Communication Networks - midas1.e-technik.tu-ilmenau.de

Communication Networks Winter 2019/20

Prof. Jochen Seitz 1

Communication Networks

Chapter 3 – Protocol Specification

Communication Networks: 3. Protocol Specification 104

Overview

1. Message Sequence Charts (MSC)

2. Finite State Machines (FSM)

3. Special Communication Scenarios

4. PDU Format

5. Example: Alternating Bit Protocol (ABP)


104

105



Example World Wide Web


1. Message Sequence Chart

Web Client

Web Server(http://www.tu-ilmenau.de/kn)

Internet

Illustration of a Telecommunication Service (repeated)



Abstract Medium MiAbstract Medium Mi

Communi-catingEntity

Ii+11


Ii+11


Ii+12


Ii+12

Communi-catingEntityIi+1

N-1


N-1


N


N

…

ServiceInterfaceof Layer i

ServiceAccessPoint (SAP)

Exchange ofServicePrimitives

106

107



Message Sequence Charts



SAP1 SAP2 SAP1 SAP2Initiator Responder Initiator ResponderMedium Medium

T-Con.Req

T-Con.Ind

T-Con.Req

T-Con.Ind

T-Con.Cnf

T-Con.Rsp

T-PAbo.Ind

T-PAbo.Ind

ServicePrimitive

t t t t

2. Finite State Machine

Basic Concept of FSM

• Finite number of states, identified by a (meaningful) name

• Explicitly marked starting state

• State transition:

• Events: Service Primitives (possibly with parameters)


Idle Next StateTriggering Event;Resulting Event

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109



Finite State Machine



T-Con.ReqT-Con.CnfT-PAbo.Ind1T-Dis.Ind

T-Con.RspT-Dis.Req

T-Con.IndT-PAbo.Ind2

.... .... ....

States for data exchange andconnection release

; T-PAbo.Ind1, T-PAbo.Ind2

T-Con.Rsp;T-Con.Cnf

T-Con.Req;T-Con.Ind

T-Con.Req;T-PAbo.Ind1

; T-PAbo.Ind1, T-PAbo.Ind2

T-Dis.Req; T-Dis.Ind

ConnectionestablishedIdle

Connectionbeing

established

TransportLayer

Connection-Oriented Services



IdleConnectionestablished

Procedures for connection release

Procedures forconnection abort

Procedures forconnection establishment

Proceduresfor data

exchange

110

111



Connection Establishment



112

Con.Req(Address of Initiator,

Address of Responder,Requested QoS,

Further Attributes,Short User Data1)

Con.Ind(Address of Initiator,Address of Responder,Requested QoS*,Further Attributes*,Short User Data1)

Con.Rsp(Address of Initiator,Address of Responder,Assurable QoS†,Further Attributes†,Short User Data2)

Con.Cnf (Address of Initiator,

Address of Responder,Assurable QoS†,

Further Attributes†,Short User Data2)

Initiator Responder * and † mean that theaccording informationmight have been altered

* and † mean that theaccording informationmight have been altered

Medium

Several Connections at One SAP



Service Access Point

ConnectionEnd Point

Con.ReqCon.IndCon.Rsp

Con.Cnf

being

established

112

113



Unconfirmed Data Transfer



Dat.Req (User Data1)

Dat.Ind (User Data1)





Medium

full duplexcommunication


Types of Data Transfer

• „normal“ data transfer: Dat.Req/Dat.Ind

• expedited data transfer: ExpDat.Req/ExpDat.Ind


ExpDat.Req(EData1)

ExpDat.Ind(EData1)

Dat.Req(NData1)

Dat.Ind(NData1)

ExpDat.Req(EData2)

ExpDat.Req(EData3)

Dat.Req(NData2) ExpDat.Ind(EData2)

ExpDat.Ind(EData3)

Dat.Ind(NData2)

Medium

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115



Confirmed Service vs. Acknowledgement

Confirmed Service

• Confirmation initiated by receiving service user

• All four types of service primitives

Acknowledgement

• Acknowledgement sent by service providing entity

• Part of the protocol



SAP1 SAP2Initiator ResponderMedium

T-Con.Req

T-Con.Ind

T-Con.Cnf

T-Con.Rsp

SAP1 SAP2Initiator ResponderMedium

T-Dat.Req

T-Dat.Ind

Connection Release

Confirmed Connection Release

• Communication partner confirms release or rejects

• Used on higher ISO/OSI layers

Unconfirmed Disconnect

• Connection release cannot be rejected

• Used on the lower ISO/OSI layers



Rel.Req(User Data1)

