osi-iso model

26
Computer S y stems (159.253) ~ 1 ~Data Communications: © P.L y ons 2004 OSI-ISO model The ISO-OSI Model Data communications Immensely complex Many manufacturers Many types of data ed tools for ilitating interconnection of heterogeneous systems ndards ucing complexity ering d a model to bring this all together Tanenbaum 3 rd edition: 28-42

Upload: maeve

Post on 12-Jan-2016

51 views

Category:

Documents


0 download

DESCRIPTION

T he ISO-OSI M odel. OSI-ISO model. Tanenbaum 3 rd edition: 28-42. Data communications Immensely complex Many manufacturers Many types of data. We need tools for Facilitating interconnection of heterogeneous systems Standards Reducing complexity Layering. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: OSI-ISO model

Computer Systems (159.253) ~ 1 ~Data Communications: © P.Lyons 2004

OSI-ISO modelThe ISO-OSI Model

Data communicationsImmensely complexMany manufacturersMany types of data

We need tools for Facilitating interconnection of heterogeneous systemsStandardsReducing complexityLayering

Need a model to bring this all together

Tanenbaum 3rd edition: 28-42

Page 2: OSI-ISO model

Computer Systems (159.253) ~ 2 ~Data Communications: © P.Lyons 2004

The model of data communicationsFacilitates communication about data communicationdiscussion of functionality in commonly understood terms

The ISO-OSI Model

Not a widely implemented set of protocolsovertaken by success of the Internet (TCP/IP)TCP/IP not completely consistent with ISO model

OSI-ISO model

Page 3: OSI-ISO model

Computer Systems (159.253) ~ 3 ~Data Communications: © P.Lyons 2004

The ISO-OSI Model

Essential ingredient in a multi-manufacturer industryTiming of standardisation critical

Too soon - research continues after standardisationresult - non-compliant systems

Too late - multimillion dollar investments in nonstandard technologyresult - non-compliant system

STANDARDISATION

activity

time

research investment

standardisation

too so

ontoo late

just right

"Apocalypse of the two elephants"

Standardisation

ElephantElephant Elephant

Page 4: OSI-ISO model

Computer Systems (159.253) ~ 4 ~Data Communications: © P.Lyons 2004

A divide and conquer approach

The ISO-OSI Model

LAYERING

Layer n Protocol entity

Layer n-1 service requests

SAP - (Service Access point)

PDUs (Protocol data units)

