Common protocols

Wireless Application Protocol

Common Protocols

Common Protocols

 Several strategies have been proposed to solve the shared
medium access problem. These strategies attempt, by various
mechanisms, to strike a balance between achieving the highest-
quality resource allocation decision and the overhead necessary
to reach this decision. These strategies can be classified in
three major categories:
 Fixed assignment
 Demand assignment
 Random assignment.

Fixed-Assignment Protocols

 In fixed-assignment strategies, each node is allocated a
predetermined fixed amount of the channel resources. Each
node uses its allocated resources exclusively without
competing with other nodes. Typical protocols that belong in
this category include
 Frequency-division multiple access(FDMA)
 Time-division multiple access (TDMA)
 Code-division multiple access(CDMA)

Demand Assignment Protocols

 The main objective of demand assignment protocols is to
improve channel utilization by allocating the capacity of the
channel to contending nodes in an optimum or near-
optimum fashion. Unlike fixed-assignment schemes, where
channel capacity is assigned exclusively to the network
nodes in a predetermined fashion regardless of their current
communication needs, demand assignment protocols ignore
idle nodes and consider only nodes that are ready to
transmit.
 Polling
 Reservation

Random Assignment Protocols

 In fixed-assignment schemes, each communicating node is
assigned a frequency band in FDMA systems or a time slot
in TDMA systems. This assignment is static, however,
regardless of whether or not the node has data to transmit.
These schemes may therefore be inefficient if the traffic
sourceis bursty. In the absence of data to be transmitted, the
node remains idle, thereby resulting in the allocated
bandwidth to be wasted. Random assignment strategies
attempt to address this shortcoming by eliminating
preallocation of bandwidth to communicating nodes.

Random Assignment Protocols

 Random assignment strategies do not exercise any
control to determine which communicating node can
access the medium next. Furthermore, these
strategies do not assign any predictable or scheduled
time for any node to transmit. To deal with
collisions,the protocol must include a mechanism to
detect collisions and a scheme to schedule colliding
packets for subsequent retransmissions.
 ALOHA
 CSMA
 CSMA/CD

Frequency Division Multiple Access
(FDMA)

 The FDMA scheme is used by radio systems to share the
radio spectrum.Based on this scheme, the available
bandwidth is divided into subchannels. Multiple channel
access is then achieved by allocating communicating nodes
with different carrier frequencies of the radio spectrum. The
bandwidth of each node’s carrier is constrained within
certain limits such that no interference, or overlap, occurs
between different nodes. The scheme requires frequency
synchronization among communicating nodes.

FDMA
 FDMA was the initial multiple-access

Code
technique for cellular systems
Separates large band into smaller
User 4
 User 3
channels. User 1
User 2

 Each channel has the ability to support
user.
Time
 Guard bands are used to separate

y
channel preventing co-channel
nc
interference ue
eq
Fr

 Narrow bandwidth (30 khz).

f1

FDMA
 FDMA is a continuous transmission scheme as compare to
TDMA because fewer bits are needed for synchronization
and framing.
 In FDMA, as unique channels are assigned to each user, so
FDMA systems have higher cell site system cost as
compared to TDMA system.
 In FDMA, both the transmitter and receiver operates at the
same time so FDMA Mobile units require duplexers. These
also increase the cost of FDMA subscriber units and base
station.

FDMA
 Advantages
– Simple to implement in terms of hardware.
– Fairly efficient with a small base population and with
constant traffic.
 Disadvantages
– Network and spectrum planning are intensive and
time consuming.
– Channels are dedicated for a single user, idle
channels add spectrum inefficiency.

Time Division Multiple Access (TDMA

 In TDMA, a radio spectrum is divided into time slots. These
time slots are allocated for each user to transmit and receive
information. The number of time slots is called a frame.
Information is transferred and received in form of frame. A
frame is consists a preamble, an information message and
trial bits.
 Preamble contains the address and synchronization
information of both subscriber and Base Station to identify
each other. Trial bits contain framing information.

Time Division Multiple Access (TDMA

 In TDMA, data transmission is not continuous and
subscriber transmitter can be turned off which result in low
battery consumption.
 In TDMA, handoff process is much simpler for a subscriber
because of discontinuous transmission.
 In TDMA, duplexers are not required because different
timeslots are used for transmission and reception.
 In TDMA, the rate of transmission is very high as compare
to FDMA.

