This document provides an overview of high-speed local area networks (LANs) and wireless LANs. It discusses the fundamentals and evolution of Ethernet, including the development of faster speeds like Fast Ethernet, Gigabit Ethernet, and 10-Gbps Ethernet. It also covers wireless LAN standards and configurations, focusing on IEEE 802.11. Key concepts explained include carrier sense multiple access with collision detection (CSMA/CD), persistence algorithms, binary exponential backoff, and wireless LAN architectures like basic service sets, access points, and extended service sets.
Carrier-sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet technology for local area networking.Carrier-sense multiple access with collision detection is a media access control method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting.
This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
4.2 Multiple Access Protocols
How to handle contention for the use of a channel.
4.3 IEEE Standards for LANs
How do the protocols of the last sections apply to real systems. Here we talk about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.
Carrier-sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet technology for local area networking.Carrier-sense multiple access with collision detection is a media access control method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting.
This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
4.2 Multiple Access Protocols
How to handle contention for the use of a channel.
4.3 IEEE Standards for LANs
How do the protocols of the last sections apply to real systems. Here we talk about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.
Carrier-sense multiple access with collision avoidance. It is a networking presentation.Here describe how CSMA/CA works to avoid collision.It helps a beginner student to understand who its work.
Carrier Sense Multiple Access With Collision Detection (CSMA/CD) Details : Me...Soumen Santra
Media Access Protocol (MAC)
Carrier Sense Multiple Access With Collision Detection (CSMA/CD)
Definition
Introduction
Features
Principle
Flowchart
Collision Mechanism
COLLISION DETECTION METHODS
Slot Time
Non-Persistent CSMA/CD
Efficiency
Advantages
Disadvantages
Detail Discussion with Mathematical Formula
This presentation gives you the basic understanding about the simplex stop and wait protocol. It contains stop and wait ARQ and algorithms for stop and wait ARQ, and simplex stop and wait ARQ. Moreover it contains the case studies to make readers understand the protocol easily.
ContikiMAC : Radio Duty Cycling ProtocolSalah Amean
Several MAC duty-cycle protocols have been proposed during the last decade to
address specific WSNs requirements and constraints such as a
low energy consumption linked to battery operated nodes
Radio Duty-Cycle (RDC) MAC protocols try to reduce the energy consumption by allowing a node to keep its radio-transceiver off most of the time.
This allow a node to avoid to keep the radio on unnecessarily, i.e when not involved in any transmission
Idle listening is used to solve such problem in which RDC MAC forces node to switch its transceiver between short active(listen) periods and long inactive (sleep) periods
. This presentation introduces a very important concept in wireless sensor network, particularly in handling transmission and reception of packets in very limited resources channel.
Carrier-sense multiple access with collision avoidance. It is a networking presentation.Here describe how CSMA/CA works to avoid collision.It helps a beginner student to understand who its work.
Carrier Sense Multiple Access With Collision Detection (CSMA/CD) Details : Me...Soumen Santra
Media Access Protocol (MAC)
Carrier Sense Multiple Access With Collision Detection (CSMA/CD)
Definition
Introduction
Features
Principle
Flowchart
Collision Mechanism
COLLISION DETECTION METHODS
Slot Time
Non-Persistent CSMA/CD
Efficiency
Advantages
Disadvantages
Detail Discussion with Mathematical Formula
This presentation gives you the basic understanding about the simplex stop and wait protocol. It contains stop and wait ARQ and algorithms for stop and wait ARQ, and simplex stop and wait ARQ. Moreover it contains the case studies to make readers understand the protocol easily.
ContikiMAC : Radio Duty Cycling ProtocolSalah Amean
Several MAC duty-cycle protocols have been proposed during the last decade to
address specific WSNs requirements and constraints such as a
low energy consumption linked to battery operated nodes
Radio Duty-Cycle (RDC) MAC protocols try to reduce the energy consumption by allowing a node to keep its radio-transceiver off most of the time.
