unit ii - mr.rajiv bhandari | "never defeat people just win … ii overview of wireless...

Unit II

Overview of Wireless Networks

10/1/2014 1 Prof. Prashant Lahane

History of wireless communication

• Guglielmo Marconi invented the wireless telegraph in 1896 – Communication by encoding alphanumeric characters in analog

signal

– Sent telegraphic signals across the Atlantic Ocean

• 1914 – first voice communication over radio waves

• Communications satellites launched in 1960s

• Advances in wireless technology – Radio, television, mobile telephone, communication satellites

• More recently – Satellite communications, wireless networking, cellular technology


What is Wireless Communication ?

• Transmitting voice and data using electromagnetic waves in open space (atmosphere)

• Electromagnetic waves • Travel at speed of light (c = 3x108 m/s)

• Has a frequency (f) and wavelength (l)

» c = f x l

• Higher frequency means higher energy photons

• The higher the energy photon the more penetrating is the radiation


Types of wireless communication

celullar wireless computer network radio service


Electromagnetic Waves & the Electromagnetic Spectrum


Electromagnetic Waves • Transverse waves without a medium!

• (They can travel through empty space)


• They travel as vibrations in electrical and magnetic fields.

– Have some magnetic and some electrical properties to them.


• When an electric field changes, so does the magnetic field. The changing magnetic field causes the electric field to change. When one field vibrates—so does the other.

• RESULT-An electromagnetic wave.


• Electromagnetic waves travel VERY FAST – around 300,000 kilometres per

second (the speed of light).

At this speed they can go around the world 8 times in one second.


• Waves or Particles?

• Electromagnetic radiation has properties of waves but also can be thought of as a stream of particles.

– Example: Light • Light as a wave: Light behaves as a transverse wave

which we can filter using polarized lenses.

• Light as particles (photons): When directed at a substance light can knock electrons off of a substance (Photoelectric effect)


• Electromagnetic Spectrum—name for the range of electromagnetic waves when placed in order of increasing frequency

RADIO

WAVES

MICROWAVES

INFRARED

RAYS

VISIBLE LIGHT

ULTRAVIOLET

RAYS

X-RAYS

GAMMA

RAYS


Notice the wavelength is long (Radio waves) and gets shorter (Gamma Rays)


RADIO WAVES

• Have the longest wavelengths and

lowest frequencies of all the

electromagnetic waves.


• Global Positioning Systems (GPS) measures the time it takes a radio wave to travel from several

satellites to the receiver, determining the distance to each satellite.


• A radio picks up radio waves through an antenna and converts it to sound waves.

– Each radio station in an area broadcasts at a different frequency.

• # on radio dial tells frequency.


MRI (MAGNETIC RESONACE IMAGING)

Uses Short wave radio waves with a magnet to create an image.


MICROWAVES

• Have the shortest wavelengths and

the highest frequency of the

radio waves.


Used in microwave ovens.

• Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.


RADAR (Radio Detection and Ranging)

• Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.


INFRARED RAYS

• Infrared= below red

• Shorter wavelength and higher frequency than microwaves.


• You can feel the longest ones as heat on your skin Warm objects give off more heat energy than cool objects.


Thermogram—a picture that shows regions of different temperatures in the body. Temperatures are calculated by the

amount of infrared radiation given off.

•Therefore people give off infrared rays.

•Heat lamps give off infrared waves.


http://www.snellinfrared.com/library/_view_msg.asp?refid=19

VISIBLE LIGHT

• Shorter wavelength and higher frequency than infrared rays.

• Electromagnetic waves we can see.

• Longest wavelength= red light

• Shortest wavelength= violet (purple) light


Spread Spectrum Systems


Spread Spectrum

• Analog or digital data

• Analog signal

• Spread data over wide bandwidth

• Makes jamming and interception harder

• Frequency hoping

– Signal broadcast over seemingly random series of frequencies

• Direct Sequence

– Each bit is represented by multiple bits in transmitted signal

– Chipping code


Spread Spectrum Concept

• Input fed into channel encoder – Produces narrow bandwidth analog signal around central frequency

• Signal modulated using sequence of digits – Spreading code/sequence

– Typically generated by pseudonoise/pseudorandom number generator

• Increases bandwidth significantly – Spreads spectrum

• Receiver uses same sequence to demodulate signal

• Demodulated signal fed into channel decoder


General Model of Spread Spectrum System


Gains

• Immunity from various noise and multipath distortion – Including jamming

• Can hide/encrypt signals – Only receiver who knows spreading code can retrieve

signal

• Several users can share same higher bandwidth with little interference – Cellular telephones – Code division multiplexing (CDM) – Code division multiple access (CDMA)


Modem Switching Techniques


Switching Techniques

• In large networks there might be multiple

paths linking sender and receiver.