TDMA
 Entire bandwidth is available to the
user for finite period of time.

Code
 Users are allotted time slots for a

4
er
channel allowing sharing of a single

3
er

Us
2
r1

Us
er
channel. e

Us
Us

 Requires time synchronization.
Time
 Each of the user takes turn in
transmitting and receiving data in a

cy
en
round robin fashion.
u
eq
Fr

How it works?
 User presses Push-to-Talk (PTT) button
 A control channel registers the radio to the closest base station.
 The BS assigns an available pair of channels.
 Unlike FDMA, TDMA system also assigns an available time slot
within the channel.
 Data transmission is not continuous rather sent and received in
bursts.
 The bursts are reassembled and appear like continuous
transmission.

TDMA Advantages & Disadvantages

 Advantages
– Extended battery life and talk time
– More efficient use of spectrum, compared to FDMA
– Will accommodate more users in the same spectrum space than an
FDMA system
 Disadvantages
– Network and spectrum planning are intensive
– Multipath interference affects call quality
– Dropped calls are possible when users switch in and out of different
cells.
– Too few users result in idle channels (rural versus urban
environment)
– Higher costs due to greater equipment

TDMA Frame Structure

 Each sequence of 8 time slots is known as a TDMA frame

Code Division Multiple Access (CDMA)

 In CDMA, all users transmit information simultaneously by
using the same carrier frequency. Each user has its own code
word, which is orthogonal to other users. To detect the
message, the receiver should know the codeword used by the
transmitter.

Code Division Multiple Access (CDMA)

 CDMA includes the following features:
 In CDMA system many users share the same frequency.
 In CDMA unlike FDMA and TDMA the number of users is
not limited. It has a soft capacity. But due to large number of
users its performance degrades.
 In CDMA, each user operates independently with no
knowledge of the other users.

CDMA

 CDMA is a spread

Code
spectrum technique used User 4

to increase spectrum User 4

efficiency. User 3

 SS has been used in User 2

military applications due to User 1
Time
anti-jamming and security.
y
nc
ue
eq
Fr

Code-Division Multiple Access (CDMA)
 We start with a data signal with rate D, Which we
call the bit data rate. We break each bit into k chips
according to a fixed pattern that is specific to each
user, called the user's code.The new channel has a
chip data rate of kD chips per second.

 Basic Principles of CDMA
– D = rate of data signal
– Break each bit into k chips
 Chips are a user-specific fixed pattern
– Chip data rate of new channel = kD

CDMA Example
 If k=6 and code is a sequence of 1s and -1s
– For a ‘1’ bit, A sends code as chip pattern
 <c1, c2, c3, c4, c5, c6>
– For a ‘0’ bit, A sends complement of code
 <-c1, -c2, -c3, -c4, -c5, -c6>
 Receiver knows sender’s code and performs electronic
decode function
 <d1, d2, d3, d4, d5, d6> = received chip pattern
 <c1, c2, c3, c4, c5, c6> = sender’s code

Su ( d ) = d1× c1 + d 2 × c 2 + d 3 × c3 + d 4 × c 4 + d 5 × c5 + d 6 × c6

CDMA Example
 User A code = <1, –1, –1, 1, –1, 1>
– To send a 1 bit = <1, –1, –1, 1, –1, 1>
– To send a 0 bit = <–1, 1, 1, –1, 1, –1>
 User B code = <1, 1, –1, – 1, 1, 1>
– To send a 1 bit = <1, 1, –1, –1, 1, 1>
 Receiver receiving with A’s code
– (A’s code) x (received chip pattern)
 User A ‘1’ bit: 6 -> 1
 User A ‘0’ bit: -6 -> 0
 User B ‘1’ bit: 0 ->6 unwanted signal ignored

CDMA
 Advantages
– Greatest spectrum efficiency:
– CDMA improves call quality by filtering out background noise, cross-talk, and
interference
– Simplified frequency planning - all users on a CDMA system use the same
radio frequency spectrum.
– Random Walsh codes enhance user privacy; a spread-spectrum advantage
– Precise power control increases talk time and battery size for mobile phones
 Disadvantages
– Backwards compatibility techniques are costly
– Currently, base station equipment is expensive
– Low traffic areas lead to inefficient use of spectrum and equipment resources