layer n+1

layer n-1

Layer n service requests

Layer n Protocol entity

layer n+1

Layer n service requests

Layer n-1 service requests

layer n-1

Layering

Page 5: OSI-ISO model

Computer Systems (159.253) ~ 5 ~Data Communications: © P.Lyons 2004

bitstreamPhysical

receiver

Application

data

Presentation

Session

Transport

Network

Data Link

Network

Data Link

router1

Network

Data Link

router1

The ISO-OSI Model

TPDU

Packet

Frame

THE SEVEN-LAYER MODELSeven layer model

APDU

PPDU

SPDU

datasender

Application

Presentation

Session

Transport

Data Link

Network

Page 6: OSI-ISO model

Computer Systems (159.253) ~ 6 ~Data Communications: © P.Lyons 2004

datasender

bitstream

Presentation

Session

Transport

Network

Data Link

Physical

Application

Network

Data Link

receiver

Application

data

bitstream

Network

Data Link

Presentation

Session

Transport

Network

Data Link

router1 router1

The ISO-OSI Model

THE SEVEN-LAYER MODELSeven layer model

Page 7: OSI-ISO model

Computer Systems (159.253) ~ 7 ~Data Communications: © P.Lyons 2004

Seven layer model

datasender

Presentation

Session

Transport

Network

Data Link

Physical

Application

Network

Data Link

Application

bitstream

Network

Data Link

bitstream

Presentation

Session

Transport

Network

Data Link

data

data

data

data

data

data

data

receiver

router1 router1

The ISO-OSI Model

THE SEVEN-LAYER MODEL

Page 8: OSI-ISO model

Computer Systems (159.253) ~ 8 ~Data Communications: © P.Lyons 2004

Seven layer model

datasender

Presentation

Session

Transport

Network

Data Link

Physical

Application

Network

Data Link

Application

Network

Data Link

Presentation

Session

Transport

Network

Data Link

receiver

router1 router1

The ISO-OSI Model

THE SEVEN-LAYER MODEL

Page 9: OSI-ISO model

Computer Systems (159.253) ~ 9 ~Data Communications: © P.Lyons 2004

Seven layer model

Administers connections, QOS, transfers error-free data (end-to-end )

Sets up and administers sessions, synchronises after upper-layer errors

Presentation

Session

Transport

Network

Data Link

Physical

Application

The ISO-OSI Model

Maps bitstream onto medium Volts, timing, mechanical specs

Transfers error-free data (point to point ) Error correction, data frames, ack frames

Delivers data Routing (tables-based, flooding), address translation

5 transport protocol classes allow for range of network service standards

Map between user sessions and transport connections

Data representation, encryption, compression

ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression

Standard data comms apps Email, WWW, file transfer

Physical

Responsibility Protocol

Data Link

Network

Transport

Session

Presentation

Application

Node-To-Node

End-to-end

Upper (user) layers

Network

THE SEVEN-LAYER MODEL

Page 10: OSI-ISO model

Computer Systems (159.253) ~ 10 ~Data Communications: © P.Lyons 2004

Seven layer modelThe ISO-OSI Model

Data Link

Administers connections, QOS, transfers error-free data (end-to-end )

Application

Transfers error-free data (point to point )

5 transport protocol classes allow for range of network service standards

Map between user sessions and transport connections

ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression

Sets up and administers sessions, synchronises after upper-layer errors

Presentation

Session

Transport

Network

PhysicalMaps bitstream onto medium Volts, timing, mechanical specs

Error correction, data frames, ack frames

Delivers data Routing (tables-based, flooding), address translation

Data representation, encryption, compression

Standard data comms apps Email, WWW, file transfer

Physical

Responsibility Protocol

Network

Transport

Session

Presentation

Application

Node-To-Node

End-to-end

Upper (user) layers

Network

THE SEVEN-LAYER MODEL

Page 11: OSI-ISO model

Computer Systems (159.253) ~ 11 ~Data Communications: © P.Lyons 2004

Layer 2 (Data Link Layer)

Data Link

Physical

Network

Bit sequence only

Link establishment and termination; messages received and for transmissionData Link

peer-to-peer (virtual) communicationlink management; transfer of error-free messages

LAYER 2 (THE DATA LINK LAYER)

real communication

The ISO-OSI Model

Page 12: OSI-ISO model

Computer Systems (159.253) ~ 12 ~Data Communications: © P.Lyons 2004

HDLCHDLC

The ISO-OSI Model

A synchronous communications techniqueAsynchronous techniques allow for clock drift between sender and receiver

Raw dataSampling times

Manchester encoding is used for asynchronous communicationsEvery bit involves a transitionData acts as its own synch pulse

Drawback; “housekeeping” transitions increase the bandwidth requirement

- AN IMPLEMENTATION OF LAYER 2

Page 13: OSI-ISO model

Computer Systems (159.253) ~ 13 ~Data Communications: © P.Lyons 2004

HDLCHDLC

The ISO-OSI Model

HDLC is synchronousSender inserts SYN character occurs at start and end of large block of data

Receiver recognises bit pattern of SYN &sets its clock to sample signal in the middle of each bit

Synchronisation must last for the whole of the current block

- AN IMPLEMENTATION OF LAYER 2

Page 14: OSI-ISO model

Computer Systems (159.253) ~ 14 ~Data Communications: © P.Lyons 2004

Asynchronous techniques use frequent synchronisation events to stay in synch

HDLC

The ISO-OSI Model

HDLC is synchronousSender inserts SYN character occurs at start and end of large block of data

