OSI model, ATM by miv12499

VIEWS: 14 PAGES: 43

									                      Seppo J. Halme




                OSI model, ATM
                     S-72.353




March 8, 2000          L353_9.shw      Slide 1 of 14
                                             Seppo J. Halme



                 Open Systems Interconnection
        3 ISO (International Standards Organization) developed jointly a
          collection of standards for computer networks known as OSI.
        3OSI standard is ISO/IEC 7498-1 or ITU-T Rec. X.200.
        3This was preceded by several “network architectures” developed
          by leading mainframe manufacturers like IBM. The first was
          Systems Network Architecture, (SNA, 1977).
        3SNA consists of six layers, each of which has a limited amount of
          tasks that can be realized independently of other layers.
            Open systems are intended as contrast to manufacturer’s proprietary systems.
             Systems are "open" to one another by virtue of their mutual use of the
             applicable standards.
        3OSI works also as conceptual basis for protocols

March 8, 2000                                 L353_9.shw                                Slide 2 of 14
                                 Seppo J. Halme



                Schematic of OSI model


                                                  (N+1)-layer
                                                      Sub
                peer (N)-layer      N-protocol       system   (N)-layer
                                                  (N-1)-layer



                             Transmission medium

March 8, 2000                     L353_9.shw                              Slide 3 of 14
                                     Seppo J. Halme



                Comments on OSI schematic
         3 The model shows connection between two systems using a
           transmission medium
         3 Each OSI system consist of a number of layers, of which the
           subsystem of the N-layer is shown. The subsystem has possibly
           many N-entities providing needed functions.
         3 The N-layer provides services to a higher (N+1)-layer and may
           need services of a lower (N-1)-layer. For this service access
           points (SAP) are needed.
         3 The N-layer communicates with a peer N-layer of a peer OSI
           system using an N-protocol.
         3 OSI has a logically similar structure for all layers.

March 8, 2000                         L353_9.shw                           Slide 4 of 14
                                    Seppo J. Halme



                Structure of N-protocol data unit

                                    (N)-SDU
                                                     The N-protocol
                (N)-PCI
                                                     data unit (N-PDU)
                                                     consists of
                                                     - N-service data
                                                     unit (N)-SDU,
                          (N)-PDU
                                                     usually same as
                                                     (N+1)-PDU, and
                                                     - N-protocol control
                                                     information (N)-PCI


March 8, 2000                        L353_9.shw                         Slide 5 of 14
                                              Seppo J. Halme



                    Services between OSI layers
                      A                                                  B
                                                               N+1
           N+1
                 N.connect.request
                                                                     N.connect.indication


                                                                             N.connect.respon
                          N.connect.confirm                    N
           N




                                                               N-1
           N-1


        (N+1)-layer puts (N)-layer requests and gives responses.
        N-layer gives (N+1)-layer indications and confirmations.
March 8, 2000                                  L353_9.shw                                   Slide 6 of 14
                                      Seppo J. Halme



                   Example: Transport layer
        3 The transport layer (N=4) serves the session layer (N=5), which is
          called the “application program”.
        3 T.CONNECT.request is used by the application program of the
          A-side to open the transport connection.
        3 This primitive (T.CONNECT.request) is submitted to the entity
          of the A-side transport layer entity through a SAP, Service Access
          Point, in practice a memory area.
        3 T.CONNECT.indication. This primitive is submitted by the peer
          entity at the B-side transport layer to the application program.
        3 With this indication the B-side application program becomes
          aware of the attempt of the A-side to open a transport connection.

March 8, 2000                          L353_9.shw                          Slide 7 of 14
                                 Seppo J. Halme



                 Transport layer continued
        3T.CONNECT.response. This primitive comes from the
         application program of the B-side to the entity of the
         transport layer, if the application program accepts the
         attempt to form a transport connection.
        3T.CONNECT.confirm. This primitive is submitted by
         the A-side transport entity to the corresponding
         application program to inform it that the transport
         connection has been successfully started.
        3The similar primitives are used at all layers with
         different functions.

March 8, 2000                     L353_9.shw                       Slide 8 of 14
                                         Seppo J. Halme



                      Comments on primitives
         3The transport connection is an administrative entity: It
          allows management of the identity numbers of trans-
          ported PDUs. Also the state of the connection is
          managed.
         3Operation is also possible in OSI without a logical
          connection, as datagrams
         3Notation:
             The first part indicates the layer pertaining to the primitive (T
              = Transport)
             The second part (CONNECT) indicates the function of the
              primitive (related to the connection concept).
             The third part tells the type of the primitive (e.g. request).
March 8, 2000                             L353_9.shw                              Slide 9 of 14
                               Seppo J. Halme



                More comments on primitives
        3All primitives may not be used, e.g. data can be
         transmitted without confirmations: For data:
         T.DATA.request & T.DATA.response
        3Primitives contain different parameters and also
         data.
        3In implementation it is not necessary to actually
         move all the data from layer to layer. It is
         sufficient to move pointers to memory areas.

