Reti di Calcolatori - Slide 23

7/30/2019 Reti di Calcolatori - Slide 23

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Dipartimento di Informatica e Sistemistica11/05/2007 GV/RC/A&P 1/

Reti di Calcolatori

Architetture e Protocolli

Giorgio Ventre

Dipartimento di Informatica e SistemisticaUniversit di Napoli Federico II


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The OSI Reference Model

The Data Link Layer


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The OSI Reference Model: the Data Link Layer

In a LAN, the Data Link Layer is responsible foroffering access to the network to/from the upper

layers

Different technical solutions depending on the type

of service required by the applications

Office automation applications might ask forsimplicity and capillarity, while factory automationneeds performance guarantees in data delivery


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The OSI Reference Model: LAN Architectures

The MAP protocol suite Manifacturing Automation

Protocol Proposed by GM to haveperformance guaranteesin data delivery for

assembly lines Based on a token-passing

mechanism to have

fairness in media access

The TOP protocol suite Technical and Office

Protocol Proposed by Boeing as astandard for officeautomation

Designed for offering wideconnectivity andcommunication to

computers Based on a contentionmechanism


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The IEEE 802 Standard Needed to provide a standard for LAN manifacturers (1980)

Compromise over different technical solutions Offers a common interface to the implementation of upper

layers protocols

802.3 802.4 802.5 802.6

802.1 Bridging

802.2Logical LinkSub-Layer

MACSub-Layer

Physical LinkLayer

DataLinkLayer

802.9

802.10 Security & Privacy

802Overview

&Architecture

802.1Management


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IEEE 802.3 - Ethernet (1983) Bus Topology with contention based access

IEEE 802.4 - Token Bus (1983) Bus Topology with token-passing mechanism

IEEE 802.5 - Token Ring (1984) Ring Topology with token passing mechanism IEEE 802.6 - DQDB (1990)

Standard for Metropolitan Area Network

IEEE 802.9 - New for Isochronous LANs


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The IEEE 802.X Standards have been adoptedor recognized by

National Bureau of Standards International Standard Organisation (ISO) as the

8802.X Standard

European Computer Manifacturer Association

They have inspired other important standards

EIA/TIA 568 for Structured Cabling Systems


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ISO/OSI, IEEE 802, EIA/TIA 568

ApplicazionePresentazione

Sessione

Trasporto

Rete

Data Link

Fisico

6

54

3

21

7

IEEE 802

WAN

EIA/TIA 568 (*)

(*) in futuro ISO/IEC 11801

PA

BX

.

.

.


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IEEE 802.1

IEEE 802.1 lo standard che contiene lespecifiche generali del progetto 802

IEEE 802.1 uno standard composto da molteparti tra cui:

802.1 Part A: Overview and Architecture802.1 Part B: Addressing Internetworking andNetwork Management

802.1 Part D: MAC Bridges


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Il Data Link nelle LAN

Le LAN sono reti di tipo broadcast in cui ognistazione a livello data link riceve i frame inviati

da tutte le altre stazioni Il data link broadcast pu essere realizzato sia

con topologie broadcast quali il bus, sia contopologie punto a punto quali l'anello

I canali trasmissivi sono sufficientemente

affidabili e non necessario in generecorreggere gli errori a questo livello


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I Sottolivelli

Per tener conto delle precedenti peculiarit ilprogetto IEEE 802 ha suddiviso il livello data link

in due sottolivelli:LLC: Logical Link ControlMAC: Media Access Control


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LLC (Logical Link Control)

linterfaccia comune a tutte le LAN verso illivello network.

I servizi e i protocolli di questo sottolivello sonodescritti nello standard IEEE 802.2


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MAC (Media Access Control)

specifico per ogni LAN e risolve il problemadella condivisione del mezzo trasmissivo

Esistono vari tipi di MAC: ad allocazione dicanale fissa o dinamica, deterministici ostatistici, ecc.