• Information may be switched as it travels

through various communication channels.

• There are three typical switching techniques

available for digital traffic.

• Circuit Switching

• Message Switching

• Packet Switching


Circuit Switching

• Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.

• Telephone switching equipment establishes a path that connects the caller to the receiver by physical connection.

• Once a connection is established, a dedicated path exists between both ends until the connection is terminated.

• Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.


Circuit switching Advantages:

• The communication channel (once established) is

dedicated.

Disadvantages:

• Possible long wait to establish a connection,

(10 seconds, more on long- distance or international

calls) during which no data can be transmitted.

• More expensive than any other switching techniques,

because a dedicated path is required for each

connection.

• Inefficient use of the communication channel,

because the channel is not used when the connected

systems are not using it.


Message Switching

• With message switching there is no need to establish a dedicated path between two stations.

• When a station sends a message, the destination address is appended to the message.

• The message is then transmitted through the network, from node to node.

• Each node receives the entire message, stores it on disk, and then transmits the message to the next node.

• This type of network is called a store-and-forward network.


Message Switching

• A message-switching node is typically a general-purpose computer.

• The device needs sufficient secondary-storage capacity to store the

incoming messages, which could be long.

• A time delay is introduced using this type of scheme due to store- and-

forward time, plus the time required to find the next node in the

transmission path.


Message Switching

Advantages:

• Channel efficiency can be greater compared to circuit-

switched systems, because more devices are sharing

the channel.

• Traffic congestion can be reduced, because messages

may be temporarily stored in route.

• Message priorities can be established due to store-and-

forward technique.

• Message broadcasting can be achieved with the use of

broadcast address appended in the message.


Message Switching

Disadvantages

• Message switching is not compatible with

interactive applications.

• Store-and-forward devices are expensive,

because they must have large disks to hold

potentially long messages.


Packet Switching

• It tries to combine the advantages of message and circuit switching and

to minimize the disadvantages of both.

• There are two methods of packet switching: Datagram and virtual

circuit.


Packet Switching

• In both packet switching methods, a message is broken

into small parts, called packets.

• Each packet is tagged with appropriate source and

destination addresses.

• Since packets have a strictly defined maximum length,

they can be stored in main memory instead of disk,

therefore access delay and cost are minimized.

• Also the transmission speeds, between nodes, are

optimized.

• With current technology, packets are generally accepted

onto the network on a first-come, first-served basis. If the

network becomes overloaded, packets are delayed or

discarded (``dropped''). 10/1/2014 40 Prof. Prashant Lahane

Packet switching • Analog signal from your phone is converted into a digital data

stream.

• That series of digital bits is then divided into relatively tiny clusters of bits, called packets.

• Each packet has at its beginning the digital address -- a long number -- to which it is being sent. The system blasts out all those tiny packets, as fast as it can, and they travel across the nation's digital backbone systems to their destination: the telephone, or rather the telephone system, of the person you're calling.

• They do not necessarily travel together; they do not travel sequentially. They don't even all travel via the same route. But eventually they arrive at the right point -- that digital address added to the front of each string of digital data -- and at their destination are reassembled into the correct order, then converted to analog form, so your friend can understand what you're saying.


Packet Switching: Datagram • Datagram packet switching is similar to message

switching in that each packet is a self-contained unit with

complete addressing information attached.

• This fact allows packets to take a variety of possible

paths through the network.

• So the packets, each with the same destination address,

do not follow the same route, and they may arrive out of

sequence at the exit point node (or the destination).

• Reordering is done at the destination point based on the

sequence number of the packets.

