Computer Networks is an interconnected collection of independent computers that are able to exchange information. The connection of these autonomous computers was firstly accomplished with copper wires, but in order to achieve greater speeds fiber optics, microwaves and communication satellites are used also.
There are five main categories of Networks:
- Local Area Networks (LANs)
- Metropolitan Area Networks (MANs)
- Wide Area Networks (WANs)
- Wireless Networks
- Internetworks
| 10m | Room | Local Area Network |
| 100m | Building | Local Area Network |
| 1Km |
Campus
| Local Area Network |
| 10Km | City | Metropolitan Area Network |
| 100Km | Country | Wide Area Network |
| 1000Km | Continent | Wide Area Network |
| 10000Km | Planet | Internet |
As far as this module is concerned LANs and Internetworks will only be discussed in detail later on of this course.
LANs are computer Networks that span a small area such as a Room, building or even a campus. There are widely used to connect workstations and personal computers. Each individual computer connected to the network has its own CPU to execute programs but is also able to access data and devices (such as printers) anywhere in the LAN. They have the advantage of transmitting information at very fast rates. Traditional LANs run at speeds 0f 10 - 100Mbit/s but the distances are limited as well as the number of computers that can be attached to a single LAN is restricted. Different kinds of topologies are possible for broadcast LANs. The most commonly used ones are shown in the figure below:
Figure 1: LAN
Topologies
- Star Topology: All devices are connected to a central hub.
- Ring Topology: All devices are connected together forming a ring. Each device is connected directly to two other devices attached to it.
- Bus Topology: The devices are connected to a central cable called the bus. Ethernet systems, discussed later on the course, use this kind of topology
Figure 2: Layers,
Protocols and Interfaces
In order to understand how the the actual communication is achieved between two remote hosts connected to the same network, a general network diagram is shown above divided into a series of layers. As it seen later on the on the course the actual number as well as their function of each layer differs from network to network. Each layer passes data and control information to the layer below It. As soon as the data are collected form the next layer, some functions are performed there and the data are upgraded and passed to the next layer. This continues until the lowest layer is reached. Actual communication occurs when the information passes layer 1 and reaches the Physical medium. This is shown with the solid lines on the diagram.
Theoretically layer n on one machine maintains a conversation with the same layer in the other machine. The way this conversation is achieved is by the protocol of each layer. Protocol is collection of rules and conventions as agreement between the communication parties on how communication is to proceed. The later is known as virtual communication and is indicated with the dotted lines on the diagram above.
As far as the above diagram is concerned another important issue to be discussed is the interface between each layer. It defines the services and operation the lower layer offers to the one above It. When a network is built decisions are made to decide how many layers to be included and what each layer should do. So each layer performs a different function and as a result the amount of information past from layer to layer is minimized.
Connection-oriented andConnectionless Services
Connection-Oriented service: The user first establishes a connection then uses the connection and then releases the connection. The sender transmits bits of information and the receiver takes them out in the same order as they were originally sent.
Connectionless: Each packet of information carries the full destination address and is routed independently from the others from the source to destination. Packets may take different routes to the destination and it is possible for two packets sent to the same destination the first one to sent can be delayed and the second one arrives first. So care must be taken in order for the all the bits arrive correctly and in the same order they were sent.
OSI Reference Model
This model employs hierarchical structure of seven layers as it is shown in figure 3 below.Click on the figure to change and rollover with the mouse for the original figure.
Figure 3: The OSI Reference Model
OSI stands for Open Systems Interconnection. It has 7-layers and attempts to abstract common features common to all approaches to data communications, and organize them into layers so that each layer only worries about the one above it and the one directly below it. Before getting into details explaining the functions and responsibilities of each layer let me clear one important statement. Although the actual data transmission is vertical, starting from the Application layer of the clients computer all the way to the Application layer of the destination computer, each layer is programmed as though the data transmission were horizontal. This can be observed by clicking on figure 3. In this figure peers are entities comprising the corresponding layers on each machine meaning that the peers that communicate using the protocol. In reality, as I stated above, no data are directly transferred from layer n on one machine to the corresponding layer on another machine.