March 8, 2000                   L353_9.shw                   Slide 10 of 14
                                 Seppo J. Halme



                    The seven layer OSI model
                A                                 B
           7.                                         7.
           6.                                         6.
           5.                                         5.
           4.               Relay system              4.
           3.               3.               3.       3.
           2.               2.               2.       2.
           1.               1.               1.       1.



                         Transmission medium
March 8, 2000                     L353_9.shw               Slide 11 of 14
                                 Seppo J. Halme



         Comments on the 7 layer OSI model
         3The OSI model is in principle abstract.
         3The layers are denoted with numbers. Other names will
          be given later.
         3The actual connection between systems A and B are
          through transmission medium.
         3A relay system may exist between A and B, e.g. a packet
          switch. It provides only lower level 1-3 services.
         3The lines refer to protocols between peer entities.



March 8, 2000                     L353_9.shw                   Slide 12 of 14
                                   Seppo J. Halme



                             Layer names
         1.     Physical layer (P)
         2.     Data link layer (L)
         3.     Network layer (N)
         4.     Transport Layer (T)
         5.     Session Layer (S)
         6.     Presentation layer
         7.     Application layer (A)

March 8, 2000                       L353_9.shw      Slide 13 of 14
                              Seppo J. Halme



                   Comments on layers
        3The layers 1-4 are jointly called transport
         protocols.
        3The layers 5-7 are called application protocols.
        3The layers need not all exist and they may have
         different names and even different functions.
        3The following descriptions are just examples and
         are related to a particular state of technology
         development.
        3OSI layers are NOT used at TCP/IP.
March 8, 2000                  L353_9.shw               Slide 14 of 14
                 Typical functions of layer entities
         3 (N)-protocol-identifier
         3 centralized or decentralized multi-endpoint-connection
         3multiplexing and demultiplexing
         3 splitting and recombining
         3 flow control
         3 segmenting and reassembling
         3blocking and deblocking
         3 concatenation and separation
         3 sequencing, acknowledgement and reset
March 20, 2000                     L353_9.shw                       Slide 15 of 17
                                 Seppo J. Halme



         Comparison of OSI and TCP/IP stacks

                    7.                            Telnet, FTP, HTTP,
                    6.                            SMTP, NNTP
                    5.
                    4.                            TCP, UDP
                    3.                             IP
                    2.                            LAN protocols
                    1.
           OSI             TCP/IP
           3Comparing functionally OSI and TCP/IP stacks
            shows similarities and differences
March 13, 2000                    L353_9.shw                           Slide 16 of 17
                  Physical layer
3The Physical Layer provides the mechanical, electrical,
  functional and procedural means to activate, maintain,
  and de-activate physical-connections for bit trans-
  mission between data-link-entities. A physical-connect-
  ion may involve intermediate open systems, each relay-
  ing bit transmission within the Physical Layer. Physical
  Layer entities are interconnected by means of a physical
  medium. (ITU-T Rec. X.200)
3 In OSI model the physical layer corresponds to a large
  part of “the rest of the world”.
            Interpretation of physical layer
                     M etallic c able           O ptic al fibre


  R eal O pe n                                                        R eal O pen
                 B                      C   E                     F
  S ys tem A                                                          S ys te m B


