OSI reference model is now considered as a primary standard for internetworking and inter computing. Today many network communication protocols are based on the standards of OSI model. In the OSI model the network/data communication is defined into seven layers.
These 7 layers further divide the tasks of moving the data across the network into subtask and hence complete one communication cycle between two computers or two network devices. Each layer is assigned a task and the task is completed independently. The OSI layers have the clear and independent characteristics and tasks.
The 7 layers of the OSI models can be divided into upper and lower layers. I have defined the characteristics, tasks and features of each layer separately.
Layer 7: Application Layer
This layer provides a means for the user to access information on the network through an application. Many user applications that need to communicate over the network interact with the Application layer protocol directly. The user applications are not part of OSI Application layer, use the networking services offered by the networking protocol suite. Application layer functions typically include identifying communication partners, and determining availability of required resources. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.
Network Devices: Gateway network device is operated on the application layer.
The Services Provided by the Application Layer.
These include FTP, FTAM, SMTP and TELNET.
FTP: File (Transfer Protocol) is the standard way to transfer files between different machines. In order to transfer files from the FTP server on the Unix/Telnet, you will need an FTP client program on your computer.
Note: Before transferring files, make sure there is enough disk space on your machine. The FTP program uses TCP/IP protocol to transfer files to and from remote nodes. Sometimes it will read the host file to determine the IP address. The TCP/IP and the OSI protocol have coexisted for sometime and will continue to coexist in the internet community because as more and more OSI hosts are fielded on the internet, the requirement for gateways between the protocol suites become more pressing.
This report will describe an application layer gateway providing interoperability between the TCP/IP file Transfer Protocol (FTP) and the OSI File Transfer, Access and Management (FTAM) protocol. The proposed application layer is based on a bi-directional set of mappings between the FTP and FTAM protocols. Since the protocols have quite different command structures, the mappings between them are not one-to-one. This report assumes the knowledge of File Transfer Protocol (FTP) [RFC959] and the File Transfer, Access and Management Protocol (FTAM) [ISO8571-1,2,3,4,5].
The Goals of Mappings.
The two important goals of mappings are to:
* Provide FTP users with as much emulated FTP capability on an FTAM Responder as possible.
* Provide FTAM users with as much emulated FTAM capability on an FTP server as possible.
Though it is anticipated that application layer gateway will be implemented on full protocol suites of both TCP/IP and OSI, at least one implementation of such a gateway ( included in the ISO Development Environment) can be configured to operate FTAM over, either OSI or TCP/IP lower layer services.
FTP Commands:
The FTP commands most frequently used are;-
ftp- starts FTP . Here if a machine is specified, a connection to that machine is made immediately. e.g. telgate.uel.ac.uk, otherwise the FTP command is displayed.
open – opens a connection to the specified machine.
cd- change directory on the remote machine.
lcd – change directory on the local machine.
pwd – print working directory on the remote machine.
ls – short directory listing on the remote machine.
lls – short directory listing on the remote machine.
dir- long directory listing on the local machine .
ascii – set ascii transfer type ( for text files)
binary – (or image) set binary transfer type ( for binary dates, executable files, gzipped files etc.
get – copy files from the remote machine to the local machine, or append “-” after the file name to display the contents of a remote file on your screen (e.g. the command get 00README – will display 00README file of current directory to your local screen.
put – copy file from the local machine to the remote machine.
mget – as for get but copies multiple files
mput – as for put but copies multiple files .
close – disconnects from the remote machine.
quit – terminates FTP session and exit.
Layer 6:Presentation Layer
The presentation layer presents the data into a uniform format and masks the difference of data format between two dissimilar systems. It also translates the data from application to the network format. Presentation layer is also responsible for the protocol conversion, encryption, decryption and data compression. Presentation layer is a best layer for cryptography.
Network Devices: Gateway Redirector is operates on the presentation layer.
Presentation Layer Functions
Here are some of the specific types of data handling issues that the presentation layer handles:
* Translation: Networks can connect very different types of computers together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines.
* Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.)
* Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the Secure Sockets Layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.
Layer 5: Session Layer
Session layer establish and manages the session between the two users at different ends in a network. Session layer also manages who can transfer the data in a certain amount of time and for how long. The examples of session layers and the interactive logins and file transfer sessions. Session layer reconnect the session if it disconnects. It also reports and logs and upper layer errors.
Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC
Network Devices: Gateway
Layer 4: Transport Layer
In computing and telecommunications, the transport layer is level four of the seven level OSI model. It responds to service requests from the session layer and issues service requests to the network layer.
The transport layer provides transparent transfer of data between hosts. It is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. In the IP protocol Stack this function is achieved by the connection oriented Transmission Control Protocol (TCP) or the datagram type User Datagram Protocol (UDP). The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.
The transport layer turns the unreliable and very basic service provided by the Network layer into one worthy of the term ‘Communication’. There is a long list of services that can be optionally provided at this level. None of them are compulsory, because not all applications want all the services available. Some can be wasted overhead, or even counterproductive in some cases.