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IEEE 802.3 (CSMA/CD)

Topologia: busCablaggio: bus, stellaArbitraggio del canale trasmissivo: tramite

contesa

Tipologia del protocollo: non deterministicoVelocit Trasmissiva: 10 Mb/s

Throughput massimo: 4 Mb/sEvoluzione della rete Ethernet proposta da

Digital, Intel, Xerox (DIX).


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IEEE 802.4 (Token Bus)

Topologia: busCablaggio: busArbitraggio del canale trasmissivo: token Tipologia del protocollo: deterministico

Velocit Trasmissiva: 10 Mb/s Throughput massimo: 8 Mb/s

Standard di rete utilizzato in ambito di fabbricaspecialmente in relazione al MAP(Manufacturing Automation Protocol)


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IEEE 802.5 (Token Ring)

Topologia: anelloCablaggio: stella o doppio anelloArbitraggio del canale trasmissivo: token Tipologia del protocollo: deterministico

Velocit Trasmissiva: 4 o 16 Mb/s Throughput massimo: 3 o 12 Mb/s

Evoluzione della rete Token Ring proposta daIBM in alternativa a Ethernet


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ISO 9314

Topologia: anelloCablaggio: doppio anello o stellaArbitraggio del canale trasmissivo: token Tipologia del protocollo: deterministico

Velocit Trasmissiva: 100 Mb/s Throughput massimo: 80 Mb/s

Primo standard per reti locali concepito peroperare su fibra ottica

(FDDI: Fiber Distributed Data Interface)


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Il pacchetto MAC

I campi principali di un pacchetto a livello MAC

sono:

IndirizzoDestinatario

IndirizzoMittente

DATI FCS

DSAP SSAP LLC-PDU CRC

OTTETTI 6 6 variabile 4


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Indirizzi MAC

Si compongono di due parti di 3 Byte: I tre byte pi significativi indicano il lotto di indirizziacquistato dal costruttore della scheda, detto

anche vendor code o OUI (Organization UniqueIdentifier). I tre meno significativi sono una numerazione

progressiva decisa dal costruttore

Esempio: una scheda con MAC address 08-00-2b-3c-07-9a una scheda prodotta da Digital inquanto il lotto 08-00-2b stato acquistato daDigital

Sono di tre tipi:Single: indirizzo di una singola stazioneMulticast: indirizzo di un gruppo di stazioniBroadcast: indirizzo di tutte le stazioni


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Indirizzo di Broadcast

Il broadcast un tipo particolare di multicast cheindica tutte le stazioni sulla rete locale

La sua codifica esadecimale ff-ff-ff-ff-ff-ff


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Ricezione

Scheda di Rete Locale

Rete locale

Transceiver

Quando la scheda LAN

deve passare la tramaricevuta alla CPU?

Transceivercable


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Ricezione

Ogni scheda di rete, quando riceve unpacchetto, lo passa ai livelli superiori nei

seguenti casi:Broadcast: sempreSingle: se il DSAP uguale a quello hardware della

scheda (scritto in una ROM) o a quello caricato dasoftware in un apposito buffer

Multicast: se stato abilitato via software


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Indirizzi di Gruppo

Servono tipicamente per scoprire i nodiadiacenti

Esistono due modi diversi di impiego:Solicitation: la stazione che interessata a scoprire

chi offre un dato servizio invia un pacchetto dimulticast allindirizzo di quel servizio. Le stazioni cheoffrono il servizio rispondono alla solicitation

Advertisement: le stazioni che offrono un serviziotrasmettono periodicamente un pacchetto dimulticast per informare di tale offerta tutte le altrestazioni


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Primitive di Servizio

A livello MAC esistono solo tre delle quattroprimitive presenti ai livelli superiori:

Request IndicationConfirm

La Response non usataConfirm viene generata o dal protocollo locale,

come indicazione dell'avvenuta trasmissione, odal protocollo remoto, come indicazionedell'avvenuta ricezione.


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Primitive di Servizio

LLC LAYER MACLAYER

CORRESPONDENTLLC LAYER

MA.DATA.requestMA.DATA.confirm

MA.DATA.request

MA.DATA.indication

MA.DATA.indication

TIMEMA.DATA.confirm

(a)

(b)

(a) usato da 802.3

(b) usato da 802.4, 802.5, FDDI


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LAN Architectures: Medium Access Control

The Medium Access Control Problem Where can we apply the control ?