Physical Layer
The physical layer has as a main function to transmit bits over a communication channel as well as to establish and terminate a connection to a communications medium. It is also responsible to make sure that when one side sends a '1' bit the other side will receive '1' bit and not '0' bit.Data Link Layer
Data link layer provides means to transfer data between network entities. At the source machine it takes the bit streams of data from the Network Layer breaks into frames and passes them to the physical layer. At the receiving end data link layer detects and possibly corrects the errors that may occur during the transmission and passes the correct stream to the network layer. It's also concerned with flow control techniques.Network Layer
This layer performs network routing, flow control and error control functions. Network routing simply means the way packets are routed from source to destination and flow control .prevents the possibility of congestion between packets which are present in the subnet simultaneously and formbottlenecks.Transport Layer
The Transport Layer has as a main task to accept data from the Session layer, split them up into smaller units and passes them to the Network layer making sure that all the pieces arrive correctly to the destination. It is the first end-to-end layer all he way from source machine to destination machine unlike the first three layers which are chained having their protocols between each machine. This is shown clearly in the diagram above.Session Layer
Session layer is responsible for controlling exchange information and for synchronization.Presentation Layer
It is responsible to translate different data formats from the representation used inside the computer (ASCII) to the network standard representation and back. Computers use different codes for representing character strings so a standard encoding must be used and is handled by the presentation layer. Generally in a few words this layer is concerned with the syntax and semantics of the information transmitted.Application layer
The upper layer of this model performs common application service for the application processes meaning that software programs are written in the application layer to handle the many different terminal types that exist and map the virtual terminal software onto the real terminal. It contains a variety of protocols and is concerned with file transfer as well as electronic mail, remote job entry and various other services of general interest.TCP/IP Reference Model
Figure below shows the OSI and TCP/IP network architectures illustrating the layers of the OSI model and introducing the corresponding layers on TCP/IP model.
OSI | TCP/IP |
| Application (layer 7) | Application |
Presentation (layer 6) | |
| Session (layer 5) | |
| Transport (layer 4) | Transport |
| Network (layer 3) | Internet |
| Data Link (layer 2) | Host-to-Network(Subnet) |
| Physical (layer 1) |
Figure 4: The TCP/IP Reference model.
TCP/IP reference model was named after its two main protocols: TCP (Transmission Control Protocol) and IP (Internet Protocol). This model has the ability to connect multiple networks together in a way so that data transferred from a program in one computer are delivered safely to a similar program on another computer.
Unlike the architecture of OSI model TCP/IP has 4 main layers as indicated in the table above. Before comparing the two models let as know proceed by exploring each layer in detail.
Host-to-Network Layer: It translates data and addresses information into format appropriate for an Ethernet Network or Token Ring Network. It uses a protocol (not specified due to lack of information concerned with this layer) in order for the host to connect to the network. Through this layer communication is achieved with physical links such as twisted pair or fiber optics carrying 1's and 0's.
Internet Layer: This layer is a connectionless internetwork layer and defines a connectionless protocol called IP. Its concerned with delivering packets from source to destination. These packets travel independently each taking a different route so may arrive in a different order than they were send. Internet layer does not care about the order the packets arrive at the destination as this job belongs to higher layers.
.Transport Layer: It contains two end-to-end protocols. TCP is a connection oriented protocol and is responsible for keeping track of the order in which packets are sent and reassemble arriving packets in the correct order. It also ensures that a byte stream originating on one machine to be delivered without error on any other machine on the internet. The incoming byte stream is fragmented into discrete messages and is passed to the internet layer. With an inverse process, at the destination, an output stream is produced by reassembling the received massage.
UDP is the second protocol in this layer and it stands for User Datagram Protocol. In contrast to TCP, UDP is a connectionless protocol used for applications operating on its own flow control independently from TCP. It is also an unreliable protocol and is widely used for applications where prompt delivery is more important than accurate delivery. such as transmitting speech or video.
Application Layer: Is the upper layer of the model and contains different kinds of protocols used for many applications. It includes virtual terminalTELNET for remote accessing on a distance machine, File Transfer Protocol FTP and e-mail (SMTP). It also contains protocols like HTTP for fetching pages on the www and others.
OSI versus TCP/IP
Till now we have discussed about the key features of each model and now we will talk about the differences and similarities behind the two models.Starting by stating the ways the models differ. A general statement will be that OSI Reference model was devised before protocols were invented so problems appeared with designing the model as designers didn't have much experience about the subject and did not know what functionality to put in each layer. In TCP/IP the protocols designed first and the model was built based on those protocols so they made an excellent fit. An apparent difference is that OSI has seven layers and TCP/IP has four. The later does not contain Session or Presentation layers simply because It was proven that are of little use to most applications in the OSI model. Another difference is that network layer on OSI model provides both connectionless and connection-oriented services but the corresponding layer in TCP/IP architecture, Internet layer, provides exclusively connectionless communications. TCP/IP model though, supports both modes in Transport layer but equivalent layer on OSI one model supports only connection-oriented.
Despite all these differences the two models have much in common. They are both based on the concept of a stack of independent protocols and the functionality of each layer is roughly similar.
Until this point we have talked about the two basic network architectures, OSI-RM and TCP/IP. You have maybe heart a lot more about the TCP/IP protocols rather than OSI ones but this doesn't mean that TCP/IP is the most advantageous network architecture to be of guide for designing new networks using new topologies. So further on of this course we will discuss the layers covered in the OSI model (minus Session and Presentation layers) from as they are more complete for discussing computer Networks.
No comments:
Post a Comment