                     D ata m ode m s            T rans ducers



3Physical layer between two open systems A and B may
 consist of different media with interfaces between them.
3The simplified example shows a metallic cable, e.g.
 subscriber line and an optical fiber used e.g. in trunk.
3The interfaces given are abstract and only the end
 interfaces relate to the physical layer.
 Physical layer services
 a. physical-connections;
 b. physical-service-data-units (bits, octets, duplex);
 c. physical-connection-endpoints (point-to-point?);
 d. data-circuit identification;
 e. sequencing;
 f. fault condition notification; and
 g. quality of service parameters.
   i. error rate, service availability, transmission rate; and transit
      delay.
1. The Physical Layer provides for the transparent trans-
   mission of bit streams between data-link-entities
   across physical-connections.
      Realization of physical layer
3Examples of interfaces are V.24 (RS-232-D) and
 X.21.
3Local area network interfaces
3Consists of lines and microcircuits together with
 controlling software.
                     Data link layer
3 The Data Link Layer provides functional and procedural means
  for (ITU-T rec. X.200)
   connectionless-mode among network-entities, and for
   connection-mode for the establishment, maintenance, and
    release data-link-connections among network-entities and for
   the transfer of data-link-service-data-units.
   A data-link- connection is built upon one or several physical-
    connections.
3 The Data Link Layer detects and possibly corrects errors which
  may occur in the Physical Layer.
3 The Data Link Layer enables the Network Layer to control the
  interconnection of data-circuits within the Physical Layer.
               Data link services
3 In connection-mode, the facilities provided by the Data
  Link Layer are (part also for connectionless mode:
   data-link-addresses;
   data-link-connection;
   data-link-service-data-units;
   data-link-connection-endpoint-identifiers;
   error notification;
   quality of service parameters; and
   reset.
     Functions within Data Link Layer
3Functions depend on mode, either connection or
 connectionless
    data-link-service-data-unit mapping;
    identification and parameter exchange;
    control of data-circuit interconnection;
    error detection;
    routing and relaying;
    data Link Layer management.
    data-link-connection establishment and release;
    connection-mode or connectionless-mode data-link data transmission;
    data-link-connection splitting;
    sequence control;
    delimiting and synchronization;
    flow control;
    error recovery;
    reset.
     Comments on Data Link Layer
3Data link realization is based on combination of hard-
  ware and software, specialized microcircuits
3HDLC (High Level Data Link Control) with alternati-
  ves: LAP-B, LAP-D, even Frame relay.
3 In LANs MAC (Medium Access Control) and LLC
  (Logical Link Control).
3Character-based BSC (Basic Mode Control procedure)
  and Bisynch
                  Network Layer
3The Network Layer (NL) provides the functional and
 procedural means for connectionless-mode or connec-
 tion-mode transmission among transport-entities.
  NL provides to higher levels independence of routing and relay
   considerations.
  The NL provides the means to establish, maintain, and termi-
   nate network-connections between open systems containing
   communicating application-entities.
  NL gives the functional and procedural means to exchange
   network-service-data-units between transport-entities over
   network-connections.
                  Network services
3The basic service of the NL is to provide the transparent
  transfer of data between transport-entities. (* no-conn.)
3 In connection-mode, the facilities provided by NL:
  network-addresses;
  network-connections;
  network-connection-endpoint-identifiers;
  network-service-data-unit transfer;
  quality of service parameters (*);
  error notification (*);
  expedited network-service-data-unit transfer ;
  reset;
  release; and
  receipt of confirmation.
                       NL Functions
   routing and relaying;
   network-connections;
   network-connection multiplexing;
   segmenting and blocking;
   error detection;
   error recovery;
   sequencing;
   flow control;
   expedited data transfer;
   reset;
   service selection;
   mapping between network-addresses and data-link addresses;
                More NL Functions
 mapping network-connectionless-mode transmissions to data-link-
  connectionless-mode transmissions;
 converting from data-link-connection-mode service to network-
  connectionless-mode service;
 enhancing a data-link-connectionless mode service to provide a network-
  connection-mode service; and
 network layer management.
            Comments on NL
3Examples are X.25 and X.21 (packet and circuit
 switched data networks).
3Realization uses special circuit cards.
3Common channel signalling, ISDN
3ATM (Asynchronous Transfer Mode)
3Several standards exist for addresses.
                  Transport Layer
3The transport-service (TS) provides transparent transfer
 of data between session-entities.
  TS relieves upper layers from any concern how reliable and
   cost effective transfer of data is achieved.
  The Transport Layer (TL) optimizes the use of the available
   network-service to provide the performance required by each
   session-entity at minimum cost.
  All protocols defined in the TL have end-to-end significance,
   where the ends are defined as transport entities having transport
   associations.
  Network-service provides data transfer from any transport-
   entity to any other, including the case of tandem subnetworks.
  TL to provides requested service quality depending on the
   quality of the network-service.
              Transport services
3The Transport Layer (TL) uniquely identifies eaion-
  entity by its transport-address.
3 In connectionless-mode service, the TL provides a
  connectionless-mode service which maps a request for
