Communication System Models

Transcription

1 Communication System Models 1

2 2 The Black Box View Block Size? Tx/Rx Ch Voltage? Char Set? Topology? Tx/Rx Many users of networks are unaware of details of network May view network as a black box service Many factors are independent of each other

3 3 Open Systems and Standards Important that black boxes conform to standards The international standards organisation (ISO) numbered ISOnnnn cover many different topics Internet request for comments (RFC) numbered RFCn often given the title of TCP/IP protocols The international telecommunications union (ITU) X and I series of standards numbered X.n and I.n, and cover anything that might run over a telephone network.

4 4 OSI Reference Model Open Systems Interconnection (OSI) model is a reference model for communication systems Standardises the function of the black boxes black-boxes provide services contents of the black-box is a service provider users are service users

5 5 OSI Model Layers Modular may replace the implementation of just one layer Number of layers is compromise between more layers to increase the modularity fewer layers so as to improve the efficiency OSI defines seven layers of abstraction Each layer needs only to be able to interface with the layer immediately below User may view communication as exchange of protocol data units (PDU) between entity at the local host with a peer entity at the remote host

6 6 Summary of the Seven Layers (1) Application Layer Procedural interface to the user and application programs that utilise the network services Presentation Layer Presentation of data transfer service of the session layer. Such services include encryption, authentication, and compression. Session Layer Provides a method to establish, maintain and terminate a channel to a process on a remote host. Transport Layer Provides an error free data and reception service, choosing the method by which PDUs are broken up or brought together to be transmit over the network.

7 7 Summary of the Seven Layers (2) Network Layer Routes the PDU over the network, choosing the switches that must relay the PDU onwards. Data Link Layer Uses the bit communication system provided by the physical layer, and provides a means for communicating data and control information. Physical Layer Concerned with the electrical and mechanical nature of the physical connection.

8 8 Layer OSI Model Implementation of PDU exchange Host Host Layer Application Presentation Session Transport Network Data Link Physical Switch Subnet Switch Transmission Medium

9 9 Worksheet: OSI Model The new WonderNet operates at 100Mbs 1, using star layout LANs to serve local hosts connected by hubs using a mesh topology. By the use of 0-5v digital signalling hardware interface costs are minimised, and the unique use of Hamming Code means that the data transfer between each host is self correcting! The addressing of hosts is automatically conducted on which socket the host is connected into the hub by, and routing of packets is then efficiently conducted by the hubs using a CL protocol. A Virtual Circuit Emulator package is provided to simulate a CO network for those applications that require it. Industry standard telephone cable and plugs used throughout minimise installation costs. Separate out the emphasised items into the physical layer data link layer network layer transport layer

10 10 Services in OSI layer. service element. service primitive H 1 Channel H 2 T.DATA.request T.DATA.confirm T.DATA.indication T.DATA.response Each layer may provide one or more service elements A service element has one or more service primitives A simple example of a layer is a CL transport layer Requires just the DATA service element

11 11 Connection Oriented (CO) H 1 Channel H 2 T.CONNECT.request T.CONNECT.confirm T.DATA.request T.DATA.confirm T.DATA.request T.DATA.confirm T.DISCONNECT.request T.CONNECT.indication T.CONNECT.response T.DATA.indication T.DATA.response T.DATA.indication T.DATA.response T.DISCONNECT.indication T.DISCONNECT.confirm T.DISCONNECT.response

12 12 Data Sent Over an Interface (N)-Entity (N)-PDU (N)-Entity (N)-Layer (N-1)-ICI (N-1)-SDU (N-1)-Entity (N-1)-SAP (N-1)-ICI (N-1)-PCI + (N-1)-SDU Interface (N-1)-PDU (N-1)-PCI (N-1)-SDU (N-1)-Layer (N-1)-Entity (N-1)-SAP

13 13 Transmission of a PDU Abbreviation Term PDU protocol data unit PCI protocol control information SAP service access point SDU service data unit ICI interface control information The SAP has an address, which uniquely identifies the (N+1)-entity One of more layer N entities will service the SAP Simple model has one (N+1)-PDU put inside one (N)-SDU Not always a 1:1 relationship between SDUs and PDUs