Random Access

 Random Access Methods
– more efficient way of managing medium access for
communicating short bursty messages
 in contrast to fixed-access schemes, each user gains access to
medium only when needed -has some data to send
 drawback: users must compete to access the medium (‘random
access’)
 collision of contending transmissions
 Random Access Methods in Wireless Networks
– can be divided into two groups:
 ALOHA based-no coordination between users
 carrier-sense based-indirect coordination -users sense
availability of medium before transmitting

Random Access
Collision Period

U ser 4

U ser 3

U ser 2 resched uled

U ser 1

Tim e

ALOHA-based Random Access

 user accesses medium as soon as it has a packet
ready to transmit
– after transmission, user waits a length of time > round-trip
delay in the network, for an ACK from the receiver
– if no ACK arrives, user waits a random interval of time (to
avoid repeated collision) and retransmits
 advantages:
– simple, no synchronization among users required
 disadvantages:
– low throughput under heavy load conditions
– probability of collision increases as number of users
increases
 max throughput = 18% of channel capacity

Slotted ALOHA
 time is divided into equal time slots –when a user
has a packet to transmit, the packet is buffered
and transmitted at the start of the next time slot
– BS transmits a beacon signal for timing, all users must
synchronize their clocks
 advantages:
– partial packet collision avoided
 Disadvantages
– throughput still quite low!
– there is either no collision or a complete collision
 max throughput = 36% of channel capacity

Reservation ALOHA
 Time slots are divided into reservation and transmission
slots / periods
– during reservation period, stations can reserve future slots in
transmission period
– reservation slot size << transmission slot size
– collisions occur only in reservation slots
 advantages:
– higher throughput under heavy loads
– max throughput up to 80% of channel capacity
 disadvantages:
– more demanding on users as they have to obtain / keep ‘reservation
list’ up-to-date
 R-Aloha is most commonly used in satellite systems
 satellite collects requests, complies ‘reservation list’ and finally
sends the list back to users

Carrier Sense Multiple Access with Collision
Detect (CSMA/CD)

 With CSMA/CD, when an Ethernet device attempts to
access the network to send data, the network interface on the
workstation or server checks to see if the network is quiet.
When the network is clear, the network interface knows that
transmission can begin. If it does not sense a carrier, the
interface waits a random amount of time before retrying. If
the network is quiet and two devices try sending data at the
same time, their signals collide. When this collision is
detected, both devices back off and wait a random amount of
time before retrying,

CSMA/CD Operation
 Carrier sense— Each computer on the LAN is always listening for traffic
on the wire to determine when gaps between frame transmissions occur.
 Multiple access— Any computer can begin sending data whenever it
detects that the network is quiet. (There is no traffic.)
 Collision detect— If two or more computers in the same CSMA/CD
network collision domain begin sending at the same time, the bit streams
from each sending computer interfere, or collide, with each other,
making each transmission unreadable. If this collision occurs, each
sending computer must be able to detect that a collision has occurred
before it has finished sending its frame.
 Each computer must stop sending its traffic as soon as it has detected the
collision and then wait some random length of time, called the
back-off algorithm, before attempting to retransmit the frame.

Carrier Sense Multiple Access
(CSMA)
 Disadvantages of ALOHA
– users do not listen to the channel before (and while)
transmitting
– suitable for networks with long propagation delays
 Carrier Sense Multiple Access
– polite version of ALOHA
– Listen to the channel before transmitting
 if sensed channel busy, back-off (defer transmission), and sense
channel again after a random amount of time
 if channel idle, transmit entire frame

Versions of CSMA

 Employs different node behaviour when
channel found busy
– non-persistent CSMA
– persistent CSMA
– 1-persistent CSMA
– p-persistent CSMA

Persistence Methods

 What should a station do if the channel is busy? What should
a station do if the channel is idle? 4 methods have been
devised to answer these questions: the I-persistent method,
the nonpersistent method, and the p-persistent method.
Figure shows the behavior of three persistence methods
when a station finds a channel busy.
 Persistent:- station sense the channel, if channel is ideal it transmits
data. if there is already some traffic going on that it does not transmit the
data. Keep sensing.

