The IEEE MAC Sub-Layer - PowerPoint Presentation

Slide1

The IEEE MAC Sub-Layer http://www.sonoma.edu/users/k/kujoory

Department of Engineering Science

ES465/CES 440, Intro. to Networking & Network Management

References

“Computer

Networks & Internet,” Douglas Comer, 6

th

ed

, Pearson, 2014,

Ch

14,

Textbook

, 5

th

ed

, slides by

Lami

Kaya (

LKaya@ieee.org

)

with some changes

.

“

Computer Networks,” A.

Tanenbaum

, 5th ed., Prentice Hall, 2011, ISBN: 13:978013212695-3.

“

Computer

& Communication

Networks,” Nader F. Mir, 2

nd

ed

,

Prentice Hall, 2015,

ISBN: 13: 9780133814743.

“Data

Communications Networking,” Behrouz A.

Forouzan

, 4th

ed

, Mc-

Graw

Hill, 2007

“Data & Computer Communications,” W. Stallings, 7th ed., Prentice Hall, 2004.

“Computer Networks: A Systems Approach," L. Peterson, B. Davie, 4th Ed., Morgan Kaufmann 2007

.Slide2

Topics Covered14.1 Introduction

14.2 A Taxonomy of Mechanisms for Multi-Access14.3 Static & Dynamic Channel Allocation14.4 Channelization Protocols14.5 Controlled Access Protocols

14.6 Random Access ProtocolsSlide3

14.1 IntroductionThis chapter

continues the discussion by examining the IEEE's MAC sublayerexplains multi-access protocolsconsiders both static & dynamic channel allocationLater chapters in this part

discuss specific networking technologies that use the access mechanisms explained hereSlide4

14.2 A Taxonomy of Mechanisms for Multi-AccessHow do multiple, independent computers

coordinate access to a shared medium?There are three broad approaches: they canuse a modified form of a multiplexing technique

engage in a distributed algorithm for controlled access, oruse a random access strategyFig. 14.1 illustrates the taxonomyincluding specific forms of each approachSlide5

Fig. 14.1

A Taxonomy of Mechanisms for Multi-Access Slide6

14.3 Static & Dynamic Channel Allocation

Channelization refers to a mapping between a given communication & a channel in the underlying systemNeed a mapping between entities & a channelreferred to as

1-to-1 & staticStatic channel allocation works well for situations where the set of communicating entities is known in advance & does not changeIn many networks, however, the set of entities using the network varies over timeAs an example, consider cellular telephones in a cityusers move, & they can turn a cell phone on & off at any timethus, the set of cell phones that are operating in the range of a given cell tower varies constantlyA dynamic channel allocation scheme is neededA mapping can be established when a new station appears, & the mapping can be removed when the station disappearsSlide7

14.4 Channelization ProtocolsChannelization protocols extend the multiplexing techniques covered in Chapter 11

Fig. 14.2 lists the main channelization techniques

These schemes have been discussed in Chapter 11 in detail14.4.1 FDMA 14.4.2 TDMA 14.4.3 CDMA Figure 14.2 The three main types of channelization.Slide8

Frequency Division Multiplexing (FDMA)In FDMA, the available bandwidth

of the common channel is divided into bands that are separated by guard bandsWas used in telephone system in analog

trunks between switchesForouzan, Ch12Slide9

Time Division Multiplexing (TDMA)In TDMA, the

bandwidth is just one channel that is timeshared between different stationsIs used in telephone system in digital trunks between switches

Forouzan, Ch12Slide10

Code Division Multiplexing (CDMA)In CDMA, one channel carries all transmissions simultaneously

.Used in CDMA cellular (Verizon)Simple idea of communication with code

Forouzan, Ch12Slide11

CDMA Enoding & DecodingSlide12

14.5 Controlled Access ProtocolsControlled access protocols provide a deterministic version of statistical multiplexing

Fig. 14.3 lists the three principal forms:

These will be discussed in the following sub-sections14.5.1 Polling14.5.2 Reservation14.5.3 Token PassingFigure 14.3 The main types of controlled access protocols.Slide13

14.5.1 Polling Polling uses a

centralized controllerwhich cycles through stations on the network & gives each an opportunity to transmit a packetAlgorithm 14.1 gives the steps a controller follows

The selection step is significant because it means a controller can choose which station to poll at a given timeThere are two general polling policies:Round robin orderRound-robin means each station has an equal opportunity to transmit packetsPriority orderPriority order means some stations will have more opportunity to sendE.g., priority order might be used to assign an IP telephone higher priority than a personal computerSlide14