This allow a node to avoid to keep the radio on unnecessarily, i.e when not involved in any transmission
Idle listening is used to solve such problem in which RDC MAC forces node to switch its transceiver between short active(listen) periods and long inactive (sleep) periods
. This presentation introduces a very important concept in wireless sensor network, particularly in handling transmission and reception of packets in very limited resources channel.
SLOTTED ALOHA and pure aloha are the category of alohaAkshathaM29
The data link layer is used in a computer network to transmit the data between two devices or nodes. It divides the layer into parts such as data link control and the multiple access resolution/protocol. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control. Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. Hence it is termed as media access control or the multiple access resolutions. ALOHA, an acronym for Advocates of Linux Open-source Hawaii Association, is a multiple access protocol that allows data to be transmitted over a public network channel. It operates within the Medium Access Control (MAC) sublayer of the Open Systems Interconnection (OSI) model.When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices. In that case, multiple stations access the channel and simultaneously transmits the data over the channel. It may create collision and cross talk. Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels.
ALOHA is a multiple access protocol for transmission of data via a shared network channel. It operates in the medium access control sublayer (MAC sublayer) of the open systems interconnection (OSI) model. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel.
this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station. Depending on the channel's state (idle or busy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by the receiver end.
In ALOHA, each node or station transmits a frame without trying to detect whether the transmission channel is idle or busy. If the channel is idle, then the frames will be successfully transmitted. If two frames attempt to occupy the channel simultaneously, collision of frames will occur and the frames will be discarded. These stations may choose to retransmit the corrupted frames repeatedly until successful transmission occurs
In pure ALOHA, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
Slotted ALOHA reduces the number of collisions and doubles the capacity of pure ALOHA. The shared channel is divided into a number of discrete time intervals
Introduction to Multiple Access Protocol.pptxupamatechverse
Data link layer is divided into two sub layers:
Logical link control (LLC) layer: The upper sub layer is responsible for data link control i.e. for flow and error control.
Media access control (MAC) layer: The lower sub layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub layer.
Safalta Digital marketing institute in Noida, provide complete applications that encompass a huge range of virtual advertising and marketing additives, which includes search engine optimization, virtual communication advertising, pay-per-click on marketing, content material advertising, internet analytics, and greater. These university courses are designed for students who possess a comprehensive understanding of virtual marketing strategies and attributes.Safalta Digital Marketing Institute in Noida is a first choice for young individuals or students who are looking to start their careers in the field of digital advertising. The institute gives specialized courses designed and certification.
for beginners, providing thorough training in areas such as SEO, digital communication marketing, and PPC training in Noida. After finishing the program, students receive the certifications recognised by top different universitie, setting a strong foundation for a successful career in digital marketing.
How to Make a Field invisible in Odoo 17Celine George
It is possible to hide or invisible some fields in odoo. Commonly using “invisible” attribute in the field definition to invisible the fields. This slide will show how to make a field invisible in odoo 17.
Embracing GenAI - A Strategic ImperativePeter Windle
Artificial Intelligence (AI) technologies such as Generative AI, Image Generators and Large Language Models have had a dramatic impact on teaching, learning and assessment over the past 18 months. The most immediate threat AI posed was to Academic Integrity with Higher Education Institutes (HEIs) focusing their efforts on combating the use of GenAI in assessment. Guidelines were developed for staff and students, policies put in place too. Innovative educators have forged paths in the use of Generative AI for teaching, learning and assessments leading to pockets of transformation springing up across HEIs, often with little or no top-down guidance, support or direction.
This Gasta posits a strategic approach to integrating AI into HEIs to prepare staff, students and the curriculum for an evolving world and workplace. We will highlight the advantages of working with these technologies beyond the realm of teaching, learning and assessment by considering prompt engineering skills, industry impact, curriculum changes, and the need for staff upskilling. In contrast, not engaging strategically with Generative AI poses risks, including falling behind peers, missed opportunities and failing to ensure our graduates remain employable. The rapid evolution of AI technologies necessitates a proactive and strategic approach if we are to remain relevant.
Acetabularia Information For Class 9 .docxvaibhavrinwa19
Acetabularia acetabulum is a single-celled green alga that in its vegetative state is morphologically differentiated into a basal rhizoid and an axially elongated stalk, which bears whorls of branching hairs. The single diploid nucleus resides in the rhizoid.