Distributed Control Centralised Control

How can we enforce the control ?

Sinchronous techniques Asinchronous techniques

Which type of control can we apply ?

Continuous Traffic Bursty Traffic

Are the assumptions still valid today ?


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Reservation Static or dynamic channel assignment Centralised or distributed control

Suited for continuous traffic Round Robin

Static or dynamic cyclic channel assignment

Centralised or distributed control For bursty and continuous traffic

Contention

Inherently distributed (local) control Dynamic channel assignment Suited for bursty traffic


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Carrier Sensing Multiple Access (CSMA) Very simple model: multiple stations connected

through a shared media (bus) Random Access Contention

Derived from the ALOHA Protocols Two-state behavior of a system

Thinking

Transmitting Contention is perceived from destruction/garbling of

data on shared media


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Non persistent If no carrier, transmit

If channel occupied, wait time T=f(Prob) 1-Persistent

If no carrier, transmit

If channel occupied, wait until again free If collision (no ack), wait time T=Random

P-Persistent If no carrier, transmit with probability P If channel occupied, wait until again free


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CSMA suffers of inefficiency due to the problem ofdetecting collisions

The effective channel bandwidth is then reduced by theexistence of a contention state for the network

The duration of this state is linked to the

Frame dimension Technique used to detect collisions

Frame Frame Frame Frame

Contention Slots Idle Contention Interval

LAN A hi M di A C l


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CSMA/Collision Detect Collisions are detected by listening over the media

while transmitting a frame If no carrier, transmit If channel occupied, wait until again free

If collision, stop and transmit collision warning Wait for time T=f(Random Backoff)

The problem is now ensuring that

collisions can be detected all the stations can hear the collision warning the duration of the collision state be shortest possible

LAN A hi E h


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LAN Architectures: Ethernet

Ethernet Developed by Xerox PARC (Metcalfe-Boggs) in late

70 with contribution from Intel and DEC High reliability (no critical dependecies on anyone of

the network components)

Low installation and operation costs Distributed control Suited for data (burst) traffic Fits the typical office environment

CSMA/CD

LAN A hit t Eth t


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Coding should allowdetection of collision(Manchester)

Frames should be longenough to ensure collisiondetection

A maximum cable length isset to avoid non detectionsdue to attenuation

The longer the frame, the

longer the collision state Binary Exponential Back-Off

to reduce collision congestion

A B

A B

A B

A Bt = 2 T

LAN A hit t Eth t


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Preamble

Start of Frame (SFD)

Destination Addr.

Source Addr.

Length

Data

Pad

Frame Check

7 Bytes

1 Byte

6 Bytes

6 Bytes

2 Bytes

< 1518 Bytes

4 Bytes

Operational Parameters Bit Rate 10 Mbps Slot Time 512 Bit/t Interframe gap 9.6 sec

Backoff limit 10 Attempt limit 16 Jam size 32 bits Maximum Frame 1518 bytes

Minimum Frame 512 bytes

The bus topology allowsmulticasting

Destination Addr. is a Group Addr.to be programmed in the stationsbroadcasting

Destination Addr. is 111111

LAN Architectures Ethernet


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SIGNALRX ?

CARRIER = ONSYNC & WAIT SFD

START RX

FCS & SIZE?

SEND FRAMEUP

DROP FRAME

N

Y

N

N

Y

DEST_ADD?

Y

WAIT PACKETFORM FRAME

CARRIERON ?

WAIT FRAME GAPTX FRAME

COLLISION?

COMPLETE TXTX = OK

ATTE_MAX = TRUETX = NOK

TX JAMATTE ++

MAX ATTE?

BACKOFF = R(ATT)WAIT BACKOFF

Y

N

Y

N

N

Y

TXAlgorithm

RXAlgorithm

I mezzi trasmissivi IEEE 802 3


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I mezzi trasmissivi IEEE 802.3

Tipo diSegmento

Num Max.di MAU

LunghezzaMassima

Min. velocitdi propagaz.