* Connection Orientated. This is normally easier to deal with than Connectionless models, so where the Network layer only provides a connectionless service, often a connection oriented service is built on top of that in the Transport layer.
* Same Order Delivery. The Network layer doesn’t generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature, so the Transport layer provides it. The simplest way of doing this is to give each packet a number, and allow the receiver to reorder the packets.
* Error ‘Free’ Data. The underlying network may well be noisy, and the data received may not always be the same as the data sent. The Transport layer can fix this: typically by providing a checksum of the data which detects if there has been a glitch of some kind. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors. This layer may also retransmit packets which have gone missing en route.
* Flow Control. The amount of memory on a computer is limited, and without flow control a larger computer might flood a computer with so much information that it can’t hold it all before dealing with it. Nowadays, this is not a big issue, as memory is cheap while bandwidth is comparatively expensive, but in earlier times it was more important. Flow control allows the receiver to say “Whoa!” before it is overwhelmed. Sometimes this is already provided by the network, but where it is not, the Transport layer may add it on.
* Byte Orientation. Rather than dealing with things on a packet-by-packet basis, the Transport layer may add the ability to view communication just as a stream of bytes. This is nicer to deal with.
* Ports. Ports are essentially ways to address multiple entities in the same location. For example, the first line of a postal address is a kind of port, and distinguishes between different occupants of the same house. Computer applications will each listen for information on their own ports, which is why you can use more than one network-based application at the same time.
On the internet there are a variety of Transport services, but the two most common are TCP and UDP. TCP is the more complicated, providing a connection and byte oriented stream which is almost error free, with flow control, multiple ports, and same order delivery. UDP is a very simple ‘datagram’ service, which provides limited error reduction and multiple ports. TCP stands for Transmission Control Protocol, while UDP stands for User Datagram Protocol.
Some things, such as connection orientation can be implemented at either Transport or Network layer. The idea is that the Network layer implements whatever set of options is easiest: for some underlying networks it is easiest to implement connectionless communication, while for others it is easiest to implement connection oriented communication. The Transport layer uses this simplest set of options to implement whatever combinations of options are actually desired.
Network Devices: The Brouter, Gateway and Cable tester work on the transport layer.
Layer 3: Network Layer
The network layer determines that how data transmits between the network devices. It also translates the logical address into the physical address e.g computer name into MAC address. It is also responsible for defining the route, managing the network problems and addressing. Router works on the network layer and if a sending device does not break the data into the similar packets as the receiving device then network layer split the data into the smaller units and at the receiving end the network layer reassemble the data.
Network layer routes the packets according to the unique network addresses. Router works as the post office and network layer stamps the letters (data) for the specific destinations.
Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.
Network Devices: Network devices including Router, Brouter, Frame Relay device and ATM switch devices work on the network layer.
Layer 2:Data Link Layer
The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network”, as opposed to an internetwork.
Data Link Layer Sublayers: Logical Link Control (LLC) and Media Access Control (MAC)
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.
Data Link Layer Functions
The following are the key tasks performed at the data link layer:
* Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.
* Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.
* Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.
* Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.
* Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.
Layer 1: Physical Layer
The physical layer is the bottom layer in the seven layer OSI (open system interconnection) reference model.
This model was developed in 1977 in order to standardize and simplify definitions relating to computer networks. It divides the networking process into seven logical layers, starting at the physical layer and ascending to the application layer (which interfaces with application programs on computers). Services and protocols (i.e., agreed-upon formats) are specified for each layer, and each layer has unique responsibilities, including passing information to the layers above and below it.
The physical layer defines all physical and electrical specifications for devices used to interface to the network, including the shape and layout of pins in connectors, voltages, cable specifications and broadcast frequencies. It provides the means for transmitting raw bits, but it is not concerned with MAC addresses, IP addresses and packets; rather, these are dealt with by layers higher in the hierarchy.
The physical layer performs services requested by the data link layer, which is the layer directly above it. Its major functions and services are: (1) the establishment and termination of connections to a communications medium (e.g., twisted pair cable or optical fiber cable), (2) conversion between the representation of digital data in computers (or other network devices) and the corresponding signals transmitted over the communications medium and (3) participation in the efficient sharing among multiple devices of the communications medium through the use of flow control and collision resolution (i.e., recovery from simultaneous transmission by two or more devices).
Devices that operate at the physical layer include repeaters, hubs, network interface cards (NICs), cables and connectors. Repeaters are used to regenerate electrical signals that have attenuated (i.e., weakened) as a result of distance. A hub is a common connection point for twisted pair or optical fiber connecting devices in a local area network (LAN).
Examples of physical layer protocols are CSMA/CD (carrier sense multiple access/collision detection), DSL (digital subscriber line) and RS-232 (which is commonly used in computer serial ports).
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{ 4 comments… read them below or add one }
Thanks for an idea, you sparked at thought from a angle I hadn’t given thoguht to yet. Now lets see if I can do something with it.
wow. thanks a lot for teach me about layers
Help full – Little about ISO/OSI layers
Little about ISO/OSI layers good..