• It is possible for a packet to be destroyed if one of the

nodes on its way is crashed momentarily. Thus all its

queued packets may be lost.


Packet Switching: Virtual Circuit

• In the virtual circuit approach, a preplanned route is

established before any data packets are sent.

• A logical connection is established when o a sender send a "call request packet" to the receiver and

o the receiver send back an acknowledge packet "call accepted

packet" to the sender if the receiver agrees on

conversational parameters.

• The conversational parameters can be maximum packet

sizes, path to be taken, and other variables necessary to

establish and maintain the conversation.

• Virtual circuits imply acknowledgements, flow control, and

error control, so virtual circuits are reliable.

• That is, they have the capability to inform upper-protocol

layers if a transmission problem occurs.


Packet Switching:Virtual Circuit • In virtual circuit, the route between stations does not

mean that this is a dedicated path, as in circuit

switching.

• A packet is still buffered at each node and queued for

output over a line.

• The difference between virtual circuit and datagram

approaches:

• With virtual circuit, the node does not need to make

a routing decision for each packet.

• It is made only once for all packets using that virtual

circuit.


Packet Switching: Virtual Circuit

VC guarantees that

• the packets sent; arrive in the order sent

• with no duplicates or omissions

• with no errors (with high probability) regardless of how they are implemented internally.


Advantages of packet switching Advantages:

1. Packet switching is cost effective, because switching

devices do not need massive amount of secondary storage.

2. Packet switching offers improved delay characteristics,

because there are no long messages in the queue

(maximum packet size is fixed).

3. Packet can be rerouted if there is any problem, such as,

busy or disabled links.

4. The advantage of packet switching is that many network

users can share the same channel at the same time. Packet

switching can maximize link efficiency by making optimal

use of link bandwidth.


Disadvantages of packet switching Disadvantages:

1. Protocols for packet switching are typically more

complex.

2. It can add some initial costs in implementation.

3. If packet is lost, sender needs to retransmit the data.

4. Another disadvantage is that packet-switched systems

still can’t deliver the same quality as dedicated circuits

in applications requiring very little delay - like voice

conversations or moving images.


Hardware Components


Transmitter

Modulator

Power Amplifier

Driver Stages

Power Stage

Antenna

Oscillator

Signal


Receiver

Low Noise Amplifier

Low Noise Amplifier

Gain-Stage Amplifier

Filter

Antenna Oscillator

Mixer

Filter


Transceiver = Transmitter + Receiver

Modulator

Power Amplifier

Antenna

Oscillator

Signal

Low Noise Amplifier

Filter

Mixer

Filter


Access Points

• An access point (AP) is a transceiver that connects to an Ethernet cable

– It bridges the wireless network with the wired network

• Not all wireless networks connect to a wired network

– Most companies have Wireless LANs (WLANs) that connect to their wired network topology


Access Points

• The AP is where channels are configured

• An AP enables users to connect to a LAN using wireless technology

– An AP is available only within a defined area


Configuring an Access Point

• Configuring an AP varies depending on the hardware

– Most devices allow access through any Web browser

– Enter IP address on your Web browser and provide your user logon name and password


Wireless Router

• A wireless router includes an access point, a router, and a switch


• A wireless router is a device that performs the

functions of a router but also includes the functions of

a wireless access point.

• It is commonly used to provide access to the Internet

or a computer network.

• It does not require a wired link, as the connection is

made wirelessly, via radio waves.

• It can function in a wired LAN (local area network),

in a wireless-only LAN (WLAN), or in a mixed

wired/wireless network, depending on the

manufacturer and model.

Wireless Router


OSI Model Layers

Physical

Data Link

Network

Transport

Session

Presentation

Application

Transmission of binary data of a medium

Transfer of units of information, framing, and error checking

Delivery of packets of information, which includes routing

Provision for end-to-end reliable and unreliable delivery

Establishment and maintenance of sessions

Data formatting and encryption

Network applications such as file transfer and terminal

emulation

OSI layer Function provided

Layer 7 - presents application to users; Layers 3-6 - provides Common Language for

communication; Layers 1-2 - provides the physical connection.


Data Link Layer

• The Data Link Layer addresses groups of bits to a device located across a single physical transmission path, called a link.