10Base5

10Base2FOIRL10baseFL10baseFB

10BaseT

100

30222

2

500

185100020002000

100

0.77 c (*)

0.65 c0.66 c0.66 c0.66 c

0.59 c

2165

95050001000010000

565

RitardoMax. (ns)

* : c = 3 x 108 m/s (velocit della luce nel vuoto)



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How Ethernet performs ? The random-based contention makes impossible to get any

bounds on the transmission delay induced However, a worst case analysis can help us in figuring out

how efficient can be an Ethernet High, constant workload from Kstations

Constant transmission probability p(no Exp. backoff)

If A is the probability that the net is alreadyacquired in a slot, then

A = K p (1 - p)k-1

For p = 1 / Kthen A = A max = (1 - 1/K) k-1

So A tends to 1 if Ktends to infinite



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The probability a contention is made ofjslots isA (1-A)

j-1

and the mean number of slots per contention isSUMj jA (1-A)

j-1 = 1/A

A slot has duration 2 then mean contention interval wisw =2 /

If Pis the mean frame duration, then we can define theEfficiency Eas

E = P / (P + w) = P / (P +2 / A)

The longer the cable, the longer the contention interval, thesmaller the Efficiency!

LAN Architectures: Token Bus


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IEEE 802.4 - Token Bus For automation of assembly lines it is essential to

have deterministic bounds on the worst casetransmission delay Dmax A token passing mechanism over a ring seems a

simple solution, since with N stations and for Tframe transmission time, we haveDmax = NT

However, a ring topology is very sensitive tobreakage and does not fit well an assembly linearchitecture.



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The solution is to define a ring-based logical topologyover a bus architecture

Each node has a Predecessor and a Successor

A node passes over the token by broadcasting it on thebus where it is taken by its Successor

P = BS = C

P = DS = A

P = AS = E

P = ES = B

P = CS = D

A B C

D E

P = PredecessorS = Successor



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75 Broadband coax

Bus or Tree structure Transmission:

Modified Basebad

(Carrierband) Broadband

Requires modulation

equipment (sometime inDTE)

MAC

ProtocolFirmware

Mod/Dem& Control

Physical

InterfaceModule

DTE

MAC

Architectural choicesinfluenced by operativeenvironment (factory)



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As in baseband, carrierbandoccupies all the cablebandwidth (1 - 5 - 10 Mbps)

Signal is modulated phase-coherent frequency shift withno phase change from 1 to 0

2 Frequencies for 1 and 0 + 3frequencies for control

All other frequencies can befiltered out (noise)

Binary 1

Binary 0

1 Bit time



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A token based MAC is very sensitive to tokencorruption and station faults

The MAC protocol must manage events such as Stations joining or leaving the ring Faulty or powering down stations Corrupted tokens/frames

There is a Tmax maximum

waiting time for DTEsTmax = 2 * (TxD + ProcD) +



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Preamble

Start Delimiter (SD)

Destination Addr.

Data

CRCEnd Delimiter

=> 1 Byte

1 Byte

1 Byte

6 Bytes

6 Bytes

< 8191 Bytes

1 Bytes

Frame Control specifies if a framecontains data or control infoIf data, contains priority and ackgrant information

If control, contains message type:Token passingToken claim

(for ring initialization)Solicit successor

(station joining - periodic)Who follows me

(lost token)Resolve contention

(multiple new stations)Set successor

(station informs predec.about leaving)

Source Addr.