14.5.1 PollingSlide15

14.5.2 ReservationIt is often used with satellite transmissionIt employs a two-step process in which each round of

packet transmissions is planned in advanceTypically, reservation systems have a central controller that follows Algorithm 14.2Slide16

14.5.2 ReservationIn the first stepeach potential sender specifies whether they have a packet to send during the next round, & the controller transmits a list of the stations that will be transmitting

In the second stepstations use the list to know when they should transmitVariations exist where a controller uses an alternate channel to gather reservations for the next round

while the current round of transmissions proceeds over the main channelSlide17

14.5.3 Token PassingIt is most often associated with

ring topologies Although older LANs used token passing ring technologypopularity has decreased, & few token passing networks remain

Imagine a set of computers connected in a ring &imagine that at any instant, exactly one of the computers has received a special control message called a tokenWhen no station has any packets to sendthe token circulates among all stations continuouslyFor a ring topology, the order of circulation is definedif messages are sent clockwise, the next station mentioned in the algorithm refers to the next physical station in a clockwise orderWhen token passing is applied to other topologies (bus)each station is assigned a position in a logical sequence& the token is passed according to the assigned sequenceSlide18

14.5.3 Token PassingTo control access, each computer follows Algorithm 14.3Slide19

14.6 Random Access ProtocolsSome LANs do not employ a controlled access mechanism

Instead, a set of computers attached to a shared medium attempt to access the medium without coordinationThe term random is used because access only occurs when a

given station has a packet to send &randomization is employed to prevent all computers on a LAN from attempting to use the medium at the same timethe descriptions of specific methods below will clarify the use of randomizationFig. 14.4 lists the three random access methods that are discussed 14.6.1 ALOHA14.6.2 CSMA/CD 14.6.3 CSMA/CASlide20

14.6 Random Access ProtocolsSlide21

14.6.1 ALOHAAn early network in Hawaii, known as

ALOHAnet, pioneered the concept of random accessthe network is no longer used, but the ideas have been extendedThe network consisted of a single

powerful transmitter in a central geographic location It is surrounded by a set of stations/computerStations had a transmitter capable of reaching the central transmitter but not powerful enough to reach all the other stationsALOHAnet used two carrier frequencies for broadcasting:one for outbound by the central transmitter to all stations &another for

inbound

by

stations to the central

transmitter

Fig. 14.5 illustration of outbound & inbound frequencies in

ALOHAnetSlide22

14.6.1 ALOHASlide23

14.6.1 ALOHAThe ALOHA protocol is straightforward:

when a station has a packet to sendit transmits the packet on the inbound frequencythe central transmitter repeats

the transmission on the outbound frequencywhich all stations can receiveTo insure that transmission is successfula sending station listens to the outbound channelif a copy of its packet arrives, the sending station moves to the next packetif no copy arrives, the sending station waits a short time & tries againWhy might a packet fail to arrive? Interference if two stations simultaneously transmitthe signals will interfere & the two transmissions will be garbledcalled a

collision

, & say that the two transmitted packets

collide

The protocol

handles a collision

by

requiring

a sender to

retransmit

each

lost

packetSlide24

14.6.2 CSMA/CDResearchers at Xerox PARC

created a random access protocol (1973)Carrier Sense Multiple Access / Collision Detect A standard (also called the DIX standard) was created in 1978 by

Digital Equipment Corporation, Intel, & XeroxIt is widely known as EthernetIt uses cable as a shared medium, instead of broadcasting radio frequency transmissions through the atmosphereEthernet uses three mechanisms to handle collisions:Carrier senseCollision detectionBinary exponential backoffSlide25

14.6.2 CSMA/CD - Carrier SenseEthernet requires each station to monitor the cable to

detect whether another transmission is already in progressthis process is known as carrier sense

it prevents the most obvious collision problems &substantially improves network utilizationA collision can occur if two stations wait for a transmission to stop, find the cable idle, & both start transmittingA small part of the problem is that even at the speed of light, some time is required for a signal to travel down the cableThus, a station at one end of the cable cannot know instantly when a station at the other end begins to transmitSlide26