• Each group of bits that the Data Link Layer transmits is called a frame.

• To form a frame, the Data Link Layer encapsulates a Network Layer packet within a header and trailer.

• The header contains the hardware address of the destination node.

• The trailer contains a Frame Check Sequence (FCS) value that the receiving node uses for error detection.

• The Data Link Layer is the only OSI layer that adds a trailer to the data it transmits.


Data Link Layer

• Each frame carries a packet of data across a single physical link.

• The encapsulated packet does not change, but a new frame is built around the packet for the trip across each link.

• Thus, we often say that the Data Link Layer is concerned with transmitting data to the next node in the network.

• Popular Data Link protocols include:

– High-Level Data Link Control (HDLC)

– Synchronous Data Link Control (SDLC)

– Link Access Procedure for D channel (LAPD), used in ISDN

– LAN protocols such as Ethernet, Token Ring, and FDDI

– WAN protocols such as frame relay, ATM, and ISDN 10/1/2014 60 Prof. Prashant Lahane

• Services Provided to the Network Layer

• Framing

• Error Control

• Flow Control

Data Link Layer Design Issues


• Provide service interface to the network layer

• Dealing with transmission errors

• Regulating data flow

• Slow receivers not swamped by fast senders

Functions of the Data Link Layer


• Unacknowledged Connectionless service

• Acknowledged Connectionless service

• Acknowledged Connection-Oriented service

Types of services provided to the Network Layer


• Losses are taken care of at higher layers

• Used on reliable medium like coax cables or optical fiber, where the error rate is low.

• Appropriate for voice, where delay is worse than bad data.

Unacknowledged Connectionless service


• Useful on unreliable medium like wireless.

• Acknowledgements add delays.

• Adding ack in the DLL rather than in the NL is just an optimization and not a requirement. Leaving it for the NL is inefficient as a large message (packet) has to be resent in contrast to small frames.

• On reliable channels, like fiber, the overhead associated with the ack is not justified.

Acknowledged Connectionless service


• Most reliable,

• Guaranteed service –

– Each frame sent is indeed received

– Each frame is received exactly once

– Frames are received in order

• Special care has to be taken to ensure this in connectionless services

Acknowledged Connection-oriented service


• Character Count

• Flag bytes with byte stuffing

• Flag bytes with bit stuffing

Framing


A character stream

(a) Without errors.

(b) With one error.

Framing with Character Count


• What if the count is garbled

• Even if with checksum, the receiver knows that the frame is bad, there is no way to tell where the next frame starts.

• Asking for retransmission doesn’t help either because the start of the retransmitted frame is not known

• No longer used

Problem with Framing with CC


Framing with byte stuffing


• Problem:

– fixed character size: assumes character size to be 8 bits: can’t handle heterogeneous environment.

Framing with byte stuffing


Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after

destuffing.

Framing with bit stuffing


Flow and Error Control


Flow Control

• Flow control coordinates the amount of data that can be sent before receiving acknowledgement

• It is one of the most important functions of data link layer.

• Flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgement from the receiver.

• Receiver has a limited speed at which it can process incoming data and a limited amount of memory in which to store incoming data.

• Receiver must inform the sender before the limits are reached and request that the transmitter to send fewer frames or stop temporarily.

• Since the rate of processing is often slower than the rate of transmission, receiver has a block of memory (buffer) for storing incoming data until they are processed.


Error Control

• Error control includes both error detection and error correction.

• It allows the receiver to inform the sender if a frame is lost or damaged during transmission and coordinates the retransmission of those frames by the sender.

• Error control in the data link layer is based on automatic repeat request (ARQ). Whenever an error is detected, specified frames are retransmitted.


Taxonomy of protocols


NOISELESS CHANNELS

Let us first assume we have an ideal channel in which

no frames are lost, duplicated, or corrupted. We

introduce two protocols for this type of channel.

Simplest Protocol

Stop-and-Wait Protocol


The design of the simplest protocol with no flow or error control


• The sender sends a sequence

of frames without even

thinking about the receiver.

• To send three frames, three

events occur at the sender site

and three events at the

receiver site.

• Note that the data frames are

shown by tilted boxes; the

height of the box defines the

transmission time difference

between the first bit and the

last bit in the frame.

