Physical Topology

The term physical topology refers to the way in which a network is laid out physically.Two or more devices connect to a link: one or more links from a topology. The topology of a network is the geometric representation of relationship of all the links and devices to one another.There are four possible basic topologies:mesh,star,bus,and ring.
In mesh topology, every device has dedicated point-to-point link to every other device.In a star topology each device has a dedicated point-to-point link only to a central controller,usually called hub.A bus topology uses multipoint link.One long cable, called bus, acts as a backbone to link all devices in a network.Nodes are connected to the bus cable by drop lines and taps(connectors).
In ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it.A signal is passed along the ring in one direction, from device to device, until it reaches its destinations.Each device in the ring incorporates a repeater.When a device receives a signal intended for another device, its repeater regenerates the bits and passed them along.
Each topology has its own advantages and disadvantages.A mesh guarantees that each connection can carry its own data load, eliminating traffic problems.It also robust in the sense that if one link becomes unusable,it does not incapacitate the entire network.The main disadvantages of mesh topology are related to the amount of cabling and the number of input/output ports required.
A ring relatively easy to install and configure.In addition,fault isolation is simplified.However,a break in the ring(such as a disabled station)can disable the entire network.The weakness can be solved by using a dual ring or a switch capable of closing off the break.Ring topology was prevalent when IBM introduced its local-area network Token Ring.Today,the need for higher-speed network has made a topology less popular.
A bus topology can be installed easily.Backbone cable can be laid along the most efficient path, then connected to nodes by drop lines of various lengths.However ,a fault or break in the bus cable stops all transmissions, between even between devices on the same side of the problem selection.Today the most common topology in high-speed  local-area networks is a star.A star topology is less expensive than a mesh topology,but has most of advantages of the latter.One big disadvantages of a star topology is the dependency of the whole topology on
one single point,the hub.If the hub goes down,the whole system is dead.However,the low cost of a star topology and ease of installation and scalability has made the star topology the only common topology.

Network Criteria

First of all i want to make a introduction that what the network is. A network is a combination of hardware and software that sends data from a location to an other. The hardware consists of the physical equipment that carries signals from one point in the network to another. The software consists of instructions that make the services that we expect from a network possible.
The network must be able to meet a number of criteria.Three most important are performance, reliability and sequrity

                   Performance

Performance can measured in many ways, including transit  time and response time.Transit time is the amount of time required for a message to travel from one device to another.Response time is elapsed time between an enquiry and a response.The performance of a network depends on a number of factor, including the number of users,the type of transmission medium, the capabilities of the connected hardware, and the efficiently of the software

                  Reliability

In addition to accuarcy of delivery, network reliability is measured by frequency of failure, the time it takes to recover from a failure, and network’s robustness

                  Sequrity

Network security issues include protecting data from unauthorized access, damage and change, and implementing policies and procedures for recovery from breaches and data losses.We are going to discus later for advanced sequrity.