Frame Control

4 Bytes



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Basic network operation1) Wait for token2) Transmit Nmax frames in queue3) Pass token to successor4) Lissen new traffic

4.1) If valid frames, then my tx was ok4.2) If frames not valid, then there is a problem

4.2.1) Rigenerate new token

4.2.2) If still nothing, my successor is dead4.2.3) Who follows me ?4.2.4) Receive Set successor & Synchronize Net



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Priority operation Four Access Classes

Class 6: Urgent Messages, Alarms Class 4: Control and Management Class 2: Routine Operations

Class 0: Low priority (e.g. file transfer, downloading) Transmission regulated by two timers

Token Hold Timer (THT)

Maximum Transmission Time for a Station High-Priority THT (HP-THT)Maximum Transmission Time for Class 6 Packets



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When the token is received, the station transmits Class6 packets until HP-THT

Then, if THT has not expired, it is set to a timer calledToken Rotation Timer (TRT) measuring the time

expired since the last token reception

The station starts transmitting lower priority packets

until the difference between THT and a fixed valuecalled Target Token Rotation Time (TTRT) is positive



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B

B

B

HP-THT > 0THT > 0

HP-THT = 0THT := TRT

HP-THT = 0

THT = TTRT

TRT = t

TRT = 1

TRT = 2

Token

Class 6

Classes 0-4

LAN Architectures: Token Ring


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IEEE 802.5 Token Ring Standard originated from/compatible with IBM LAN

Based on a ring topology Stations connected through a serial line, not via a

bus

Media Access Control based on a token mechanism Bandwidth achievable 4 - 16 Mbps Star topology possible with concentrators



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A

D

C

B

F

A

D

C

B

T

A receivesthe Token

A

D

C

B

F

A tx aFrame to D

Frame returnsto Athrough D andis eliminated

A

D

C

B

T

A releasesthe Token to B Early Token

Release

A

D

C

B

F

A tx aFrame to D andreleases immediatelythe token

F

T



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The network media is a 2 pairs shielded cable The signal coding is Differential Manchester

The physical architecture can be a point-to-point ring orthrough a concentrator

MAC

ProtocolFirmware

TrunkCouplingUnit

DTE

MACRTR R RTT T

HUB

Drop Connection

Ring In Ring Out



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Since the ring is realised through point-to-point lines,the TCU must guarantee data by-passing when astation is inactive.

When it is not transmitting, a station is inlistening mode - 1 bit copy mechanism

Listening Mode

1 Bit Buffer / Copy Delay

In Out

DTE

Tx Mode

In Out

DTE

To/From Upper Layers



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Start Delimiter

Access Control

Destination Addr.

Data

CRC

End Delimiter

1 Byte

1 Byte

1 Byte

6 Bytes

6 Bytes

< 5000 Bytes

1 Bytes

Source Addr.

Frame Control

4 Bytes

Data Frame Format

Start Delimiter

Access Control (AC)

=> 1 Byte

1 Byte

1 ByteEnd Delimiter

Token Frame Format

Frame Status 1 Bytes

Access Control:Token, Monitor, Priority, Reserv.

Frame Control:For ring management

Frame Status:A bit - On, Destination OK

C bit - On, Frame OK



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Token Ring has also a priority mechanism: Each token has a priority assigned

When a station receives a token with priority P, itcan only transmit frames with priority F > = P

A station can book a token with a certain priority Mechanisms exists in order to avoid abuses This priority mechanism is not fair as the one

adopted in Token Bus



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Each MAC Layer maintains two sets of values: Priorities:

Pm: Max priority among the waiting frames Pr: Priority of the previous token Rr: Reservation contained in the previous token

Stacks: Sr: Stack for the previous Pr Sx: Stack for new priorities P



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MAC assignes priority to any tx frame P = Current token priority Pr

R = 0 (to be set by other stations) MAC generates a new token with:

P = Pr, R= Max(Rr,Pm) P constantif there are no more transmittable frames or noreservation request with higher P

P = Max(Rr,Pm), R = 0 P increasesif there are still transmittable frames pending orthere is a reservation request for higher priority



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We now need a mechanism to lower the priority:

The station who raises P becames thestacking station, i.e. a station keeping track of P This station will be responsible for

raising the priority for new reservations keeping it the same lowering it, as soon as new reservations ask for lower

priorities



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Ring Management - Inizialization When switched on, a station makes a Duplication Address

Test (DAT)

If OK, it send out a Standby Monitor Present (SMP) to informthe immediate downstream station about its existence.