14.6.2 CSMA/CD – Collision Detection

To handle collisionseach station monitors the cable during transmission

If the signal on the cable differs from the signal that the station is sendingit means that a collision has occurredthe technique is known as collision detectionwhen a collision is detected, the sending station aborts transmissionMany details complicate Ethernet transmission, e.g.,After a collision, transmission does not abort until enough bits have been sent to guarantee that the collided signals reach all stations, alsoAfter a transmission, stations must wait for an interpacket gap (9.6 micro-sec for a 10 Mbps Ethernet) to insure that all stations sense an idle network & have a chance to transmit

A min

pause required

between

frames, depending on the encoding used & physical

layer;

the pause may be necessary to allow for receiver clock recovery, permitting the receiver to prepare for another frame

.

https://en.wikipedia.org/wiki/

Interpacket_gapSlide27

14.6.2 CSMA/CD – Binary Exponential Backoff

Binary Exponential Backoff After a collision occurs

a computer must wait for the cable to become idle again before transmitting a frameRandomization is used to avoid having multiple stations transmit simultaneously as soon as the cable is idleThe standard specifies a maximum delay, d, & requires each station to choose a random delay less than d after a collision occursWhen two stations each choose a random valuethe station that chooses the smallest delay will proceed to send a packet & The network will return to normal operationIn the case where two or more computers happen to choose nearly the same amount of delay

they will both begin to transmit at nearly the same time

producing a second

collisionSlide28

14.6.2 CSMA/CD – Binary Exponential Backoff

To avoid a sequence of collisionsEthernet requires each computer to double the range from which a delay is chosen after each collision

a computer chooses a random delay between 0 - d after one collisiona random delay between 0 - 2d after a second collisiona random delay between 0 - 4d after a third collisiona random delay between 0 – [2(n-1)]d after nth collision.After a few collisions, the range from which a random value is chosen becomes

large

Thus, some computer will choose a random delay shorter than the others, & will transmit without a collision

Doubling the range of the random delay after each collision is known as

binary exponential

backoffSlide29

14.6.2 CSMA/CD – Binary Exponential Backoff

By using exponential backoffan Ethernet can recover quickly after a collisionbecause each computer agrees to wait longer times between attempts when the cable becomes busyEven in the unlikely event that two or more computers choose delays that are approximately equal

exponential backoff guarantees that contention for the cable will be reduced after a few collisions The combination of techniques described above is known by the name Carrier Sense Multi-Access with Collision Detection (CSMA/CD)Algorithm 14.4 summarizes CSMA/CDSlide30

14.6.3 CSMA/CDSlide31

14.6.3 CSMA/CD - Collision Avoidance

CSMA/CD does not work as well in wireless LANs

because a transmitter used in a wireless LAN has a limited rangeA receiver that is more than δ away from the transmitter will not receive a signal, & will not be able to detect a carrierConsider three computers with wireless LAN hardware positioned as Fig. 14.6 illustrates Slide32

14.6.3 CSMA/CD - Collision Avoidance

In Fig. 14.6, computer1 can communicate with computer2, but cannot receive the signal from computer

3Thus, if computer3 is transmitting a packet to computer2, computer1's carrier sense mechanism will not detect the transmissionSimilarly, if computer1 & computer3 simultaneously transmit, only computer2 will detect a collisionThe problem is sometimes called the hidden station problembecause some stations are not visible to othersWireless LANs use a modified access protocol known as CSMA with Collision Avoidance (CSMA/CA)The CSMA/CA triggers a brief transmission from the intended receiver before transmitting a packetSlide33

14.6.3 CSMA/CD - Collision Avoidance

The idea is that if both the sender & receiver transmit a messageall computers within range of either will know a packet transmission is beginningFig. 14.7 illustrates the sequenceSlide34

14.6.3 CSMA/CD - Collision Avoidance

In Fig. 14.7computer3 sends a short message to announce that it is ready to transmit a packet to computer

2, &computer2 responds by sending a short message announcing that it is ready to receive the packetall computers in range of computer3 receive the initial announcement &all computers in the range of computer2 receive the responseas a result, even though it cannot receive the signal or sense a carrier, computer1 knows that a packet transmission is taking placecomputer3 transmits its packetSlide35

14.6.3 CSMA/CD - Collision Avoidance

Collisions of control messages can occur when using CSMA/CA, but they can be handled easilyE.g., if computer1 & computer

3 each attempt to transmit a packet to computer2 at exactly the same timetheir control messages will collideWhen a collision occurs, the sending stations apply random backoff before resending the control messages. Because control messages are much shorter than a packet, the probability of a second collision is low