Friday, December 12, 2014
DATA COMMUNICATION & NETWORKING
Data Communication deals with the transmission of digital data from one device to another. Data is transferred through a pathway called as communication channel which can be physical wire connecting the devices or may be unguided media like laser, microwave etc.
►A communication channel has a source or transmitter at one side and a designation or receiver at another side of the network. The source of data origination is single but there may be multiple receivers.
A communication channel is of 3 types:
►Simplex: In this, communication is unidirectional i.e. one of the two devices can transmit the data and the other can only receive the data. For eg. Radio broadcasting, television broadcasting etc.
►Half duplex: In this communication is bidirectional. Either of the devices can act as transmitter or receiver but only one device can transmit the data at one time. For eg. Walkie talkie.
►Full Duplex: Here also communication is in both directions and both the devices can simultaneously transmit the data. For eg. Telephone conversation.
►Different types of network are: LAN, MAN and WAN.
A LAN (local area network) is a group of computers and network devices connected together, usually within the same building. By definition, the connections must be high speed and relatively inexpensive (e.g., token ring or Ethernet).
A MAN (metropolitan area network) is a larger network that usually spans several buildings in the same city or town.
A WAN (wide area network), in comparison to a MAN, is not restricted to a geographical location, although it might be confined within the bounds of a state or country. A WAN connects several LANs, and may be limited to an enterprise (a corporation or an organization) or accessible to the public. The technology is high speed and relatively expensive. The Internet is an example of a worldwide public WAN.
►Now let’s have a look at some basic differences between these three...
Area:
LAN: Covers small area.
WAN: Covers large geographical area
MAN: Covers larger than LAN but smaller than WAN
Error Rates:
LAN: Lowest
WAN: Highest
MAN: Moderate
Transmission speed:
LAN: High Speed
WAN: Low Speed
MAN: Moderate speed
Cost:
LAN: Uses inexpensive equipment
WAN: Uses most expensive equipment (Connecting systems via internet)
MAN: Uses moderately expensive equipment (Connecting two systems in different buildings)
►The Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO).
Seven layers of OSI Model are:
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data link Layer
Physical Layer
►Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer network. There are two basic categories of network topologies:
►Physical topologies and logical topologies.
Physical topology is the placement of the various components of a network, including device location and cable installation, while logical topology illustrates how data flows within a network, regardless of its physical design. Various types of topologies are:
Bus Topology
Star Topology
Ring Topology
Mesh Topology
Tree Topology
Hybrid Topology
►Bus Topology is a network topology in which nodes are connected in a daisy chain by a linear sequence of buses.
The bus can only transmit data in one direction, and if any network segment is severed, all network transmission ceases.
Advantages:
---Easy to connect a computer or peripheral to a linear bus.
---Requires less cable length than a star topology.
---It works well for small networks.
Disadvantages:
---Entire network shuts down if there is a break in the main cable.
---Terminators are required at both ends of the backbone cable.
---Difficult to identify the problem if the entire network shuts down.
---Not meant to be used as a stand-alone solution in a large building.
---It is slow when more devices are added into the network.
---If a main cable is damaged then network will fail or be split into two networks.
BUS TOPOLOGY
Star Topologies are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages. This consists of a central node, to which all other nodes are connected; this central node provides a common connection point for all nodes through a hub. In star topology, every node (computer workstation or any other peripheral) is connected to a central node called a hub or switch.
STAR TOPOLOGY
Advantages:
►Better performance: Star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices.
►Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straightforward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.
►Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.
►Easy to detect faults and to remove parts.
►No disruptions to the network when connecting or removing devices.
►Installation and configuration is easy since every one device only requires a link and one input/output port to connect it to any other device(s).
Disadvantages:
►Reliance on central device: Star Topology relies on the central device (the switch, hub or computer) and if this device fails the whole network will fail in turn.
►Higher costs: The need for a central device increases costs compared to the bus and ring topologies. The star topology also requires more cable when using Ethernet cables than ring and bus topologies.
►Limited capacity for nodes: As this type of network needs all connections to go through a central device the amount of nodes in a network is limited by this factor whereas bus and ring topologies are not limited in such a way.
►Ring Topology is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring.
Data travel from node to node, with each node along the way handling every packet.
Advantages:
►Very orderly network where every device has access to the token and the opportunity to transmit.
►Performs better than a bus topology under heavy network load.
►Does not require a central node to manage the connectivity between the computers.
►Due to the point to point line configuration of devices with a device on either side (each device is connected to its immediate neighbor), it is quite easy to install and reconfigure since adding or removing a device requires moving just two connections.
►Point to point line configuration makes it easy to identify and isolate faults.
Disadvantages:
►One malfunctioning workstation can create problems for the entire network. This can be solved by using a dual ring or a switch that closes off the break.
►Moving, adding and changing the devices can affect the network.
►Communication delay is directly proportional to number of nodes in the network.