Rel.Rsp(Indication±,User Data2)

Rel.Ind (User Data1)

Rel.Cnf(Indication±,User Data2)

Medium

Dis.Req

Dis.Ind

Medium

116

117



Connection Abort

Provider Abort (PAbo)

• Provider cannot keep up the requested service anymore

User Abort (UAbo)

• User cannot keep up the association anymore



PAbo.Ind(Reason)

PAbo.Ind(Reason)

UAbo.Ind(User Data)

UAbo.Req(User Data)

Telecommunication Protocol


4. PDU Format

Abstract Medium Mi

Ii+12Ii+12Ii+1

1Ii+11

Abstract Medium Mi-1Abstract Medium Mi-1

Ii2Ii2Ii

1Ii1 Protocol

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119



Example: Browsing the Internet


4. PDU Format

ProtocolEntity

ProtocolEntity

ServiceUser Basic Service

User of a Web Browser

Web Server

ServiceUser

T-Con.Req

T-Con.Ind

T-Con.RspT-Con.Cnf

T-Dat.Req

T-Dat.Ind

T-Dat.Ind

T-Dat.Req

Present Page Contentwww.ieee.org

Page Requestwww.ieee.org/index.html

Deliver Page Contentwww.ieee.org/index.html

Establish transportconnection Accept

transportconnection

Send URL of therequested page

Send the contentsof the requestedweb page

Relay URL toweb server

Relay page content to web browser

HTTP Entity HTTP Entity

Inputwww.ieee.org

4. PDU Format

Defining the PDU Format

• For defining the PDU format

▪ define (max.) length of PDU

▪ define the fields in the PCI (header) according to the protocol functions implemented in the protocol

❖addresses

❖numbers and acknowledgements

❖ length

❖ information for flow control or load control

❖ checksum

❖…

• Formal definition required (syntax and semantics) Abstract Syntax Notation 1 (ASN.1)


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121



5. Alternating Bit Protocol

Example: Alternating Bit Protocol (ABP)

• Confirmed data transfer service based on the unconfirmed data transfer service of the lower layer

• Service primitives to exchange User Data:

▪ Dat.Req (User Data)

▪ Dat.Ind (User Data)

▪ Dat.Rsp (User Data)

▪ Dat.Cnf (User Data)

• Alternating acknowledgement number (0 and 1) to distinguish between message loss and acknowledgement loss


ABP Service in a Message Sequence Chart



SAPAABP

SAPBABP

Dat.Req(XX)

Dat.Cnf(YY)

Dat.Req(ZZ)

Dat.Cnf(UU)

Dat.Ind(XX)

Dat.Rsp(YY)

Dat.Ind(ZZ)

Dat.Rsp(UU)

Medium O

122

123



Finite State Machine forEntity S▪ Initiates data transfer

▪ Starts with bit “b” set to 0

▪ Incoming message also requires “b” = 0

▪ Bit “f” signals whether error was detected by abstract medium L:

▪ f=0: no error

▪ f=1: error

Communication Networks: 2. Fundamentals 124


Entity SEntity SStart &

IdleState

AwaitingAck 0

AwaitingAck 1

IdleState

L-Dat.IndBA(0,<m,1>);Dat.Cnf(m)

Dat.Req(n);L-Dat.ReqAB(<n,0>)

L-Dat.IndBA(0,<m,0>);Dat.Cnf(m)

Dat.Req(n);L-Dat.ReqAB(<n,1>)

L-Dat.IndBA(0,<m,1>)orL-Dat.IndBA(1,<m,b>);L-Dat.ReqAB(<n,0>)

L-Dat.IndBA(0,<m,0>)or

L-Dat.IndBA(1,<m,b>);L-Dat.ReqAB(<n,1>)

Abstract Medium with ABP

Abstract Medium L

SAP AABP

SAP AL

L-Dat.ReqAB(<n,b>) L-Dat.IndBA(f,<m,b>)

Dat.Req(n) Dat.Cnf(m)

Entity S Described by Message Sequence Chart



SAPAABP

SAPAL

L-Dat.ReqAB(<XX,0>)

L-Dat.IndBA(1,<YY,0>)

Medium U

Dat.Req(XX)

Dat.Cnf(YY)

Dat.Req(ZZ)

Dat.Cnf(UU)

<XX,0>

<YY,0>Error

<XX,0>

<ZZ,1>

<UU,1>

<YY,0>

Entity S

124

125



BodyHeader


PDU Definition for ABP

• Byte 0: sequence bit „b“ in the lowest bit

• Bytes 1 and 2: length of user data (positive binary number)

• Remaining bytes: user data n


xx xx xx xx xx xx xx bb ll ll ll ll ll ll ll ll ll ll ll ll ll ll ll ll nnnnnnnnnnnnnnnn nnnnnnnnnnnnnnnn

76543210 76543210 76543210 76543210 76543210

Octet 0 Octet 1 Octet 2 Octet 3 Octet 3+l-1

lLength of User Data

nUser Data as submitted by O

...

