IT TUTOR

INFORMATION TECHNOLOGY

NETWORK OPERATING SYSTEMS

Network Operating Systems

The exam specification indicates that an understanding of the following networking terms will be required: Client Server, Application Server, File and Print server, Peer to Peer system. This is a reasonably straight forward subject and the terms are explained in the next pages.

Client Server
In a client server environment the server responds to requests from client computers for data and processing power. In this scenario the client computer is responsible for gathering requests for data to be manipulated and then displaying the result.

Peer to Peer
There are no servers in the network, every machine is equal and may make use of resources on any other. Peer to Peer networks are often called workgroups. Security in this environment is implemented through share level security, i.e. passwords being set on each resource being shared. This configuration is suited to networks of up to 10 clients, beyond this number the administration becomes more difficult because of its distributed nature.

File and Print Server

File and print servers are simpler than client server networks. The file and print server is used as a central share machine for storing files, printers and other hardware. The original PC networks were file and print networks, at the time printers and hard disks were both expensive and a company could share resources more easily with these networks. Even when the cost of hard disks began to reduce the first CD-ROMS were also expensive, so installing them on a server meant the resource could be shared more easily. These File and Print networks failed to offer true application support. For example a multi-user database might store the shared file on the server, yet all processing was completed on the client.

Application Server

NT grew from its origins of a file and print server and moved into the application server environment. It was as an application server that NT began to dominate the market. There were other companies offering robust scalable file and print networks, but these companies could not compete with NT as an application server. While the network operating system was battled out in the file and print arena NT had stiff competition from NetWare. NT as an application server offers Exchange and SQL amongst a number of other applications that run on NT server.

OSI Reference Model

One of the most important areas to understand in networking technologies is that of the OSI reference model. You will need to make sure that your understanding of the OSI model is complete before taking the exam. Although you may not find the model comes up in as many questions as you might expect, it is not until you fully understand this model that you should consider yourself ready for the exam.

The Open Systems Interconnect model was developed in 1978 by the ISO (International Organisation for Standards). The OSI model does not itself specify particular communication standards to allow network devices to communicate, but rather provides a simple model against which systems can be compared. The model is divided into seven manageable and definable levels, each with a defined responsibility.

You will find as you spend more time with networking software that text books often refer to the OSI reference model, so don't treat this as a purely academic exercise. For the exam make sure that you can go into the examination room and write out the layers in the model, the responsibility of each layer and the key networking hardware which works at each layer.

Physical Layer

The data units at this layer are called bits. This layer defines the mechanical and electrical definition of the network medium and network hardware. This includes how data is merged onto the cable and retrieved from it. An example of the Physical layer is RS-232.

The main network device found at the Physical layer is a repeater. The purpose of a repeater (as the name suggests) is simply to receive the digital signal, reform it, and retransmit the signal. It should be easy to see that this has the effect of increasing the maximum length of a network, which would have been reached due to signal degradation if a repeater was not available. It should also be possible to anticipate the requirements of a repeater, it is simply regenerating cleaner digital signal so doesn't have to understand anything about the information it is transmitting, processing on the repeater is non-existent.

Data Link Layer

The data link layer groups the bits that we see at Physical layer into Frames. Primarily responsible for error-free delivery of data we see the DATA LINK layer as split into two, the Logical Link Control and Media Access Control. Although this split may not be necessary for the exam it is important to be aware if you see reference to the LLC that this is actually part of the data link layer.

The main network device found at the data link layer is a bridge. This device works at a higher layer than the repeater and therefore is a more complex device. It has some understanding of the data it receives and can make a decision based on the frames it receives as to whether it needs to let the information pass, or can remove the information from the wire. This means that the amount of traffic on the cable can be reduced and therefore the usable bandwidth increased.

Network Layer

The unit of data at the network layer is called a packet. This layer is responsible for ROUTING. It examines network addresses in the data instead of physical addresses seen in the Data Link layer. It makes a decision as to where to route the packet based on information and calculations from other routers, or according to static entries in the routing table.

The main device found at the Network layer is a router.

OSI Reference Model

Transport Layer

The transport layer is responsible for the reliable delivery of data. The mechanism for this is often a connection-oriented delivery where both communicating devices go through a process where they come to expect data from the other. Once the transfer of data is complete they go through a 'teardown' phase which breaks the connection between the machines. There is more overhead involved in this reliable delivery of data, but it ensures that errors are recognised and recovered from.

Some applications do not require this level of reliability and make use of connectionless delivery where the above connection-building process is not performed. This in turn reduces the level of overhead involved. For example, a file transfer over the Internet would probably make use of reliable connection oriented delivery, whereas real time audio information (e.g. an Internet radio show) can transmit using connectionless delivery. If some information is missed the first time, then being realtime audio it is lost forever (you don't want it 5 seconds late and out of sequence).

Session Layer

This layer is about establishing connections, authenticating access, co-ordination and synchronisation. The session layer is responsible for managing dialog between two devices, for example the session layer determines whether the communication will be simplex, half-duplex or duplex
Presentation Layer

This layer is responsible for ensuring the data is in a format that is understood by both sender and recipient. This layer is also responsible for data encryption and data compression.

OSI Reference Model

Application Layer

The Application layer is probably the most easily misunderstood layer of the model. This layer deals with providing the network services to the end user, but is not the software running on their machine (Word for instance). Server messaging blocks (SMB’s) exist at the application layer.

Each layer, with the exception of the physical layer, adds information to the data as it travels from the application down to the physical layer. This extra information is called a header.