An Introduction To Network Topology

In the context of a communication network, the term topology refers to that way in which the end points, or stations, attached to the network are interconnected or it is the arrangements of systems in a computer network. It can be either physical or logical. The physical topology refers that, a way in which a network is laid out physically and it will include the devices, installation and location. Logical topology refers that how a data transfers in a network as opposed to its design.

The network topology can be categorized into bus, ring, star, tree and mesh.

Hybrid networks (They are the complex networks, which can be built of two or more topologies).

Bus Topology

A Bus topology is characterized by the use of a multi point medium. A long and single cable acts as a backbone to connect all the devices in a network. In a bus topology, all computers or stations attach through the appropriate hardware interfacing known as a tap, directly to a bus network. Full duplex operation between the station and tap allows data to transmit onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus. Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. A bus network work best with a limited number of computers.

Advantages

Bus topology can install very easily on a network.

Cabling will be less compare to other topologies because of the main backbone cable laid efficiently in the network path.

Bus topology suited for a small network.

If one computer fails in the network, the other computers are not affected they will continue to work.

It is also less expensive than star topology.

Disadvantages

The cable length will limited and there by limits the number of stations.

If the backbone cable fails, the entire network will goes down.

It is very difficult to trouble shoot.

Maintenance cost is very high in a long run.

Terminators are required for both the ends of the cable.

Ring topology

The ring topology the network consists of dedicated point to point connection and a set of repeaters in a closed loop. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. It may be clock wise or anti clock wise. When a device receives a signal intend for another device, its repeater generates the bits and passes them along. As with the bus and tree, data are transmitted in frames. As a frame circulates past all the other stations, the destination station recognize its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. These topologies are used in school campuses and some office buildings.

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Figure (2) – Bus topology

Advantages

It performs better than star topology under heavy work load

For managing the connection between the computers, there is no need for the network server.

It is cheaper than star topology because of less wiring.

By adding the token ring in the network, can create large network.

Very order network because all the devices has a access to the token ring and opportunity to transmit.

Disadvantages

A break in the ring (such as a disabled station) can disable the entire network.

It is much slower than an Ethernet network with under normal load.

Any moves, changes and adds of the devices can affect the network.

Network connection devices like (Network adapter cards and MAU) are much more expense than Ethernet cards.

Star Topology

In a star topology, each station is directly connected to a common node called hub. Unlike a mesh technology, the devices are not directly linked to one another. A star topology does not allow direct traffic between devices. The controller act as an exchange, like if one device wants to send to another, it sends the data to the controller, which then relays the data to the connected device. In a star, each device needs only one link and one I/O port to connect it to any number of others. The star topology is used in local area networks (LAN) and sometimes high speed LAN often uses a star topology with central hub.

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Advantages

If one link fails in the star topology, only that link is affected. All other links remain active.

It is easy to identify the fault and fault isolation.

Easy to expand the network in the star topology.

No disruptions to the network when connecting or removing devices.

It is very easy to manage because of its simplicity in the function.

Disadvantages

In a star topology, if the hub goes down, the entire network will fails.

It requires more cable length compared to the linear bus topology.

It is much more expensive than bus topology, because of the cost of the hubs.

Tree Topology

A tree topology is the generalization of the bus topology. It integrates the multiple star topologies together on to a bus. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the head end. The branches in turn may have additional branches to allow quite complex layouts. A transmission from any station propagates throughout the medium and can be received by all other stations. This topology will allow for the expansion of an existing network.

Advantages

Tree topology is well supported by the hardware and software vendors.

Point to point wiring for each and every segments of the network.

It is the best topology for the branched networks.

Disadvantages

It is more expensive because more hubs are required to install the network.

Tree topology is entirely depends upon the backbone line, if it fails then the entire network would fail.

It is very difficult to configure and wire than other network topologies.

In a tree topology, the length of network depends on the type of cable being used.

Mesh Topology

In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to other node. Node 1 must be connected to n-1nodes, node 2 must be connected to n-1nodes, and finally node n must be connected n-1 nodes. However, if each physical link allows communication in both directions, we can divide the number of links by 2.In other words we can say that in a mesh topology, we need n (n-1)/2.

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Figure (5 – Mesh topology

Suppose if we are connecting 15 nodes in a mesh topology, then the number of cables required;

DA = n (n-1)/2 DA = Number of cables

= 15 (15 – 1)/2 n = Node

= 15*14/2

= 15*7

= 105

Therefore, the total number of cables required for connecting 15 nodes = 105.

Advantages

There is no traffic problem because of the dedicated link in the mesh network.

Mesh topology is robust. If one link becomes unusable. It does not incapacitate the entire system.

Point-to-point links make full identification and fault isolation easy.

Security or privacy for data travels along the dedicated line.

Network can be expanded without any disruptions to the users.

Disadvantages

Installation and reconnection are difficult.

Large amount of cabling and the number of I/O ports required

Sheer bulk of the wiring can be greater than the available space can accommodate.

The hardware required to connect each link can be prohibitively expensive.

Hybrid Topology

A network can be hybrid, which uses two or more network topologies together in a network. For example, we can have a main star topology with each branch connecting several stations in a bus topology.

