LOCAL AREA NETWORK (LAN) AND TOPOLOGIES

LOCAL AREA NETWORK (LAN) 

Definition: 

• A local area network (LAN) supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home.
•  A LAN is useful for sharing resources like files, printers, games or other applications.
• A LAN in turn often connects to other LANs, and to the Internet or other WAN.
• Most local area networks are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs.
• Wireless LAN and other more advanced LAN hardware options also exist.

LAN Capabilities

• One of the main capabilities of a local area network is resource sharing, such as data and expensive peripherals. 
• This ability to share resources can mean a decrease in the cost of an individual workstation, since not every workstation may need its own printer or hard disk.
• A LAN may also provide better reliability and availability than a centrally controlled network. The failure of a workstation does not affect other users on the LAN unless the failure point is a server. A workstation on a LAN usually has more flexibility and function than a fixed function terminal connected to a host system. 
• For example, the LAN user can frequently work as a stand-aloneworkstation, share resources on the LAN, or be in session with systems externalto the LAN. 
• Because the LAN attachments must have some degree of intelligence to support the LAN protocols, they may also provide management or problem determination support through those protocols.

Planning for a LAN

In planning for a local area network consider the following:

·          The organization′s objectives
·        End-user requirements
·        Number and location of workstations
·        Requirements for special functions or capabilities such as       backup
·        Security
·        Network and systems management

BUS TOPOLOGY

• The bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus.
• Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus.
• A transmission from any station propagates the length of the medium in both directions and can be received by all other stations.
•  At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.

TREE TOPOLOGY

1. The tree topology is a generalization of the bus topology.
2.  The transmission medium is a branching cable with no closed loops.
3. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches.
4. The branches in turn may have additional branches to allow quite complex layouts. Again, a transmission from any station propagates throughout the medium and can be received by all other stations.
5. Two problems present themselves in this arrangement. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended.
6. Second, a mechanism is needed to regulate transmission.

RING TOPOLOGY

1. In the ring topology, the network consists of a set of repeaters joined by point-topoint links in a closed loop.
2. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater.
3. The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way.
4. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).
5. Each station attaches to the network at a repeater and can transmit data onto the network through that repeater.
6. As with the bus and tree, data are transmitted in frames.
7. As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by.
8. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

STAR TOPOLOGY

1. In the star LAN topology, each station is directly connected to a common central node. 
2. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception.
3. In general, there are two alternatives for the operation of the central node.
4. One approach is for the central node to operate in a broadcast fashion.
5. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links.
6. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit.
7. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station. 

LAN COMPONENTS

Network Adapter

• A computer needs a network adapter to connect to a network. It converts computer data into electronic signals.
• It listens for silence on the network cable and applies the data to it when it has an opportunity.
• The network access element of its job is called Media Access Control, or MAC.
• The physical address of every computer on a network is called its MAC address. The MAC address is the network adapter's serial number.
• Most computers are shipped with the network adapter integrated into the motherboard. The wireless equivalent is called a Wireless Network Interface Controller.

Network Medium

• Wired networks need cable.
• The most common form of cable used in networks is called the "Unshielded Twisted Pair."
• In PC shops, it is generally just referred to as "network cable" or "Ethernet cable."
• Ethernet is the most widely implemented set of standards for the physical properties of networks.
• UTP is so closely identified with Ethernet that it is often given that name.
• Other cable types used for networks are twin-axial, Shielded Twisted Pair and single-mode and multi-mode fiber optic cable.
• Wireless networks don't need cable; they send data on radio waves generated by the WNIC.

Cable Connectors

• In wired networks, the most common form of connector is the RJ45.
• Every computer with networking capabilities has an RJ45 port. This is sometimes called a "network port" or an "Ethernet port."
• The RJ45 plug looks like a slightly larger telephone plug and connects the Unshielded Twisted Pair or the Shielded Twisted Pair cable.

Power Supply

• Both wired and wireless networks need a power supply.
• A wireless network uses the current to generate radio waves.
• A cabled network sends data interpreted as an electronic pulse.

Hub/Switch/Router

• In wired networks, one computer cannot connect to many others without some form of splitter.
• A hub is little more than a splitter.
• It repeats any signals coming into one of its ports out onto all its other ports.
• A cable leads from each port to one computer.
• A switch is a more sophisticated version of a hub.
• It only sends the signal on to the computer with the address written in the arriving message.
• Routers are much more complicated and are able to forward messages all over the world.
• Larger networks sometimes use routers for their LAN traffic.
• The wireless networking device is called a "wireless router."

Network Software

• Software on a communicating computer packages data into segments and puts that data into a structure called a "packet."
• The source and destination addresses of the packet are written into the header of the packet.
• The receiving computer needs to interpret these packets back into meaningful data and deliver it to the appropriate application.