Receiver recognises bit pattern of SYN &sets its clock to sample signal in the middle of each bit

operates between adjacent nodes in a network

need not operate over a reliable medium

may be full duplex

uses sliding window acknowledgement

is responsible for error-free data transfer

HDLC

Page 15: OSI-ISO model

Computer Systems (159.253) ~ 15 ~Data Communications: © P.Lyons 2004

FRAMEHDLC

The ISO-OSI Model

FLAG 01111110

ADDRESS FIELDOctet (8-bit sequence) specifying destination terminal for frame when using multidrop line

CONTROL FIELDOctet containing sequencing and protocol information

INFORMATION FIELDANY combination of 0 or more bits

FRAME CHECK10-bit CRC check on everything between flags

The HDLC Frame

Page 16: OSI-ISO model

Computer Systems (159.253) ~ 16 ~Data Communications: © P.Lyons 2004

HDLC

The ISO-OSI Model

CONTROL FIELDOctet containing sequencing and protocol information

Distinguishes betweenInformation framesNumbered supervisory framesUnnumbered supervisory frames

CONTROL FIELD

HDLC Control Field

Page 17: OSI-ISO model

Computer Systems (159.253) ~ 17 ~Data Communications: © P.Lyons 2004

HDLC Information Frame

Distinguishes betweenInformation framesNumbered supervisory framesUnnumbered supervisory frames

HDLC

The ISO-OSI Model

CONTROL FIELD

Poll/final bitSend count Receive count0

0 denotes information frame

Send count is sequence number of current frameWhen frames arrive correctly, receiving station stores arriving send count + 1 as receive count Arriving frame’s send count should always equal receiving station’s receive countReceiving station does not increment its receive count till arriving frame checks out

Receive count in the HDLC frame is seq no. of next frame expected by the senderReceiver compares incoming receive count with its send countSends frames starting with incoming receive count value

P/F bit is set by primary station when polling, by secondary when finished

: INFORMATION FRAME

Page 18: OSI-ISO model

Computer Systems (159.253) ~ 18 ~Data Communications: © P.Lyons 2004

HDLC Numbered Supervisory FrameThe ISO-OSI Model

CONTROL FIELD

Poll/final bit

Receive count

HDLC : NUMBERED SUPERVISORY FRAME

Send count01 0 Function

10 denotes numbered supervisory frameFrame carries information payload (hence number), and specifies a supervisory function

00 Receive Ready 01 Reject

notification of sequence errorMust retransmit all frames from Receive Count onwards

10 Receive Not Ready11 Selective Reject

need only retransmit specified frame

Function field

: INFORMATION FRAME

Page 19: OSI-ISO model

Computer Systems (159.253) ~ 19 ~Data Communications: © P.Lyons 2004

HDLC Unnumbered Supervisory Frame: UNNUMBERED SUPERVISORY FRAME

The ISO-OSI Model

CONTROL FIELDHDLC

Poll/final bit

function Part BReceive count1 0 functionpart A

1 1 Function

11 denotes unnumbered supervisory frame

5-bit code for network housekeepingReset countersDisconnectQuery identityTestetc

Function field

: NUMBERED SUPERVISORY FRAME

Page 20: OSI-ISO model

Computer Systems (159.253) ~ 20 ~Data Communications: © P.Lyons 2004

ProblemHDLC’s flag sequence is 01111110Data may include 01111110arbitrary length, so flag’s position can’t be predictedconflict if data gets mistaken for flag

The ISO-OSI Model

HDLC

Solution Accept data containing flag sequence from level 3Deliver data containing flag sequence to level 3BUT at level 2 transmitter, add an extra bit to the data prevents flag sequence from occurring in data part of transmitted bit stream