Ring Management - Standby Monitor

Each ring has a monitor station, to check token passing andtransmitting periodic Active Monitor Present (AMP) signals In case there are no AMP, a station can send a Claim Token

(CT) frame to become the monitor (FCFS approach)



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Ring Management - Active Monitor When a station becomes the new monitor, it starts a clock

and sends out a Purge (PRG) frame to eliminate tokens and

frames It then sends out a AMP signal to inform the other stations,and creates a new token

The other stations move to a Standby Monitor State

Ring Management - Beaconing When a station receives no AMP or frames in an interval, it

transmits a Beacon (BCN) signal to verify the existence of aconnection

If the signal is not received back from the station, then thering is interrupted somewhere

LAN Architectures: High Speed LANs


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High Speed Lans The Performance Problem -

More stations More applications More usage

More media More capillarity

Demand for More Bandwidth

This must cope with the technological limitations ofsome LAN architectures



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An example: improving Ethernet performance If Tc is the delay to detect a collision, and Tix the

delay to transmit a frame, whenTc


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Case 1:Tp = 25 sec (Lenght = 2.5 Km)Fs = 10.000 bits (Frame size)

B = 10 Mbps (Bandwidth) Tp / Tix = 25 / 1000

Tc = 50 sec N = Tc x B = 500 (Bits wasted if collision)

Case 2: B = 100 Mbps, N = 5000 Case 3: B = 200 Mbps, N = 10000 (Frame)

CSMA/CD is not suited!



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FDDI : ISO 9314 Token based MAC

Fibre based (Copper based available - CDDI) Ring 1: Primary Ring Ring 2: Secondary Ring - Backup / Add. Tx

Single Attachment Stations (SAS) Dual Attachment Stations (DAS)

Longer distances - Higher speed



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M

S

M

S

A B

A

BA

B

Optical CouplingUnit

SAS

DAS

Wiring Concentrator

Media InterfaceConnector

Primary Ring

Secondary Ring

Primary Ring

Master Key

Slave Key



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FDDI is used as a High-Speed LAN for clustering top-end computing

systems (NOW)

as a High-Speed backbone for LAN interconnection

FDDI

Local Area Network

Bridge

Campus Network

BuildingBackbone



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Operation Mode At any time, only one ring is active and provides

timing 4B/5B Manchester Coding MAC based on ETR - Token Ring

Priority mechanism similar to Token Bus Isochronous FDDI defined but not standard

based on allocation of time-slots to isochronous

media - Synchronous Allocation Time (SAT)

LAN Architectures: Bridging


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The diffusion of LANs brings in severalproblems:

How to connect multiple LANs with similartechnologies How to divide a single LAN into separate entities

How to connect multiple LANs with differenttechnologies

How to limit and control the traffic flowing from LAN

to LAN (load balancing and security)



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Backbone LAN

Buiding C

LAN 3

Buiding D

LAN 4

Bridge

Office LAN

ComputerBuiding A

LAN 1 Buiding B

LAN 2

Bridging keeps local all the frames to systems in the sameLAN and propagates only frames for external systems



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Connecting LAN

Buiding A

LAN 3

BridgeOffice LAN

Computer

Buiding B

LAN 2

Bridging is a solution also for the existing limitationsover the extension of a LAN

2.5 Km

2.5 Km

2.5 Km



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Backbone LAN

Buiding C

LAN 3

Buiding D

LAN 4

Bridge

Office LAN

ComputerBuiding A

LAN 1 Buiding B

LAN 2

Bridging keeps local all the problems due tofaulty networks or systems

Missing

Terminator

Faulty

Transceiver

Bridging Operation


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NET

LLC

MAC

PHY

NET

LLC

MAC

PHY

PKT

802.3 PKT

802.3 PKT

PKT

PKT

802.3 PKT

802.3 PKT

PKT

PKT

802.4 PKT

802.4 PKT

802.4 PKT

802.4 PKT

CSMA/CD Lan 802.4 Lan

Bridge

Host A Host B

Differences in formats and protocols make bridginga complex task.

