LAN INSTALLATION AND PERFORMANCE
Once the LAN has been selected based on the requirements of the organization, it must be installed by the people within the organization. However, consultants can be hired to do all or most of the work or to oversee the portions, such as installing wire or cable, that employees may not be able to do themselves. Several suppliers of LAN hardware or software may have been contacted in the course of evaluating and selecting the LAN, and these companies may offer installation and maintenance services. Negotiating the installation should have been considered part of the LAN's total cost calculation.
If the basic work is to be done by an outside company, it is a good idea for the organization's LAN administrator to be involved in the installation process. By being

involved the administrator can gain detailed knowledge of the LAN and its components, and will be in a better position to handle problems and make changes afer it is operational. Important tasks of installing a LAN are given below:
• Install:
New workstations
NIC's on existing workstations Wiring or cabling
Server hardware
Bridges, souters, brouters, or gateways LAN software
• Determine the access and capability required by each user • Document the LAN's hardware and software configuration • Train the users
• Using the LAN and its new capabilities • Troubleshoot any startup problems.
As with any project, it is beneficial to have a project plan that identifies the tasks (in detail), the person or group responsible for performing the task, the date for starting the task, and the target date for completing the task.
The performance of LAN is based on several factors, including the protocol that is used, the speed of the transmissions. the amount of traffic, the error rate, the efficiency of the LAN software, and the speed of server computers and disks.

  ETHERNET: IEEE STANDARD 802.3
IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation, Intel Corporation and Xerox. This was called Ethernet.Ethernet has a bus topology. Stations contend for the segment using a form of the CSMA/CD contention protocol. It is commonly used to connect PCs, workstations, printers and file servers and even mainframes.
To fully understand the IEEE 802.3 standard (and other LAN standard as well), it is important to understand where it fits in a layered design and how it relates to other topics. Network operations typically are defined by the lowest three layers of OSI model, that is, the physical, data link and network layers. The data link layer performs services for the network layer and assumes the existence of the physical layer. Specifically, the data link layer is responsible for accurate communication between two nodes in a network. This involves frame formats, error checking and flow control. Data link layer is further divided into two sublayers:
(1) Logical Link Control (LLC)
(2) Medium Access Control (MAC).
Figure 6.3 illustrates data link layer refinement. Logical link control handles logical links between the stations, whereas MAC controls access to the transmission medium. IEEE 802.2 standard is an LLC and is based on the HDLC protocol. IEEE 802.3 standard is an MAC protocol.

Medium access sublayer
Network can be divided into two categories: point-to-point network and broadcast network. In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it.
To make this point clear, consider a conference call, in which six people, on six different telephones are all connected together so that each one can hear and talk to all the others. It is very likely that when one of them stops speaking two or more will start talking at once, leading to chaos. In a face-to-face meeting, chaos is avoided by external means, for example, at a meeting, people may raise their hands to get permission to speak. When only a single channel is available, determining who should go next is much harder.
Many protocols for solving the problem are known. In literature, broadcast channels are sometimes referred to as multi-access channels or random access channels. The protocols used to determine who goes next on multi-access channel belong to a sublayer of the data link layer called the multiple access protocols. Many algorithms for allocating multiple access channel are known. Some of these are: Pure Aloha
Slotted Aloha Carrier Sense Multiple Access (CSMA) CSMA with Collision Detection.

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