ISO OSI REFERENCE MODEL

1.0 Introduction to ISO OSI REFERNCE model

The International Standards Organization (ISO) Open System Interconnect (OSI) Reference Model was developed to provide a framework for understanding how information is sent from one computer to another. The model is so called because it deals with connecting open systems - that is, the system is open for communication with other systems. The OSI model describes seven layers, with each layer intended to provide a well-defined service in order to ensure data has been successfully transmitted between devices. These layers are accurately defined and can be represented as:
* OSI Application Layer
* OSI Presentation Layer
* OSI Session Layer
* OSI Transport Layer
* OSI Network Layer
* OSI Data Link Layer
* OSI Physical Layer

The OSI model describes the flow of information through a network as moving down the seven layers on the transmitting device, through the intermediate nodes of a network service, and up through the seven layers on the receiving device.
Since the OSI model is an abstract concept, it is not actually used in data networks. However, ISO and other standards organizations have developed and documented specific protocol standards that can be implemented for each layer. In some cases several protocol standards are available for each layer.
As an abstract model, these layers serve the important function of providing a framework and set of terms that allows discussion about data networking without the ambiguity that would result if such a model were not available. The OSI model has great value as a guide to how to think about data networking.

Let us consider the model as we go from the outermost layer and work our way inwards.
1.1 OSI Application Layer

This layer provides high level services to applications, like virtual network terminal, file transfer, X.400 e-mail, X.500 directory services and other general facilities.
1.1.1 OSI Application Layer Details
What users are really interested in are applications. In OSI terminology, applications are executed by application processes. The portion of the these processes that handles the communications aspects are called application entities. Since an application may have diverse communications needs, it is possible for an application to have multiple application entities.
There are a wide range of applications and each has its own specific needs. However, some of functions are generally useful to many applications. OSI approaches this by decomposing the application entities into elements. An application entity contains one or more application service elements (ASE). The interaction between the ASEs and the underlying OSI services specify the application protocols.

1.2 OSI Presentation Layer

This layer ensures everyone speaks the same language, EBCDIC and ASCII conversions, data compression, cryptography and other functions required often enough to warrant implementing a general solution.
1.2.1 OSI Presentation Layer Details
The presentation layer is layer 6 of the OSI model. Data communications deal with information transfer. The meaning of the information transferred, the semantics, is the responsibility of the application layer, specific to the individual applications. However, the precise bit pattern to be transferred is the responsibility of the presentation layer. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.
When an application specifies semantics, it specifies it in an abstract representation, also known as an abstraction syntax notation. For a given abstract syntax definition, there are many possible encoding rules to encode the information into specific bit patterns. Each encoding rule results in a different transfer syntax. Currently, ITU-T and ISO have defined an abstract syntax notation, called Abstract Syntax Notation One (ASN.1). Associated with ASN.1 is a corresponding set of encoding rules, the Basic Encoding Rule (BER). ISO also recognizes there may well be the need for more than one abstract syntax notation. For example, there may be a different abstract syntax to handle graphic information.

1.3 OSI Session Layer Details

The session layer is responsible for the management of the conversation between the two end-applications. There are three major concepts in the session layer:

1. Token
,
2. Sync point and
3. Activity.
Many applications are actually half-duplex in nature in the sense that the two end-applications take turns speaking.
Tokens are used to regulate whose turn it is to speak.
Sync points are check points of the conversation. There are two types of sync points:
1. Major and
2. Minor.
Major sync points subdivide the conversation into dialogue units and thereby give a structure to the conversation. When a major sync point is reached, an application may not send more data until it is acknowledged. For recovery purposes, it is not possible to recover beyond the last acknowledged major sync point.
Minor sync points are used to further structure a dialogue unit. They provide more flexibility for resynchronization.
Activities give a further level of structuring to a conversation. An activity can be interrupted and resumed later. The relationship between sessions and activities is not fixed. There can be many activities in a session while another activity can span many sessions.

1.4 OSI Transport Layer Details

The transport layer is responsible for the reliable transfer of data between two end-stations. It provides the following basic services:
1. Transport connection establishment,
2. End-to-end data transfer and
3. Connection release.
To provide these services, the following functions are identified:
* Multiplexing of transport connections onto network connections
* End-to-end connection establishment and release
* End-to-end error detection and recovery
* End-to-end sequencing control
* End-to-end flow control
* Supervisory functions
* Expedited data transfer
Similar to the network layer, there are two modes of transport services:
* Connection mode and
* Connectionless.
The connection mode transport service is defined in two standards:

* Basic Connection-mode Transport Service, ISO 8072
* Basic Connection-mode Transport Protocol, ISO 8073
These are the first two OSI standards to reach International Standard status. The following focuses on the connection mode standards.
It is the transport layer that bridges the grade of service required by an application and the grade of service provided by the network. Therefore, the amount of work that the transport layer has to perform depends on the quality of the underlying network. There are five classes of transport services, TP0 to TP4. TP0 is the simplest while TP4 is the most complicated.
In the connectionless mode, there is no connection establishment and release. An implication of this is that there is no explicit negotiation of grade of service (or quality of service).
The Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a statefull protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. Usually UDP is preferred because of its speed and statelessness. Statelessness simplifies the crash recovery.

