Network Topology

Network Topology:

Topology is the layout of connected devices on a network. Think of it as the logical "shape" of the network wiring. This logical shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle, but it would be highly unlikely to find an actual ring topology there.

'Logical' means how it looks as a pure design concept, rather than how it actually looks physically. e.g. the topology pictures you will see have nice straight lines between bits of the network, they don't try to show all the corners that need to be turned and holes that have to be drilled in a real cable installation.

Each topology has its advantages and disadvantages: usually related to cost, complexity, reliability and traffic.

There are five basic types of topologies are normally used to implement network:

•          Bus
•          Ring
•          Star
•          Tree
•          Mesh

Bus Topology :

The physical Bus Network Topology is the simplest and most widely used of the network designs. It consists of one continuous length of cable (trunk) that is shared by all the nodes in the network and a terminating resistor (terminator) at each end that absorbs the signal when it reaches the end of line. Without a terminator the electrical signal would reach the end of copper wire and bounce back, causing errors on the network.

Bus networks usually use coaxial cables that connect to each of the computers through T-shaped connectors. A terminator specific to the type of cable used placed on each end node of the network. Since the bus network is nothing more than a set of cables, connectors, and terminators, the signal is not amplified when traveling through the wiring.

Bus networks are easy to assemble and expand. They only require a small amount of cable, compared to other network topologies. However, bus networks can suffer cable breakage, loss of information in the connectors and deficiencies in the necessary wiring length, often difficult to resolve. Any physical problem in the network, such as a loose connector, can crush the entire bus network.

Advantages of Bus Topology:

·         It uses established standards and it is relatively easy to install and the use for small networks.

·         It requires fewer medium than other topologies.

·         Failure of one node does not affect the network functioning.

·         Cost is less as only one main cable is required and least amount of cable is required to connect computers.

·         Expansion is easier. New node can be easily added by using a connector.

Disadvantages of Bus Topology:

·         If the main central line fails the entire network collapses.

·         The bus networks are difficult to reconfigure, especially when the acceptable number of connections or maximum distances have been reached.

·         They are also difficult to troubleshoot because everything happens on a single media segment.

·         Sharing a single communication channel results in slower access time.

Ring Topology:

The physical ring Topology is a circular loop of point-to-point links. Each device connects directly to the ring or indirectly through and interface device or drop cable. Message travel around the ring from node to node in a very organized manner. Each workstation checks the message for a matching destination address. If the address doesn't match the node simply regenerates the message and sends it on its way. If the address matches, the node accepts the message and sends a reply to the originating sender.

• In ring topology, the various nodes are connected in form of a ring or circle (physical ring), in which data flows in a circle, from one station to another station.

• It has no beginning or end that needs to be terminated.

• In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

• There are two kinds of ring topologies:

1. Single Ring

2. Dual Ring

•. In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

• There are two kinds of ring topologies:

1. Single Ring

2. Dual Ring

1. Single ring - In single ring network, a single cable is shared by all the devices and data travel only in one direction.

Each device waits for its turn and then transmits. When the data reaches its destination, another device can transmit.

2. Dual ring: This topology uses two rings to send the data, each in different direction. Thus allowing more packets to be sent over the network.

Advantages of Ring Topology

The advantages of Ring Topology are:

1.They are very easy to troubleshoot because each device incorporates a repeater.

2.A special internal feature called beaconing allows troubled workstations to identify themselves quickly.

There is no master computer on controller. Every computer has equal chance to place the data and access the token.

There are no collisions.

Data packets travel at greater speeds.

It is easier to locate the problems with device and cable i.e. fault isolation is simplified. If one device does not receive a signal within a specified time, it can issue an alarm. This alarm alerts the network operator to the problem and its location.

Disadvantages of Ring Topology

The disadvantages of ring topologies are:

A ring network requires more cable than a bus network.

A break in cable ring brings down entire network (in case of single ring).

Adding or removing the node disturbs the network activity.

In ring network, communication delay is directly proportional to the number of nodes in the network. Hence addition of new nodes in the network also increases communication delay.

It is considerably difficult to install and reconfigure ring Topology

Media failure on unidirectional or single loop causes complete network failure.

Star Topology :

The physical star Topology uses a central controlling or hub with dedicated legs pointing in all directions – like points of a star. Each network device has a dedicated point-to-point link to the central hub. There is no direct link between these computers and the computers can communicate via central controller only. This strategy prevents troublesome collisions and keeps the lines of communications open and free of traffic. Since in the star topology each computer on the network uses a different cable connection, this type of topology is expandable, only limited by the number of ports available in the hub (although it is possible to join several hubs to increase the number of ports ). The expansion of a star topology network does not present any difficulty, since adding another computer to the network means nothing more than placing a cable between the computer and the hub. In fact, the rest of the network users will not even notice the extension.

The routing function is performed by the central controller which centrally controls communication between any two computers by establishing a logical path between them. It means that if one computer A wants to send data to another computer B, Computer A sends the data to the controller & this controller then sends the data to computer B.

This Topology, obviously, require a great deal of cabling. This design provides an excellent platform for reconfiguration and trouble-shooting. Changes to the network are as simple as plugging another segment into the hub and a break in the LAN is easy to isolate and doesn't affect the rest of the network.