14 14 Efficiency of OSI Model In a particular implementation of the OSI model, each of the layers uses a protocol control information (PCI) of four bytes. Calculate the total data transfer efficiency for application data sizes: 1. 4 bytes bytes Bottom layer PDU will contain 4 7 = 28 bytes that belongs to the headers 1. ρ = = ρ = = 0.97

15 15 Segmentation Layer (N+1) (N)-ICI (N)-SDU Interface SAP Layer (N) (N)-PCI a (N)-SDU a (N)-PCI b (N)-SDU b Necessary where size((n+1)-pdu) > size((n)-sdu) (N)-PDU = (N+1)-PDU (N)-SDU for each (N+1)-PDU Segmentation commonly called fragmentation

16 16 Blocking Layer (N+1) (N)-ICI 1 (N)-SDU 1 (N)-ICI 2 (N)-SDU 2 Interface SAP SAP Layer (N) (N)-PCI (N)-SDU 1 (N)-SDU 2 Possible when size((n+1)-pdu) < size((n)-sdu) (N+1)-PDU = (N)-SDU (N+1)-PDU fit in each (N)-PDU reduces (N)-PDUs sent less overheads at the receiver

17 17 Concatenation Layer (N+1) (N)-ICI 1 (N)-SDU 1 (N)-ICI 2 (N)-SDU 2 Interface SAP SAP Layer (N) 7 (N)-PCI 1 (N)-SDU 1 (N)-PCI 2 (N)-SDU 2 similar analysis to blocking more wasteful in using multiple PCI fields

18 18 Segmentation and Blocking in the OSI Model Size in Bytes Layer ICI PCI SDU Transport Network Data Link How many N-PDUs are required to send one T-PDU? TPDU= =64064 fragmentation NP DU = = How many D-PDUs are required to send one T-PDU? NPDU= =1012 blocking for one DPDU: NP DU = = 2 To transport one TPDU: DP DU = 65 2 = 33

19 19 Worksheet: Passing Data Through OSI Layers 1. Complete the missing information in the following table Size in Bytes Layer ICI PCI SDU PDU (N+1) (N) (N-1) Can a single (N)-PDU carry a single (N+1)-PDU? If not, what must be done? 3. How many (N)-PDUs will be used to carry one (N+1)-PDU? 4. How many (N)-PDUs will be used to carry two (N+1)-PDUs? 5. How many (N-1)-PDUs will be used to carry one (N+1)-PDU?

20 20 CO Services: Direct Level N+1 (N)-CEP Level N (N)-Entity (N-1)-CEP Level N-1 SAP contains one or more connection end-point (CEP), each identified by a channel number in direct layers each (N)-CEP is served by exactly one (N)-connection

21 21 CO Services: Multiplexing Level N+1 (N)-CEPs Level N (N)-Entity (N-1)-CEP Level N-1 Reduces the overheads in serving several connections

22 22 CO Services: Splitting Level N+1 (N)-CEP Level N (N)-Entity (N-1)-CEPs Level N-1 Increases the bandwidth available to the (N)-CEP

23 23 OSI Model Review of Layers Layer Host Host Layer Application Presentation Session Transport Network Data Link Physical Switch Subnet Switch Transmission Medium

24 24 Application Layer Provides the interface to the communications systems provide interfaces which are support a range of specific application programs or devices e.g. virtual terminal interface for a network allows programs such as editors to be written independent of the actual type of terminal that is to be connected Other examples of services include , file transfer and facsimile.

25 25 Presentation Layer Convert between abstract data syntax for applications and transport data syntax for transport over the network conversion between different character sets, such as when an IBM mainframe using EBCDIC code uses an ASCII based network. marshaling of data into a standard format, including such things as conversion between big endian and little endian representations of integers. encryption of data so that only the remote host is able to read the information being sent by the host. compression of data to minimise the network bandwidth and transmission time of the information.

26 26 Session Layer A CO model will provide the user with the notion of a virtual circuit There are additional procedures to be followed in order that two hosts may engage in communication synchronisation points which record sufficient information during a long communication, which enables the communication to restart at that point remote login which provides authentication to the two hosts are indeed communicating with each other, and ensures that the user is allowed to access remote host. exception handling of events in the lower level which are not part of the correct operation of data communication.