Introduction to AI for Nonprofits with Tapp NetworkTechSoup
Dive into the world of AI! Experts Jon Hill and Tareq Monaur will guide you through AI's role in enhancing nonprofit websites and basic marketing strategies, making it easy to understand and apply.
Read| The latest issue of The Challenger is here! We are thrilled to announce that our school paper has qualified for the NATIONAL SCHOOLS PRESS CONFERENCE (NSPC) 2024. Thank you for your unwavering support and trust. Dive into the stories that made us stand out!
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
Letter from the Congress of the United States regarding Anti-Semitism sent June 3rd to MIT President Sally Kornbluth, MIT Corp Chair, Mark Gorenberg
Dear Dr. Kornbluth and Mr. Gorenberg,
The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
This Congress will not stand idly by and allow an environment hostile to Jewish students to persist. The House believes that your institution is in violation of Title VI of the Civil Rights Act, and the inability or
unwillingness to rectify this violation through action requires accountability.
Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
students that opportunity and have been hijacked to become venues for the promotion of terrorism, antisemitic harassment and intimidation, unlawful encampments, and in some cases, assaults and riots.
The House of Representatives will not countenance the use of federal funds to indoctrinate students into hateful, antisemitic, anti-American supporters of terrorism. Investigations into campus antisemitism by the Committee on Education and the Workforce and the Committee on Ways and Means have been expanded into a Congress-wide probe across all relevant jurisdictions to address this national crisis. The undersigned Committees will conduct oversight into the use of federal funds at MIT and its learning environment under authorities granted to each Committee.
• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
"Protectable subject matters, Protection in biotechnology, Protection of othe...
Dc ch09 : high speed la ns and wireless lans
1. 1
1
1
1 tohttps://github.com/syaifulahdan/
DATA COMMUNICATION
High Speed LANs and Wireless LANs
Data Communications and Networking
Fundamentals of Ethernet
ALOHA, slotted ALOHA, CSMA
CSMA/CD
Ethernet Examples
10-Mbps Ethernet
Fast Ethernet
Gigabit Ethernet
10-Gbps Ethernet
802.11 Wireless LANs
Reading:
Book Chapter 9
Data and Computer Communications, 8th edition
By William Stallings
2. Data Communications and
Networking
Chapter 9
High Speed LANs and Wireless LANs
References:
Book Chapters 16 and 17
Data and Computer Communications, 8th edition
By William Stallings
4. 4
4
4
4 tohttps://github.com/syaifulahdan/ 4
Ethernet
• Most widely used high-speed LANs
—Ethernet (10Mbps, 100Mbps, 1Gbps, 10Gbps)
—Fibre channel
—High-speed wireless LANs
• Ethernet protocol is developed by IEEE 802.3
standards committee, consisting of
—Medium Access Control (MAC) Layer (CSMA/CD protocol)
• the key part of this chapter
—Physical Layer
• Earlier MAC schemes:
—ALOHA
—Slotted ALOHA
—CSMA
5. 5
5
5
5 tohttps://github.com/syaifulahdan/ 5
ALOHA
• ALOHA protocol is developed for packet radio networks, but applicable
to any shared transmission medium.
— A number of stations share the transmission medium. Two or more
simultaneous transmissions will cause a collision.
• Sender
— When station has frame, it sends
— Station listens for an amount of time
— If its hears an ACK, fine. If not, it retransmits the frame after a random
time
— If no ACK after several transmissions, it gives up
— Frame check sequence can be used for error detection
• Receiver
— If frame is OK and address matches receiver, sends ACK
— Otherwise, ignores this frame and does nothing
• Frame may be damaged by noise or by another station transmitting at
the same time (collision). Overlap of frames also causes collision.
• ALOHA is simple, but very inefficient
— Assuming random traffic, the maximum channel utilization is only about
18%
7. 7
7
7
7 tohttps://github.com/syaifulahdan/ 7
Slotted ALOHA
• To improve efficiency, a modification of
ALOHA, known as slotted ALOHA, was
developed.
