscriber, in CDMA all
subscribers in a cell use the same
frequency simultaneously.
To separate the signals, each
subscriber is assigned an orthogonal
code called chip.
14. SDMA (Space Division Multiple
Access)
Along with TDMA,FDMA, and CDMA,
we need to make use of the space
effectively.
SDMA is a technique where we use
different parts of the space for
multiplexing.
In SDMA antennas are highly
directional, allowing duplicate
frequencies to be used at the same
time for multiple surface zones on earth.
SDMA requires careful choice of zones
for each transmitter.
