service and protocol (i) · sap. vertical communication delivered by peer entities in layer (n)...

Modeling and Simulation of Communication Systems and Networks

Summer Term 2010

3. Finite State Machines 1

Modeling and Simulation of

Communication Systems and Networks

Service and Protocol (I)

Modeling & Simulation - 3. Finite State Machines 2

Service User

1

Service User

2

Service Provider

1

Service Provider

2


Summer Term 2010


Service and Protocol (II)


Peer entities on one layer

Utilize the service of the layer below (if there is one)

Offer their service to the layer above (if there is one)

Only know the interface to the service beneath, but NOT its

implementation

Communicate with each other according to specific rules

Receive input

Generate events

Thus, the service is delivered by cooperation of the peer

entities in one layer

Definition of “Service”


Service = Set of functions in one layer.

Delivered by co-operating / communicating entities of the layer, which obey a specific protocol.

Offered at the service access point of the layer.

Defined by a set of service primitives and rules for their usage.

Utilized or indicated by service primitives:

Request

Indication

Response

Confirmation


Summer Term 2010


Illustration of “Service”


Abstract Medium Mi

Commu-

nicating

Entity

Ii+11

Commu-

nicating

Entity

Ii+12

Commu-

nicating

Entity

Ii+1n-1

Commu-

nicating

Entity

Ii+1n

...

Service Primitive

Service

Interface Di

Service

Access

Point

Layer (N)

Layer (N)-Service


Set of functions offered by layer

(N) at its Service Access Point (N)-

SAP.

vertical communication

Delivered by peer entities in layer

(N) that communication with each

other utilizing (N-1)-service.

horizontal communication

How the (N)-service is achieved

remains hidden in Layer (N).

(N) - SAP

(N+1)-entity

(N)-entity

(N) - SAP

(N+1)-entity

(N)-entity


Summer Term 2010


The Idea Behind Protocols


Rules for concurrently running, but co-operating entities

Enhancement of the quality and the functions of the layer below

Distributed algorithm

Problem: failures and faults must be considered

Specification

In most cases only for two entities

Finite state machines

Message-sequence-charts (MSC)

Finite State Machine


TConReq

TConCnf

TPAboInd1

TDisInd

TConRsp

TDisReq

TConInd

TPAboInd2

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

States for data exchange and

connection release

; TPAboInd1, TPAboInd2

TConRsp;

TConCnf

TConReq;

TConInd

TConReq;

TPAboInd1

; TPAboInd1, TPAboInd2

TDisReq; TDisInd

Connection established Idle

Connection being

established


Summer Term 2010


Communication Networks -11. The Internet 9

Clo

se P

assive

Clo

se A

cti

v

Co

nn

ec

tio

n E

stab

lish

me

nt

Example: TCP Connection Management Closed

Listen

SYN rcvd SYN sent

Estblshd

FIN wait1

FIN wait2

Closed

Closing

Timed wait

Close wait

Last ACK

Connect; SYN

Close; -

Listen; - Close; -

SYN; SYN+ACK

RST; - Send

SYN; SYN+ACK

(synchronous)

ACK; -

SYN+ACK; ACK

Close; FIN Close; FIN FIN; ACK

FIN; ACK

Close; FIN

FIN; ACK

ACK; - ACK; -

ACK; -

FIN+ACK;

ACK

(Timeout; -)

Example: Alternating Bit Protocol


The Alternating Bit Protocol implements a service for confirmed data

transfer based on a service for unconfirmed data transfer.

Service primitives are:

DtReq;

DtInd;

DtRsp;

DtCnf.

Acknowledgment is done with alternating acknowledgment numbers 0

and 1. Thus, one can distinguish between message and acknowledgement

loss.

After the confirmed data transfer has been completed, the protocol

entity enters the idle state until the next data transfer is requested.


Summer Term 2010


Service Description with a

Message Sequence Chart


SAP

AO

SAP

BO

ODtReq(XX)

ODtCnf(YY)

ODtReq(ZZ)

ODtCnf(UU)

ODtInd(XX)

ODtRsp(YY)

ODtInd(ZZ)

ODtRsp(UU)

Sender Receiver

Medium O

Finite State Machine - Entity S


Entity S

Start and

Idle

State

Awaiting

Ack 0

Awaiting

Ack 1

Idle

State

UDtIndBA(0,<m,1>);

ODtCnf(m)

ODtReq(n);

UDtReqAB(<n,0>)

UDtIndBA(0,<m,0>);

ODtCnf(m)

ODtReq(n);

UDtReqAB(<n,1>)

UDtIndBA(0,<m,1>)

or

UDtIndBA(1,<m,b>);

UDtReqAB(<n,0>)

UDtIndBA(0,<m,0>)

or

UDtIndBA(1,<m,b>);

UDtReqAB(<n,1>)

Medium O

Medium U

SAP AO

SAP AU UDtReqAB(<n,b>) UDtIndBA(f,<m,b>)

ODtReq(n) ODtCnf(m)


Summer Term 2010


Entity S Described in a

Message Sequence Chart


SAP

AO

SAP

AU

UDtReqAB(<XX,0>)

UDtReqAB(<XX,0>)

UDtIndBA(1,<YY,0>)

UDtIndBA(0,<YY,0>)

UDtReqAB(<ZZ,1>)

UDtIndBA(0,<UU,1>)

Medium U

ODtReq(XX)

ODtCnf(YY)

ODtReq(ZZ)

ODtCnf(UU)

<XX,0>

<YY,0> Error

<XX,0>

<ZZ,1>

<UU,1>

<YY,0>

Entity S

Definition of PDUs


Octet 0: contains sequence number b in the lowest bit

Octet 1+2: codes the length of the user data

Remaining Octets: user data n

x x x x x x x b i i i i i i i i i i i i i i i i n n n n n n n n n n n n n n n n

7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0

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

l

Length of User Data

n

User Data as submitted by O

Example: PDU format <n, b>

...

