Auto-sensing

- Autosensing is a feature of so-called "10/100" Ethernet hubs, switches, and NICs.
- Compatible Ethernet speeds can be selected using low-level signaling techniques probing the capability of the network.
- Autosensing was developed to make the migration from traditional Ethernet to Fast Ethernet products easier.

When first connected, 10/100 devices automatically exchange information with each other to agree on a common speed setting. The devices run at 100 Mbps if the network supports it, otherwise they drop down to 10 Mbps to ensure a "lowest common denominator" of performance. Many hubs and switches are capable of autosensing on a port-by-port basis; in this case, some computers on the network may be communicating at 10 Mbps and others at 100 Mbps. 10/100 products often incorporate two LEDs of different colors to indicate the speed setting that is currently active.

Auto-sensing is an active method of determining link mode. Each interface is expected to transmit specific information in a specific format. If an interface that is expecting to use auto-sensing does not receive this information from the other side, it assumes the other side cannot detect or change its mode.

Ethernet Hubs

A hub connects multiple devices together. Ethernet hubs, and are most commonly used in computers for networking purposes. Ethernet hubs are available in different types, depending on the speed of the network connection or broadband speed. The number of ports an Ethernet hub supports also varies. Older Ethernet hubs were relatively large in size and sometimes noisy as they contained built in fans for cooling the unit. Newer devices are much smaller, designed for mobility, and noiseless.

Working of an Ethernet Hub

The main purpose of the Ethernet hub is to transmit the large pockets or cluster of data it receives from one computer onto another through all the ports connected to it. Ethernet uses a protocol called CSMA/CD, which stands for Carrier Sense, Multiple Access with Collision Detection.

- Carrier Sense - When a device connected to an Ethernet network wants to send data it first checks to make sure it has a carrier on which to send its data.
- Multiple Access - This means that all machines on the network are free to use the network whenever they like so long as no one else is transmitting.
- Collision Detection - A means of ensuring that when two machines start to transmit data simultaneously, that the resultant corrupted data is discarded, and re-transmissions are generated at differing time intervals.

Hubs and their types

Hubs have become an integral part of various network and business systems. Hub, sometimes referred to as a concentrator or repeater Hub, refers to a networking component which acts as a convergence point of a Network, allowing the transfer of data packets.

Characteristics of Hubs :
- Hub is a small plastic box which takes its power from an ordinary wall outlet.
- Multiple computers are joined through a hub.
- On this network segment, all computers can communicate directly with each other.
- A hub includes a series of ports that each accept a network cable.
- Hubs are the layer 1 devices while switches and routers are layer 2 and layer 3 devices respectively.
- Hubs do not read any of the data passing through them and are not aware of their source or destination. Essentially, a hub simply receives incoming packets, possibly amplifies the electrical signal, and broadcasts these packets out to all devices on the network - including the one that originally sent the packet.

Types of Hubs :
- Passive Hubs : They do not amplify the electrical signal of incoming packets before broadcasting them out to the network. Their contribution in enhancing the performance is very less. It does not help in any way in the troubleshooting operations. Most of the passive hubs are easily obtainable at a lesser cost.
- Active Hubs : They amplify the incoming signals. An active hub is sometimes referred to as multiport repeater. An active hub takes a larger role in Ethernet communications with the help of technology called store & forward. If the data received being weak but readable, the active hub restores the signal before rebroadcasting the same. An active hub provides information n devices on the network which are not yet fully functional.
- Intelligent Hubs : This hub typically behaves like a stack. It is built in such a way that multiple units can be placed one on top of the other to conserve space. It has the ability to manage the network from one central location. With the help of an intelligent hub, one can easily identify, diagnose problems and even come up with remedial solutions.

Bus Network Topology

A bus network is an arrangement in a local area network (LAN) in which each node (workstation or other device) is connected to a main cable or link called the bus. Bus networks are the simplest way to connect multiple clients, but may have problems when two clients want to transmit at the same time on the same bus. A true bus network is passive – the computers on the bus simply listen for a signal; they are not responsible for moving the signal along.
The bus topology makes the addition of new devices straightforward. The term used to describe clients is station or workstation in this type of network. Bus network topology uses a broadcast channel which means that all attached stations can hear every transmission and all stations have equal priority in using the network to transmit data.



Advantages :
* Easy to implement and extend.
* Well-suited for temporary or small networks not requiring high speeds (quick setup).
* Cheaper than other topologies.
* Cost effective; only a single cable is used.
* Easy identification of cable faults.
* Reduced weight due to fewer wires.

Disadvantages :
* Limited cable length and number of stations.
* If there is a problem with the cable, the entire network goes down.
* Maintenance costs may be higher in the long run.
* Performance degrades as additional computers are added or on heavy traffic.
* Proper termination is required.
* Significant Capacitive Load.
* It works best with limited number of nodes.
* It is slower than the other topologies.

Star Network Topology

Star Topology is the most common type of network topology that is used in homes and offices. In the Star Topology there is a central connection point called the hub which is a computer hub or sometimes just a switch.
In local area networks where the star topology is used, each machine is connected to a central hub. In contrast to the bus topology, the star topology allows each machine on the network to have a point to point connection to the central hub. All of the traffic which transverses the network passes through the central hub.



Advantages of Star Topology :
- A Star Network Topology is very easy to manage because of its simplicity in functionality.
- The problems can be easily located logically in a Star Topology and therefore is easy to troubleshoot also.
- The Star Topology is very simple in format so it is very easy to expand on the Star Topology.

Disadvantages of Star Topology :
- The Star Topology is fully dependent on the hub and the entire working of the network depends on the hub or the switch.
- If there are many nodes and the cable is long then the network may slow down.

Extended Star Network :
A type of network topology in which a network that is based upon the physical star topology has one or more repeaters between the central node and the peripheral or 'spoke' nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes.

Distributed Star Topology :
A type of network topology that is composed of individual networks that are based upon the physical star topology connected together in a linear fashion – i.e., 'daisy-chained' – with no central or top level connection point (e.g., two or more 'stacked' hubs, along with their associated star connected nodes or 'spokes').

Hierarchical or Tree Network Topology

In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.
This type of topology suffers from the same centralization flaw as the Star Topology. If the device that is on top of the chain fails, consider the entire network down.Obviously this is impractical and not used a great deal in real applications.Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the 'branching factor' of the hierarchical tree.

- A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central 'root' node and only one hierarchical level below it would exhibit the physical topology of a star.
- The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network.
- If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy. Such a type of network topology is very useful and highly recommended.

What are Routers, Switches, Hubs — Is There a Difference between them ?

Each device has its own function in a network environment. Two reasons confusion exists are
(1) all three are simple boxes with several plugs that accept cables and
(2) at times the functions of each device are rolled into one single device.

A hub, also known as a repeater, is a simple device used for years to connect all nodes, or computers, on a network to a central location. Each node on a network has a unique hardware address called a MAC address. A hub is known as a repeater because when a packet of data, or frame, is sent through the hub, it is repeated to each and every computer on the network.

A switch-based network is one that utilizes switches instead of hubs. A switch is a major upgrade to a hub. Instead of sending all network data to each and every network node, the switch will analyze the MAC address and determine where to send the data. Network bandwidth is not wasted by sending every frame to every port.

Routers not only provide connections to the internet, they also protect the LAN from the Internet. The router could block any Packet that has a destination address outside of the LAN. In short, a router can perform many of the same functions as switches and hubs, but it has address translation and filtering capabilities.