CSMA: Persistence Methods

 Behavior of 1-persistent, Nonpersistent, p-persistent method

Persistence Methods
 I-Persistent The I-persistent method is simple and straightforward. In
this method,after the station finds the line idle, it sends its frame
immediately (with probability I).This method has the highest chance of
collision because two or more stations may find the line idle and send
their frames immediately.
 Nonpersistent:- In the nonpersistent method, a station that has a frame
to send senses the line. If the line is idle, it sends immediately. If the line
is not idle, it waits a random amount of time and then senses the line
again. The nonpersistent approach reduces the chance of collision
because it is unlikely that two or more stations will wait the same
amount of time and retry to send simultaneously. However, this method
reduces the efficiency of the network because the medium remains idle
when there may be stations with frames to send.

Persistence Methods
 p-Persistent :- The p-persistent method is used if the channel has time
slots with a slot duration equal to or greater than the maximum
propagation time. The p-persistent approach combines the advantages of
the other two strategies. It reduces the chance of collision and improves
efficiency. In this method, after the station finds the line idle it follows
these steps:
 1. With probability p, the station sends its frame.
 2. With probability q = 1 - p, the station waits for the beginning of the
next time slot and checks the line again.
 a. If the line is idle, it goes to step 1.
 b. If the line is busy, it acts as though a collision has occurred and uses
the backoff procedure

Flow diagram for 1-persistent, Nonpersistent, p-persistent method

CSMA/CA (Collision Avoidance)
 Invented for wireless network where we cannot detect collisions
Collision are avoided through the use of CSMA/CA’s three strategies:
the interframe space, the contention windows, and acknowledgement

 IFS can also be used to define the priority of a station or a frame If the
station finds the channel busy, it does not restart the timer of the
contention window; it stops the timer and restarts it when the channel
becomes idle

CSMA / Collision Avoidance
 Used where CSMA/CD cannot be used
– e.g. in wireless medium collision cannot be easily detected
as power of transmitting overwhelms receiving antenna
– CSMA/CA is designed to reduce collision probability at points
where collisions would most likely occur
 when medium has become idle after a busy state, as several
users could have been waiting for medium to become available
– key elements of CSMA/CA:
 IFS –interframe spacing –priority mechanism–the shorter the
IFS the higher the priority for transmission
 CW intervals –contention window –intervals used for contention
and transmission of packet frames
 Backoff counter–used only if two or more stations compete for
transmission

CSMA/CA Algorithm

Fram e to
transm it

Med ium No Wait until
Id le? Trans end s

Yes
Wait IFS
Wait IFS

S till No If m ed ium becom es busy d uring the backoff
S till No Id le? tim e, the backoff tim er is halted and
Id le?
Yes resum es when the m ed ium becom es id le.
Yes Exp b/o while
Transm it fram e Med ium id le

Transm it fram e

Demand Assignment Protocols

 D e m a n d A s s ig n m e n t
P r o t o c o ls

 Polling
 Reservation

Polling

 A widely used demand assignment scheme is polling. In this
scheme, a master control device queries, in some
predetermined order, each slave node about whether it has
data to transmit. If the polled node has data to transmit, it
informs the controller of its intention to transmit. In
response, the controller allocates the channel to the ready
node, which uses the full data rate to transmit its traffic. If
the node being polled has no data to transmit, it declines the
controller’s request. In response, the controller proceeds to
query the next network node.

Polling

 The major drawback of polling is the substantial overhead
caused by the large number of messages generated by the
controller to query the communicating nodes. Furthermore,
the efficiency of the polling scheme depends on the
reliability of the controller.

 The main advantage of polling is that all nodes can receive
equal access to the channel.

Reservation

 The basic idea in a reservation-based scheme is to set some time slots
for carrying reservation messages. Since these messages are usually
smaller than data packets, they are called minislots. When a station has
data to send, it requests a data slot by sending a reservation message to
the master in a reservation minislot.
 In a reservation-based scheme, if each station has its own reservation
minislot,collision can be avoided. Moreover, if reservation requests
have a priority field, the master can schedule urgent data before delay-
insensitive data.
 Packet collisions can happen only when stations contend for the
minislot, which use only a small fraction of the total bandwidth. Thus,
the largest part of the bandwidth assigned to data packets is used
efficiently.

Spread Spectrum
 Spread spectrum is designed to be used in wireless applications.
 The spread spectrum technique was developed initially for
military and intelligence requirements. The essential idea is to
spread the information signal over a wider bandwidth to make
jamming and interception more difficult.
 In wireless applications, all stations use air (or a vacuum) as the
 medium for communication. Stations must be able to share this
medium without interception and jamming.