Bridging Operation


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A long list of problems Different frames:

translation fragmentation

Different data rates:

buffering timing control from faster to slower

Different MACs

token vs. collision with different efficiency Different maximum frame length

no solution!

Bridging Disadvantages


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Store&Forward behaviour with additional delayintroduced

No flow-control at the MAC sublayer, withpossible buffer overflow when the output LAN isoverloaded

Differences in frame types demand for frameconversion, with possible multiple errors

introduced during frame relaying

Bridging Operation


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802.3 to 802.3: only load differences 802.4 to 802.3

Priority bits missing (bridge lie) Acknowledgement request (bridge lie)

802.5 to 802.3 A&C bits missing (bridge lie) Priority bits (bridge lie)

802.4 to 802.4 Acknowledgement request (bridge lie)

802.5 to 802.4

A&C bits Priority 802.5 to 802.5

A&C bits

Bridging Operation


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Transparent bridgingthe presence of one or multiple bridges is

transparent to the stations Bridges automatically initialize and configure

dynamically with no external intervention A bridge can have two or more bridge ports. In

this last case it is called a multiportbridge.

Bridges operate in promiscuous mode, i.e. theyreceive and buffer all frames received

Bridging Operation


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A bridge includes a MAC chipset with a portmanagement software

MACChipset

MACChipset

MemoryBuffers

PortMngmt

ProtocolEntity

Forw.DataBase

Station

Address

Port

Num.

Port 1 Port 2

Bridge learning


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The Forwarding Data Base (FDB) is formed bydynamic learning and interaction with otherbridges A newly inserted bridge has an empty FDB For each received frame, the source address and

the entry port are stored in the FDB

To ensure delivery, the frame is forwarded over allthe ports (flooding)

The procedure is repeated for each frame from anewsource

OK for static scenarios: no moving stations/links

Bridge learning


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The Inactivity Timer (IT) is a value in the FDB thatrecords the silence time of a source.

When IT > Tmax the entry is removed

In this way the FDB is formed only of active stationsand has a limited size

A spanning tree algorithm is used to evaluate theproper forwarding route in case of multiple paths

In large networks, stations can move and links can

change state so a new spanning tree must be created

Topology Initialization


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Bridges exchange regularly special frames calledBridges Protocol Data Units (BPDU). These frames arenot directly forwarded, but used to produce new

BPDUs Each bridge has an Id and a priority. The root bridge

has the highest priority and smallest Id

A new bridge starts to operate in the rootstate andtransmists over all ports the Configuration BPDUs

Id of the current root (itself initially) Path cost to the root (0 initially) Id of the transmitter Id of the port

Topology Initialization


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When a bridge receives a BPDU it compares the rootPriority and Id with its own values.

If this is the case, the bridge sets the new value for theroot and computes the new cost for the root path

A new Configuration BPDU is formed and forwarded toall the other ports

By knowing the costs to the root for each port, a bridgeis designatedto be its way to the root. It will be the onlyfrowarding bridge in that segment

The Id value is again the tie breaker value

Topology change


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The root bridge will transmit regularly ConfigurationBPDUs. These are processed by the bridges on thepath to inform all other bridges about changes

A Message Age Timer is kept for each port. In case ofa port/link failure, the MAT expires and a procedure isactivated to change topology

Topology Change Notification BPDUs are exchangedto inform the root about port status modifications Exists a procedure also to change the root and the

designated bridge

Il progetto IEEE 802: il sottolivello LLC


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FDDI802.3 802.4 802.5

FDDI

802.2 Logical Link ControlISO 8802.2

802.6

LIVELLONETWORK

LIVELLO

DATA LINK

LIVELLOFISICO

LLC

MAC

CSMA/CD TOKENBUS

TOKENRING

DQDB

Interfaccia unificata con il livello network

Tecnologie trasmissive differenziate

ISO8802.3

ISO8802.4

ISO8802.5

ISO8802.6

ISO9314

(Local and Metropolitan Area Network)

Il sottolivello LLC


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Definisce i protocolli usati per realizzare una opi connessioni logiche su di un singolo mezzofisico

