Chapter 7: Local Area Networks Part 1

Introduction:

A local area network is a communication network that interconnects a variety of data communications devices within a small geographic area and transmits data at high data transfer rates. The devices that are implied in this definition include: personal computers, computer workstations, mainframe computers, peripheral devices, printers, and modems. The portion of the definition that discusses the "small geographic area" is explaining that a LAN can be a small room, multiple rooms, or multiple floors within a building, and even multiple building on a single campus. The transferring of data at high data rates start from 10 million bits per second to 10 billion bits per second.

Primary function of local area networks:

Users expect a LAN to perform various activities that include: access to internet, file serving, database and application serving, print serving, e-mail serving, process control and monitoring, and distributed processing. Three of the most common application areas for local area networks are office environments, academic environments, and manufacturing environments.

Advantages and disadvantages of local area networks:

Some of the advantages of LANs are their abilities to share resources, independence of system evolution and vice versa, high transfer rates, and low error rates. The disadvantages of local area networks include: high costs for the hardware, operating systems, and software that run the networks; issues with purchasing the wrong user license, and the amount of hours that must dedicate to attending and servicing the networks. The last downfall of a LAN is if an employee upgrades a server, cabling, or network operating system, some other part of the network may need to be improved because of the recent upgrade. A local area network is only as strong as its weakest link.

First local area network: The BUS/TREE:

The bus\tree local area network (bus LAN) was the first physical design when LANs were first available. This network is consisted of a simple cable, or bus, to which all devices are attached. Each device that is attached to the BUS/Tree network is required to have a "tap," which is also called a "passive device." A tap does not alter the signal and does not require electricity to operate. The workstation will contain a "network interface card" (NIC), which is an electric device that performs the necessary signal conversions and protocol operations that allow the workstation to send and receive data.

There are two different types of signaling technologies that can implemented with the BUS/Tree network: baseband signaling and broadband signaling. The baseband signaling uses a digital signal to transmit data over the bus. Baseband transmission is also bidirectional. Broadband technology uses analog signaling in the form of division multiplexing to divide the available medium into multiple channels. The disadvantage with both technologies is that both are difficult to add new workstations if there is no tap for the existing computer.

A more modern LAN:

Star-wired bus LAN is considered to be the most popular configuration for a local area network.  The network logically acts like a bus, but it physically looks like a star. The logical design of a network determines how the data moves around the network from workstation to workstation. The physical design is the pattern the network is formed in. With the networks that contain "hubs" is the medium access control protocol, which is software that allows a device to place data onto a hub-based local area network. There are two basic categories of medium access control protocols for LANs which are: contention-based protocols, and round-robin protocols.

Contention-based protocol is a first-come, first-serve protocol. The first station to recognize that no other station is transmitting data and place its data onto the medium is the first station to transmit. The most popular contention-based protocol is the carrier sense multiple access with collision detection (CSMA/CD). A collision occurs when two workstations listen to the medium at the same time and hear nothing, provoking both workstations to transmit at the same time.

Switches:

Switches use addresses and processing power to direct a frame out of a particular port, thus reducing the amount of traffic on the network. These devices can be used to interconnect multiple workstations on a local area network or multiple LANs.

Virtual LANs:

A virtual LAN (VLAN) is a logical subgroup within a local area network created via switches and software rather than by moving wiring from one network device to another.