The OSI Model

The Open System Inter connection (OSI) reference model was developed by the International Organization for Standardization (ISO)2 as a model for a computer protocol architecture and as a frame work for developing protocol standards. The purpose of the OSI model is show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding a network architecture that is flexible, robust and interoperable. The OSI model is a layered frame work for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process moving information across a network.

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.

Upper Layers of the OSI Models are;

Application layer

Presentation layer

Session layer

The upper layers of the OSI model designate the application issues, presentation session stages and generally are implemented only in software. The highest layer, (the application layer) is close to the end user. These upper layers are act as an interface between the user and the computer. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model.

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Examples of upper layer technologies in the OSI model are SNMP, FTP, and WWW etc.

Lower Layers of the OSI Model

Transport layer

Network layer

Data link layer

Physical layer

The lower layers of the OSI model provide network specific functions like data transport issues (flow control, addressing and routing). The lower layers of the OSI model (the physical layer and the data link layer) are implemented in hardware and software also. Examples of lower layer technologies in the OSI model are TCP, UDP, IP, IPX etc.

Application layer

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. The application layer provides specific services like network virtual terminal, file transfer, access and management, mail services and directory services.

Network virtual terminal: A network virtual terminal is a software version of physical terminal, and it allows a user to log on to a remote host.

File transfer, access and management: This application allows a user to access files in a remote host (to make changes, read data), to retrieve files from a remote computer for use in the local computer and to manage or control files in a remote computer locally.

Mail services: The application provides the basis for e-mail forwarding and storage.

Directory services: This application provides distributed database source and access for global information about various objects and services.

Presentation layer

The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. The presentation layer is responsible for the translation, compression and encryption. Messages are sending between the layers.

Translation: The process in two systems are usually exchanging in the form of character strings, numbers, and so on. The information is changed into bit streams before being transmitted. The presentation layer at the sender changes the information from its sender dependent format into a common format. On the receiving machine, the presentation layer changes the common format into its receiver-dependent format.

Encryption: Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform message back to its original form.

Compression: Data compression reduces the number of bits contained in the information. It becomes particularly important in the transmission of multimedia such as text, audio and video.

Session layer

The session layer is the network dialog controller. It establishes, maintains and synchronizes the interaction among communicating systems. These layers have specific responsibilities include the following;

Dialog control: The session layer allows two systems to enter into a dialog. It allows the communication between twp processes to take place in either half duplex (one way at a time) or full duplex (two ways at a time) mode.

Synchronization: The session layer allows a process to add check points, or synchronization points, to a stream of data.

Examples for session layers are MPEG, JPEG, MIDI, NCP etc.

Transport layer

The transport layer is responsible for process to process delivery of the entire message. The transport layer is responsible for the delivery of a message from one process to another. A process is an application program running on a host. The transport layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. It also has some specific responsibilities mentioned below;

Service-point addressing: The transport layer includes a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer,; the transport layer gets the entire message to the correct process on that computer.

Segmentation and reassembly: A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layers to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

Connection control: The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. If a connection oriented transport layer make a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred the connection is terminated.

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Flow control: The transport layer is responsible for the flow control. However, flow control at this layer is performed end to end rather than across a single link.

Error control: Transport layer is also responsible for the error control. Error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error.

These layers using the TCP/IP and UDP protocols.

Network layer

The network layer is responsible for the source to destination delivery of a packet, possibly across multiple networks (links). This layer ensures that each packet gets from its point of origin to its final destination. Network layers also have other responsibilities include the following;

Logical addressing: If a packet passes the network boundary, it needs another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

Routing: When independent networks are connected to create internetworks or a large network, the connecting devices route or switch the packets to their final destination.

Data link layer

The data link transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer. It also has other responsibilities include the following;

Framing: The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Physical addressing: The data link layer adds a header to the frame to define the sender and/or receive of the frame. If the frame is intend for a system outside the sender’s network, the receiver address is the address of the device that connects the network to the next one.

Flow control: If the rate at which the data are absorbed by receiver is less than the rate at which data are produced in the sender, the data link layer impose a flow control mechanism to avoid overwhelming the receiver.

Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

Access control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

Data link contains two sub layers; LLC (Logical Link Control) and MAC (Medium Access Control).LLC is the upper sub layer, which maintains and establishes the communication links to the device. And it also responsible for the frame error control and addressing.MAC is the lower sub layer of the data link layer. It controls how the devices sharing the media channel.

Physical layer

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.

The physical layer is also concerned with the following:

Physical characteristics of interface and medium: Physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium.

Representation of bits: This layer data consists of a stream of bits with no interpretation. To be transmitted, bits must be encoded into signals electrical or optical. The physical layer defines the type of encoding.

Data rate: The transmission rate the number of bits sent each second- is also defined by physical layer. In other words physical layer defines the duration of a bit, which how long it lasts.

Synchronization of bits: The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level.

Line configuration: The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration a link is shared among several devices.

Physical topology: The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology, a star topology, a ring topology, a bus topology, or a hybrid topology.

Transmission mode: The physical layer also defines the direction of transmission between two devices: simplex, half duplex, or full duplex.

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