• Time is organized into uniform slots whose
size equals the frame transmission time
—Need a central clock (or other sync mechanism)
• Transmission begins only at a slot boundary
—Consequence: frames either miss or overlap totally
• Maximum channel utilization can be
improved to about 37%
9. 9
9
9
9 tohttps://github.com/syaifulahdan/ 9
CSMA
• Why ALOHA and slotted ALOHA are so inefficient?
—Stations don’t check the channel status. They just send out
frames without considering whether the channel is free or
not, which creates too many collisions.
• In fact, it is not difficult for a station to “sense” the
channel status (free or not).
• CSMA: Carrier Sense Multiple Access
—Stations listen to the channel (carrier sense)
—Stations “know” whether the channel is free or not
—Stations transmit only if the channel is free
—Collisions become rare
• Only if two or more stations attempt to transmit at about the
same time, collisions could happen.
10. 10
10
10
10 tohttps://github.com/syaifulahdan/ 10
CSMA (Cont.)
• In traditional LANs, propagation time is much less than frame
transmission time
— Remark: this may not be true for 1Gbps and 10Gbps Ethernet
• All stations know that a transmission has started almost
immediately by “carrier sense”
• Details of CSMA
— Stations first listen for clear medium (carrier sense)
— If medium is idle, transmit the frame
— If two or more stations start at about the same instant, there will
be a collision.
• To account for this, a station waits for an ACK
• If no ACK after a reasonable time, then retransmit
• What should a station do if the medium is found busy?
— Three different approaches: nonpersistent CSMA, 1-persistent CSMA,
and p-persistent CSMA
11. 11
11
11
11 tohttps://github.com/syaifulahdan/ 11
Nonpersistent CSMA
• A station wishing to transmit listens to the medium
and obeys the following rules:
1. If medium is idle, transmit; otherwise, go to step 2
2. If medium is busy, wait an amount of time drawn from a
probability distribution and repeat step 1
• The use of random delays reduces probability of
collisions
— Consider two stations become ready to transmit at about
the same time while another transmission is in progress
— If both stations delay the same amount of time before
retrying, both will attempt to transmit at same time
collision
• Drawback:
— Capacity is wasted because medium will generally remain
idle following the end of a transmission, even if there are
one or more stations waiting to transmit.
12. 12
12
12
12 tohttps://github.com/syaifulahdan/ 12
1-persistent CSMA
• To avoid idle channel time, 1-persistent protocol can
be used
• A station wishing to transmit listens to the medium
and obeys the following rules:
1. If medium is idle, transmit; otherwise, go to step 2
2. If medium is busy, continue to listen until the
channel is sensed idle; then transmit immediately.
• 1-persistent stations are selfish
• Drawback:
— If two or more stations are waiting to transmit, a collision
is guaranteed.
13. 13
13
13
13 tohttps://github.com/syaifulahdan/ 13
p-persistent CSMA
• p-persistent CSMA is a compromise that attempts to
reduce collisions, like nonpersistent, and reduce idle
time, like 1-persistent
• Rules:
1. If the medium is idle, transmit with probability p, and
delay one time unit with probability (1 – p)
• The time unit is typically equal to the maximum propagation
delay
— If the medium is busy, continue to listen until the channel
is idle and repeat step 1
— If transmission is delayed one time unit, repeat step 1
• Question:
— What is an effective value of p?
14. 14
14
14
14 tohttps://github.com/syaifulahdan/ 14
Value of p?
• Assume n stations are waiting to transmit while a transmission
is taking place
• At the end of transmission, the expected number of stations
attempting to transmit is the number of stations ready (i.e., n)
times the probability of transmitting (i.e., p): np
• If np > 1, on average there will be a collision
— Repeated attempts to transmit almost guarantee more collisions
— Retries compete with new transmissions from other stations
— Eventually, all stations try to send
• Continuous collisions; zero throughput
• So np should be less than 1 for expected peaks of n
• Drawback of p-persistent:
— If heavy load is expected, p should be small such that np < 1.
However, as p is made smaller, stations must wait longer to
attempt transmission, which results in very long delays.