Simplest Protocol


Design of Stop-and-Wait Protocol


• The sender sends

one frame and waits

for feedback from

the receiver.

• When the ACK

arrives, the sender

sends the next

frame.

Stop-and-Wait Protocol


NOISY CHANNELS

Although the Stop-and-Wait Protocol gives us an idea of

how to add flow control to its predecessor, noiseless

channels are nonexistent. We discuss three protocols in

this section that use error control.

Stop-and-Wait Automatic Repeat Request

Go-Back-N Automatic Repeat Request

Selective Repeat Automatic Repeat Request


Error correction in Stop-and-Wait ARQ is

done by keeping a copy of the sent frame

and retransmitting of the frame when the

timer expires.

Note


Sequence Numbers

• Frames from a sender are numbered sequentially.

• We need to set a limit since we need to include the sequence number of each frame in the header.

• If the header of the frame allows m bits for sequence number, the sequence numbers range from 0 to 2 m – 1. for m = 3, sequence numbers are: 1, 2, 3, 4, 5, 6, 7.

• We can repeat the sequence number.

• Sequence numbers are:

0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, …


In Stop-and-Wait ARQ, we use sequence

numbers to number the frames.

The sequence numbers are based on modulo-2

arithmetic.

Note


In Stop-and-Wait ARQ, the

acknowledgment number always

announces in modulo-2 arithmetic the

sequence number of the next frame

expected.

Note


Design of the Stop-and-Wait ARQ Protocol


Stop-and-Wait ARQ Protocol

• Frame 0 is sent and

acknowledged.

• Frame 1 is lost and resent

after the time-out.

• The resent frame 1 is

acknowledged and the timer

stops.

• Frame 0 is sent and

acknowledged, but the

acknowledgment is lost.

• The sender has no idea if the

frame or the acknowledgment

is lost, so after the time-out, it

resends frame 0, which is

acknowledged.


In the Go-Back-N Protocol, the sequence

numbers are modulo 2m,

where m is the size of the sequence

number field in bits.

Note


Send (sliding) window for Go-Back-N ARQ


The send window is an abstract concept

defining an imaginary box of size 2m − 1

with three variables: Sf, Sn, and Ssize.

Note


The send window can slide one

or more slots when a valid acknowledgment

arrives.

Note


Receive (sliding) window for Go-Back-N ARQ


The receive window is an abstract concept

defining an imaginary box

of size 1 with one single variable Rn.

The window slides

when a correct frame has arrived; sliding

occurs one slot at a time.

Note


Design of Go-Back-N ARQ


Window size for Go-Back-N ARQ


In Go-Back-N ARQ, the size of the send

window must be less than 2m;

the size of the receiver window

is always 1.

Note


• This is an example of a case where

the forward channel is reliable,

but the reverse is not.

• No data frames are lost, but some

ACKs are delayed and one is lost.

• The example also shows how

cumulative acknowledgments can

help if acknowledgments are

delayed or lost.

• After initialization, there are seven

sender events.

• Request events are triggered by

data from the network layer;

arrival events are triggered by

acknowledgments from the

physical layer.

• There is no time-out event here

because all outstanding frames are

acknowledged before the timer

expires. Note that although ACK 2

is lost, ACK 3 serves as both ACK

2 and ACK 3.


• NEXT slide shows what happens when a frame is lost. Frames

0, 1, 2, and 3 are sent. However, frame 1 is lost.

• The receiver receives frames 2 and 3, but they are discarded

because they are received out of order.

• The sender receives no acknowledgment about frames 1, 2, or

3. Its timer finally expires.

• The sender sends all outstanding frames (1, 2, and 3) because

it does not know what is wrong.

• Note that the resending of frames 1, 2, and 3 is the response

to one single event. When the sender is responding to this

event, it cannot accept the triggering of other events. This

means that when ACK 2 arrives, the sender is still busy with

sending frame 3.

Example of Forward Channel Not Reliable


• The physical layer must wait until this event is

completed and the data link layer goes back to its

sleeping state.

• We have shown a vertical line to indicate the delay.

It is the same story with ACK 3; but when ACK 3

arrives, the sender is busy responding to ACK 2.