...

...


Summer Term 2010



ASN.1 ASN.1 (Abstract Syntax Notation 1) is a standard defined

by ISO ASN.1 allows

definition of data types

specification of values

Data types are subdivided into 4 classes: Universal: Globally defined data types, e. g. Integer

Application: Data types that are defined in some othe (application oriented) standard (e. g. FTAM, MHS)

Private: Data types which have been defined for some specific non-standard application

Context-Specific: Some shorter represenation of data types that are valid only within a given (application) context

ASN.1: Universal Data Types


Primitive data types:

Boolean, Integer, Bit String, Octet String, IA5 String, …

Constructed data types:

Sequence: Ordered list of data types (cf. record in PASCAL)

Set: Unordered set of data types

Sequence OF: Ordered list of elements of the same data type

(cf. array in PASCAL)

Set OF: Unordered set of elements of the same data type

Choice: Unordered set of data types, of which some may be chosen

(cf. variant record in PASCAL)

Example:

Staff_Member ::= Set { name IA5String,

year_of_birth Integer,

personnel_nr Integer }


Summer Term 2010



ASN.1: Coding & Decoding Basic Encoding Rules, BER:

Bit 7 6 5 4 3 2 1 0

Type Length Value

Example: Integer Value 5

Universal

Primitive 2

Integer

0 0 0 0 0 0 1 0

Type Length

0 0 0 0 0 0 1 0

1

0 0 0 0 0 1 1 0

Value

5

Class of Type 00: Universal 01: Application 02: Context Specific 03: Private

Sort of type 0: Primitive 1: Constructed

Tag 0..30: as given 31: next (complete) byte contains tag

Complete Example


Medium O

Medium U

ODtReq(n) ODtCnf(m) ODtInd(n) ODtRsp(m)

Entity

S

Entity

R

AU BU

AO BO

UDtReqAB(n) UDtIndBA(f,m) UDtReqBA(m) UDtIndAB(f,n)


Summer Term 2010


Service O


Medium O

ODtReq(n) ODtCnf(m) ODtInd(n) ODtRsp(m)

AO BO

Awaiting

Response

Start and

Idle State

ODtReq(n); ODtInd(n)

ODtRsp(m); ODtCnf(m)

Basic Service U


Medium U

UDtReqAB(n) UDtIndBA(f,m) UDtIndAB(f,n) UDtReqBA(m)

AU BU

Awaiting

Transfer

Start and

Idle State

UDtReqAB(n); UDtIndAB(0,n)

UDtReqAB(n); UDtIndAB(1,x)

UDtReqBA(m); UDtIndBA(0,m)

UDtReqBA(m); UDtIndBA(1,y)


Summer Term 2010


Entity R


Entity R

Received

Ack 0

Idle

State

Start and

Idle

State

Received

Ack 1 UDtIndAB(0,<n,1>);

ODtInd(n)

ODtRsp(m);

UDtReqBA(<m,0>)

UDtIndAB(0,<n,0>);

ODtInd(n)

ODtRsp(m);

UDtReqBA(<m,1>)

UDtIndAB(0,<n,0>)

or

UDtIndAB(1,<n,b>);

UDtReqBA(<m,0>)

UDtIndAB(0,<n,1>)

or

UDtIndAB(1,<n,b>);

UDtReqBA(<m,1>)

Medium O

Medium U

SAP BO

SAP BU UDtReqBA(m) UDtIndAB(f,n)

ODtRsp(m) ODtInd(n)

Complete Message Sequence Chart


SAP

AO

SAP

AU

UDtReqAB(<XX,0>)

UDtReqAB(<XX,0>)

UDtIndBA(1,<YY,0>)

UDtIndBA(0,<YY,0>)

UDtReqAB(<ZZ,1>)

UDtIndBA(0,<UU,1>)

ODtReq(XX)

ODtCnf(YY)

ODtReq(ZZ)

ODtCnf(UU)

<XX,0>

<YY,0> Error

<XX,0>

<ZZ,1>

<UU,1>

<YY,0>

UDtIndAB(0,<XX,0>) ODtInd(XX)

ODtRsp(YY)

ODtInd(ZZ)

ODtRsp(UU)

UDtReqBA(<YY,0>)

UDtIndAB(0,<XX,0>)

UDtReqBA(<YY,0>)

UDtIndAB(0,<ZZ,1>)

UDtReqBA(<UU,1>)

SAP

BU

SAP

BO

Entity S Entity R Medium U


Summer Term 2010


References


Halsall, F. (1996). Data Communications, Computer Networks, and Open Systems. Harlow, England; Reading, Massachusetts; Menlo Park, California, Addison-Wesley.

Lynch, W. (1968). "Reliable full-duplex file transmission over half-duplex telephone lines“. Communications of the ACM 11 (6): 407–410.

Popovic, M. (2006). Communication Protocol Engineering. Boca Raton, CRC Press Inc.

Tanenbaum, A. S. (2002). Computer Networks. Upper Saddle River, New Jersey, Pearson Education.

Venkataram, P. and S.S. Manvi (2004). Communication Protocol Engineering. Delhi, Prentice-Hall of India Pvt. Ltd.

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