15. 15
15
15
15 tohttps://github.com/syaifulahdan/ 15
CSMA/CD:
CSMA with Collision Detection
• The problem of CSMA:
— A collision occupies medium for the duration of a frame
transmission, which is not good for long frames.
• Collision Detection:
— Stations listen whilst transmitting. If collision is detected, stop
transmission immediately.
• Rules of CSMA/CD:
1. If the medium is idle, transmit; otherwise, go to step 2
2. If the medium is busy, continue to listen until the channel is
idle, then transmit immediately
3. If a collision is detected during transmission, transmit a brief
jamming signal to assure that all stations know that there has
been a collision and then cease transmission
4. After transmitting the jamming signal, wait a random amount
of time, referred to as backoff, then attempt to transmit again
(repeat from step 1)
17. 17
17
17
17 tohttps://github.com/syaifulahdan/ 17
Collision Detection
The amount of time that it takes to detect a collision is no greater than twice the end-to-end
propagation delay.
Frames should be long enough to allow collision detection prior to the end of transmission. If
shorter frames are used, then collision detection does not occur.
For 10 and 100Mbps Ethernet, the frame length is at least 512 bits.
For 1Gbps Ethernet, the frame length is at least 4096 bits, using carrier extension or frame bursting.
18. 18
18
18
18 tohttps://github.com/syaifulahdan/ 18
Which Persistence Algorithm?
• IEEE 802.3 uses 1-persistent
• Both nonpersistent and p-persistent have
performance problems
• 1-persistent seems to be more unstable than
p-persistent, due to the greed of the stations
• But wasted time due to collisions is short (if
frames are long relative to propagation delay)
• With random backoff, stations involved in a
collision are unlikely to collide on next tries
—To ensure backoff maintains stability, IEEE 802.3
and Ethernet use binary exponential backoff
19. 19
19
19
19 tohttps://github.com/syaifulahdan/ 19
Binary Exponential Backoff
• Rules of binary exponential backoff:
— A station attempts to transmit repeatedly in the face of repeated
collisions
— For the first 10 attempts, the mean value of the random delay is
doubled
— The mean value then remains the same for 6 additional attempts
— After 16 unsuccessful attempts, the station gives up and reports an
error
• As congestion increases, stations back off by larger and larger
amounts to reduce the probability of collision.
• 1-persistent algorithm with binary exponential backoff is
efficient over a wide range of loads
— At low loads, 1-persistence guarantees that a station can seize
channel as soon as the channel goes idle
— At high loads, it is at least as stable as the other techniques
• Problem: Backoff algorithm gives last-in, first-out effect
— Stations with no or few collisions will have a chance to transmit
before stations that have waited longer
20. 20
20
20
20 tohttps://github.com/syaifulahdan/ 20
IEEE 802.3 Frame Format
Preamble: 7 octets of 10101010
SFD: 10101011
Length: the maximum frame size is 1518 octets, excluding the preamble and SFD.
Pad: octets added to ensure that the frame is long enough for collision detection
FCS: 32-bit CRC, based on all fields except preamble, SFD, and FCS
≥ ≥
21. 21
21
21
21 tohttps://github.com/syaifulahdan/ 21
10Mbps Specification
(Ethernet)
• IEEE 802.3 defined a number of physical configurations.
• The nation follows the rule of
— <data rate in Mbps><Signaling method><Max segment length in
hundreds of meters>
22. 22
22
22
22 tohttps://github.com/syaifulahdan/ 22
100Mbps Fast Ethernet
• Fast Ethernet refers to a set of specifications developed by IEEE 802.3
committee to provide a low-cost, Ethernet-compatible LAN operating at 100
Mbps.
— Use IEEE 802.3 MAC protocol and frame format
• 100BASE-X refers to a set of options that use two physical links between nodes:
one for ttransmission and one for reception
— 100BASE-TX for twisted pair
— 100BASE-FX for optical fiber
• 100BASE-T4 can use Category 3 UTP cable
• Uses four twisted-pair lines between nodes
• Data transmission uses three pairs in one direction at a time
• Star-wire topology
— Similar to 10BASE-T
23. 23
23
23
23 tohttps://github.com/syaifulahdan/ 23
Gigabit Ethernet
• The Gigabit Ethernet uses the same CSMA/CD frame
format and MAC protocol as used in the 10Mbps and
100Mbps version of IEEE 802.3.