• It happens again when ACK 4 arrives.

• Note that before the second timer expires, all

outstanding frames have been sent and the timer is

stopped.

Example of Forward Channel Not Reliable

(cont…)


Example of Forward channel not reliable


Stop-and-Wait ARQ is a special case of Go-

Back-N ARQ in which the size of the send

window is 1.

Note


Send window for Selective Repeat ARQ


Receive window for Selective Repeat ARQ


Design of Selective Repeat ARQ


Selective Repeat ARQ, window size


In Selective Repeat ARQ, the size of the

sender and receiver window

must be at most one-half of 2m.

Note


108

The Medium Access Control Sublayer

10/1/2014 Prof. Prashant Lahane

109

The Medium Access Control Sublayer

• In the literature, broadcast channels are sometimes referred to as multiaccess channels or random access channels.

• The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.


110

The Channel Allocation Problem

• To allocate a single broadcast channel among competing users, we can use:

– Static Channel Allocation in LANs and MANs

– Dynamic Channel Allocation in LANs and MANs


111

Static Channel Allocation in LANs and MANs

• Frequency Division Multiplexing (FDM) is an example of static channel allocation where the bandwidth is divided among a number of N users.

• When there is only a small and constant number of users, each of which has a heavy (buffered) load of traffic (e.g., carriers' switching offices), FDM is a simple and efficient allocation mechanism.


Static Channel Allocation (cont…)

However, when the number of senders is large

and continuously varying or the traffic is bursty,

FDM presents some problems.

1) when fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted.

2) when more users wants to communicate, those who have not been assigned a frequency will be denied permission.

3) even assuming that the number of users could somehow be held constant at N, each user traffic usually changes dynamically over time.


113

Dynamic Channel Allocation in LANs and MANs

1. Station Model: N independent stations (terminals) exists.

2. Single Channel Assumption: A single channel is available for all communication (send and receive)

3. Collision Assumption: If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled (collision). No errors other than those generated by collisions assumed to exist.


Dynamic Channel Allocation (cont…)

4. (a) Continuous Time: Frame transmission can begin at any instant.

(b) Slotted Time: Frame transmissions always begin at the start of a slot where the time is divided into discrete slots.

5. (a) Carrier Sense: Stations can tell if the channel is in use before trying to use it.

(b) No Carrier Sense: Stations cannot sense the channel before trying to use it.


Multiple Access Protocols


Data link layer divided into two functionality-oriented sublayers

Link Layer Control (LLC)

MAC

Responsible for error

and flow control

Control

Responsible framing

and MAC address and

Multiple Access Control


Multiple Access

Problem: When two or more nodes transmit at the same time, their frames

will collide and the link bandwidth is wasted during collision

How to coordinate the access of multiple sending/receiving nodes to the

shared link???

• Solution: We need a protocol to coordinate the transmission of the active

nodes

• These protocols are called Medium or Multiple Access Control (MAC)

Protocols belong to a sublayer of the data link layer called MAC (Medium

Access Control)

• What is expected from Multiple Access Protocols:

– Main task is to minimize collisions in order to utilize the bandwidth by:

• Determining when a station can use the link (medium)

• what a station should do when the link is busy

• what the station should do when it is involved in collision


Taxonomy of multiple-access protocols

For wireless not

included with us


Random Access

• Random Access (or contention) Protocols: – No station is superior to another station and none

is assigned the control over another.

– A station with a frame to be transmitted can use the link directly based on a procedure defined by the protocol to make a decision on whether or not to send.


ALOHA Protocols • Was designed for wireless LAN and can be used for any

shared medium

• Pure ALOHA Protocol Description – All frames from any station are of fixed length (L bits)

– Stations transmit at equal transmission time (all stations produce frames with equal frame lengths).

– A station that has data can transmit at any time

– After transmitting a frame, the sender waits for an acknowledgment for an amount of time (time out) equal to the maximum round-trip propagation delay = 2* tprop(see next slide)

– If no ACK was received, sender assumes that the frame or ACK has been destroyed and resends that frame after it waits for a random amount of time

– If station fails to receive an ACK after repeated transmissions, it gives up

– Channel utilization or efficiency or Throughput is the percentage of the transmitted frames that arrive successfully (without collisions) or the percentage of the channel bandwidth that will be used for transmitting frames without collisions

– ALOHA Maximum channel utilization is 18% (i.e, if the system produces F frames/s, then 0.18 * F frames will arrive successfully on average without the need of retransmission).