...

...

Complete Example



Abstract Medium with ABPAbstract Medium with ABP

Abstract Medium LAbstract Medium L

Dat.Req(n) Dat.Cnf(m) Dat.Ind(n)Dat.Rsp(m)

EntityS

EntityS

EntityR

EntityR

AL BL

AABP BABP

L-Dat.ReqAB(n) L-Dat.IndBA(f,m) L-Dat.ReqBA(m) L-Dat.IndAB(f,n)

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127



Service Provided by the Two ABP Entities



Abstract Medium with ABPAbstract Medium with ABP

Dat.Req(n) Dat.Cnf(m) Dat.Ind(n)Dat.Rsp(m)

AABP BABP

AwaitingResponse

Start andIdle State

Dat.Req(n); Dat.Ind(n)

Dat.Rsp(m); Dat.Cnf(m)

Service Provided by Abstract Medium L



Abstract Medium LAbstract Medium L

L-Dat.ReqAB(n) L-Dat.IndBA(f,m) L-Dat.IndAB(f,n)L-Dat.ReqBA(m)

AU BU

AwaitingTransfer

Start andIdle State

L-Dat.ReqAB(n); L-Dat.IndAB(0,n)

L-Dat.ReqAB(n); L-Dat.IndAB(1,x)

L-Dat.ReqBA(m); L-Dat.IndBA(0,m)

L-DatReqBA(m); L-Dat.IndBA(1,y)

128

129



Finite State Machine for Entity R• Responds to entity S

• Cannot initiate communication

• First incoming message (from abstract medium L) expected to have bit „b“ set to 0



Entity REntity R

ReceivedAck 0

IdleState

Start &Idle

State

ReceivedAck 1 L-Dat.IndAB(0,<n,1>);

Dat.Ind(n)

Dat.Rsp(m);L-Dat.ReqBA(<m,0>)

L-Dat.IndAB(0,<n,0>);Dat.Ind(n)

Dat.Rsp(m);L-Dat.ReqBA(<m,1>)

L-Dat.IndAB(0,<n,0>)orL-Dat.IndAB(1,<n,b>);L-Dat.ReqBA(<m,0>)

L-Dat.IndAB(0,<n,1>)or

L-Dat.IndAB(1,<n,b>);L-Dat.ReqBA(<m,1>)

Abstract Medium with ABP

Abstract Medium L

SAP BABP

SAP BL

L-Dat.ReqBA(m) L-Dat.IndAB(f,n)

Dat.Rsp(m) Dat.Ind(n)

Complete Message Sequence Chart



SAPAABP

SAPAL

L-Dat.ReqAB(<XX,0>)

L-Dat.IndBA(1,<YY,0>)

Dat.Req(XX)

Dat.Cnf(YY)

Dat.Req(ZZ)

Dat.Cnf(UU)

<XX,0>

<YY,0>Error

<XX,0>

<ZZ,1>

<UU,1>

<YY,0>

L-Dat.ReqBA(<YY,0>)

L-Dat.IndAB(0,<XX,0>)

L-Dat.ReqBA(<YY,0>)

SAPBL

SAPBABPEntity S Entity RMedium L

ODtInd(XX)

ODtRsp(YY)

ODtInd(ZZ)

ODtRsp(UU)

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References

• Lockemann, Peter C.; Krüger, Gerhard; Krumm, Heiko (1993): Telekommunikation und Datenhaltung. München, Wien: Carl Hanser Verlag.

• Lynch, William C. (1968): Reliable full-duplex file transmission over half-duplex telephone lines. In: Communications of the ACM 11 (6), S. 407–410.

• Popovic, Miroslav (2006): Communication Protocol Engineering. Boca Raton, FL, USA: CRC Press.

• Seitz, Jochen; Debes, Maik (2016): Kommunikationsnetze. Eine umfassende Einführung. Anwendungen – Dienste – Protokolle. Ilmenau: Unicopy Campus Edition.

• Venkataram, Pallapa; Manvi, Sunilkumar S. (2008): Communication Protocol Engineering. New Delhi: Prentice-Hall of India.


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Communication Networks - midas1.e-technik.tu-ilmenau.de