• For shared-medium hub operation, there are two
enhancements to the basic CSMA/CD
—Carrier extension
• Appends a set of special symbols to the end of short MAC
frames so that the resulting block is at least 4096 bit-times in
duration (512 bit-times for 10/100Mbps)
—Frame bursting
• Allows for multiple short frames to be transmitted consecutively
(up to a limit) without giving up control for CSMA/CD between
frames. It avoids the overhead of carrier extension when a
single station has a number of small frames ready to send.
• Not necessary for Ethernet switches because there is
no contention for a shared medium.
25. 25
25
25
25 tohttps://github.com/syaifulahdan/ 25
10-Gbps Ethernet
• 10-Gbps Ethernet can be used to provide high-speed, local
backbone interconnection between large-capacity switches.
— Our departmental Ethernet uses 10-Gbps Ethernet at the backbone.
27. 27
27
27
27 tohttps://github.com/syaifulahdan/ 27
Wireless LANs
• Wireless LAN makes use of a wireless
transmission medium.
• Wireless LAN applications
—LAN Extension
—Cross-building Interconnection
—Nomadic Access
—Ad Hoc Networking
• An ad hoc network is a peer-to-peer network (without
centralized server) set up temporarily to meet some
immediate need.
30. 30
30
30
30 tohttps://github.com/syaifulahdan/ 30
Wireless LAN Technology
• Infrared (IR) LANs
—An individual cell of an IR LAN is limited to a single room,
because infrared light does not penetrate opaque walls
• Spread spectrum LANs
—The most popular type
—In United States, three microwave bands for unlicensed use
• 915-MHz band (902-928 MHz) ---- 26 MHz of bandwidth
• 2.4-GHz band (2.4-2.4835 GHz) ---- 83.5 MHz of bandwidth
• 5.8-GHz band (5.725-5.825 GHz) ---- 100 MHz of bandwidth
31. 31
31
31
31 tohttps://github.com/syaifulahdan/ 31
IEEE 802.11
• IEEE 802.11 is devoted to wireless LANs.
— Consists of MAC and physical layer protocols for wireless LANs
• The Wi-Fi Alliance (Wi-Fi: Wireless Fidelity)
— An industry consortium
— To certify interoperability for 802.11 products
• IEEE 802.11 Architecture
— The smallest building block is Basic Service Set (BSS)
• A number of stations executing the same MAC protocol
• Shared wireless medium
• BSS corresponds to a cell
— A BSS may be isolated, or may connect to a Backbone Distribution
System (DS) through an Access Point (AP)
• AP functions as a bridge and a relay point
• AP could be a station which has the logic to provide DS services
• AP corresponds to a Control Module (CM)
• DS can be a switch, wired network, or wireless network
— An Extended Service Set (ESS) consists of two or more BSSs
interconnected by a DS.
34. 34
34
34
34 tohttps://github.com/syaifulahdan/ 34
802.11 Physical Layer
• Issued in four stages
• First part in 1997
— IEEE 802.11
— Includes MAC layer and three physical layer specifications
— Two in 2.4-GHz band and one infrared
— All operating at 1 and 2 Mbps
• Two additional parts in 1999
— IEEE 802.11a
• 5-GHz band, data rate up to 54 Mbps
— IEEE 802.11b
• 2.4-GHz band, data rate at 5.5 and 11 Mbps
• Most recent in 2002
— IEEE 802.11g extends IEEE 802.11b to higher data rates, up to 54
Mbps
• At present
— IEEE 802.11n: data rate up to hundreds of Mbps
35. 35
35
35
35 tohttps://github.com/syaifulahdan/ 35
Medium Access Control
• MAC layer has three functions
—Reliable data delivery
• Different from Ethernet, wireless LANs suffer from
considerable unreliability.