: BIT STUFFINGHDLC Bit Stuffing

Page 21: OSI-ISO model

Computer Systems (159.253) ~ 21 ~Data Communications: © P.Lyons 2004

0111111x

x10111110

The ISO-OSI Model

HDLC: BIT STUFFING01

1111

1x

HDLC HDLC

HDLC Bit Stuffing

Page 22: OSI-ISO model

Computer Systems (159.253) ~ 22 ~Data Communications: © P.Lyons 2004

ARM Asynchronous Response ModeSecondary may be polled butmay initiate transmission without being polledmay result in contention (cf. CSMA)

The ISO-OSI Model

HDLC: MODES

NRM Normal Response ModeSecondary is polled, starts transmitting frame sequenceSecondary sets F bit in final frameMay not transmit again till polled again

ABRM Asynchronous Balanced Response ModeTwo stations:Each sends as primary, receives as secondary

HDLC Modes

Page 23: OSI-ISO model

Computer Systems (159.253) ~ 23 ~Data Communications: © P.Lyons 2004

HDLC Sliding Window Acknowledgement

Poll/final bitSend count Receive count

00

0000 00

The ISO-OSI Model

HDLC: SLIDING WINDOW ACKNOWLEDGEMENT

0

1

2

34

5

6

7

Send count0

1

2

34

5

6

7

Receive count0

1

2

34

5

6

7

Send count0

1

2

34

5

6

7

Receive count

00

frame.sendCount := sendCountframe.sendCount := sendCountframe.receiveCount := receiveCountframe.sendCount := sendCountframe.receiveCount := receiveCountframe.Data := data

frame.sendCount := sendCountframe.receiveCount := receiveCountframe.Data := data…frame.send

frame.sendCount := sendCountframe.receiveCount := receiveCountframe.Data := data…frame.sendsendCount++

If checksOutOK(frame.CRC) thenIf frame.sendCount = receiveCount then begin Frame.accept sendCount := frame.receiveCount receiveCount++ end

11

Page 24: OSI-ISO model

Computer Systems (159.253) ~ 24 ~Data Communications: © P.Lyons 2004

abbreviated representation of admin. information transfer and updating

The ISO-OSI Model

HDLC: FRAME TRANSFER DIAGRAMS

Symbol Meaning

I(0,0) Information Frame, SN = 0, RN = 0, P/F bit = FALSE

I(1,0)P Information Frame, SN = 1, RN = 0, Poll

RR(4)F Receive Ready Frame, no SN, RN = 4, Final

HDLC Frame Transfer Diagrams

Page 25: OSI-ISO model

Computer Systems (159.253) ~ 25 ~Data Communications: © P.Lyons 2004

SecondaryPrimary

Next frame to send

N(S)

Next frame to receive

N(R)0 0 0 0

1 0 0 1

I(1,0)

Time

2 0 0 2I(2,0),P3 0 0 3I(0,3)

3 1 1 3I(1,3)F

. .. .3 2 .. ..2 3

NRM (error-free operation)

I(0,0)

Next frame to receive

N(R)N(S)

Next frame to send

The ISO-OSI Model

HDLC: FRAME TRANSFER DIAGRAMSHDLC Frame Transfer Diagrams

Page 26: OSI-ISO model

Computer Systems (159.253) ~ 26 ~Data Communications: © P.Lyons 2004

(transmission error):NRM NRM

(error-free operation)

The ISO-OSI Model

HDLC: FRAME TRANSFER DIAGRAMS

SecondaryPrimary

Next frame to send

N(S)

Next frame to receive

N(R)5 3 3 5

6 3 3 6I(6,3) Time

0 3 3 6

I(7,3)

1 3 3 6I(3,6),F

6 4 4 6I(6,4)F

. .. .7 4 .. ..4 7

I(5,3)

Next frame to receive

N(R)N(S)

Next frame to send

7 3 3 6Error: ignore

Incorrect N(S): ignore

I(0,3),PIncorrect N(S): ignore data, accept P bit

HDLC Frame Transfer Diagrams