1.5 OSI Network Layer Details

The responsibility of the network layer is to move packets from one end-station to another. It insulates the upper layer protocols from the inner workings of the physical network so that they can concentrate on end-to-end matters. In OSI, there are two methods of providing network services: Connection Oriented Network Service (CONS) and ConnectionLess Network Service (CLNS). Each method has merits, and different technologies may suit one method better than others.
OSI recognizes that a network is likely to consist of a number of subnetworks. Each subnetwork is based on a different technology to satisfy diverse user-functional and economic needs. Of course, these subnetworks are interconnected together to form a single network which will provide one or both of the specified services: CONS and/or CLNS.
1.5.1 Sublayers
OSI subdivides the network layer into three sublayers:

* Subnetwork Access Sub-Layer -- The protocol in this sub-layer is the native protocol of a subnetwork. The subnetwork may or may not provide all the services specified by OSI.

* Subnetwork-Dependent Convergence Sub-Layer -- This sub-layer is conceptually above the subnetwork access sub-layer. It deals with moving data between two end-points on the same subnetwork. It augments the subnetwork access sub-layer so that all the services specified by OSI, either CONS and/or CLNS, is available to the end-points for communications over the same subnetwork.

* Subnetwork-Independent Convergence Sub-Layer -- This sub-layer is conceptually above the subnetwork-dependent convergence layer and deals with the moving data between end-points that are on different subnetworks. It ensures the overall network will provide one or both of the services (CLNS and/or CONS) specified by OSI.
Internetwork Protocol (IP) is normally used as its network layer interface. IP is responsible for routing, directing datagrams from one one network to another. Network layer may have to break large datagrams, larger than Maximum Transmission Unit (MTU), into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129:79:16:40. The leading 1-3 bytes of the IP identifies the network and the remaining bytes identifies the host on that network. The first two bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet and host respectively.
Even though IP are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address.

1.6 OSI Data Link Layer Details

The Data Link Layer oversees the data transfer and defines the format of data on the network between devices that are connected to the same physical media. The simplest case is two devices connected with a circuit in a point-to-point fashion. There are many well-established protocols that can provide communication over a point-to-point connection. One of the most common is the HDLC/LAPD family. In LAPD, data is transmitted in frames and the protocol provides the following functions:

* Delimits frames
* Maintains frame integrity
* Provides data transparency
* Provides error detection
* Provides link layer multiplexing
* Provides retransmission of frames for error recovery
* Provides flow control
* Provides link management functions
A more complex example is a multipoint circuit. HDLC does support this configuration via polling which is a form of media access control. That is to say that the polling algorithm arbitrates who, among all the tributaries, has the access to the circuit.
With the advent of local-area networks (LANs), where many devices share the same physical media, the issue of media access becomes more prominent. Therefore, in the IEEE 802 LAN standards, the data link layer is subdivided into two sublayers:

* The Media Access Control (MAC) sub-layer deals with how the devices access the media for the different technologies

* The Logical Link Control (LLC) sub-layer is conceptually above the MAC layer. The LLC provides the traditional link layer functions as described above.
HDLC/LAPD was specified when most circuits were analog with relatively high error rates. Under those conditions, a reliable and sturdy link layer protocol is both desirable and necessary. Current digital transmission technologies, in particular optical fiber, have very low error rates. Frame relay takes advantage of this fact and simplifies the network access protocol by discarding some the traditional link layer functions, namely:

* error recovery
* flow control
* link management
These three functions are the most demanding of the link layer functions. It is this simplification that allows frame relay to support high access speeds and achieve high throughput. This is also why frame relay is referred to as a layer 1.5 protocol.
When we talk about the format of the data, we consider the packet definition. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. An host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself. Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.

1.7 OSI Physical Layer

This layer transports bits over a communications media or channel. It typically deals with type of connector, signal strength and how to access media. 1.7.1 OSI Physical Layer Details
It is up to the physical layer to make use of transmission media to transmit the data. It must provide means to activate, maintain and deactivate the physical link/media between media. How a physical device accesses the media depends on the nature of the media and the technology used. The major concern here is what the physical interface/access method to the media should be. The physical interface has four important characteristics:

* Mechanical
* Electrical
* Functional
* Procedural
Examples of physical interface standards are RS-232-C, RS-449/422/423, and X.21.
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.

1.8 Data Transmission in the OSI model:

The sender has some information that is to be transported to the receiver. Assuming that the sender and the receiver are working on a structured OSI model, we could very well define the following process.
The sending machine passes the data to the application layer, which then attaches the application header (AH) in front of the data and passes it to the presentation layer. The presentation layer may transform this and possibly add a header to the front, giving the resultant stream to the seccion layer. This process is repeated until the data reaches the physical layer, where they are actually transmitted to the receiving machine.
On that machine, the various headers are then stripped off one by one as the message propagates up the layers untill the data reaches the receiver's application layer.
The basic theory is that the actual data flow is in the vertical format, but the layers are programmed as if the data flow is horizontal. Therefore the data transfer is more transparent to the lower layers as to what happens at the higher end.

Thus through this paper, I have studied and revealed all the possible technicalities available to me through my resources. I would like to acknowlegde the usage of the following:
Sites:
1. http://www.nodec.com/tech/iso.htm
2. http://www.uwsg.indiana.edu/usail/network/nfs/layers.htm

E-Mail: Viral Shah at vshah@poboxes.com

URL: https://members.tripod.com/~vshah
Text - Copyright © 1997, Viral Shah.
Last Revised - April 27th, 1997