—Access control
• Distributed access
• Centralized access
—Security
36. 36
36
36
36 tohttps://github.com/syaifulahdan/ 36
Reliable Data Delivery
• 802.11 physical and MAC layers subject to unreliability
— Noise, interference, and other propagation effects result in loss of frames
— Even with error-correction codes, frames may not successfully be received
• 802.11 includes frame exchange protocol
— Station receiving frame returns acknowledgment (ACK) frame
— The exchange of these two frames is treated as atomic unit
• Not interrupted by any other station
— If no ACK frame received within short period of time, retransmit
• To further enhance reliability, four-frame exchange may be used
— Source issues a Request to Send (RTS) frame to destination
— Destination responds with Clear to Send (CTS)
— After receiving CTS, source transmits data
— RTS and CTS notify other stations that a transmission is under way, so they
refrain from transmission in order to avoid collision
— Destination responds with ACK
• The purpose of RTS/CTS
— The collision time is shortened, because collision only occurs on the RTS
frame, which is very short compared with data frames
37. 37
37
37
37 tohttps://github.com/syaifulahdan/ 37
Hidden Terminal Problem
A B C
Consider the effect of RTS/CTS:
RTS alerts all stations within range of source (i.e., A) that
exchange is under way; CTS alerts all stations within
range of destination (i.e., B).
A and B can hear each other. B and C
can hear each other. But A and C cannot
hear each other.
When A is sending data to B, C cannot
sense this activity and hence C is
allowed to send data to B at the same
time. This will cause a collision at B.
collision
38. 38
38
38
38 tohttps://github.com/syaifulahdan/ 38
Medium Access Control
• Two sublayers
• Lower sublayer is distributed coordination function
(DCF)
—Uses a contention algorithm to provide access to all traffic
• Higher sublayer is point coordination function (PCF)
—Uses a centralized algorithm
—Contention free
—Implemented on top of DCF
• Remark: PCF has not been popularly implemented in
today’s 802.11 products. DCF is widely used.
39. 39
39
39
39 tohttps://github.com/syaifulahdan/ 39
Distributed Coordination
Function: CSMA/CA
• DCF sublayer uses CSMA/CA protocol, where CA refers to as Collision
Avoidance
1. A station with a frame to transmit senses the medium. If the medium is idle, it
waits to see if the medium remains idle for a time equals to a delay called
Interframe Space (IFS). If so, the station may transmit immediately.
2. If the medium is busy, the station defers transmission and continues to monitor the
medium until the current transmission is over.
3. Once the transmission is over, the station delays another IFS. If the medium remains
idle for this period, then the station backs off a random amount of time and again
senses the medium. If the medium is still idle, the station may transmit. During the
backoff time, if the medium becomes busy, the backoff timer is halted and resumes
when the medium becomes idle.
4. If the transmission is unsuccessful, which is determined by the absence of an ACK,
then it is assumed that a collision has occurred.
• To ensure that backoff maintains stability, binary exponential backoff is used.
1. Why not collision detection?
— Collision detection is not practical on wireless networks
— The dynamic range of wireless signals is very large
1. The transmitting station cannot distinguish incoming weak signals from noise and/or
effects of own transmission
40. 40
40
40
40 tohttps://github.com/syaifulahdan/ 40
IEEE 802.11 MAC Frame Format
• Frame Control: Indicates the type of frame
• control, management, or data
• Control Frames:
• RTS, CTS, ACK (Acknowledgement), etc.
• Management Frames:
• to manage communications between stations and APs.
41. 41
41
41
41 tohttps://github.com/syaifulahdan/ 41
MAC Frame Fields
• Duration/Connection ID:
— If used as duration field, indicates the time (in s) the channel will be
allocated for successful transmission of a MAC frame
— In some control frames, contains association or connection identifier
• Addresses:
— The number and meaning of the 48-bit address fields depend on context
— source address, destination address, transmitter address, receiver address
• Sequence Control:
— 4-bit fragment number subfield used for fragmentation and reassembly
— 12-bit sequence number used to number frames between given transmitter and
receiver
• Frame Body:
— MSDU or a fragment of an MSDU
• Frame Check Sequence:
— 32-bit cyclic redundancy check