Maximum Propagation Delay • Maximum propagation delay(tprop): time it takes for a bit of a frame

to travel between the two most widely separated stations.

The farthest

station

Station B

receives the

first bit of

the frame at

time t= tprop


Procedure for ALOHA protocol


Critical time for pure ALOHA protocol

If the frame transmission time is T sec, then the vulnerable

time is = 2 T sec.

This means no station should send during the T-sec before this

station starts transmission and no station should start sending

during the T-sec period that the current station is sending.

Tfr= Frame

Transmission time


Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times. 10/1/2014 124 Prof. Prashant Lahane

125

ALOHA network


126

Procedure for ALOHA protocol


Random Access – Slotted ALOHA

• Time is divided into slots equal to a frame transmission

time (Tfr)

• A station can transmit at the beginning of a slot only

• If a station misses the beginning of a slot, it has to wait

until the beginning of the next time slot.

• A central clock or station informs all stations about the

start of a each slot

• Maximum channel utilization is 37%


In danger time for slotted ALOHA protocol


Random Access – Slotted

ALOHA


Advantage of ALOHA protocols

• A node that has frames to be transmitted can transmit

continuously at the full rate of channel (R bps) if it is

the only node with frames

• Simple to be implemented

• No master station is needed to control the medium

Disadvantage

• If (M) nodes want to transmit, many collisions can

occur and the rate allocated for each node will not be on

average R/M bps

• This causes low channel utilization 10/1/2014 130 Prof. Prashant Lahane

Random Access – Carrier Sense Multiple Access (CSMA)

• To improve performance, avoid transmissions that are certain to cause

collisions

• Based on the fact that in LAN propagation time is very small

• If a frame was sent by a station, All stations knows immediately so they can

wait before start sending

– A station with frames to be sent, should sense the medium for the

presence of another transmission (carrier) before it starts its own

transmission

• This can reduce the possibility of collision but it cannot eliminate it.

– Collision can only happen when more than one station begin transmitting

within a short time (the propagation time period)


Random Access – Carrier Sense Multiple Access

(CSMA)

Vulnerable time for CSMA is the maximum propagation time

The longer the propagation delay, the worse the performance of the

protocol because of the above case.


CSMA/CD (Collision Detection)

CSMA has an inefficiency:

If a collision has occurred, the channel is unstable until

colliding packets have been fully transmitted

CSMA/CD (Carrier Sense Multiple Access with Collision

Detection) overcomes this as follows:

While transmitting, the sender is listening to medium for

collisions.

Sender stops transmission if collision has occurred

reducing channel wastage .

CSMA/CD is Widely used for bus topology LANs (IEEE 802.3,

Ethernet).


of its own

signal, it means collision occurred


CSMA/CD Protocol

• If a collision is detected by a station during

its transmission then it should do the

following:

– Abort transmission and

– Transmit a jam signal (48 bit) to notify other stations

of collision so that they will discard the transmitted

frame also to make sure that the collision signal will

stay until detected by the furthest station

– After sending the jam signal, backoff (wait) for a

random amount of time, then

– Transmit the frame again


CSMA/CD

Restrictions of CSMA / CD:

Packet transmission time should be at least as long as

the time needed to detect a collision (2 * maximum

propagation delay + jam sequence transmission time)

Otherwise, CSMA/CD does not have an advantage over

CSMA


Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

• Procedure

– Similar to CSMA but instead of sending packets control frames are exchanged

– RTS = request to send

– CTS = clear to send

– DATA = actual packet

– ACK = acknowledgement



• Advantages

– Small control frames lessen the cost of collisions (when data is large)

– RTS + CTS provide “virtual” carrier sense which protects against hidden terminal collisions (where A can’t hear B)

A B



• Disadvantages

– Not as efficient as CSMA-CD

– Doesn’t solve all the problems of MAC in wireless networks (more to come)


unit ii - mr.rajiv bhandari | "never defeat people just win … ii overview of wireless...

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