  transmission of a transport-service-data-unit onto a
  request to the connectionless-mode network-service.
3 In connection-mode, the transport-service provides the
  means to establish, maintain, and release transport-
  connections. Transport-connections provide duplex
  transmission between a pair of session-entites (through
  transport-SAPs).
                      TL facilities
3 transport-connection establishment;
3 transport-connection release;
3data transfer;
  The TL provides the means by which transport-service-data-
   units of arbitrary length are delimited and transparently
   transferred in sequence from one sending transport-service-
   access-point to the receiving transport-service-access-point over
   a transport-connection. This service is subject to flow control.
3 expedited data transfer; and
3 suspend facility.
                        TL Functions
3 In connection mode TL provides functions: (* no-conn.)
  mapping transport-address onto a network-address (*);
  multiplexing (end-to-end) transport-connections onto network-connections
   (*);
  establishment and release of transport-connections;
  end-to-end sequence control on individual connections;
  end-to-end error detection and any necessary monitoring of the quality of
   service (*);
  end-to-end error recovery;
  end-to-end segmenting, blocking, and concatenation;
  end-to-end flow control on individual connections;
  supervisory functions (*);
  expedited transport-service-data-unit transfer; and
  suspend/resume.
             Comments on TL
3Transport layer may offer several classes of
 service.
3OSI Transport Protocol class 4 provides also
 error control.
                    Session Layer
3The purpose of the Session Layer (SL) is to allow co-
 operating presentation-entities to organize and to syn-
 chronize their dialogue and to manage their data
 exchange.
  SL provides services to establish a session-connection between
   two presentation-entities.
  SL supports orderly data exchange interactions.
  SL releaseS the connection in an orderly manner.
  For connectionless-mode communication SL provides mapping
   of transport-addresses to session-addresses.
  The session-connection exists until it is released by either the
   presentation-entities or the session-entities.
  The initiating presentation-entity designates the destination
   presentation-entity by a session address. (many-to-one
   correspondence)
                 Session services
 session-connection establishment;
 session-connection release;
 normal data transfer;
 expedited data transfer;
 token management;
 session-connection synchronization;
 exception reporting;
 activity management;
 typed data transfer;
 resynchronization.
       Comments on session layer
3Session layer orders large scale distributed working.
3Token is used to allow the parties to take turns.
3Synhronization points are used to mark how far the
 work has proceeded without any problems. If the work
 is stopped by a fault, it can be started again from a
 previous synchronization point.
                Presentation Layer
3The Presentation Layer (PrL) provides for the
 representation of information that application-entities
 either communicate or refer to in their communication.
3PL provides for common representation of the data
 transferred between application-entities.
  Application-entities need not concern with "common"
   representation of information.
3The PL ensures that the information content of the
 Application Layer data is preserved during transfer.
  PL is informed of the abstract syntaxes that are to be em-
   ployed. Knowing the set of abstract syntaxes to be used by the
   application-entities, the Presentation Layer is responsible for
   selecting mutually acceptable transfer syntaxes.
                  PL functions
 negotiation and re-negotiation of transfer syntax;
 representation of the abstract syntax chosen by the
  application-entities in the transfer syntax negotiated or
  renegotiated, including format and special purpose
  transformations (for example, data compression);
 restoration of previously negotiated syntaxes on the
  occurrence of certain events;
 use of session-services.
              Comments on PL
3PL alone is rarely used, instead its services are
 merged in the Application Layer using ASN.1
 (Abstract Syntax Notation)
            Application Layer (AL)
3As the highest layer in the Reference Model of Open
 Systems Interconnection, the Application Layer provi-
 des the sole means for the application process to access
 the OSIE.
  AL has no boundary with a higher layer.
  The aspects of an application process which need to be taken
   into account for the purpose of OSI are represented by one or
   more application-entities.
  An application-entity represents one and only one application
   process in the OSIE.
  Different application processes may be represented by applicat-
   ion-entities of the same application-entity-type.
  An application process may be represented by a set of applicat-
   ion-entitites.
                     AL facilities
 identification of the intended communication partners (for
  example by name, by address, by definite description, by
  generic description);
 determination of the acceptable quality of service (for example
  response time, tolerable error rate, cost vis-a-vis the previous
  considerations);
 synchronization of cooperating applications;
 agreement on responsibility for error recovery;
 agreement on security aspects (e.g. authentication, access
  control, data integrity);
 selection of mode of dialogue; and
 identification of abstract syntaxes.
              Comments on AL
3AL has a very rich structure
3The services may be provided by CASE (Common
 Applications Service Element) or by SASE (Specific
 Applications Service Element).
3CASEs include ACSE, ROSE and CCR.
3SASEs include FTAM, MHS, MMS, JTM, DS
3The application programs call AL if communication
 services are required.

								
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