A network configuration is also known as a network topology. A network topology is the shape or the physical connectivity of the network. The following three objectives are to be met while designing a topology:
a) Provide maximum possible reliability to assure proper recipient of all traffic
b) Route the traffic across the least-cost path within the network between the sender and receiver
c) Give the end user the best possible response time and throughput.
The various common topologies are as follows:
a) Hierarchical Topology:
It is one of the most common topology. Here, the software required to control the network is relatively simple. But also contains the potential bottleneck problems as well as the reliability problem. If the upper layer machine fails, the network capabilities are lost completely. Hierarchical topology is also known as “vertical network” or “tree network”.
b) Bus Topology:
Bust topology is pretty popular in LAN. It is relatively simple to control traffic flow between and among machines on the network since the bus permits all stations to receive every transmission, i.e. a single station broadcasts to multiple stations. The main drawback of this topology is that it uses only one communication channel to serve all the devices on the network. Consequently in case of the failure of communication channel, the entire network is down. And the next problem is the difficulty in isolating faults to any one particular component inside the bus.
c) Star Topology:
At present, it is the most widely used topology for data communication systems. This came into existence since 1960s and is easy to control as the software is not so complex and the traffic flow is simple. All traffic originates from the hub of the star, the central node, which is responsible for routing traffic to the other components as well as for fault isolation. Fault isolation is relatively simple in a star network because the lines can be isolated to identify the problem. However, the star network is subjected to potential bottleneck and failure problems at the hub.
d) Ring Topology:
The data flow in only one direction, with one single station receiving the signal and relaying it to the next station on the ring. It is more user friendly since bottlenecks, such as those found in the hierarchical or star systems are very uncommon. Here, the token circumnavigates throughout the entire ring and the transmitting node holds the token and sends the network packet. Once done, the token is passed to an adjacent node.
To reduce the design complexity, most networks are organized as a series of layers or levels. The name, number, content and function of each layer differ from network to network. However in all networks, the purpose of each layer is to offer certain services to the higher layers.
A protocol is an agreement between the communicating parties on how communication is to proceed. Let a layer “n” on one machine carries on conversation with layer ”n” on another machine. The rules and conventions used in thi8s conversation are collectively known as protocol. Practically, no data are directly transferred from layer ”n” on one machine to layer ”n” on another machine. Instead, each layer passes data and control information to the adjacent layer below it, until the lowest layer i.e. physical medium is reached through which an actual communication occurs. Between each layer, there is an interface that defines which primitive operations and services the lower layer offers to the upper one. A set of layers and protocols is called network architecture. Its specification must contain enough information to allow write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol. Since a certain system uses a list of protocols, one protocol per layer is called a protocol stack.
Design issues for the layers:
In some systems, data flow only in one direction known as simplex communication, data flow in either direction but not simultaneously known as half-duplex communication and those data flow in both directions at once, known as full-duplex communication.
Interfaces and services:
The function of each layer is to provide services to the layer above it. The active elements in each layer are called entities. It can either be a software entity (say a process) or a hardware entity (say an intelligent I/O chip). Entities in the same layer on different machines are known as peer entities.
The entities in layer ”n” implement a service used by layer n+1, where “n” is known as the “service provider” and the layer “n+1” is known as “service user”.
Service Active Points:
The layer “n” SAPs are the places where layer n+1 can access the service s offered. Each SAP has an address that uniquely identifies it.
Interface Data Unit:
At a typical interface, the layer “n+1” entity passes an IDU to the layer “n” entity through SAP. The IDU consists of an SDU and some control information. The SDU is the information passed across the network to the peer entity and then up to layer “n+1”. It should be noted that the control information is required to help the lower layer do its job but is not part of data itself. In order to transfer the SDU, the layer “n” entity may have to fragment it into several pieces, known as PDU (Protocol Data Unit) such as a packet. The PDU headers are used by the peer entities to carry out their peer protocol.
Connection oriented and connectionless services
Layers can offer two different types of service to the layers above them.
Connection oriented service is similar to telephone system. To use a connection oriented network service, the service user first establishes a connection, uses it and then releases the connection. It acts like a tube, where the sender pushes objects (bits) in at one end and the receiver takes them out in the some order at other end. In contrast, connectionless service is similar to postal system, where each message carries the full destination address and each one is routed through the system independent of all the others, here the first sent message can be delayed and later may arrive first. But it is not possible in connection-oriented service.
Unreliable connectionless service is often called datagram service, in analogy with telegram service that does not provide an acknowledgement back to the sender.
A service is formally specified by a set of primitives (operations) available to a user or other entity to access the service. These primitives tell the service to perform some action or report on an action taken by a peer entity.
The relationship of services to protocols:
A service is a set of primates (operations) that a layer provides to the layer above it. The service defines what operations the layers are prepared to perform on behalf of its users. A service relates to an interface between two layers, with the lower layer being the service provider and the upper layer being the service user.
A protocol is a set of rules governing the format and meaning of the frames, packets or messages that a re exchanged by the peer entities within a layer. Entities user protocols in order to implement their service definitions. A protocol relates to the implementation of the service and as such is not known to the user of the service.
Wireless LAN: A system of portable computers that communicate by radio can be regarded as a wireless LAN. These have different properties than conventional LAN and require special MAC sublayer protocols. A common configuration for a wireless LAN is an office building with base stations strategically placed around the building. All the base stations are wired together using copper or fiber. If the transmission power of the base stations and portables is adjusted to have a range of 3 or 4 meters, then each room becomes a single cell, and the entire building becomes a large cellular system. But when a receiver is within range of two active transmitters, the resulting signal will generally be garbled and useless.
IEEE named a wireless LAN as 802.11 and a common name for it is WiFi. Wireless LAN had to be basically operated in two modes:
- In the presence of base station
- In the absence of base station
In first mode, all communication had to go through the base station, called an access point, whereas in the later mode, the computers communicate each other directly. This mode is also known as ad hoc networking. By mid 1990s, as the Ethernet had already started to dominate the LAN, 8002.11 was decided to make compatible with Ethernet above the data link layer such that it should be possible to send an IP packet over the wireless LAN the same way a wired computer sent an IP packet over Ethernet. But in the physical and data link layers, there are several inherent differences between the Ethernet and a wireless LAN which had to be dealt with.
First, a computer on Ethernet always listens to the ether before transmitting. Only if the ether is idle, the computer begins transmitting. But it is not practicable in case of wireless LAN.
Second, a radio signal can be reflected off solid objects, so it may be received multiple times (along multiple paths), and hence such interference is known as multipath fading.
Third, most of the application will not be aware of mobility. For example, a Windows 95 machine may have one specific default printer. Which the computer is taken into new environment, it is unable to print at all until the changes are made.