►Bandwidth is shared on all links between devices.
►More difficult to configure than a Star: node adjunction = Ring shutdown and reconfiguration
Ring Topology
►Mesh Topology is a network topology in which each node (called a mesh node) relays data for the network. All nodes cooperate in the distribution of data in the network.
Mesh Topology
Advantages:
►Data can be transmitted from different devices simultaneously. This topology can withstand high traffic.
►Even if one of the components fails there is always an alternative present. So data transfer doesn’t get affected.
►Expansion and modification in topology can be done without disrupting other nodes.
Disadvantages:
►There are high chances of redundancy in many of the network connections.
►Overall cost of this network is way too high as compared to other network topologies.
►Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.
Tree Topology
Tree Topology integrates the characteristics of Star and Bus Topology. Earlier we saw how in Physical Star network Topology, computers (nodes) are connected by each other through central hub. And we also saw in Bus Topology, work station devices are connected by the common cable called Bus. After understanding these two network configurations, we can understand tree topology better. In Tree Topology, a number of Star networks are connected using Bus. This main cable seems like a main stem of a tree, and other star networks as the branches. It is also called Expanded Star Topology.
Advantages:
►It is an extension of Star and bus Topologies, so in networks where these topologies can't be implemented individually for reasons related to scalability, tree topology is the best alternative.
►Expansion of Network is possible and easy.
►Here, we divide the whole network into segments (star networks), which can be easily managed and maintained.
►Error detection and correction is easy.
►Each segment is provided with dedicated point-to-point wiring to the central hub.
►If one segment is damaged, other segments are not affected.
Disadvantages:
►Because of its basic structure, tree topology, relies heavily on the main bus cable, if it breaks whole network is crippled.
►As more and more nodes and segments are added, the maintenance becomes difficult.
►Scalability of the network depends on the type of cable used.
Hybrid Topology
Hybrid, as the name suggests, is mixture of two different things. Similarly in Hybrid Topology, we integrate two or more different topologies to form a resultant topology which has good points (as well as weaknesses) of all the constituent basic topologies rather than having characteristics of one specific topology. This combination of topologies is done according to the requirements of the organization.
For example, if there exists a ring topology in one office department while a bus topology in another department, connecting these two will result in Hybrid topology. Remember connecting two similar topologies cannot be termed as Hybrid topology. Star-Ring and Star-Bus networks are most common examples of hybrid network
Advantages:
Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network.
Scalable: It’s easy to increase the size of network by adding new components, without disturbing existing architecture.
Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high.
Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology.
Disadvantages:
Complexity of Design: One of the biggest drawback of hybrid topology is its design. It’s not easy to design this type of architecture and it’s a tough job for designers. Configuration and installation process needs to be very efficient.
Costly Hub: The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down.
Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems, sophisticate network devices, etc.
Hybrid Topology
►Internetworking Devices
These are used to connect different devices in the network or to connect two or more different networks. Following devices are used for interconnection:
Modem
Hub
Switch
Repeater
Router
Gateway
Bridge
Modem: Modem stands for Modulator-Demodulator. It is used to connect computers for communication via telephone lines.
Hub: It works at the Physical layer. It just acts like a connector of several computers i.e. simply connects all the devices on its ports together. It broadcasts all the data packets arriving at it with no filtering capacity.
Switch: It works at the Data Link Layer. It is used for dividing a network into segments called subnets. It provides filtering of data packets and prevents network traffic also.
Repeater: It operates at the Physical Layer. It is used to amplify a signal that has lost its original strength so as to enable them to travel long distances. It can only join the networks that transmit similar data packets. It does not have filtering capacity i.e. all data including noise is amplified and passed on in the network so don’t help in reducing network traffic.
Router: It works at the Network Layer and is used to connect different networks that have different architectures and protocols. It sends the data packets to desired destination by choosing the best path available thus reducing network traffic. It routes the data packets using the routing table that contains all the Information regarding all known network addresses, possible paths and cost of transmission over them. Availability of path and cost of transmission decide sending of data over that path. It is of 2 types: static (manual configuration of routing table is needed) and dynamic (automatically discovers paths).
Gateway: It operates in all the layers of the network architecture. It can be used to connect two different networks having different architectures, environment and even models. It converts the data packets in form that is suitable to the destination application. The two different networks may differ in types of communication protocols they use, language, data formats etc.
Bridge: They are used two connect two LANs with the same standard but using different types of cables. It provides an intelligent connection by allowing only desired messages to cross the bridge thus improving performance. It uses physical addresses of the packets for this decision. It works on Data Link Layer of the OSI model. A bridge uses Spanning tree Algorithm for data transmission so as to avoid loops in the network.
►An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication.
►IP addresses are binary numbers, but they are usually stored in text files and displayed in human-readable notations, such as 172.16.254.1 (for IPv4), and 2001:db8:0:1234:0:567:8:1 (for IPv6).
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