LLC deve essere utilizzabile con uno qualunquedei MAC

PDU del LLC


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DESTINATION

ADDRESS

SOURCE

ADDRESS

CONTROL INFORMATION

1 1 1 o 2 variabileOTTETTI

Il campo control permette lesistenza di tre tipi dipacchetti:

Unnumbered Supervisor Information


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Livello 3

OSI

Livello 3

TCP/IP

Altro

livello 3

Sottolivello MAC

Sottolivello LLC

Scelta basatasu LLC-DSAP

SAP di LLC


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Servono ad indicare quale protocollo di livellosuperiore ha originato il pacchettoSono grandi un Byte

due bit I/G e U riservati64 indirizzi singoli, globali definibili

ff broadcast00 data link layer itself

1 0 1 1 1 1 01

I/G (Individual=0, Group=1)U (Non Universal=0, Universal=1)

SAP LLC Universal


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Gli indirizzi globali sono assegnati dallISO soloper i protocolli progettati da un comitato distandardizzazione

Esempi di SAP-LLC


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La codifica 0FEH indica il protocollo ISO 8473Internet Protocol

La codifica 042H indica il protocollo IEEE802.1D Spanning Tree Configuration La codifica 0AAH indica un pacchetto LLC

speciale detto SNAP

0FEH 0FEH UI

1 1 1 mOTTETTI

DSAP SSAP CONTROL INFO

Servizi LLC


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LLC offre al Livello 3 i seguenti tipi di servizio:Tipo 1: Unacknowledged Connectionless Service

Tipo 2: Connection Oriented ServiceTipo 3: Semireliable Service

LLC Tipo 1


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Unacknowledged Connectionless ServiceServizio non connesso

Non esiste un acknowledge (ACK)Se il messaggio perso o rovinato dai disturbi

non viene fatto nessun tentativo, a questo livello,

di recuperare l'erroreNessuna procedura di controllo di flusso: le

trame non sono numerate La trasmissione pu essere punto-punto,

multipunto o broadcast

Primitive per LLC tipo 1

Q i i LLC l li i hi d


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Questo servizio LLC molto semplice e richiedesolo due primitive:L.DATA.requestL.DATA.indication

USER LLCLAYER

CORRESPONDENTUSER

L.DATA.request

L.DATA.indication

LLC tipo 2

S i i


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Servizio connesso il servizio pi sofisticato che il livello data link

possa offrire al livello networkSorgente e destinazione aprono una connessione

prima di trasferire i dati e la chiudono al termine

I frame sono numerati e il livello 2 garantisce che:ogni frame inviato sia ricevuto correttamenteogni frame sia ricevuto esattamente solo una volta

tutti i frame siano ricevuti nell'ordine correttoEsistono meccanismi di controllo di flusso (flow

control)

Primitive LLC tipo 2

I t i d ll C i


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Instaurazione della Connessione:L.CONNECT.requestL.CONNECT.indicationL.CONNECT.confirm

Trasferimento dei dati:L.DATA_CONNECT.requestL.DATA_CONNECT.indicationL.DATA_CONNECT.confirm

Chiusura della Connessione:L.DISCONNECT.requestL.DISCONNECT.indicationL.DISCONNECT.confirm

LLC tipo 2

R t d ll C i


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Reset della Connessione:L.RESET.request

L.RESET.indicationL.RESET.confirm

Flow control:

L.FLOWCONTROL.requestL.FLOWCONTROL.indication

LLC tipo 2

Il t l l 1 2 B t


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Il campo control pu essere lungo 1 o 2 Byte Il campo control lungo 2 Byte:

I: information il pacchetto di dati. I numeri disequenza sono lunghi 7 bitRR: Receiver Ready un ACK frame quando non

esiste traffico per piggybackingRNR: Receiver Not Ready come RR, ma invita il

trasmettitore a sospendere la trasmissione

REJ: Reject indica un errore di trasmissione. Ilmittente deve ritrasmettere tutti i pacchetti a partireda quello errato

The ultimate winner in the LAN war


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Reti di Calcolatori - Slide 23

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Transcript of Reti di Calcolatori - Slide 23