27 27 Transport Layer Join between application oriented layers 5 7 and network oriented layers 1 3 Can provide implementation of a CO service if lower levels provide a CL service Performs segmentation, blocking and concatenation There are five quality of service (QOS) levels: level 0 providing only a basic transport of data level 4 providing full error and flow control If the lower layers are unreliable (e.g. PSTN), then need higher QOS than if lower levels could be relied upon (e.g. ISDN)

28 28 At this layer we must deal with the details of network topology Network Layer process of establishing and clearing a route across the network A set of nodes is called a subnetwork or subnet subnets may be connected together internetworking at boundaries we will deal with the variations in protocols and PDU sizes used by different subnets subnets are often owned by different organisations call charging

29 29 Data Link Layer Provides a mechanism for data transfer between nodes of the network error and flow control are often performed at this level (but may also be performed in layers 3 and 4) provide protocols for the sending and acknowledgement of blocks of data control access to the transmission medium

30 30 Physical Layer Defines the physical and electrical properties of the channel the type of transmission medium that implements the channel the signalling levels used (e.g. use 0V and 5V) duplex, half-duplex or simplex use of the channel shape of connectors and number of pins

31 31 Summary of OSI Model OSI model defines modular approach to implementing communication systems. Defines what each module should do services that modules provide to each other number of layers is a trade-off between modularity and efficiency entities at each layer communicate via messages in the form of PDUs implementation of the protocol is via calls to services of the layer below upper three layers concerned with applications, lower three layers with the network. Joined by the transport layer.

32 32 TCP/IP Model transmission control protocol/internet protocol (TCP/IP) OSI TCP/IP General Framework Specific Model ISO Standards X.nnn RFCnnnn c 1983 c 1974 Covers all aspects Network layer and above Telecoms industry US defence (ARPANET) basis for the Internet internetworking Intranet using Internet protocols within organisation

33 33 Relationship between OSI 7-layer and TCP/IP model OSI Layer TCP/IP Layer Application Presentation Session Transport Network Data Link Application Transport Internet Network Data Link FTP, HTTP, SMTP,... TCP, UDP, ICMP,... IP Physical Physical

34 34 IP Protocol S 3 H 5 H 2 S 5 S 2 H 4 H 1 S 4 S 1 H 3 CL: exchanges PDUs called datagrams IPv4 used at present (e.g. IP address ) IPv6 (aka IPng) being introduced IP addresses are the SAP of the TCP/IP internet layer

35 35 IP Protocol S 3 H 5 H 2 S 5 S 2 H 4 H 1 S 4 S 1 H 3 Use of CL allows for robust handling of failures

36 36 IP Protocol S 3 H 5 H 2 S 5 S 2 H 4 H 1 S 4 S 1 H 3 As long as there is some route through the network, communication can contiue

37 37 Transport Layer Protocols The true defining part of TCP/IP is the IP layer Various protocols may be used in the transport layer user datagram protocol (UDP): CL service brings the raw IP service up to AL supports point-to-point and multicast communication transmission control protocol (TCP) CO service reliable exchange PDUs called segments only supports point-to-point communication

38 38 Ports and IP addresses in TCP/IP TCP/IP Layer Protocol PDU Name application SNMP Sun RPC telnet ftp message transport UDP TCP datagram segment internet X.X.X.X IP datagram application will access the network via a port on TL port+ip address=socket the SAP of TCP/IP transport layer.

39 39 Worksheet: TCP/IP TCP/IP Layer H 1 H 2 PDU Nam application telnet telnet telnet SNMP Sun RPC telnet messag transport TCP UDP TCP datagra segmen internet IP IP datagra

40 40 Obtaining a Socket For a Service To reach remote service involves finding its socket find the IP address for the host, given its name performed by AL service called domain name service find the port number for the service on the host first 1024 numbers (i.e. 0 to 1023) are allocated as well-known ports for well-known services port numbers above 1023 may be used for any application well-known port numbers only apply to the server end of the communication client process using the service must be allocated an unused port number on the local host

41 41 What Next? OSI and TCP/IP models act as framework for other material in course Error handling Datalink Layer Codes and compression Presentation Layer Next we look at the Physical Layer