Spread Spectrum
 To achieve these goals, spread spectrum techniques add
redundancy; they spread the original spectrum needed for
each station. If the required bandwidth for each station is B,
spread spectrum expands it to Bss' such that Bss » B. The
expanded bandwidth allows the source to wrap its message
in a protective envelope for a more secure transmission
 Eg:. expensive gift. We can insert the gift in a special box to prevent it
from being damaged during transportation, and we can use a superior
delivery service to guarantee the safety of the package.

Spread Spectrum
 Spread spectrum achieves its goals through two principles.
 1. The bandwidth allocated to each station needs to be, by
far, larger than what is needed. This allows redundancy.

 2. The expanding of the original bandwidth B to the
bandwidth Bss must be done by a process that is independent
of the original signal. In other words, the spreading process
occurs after the signal is created by the source.

Spread Spectrum
 After the signal is created by the source, the spreading
process uses a spreading code and spreads the bandwidth.
 The spreading code is a series of numbers that look random,
but are actually a pattern.

Spread Spectrum
 Input is fed into a channel encoder that produces an analog
signal with a relatively narrow bandwidth around some
center frequency. This signal is further modulated using a
sequence of digits known as a spreading code or Spreading
sequence.

Spread Spectrum
 Spread-spectrum techniques are methods by which a signal (e.g. an
electrical, electromagnetic signal) generated in a particular bandwidth is
deliberately spread in the frequency domain, resulting in a signal with a
wider bandwidth. These techniques are used for a variety of reasons,
including the establishment of secure communications, increasing
resistance to natural interference and jamming, to prevent detection, and
to limit

Spread Spectrum

 Types of spreading:
– direct sequence spread spectrum (DSSS)
– frequency hopping spread spectrum (FHSS)

Frequency hopping spread spectrum
(FHSS)

 The frequency hopping spread spectrum (FHSS) technique uses M
different carrier frequencies that are modulated by the source signal.

 At one moment, the signal modulates one carrier frequency; at the next
moment, the signal modulates another carrier frequency.

 If an intruder tries to intercept the transmitted signal, she can only
 access a small piece of data because she does not know the spreading
sequence to quickly adapt herself to the next hop.

Frequency Hoping Spread Spectrum
(FHSS)
 Signal is broadcast over seemingly random series of radio
frequencies
– A number of channels allocated for the FH signal
– Width of each channel corresponds to bandwidth of input signal
 Signal hops from frequency to frequency at fixed intervals
– Transmitter operates in one channel at a time
– Bits are transmitted using some encoding scheme
– At each successive interval, a new carrier frequency is selected
 Channel sequence dictated by spreading code

 Receiver, hopping between frequencies in synchronization
with transmitter, picks up message
 Advantages
– Attempts to jam signal on one frequency succeed only at knocking
out a few bits

 Frequency Hopping Spread Spectrum
(FHSS)
FHSS

Frequency Hopping Spread
Spectrum (FHSS)

 Suppose we have decided to have eight hopping frequencies.
This is extremely low for real applications and is just for
illustration.
 In this case, Mis 8 and k is 3. The pseudorandom code
generator will create eight different 3-bit patterns. These are
mapped to eight different frequencies in the frequency table

FHSS Performance Considerations

 Large number of frequencies used
 Results in a system that is quite resistant to jamming
– Jammer must jam all frequencies
– With fixed power, this reduces the jamming power in any
one frequency band

Direct Sequence Spread Spectrum
(DSSS
 The direct sequence spread spectrum technique also
expands the bandwidth of the original signal, but the process
is different.
 In DSSS, we replace each data bit with n bits using a
spreading code. In other words, each bit is assigned a code
of n bits, called chips, where the chip rate is n times that of
the data

(DSSS)
 Each bit in original signal is represented by multiple bits in
the transmitted signal
 Spreading code spreads signal across a wider frequency band
– Spread is in direct proportion to number of bits used
 One technique combines digital information stream with the
spreading code bit stream using exclusive-OR

DSSS

 Direct Sequence Spread Spectrum (DSSS)
 Replace each data bit with n bits using a spreading code
 Each bit is assigned a code of n bits called chips

(DSSS)

Common protocols

More Related Content

What's hot

Viewers also liked

Similar to Common protocols

More from Mr SMAK

Recently uploaded

Common protocols

Editor's Notes