Advantages of Star Topology

The benefits of star topology are:

1. It is easier to add new node or modify any existing node without disturbing network i.e. expansion is easier.

2. Addition of new node does not increase communication delay.

3. If any local computer or link fails, the entire system does not collapse. Only that link or computer is affected.

4. It is easy to find device and cable problems i.e. fault identification and isolation is easier.

5. Media faults are automatically isolated to the failed segment.

Disadvantages of Star Topology

The disadvantages are considered as follows:

1. If the central controller or hub fails, entire system collapses.

2. Cabling cost is more as each node is connected individually to the hub.

3. Requires more cable than most topologies

4. Moderately difficult to install.

Mesh Topology:

The mesh network topology uses redundant connections between the node on the network, applying a fault tolerance strategy. Each node included in the network connected to the rest of the node, which explains why this type of topology requires extensive wiring. This type of topology can cope with the failure of one or two segments of the network without interrupting traffic since it has redundant lines.

Mesh networks are more expensive and difficult to install than other types of network topologies due to the large number of connections they require. In most cases, networks that use this redundant connection strategy included within the broader hybrid networks. In a hybrid network, only the most essential and crucial servers and computers configured with redundant connections. In this way, the fundamental segments of the corporate network protected without using multiple lines for each of the computers connected to the network.

Advantages of Mesh Topology

1. It is robust as the failure of one node does not collapse the entire system. If one link fails, the entire system continues to work.

2. There is no traffic congestion problem as dedicated links are being used.

3. Dedicated links ensure faster transmission without any delay.

4. Dedicated links also ensure data privacy and security.

5. Point to point links makes fault identification and isolation easier.

Disadvantages of Mesh Topology

1. Connecting each device to every other device in the network makes installation and reconfiguration difficult.

2. It has high cabling cost as n (n-l)/2 links are required to connect n nodes.

Tree Topology:

Tree or Hierarchical Topology: The type of  Topology in which a central 'root' node, the top level of the hierarchy, is connected to one or more other nodes that are one level lower in the hierarchy i.e., the second level, with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy, i.e., the third level, connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy – the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number, f, of nodes connected to it at the next lower level in the hierarchy, the number, f, being referred to as the 'branching factor' of the hierarchical tree

Advantages:

Supported by several hardware and software venders.

It allows more devices to be attached to a single central hub and can therefore increases the distance a signal can travel between devices.

It allows the network to isolate and prioritize communication from different computers i.e. the computers attached to one secondary hub can be given priority over the computers attached to another secondary hub.

Disadvantages:

• Overall length of each segment is limited by the type of cabling used.

• If the backbone line breaks, the entire segment goes down.

• More difficult to configure and wire than other topologies.

• It has higher cabling cost in setting up a tree structure.

Hybrid Topology

The hybrid Topology is a type of Topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies in a single network that is composed of one or more interconnections of two or more networks that are based upon the same physical topology.

When two hubs of different topologies are joined so that the devices attached to them can communicate as in figure, it is called a Star-Bus network.

Network Layer Design Issues:

Store-and-Forward Packet Switching:

The major components of the system are the carrier's equipment (routers connected by transmission lines), shown inside the shaded oval, and the customers' equipment, shown outside the oval.

Host H1 is directly connected to one of the carrier's routers, A, by a leased line. In contrast, H2 is on a LAN with a router, F, owned and operated by the customer. This router also has a leased line to the carrier's equipment.

We have shown F as being outside the oval because it does not belong to the carrier, but in terms of construction, software, and protocols, it is probably no different from the carrier's routers.

Figure. The environment of the network layer protocols.

This equipment is used as follows.

A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-to-point link to the carrier. The packet is stored there until it has fully arrived so the checksum can be verified.

Then it is forwarded to the next router along the path until it reaches the destination host, where it is delivered. This mechanism is store-and-forward packet switching.

Services Provided to the Transport Layer:

The network layer provides services to the transport layer at the network layer/transport layer interface. An important question is what kind of services the network layer provides to the transport layer.

The network layer services have been designed with the following goals in mind.

1.      The services should be independent of the router technology.

2.      The transport layer should be shielded from the number, type, and topology of the routers present.

3.      The network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs.

Given these goals, the designers of the network layer have a lot of freedom in writing detailed specifications of the services to be offered to the transport layer. This freedom often degenerates into a raging battle between two warring factions.

The other camp argues that the subnet should provide a reliable, connection-oriented service. They claim that 100 years of successful experience with the worldwide telephone system is an excellent guide. In this view, quality of service is the dominant factor, and without connections in the subnet, quality of service is very difficult to achieve, especially for real-time traffic such as voice and video.

These two camps are best exemplified by the Internet and ATM. The Internet offers connectionless network-layer service; ATM networks offer connection-oriented network-layer service. However, it is interesting to note that as quality-of-service guarantees are becoming more and more important, the Internet is evolving.

Connection Oriented Communication Services:

There is a sequence of operation to be followed by the users of connection oriented service. These are:

1.      Connection is established.

2.      Information is sent.

3.      Connection is released.

In connection oriented service we have to establish a connection before starting the communication. When connection is established, we send the message or the information and then we release the connection.

Connection oriented service is more reliable than connectionless service. We can send the message in connection oriented service if there is an error at the receivers end. Example of connection oriented is TCP (Transmission Control Protocol) protocol.

Connection less Services:

It is similar to the postal services, as it carries the full address where the message (letter) is to be carried. Each message is routed independently from source to destination. The order of message sent can be different from the order received.

In connectionless the data is transferred in one direction from source to destination without checking that destination is still there or not or if it prepared to accept the message. Authentication is not needed in this. Example of Connectionless service is UDP (User Datagram Protocol) protocol.

Difference between Connection oriented and Connectionless service:

In connection oriented service authentication is needed, while connectionless service does not need any authentication.

Connection oriented protocol makes a connection and checks whether message is received or not and sends again if an error occurs, while connectionless service protocol does not guarantees a message delivery.

Connection oriented service is more reliable than connectionless service.

Connection oriented service interface is stream based and connectionless is message based.